Review pubs.acs.org/CR
Fluorescence and Colorimetric Chemosensors for Fluoride-Ion Detection Ying Zhou,†,‡ Jun Feng Zhang,§ and Juyoung Yoon*,† †
Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea Key Laboratory of Medicinal Chemistry for Natural Resource, School of Chemical Science and Technology, Yunnan University, Kunming, 650091, P. R. China § College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming 650500, P. R. China ‡
Corresponding Author Notes Biographies Acknowledgments Abbreviations References
1. INTRODUCTION In recent years, the development of methods to recognize and sense biologically and environmentally important species has emerged as a significant target in the field of chemical sensors.1−12 Although anions are known to play important roles in a wide range of chemical and biological processes, they have become a target of investigations aimed at sensor development only in the past decade.13−28 Among these anions, fluoride, having the smallest ionic radius, highest charge density, and a hard Lewis basic nature, has arisen as an attractive target for sensor designs owing to its association with a diverse array of biological, medical, and technological processes. Consequently, sensing and recognition of fluoride ion has become a highly popular topic of interest to scientist whose research focuses on host/guest and supramolecular chemistry. Knowledge about the importance of fluoride ions in biological and medical processes has grown in recent years. It is now known that this anion plays a role in dental health29 and has potential use for the treatment of osteoporosis.30,31 Fluoride is easily absorbed by the body but is excreted slowly. As a result, overexposure to fluoride can lead to acute gastric and kidney problems.32 In several underdeveloped countries, excessive fluoride levels in drinking water have been linked to the debilitating bone disease fluorosis. Fluoride also plays an important role in a number of military applications. For example, the refinement of uranium in nuclear weapons manufacturing utilizes fluoride.33 GB, more commonly known as sarin (isopropyl methylphosphonofluoridate), is a nerve agent employed by the Aum Shinrikyu cult in terrorist attacks in Tokyo in 1995. Fluoride is a product of the hydrolysis of sarin (and soman, a related G-type agent), and consequently, the ability to monitor fluoride in victims and surrounding environments after a terrorist incident using this nerve gas is of great value.34,35
CONTENTS 1. Introduction 2. Sensors Based on the Interactions between Fluoride Ions and Lewis Acids 2.1. Sensors Based on F−-Promoted Cleavage of Silicon−Oxygen Bonds 2.2. Sensors Based on F−-Promoted Cleavage of Silicon−Carbon Bonds 2.3. Sensors Based on the Interactions between Fluoride Ions and Boronic Acid Moieties 2.4. Sensors Based on Displacement Ensembles Using Metal Complexes 3. Sensors Utilizing Hydrogen-Bond Interactions 3.1. Sensors Bearing Thiourea Binding Sites 3.2. Sensors Bearing Urea Binding Sites 3.3. Sensors Bearing Amides as Binding Groups 3.4. Sensors Bearing Sulfonamides as Binding Sites 3.5. Sensors Bearing Imidazoline or Imidazole Groups 3.6. Sensors Bearing Indole Groups 3.7. Sensors Bearing Pyrrole Groups 3.8. Sensors Bearing Schiff Base Groups 3.9. Sensors Bearing Other Groups 4. Reaction-Based Sensors for the Detection of Fluoride Ion 5. Fluoride-Ion Sensors Based on Polymers 6. Fluoride-Ion Sensors Based on Quantum Dots and Gold Nanoparticles 7. Fluoride-Ion Detectors Based on Mesoporous Silica or Silica Particles 8. Conclusions Author Information © 2014 American Chemical Society
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Received: January 23, 2013 Published: March 25, 2014 5511
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Figure 1. Cleavage of the SiO bond of 1 by F− and structures of 2−4.
Despite the growing interest, the development of fluoride probes and anion sensors in general has been particularly challenging. Specifically, the requirement that these detection systems exhibit anion selectivities is a hurdle that needs to be overcome. Among several widely used fluoride-detecting and -sensing techniques, including the electrode method,36 19F NMR analysis,37 and colorimetric (UV) and fluorescence sensing, electrochemical systems have become the most wellestablished. However, this approach has major disadvantages associated with the need for fragile instrumentation and timeconsuming manipulations.38 In addition, 19F NMR spectroscopy can be used reliably to detect only micromolar levels of fluoride. Moreover, neither the electrochemical nor the NMR approach can be miniaturized for use in studying biological processes in vivo. As a result of these deficiencies, colorimetric and fluorescence sensor systems have attracted the greatest attention because they have been shown to have high sensitivities, with detection limits as low as sub-parts-permillion, and the capability of being employed for intracellular fluoride monitoring. Over the past two decades, a large number of optical sensors have been developed for detecting fluoride. For the most part, the methods employed in these sensors rely on strong interactions taking place between Lewis-acidic boron and fluoride, as well as hydrogen bonding and other types of interactions involving fluoride as a participant or a disrupter. In addition, reaction-based chemodosimeters have been devised for the selective detection of fluorides. In this review, recent studies aimed at the development of fluorescence and colorimetric chemosensors for fluoride are discussed using a format in which sensor types are classified on the basis of their topological and structural classes as well as the chemistry leading to optical responses. The topics covered include reaction-based chemosensors that utilize F−-promoted cleavage reactions; boron−fluoride interaction-based chemosensors; hydrogen-bond and π−π interaction-based chemosensors containing thiourea/urea, amide/sulfonamide, imidazoline/ imidazole, indole, pyrrole, Schiff base, and other groups as binding units; chemosensors bearing metal ion binding sites; and finally chemosensors based on polymers, quantum dots,
gold nanoparticles, mesoporous silica, silica particles, and other types of materials.
2. SENSORS BASED ON THE INTERACTIONS BETWEEN FLUORIDE IONS AND LEWIS ACIDS One important approach to the design of fluoride-selective optical sensors relies on unique interactions that occur between fluoride and Lewis acids. One general strategy of this type employs familiar, fluoride-promoted SiO and SiC cleavage reactions, owing to the large differences among SiC, SiO, and SiF bond dissociation energies (69, 103, and 141 kcal/ mol,39 respectively). Reactions that form SiF bonds at the expense of cleavage of SO and SiC bonds take place both rapidly and nearly irreversibly. Another approach takes advantage of the high propensity to form fluoroborate anions through reactions of borates with the fluoride anion. In the design of these types of fluoride chemodosimeters, the SiF and BF bond-forming processes generate fluorophores or chromophores from nonoptically responsive precursors. Several thorough reviews describing fluoride chemosensors that are based on unique interactions between fluoride and boron have been written by Gabbai ̈ and co-workers.40 Because of the earlier coverage, this topic will receive only brief attention in the current review. In another family of sensors discussed in this section, interactions between various metal ions and fluoride are used to promote displacement reactions that lead to fluorescence or color responses. 2.1. Sensors Based on F−-Promoted Cleavage of Silicon−Oxygen Bonds
Recently, great effort has been directed toward the development of reaction-based chemodosimeters for the detection of metal ions, reactive anions, oxygen species, thiols, and other important substances. Even though they often participate in irreversible chemical processes, chemodosimeters typically display higher selectivities than do their chemosensor counterparts. Silyl groups, which are widely used as protection groups for hydroxyl-containing substances, have been used as key reactive elements in fluoride-selective chemodosimeters. In this section, recent examples of fluoride-promoted selective SiO cleavage-based chemodosimeters for fluorides are discussed. 5512
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The tert-butyldiphenylsilane (TBDPS) group has become a common electrofugal group in reaction pathways that serves as the basis of fluoride-sensing systems. In 2007, Kim and Hong described chemodosimeter 1 (see Figure 1), which releases resorufin through substitution by F− (TBA+ and Na+ salts) at the silicon center.41 As a consequence of this reaction, CH3CN/H2O (50:50, v/v) and acetonitrile solutions of 1 treated with fluoride undergo dramatic changes in their UV−vis absorption spectroscopic properties corresponding to a change in color from yellow to pink. Also, upon the addition of F− to a solution of 1 in CH3CN, the fluorescence emission intensity at 591 nm increases by 500-fold and then reaches a maximum value after addition of 1400 equiv of F−. In CH3CN/H2O, the fluorescence intensity at 589 nm promoted by reaction of 1 with fluoride is enhanced by 200-fold after addition of 3000 equiv of F−. In the sensing mechanism, the displacement reaction of fluoride at the silyl ether center takes place to produce a phenolate anion, which then undergoes fragmentation to form p-quinomethane and resorufin, the latter of which serves as the chromophore and fluorophore responsible for the light absorption and emission changes. Two years later, Kim et al. synthesized another fluoride sensor, 2, that contains a tertbutyldiphenylsiloxy group as a trigger and a coumarin as the optical signaling group. This fluoride-ion probe, whose fluorescence intensity at 461 nm in HEPES buffer (pH 7.4) increases with increasing fluoride concentrations, was used for fluorescence cell bioimaging42 and for the detection of NaF in A549 human epithelial lung cancer cells under physiological conditions. A near-infrared (NIR) fluorescence chemodosimeter for F− that incorporates a tert-butyldiphenylsiloxy group and a dicyanomethylene-4H-chromene moiety was developed by Cao et al.43 In DMSO/H2O (95:5, v/v) solution, F− triggers specific SiO cleavage of 3 and promotes a dramatic color change from light yellow to blue. In addition, a significant turnon fluorescence response takes place at 718 nm corresponding to a 1000-fold intensity enhancement upon addition of 400 equiv of F−. Hu et al. developed the closely related sensor 4, which has a dual-wavelength fluorescence sensing capability.44 Using 4, fluoride ions can be detected at the 100 ppb level by determining the disappearance of blue-violet fluorescence and the appearance of bright yellow fluorescence, changes that can be observed by the naked eye. Specifically, addition of F− to 4 results in SiO bond cleavage to generate a benzothiazole derivative that exhibits a bright yellow emission at 560 nm in water. This effort also demonstrated that this sensor system can be absorbed on a paper sheet that can be used as a convenient and reliable tool for detecting F−. The new ICT-based fluorescent chemodosimeter 5 for the detection of F−, which contains a protected form of benzothiazolium hemicyanine dye as a latent fluorophore, was developed by Zhang’s group (see Figure 2).45 Reaction of 5 with F− triggers cleavage of the SiO bond and releases the long-wavelength-fluorescing product 6. Gradual addition of F− to a solution of 5 in an ethanol/water (3:7, v/v) mixture containing PBS causes a 110-nm red shift in the absorption maximum and an associated color change from light yellow to orange. Concurrently, F− addition causes a change in the wavelength of the emission maximum from 500 to 558 nm. The probe, which has a detection limit of 0.08 mM, was used for bioimaging of F− in live RAW 264.7 macrophage cells.
Figure 2. Proposed reaction mechanism of 5 and F−.
Wei et al. developed another novel fluorescent probe, 7, for F− detection (TBA+ and Na+ salts; Figure 3) that operates on the basis of a desilylation-triggered chromogenic reaction in 100% water.46 Probe 7 exhibits an excellent selectivity for the F− ions with a detection limit of 10.5 μM. The SiO-bondcleaving reaction of fluoride with 7 to produce a quinone product is responsible for the optical response of this sensor. This reaction, employing fluoride at concentrations as low as 1.5 mg L−1, produces a color change that is visible to the naked eye. The sensor was utilized in the reported study for imaging F− in HepG2 cells. The triisopropylsilane (TIPS) group, in the form of silyl ethers, has also been employed as a trigger in F−-sensing systems. For example, Kim’s group developed the highly selective naphthalimide-based colorimetric and ratiometric fluorescence probe 8 (Figure 4). Importantly, this sensor displays both one- and two-photon ratiometric changes.47 At lower F− concentrations in MeCN, the intensity of the absorption band of 8 at 365 nm decreases, and a new band with a shoulder at 487 nm appears. As the F− concentration is increased, the intensity of the shoulder at 487 nm decreases, and a new absorption maximum at 425 nm is generated with a corresponding dramatic color change from colorless to jadegreen. In addition, when F− is added to a MeCN solution of 8, the intensity of the emission band at 449 nm decreases, and the band at 508 nm increases. The remarkable fluorescence behavior of 8 is associated with a 220-fold change in the ratio of the emission intensities at 449 and 508 nm that corresponds to a sigmoidal dependence on the F− concentration. Sokkalingam and Lee developed another coumarin-based probe to quantitatively determine F− concentrations in both acetonitrile and water solutions. The sensor, 9 (Figure 5), operates through “turn-on” chromogenic and fluorogenic dual modes that are triggered by fluoride-promoted cleavage of the SiO bond.48 Addition of increasing concentrations of F− anion (as its TBA salt) to an acetonitrile solution of 9 results in a dramatic color change from colorless to yellow along with the appearance of strong fluorescence emission at 500 nm. The estimated detection limit of this sensor for F− was found to be 50 nM. An easy-to-prepare, short-response-time test system, obtained by simply dipping paper into a solution of a TIPSprotected coumarin derivative, was used to detect F−. Bozdemir et al. fabricated two BODIPY derivatives, 10 and 11 (Figure 6), containing silyl-protected phenolic groups that serve as sensors for fluoride in solution and a poly(methylmethacrylate) matrix.49 Reaction of 10 with fluoride liberates the phenolate group, which serves as a quencher of BODIPY fluorescence at 506 nm. In contrast, fluoride reaction 5513
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Figure 3. Cleavage of the SiO bond in probe 7 by F−.
Figure 4. Mechanism of the reaction of compound 8 with F−.
with styryl-conjugated, silyl-protected BODIPY derivative 11 triggers strong ICT associated with a gradual decrease in the intensity of the absorption band at 560 nm, a concurrent increase in a band at 682 nm, and a purple-to-green color change. Cao et al. described the new coumarin−BODIPY derivative 12 to detect F−, which operates by a siloxy ether deprotection mechanism.50 In the presence of F− in DMSO, 12 undergoes loss of the TIPS group in conjunction with a 88-nm red shift in
Figure 5. Cleavage of the SiO bond of 9 by F−.
Figure 6. Structures of 10 and 11 and cleavage of the SiO bond of 12 by F−. 5514
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Figure 7. Structures of 13−16 and proposed mechanisms of the reactions of 13 and 16 with F−.
of 17 displayed a 3-fold fluorescence increase at 460 nm. The mechanism of fluorescence turn-on in this system is based exclusively on the SiO bond cleavage reactions of 17, which can be effected by F− under mild conditions. Finally, the tris(Nsalicylideneamine) derivative 17 was shown to be a sensitive F− chemosensor. Treatment of a CH2Cl2 solution of 17 (Figure 8) with fluoride was observed to produce a 3-fold increase in fluorescence intensity at 460 nm.
the absorption maximum and a dramatic ratiometric fluorescence response. Specifically, the color of the solution changes from pink to red, and the emission color changes from orange to blue. These optical changes were found to be in accord with the results of density functional theory and timedependent density functional theory calculations. Finally, the detection limit of 12 for F− was determined to be 0.12 × 10−6 M. A wide variety of other types of F− sensors have been developed based on the t-butyldimethylsilyl ether cleavage strategy (see Figure 7). For example, the colorimetric and ratiometric fluorescent chemosensor 13, with a detection limit below 1.0 μM, was prepared by Bai’s group.51 This sensor in THF undergoes a color change from colorless to yellowishgreen (from 313 to 363 and 410 nm), and the intensity of the emission band at 403 nm gradually decreases, being replaced by a new peak at 520 nm [blue fluorescence (λmax = 403 nm) changes to yellowish-green (λmax = 520 nm)]. In addition, naphthalimide derivative 14 has been developed as a colorimetric chemosensor for fluoride with a detection limit of 0.59 μM.52 Addition of fluoride ion to a CH3CN solution of 14 results in a color change from colorless to yellow associated with a decrease in the intensity of the absorption band at 362 nm and growth of a new band at 474 nm. Chemodosimetric gelation system 15, which is transformed into a fluorescent gel in the presence of F− in aqueous media, was developed by Park et al.53 Selective cleavage of a silyl ether protecting group in the sensor gives rise to a turn-on fluorescence at 405 nm and a substance that undergoes sol-to-gel transition in methanol/ HEPES buffer (1:1, v/v; pH 7.40). An intelligent reaction-based relay recognition strategy served as the basis for the design of F− sensor 16 by Dong et al.54 Treatment of this probe with F− generates a prominent decrease in the intensity of the fluorescence band at 360 nm and the growth of a new peak at 460 nm. A tris(Nsalicylideneamine)-derived dynamic fluorophore was developed by Jiang et al.55 Upon addition of F−, a CH2Cl2 solution sample
Figure 8. Structure of 17.
2.2. Sensors Based on F−-Promoted Cleavage of Silicon−Carbon Bonds
In this section, chemosensors for fluoride-ion detection that are based on SiC bond cleavage reactions are discussed. It is known that anionic fluoride reacts selectively with compounds containing the CCSiMe3 group to generate the corresponding terminal acetylenes. This reaction occurs more rapidly than do SiO bond-cleavage processes. One example of a BODIPY dye-containing sensor of this type was developed by Rao et al. for the selective colorimetric and fluorescence detection of fluoride ion (see Figure 9).56 5515
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Figure 9. Structure of 18 and reaction-based chromogenic and fluorescence sensing of F− by 19.
Figure 10. Structure of 20 and proposed sensing mechanism for the interaction of 21 with F−.
Addition of F− ions to 18 in CH2Cl2 results in ratiometric changes corresponding to a decrease in the intensity of the absorption band at 571 nm and the appearance of a new band at 551 nm. This process also takes place concurrently with clear ratiometric changes in the fluorescence spectra involving transformation of the emission from bright orange to green and associated with a decrease in the intensity of the band at 584 nm. Fu et al. created the related BOIDPY dye-containing chemodosimeter 19 (see Figure 9),57 which operates on the basis of a fluoride-promoted desilylation of a silylacetylene and has a limit of detection of 67.4 nM. Addition of F− to an acetone solution of 19 leads to a decrease in the intensity of the absorption band at 555 nm and the appearance of a new peak at 538 nm, processes that occur concomitantly with a decrease in the fluorescence intensity at 571 nm and an increase in the emission intensity at 554 nm (orange to green). The substituted 1,8-naphthalene diimide 20 bearing trimethylsilylacetylene moieties serves as an effective colorimetric and fluorescence chemodosimeter for fluoride-anion detection (see Figure 10).58 An immediate and significant visible change from yellow to dark brown occurs upon addition of F− ions to a solution of this probe. Likewise, a gradual fluoride-promoted increase in the fluorescence intensity of 20 at 329 and 455 nm takes place concurrently. The pyrene derivative 21, containing four trimethylsilylethynyl substituents, was designed by Lu et al.59 as a chromogenic and fluorescence chemodosimeter for fluoride ion. Addition of F− to a THF solution of 21 results in the removal of the trimethylsilyl substituents and causes a blue shift in both the absorption and fluorescence maxima of the probe.
Specifically, a significant visible color change from light green (absorption bands at 436 and 410 nm) to colorless (417 and 392 nm) and a blue-to-purple emission change take place in this system. Moreover, a test paper for fluoride containing this probe is easily made by immersing filter paper into a THF solution of 21. 2.3. Sensors Based on the Interactions between Fluoride Ions and Boronic Acid Moieties
Addition of fluoride ion to triarylboranes results in the formation of fluoroborate anions. Furthermore, as shown in Figure 11, fluoride also readily adds to arylboronic acids.60
Figure 11. Mechanism for the binding of fluoride by a boronic acid moiety.
These unique reactions of fluoride ion with boron moieties coupled with changes in the electronic properties of the boron substituents have been cleverly adopted to develop optical sensors for fluoride by various groups.61 In 2002, six new fluorescence probes, 22−27 (Figure 12), that operate on the basis of interactions between fluoride and boronic acid groups were developed for fluoride sensing.62 All of these substances display spectral shifts and/or intensity changes in the presence of fluoride in water/methanol (2:1 v/ v) solutions, which enable them to be employed as wavelengthratiometric probes. 5516
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Figure 12. Structures of 22−27.
between fluoride and imidazolium groups. This stabilization causes the binding constant of F− with the imidazolium moiety of 29 to be (2.3 ± 0.2) × 106 M−1 in acetonitrile, much larger than that between fluoride and 28. Hydrogen bonding between imidazolium CH and F− in the complex promotes a blue shift of the emission maximum from 445 to 370 nm that is associated with a significant color change. Utilizing this strategy, Badugu et al. developed the new water-soluble quinolinium salt fluorescence probe 30 (Figure 13), which, in aqueous solution, displays ratiometric and colorimetric changes in the presence of fluoride.65 As the concentration of fluoride increases, the absorption band of 30 at 388 nm decreases, whereas the intensity of a new band at 342 nm increases. Similar changes are seen in the fluorescence emission of 30, where the intensity of the band at 546 nm decreases whereas the emission band at 450 nm increases as a function of added fluoride. These and other results demonstrate that the probe 30 responds well to fluoride even when high concentrations of other species are present in aqueous solutions, such as in a biological cocktail consisting of 50 mM glucose, 50 mM aqueous Cl−, and 5 mM fructose. The fluorescein boronic acid probe 31 (Figure 14) was investigated by Swamy et al. as a fluorescence chemosensor for
The fluoride sensor 28 (Figure 13), bearing boronic acid and imidazolium moieties and displaying selective and ratiometric
Figure 13. Structures of 28−30 and proposed mechanism for the reaction of 28 with F−.
fluorescence changes upon addition of fluoride ions, was developed by Xu et al.63 Addition of F− to a CH3CN solution of 28 causes a blue shift in the emission maximum from 440 to 372 nm, which leads to a significant emission color change that is visible to the naked eye. In CH3CN/HEPES (95:5, v/v), F− addition first promotes an increase in the intensity of the emission band of 28 at 440 nm, and then the behavior seen in pure acetonitrile is observed. These findings demonstrate that 28 displays a mutlistep mode of fluoride activation. Specifically, addition of two fluorides to the boron center occurs first and is followed by formation of a (CH)+···F−-type ionic hydrogenbonded complex with the imidazolium moiety. Further studies by Jun et al. further uncovered the new ratiometric fluoride-ion fluorescence probe 29, containing one boronic acid moiety and two imidazolium groups.64 The boron center participates in a cooperative manner with imidazolium to bind F−, with the initially formed BF complex stabilizing the interaction
Figure 14. Proposed mechanism for the reaction of 31 with F−.
F−.66 This sensor participates in an off−on-type fluorescence change that is promoted by blocking fluoresceinamine PET quenching of the singlet excited state induced by the reaction between fluoride and the boronic acid moiety. The apparent association constant for formation of the fluoride−31 complex was determined to be 9.2 × 1010 M−3. Koskela et al. fabricated the two crown-ether-based sensors 32 and 33 (Figure 15), which behave like an AND logic gate involving potassium and fluoride binding as inputs and 5517
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in association with the production of an absorption band with λmax at 571 nm. The formed BA−F− complex has a 1:1 stoichiometry and a binding constant of (1.93 ± 0.53) × 105 M−1. Galbraith et al. designed a new family of boronate blocked phenols including 36−39 (Figure 17) as colorimetric detectors for fluoride and other anions.70 Addition of an excess of TBAF to chloroform solutions of each of these substances promotes a dramatic color change from colorless to yellow (36, 38) or to purple (37, 39). In the cases of 37−39, addition of TBAF causes complete quenching of fluorescence, which is attributed to fluoride-mediated deprotonation. Finally, addition of chloride ion to 39 in a dichloromethane solution produces a selective decrease in the fluorescence intensity as a consequence of associative hydrogen bonding between chloride and two boronic acid groups. Rao and Ravikanth explored the fluoride-sensing properties of 40 (Figure 18), the B(OH)2 complex of the expanded coremodified porphyrin oxasmaragdyrin.71 The results of this effort showed that addition of fluoride to a CH2Cl2 solution of 40 results in a decrease in the intensity of the absorption bands at 446, 475, and 705 nm, concurrent with an increase in new bands at 455, 487, and 720 nm. Using the Benesi−Hildebrand equation, the binding constant for a 1:1 receptor/anion complex formation was determined to be 1.3 × 104 M−1. The addition of increasing amounts of TBAF to a solution of 40 results in quenching of the emission band at 713 nm. The novel fluoride sensor 41 (Figure 19), with a lower-rim boron-substituted calixarene structure, was devised by Arimori et al.72 Titration of a chloroform solution of this substance with Bu4NF results in a decrease of the fluorescence in association with the formation of a bidentate 1:1 “endo” complex, which has a large binding constant (log K1 = 6.3 ± 0.4). The aryl boronate 42 (Figure 19), having a 4-(N,N-methylamino)benzonitrile structure, was reported to be a selective F− sensor.73 In the presence of F−, the absorption bands in the range of 270−320 nm of the sensor undergo a concentrationdependent blue shift. Based on this absorption response, the binding constant of 42 with F− was determined to be (3.4 ±
Figure 15. Structures of 32 and 33.
fluorescence enhancement as the output.67 Cooperative complexation between the metal cation and fluoride anion enables these probes to display selective binding with potassium fluoride over other similar salts. The formation constants (Kobs) between KF and 32 and 33 were determined to be 1 × 10−4 and 320 M−1, respectively. Interestingly, fluorescence from the complex can be “switched off” by removal of the potassium cation from the benzocrown ether receptors of sensors 32 and 33 through addition of the more strongly potassium-cation-binding [2.2.2]-cryptand. The fluoride-selective chemosignaling system consisting of a mixture of the merocyanine dye 34 and phenylboronic acids (PBA−Rs) (Figure 16) was investigated by Cha et al.68 The mechanistic basis for the operation of this sensor involves fluoride-induced displacement of the dye from a complex formed between it and the PBA−Rs in association with the formation of a chromogenic signal. In CH3CN, the stabilities of the complexes of PBA−Rs with 34 were found to be in the following order: PBA−CHO > PBA−NO2 > PBA−CHO-4 > PBA−H > PBA−OCH3. Polymer-bound PBA complexed with 34 also exhibits a fluoride-selective UV−vis absorbance signal with a detection limit of 9 × 10−6 M. A similar displacementbased assay, involving the reaction of Brooker’s merocyanine (BM) and phenylboronic acid (BA), 35 (Figure 16), with fluoride was developed.69 BM, which is violet in acetonitrile solution, becomes colorless when it is present in the BA−BM complex. Only fluoride, among strongly nucleophilic anions, reacts with BA−BM to displace BM and generate a violet color
Figure 16. Proposed mechanism for the reactions of 34 and 35 with F−. 5518
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Figure 17. Structures of 36−39 and mechanism of fluoride-mediated deboronation (38 or 39) or deprotonation (36 or 37) followed by atmospheric oxidation to produce a colored radical.
around 402 nm, and a new emission band around 343 nm simultaneously appears. The source of these optical changes is proposed to be a reduction in ICT interactions with the boronate moiety upon formation of the negatively charged fluoroboronate product. The closely related substance 43 (Figure 19), containing the DMABN (4-N, N-dimethylaminobenzonitrile) structure, was designed as a new ratiometric fluorescence sensor for F− on the basis of expected changes in twisted-intramolecular-chargetransfer (TICT) quenching.74 The results show that 43 displays dual fluorescence with emission bands at 464 and 352 nm. Addition of F− to a solution of 43 causes a decrease in the intensity of fluorescence at 464 nm and an increase in the band at 352 nm. The corresponding binding constants of 43 with F− are estimated to be 1.47 × 104, 2.37 × 103, and 2.10 × 103 M−1 in DMF, THF, and CH2Cl2, respectively.
Figure 18. Structure of 40.
2.4. Sensors Based on Displacement Ensembles Using Metal Complexes
Another strategy for the design of fluoride-ion chemosensors utilizes a displacement approach in which dyes weakly bound to metal ions are released in conjunction with the generation of an optical signal. An example of this strategy is found in studies of the water-soluble fluoride-ion-signaling fluorescence probe 44 (Figure 20) by Sathish et al. in 2007.75 This probe operates through a mechanism involving the exchange of two Alizarin Red S (ARS) molecules coordinated to a central Al(III) ion by fluoride-ion-promoted substitution. The binary complex 44 serves as a sensitive signaling system by undergoing a decrease in its pale orange-colored fluorescence of 44 associated with emission at 575 nm (excitation at 435 nm) in the presence of fluoride ion in the concentration range from 5 × 10−6 to 3 × 10−4 M (detection limit of 0.1 mg L−1). In 2008, these workers described the related sensor 45 (Figure 20), comprising an Al(III) complex with 5-hydroxyflavone (5HF) and coordinated MeOH molecules.76 Successively increasing the fluoride-ion concentration results in a decrease in the intensities of the absorption bands of the complex at 289 and 397 nm and concurrent increases in bands associated with free 5HF at 269 and 336 nm. In addition, the subsequent ligand-exchange reaction of the complex with fluoride ion forms 46 in
Figure 19. Structures of 41−43 and schematic of F− sensor 43 based on TICT.
0.2) × 105 M−1 in THF. In addition, addition of F− also promotes a decrease in the intensity of the emission band 5519
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Figure 20. Structure of 44 and proposed mechanism for the detection of F− by 45.
Figure 21. Proposed binding mode for the detection of F− by 47 and “off−on” fluorescence sensory system 48 for the detection of F− in aqueous media.
conjunction with an increase in the fluorescence at 420. This system can be employed to detect F− at concentrations ranging from 5 × 10−5 to 7 × 10−4 M. A novel chemosensing ensemble probe, 47−Fe (Figure 21), for the detection of fluoride ions was developed by Fu et al.77 Addition of Fe3+ to the free anthracene derivative 47 in aqueous solution at pH 3.5 results in the generation of 47−Fe, a process that causes a decrease in the intensity of the absorption band of 47 at 259 nm and the formation of new red-shifted absorption bands at 350−420 nm. Reaction of the complex with fluoride
promotes regeneration of the absorption and fluorescence of 47, the latter of which is quenched in the complex 47−Fe. Lu et al. developed a simple and efficient method for the selective detection of fluoride anions in neutral aqueous media that is based on the reversible formation of a metal complex (Figure 21).78 This off−on-type sensor system is based on the specific affinity of a thiohydrazide fluorescent receptor for ferric ions associated with the formation of the complex 48−Fe(III). Generation of a 48−Fe3+ complex results in fluorescence quenching of 48 at 400 nm. Upon addition of fluoride, Fe3+ is 5520
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displays an intense blue fluorescence at 460 nm, which decreases upon addition of fluoride ions. This quenching effect is believed to be caused by a ligand-exchange process in which flavonol coordinated to Zr(IV) is replaced by fluoride ions.81 This signaling system can be used to detect F− in the concentration range from 1 × 10−3 to 3 × 10−6 M without any significant interference from other anions. Matsunaga et al. devised a related complex consisting of Zr−EDTA and 3hydroxy-2′-flavone (FS) (Figure 23).82 A solution of this complex in water displays an intense blue fluorescence at 460 nm, whose intensity decreases upon addition of F−. The mechanism for the fluoride-detection process involves ligand exchange of FS bound to Zr−EDTA. This sensor method is both rapid and selective, and it enables F− detection at concentrations ranging from 5 × 10−6 to 1 × 10−3 M. A closely related detection method takes advantage of the transformation of the blue color of PV−Zr−EDTA to orangered of the 1:1 complex in the presence of fluoride (Figure 24).83 A ligand-exchange reaction of PV coordinated to Zr(IV) with fluoride is responsible for this optical change, which can be used to determine fluoride concentrations ranging from 1.5 × 10−5 to 1.5× 10−4 M−1, with a detection limit of 4.5234 × 10−4 M−1. This chemosensor has been used to detect fluoride in industrial effluents. Another simple ligand-exchange-based method for selective fluorescence detection of fluoride ions utilizes a ternary complex containing Zr(IV), EDTA, and 8-hydroxyquinoline (Figure 25).84 The complex undergoes an exchange reaction with F− in the concentration range from 6 × 10−7 to 8 × 10−4 M (detection limit of 12 ppb) in association with a decrease in the intensity of the fluorescence band at 532 nm. A turn-on-type fluoride-selective fluorescence chemosensor relies on the ligand-exchange reaction between fluoride anion and a flavonol-coordinated Zr−EDTA complex (Figure 26).85 The fluorescence intensity at 554 nm of a solution of Zr− EDTA−51 increases by a factor of 6 upon addition of fluoride ion in aqueous solution at physiological pH values. Wang and co-workers prepared the red-emitting ligand 52 and studied its complex with Zr−EDTA (Figure 27), which serves as a highly selective and sensitive sensor for fluoride anions in ethanol/water solution at physiological pH values (6−8).86 The ratiometric signal used for fluoride-selective sensing is a consequence of inhibition of an ESIPT process caused by a ligand-exchange reaction occurring between fluoride anions and flavonol coordinated to Zr−EDTA. The intensity of the absorption peak at 510 nm gradually decreases, and a new peak at 460 nm emerges upon the addition of fluoride anions to the aqueous solution of equimolar amounts of Zr−EDTA and 52. Moreover, addition of F− to the solution of Zr−EDTA−52 causes a decrease in the intensity of emission
displaced from the in situ formed complex, a process that leads to recovery of the fluorescence of the receptor. A low detection limit of 140 μM and a high selectivity of F− over AcO− were reported for this system. Rochat and Severin developed the simple metal-chelationbased sensor 49 (Figure 22) for the detection of fluoride in
Figure 22. Structures of 49 and 50.
water (pH 7.0, MOPS buffer).79 Complexation with Ca2+ results in quenching of the fluorescence of 49 at 370 nm. Addition of fluoride to the complex leads to partial precipitation of CaF2 and an enhancement in fluorescence. The intensity of the emission was found to be dependent on the concentration of fluoride over the range of 0.5−5.0 mM with a detection limit of 0.30 mM. Importantly, this fluoride assay technique has been used for the analysis of dental health products, such as mouthwashes and toothpastes. A fluoride sensor based on 1,3-di-p-nitrophenylazocalix[4]arene-calix[4]pyrrole (50) was described by Thiampanya et al.80 In CH3CN, the color of a solution of 50 (Figure 22) changes from light orange to light sky blue, blue, light pink, and light yellow upon addition of F−, CH3CO2−, PhCO2−, and H2PO4− ions, respectively. When Ca2+ is added to a solution of the complex formed with fluoride, 50−F−, the F− ions are removed to form the CaF2 salt, which results in revival of the original absorption spectrum of 50. It has been demonstrated that Zr−EDTA binds fluoride ion and that this phenomenon can be employed to create a fluoride-ion ratiometric fluorescent sensor. An aqueous solution of [Zr(H2O)2EDTA]·2H2O and 3-hydroxyflavone (flavonol)
Figure 23. Proposed binding mode for the detection of F− using a flavonol-coordinated Z−EDTA complex. 5521
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Figure 24. Proposed binding mode with F− using PV−Zr−EDTA.
3.1. Sensors Bearing Thiourea Binding Sites
It is known that sulfur forms weaker hydrogen bonds than does oxygen, a fact that makes it much easier to align the thiourea functionality for complex formation with fluoride ion in comparison than to its urea counterpart. Also, because thioureas have higher NH acidities than do ureas, they serve as better receptors for F−. These phenomena have been utilized advantageously in the design of novel fluoride-ion chemosensors. For example, the efficient colorimetric sensor 53 (Figure 28), containing a thiourea moiety as a binding unit and pnitrophenylhydrazine as a signaling unit, was developed by Shao et al.88 Addition of fluoride to a DMSO solution of 53 results in a decrease in the intensity of the absorption band at 372 nm and an increase in the band at 532 nm, in association with a color change from light to deep purple. The association constant for complex formation between 53 and fluoride was determined to be (7.90 ± 0.16) × 104 M−1. Similar spectral changes take place in DMSO/H2O (95:5, v/v) solutions. To demonstrate its potential practical applications, this sensor was employed to detect F− in toothpaste. Devaraj et al. designed the two thiourea-containing receptors 54 and 55, that were shown to exhibit selective recognition toward F− over other halide anions.89 The F− selectivity for of these probes was attributed mainly to the hydrogen-bond interaction. Addition of TBAF to CH3CN solutions of 54 and 55 promotes color changes from colorless to brown and yellow, respectively. The binding constant for F− complexation of the receptors 54 and 55 were estimated to be (1.2 ± 0.23) × 103 and (3.3 ± 0.02) × 103 M−1, respectively, the difference being attributed to the presence in the latter of the OH group, which serves as an extra binding moiety. Wu et al. synthesized the structurally simple, thioureacontaining colorimetric sensor 56 (Figure 29).90 Addition of F−
Figure 25. Proposed mechanism for the detection of F− using a Zr(IV)−EDTA−8-hydroxyquinoline complex.
at 470 nm, which is related to the inhibition of ESIPT, as well as a blue shift of the emission band at 610 to 570 nm.
3. SENSORS UTILIZING HYDROGEN-BOND INTERACTIONS Whereas reaction-based probes and unique boron fluoridebased probes have been actively investigated in recent years, receptors utilizing conventional hydrogen bonds have been extensively evaluated for many years. During the past decade, fluorescent or colorimetric receptors utilizing hydrogen bonding have been developed and studied for the selective recognition of fluorides. For example, the urea group contains two relatively acidic NH groups as a rigid binding site with fluoride. In some cases, the relatively strong base fluoride can deprotonate these acidic amidic NH groups, as the pKa of HF is reported to be 1.5.87 This section is categorized according to the various hydrogen-bond donors, such as thiourea, urea, amide, sulfonamide, imidazoliums/imidazole, indole, pyrrole, and Schiff base.
Figure 26. Proposed binding mode for the detection of F− using a flavonol-coordinated Zr−EDTA complex. 5522
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Figure 27. Proposed binding mode of 52 with F−.
CH3CO2− > H2PO4− > HSO4− > Cl− > Br−. The results of AM1 calculations suggest that the most stable configuration of 56 is one that has the Z−E conformation, in which a sixmembered ring is formed through intramolecular hydrogen bonding. Two chromogenic thiourea-based sensors, 57 and 58, containing a p-nitrophenyl group as the signaling unit were reported by Kumar et al.91 The color of DMSO solutions of these sensors changes from pale yellow to an intense red upon addition of both fluoride and cyanide ions in association with a decrease in the intensity of the absorption peak at 364 nm and a concurrent increase in a new band centered at 466 nm. Analysis of the absorption spectral changes indicates that the complexes formed between the sensors and fluoride have 1:2 stoichiometric compositions. Applications of 57 and 58 were demonstrated by their use in detecting fluoride ions in toothpaste and potassium cyanide in aqueous medium using the naked eye. A family of highly sensitive thiocarbonohydrazone anion chemosensors, including 59−62 (Figure 30), was prepared and studied by Han et al.92 The results showed that sensors 59 and 60 undergo two stepwise deprotonation reactions of the thioamide framework in the presence of F−. In contrast, sensor 61 undergoes only a single deprotonation of the two NH groups. Addition of 0.4 equiv of F− to a colorless solution of 59 causes development of reddish color, whereas addition of 20 equiv of F− changes the solution color to blue. In contrast, 100 equiv of F− is required to promote the development of a blue color for sensor 60. Finally, the color of a solution of 62 turns from light yellow to red in the presence of F−. The efficient colorimetric chemosensor 63 (Figure 31), containing a thiourea binding site and 2-amino-6-nitro-
Figure 28. Structures of 53−55.
Figure 29. Structures of 56−58.
to a solution of 56 in acetonitrile causes a color change from colorless to yellow. The fluoride-ion selectivity of this probe is revealed in the association constants of formation of anionic complexes with 56 that lie in the following order: F− ≫
Figure 30. Structures of 59−62. 5523
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Calixarenes are ideal macrocyclic platforms for the creation of host molecules with ion-sensing properties. The new thiacalix[4]arene-based fluorescence sensor 69 (Figure 33),
Figure 31. Proposed mechanism for the reaction of 63 with F−.
benothiazole group, was investigated by Misra et al.93 This chemosensor, which displays a naked-eye-detectable change, selectively recognizes fluoride and acetate anions in dry DMSO solutions. Addition of AcO− or F− results in the disappearance of the ICT band of 63 at 392 nm and the formation of a new absorption peak in the yellow region of the spectra at 457 nm. In association with these spectral changes, the color of the solution changes from light yellow-green to red. The association constants for 63 and these anions were found to be on the order of 1 × 107. The thiourea- and urea-functionalized 4-amino-1,8-naphthalimides 64−66 (Figure 32), whose design is based on the
Figure 33. Sensing mechanism of 69 for the detection of F−.
bearing two naphthylthiourea groups, was explored by Kumar et al.96 In this probe, intermolecular binding interactions between F− ions and the NH protons of thiourea moieties trigger intramolecular π−π interactions of naphthyl groups, which leads to excimer formation and emission. The addition of increasing amounts of F− in DMSO induces a decrease in the absorption band of 69 at 257 nm and the formation of new redshifted absorption band at 360 nm concurrent with a significant increase in the intensity of a fluorescence emission band at 385 nm with a shoulder at 412 nm. Fluoroionophore 69 displays a high selectivity for F− with a detection limit of 2.6 × 10−7 M. Raposo et al. developed the family of heterocyclic thiosemicarbazone dyes 70−78 (Figure 34), which serve as fluoride-anion sensors.97 Titration of receptors 70−78 with fluoride brings about a small bathochromic shift of their absorption band as a result of coordination of the anion with the thiourea group. This is followed the growth of new redshifted bands associated with a color change from yellow or pale yellow to orange-red owing to deprotonation as the fluoride-ion concentration is further increased. In general, receptors 70−75 display enhanced fluorescence emission upon addition of moderate amounts of fluoride anion and quenching of the emission band at higher anion concentrations simultaneous with the growth of a new band at longer wavelengths. Two new chemosensors, 79 and 80 (Figure 35), were designed based on the photochromic dithienylcyclopentene moiety by Tian’s group.98 These probes, which operate through a thioamide proton deprotonation mechanism, respond to F− by undergoing decreases and red shifts of their absorption bands at about 350 nm. The fluorescence intensities of 79 and 80 also decrease upon addition of F−. Specifically, when F− is added to solutions of 79 and 80, the intensities of their emission bands at about 583 nm increase significantly and undergo a bathochromic shift of about 18 nm. The unique supramolecular sensing system 81 (Figure 36) for fluoride ion was constructed from new bispyrenyl thioureas linked by poly(ethylene glycol) (PEG) chains and methoxy benzene pyrene thiourea.99 Upon binding with F− in CHCl3, 81 exhibits a strong excimer emission band (IE) at 500 nm and a weak monomer emission band (IM) at 398 nm. Analysis of a Job plot shows that 1:1 and 2:2 complex formations take place between 81 and F−. The IE/IM ratios of the 81·F− complex are dependent on the concentration of 81, implying that
Figure 32. Structures of 64−68.
fluorophore−spacer−receptor principle, were reported by Duke and Gunnlaugsson94 to be selective fluoride detectors in DMSO. Emission from 65 and 66 is quenched by fluoride as a consequence of F−-binding-enhanced SET from the receptor to the excited state of the fluorophore. In addition, red shifts take place in the absorption maxima of these sensors upon addition of fluoride. Two anion sensors, 67 and 68 (Figure 32), which contain 4amino-1,8-naphthalimide fluorophores and diarylthiourea receptors, connected through a methylene spacer to a naphthalimide group, were described by Veale and Gunnlaugsson.95 Additions of acetate, phosphate, and fluoride were shown to quench the fluorescence of these sensors to different extents. When treated with F−, very effective quenching (ca. 90%) of the fluorescence band of 67 occurs at 528 nm, but no change in the absorption band takes place. For 68, titration with F− also causes quenching of the excited-state emission band at 533 nm, but to a lesser extent than for 67 (ca. 50%). Significant changes occur in the case of 67 at high concentrations of F−, where a long-wavelength absorption band rises at 529 nm along with a second transition band at 332 nm. These results demonstrate that bidirectional SET quenching is responsible for sensing by these naphthalimide-based anion probes. 5524
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Figure 34. Structures of 70−78.
Figure 35. Sensing mechanism of 79 and 80 for the detection of F−.
Figure 36. Possible structure of the 81−F− complex.
intermolecular excimer formation takes place. Probe 81 exhibits an extremely large binding constant (1 × 104) and sensitivity (detection limit as low as 46.2 g/L) toward F−. New ferrocene-based receptors for F− were developed by Devaraj et al. (Figure 37).100 The absorption peak of one of these, 82, in CH3CN at 480 nm is blue-shifted by 15 nm upon addition of F−. Simultaneously, a significant increase in the intensity of fluorescence emission at 438 nm occurs. Similar observations were made in studies with the related sensor 83, which displays a higher fluoride binding constant [(24.0 ± 0.12) × 103 M−1] than does 82 because of the presence of the nitro group. The results of electrochemical titrations of 82 and 83 in CH3CN reveal that the addition of F− anion to 82 alters its oxidation potential cathodically from 690 to 600 mV. Receptor 83 shows a similar response to fluoride, experiencing a cathodic shift in oxidation potential from 790 to 690 mV. The cathodic shifts are a result of the fact that the oxidation process becomes easier owing to electrostatic stabilization by the negatively charged ions present in the fluoride complexes. Jun et al. devised the new anthracene-linked thiourea-based fluoride chemosensors 84−86 (Figure 37).101 The X-raycrystallography-characterized thiourea derivatives in DMSO
Figure 37. Structures of 82−87.
display charge-transfer absorption peaks at around 430 nm in the presence of fluoride. In addition, new peaks at ca. 568 nm arise in their fluorescence spectra upon addition of F−, corresponding to a change from blue to orange fluorescence. 5525
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Figure 38. Structures of 88−91.
two ions, F− and HSO4−. The remarkable color change from colorless to yellow and fluorescence quenching upon fluoride addition can be used to discriminate F− from other anions using 91.
The novel bisenaminone-containing thiourea 87 (Figure 37) serves as an optical signaling agent for F−, AcO−, and H2PO4−.102 Addition of F− beyond 0.4 mM to an acetonitrile solution of 87 results in a color change from colorless to an intense green that is visible to the naked eye. This change takes place concurrently with the rise of a new absorption peak at 460 nm and an emission peak at 500 nm. Lee et al. devised the anion sensor 88 (Figure 38), which contains a biaryl−thiourea structure and exhibits a fluorescence emission enhancement upon hydrogen-bond-mediated complexation with F−.103 Addition of increasing amounts of F− concentration up to 2.5 equiv results in a 2.4-fold fluorescence enhancement of 88. Addition of further amounts of F− causes a reversal of the emission increase, suggesting that both 1:1 and 1:2 host/guest complexes are formed. The association constants of the 1:1 and 1:2 complexes were calculated to be 1.08 × 104 M−1 and 2.28 × 107 M−2, respectively. The chromogenic sensor 89 (Figure 38) for F−, which does not appear to contain any defined chromophore, was studied by Vázquez et al.104 Addition of F− to a solution of 89 results in a decrease in the intensity of the absorption band at 324 nm and the appearance of a new band at 412 nm in association with the development of a yellow color. These changes are likely consequences of strong interactions between F− and the internal NH fragment of the thiourea group, which enhances π delocalization over the receptor’s framework and causes the π−π* transition to shift from the UV to the visible region. A new binaphthol thiourea derivative 90 (Figure 38) was prepared and demonstrated to be a fluoride-ion sensor by Tang et al.105 Upon addition of F−, the color of a CH3CN solution of 90 changes from colorless to yellow, and the absorption maximum red shifts from 340 to 414 nm. Concurrently, a gradual increase in the intensity of fluorescence at 338 nm takes place. The binding constant for formation of a 2:1 complex between fluoride and 90 was determined to be 3.02 × 104 M−2. Lu et al. recently explored the simple sensor 91 (Figure 38), which displays a highly fluoride-anion-selective absorption and fluorescence response in DMSO.106 Because the fluorideinduced chromogenic changes can be completely reversed by addition of HSO4−, receptor 91 operates as a reversible colorimetric and fluorescence switch with complementary “IMPLICATION/INHIBIT” logic function by inputting these
3.2. Sensors Bearing Urea Binding Sites
In 2002, Kim and Yoon described the bisurea anthracene 92 (Figure 39), whose fluorescence in acetonitrile/DMSO (9:1, v/
Figure 39. Structures of 92−94.
v) is selectively quenched by fluoride ions through an SET process.107 For 92, the anthracene moiety in this probe acts not only as a fluorophore but also as a template for introducing binding selectivity. The association constant of the 92−F− complex was calculated to be 71270 M−1. In 2004, Kim’s group also developed the related anthracene derivatives 93 and 94 (Figure 39), which serve as a colorimetric sensor and fluorescence chemosensor, respectively, for fluoride ion.108 Probe 93 displays an absorption peak at 485 nm that is a redshifted by 129 nm upon addition of fluoride or pyrophosphate ions, which causes a color change from pale yellow to red. Upon the addition of F− to 94 in DMSO, fluorescence quenching takes place. The results of this study demonstrated that the anthracene 9-H functions as a CH hydrogenbonding donor in addition to the traditional urea NH hydrogen-bonding donor. In 2003, Cho et al. developed chemosensor 95 (Figure 40), which contains two phenylurea groups at the 1,8-positions of a naphthalene moiety, and demonstrated that it interacts with the fluoride anion through strong hydrogen bonding.109 In CH3CN/DMSO (9:1, v/v), 95 forms a complex with F− in 5526
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Figure 40. Structures of 95−100.
Figure 41. Structures of 101−104.
fluoride ion. The origin of the color changes in these probes is attributed to charge-transfer interactions that take place between the electron-rich donor units and the electrondeficient p-nitrophenyl and azophenyl moieties. The bisurea-functionalized naphthalene organogelator 100 (Figure 40), which operates through cooperative hydrogen bonding and π−π stacking interactions, was developed by Yang et al.113 This sensor undergoes a strong red shift in its emission when it is in its gel state compared to when it is in solution. Both the gel−sol transition and the fluorescent emission change are reversibly controlled by temperature changes or addition of F− and protons. The effect of fluoride anions on the fluorescence and gel−sol processes are a consequence of the dissociation of intermolecular hydrogen bonds by bonding of
association with the disappearance of an absorption peak at 345 nm and the formation of a red-shifted peak at 408 nm. This study also demonstrated that the fluoride selectivity of the probe is a result of the fact that fluoride binds much closer to the amide protons than do the other halides.110 The same probe was also reported to be a fluoride-selective receptor by Xu and Tarr at nearly the same time.111 Likewise, a series of very closely related naphthalene−urea derivatives was also described by Nam’s group in 2005.112 Distinct color changes take place when 96−98 are treated with fluoride ions. For example, the pale yellow solution of 96 in DMSO becomes red when fluoride ions are added. A similar red color was observed to arise when fluoride ions are added to azophenyl derivative 97, whereas a blue color is generated for 98 in the presence of 5527
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fluoride anions to the urea groups of the gelator. Meanwhile, the sol form can be returned to the gel state by addition of trifluoroacetic acid. Anthraquinone derivatives have also been utilized as both templates and signaling groups in novel F− sensors. For example, Cho et al. devised chromogenic anthraquinone 102 (Figure 41), which contain tetra-urea moieties as anion binding sites.114 Addition of fluoride to 102 (blue solution becomes green) in DMSO causes a UV−vis spectral change similar to that of 101 (blue solution becomes dark green), suggesting that fluoride binding occurs equally well at the amino site as at the urea moiety. Related receptors for selective fluoride-ion sensing, containing anthraquinone chromogenic signaling subunits and urea/thiourea binding sites, were previously reported.115 In the presence of fluoride ion, a DMSO/CH3CN (1:9, v/v) solution of 103 (Figure 41) does not undergo a color change even when allowed to stand at room temperature for more than 24 h. However, the color of the solution changes to pale red when the temperature of the mixture is raised to 60 °C. The results of titration experiments using a DMSO/CH3CN (1:9, v/v) solution show that the related sensor 104 undergoes an immediate color change upon addition of fluoride at room temperature. The results of calculations show that intramolecular NH···O hydrogen bonding in 103 is strong and it prevents fluoride ion from complexing with the NH donor atoms. Because sulfur forms weaker hydrogen bonds, 104 acts as a better receptor for F− at room temperature. Upon complexation with F−, a color change from yellow to pale red for 103 and to purple for 104 takes place, concurrently with red shifts of 108 and 102 nm, respectively, in their absorption bands. The association constants with F− were calculated to be (4.4 ± 0.2) × 105 and (8.2 ± 0.5) × 105 M−1 for receptors 103 and 104, respectively. Calix[4]arenes have been employed as unique supramolecular receptors for anion recognition. The chemosensor 105 (Figure 42), containing a urea binding site on the lower rim of a 1,3-alternate calix[4]arene structure was synthesized by Nam and co-workers.116 These workers observed that addition of Pb2+ ion to a solution of 105 in acetone-CHCl3 (v/v = 7:3)
promotes quenching of the monomer and excimer bands owing to reverse SET from the pyrene units to the urea groups. The quenching is reversed by addition of F− to the Pb2+-105 complex. These results demonstrate that fluoride ions bind to the amide protons in 105, thereby disrupting metal cation complexation. A related, colorimetric anion sensor 106, containing four amidourea receptors in the lower rim of a calix[4]arene structure, has been described.117 This probe contains a highly symmetrical and preorganized cavity for anion recognition through hydrogen bonding. Anions such as F− and AcO− undergo strong 1:2 binding with this probe, with F− recognition being most likely the results of formation of HF2− upon deprotonation of the urea moiety. Because the color changes from colorless to red as a consequence of the disturbance in the ICT nature of the anion receptor, probe 106 can be employed as a naked eye sensor for F−. The nitronaphthalene urea derivative 107 (Figure 43), synthesized by simple reaction of 1-naphthylamine and its corresponding isocyanate, was described by Nam and coworkers.118 A pale yellow DMSO solution of this probe becomes red when fluoride ions are added. In contrast, addition of acetate and benzoate to a solution of this sensor causes a color change to brown-yellow. Two related anion receptors, thiourea 108 and urea 109 (Figure 43), were synthesized by Kim et al.119 Addition of TBAF to DMSO solutions of receptors 108 and 109 causes dramatic color changes from colorless to yellow, as a result of fluoride-ion-induced hydrogen bonding and deprotonation of the NH acidic protons. In 2004, Boiocchi et al. reported that F− forms a strong Hbond with the NH hydrogens of the urea subunit in 110 (Figure 43).120 Moreover, proton transfer takes place from the urea group in the presence of a second F− ion, resulting in the formation of HF2−. The chemical process occurs in conjunction with the appearance of a charge-transfer absorption band of 110 in the visible region and a dramatic color change. In 2005, Esteban-Gómez et al. evaluated 110 along with the related substance 111 (Figure 43) more thoroughly.121 In the presence of fluoride, urea-based receptor 111, which contains electronwithdrawing chromogenic substituents, undergoes stepwise deprotonation of the two NH protons in a DMSO solution. Upon addition of an excess of TBAF, the color of the DMSO solution of 110 turns from yellow to red, and then a blue color develops at higher fluoride concentrations. Receptor 111 undergoes a similar two-step deprotonation process that results in pale yellow-to-orange and orange-to-violet color changes. Two colorimetric chemosensor, 112 and 113 (Figure 44), macrocyclic amino ether frameworks linked to three nitrophenylurea groups, were described by Aldrey et al.122 Treatment of 112 and 113 with TBAF in DMSO solutions leads to the formation of a new absorption band centered at ca. 480 nm and a decrease in the intensity of the band at 348 nm for 112. The colors of solutions of both receptors change from light yellow to brown in the presence of F−, CN−, and H2PO4−. The stability constants for the interactions of chemosensors 112 and 113 with anions are in the order F− > OH− > CN− > H2PO4−. The selectivity is a consequence of the size of the macrocyclic cavity and the flexibility of the pendant urea arms. The reversible assembly and disassembly processes of a 2ureido-4[1H]-pyrimidinone quadruple hydrogen-bonded AADD supramolecule has been exploited as the foundation of a new F− detector (Figure 45).123 In the absence of fluoride ions, the fluorescence spectrum of a CH2Cl2 solution of the hydrogen-bonded complex exhibits three typical emission
Figure 42. Structures of 105 and 106. 5528
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Figure 43. Structures of 107−111.
Figure 44. Structures of 112 and 113.
Figure 46. Structures of 115−117.
Figure 45. Proposed mechanism for the reaction of 114 with F−.
units spaced by a flexible diethylenetriamine fragment.125 In both DMSO and CH3CN, the fluorescence emission of 117 in the visible range (400 nm) was found to be perturbed by the presence of anions. The emission of 117 is quenched by acetate, chloride, and pyrophosphate, whereas it is enhanced by fluoride. It is presumed that 117 forms a stronger H-bond with F−, making the nitrogen lone-pair electrons more available for conjugation with the coumarin ring, thus favoring formation of an ICT emissive state. In contrast, weaker H-bond interactions with acetate, chloride, and pyrophosphate increase formation of the flexible, less emissive TICT while hindering the more rigid and sterically hindered ICT state. Ren et al. evaluated the fluoride-sensing properties of two dimers, 118 and 119 (Figure 47), that contain perylene fluorophores linked by urea bridges.126 Both the absorption and fluorescence spectra of 118 and 119 in THF solution are nearly the same, indicating that the two dimers have nearly identical configurations. Addition of F− to these solutions results in quenching of the perylene emission, as a likely consequence of F− coordination, which decreases the oxidation potential of the urea donors, causing SET of the singlet excited perylene groups to become more efficient. The F− association constants were
bands associated with the anthracene fluorophore at 397, 420, and 445 nm. Introduction of fluoride ion into the solution leads to the generation of monomeric species 114 and a dramatic reduction in the fluorescence intensity but not the positions of the emission maxima. The association constant for 114 and F− was calculated to be 13200 M−1. Wu et al. developed a new type of fluorescent anion sensor that is based on inhibition of an excited-state intramolecular proton process.124 The ESIPT process in which 115 (Figure 46) participates can be readily blocked by deprotonation of the sulfonamide unit by the addition of basic anions such as F−, CH3COO−, and H2PO4−. For 116, which is a good hydrogenbond donor, the ESIPT process is inhibited by either the fluoride-induced deprotonation of the urea unit or the formation of a strong CH3COO−−urea intermolecular hydrogen-bond complex. These two types of inhibition mechanisms result in different ratiometric emission responses, where the peak of 116 at 554 nm shifts to 510 nm upon F− binding and to 414 nm after CH3COO− binding. The anion fluorescence chemosensor 117 (Figure 46), described by Ambrosi et al., contains two coumarin−urea 5529
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corresponding to a color change from light yellow to red are clearly visible to the naked eye. 3.3. Sensors Bearing Amides as Binding Groups
A number of sensors bearing amide moieties as binding groups and the calixarene framework have been explored. In 2005, Kim et al. described the calixarene-based fluorescent chemosensor 122 (Figure 48), which contains two fluorogenic pyrene units conjugated to amide groups as guest recognition sites.129 Complexation of F− to 122 causes a red shift in its absorption band to 400 nm in conjunction with an enhancement and blue shift of the excimer emission to 470 nm, which are attributed to ground-state pyrene dimer-like formation. The association constant of 122 with F− was calculated to be 2.5 × 102 M−1. Based on the above findings, calix[4]arene derivatives 123 and 124 (Figure 48) bearing 4-nitrophenylazo and pyrene moieties were prepared by Kim’s group.130 F− was shown to bind to the amide groups (NH) of 123, resulting in a change in the characteristic excimer emission peak at 480 nm and the formation of a new emission peak at 460 nm. In contrast, the addition of F− to a solution of 124 promotes a change in the characteristic absorption spectrum corresponding to a color change from yellow to blue owing to complex formation with the hydroxyl protons (OH). In 2006, Kim’s group described the results of studies with the calix[4]arene-based anion sensor 125 (Figure 49), which contains two coumarin units attached through amide groups that serve as binding sites.131 Titration of a solution of 125 with F− causes a decrease in intensity and the formation of a redshifted absorption band at 349 nm and new band at 408 nm. The emission band of the probe at 420 nm decreases upon addition of F− and is eventually quenched, and a weak band arises at 508 nm. The association constant (Ka) of 125 for F− was determined to be 1.08 × 104 M−1. Kim’s group extended this study to the chemosensor 126 (Figure 49), which bears two amidoanthraquinone groups (1-AAQ) at the lower rim of p-tert-butylcalix[4]arene.132 A red-shifted absorption band centered at 510 nm arises along with a concomitant decrease in the band at 405 nm of 126 in CH3CN as a function of F− concentration. This change promotes a color change from pale yellow to deep yellow. New bands at 527 and 560 nm appear in the emission spectrum of the probe, associated with a fluorescence color change from colorless to green, upon
Figure 47. Structures of 118−121.
calculated to be 1.03 × 105 M−1 for 118 and 1.017 × 105 M−1 for 119. The phthalimide−urea chemosensor 120 (Figure 47) was shown to display a selective fluorescence response to F−.127 An acetonitrile solution of 120 displays an intense emission band at 430 nm, which is attributed to the aminophthalimide fluorophore. The addition of F − promotes significant quenching of the fluorescence at 430 nm and the appearance of a weak red-shifted emission band in the 520−550-nm range. These findings indicate that the complex between 120 and F− forms through hydrogen-bonding interactions rather than receptor deprotonation. The 1,8-naphthalimide urea sensor 121 (Figure 47) was devised as a colorimetric and fluorescence sensor for anions.128 The 3-position of the naphthalimide ring contains a trifluoromethyl-substituted phenylurea moiety as the anion recognition site. Upon addition of 25 equiv of F− to a DMSO solution of 121, large changes take place in the absorption spectrum at 495 nm. In addition, the intensity of the absorption band of the probe at 327 nm is enhanced dramatically, accompanied by a large decrease in the absorbance band centered at 272 nm. These changes in the absorption spectrum
Figure 48. Structures of 122−124. 5530
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Figure 49. Structures of 125 and 126 and proposed mechanism for the inhibition of ESIPT in 126 induced by F−.
Figure 50. Proposed mechanism for the reaction of 127 with F−.
addition of F− ion. These results demonstrate that the ESIPT process of 126 is inhibited by fluoride-induced H-bonding, followed by deprotonation of NH of the 1-AAQ group. In 2007, the novel calix[4]arene 127 (Figure 50) bearing one 2,3-naphthocrown-6 and two coumarin amide units at the lower rim in the partial-cone conformation was developed by Kim and co-workers as a colorimetric and FRET-based fluorometric sensor for F− and Cs+ ions.133 Intramolecular FRET arising from naphthalene emission and coumarin absorption results in highly selective fluorescence response toward F−. The addition of F− induces a significant red shift in the coumarin absorption from 344 to 435 nm. Moreover, the addition of a large excess of F− (for example, 1000 equiv) results in coumarin emission quenching at 536 nm, which is presumably a result of single
electron transfer (SET) from F− to the excited coumarin fluorophore. Calixarene-based fluorescence receptors 128 and 129 (Figure 51) displaying excimer emission were designed to incorporate pyrene as the fluorogenic unit.134 The ionophoric groups in these substances strongly interact with only F−, and this interaction results in a new absorption band around 430 nm and an observable color change from colorless to yellow. The new band is a consequence of ICT promoted by deprotonation of OH groups. In acetonitrile/chloroform (9:1), the emission intensities of 128 and 129 are quenched upon addition of increasing amounts of F−. The new C-linked peptidocalix[4]arene 130 (Figure 51), functionalized with four L-alanine and dansyl units at the upper rim, was investigated previously.135 A decrease in the fluorescence intensity of this probe upon the 5531
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Figure 51. Structures of 128−131.
addition of F− was observed. The results of fluorescence titration experiments show that the association constant for 130 binding F− is 29500 M−1. The new pyrene-appended probe based on a thiacalix[4]arene in a 1,3-alternate conformation (131) was prepared by Kumar et al.136 Addition of anion F− to THF solution of this sensor causes quenching of monomer emission at 386 nm and slightly increases excimer emission at 475 nm. These phenomena are associated with anion coordination with the NH amides through hydrogenbonding interactions, which induces the SET process, resulting in a fluorescence quenching effect. Receptor 131 was also used in a displacement assay for the fluorimetric recognition of F−. In this assay, addition of Fe3+ to a CH3CH2OH solution of 131 induces quenching of pyrene monomer emission as a result of the formation of the 131−Fe3+ complex. The subsequent addition of anion F− promotes recovery of the pyrene fluorescence caused by the release of free 131. Hu et al. developed the new turn-on fluorescence chemosensor 132 (Figure 52) as an anthracene diamine derivative incorporating two indole units.137 Upon mixing 132 with F− in DMSO/CH2Cl2 (1:9, v/v), an increase in the intensity of fluorescence at 430 nm takes place. The results of a Job plot indicate that a 1:1 complex between 132 and F− forms with an association constant of 7.3 × 104 M−1. These results indicate that fluoride anion participates in strong hydrogen-bonding interactions not only with the amide NH groups but also with the indole NH and the 9-H of anthracene. Rajamalli and Prasad described the low-molecular-weight gel (LMWG) 133 (Figure 52), which consists of an anthracene chromophore attached to a polybenzyloxy arene group through
Figure 52. Proposed binding modes of 132 and 133 with F−.
an acylhydrazone linkage.138 A remarkable 590-fold fluorescence enhancement, referred to as a gel-induced enhanced emission (GIEE), occurs upon TBAF-promoted gelation of this probe. The presence of TBAF not only changes the color of the probe but also disrupts the preformed gel through slow diffusion of the anion, a change that can be observed using the naked eye. Analysis of a Job plot suggests that a complex with a 5532
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Figure 53. Structures of 134−137.
The new sensor for naked-eye detection of F− 137 (Figure 53), which is based on a perylene diimide platform, was explored recently.142 Addition of fluoride to a DCM solution of 137 results in an observable color change from red to blackishgreen as a consequence of a 151-nm red shift in the absorption maximum that corresponds to a detection limit of 0.14 mM. The mechanism for the process leading to this change involves proton transfer from a NH hydrogen of the sensor to fluoride anion. Two biphenyl-like colorimetric anion sensors, 138 and 139 (Figure 54), were developed by Costero et al.143 Selective color
1:1 stoichiometric ratio between the gelator and anion is formed and has an association constant of 2.9 × 104 M−1. In 2005, Liu and Tian devised 4-benzoylamido-N-butyl-1,8naphthalimide, 134 (Figure 53), to be both a colorimetric and fluorescence selective chemosensor for fluoride ion in CH3CN.139 Using this probe, fluoride can be detected at concentrations ranging from 20 to 100 μM. An observable colorless-to-yellow color change and blue-to-orange emission color change takes place upon addition of fluoride to a solution of 134, changes that are attributed to deprotonation of the 4amide moiety of the naphthalimide fluorophore. In 2006, Tian and co-workers also described a multiple switching process involving photographic images of the naphthalimide derivative 135 under the cooperative effects of light, thermal, fluoride ions, and protons.140 Upon addition of F− to a THF solution of this probe, the intensity of the absorption band around 330 nm progressively increases, and a new band with a maximum at around 495 nm forms. One possible explanation for this phenomenon involves deprotonation of the amino moiety by F−, which results in the development of negative charge density on the amide nitrogen. A fluoride-ion-triggered dual-fluorescence molecular switch based on the naphthalimide-winged zinc porphyrin 136 (Figure 53) was also developed by Li et al.141 When fluoride is added to a THF solution of this sensor, fluorescence arising from the zinc porphyrin unit of 136 can be regulated in an on−off manner by selective excitation at 365 nm and in an off−on manner by selective excitation at 504 nm. The obvious color change from pale to dark orange, induced by intermolecular proton transfer in the NH fragments, is clearly visible to the naked eye.
Figure 54. Structures of 138 and 139.
changes from colorless to orange take place when these probes are in the presence of fluoride. These changes are associated with the appearance of a broad absorption band centered at ca. 450 and 479 nm for 138 and 139, respectively. Two different types of products are generated by reaction of fluoride with these sensors. One species is a coordination complex, and the other is a salt generated by NH deprotonation, the latter of which induces a conformational change that gives rise to color development. 5533
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sensor has a layered structure as a result of π−π stacking and the CH···π interactions between the ferrocene moieties and naphthalene rings. In the presence of F− and H2PO4−, the emission intensity of 143 at 490 nm is enhanced significantly. The results of 1H NMR titration experiments demonstrate that addition of F− induces the deprotonation of the NH groups in 143. The closely related ferrocene-based receptor 144 (Figure 56) was designed to undergo hydrogen-bonding interactions with anions.146 Observations made in UV−vis spectral titration experiments showed that 144 forms a 1:1 complexes with F− in DMSO. Upon addition of F−, the absorption maximum of the probe at 394 nm undergoes a large red shift to 520 nm in association with a dramatic change in color from brown to purple. This change is easily observed by the naked eye and is selective for F− as compared to other anions. Zhao et al. devised the fluorescence sensor 145 (Figure 57), which can be used to detect fluoride ion through an excitedstate intermolecular proton transfer (ESIPT) mechanism.147 Addition of fluoride to a DMSO solution of 145 results in a decrease in the intensities of the absorption bands at 330 and 370 nm and the appearance of new bands at 440, 465, and 495 nm. In addition, a large enhancement in the intensity of a redshifted emission band at 520 nm takes place. The association constant for the formation of the 1:1 complex between fluoride and 145 was determined to be given by log Kassoc = 3.78 ± 0.2. The pyridinium amide receptor 146 (Figure 57), containing two anthracene moieties, was demonstrated to be a fluorescence chemosensor for H2PO4−, F−, and AcO−.148 The intensity of the monomer emission from this probe is quenched significantly by F− and AcO−. Quenching of the excited state of 146 by F− is attributed to the generation of strong interactions in the open cleft initially through NH···F− and CH···F− hydrogen-bond formation. The new gelator 147 (Figure 57) was reported to form a thermally reversible gel in DMSO/H2O and an ultrasoundstimulated gel in DMSO.149 Formation of these gels is attributed to cooperative intermolecular hydrogen bonding and π−π stacking and cyano interactions. Addition of fluoride not only changes the color of the probe from colorless to yellow but also disrupts the preformed gel into a solution. Moreover, the reversible gel−sol transition and noticeable color change can be controlled by addition of fluoride and protons.
The fluoride-sensing properties of the new chromogenic receptors 140−142 (Figure 55), containing 2-nitrophenyl and
Figure 55. Structures of 140−142.
3,5-dinitrophenyl groups appended to amide and in secondary amine positions, have been investigated.144 Upon addition of fluoride to solutions of 140 and 141 in acetonitrile, a yellow color develops. In contrast, the color of an acetonitrile solution of 142 turns intense purple color in the presence of fluoride, a change that can be detected by the naked eye even at parts-permillion concentrations of fluoride. The binding constants (Ka) of 140−142 with fluoride were determined to be 6.8 × 103, 8.1 × 103, and 1.9 × 104 M−1, respectively. The color changes taking place with these probes are most likely a result of the formation of hydrogen-bond complexes between the amide groups and F−. Zhang et al. described the host/guest complexation of anions with the ferrocenenylphalene dyad 143 (Figure 56).145 This
Figure 56. Structures of 143 and 144.
Figure 57. Structures of 145−147. 5534
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A solution of the naphthol amide sensor 151 (Figure 59) in acetonitrile was observed to undergo a red shift in its fluorescence emission upon addition of anions such as F−, AcO−, and H2PO4−.153 The magnitude of the shift is dependent on the basicity of the anion. A new, red-shifted emission band appears at 515 nm in the presence of F− as a consequence of hydrogen-bonding interactions involving the amide NH and phenol OH groups. The novel turn-on fluorescence colorimetric sensor for fluoride 152 (Figure 59) was described by Li et al.154 Addition of F− to a DMSO solution of 152 results in enhancements in the fluorescence intensities at 338 and 352 nm that can be detected by the naked eye. Similarly, AcO− and H2PO4− also enhance the fluorescence of the probe but to a much lower extent than does F−. The color of the solution of 152 turns to red from yellow upon addition of fluoride. These optical changes are attributed to a fluoride-anion-induced increase of the charge density and rigidity of 152. The new pyrrole 2,5-diamide 153 (Figure 60) contains 3,5dinitrophenyl groups appended to the amide positions and an anion hydrogen-bonding cleft.155 Fluoride addition to an acetonitrile solution of this sensor results in the development of a deep blue color, which is attributed to a deprotonation process in which fluoride acts as a base. The subsequent chargetransfer interaction occurring between the deprotonated pyrrole and the nitroaromatic groups causes the optical changes. The chemosensor 154 (Figure 60), with a pyreneamide bipyridine structure, was studied by Jeon et al.156 In acetonitrile, 154 undergoes turn-on fluorescence at 420 nm in the presence of fluoride ion, and the colorless solution becomes orange. The association constant (K) of the complex formed between 154 and F− was determined to be 2.0 × 104 M−1. Xiong et al. demonstrated that the tetrathiafulvalene (TTF) derivative 155 (Figure 60), containing a boron dipyrromethene (BODIPY) moiety, was observed to display selective optical and electrochemical changes in the presence of fluoride ion. 157 Intermolecular NH···F and CH···F hydrogen bonding plays a key role in the fluoride-anion-recognition process. A significant decrease in the fluorescence intensity of 155 at 527 nm takes place upon addition of increasing amounts of F−, whereas weak quenching occurs in the presence of CH3COO− and H2PO4− under the same conditions. Addition of F− leads to a color change from orange to amaranth (rose-red), accompanied by the formation of a new broad absorption band centered at 580 nm and a decrease in the BODIPYcharacteristic peak. Piatek and Jurczak developed the amide-based macrocycle 156 (Figure 61) for the colorimetric detection of F−, AcO−,
Džolić et al. evaluated the sensor properties of the new anthraquinone-derived oxalamide gelator 148 (Figure 58).150
Figure 58. Structures of 148−150.
This substance displays an excellent gelation ability toward aromatic solvents and alcohols at very low concentrations. The gel phase 148−EtOH is responsive to fluoride, which promotes a color change from yellow to orange. In addition, fluoride prevents the gelation of 148 in p-xylene. Interestingly, simple contact between fluoride and the 148−p-xylene gel leads to a gel-to-sol transition. The flavone derivative 149 (Figure 58) was designed for the recognition of fluoride ions.151 Upon progressive addition of TBAF to a solution of this sensor, the intensity of the 317-nm absorption band decreases slightly, and a new band arises at 408 nm, which was accompanied by a color change from colorless to yellow. Meanwhile, a significant decrease in the intensity of the fluorescence band occurs, accompanied by the emergence of a new emission band centered at 590 nm. The selectivity of the probe for fluoride over other halides is attributed to the higher basicity of fluoride, and the spectral changes are attributed to proton abstraction rather than the more common hydrogen-bonding mechanism. Lin et al. explored the interactions of halide and benzoate anions with the 4,5-diacetamidoacridine-9(10H)-one 150 (Figure 58).152 Whereas chloride and bromide exhibit simple association with this substance to form 1:1 complexes, both fluoride and benzoate also form complexes but then deprotonate the acidic acridinone NH proton of 150 using a second equivalent. Upon addition of F− to a solution of the probe, the initial absorption peak at 435 nm disappears and is replaced by new peaks at about 465 and 495 nm, along with a much less intense peak near 530 nm.
Figure 59. Structure of 151 and proposed binding mode of 152 with F−. 5535
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Figure 60. Structures of 153−155.
Figure 61. Structures of 156−158.
and H2PO4− ions in both DMSO and acetonitrile solutions.158 In DMSO, the initially colorless solution of this probe turns yellow when exposed to any of these ions. However, addition of fluoride, acetate, and dihydrogen phosphate ions to acetonitrile solutions of 156 results in the development of turquoise (λ = 408, 700 nm), yellow (λ = 378 nm), and purple (λ = 537, 393 nm) colors, respectively. These visual changes are completely consistent with the association constants of 156 with these
anions in acetonitrile, which are in the following order: F− ≫ H2PO4− ≫AcO− > Cl− ≈ HSO4− ≫ Br−. The new dendritic gelator 157 (Figure 61), consisting of carbazole building blocks, was developed by Xu et al. This sensor forms a supergel in DMSO and in some DMSOcontaining mixed solvents.159 The organogel of 157 enables naked-eye recognition of F−, because it is destroyed in the presence of 4 equiv of F−, a process that is accompanied by an enhancement in the emission at 420 nm. This phenomenon 5536
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Figure 62. Structures of 158−160.
Figure 63. Structures of 161−164.
Figure 64. Structures of 165−167.
results from the interaction of F− with 157 through H-bonding, which leads to an increase in molecular coplanarity. The selectivity of 157 toward F− is a consequence of the size of the ion and the fact that larger anions fail to interact with the amide groups in 157 because of steric hindrance.
hydroxy-2-methylquinoline, which is nonfluorescent because of efficient radiationless relaxation to the ground state through intra- and intermolecular ESPT. However, a new hydrogen bond is formed between the OH group and F−, resulting in color and fluorescence changes caused by a new charge-transfer interaction between the F-bound OH and the electron-deficient dibrominated quinoline. The anion receptor 159 (Figure 62), containing anthracene and TTF units, was prepared and studied by Lu et al.161 This substance, which displays a unique selectivity for fluoride relative to other anions, undergoes a dramatic fluorescence enhancement upon addition of fluoride under neutral conditions. The 159−F− complex in CH2Cl2 was found to
3.4. Sensors Bearing Sulfonamides as Binding Sites
The chemosensor 158 (Figure 62), consisting of four sulfonamide groups linked to two dansyl moieties, was developed by Bao et al.160 Addition of 2 equiv of fluoride to a CH3CN solution of this sensor promotes a ca. 10-nm blue shift and an increase in the fluorescence band. Upon addition of F−, 158 undergoes deprotection to give 5,7-dibromo-85537
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anions were found to be in the following orders: for 166, CN− > BzO− > AcO− > F−; for 167, F− > AcO− > CN− > BzO−.
have a 1:2 stoichiometry and an association constant of 1.51 × 108 M−2. Two fluoride ions participate in effective hydrogenbonding interactions not only with the protons of the amide but also with the 9-H of anthracene. This bonding rigidifies the receptor, which leads to a reduction in the efficiency of SET quenching. Bhalla et al. developed the linked thiacalix[4]arene−dansyl moiety fluorescence sensor 160 (Figure 62), which is highly selective for Cu2+ and F−.162 In CH3CN/CH2Cl2 (1:1, v/v), the presence of Cu2+ induces the formation of a 1:1 metal−160 complex that exhibits intense 71-nm-blue-shifted emission at 433 nm. The fluorescence enhancement by Cu2+ is a result of the reduction in the electron-donating ability of the two nitrogens. The fluorescence emission at 504 nm is quenched in the presence of F− because of deprotonation of the amide moiety to produce a better donor for transferring electron quenching of the dansyl moiety. This was the first example of a thiacalix[4]arene-1,3-diamine scaffold combined with a dansyl moiety to produce a fluorescent sensor for copper and fluoride ions. Interactions of anions with the sulfonamide-containing Ru(II)- and Co(II)-based receptors 161−164 (Figure 63) were studied by Shang et al.163 Addition of AcO− and F− to a solution of the ruthenium complex 161 results in the formation of three new absorption bands at 380, 475, and 500 nm, which is associated with a color change from orange-red to red. The results of solution studies demonstrated that 1:1 complexes between 161 and the two anions are generated as a result of hydrogen-bonding interactions. Nearly the same response takes place with 162. The anion-binding abilities of 161 and 162 are in the order AcO− > F− > H2PO4− > OH− > Cl− > Br− > I−. Additionally, the cobalt complex 163 in DMSO displays an absorption band at 471 nm, whose intensity increases upon the addition of AcO− and F− anions. The binding abilities of 163 and 164 were found be in the order F− > AcO− > H2PO4 > − OH > Cl− > Br− > I−. The core-substituted naphthalene diimide sensor 165 (Figure 64), bearing a bis-sulfonamide group, was described by Bhosale et al.164 In CHCl3, about 97% quenching of the fluorescence of this probe at 635 and 696 nm takes place upon the addition of TBAF. The stoichiometry of the complex formed between the probe and fluoride was determined to be 2:1, and the association constant was determined to be 4.1 × 105 M−2. In DMSO, a significant bathochromic shift of about 100 nm in the absorption band of 165 occurs upon complexation with fluoride, corresponding to the development of a green color. Analysis of a Job plot shows that a 1:1 complex is formed in DMSO with an association constant of (1.2 ± 0.5) × 106 M−1. The two novel quinoxalinophenanthrophenazine-based anion sensors 166 and 167 (Figure 64) were found to be effective fluorescence sensors for several anions.165 Addition of anions to a solution of 166 causes a decrease in the absorbance at 280 nm and an enhancement of the absorbance bands at 355 and 450 nm. Titrations of this sensor with acetate, benzoate, cyanide, and fluoride anions result in an increase in the emission intensity along with a bathochromic shift of the maximum to 550 nm. When acetate, benzoate, cyanide, and fluoride are added to solutions of 167, the intensity of emission at 450 nm decreases. Theoretical modeling of 1:1 anion complexes with 166 and 1:2 anion complexes with 167 was carried out. The binding constants of 166 and 167 to various
3.5. Sensors Bearing Imidazoline or Imidazole Groups
Cui et al. developed the new type of anion sensor 168 (Figure 65), which consists of a Ru(II)-bpy moiety as a chromophore
Figure 65. Proposed mechanism for the detection of F− and OAc− by 168.
and the 2,2′-biimidazole (H2biim) as the anion receptor.166 In CH3CN solution, sensor 168 serves as a donor for hydrogen bonding with Cl−, Br−, I−, NO3−, HSO4−, H2PO4−, and OAc− anions, resulting in a color change from yellow to orangebrown. Fluoride ion reacts to deprotonate the NH group of the H2biim ligand owing to the formation of the highly stable HF2− anion. The stepwise deprotonations of the two NH fragments induce a vivid color change from yellow to orangebrown and then to violet. Two new imidazo[4,5-f ]-1,10-phenanthroline-containing Ru(II) (169) and Re(I) (170) complexes (Figure 66), which serve as colorimetric and fluorescence chemosensors for F−, were recently investigated.167 A solution of probe 170 in DMSO undergoes an immediate naked-eye-visible change from yellow to pink when fluoride anion is added. In contrast, the emission intensity of 169 in DMSO at 512 nm is strongly enhanced upon the addition of F−. The effective anion sensor 171 (Figure 66) was devised for selective recognition of F− and OAc− by Kundu et al.168 Sensing by this substance is based on the oxidative generation of 171+, which undergoes selective colorimetric changes in the presence of F− and OAc− anions. The MLCT band of 171+ at 498 nm is red-shifted to 538 nm with a distinct color change from reddish-orange to pink upon addition of these anions. In addition, the emission band of 171+ at 716 nm is quenched in the presence of 1 equiv of F− or OAc−. In CH3CN, the binding constants of 171+ with F− and OAc− were determined to be log K = 7.61 and log K = 7.88, respectively. The binol-based receptor 172 (Figure 67) can be employed to detect F− anion through color changes and F− and AcO− through fluorescence changes.169 In CH3CN/DMSO (9:1 v/v), the color of 172 solution changes from colorless to yellow upon the addition of 50 equiv of F− and to pale yellow upon the addition of 50 equiv of AcO−. The distinct chromogenic response of 172 is attributed to deprotonation of the imidazolium moieties by F−. In contrast, AcO− interacts with the imidazolium moieties only through the formation of hydrogen bonds. Probe 172 displays an emission band at 370 nm in acetonitrile that undergoes a bathochromic shift to 474 and 454 nm upon addition of F− and AcO−, respectively. Receptors 173 and 174 (Figure 67), which participate in highly efficient intramolecular ESPT, can be used for the highly selective detection of fluoride ions.170 Upon addition of fluoride to a solution of 173 in CH3CN, a new absorption band appears between 400 and 440 nm with a concurrent color change from 5538
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Figure 66. Structures of 169−171.
Figure 67. Structure of 172 and proposed binding modes of 173 and 174 with F−.
colorless to yellow. Addition of increasing concentrations of fluoride to the solution of 173 promotes generation of two new emission bands at 425 and 500 nm. In the case of 174, the intensity of the fluorescence peak at 586 nm gradually decreases in concert with the appearance of a new blue-shifted emission band at 515 nm. Because of its on−off−on behavior, 174 can be used for the ratiometric determination of F−. The new benzoimidazole naphthalimide 175 (Figure 68) was devised to sense F−.171 Upon addition of fluoride anions to a THF solution of this substance, the absorption band at 378 nm progressively decreases concurrent with the appearance of a new band at 476 nm. These changes result in an observable color change from colorless to magenta that can be seen by the
Figure 68. Structures of 175 and 176.
naked eye. In addition, a significant decrease takes place in the emission band at 478 nm, and the emission maximum is redshifted to 603 nm. These absorption and fluorescence changes are a result of deprotonation by fluoride anions of the amino moiety located in the benzoimidazole group. 5539
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Figure 69. Proposed mechanism for the detection of F− by 177 and structures of 178 and 179.
Figure 70. Proposed mechanism for the interaction of 180 with F−.
The new fluorescence sensor 176 (Figure 68), designed for the detection of fluoride by Wang et al.,172 contains a similar Nimidazolyl-1,8-naphthalimide moiety. A solution of this substance in CH2Cl2 displays strong fluorescence owing to the formation of a coplanar geometry as a result of intramolecular H-bonding between the naphthalimide and imidazole moieties. Addition of F− disrupts this H-bonding, which results in significant fluorescence quenching at 450 nm. In addition, the results of nonlinear regression analysis indicate that the stoichiometry of the 176−F− complex is 1:1 with an association constant of log Ka = 5.31 ± 0.07. Xu et al. provided the first concrete theoretical and experimental evidence for the existence of anion−π interactions between electron-rich alkylbenzene rings and fluoride ion in CH3CN.173 Probe 177 (Figure 69), which contains a cyclophane cavity bridged by three naphthoimidazolium groups, selectively interacts with fluoride ion through a combination of anion−π interactions and (CH)+···F−-type ionic hydrogen bonding in the form of a sandwich complex. Upon addition of 1 equiv of F−, a strong increase in the fluorescence emission band of the probe centered at 385 nm occurs, and the band at 474 nm decreases. The results of fluorescence titration experiments showed that the ratio of the emission intensities at 375 and 474 nm are dependent on the concentration of F−.
The analogous bisbenzimidazolium cyclophane receptors 178 and 179 (Figure 69) were described by Amendola et al.174 F− in CH3CN interacts with the three coplanar imidazolium CH hydrogens in each probe to give stable complexes. In both cases, upon the addition of F−, the absorbance at 240 nm increases continuously, and a broad defined band centered at 300 nm is formed. Probe 178 is a completely specific receptor for fluoride. Other anions, such as Br− and NO3−, cannot be accommodated in a coplanar fashion in the triangular structure of 178. Although the cage complex [178···F]2+ is stable in the presence of excess F−, the related tripodal complex [179···F]2+ decomposes as a consequence of deprotonation of the imidazolium CH protons and the formation of very stable [HF2]− species. Zhang et al. devised the novel Y-shaped imidazole derivative 180 (Figure 70) to be a single- and two-photon-excited fluorescence chemosensor for fluoride anions.175 Upon the addition of F− to a CH3CN solution of this sensor, the emission color changes from green to brown, as a result of deprotonation of the imidazole core. Upon complexation with fluoride anion, the maximum wavelength of the two-photon-excited fluorescence (TPEF) peak shifts from 490 to 565 nm, indicating that 180 is an excellent ratiometric TPEF sensor for fluoride anion, with a K value of 7.3 × 103 M−1. 5540
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Figure 71. Structures of 181−184.
The N-arylimidazolium-based chromofluorescent probe 181 (Figure 71) was developed as a selective, ratiometric sensor for F−.176 Upon gradual addition of TBAF to a solution of 181 in CH3CN/DMSO (20:1, v/v), the absorbance at 405 nm undergoes a gradual decrease in intensity with a concomitant increase at 480 nm and an isosbestic point at 440 nm in association with a color change from light yellow to orange. Upon addition of F−, the fluorescence intensity of the probe at 480 nm disappears, a new red-shifted fluorescence emission band at 580 nm appears, and the emission color changes from green to red. Peng et al. designed the two ratiometric fluorescent chemosensors 182 and 183 for fluoride.177 The spectral responses of these probes to fluoride in acetonitrile includes an approximately 100-nm red shift in the absorption and fluorescence emission bands and significant ratiometric fluorescent responses (Rmax/Rmin) of 88 for sensor 182 and 548 for sensor 183. The selectivity for F− can be tuned by changing the electronic push−pull properties of substituents on the phenyl para position of the sensor. As a result, 182 displays a higher selectivity than does 183. 2,6-Bis(2-benzimidazolyl)pyridine 184 (Figure 71) was developed as a highly selective and sensitive (concentrations as low as 10−7 M) chemosensor for the detection of fluoride ions with high selectivity.178 Upon the addition of fluoride to a CH3CN solution of the probe, the absorption band at 327 nm progressively decreases and broadens, and a new peak at 311 nm arises. The association constant (Ka) was calculated to be 439.49 M−1, suggesting that strong hydrogen bonding takes place between 184 and fluoride. Significant quenching of the 375-nm emission band and the development of a new band at 434 nm occur upon the addition of TBAF, indicating that a hydrogen-bonded complex between TBAF and 184 is formed. Unlike previously described optical methods for CO2 sensing, which depend primarily on the acidic properties of this gas, a new approach has been developed recently in which anions are used as activators.179 The sensor 185 (Figure 72) consists of a benzobisimidazolium group that, in the absence of exogenous base, undergoes a deprotonation reaction with fluoride. Among other anions, F− reaction with 185 in acetonitrile induces significant bathochromic shifts of the absorption bands at 290 to 344 nm, which is ascribed to the formation of strong (CH)+···F− bonds. In addition, fluorescence quenching takes place upon addition of F−. When the fluoride-treated sensor is exposed to CO2 gas, fluorescence is recovered, and a hypochromic shift in absorption peak back to 290 nm takes place. These observations illustrate how this novel fluoride-activated system senses CO2 in the form of turn-on fluorescence and a colorimetric response.
Figure 72. Proposed reactions that occur when 185 is treated first with F− and then with either CO2 or CS2.
3.6. Sensors Bearing Indole Groups
Receptors containing maleimide dyes as a signaling subunits and indoles (186 and 187, Figure 73) and pyrroles (188, Figure 73) as binding sites have been developed by Lin et al.180 For example, the colors of solutions of 186 and 187 change from yellow (absorption band at 474 nm) to orange (505 nm) in the presence of F− and H2PO4−, suggesting that the two indolyl NH groups chelate the anions through hydrogen-bonding interactions. The colors of dichloromethane solutions of 188 change from red (absorption band at 516 nm) to blue (580 nm) upon the addition of F− and CN−. Also, the emission band of 188 at 646 nm is gradually quenched and shifted to 705 nm upon addition of these anions as a consequence of deprotonation of one of the pyrrole groups. In 2006, Shao’s group described the new substance 189 (Figure 73), containing an acidic H-bond-donor moiety and a basic H-bond-acceptor moiety, which acts as a selective colorimetric sensor for either F− or HSO4−.181 In CH3CN, F− induces a color change of this probe from yellow to red, associated with the disappearance of the absorption band at 423 nm and the formation of a new red-shifted band at 517 nm. In aqueous solvent systems (CH3CN/H2O), a significant color change takes place only upon addition of HSO4−. A deprotonation/protonation reaction of 189 is responsible for the dramatic color changes promoted by both F− and HSO4−. 5541
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Figure 73. Structures of 186−193.
and two new peaks at 540 and 570 nm appear. In addition, fluoride induces an increase in the intensity of the fluorescence emission of 194, together with a red shift from 410 to 435 nm. The association constant between 194 and F− was determined to be 1.02 × 104 M−1. Indole derivatives 195−197 (Figure 75) were designed by Pereira et al.185 for the fluorescence recognition of F−. In
In 2011, the tris(indolyl)methene receptors 190−192 (Figure 73), containing conjugated bisindole skeletons, were reported by Shao’s group.182 These sensors selectively detect F− using a two-stage proton-transfer pathway that induces dramatic color changes. The first step in the double-deprotonation process promoted by F− has a higher dissociation constant than does the second step, which is attributed to a decrease in the NH proton acidity following monodeprotonation. In the presence of F−, the color of a solution of 190 gradually changes from yellow to light pink and then to orange. In the case of 191, two distinct color changes occur from orange to light pink and then to yellow in the presence of F−. Owing to the increase in the acidity of indole NH groups caused by the presence of electronwithdrawing groups, the nitro-substituted derivative 192 display a much higher binding affinity for F− than do 190 and 191. In 2012, Mallick et al. described the fluoride chemosensor 193 (Figure 73), which has a structure that is closely related to that of 189.183 Upon addition of increasing concentrations of fluoride in CH3CN, the absorption band of this probe at 530 nm blue shifts by about 10 nm, and the band at 425 nm increases. These changes enable the sensor to be used for the detection of fluoride even at submicromolar levels and as high as 2 mM. Based on their previous studies, Shao’s group developed the indole hydrazone-based colorimetric sensor 194 (Figure 74), which displays a colorimetric response and turn-on fluorescence in the presence of fluoride anions.184 Upon addition of F−, the band of the probe at 425 nm progressively decreases,
Figure 75. Structures of 195−197.
CH3CN solutions, the sensors display emission bands in the 340−420-nm region. Upon addition of F− but not Cl−, Br−, AcO−, or HSO4−, quenching of the emission of the probes in the 340−420-nm range takes place, along with the appearance of a red-shifted band at 465 nm for 195, 515 nm for 196, and 510 nm for 197. The new red-shifted emissions are ascribed to the deprotonated form of the receptors generated by reaction with basic F−. In an investigation of the interesting indole-linked quinolone derivative 198 (Figure 76), Panzella et al. observed that this material can be employed as a turn-on fluorescent sensor for fluoride.186 Upon addition of increasing concentrations of F− to a CH3CN solution of this substance, a colorless-to-yellow color change takes place in association with the appearance of an absorption band at 414 nm. In CH3CN, addition of F− results in the appearance of an emission band at 489 nm. The stoichiometry of the fluoride−198 interaction was determined to be 2:1 based on analysis of a Job plot. The bisindole diazine 199 (Figure 77) has been identified as a novel fluoride sensor.187 Addition of F− to a DMSO solution of 199 leads to the disappearance of the absorption band at 352 nm and the formation of a new band at 409 nm. This red shift induces a color change from colorless to yellow. Upon addition of increasing quantities of F− anion, a new emission band at 455 nm appears in association with blue-colored fluorescence.
Figure 74. Proposed binding mode of receptor 194 with F−. 5542
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Figure 76. Proposed binding mode of 198 with F−.
These studies indicate that a 1:2 host/guest complex forms as a result of hydrogen-bonding interactions between the NH protons of the indole groups and the anion. Kim et al. devised chemosensor 200 (Figure 77) in which the indole moiety acts as a proton-donor unit and the furan moiety acts as an acceptor.188 Upon addition of fluoride to this sensor in acetonitrile, the absorption band at 523 nm progressively shifts to 545 nm. and the color changes from orange-red to purple-pink as a consequence NH deprotonation. In conjunction with the color change, fluorescence of the probe at 568 nm is dramatically quenched. Lozano et al. developed organogelator 201 (Figure 77), which contains asparagine and tryptophan residues in the outer core and a central piperazine with two 1,3,5-triazine units in the inner core.189 In acetonitrile (2%, w/v), the 201 gel undergoes a rapid nakedeye-observable gel-to-sol transition upon addition of TBAF. Concurrently, quenching of the emission band at 340 nm takes place. The gel−sol transition was rationalized in terms of the dissociation of intermolecular hydrogen bonds caused by bonding of fluoride anions to the asparagine amide groups of the gel form. The ICT-based chemosensor 202 (Figure 77), which contains a π-rich indole as the H-donor and the cyanoquinazolinone ring as the π-acceptor, was recently developed.190 Incremental addition of TBAF causes the absorbance band of the probe at 412 nm to decrease gradually as a red-shifted maximum at 524 nm appears. This change enables naked-eye detection of a color change from yellow to red. In DMSO,
Figure 77. Structures of 199−203.
Figure 78. Structures of 204−208. 5543
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was shown that F− can displace BM through the formation of a complex with 211, concurrent with the development of a violet color of the solution. Addition of HSO4− transforms the solution to almost colorless because it deprotonates the receptor site in 211 and protonates the dye. These changes enable this sensor system to be used for visual detection of F− and HSO4−. The dipyrrolemethane-containing compounds 212−215 (Figure 81) were shown to be colorimetric fluoride sensors in CH3CN.197 A new absorption peak appears at 490 nm when 212 forms a complex with F−, and the absorption peak of 213 at 330 nm is shifted to 510 nm when fluoride ions are added to compound 213. For compound 214, the absorption peak at 413 nm is shifted to 501 nm in the presence of this anion. Both 212 and 215 display dramatic color changes from colorless to yellow-red upon addition of fluoride, and 213 and 214 also undergo distinct color changes from light yellow to deep red when F− is added. Electrostatic interactions between the pyrrole NH groups of the dipyrrolemethane moiety and fluoride anion have been shown to be responsible for these changes. The efficient colorimetric fluoride sensors 216 and 217 (Figure 81), incorporating various numbers of hemiquinone and pyrrole moieties, were described by Wang et al.198 Addition of only fluoride to a solution of 216 in DMSO induces a gradual decrease in the absorption peak at 496 nm and the appearance of a new peak at 568 nm. In addition, a color change occurs from orange to blue as a result of deprotonation of the NH moiety. Based on the unique bis-deprotonation process of 217, addition of F− results in a decrease of the absorption bands at 459 and 571 nm and the increase of a new band at 740 nm in conjunction with a color change from winered to gray. The detection limits for F− in DMSO were determined to be 2 × 10−4 and 2 × 10−5 M for 216 and 217, respectively. The colorimetric sensor 218 (Figure 82) containing a dipyrromethane (donor)−7,7′,8,8′-tetracyanoquinodimethane (acceptor) charge-transfer dyad displays excellent selectivity for the detection of F−.199 The sensing mechanism was ascribed to the interruption of charge transfer between the donor and acceptor in the presence of F−. Upon addition of F− to a MeCN/H2O (1:1, v/v) solution of the probe, the intensity of the CT band at 612 nm decreases, and a new band appears at 510 nm. In addition, a distinct colorimetric change from intense blue to light pink takes place. The pyrazole-based fluorescent sensor 219 (Figure 83) was studied as a fluoride-anion detector in organic (DMSO) and aqueous [DMSO/H2O (95:5, v/v)] solutions.200 Sensor 219 displays changes in both its UV−vis absorption and fluorescence spectra upon addition of F−. Addition of F− induces a red shift of the main absorption band of 219 from 342 to 368 nm in association with a dramatic, 607-fold enhancement of the intensity of blue emission at 424 nm. The stoichiometry of the complex formed between 219 and F was determined to be 1:2, and the stability constants of the complex were determined to be log K1 = 4.67 ± 0.16 and log K2 = 4.68 ± 0.05. The two efficient colorimetric anion sensors 220 and 221 (Figure 83), with a pyrrole NH moiety as the binding site and a nitro group as the signaling unit, have been explored.201 The receptors undergo color changes to yellow and permanganate upon the addition of F− and OH−, respectively. The nitro group in receptor 221 causes it to serve as a superior sensor in
emission of the probe at 489 nm is replaced by a new redshifted emission band at 551 nm in a manner that enables fluorescence ratiometric detection. The detection limit of 202, determined using the colorimetric response, was found to be 1.096 × 10−6 M. The BODIPY−indole conjugate 203 (Figure 77) serves as a colorimetric and fluorometric probe for selective and sensitive detection of F−.191 This sensor in CH3CN interacts with F− in a 1:1 stoichiometric manner through a hydrogen-bonding interaction between the indole NH and F−. The addition of F− to 203 leads to a decrease in the 599-nm absorption band along with an increase in a 718-nm band, leading to a clear color change from blue to green. In addition, the orange fluorescence of 203 at 624 nm is quenched by fluoride. The association constant between 203 and F− was determined to be 7.8 × 102 M−1. 3.7. Sensors Bearing Pyrrole Groups
In 2002, the cobalt(III) complex 204 (Figure 78) was described by Mizuno et al.192 When a solution of 204 is treated with increasing quantities of TBAF, the absorption peaks at 323 and 525 nm decrease, and a new peak at 652 nm appears, in conjunction with a color change from red-pink to pale purple. Chemosensors bearing dipyrrolyl motifs as recognition sites and tetrathiafulvalene redox-active moieties have also been investigated.193 For example, the receptor 205 (Figure 78) shows specific optical signaling for fluoride by undergoing significant changes in its absorption spectrum and dramatic enhancement in the intensity of emission. The electrooptical sensors 206−208 (Figure 78), containing conjugated chromophores, display colorimetric changes as well as redox activities in response to anions.194 A unique anion-specific response was observed for fluoride, pyrophosphate, and acetate. Distinct colorimetric changes from yellow to blue (for 206), brown (for 207), and purple (for 208) and changes in electrochemical properties occur for these sensors in CH3CN upon addition of F−. Pyreno[2,1-b]pyrrole 209 and its dimeric derivative 210 (Figure 79) are reported to display excellent selectivity and
Figure 79. Structures of 209 and 210.
sensitivity for the detection of fluoride ions.195 Reversible hydrogen bonding with fluoride ion produces obvious visible colorimetric and fluorescent changes. In DMSO, the colorless solution of 209 turns grass-green upon addition of F−. In DMSO solution, 210 also undergoes a dramatic color change from greenish-yellow to bright red upon fluoride addition. In acetonitrile, growth of a new absorption band of 209 at 450 nm takes place only when F− ion is added in excess amounts. The association constants in CH3CN were determined to be Kassoc = 2 × 103 M−1 and Kassoc = 2 × 104 M−1 for 209 and 210, respectively. The formation of a 211−BM species (Figure 80) that acts as an anionic chromogenic sensor has been described.196 Upon addition of the cyclic polypyrrole 211 to a violet solution of the zwitterion BM in acetonitrile, the color changes to orange. It 5544
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Figure 80. Proposed binding mode of receptor 211 with anions.
Figure 83. Structures of 219−221.
Figure 81. Structures of 212−217.
that it exhibits stronger binding toward F− and OH−. The emission intensity at 398 nm for 221 gradually increases with an increase in F− concentration, which is caused by the formation of an intermolecular hydrogen bond (NHF). 3.8. Sensors Bearing Schiff Base Groups
Two new chromogenic receptors containing a nitro group as a signaling unit and OH and NH groups as binding sites were described by Saravanakumar et al.202 Both receptors 222 and 223 (Figure 84) display colorimetric responses to fluoride ions in CH3CN. Addition of F− to 222 gradually results in the
Figure 84. Structures of 222 and 223 and proposed mechanism for the F−-sensing process by 224.
Figure 82. Proposed mechanism for the interaction of 218 with F−. 5545
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Figure 85. Structures of 225−228 and possible structure of the complex formed between 229 and F−.
concentrations of these anions increase, the peak at 342 nm of the free receptor gradually disappears and is replaced by a new absorption peak at 450 nm, with a corresponding color change from light yellow to deep yellow that is visible to the naked eye. This change is likely caused by the development of a chargetransfer state upon interaction between the proton of the amine or phenolic group and the acceptor groups. The detection limits of this probe were determined to be 1.20 × 10−6, 1.23 × 10−6, and 3.20 × 10−6 M for fluoride, acetate, and dihydrogen phosphate, respectively. In 2007, Wu et al. described the results of studies with receptors 230−232 (Figure 86) containing p-nitrophenyl
development of a light orange color associated with generation of new absorption peaks at 369 and 457 nm. Upon further addition of F−, the orange color becomes more intense. For receptor 223, addition of fluoride causes the intensities of the absorption peaks at 233, 301, and 362 nm to decrease and the color of the solution to turn fluorescent yellow in association with the development of new peaks at 417 and 362 nm. The simple salicylaldehyde derivative 224 (Figure 84) was designed for fluoride sensing in DMSO and DMSO/H2O (9:1, v/v).203 In DMSO/H2O (9:1, v/v), addition of fluoride promotes a decrease in the intensity of the absorption band at 430 nm and a concomitant increase in a peak at 502 nm, which is attributed to charge transfer (CT) between the anionbound NH and OH units and the electron-deficient NO2 moiety. This process leads to a color change from yellow to red that can be seen with the naked eye. The colorimetric receptors 225−227 (Figure 85) based on anthraquinone fluorophores were synthesized by Devaraj et al.204 In DMSO, the presence of TBAF induces dramatic color changes of 225−227 from light pink to dark pink, brown, and golden yellow, respectively. The binding constants (Ka) for 225−227 with fluoride were determined to be 9.2 × 103, 5.78 × 103, and 1.90 × 104 M−1, respectively. Addition of fluoride ions to acetonitrile solutions of these sensors promotes a change in the color of the solutions from pale red to dark red (for 225 and 226) and reddish-brown (for 227). In CHCl3, the yellow solutions of these substances turn pink (for 225) and reddishbrown (for 226 and 227) in the presence of F−. These color changes are likely a consequence of the formation of hydrogen bonds between the OH groups and fluoride ions. The coumarin-based hydrazone receptor 228 (Figure 85) was designed to be a selective, naked-eye-detectable sensor for fluoride ion in aqueous DMSO (5:95, v/v).205 This compound can also be used to detect fluoride in a toothpaste sample using the naked eye. UV−vis spectral measurements showed that the absorption maximum of 228 undergoes a bathochromic shift from 455 to 514 and 484 nm in the presence of fluoride and acetate, respectively. Upon addition of F−, 228 develops a red color, whereas the addition of acetate produces a faint pink color. Nonlinear fits of the UV−vis titration data were used to determine that the association constant of a 1:1 complex of 228 with fluoride is (3.60 ± 0.47) × 105 M−1. Hijji and co-workers described the Schiff base 229 (Figure 85), which undergoes a color change upon addition of fluoride, acetate, and dihydrogen phosphate (DHP) ions.206 As the
Figure 86. Structures of 230−232 and proposed host/guest binding mode of 233 with F− in DMSO.
signaling subunits linked to phenol and hydrazone groups.207 In DMSO solutions, the three receptors display color changes from orange (absorption band at 440 nm) to magenta upon the addition of F−, AcO−, and H2PO4− anions. However, in DMSO/water (9:1, v/v), only 230 and 231 produce color changes in the presence of F− and AcO−, whereas only H2PO4− is able to induce a color changes in the case of 230. Finally, a selective response to AcO− is displayed by only 230 and 231 in 5546
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Figure 87. Structures of 234−236 and proposed mechanism for F− sensing by 234 and 235.
DMSO/water (7.5:2.5, v/v). Two years later, the artificial tweezer 233 (Figure 86), consisting of a symmetric 1,3phenylenedi(carbonylhydrazone)-based structure, was developed as a sensor by Wei’s group.208 The ability of this substance to recognize anions in DMSO and DMSO/H2O (85:15, v/v) was investigated by UV−vis spectroscopy. The results showed that 233 in both solvent systems undergoes a marked color change from colorless to yellow upon addition of AcO−, F−, and H2PO4−. In pure DMSO, the binding constant of 233 for F− (4.65 × 105 M−1) was found to be larger than those for AcO− and H2PO4−. The phenolic hydroxyl-based sensors 234−236 (Figure 87), bearing Schiff base groups, were described by Huang et al.209 These probes undergo naked-eye-detectable changes in color from light yellow to orange-red (234), intense purple (235), and yellow (236) in the presence of F− in DMSO. Sensor 234 takes part in a stepwise deprotonation of the two OH fragments when it interacts with excess fluoride. Upon addition of F− to a solution of 234, the intensity of the absorption peak at 332 nm decreases, and a new absorption peak at 440 nm appears. In the case of 236, addition of an excess of F− promotes a blue shift in the absorption band at 440 to 420 nm. The selectivity of anion-induced color changes for 234− 236 was determined to be F− > AcO− ≫ H2PO4− > Cl−, Br−, I−. Two salicylaldehyde-based colorimetric and turn-on fluorescence chemosensors, 237 and 238 (Figure 88), were developed by Shao’s group.210 Both receptors show a unique selectivity for F− over other anions in DMSO solutions. Titration of 237 with fluoride anions results in a bathochromic shift of the absorption-band maximum at 461 nm. Upon addition of fluoride, the two absorption bands at 269 and 331 nm of 238 in DMSO disappear, and a new band at 435 nm arises in association with a color change from colorless to yellow. The association constants of receptors 237 and 238 with fluoride anions were determined to be 4.2 × 103 and 5.5 × 103 M−1, respectively, by Benesi−Hildebrand analysis. The cleft-like receptor 239 (Figure 88) was shown to selectively recognize F− through both color changes and an increase in emission intensity.211 Addition of F− to a solution of this sensor in DMSO induces a moderate decrease in the intensity of the absorption band at 326 nm (colorless) and the appearance of a new band at 425 nm (yellow-green). This process is ascribed to the formation of a 1:1 host/guest complex through multiple hydrogen-bonding interactions of F− with the amide and hydroxyl groups. Similar spectral changes occur upon the addition of AcO− to a DMSO solution of 239,
Figure 88. Proposed binding modes of receptor 237 and 238 with F− and structure of 239.
but the color change is less intense. In addition, upon addition of F− and AcO−, the emission band of 239 at 463 nm shifts to 475 nm, and its intensity is enhanced. These emission enhancements are associated with blocking of the excited intramolecular proton-transfer process. The tripodal receptor 240 (Figure 89), containing three catechol subunits, exhibits a selective chromogenic response to the F− anion.212 In DMSO, 240 displays an absorption band centered at 274 nm and two shoulders at 306 and 353 nm. Addition of F− induces a slight hypsochromic shift of the highest-energy band, whereas the shoulder at 353 nm disappears and a new band appears at 433 nm. These changes promote a color change from pale to deep yellow. Deprotonation of the hydroxyl moieties of the catechol was found to be responsible for the color change. The negative charge generated in the deprotonation process results in an increase in the dipole moment and stabilizes the excited state, causing a significant red shift of the visible band. Mahapatra et al. synthesized the new tripodal receptors 241− 245 (Figure 89), which display distinct color changes only in the presence of fluoride ions in CH3CN solutions.213 For tripodal receptors 241 and 243−245, dramatic color changes 5547
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Figure 89. Structures of 240−245 and proposed host/guest binding mode of 246−248 with F−.
249 at 450 nm decreases upon the addition of fluoride anion, a new peak appears at 610 nm, and the color changes from yellow to green. The interaction between 250 and F− was found to be a consequence of initial hydrogen-bond formation, which is then followed by NH deprotonation and the formation of a hydrogen bond between the F− ion and the nearest CH group in the Schiff base. Interaction between receptor 249 and F−, AcO−, and H2PO4− is accompanied by color changes from yellow to violet or red. The binding abilities of receptors 249 and 250 with various anions are in the order AcO− > F− > H2PO4− ≥ Cl−, Br−, I−. Probes that contain multiple hydrogen-bonding donors, including hydrazine, hydrazone, and hydroxyl groups, as potential anion-interacting sites were reported by Chen et al.216 DMSO solutions of both 251 and 252 (Figure 91) are
take place from light pale yellow to orange, whereas 242 displays a noticeable deep color change from yellow to deep purple in the presence of 5 equiv of F−. Among these probes, 242 has the largest Kassoc value (3.3311 × 104 M−1) for fluoride and a detection limit of 1.00 × 10−5 M. The results of this study showed that deprotonation rather than H-bonding is the key factor for triggering the observed chromogenic effects for these receptors. Salicylaldimine-based Schiff base receptors 246−248 (Figure 89), containing different para-N-aryl substituents, were described by Sivakumar et al.214 Addition of F− to acetonitrile solutions of these probes promotes a dramatic color change from colorless to pale yellow (246), yellow (247), and orange (248). Generation of the new absorption peaks above 400 nm for the receptors 246 and 247 is attributed to the formation of the keto form of the receptors, which results from hydrogenbonding interactions with fluoride anion. The binding constants (Ka) of fluoride complexes of 246−248 were determined to be 2.787 × 103, 5.684 × 103, and 2.53 × 104 M−1, respectively. Finally, addition of fluoride to solutions of these receptors markedly enhances the intensity of emission at around 490 nm. Among the three, the receptor containing the NO 2 substituent (248) displays the highest fluorescence enhancement in the presence of fluoride. The hydrazone derivatives 249 and 250 (Figure 90), which contain anthracene fluorophores, were designed as fluoride sensors by Shang and Xu.215 In DMSO, the absorption band of
Figure 91. Structures of 251−253.
orange and display fairly weak fluorescence at room temperature. However, these probes undergo noticeable colorimetric and fluorescent responses only to F−, OAc−, and H2PO4−. The general trend of the sensitivity to these anions is in the order F− > OAc− > H2PO4− > Cl− > Br− ≈ I−. A 1:2 binding stoichiometry was found to exist for 251 with OAc− and H2PO4− and for 252 with F−, OAc−, and H2PO4−. The formation of an aggregated supramolecular complex upon addition of fluoride was found to be responsible for the observed optical responses.
Figure 90. Structures of 249−250 and proposed interaction of 249 with F−. 5548
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Figure 92. Structures of 254−263.
between AcO− and F−. In contrast, 266 and 267 exhibit high selectivity toward fluoride ions, and 266 undergoes a blue-toorange fluoride-anion-induced color change. Discrimination between fluoride and other anions by 266 and 267 is a consequence of their abilities to form 1:2 host/guest complexes. Zhao et al. designed the 2,2′-bispyridine (bipy) ruthenium complex 268 (Figure 94), which also contains a 3-hydroxyl-2naphthoylhydrazine recognition moiety.220 The OH group in the naphthalene fluorophore leads to a turn-on ESIPT signaling transduction. Addition of F− to a solution of this probe causes a dramatic change in its UV−vis spectrum, with the band at 345 nm decreasing progressively as the band at 440 nm increases. The association constant (log K) for the fluoride complex with 268 was determined to be 5.76 ± 0.07. A solution of 268 in CH3CN containing fluoride and excited at 355 nm displays a new emission band at about 530 nm. The sensing properties of the related fluoride sensor 269 (Figure 94), which contains a Ru-bipy fluorophore and a 2,4-dinitrophenylhydrazone chromophore, have also been explored.221 Addition of F− to a solution of this probe promotes the appearance of a new intense absorption band centered at about 580 nm and an accompanying dramatic change in color from yellow to magenta. The association constant (log K) of the complex formed in this case was determined to be 6.71 ± 0.04. Upon the addition of F−, the intensity of the emission of 269 is significantly enhanced. An easy-to-prepare test paper containing this sensor was used to detect F− in aqueous media, demonstrating the potential use of this system for detecting F− in natural aqueous environments without the need for sophisticated instrumentation. Lin et al. developed a naked-eye detection system consisting of 270 (Figure 94) for fluoride ions.222 Addition of F− to a solution of this substance in CH3CN causes a dramatic change in the color from orange to blue-violet, which is accompanied by an increase in a new intense absorption band at about 580 nm. The formation of a complex with 1:1 stoichiometry occurs in this case with an association constant of log K = 6.23 ± 0.03. The presence of F− also induces a significant increase in the luminescence intensities of Ru-based receptor 270 at 630 nm. Also, a test paper loaded with this probe exhibits color changes that depend on the fluoride concentration at pH ∼7 in water with a low detection limit of about 10 ppm (10 mg L−1). The two ruthenium(II) polypyridyl complexes 271 and 272 (Figure 95) with pendant phenol/catechol groups were shown to function as colorimetric sensors for fluoride ions.223 Addition of F− to CH3CN solutions of 271 and 272 causes a decrease in
The fluorescent bischromophoric dye 253 (Figure 91), combining anthracene and naphthalic anhydride units, has been investigated as a fluoride sensor.217 This substance in DMSO displays a FRET-mediated emission signal at 535 nm with a high energy-transfer efficiency (ET = 93%) and quantum yield (Φ = 0.155). Upon addition of F− and Hg2+ ions to this solution, the FRET signal decreases along with simultaneous enhancement of the anthracene emission band at 417−437 nm. In addition, the yellow-green color of the solution changes to dark red. A Job plot analysis based on fluorescence titration data suggests that a 1:1 complex is formed with an association constant of 2.24 × 104 M−1. Bose et al. prepared the series of new symmetrically functionalized guanidinium chlorides 254−263 (Figure 92) and evaluated their ability to sense anions in MeCN/DMF (24:1, v/v).218 Addition of only fluoride causes vivid color changes from colorless to yellow (254, λmax = 454 nm), red (255, λmax = 510 nm), reddish-orange (261, λmax = 468 nm), and finally blue (256, λmax = 600 nm; 262, λmax = 642 nm) and pink (263, λmax = 932 nm). Probe 262 can be used to sense both F− and AcO− colorimetrically with a detection limit of ∼6.25 × 10−5 M, whereas the indole-bearing derivative 263 displays a selective color change in the presence of F− that results in NIR absorption at about 930 nm. 3.9. Sensors Bearing Other Groups
In fluoride sensors 264−267 (Figure 93), designed by Fillaut et al.,219 alkynyl ruthenium complexes bearing terminal hydrogenbonding receptors are employed as luminophore recognition groups. Sensors 264 and 265, which contain only one recognition site, proved to be inefficient for distinguishing
Figure 93. Structures of 264−267. 5549
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Figure 94. Structures of 268 and 269 and proposed binding mode of receptor 270 with F−.
Figure 95. Structures of 271−274.
assigned to the naphthalimide π−π transition, appears. F− recognition was also naked-eye-visible in the form of a clear color change from yellow to red. The absorption spectral changes can be reversed by addition of competitive hydrogenbonding solvents such as EtOH and water. The sensing mechanism in this case is a result of the initial hydrogen bonding between the anion and the amine, which results in deprotonation of the acidic 4-amino-1,8-naphthalimide moiety. The trinuclear star-shaped organometallic host 274 (Figure 95), containing a π-acidic interior cavity, serves as a colorimetric sensor for F−.225 Acetonitrile solutions of receptor 274 display intense ligand-localized π−π* transitions at 250−
the emission intensity at 605 nm for 271 and 600 nm for 272. Detectable changes in colors from yellow to dark red for 271 and to red for 272 also occur when fluoride is added. The corresponding equilibrium constants for complexes of these probes with F− are 6.56 × 103 M−1 for 271 and 5.16 × 105 M−1 for 272. Selective sensing of fluoride was achieved by detecting fluorescence quenching of the metal-to-ligand charge transfer (MLCT) emission of the 1,8-naphthalimide ruthenium conjugate 273 (Figure 95).224 Upon addition of F− to a solution of this substance, the long-wavelength absorption band at 432 nm is red-shifted to 460 nm, and a shoulder at 340 nm, 5550
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Figure 96. Structures of 275−279 and proposed binding mode and deprotonation of 280 with fluoride and hydroxide ions.
Wang and co-workers synthesized the oxacalix[2]arene[2]triazine azacrown 281 (Figure 97) bearing an azacrown moiety and an electron-deficient V-shaped cleft formed by two triazine rings that serves as a receptor to interact with ion pairs.228 The fluorescence spectrum of 281 undergoes changes upon addition of TBAF with an enhancement occurring in the fluorescence band at 425 nm. The association constant of the complex between 281 and fluoride was determined to be 6.59 × 103 M−1. Observations made in ion-pair recognition tests showed that binding of [281·Pb]2+ with fluoride is comparable to that of 281 (Ka = 6.59 × 103 M−1) and that a more than 23-fold increase in the binding constant from 6.59 × 103 M−1 [281·F]− to 1.53 × 105 M−1 [Zn·281·F]+ takes place. Wang and coworkers also reported that tetraoxacalix[2]arene[2]triazine azacrowns 282 and 283 (Figure 97) also interact with fluoride anions, leading to cavity changes in the host molecules.229 For the syn forms, fluoride serves as a base to deprotonate two amino groups, resulting in the formation of an intramolecular hydrogen bond between the unprotonated and deprotonated amino groups. This interaction causes the two benzoate ester rings in the tetraoxacalix[2]arene[2]triazine azacrown backbone to move farther apart, thus promoting a decrease in the fluorescence intensity at 340 nm. With an increase in the concentrations of added n-Bu4NF and n-Bu4NOH, the emission at 477 nm decreases and eventually disappears, in concert with an increase of the emission band in the range of 378−396 nm. The new naphthalimide calix[4]arene 284 (Figure 97) was devised by Xu et al.230 as a highly selective fluorescent chemosensor for Cu2+ and F−. In acetonitrile, addition of Cu2+
310 nm and a weak MLCT band at 320 nm. Addition of increasing quantities of F− induces the appearance of an absorption band at 470 nm and an associated color change from yellow to orange. The color changes were ascribed to binding of F− with the π-electron-deficient triazine moiety, which perturbs the electronic properties of 274. In addition, the emission intensity of 274 at 574 nm is selectively quenched in the presence of F−. The azocalix[4]arene derivatives 275−279 (Figure 96), containing one or two azophenol chromophores and different binding sites, were designed by Chen and Chen.226 These substances in CH3CN recognize anions by means of hydrogenbonding interactions. Upon addition of F−, a solution of 275 undergoes a significant color change from light yellow to blue. Probe 276 also displays a noticeable color change from light yellow to blue upon the addition of F− and to violet-blue when the same amount of AcO− is added. Owing to their colorimetric properties, 277−279 can be used for the detection of F−, AcO−, and H2PO4−. The neutral calix[4]arene receptor 280 (Figure 96) with hydrazone moieties located on the lower rim exhibits a prominent naked-eye-visible color change and significant bathochromic shifts when F− is present.227 Interaction of 280 with F− promotes a bathochromic shift of 143 nm and the appearance of a new absorption peak at 549 nm, associated with a distinct color change from yellow to dark purple. Receptor 280 selectively recognizes fluoride ions through initial H-bonding interactions and subsequent deprotonation with a Ka value of 9.9 × 106 M−1. 5551
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Figure 97. Structures of 281−284.
or F− induces selective quenching of fluorescence. Analysis of a Job plot of data for binding between 284 with F− shows that a 1:1 complex is formed with an association constant of 1.1 × 104 M−1. In the complex, four NH and OH hydrogens in the rigid binding pocket of 284 participate in favorable hydrogenbonding interactions with F−. On the other hand, the two nitrogens in the naphthalimide moiety and the oxygens in the calixarene ring provide a binding pocket for Cu2+. A colorimetric displacement assay for F− and H2PO4− in DMSO/H2O (1:1, v/v) consisting of a complex between 2,6dichlorophenol indoo cresol sodium salt (DIC, Figure 98) and 285 was evaluated.231 The complex has an absorption peak at 625 nm. Addition of fluoride causes this peak and the blue color to disappear as a consequence of a process in which DIC is displaced from the complex. The colorimetric sensor for fluoride ions 286 (Figure 99), which contains an isomerizable enol−keto moiety as the recognition site and a phenothiazine group as the chromogenic center, was described by Han and Zhao.232 In the presence of F− in aprotic solvents such as CHCl3 and MeCN, the absorption band at 499 nm decreases with a concurrent increase in the band at 360 nm. These changes promote a color
change from red to light yellow that is visible to the naked eye. Fluoride-binding-governed enol−keto tautomerization is suggested to be responsible for the anion-sensing ability of 286. The fluorophore−spacer−receptor−spacer−fluorophore dyad 287 (Figure 100) was devised by Pischel et al.233 to serve as a fluoride sensor. Fluorescence of the probe occurs from the tertiary amine donor moiety in its unprotonated form. Quenching is blocked by protonation of the amine, which results in enhanced singlet−singlet energy transfer between the 1,8-naphthalimide and 4-amino-1,8-naphthalimide groups. In acetonitrile, addition of fluoride induces deprotonation of the 4amino group on the naphthalimide moiety, leading to very efficient (ca. 93%) fluorescence quenching. By using fluoride anions as degenerate inputs, a ternary NOR logic gate is established for 287. Fluoride receptor 288 (Figure 101) containing anthracene was described by Huang et al.234 This probe was shown to selectively recognize fluoride in DMSO and a DMSO/H2O (95:5, v/v), undergoing a decrease in the intensity of the absorption band at 482 nm and an increase in the band at 672 nm. The process can be observed by the naked eye in the form of a sharp color change from light red to dark brown. The 5552
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Figure 98. Proposed mechanism for the sensing of F− by 285.
Figure 99. Proposed F−-sensing mechanism of 286.
Figure 100. Proposed F−-sensing mechanism of 287.
fluorescence emission of 288 is quenched effectively by F− with a detection limit of 2 × 10−6 M. Moreover, receptor 288 can be employed to detect fluoride in toothpaste even at low F− concentrations.
The fluoride-sensing properties of the new colorimetric chemosensor 289 (Figure 102), consisting of a coumarin moiety linked to a p-nitrophenylhydrazone group, were investigated by Zhuang et al.235 Upon addition of F− to an 5553
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The binol derivative 296 (Figure 105) was found by Chen and Leung to be highly fluorescent in the visible region.238 Among the TBA salts tested, only that with F− was found to be an effective fluorescence quencher of this substance. Quantitative analysis of the fluorescence titration data revealed that the response of 296 to anions can be classified into two categories: F− and Cl form 2:1 complexes with 296, whereas the oxo anions OAc−, H2PO−, and TsO− produce 1:1 complexes with this substance. Upon addition of F− to a solution of 296, the absorption band at 375 nm increases and reaches a maximum after 2 equiv has been added, indicating again that a (296·F2)2− complex is formed. Mallick et al. observed that norharmane (NHM) serves as a ratiometric sensor for F− and HSO4− (see Figure 106).239 In aqueous solution, NHM exists in four different protonated forms. In the presence of strong bases such as F−, hydrogenbond formation occurs with acidic hydrogens in this substance, and the relatively strong acid (HSO4−) can easily protonate the basic sites of this substance. Both processes lead to optical changes. Addition of TBAF to an acetonitrile/water (5:1, v/v) mixture of this sensor leads to formation of a new absorption band around 375−425 nm and a fluorescence band around 425−475 nm. The unusually structured, cage-type bisenaminoimine 297 (Figure 107) was designed by Dudek et al. for recognition of F−.240 This substance undergoes a distinct color change and luminescence intensity increase upon the addition of F− in DMSO. The color change from orange to red and orange luminescence at 576 nm arises in the presence of fluoride. It was demonstrated that deprotonation of the bisenaminoimine group in the macrocycle takes place to cause these changes, a conclusion that was supported by the results of DFT calculations. An unidentified, irreversible reaction between the BODIPY dye 298 (Figure 108) and TBAF in acetone solution leads to the disappearance of the absorption band at 599 nm and the emission peak at 686 nm, concurrent with an increase in the absorption peak intensities at 452 and 482 nm.241 These spectral changes correspond to a color change of the solution from light blue to colorless and a fluorescence color change from red to blue. Another BODIPY dye, 299 (Figure 108), bearing a phenolic OH in the meso-phenyl moiety was designed and studied by Wang’s group for the colorimetric and fluorescence detection of F− ions by means of OH···F hydrogen-bonding interactions.242 Upon addition of F−, the absorbance of 299 at 502 nm decreases, and a gradual increase in the absorption band at 488 nm takes place with an isosbestic point at 493 nm. In addition, in response to F−, the
Figure 101. Proposed binding mode of 288 with F−.
acetonitrile solution of this sensor, the absorption peak at 475 nm decreases, and new absorption peaks at 620 and 650 nm are generated, in association with an observable color change from yellow to blue. In the proposed mechanism for this process, F− forms a hydrogen bond with 289, which results in [FH F]−-induced deprotonation of the hydrazine moiety. Tian’s group developed three new diketopyrrolopyrrole (DPP) derivatives, 290−292 (Figure 103), as fluorescent sensors for fluoride anions.236 The F−-recognition mechanism for these substances is assumed to involve intermolecular proton transfer of a lactam hydrogen in the DPP moiety to fluoride anion. For receptors 290−292 in DCM, addition of fluoride results in an orange-to-red color change concurrent with a large bathochromic shift (80 nm) and a yellow-to-red change in the emission color. Guha and Saha employed supramolecular interactions [anion−π and charge/electron transfer (CT/ET)] of fluoride ions with π-electron-deficient colorless naphthalene diimide (NDI) receptors to design fluoride sensors 293−295 (Figure 104).237 Strong electronic interactions between lone-pair electrons of F− and π* orbitals of the NDI unit lead to a F− → NDI ET event that produces the orange NDI•− radical anion. As a result, a colorless solution of 293 turns orange upon addition of F− (≤5 equiv) and pink at higher F− concentrations (>5 equiv), associated with formation of the NDI2− dianion. The color change exhibited by 293 is attributed to further fluoride-promoted reduction of NDI•− to produce NDI2− dianion. The preorganized NDI receptor 294 displays a similar two-step spectroscopic change in the presence of F−. In addition to forming an NDI/F−/NDI sandwich complex, 294 and 295 bind to a second F− ion in the amide cavities through H-bonding interactions (Figure 104). This event significantly improves their selectivity and sensitivity for F−, allowing F− to be detected at nanomolar concentrations in DMSO/H2O (85:15, v/v).
Figure 102. Proposed mechanism for the interaction of 289 with F−. 5554
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Figure 103. Structures of 290−292.
Figure 104. (Top) Structures of 293−295. (Bottom) Graphical illustrations of (a) anion−π and CT interactions between F− and NDI receptor generating a fluorochromogenic response through an F− → NDI ET event and (b) stepwise F− recognition by preorganized receptors (PRs) through (i) π−anion−π and (ii) H-bonding interactions.
fluorescence intensity of 299 decreases and undergoes a minor
intramolecular SET from the meso-phenyl moiety to the excited BODIPY chromophore. In 2008, Kumar’s group synthesized chemosensors 300 and 301 (Figure 108), which contain a quaternary ammonium cation and a NH group as recognition sites and an
blue shift. The spectroscopic changes are likely a consequence an OH···F hydrogen-bonding interaction that enhances the electron-donating ability of phenolic OH and facilitates 5555
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Figure 105. Difluoride complexation of 296.
Figure 106. Different acid−base equilibria for norharmane: CN, cation−neutral; NA, neutral−anion; ZA, zwitterion−anion; and CZ, cation−zwitterion.
anthracene-9,10-dione as both a chromogenic and fluorescent moiety.243 This substance can be used for the detection of fluoride ions at concentrations ranging from 500 to 1200 μM. A change in the color of the solution of 300 in CH3CN/DMSO (9:1, v/v) from yellow to pink occurs upon addition of F−, and a similar change takes place with 300. The dual emissions at 580 nm for free 300 and 301 and at 510 and 540 nm for 300 and 301, respectively, with F− enable these sensors to be used for ratiometric analysis of fluoride ions. The structurally similar F− probe 302 (Figure 108), bearing a pyridinium unit conjugated to an anthrapyridone moiety, was described by the same group in 2010.244 In a CH3CN/DMSO (20:1, v/v) solution, this probe undergoes a dual color change in the presence of fluoride. Specifically, an absorbance develops at 490 nm, generating an orange color, at F− ion concentrations between 5 and 75 μM, and a band at 630 nm associated with a green color arises when the concentration of fluoride is between 75 and 600 μM. Fluoride ions also induce an increase
Figure 108. Structures of 298−302.
in the intensity of the fluorescence by a factor of 25 at 510 nm, enabling selective determination of fluoride concentrations between 0.5 and 5 μM, which is a desirable range for assessment of the maximum contaminant level of F− allowed in drinking water (4 ppm). The unmodified fluorescein 303 (Figure 109) behaves as a fluorescence chemosensor for F− detection.245 The addition of F− to a solution of this substance results in the appearance of a strong absorption peak at 514 nm, along with a clear color change from colorless to yellow-green. An emission enhancement at 532 nm of about 2100-fold also occurs. These
Figure 107. Proposed interaction between 297 and F− and formation of zwitterionic species. 5556
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Figure 109. Structures of 303−305 and binding mode of 305 with F−.
Figure 110. Structures of 306−311.
and 1:1 in 1−6 M water/acetonitrile mixtures.248 Addition of F− and CH3COO− to acetonitrile solutions of this probe induces the generation of new absorbance bands at 443 nm in concert with a shift of the peak at 259 to 275 nm. In addition, an increase in the emission intensity takes place that is reflective of an increase in a broad emission band between 470 and 740 nm with an isoemissive point. Studies of the effect of the water content in acetonitrile on F− binding showed that the extent of tautomerization is significantly diminished when the concentration of water is increased to 2 M. This phenomenon is a consequence of the fact that enrichment of the solvate shell in water decreases the binding power of F− and lessens its effect on the electron density of the heterocyclic ring. Oxadiazole 307 (Figure 110) is able to sense fluoride anions in DMF solution through changes in both its absorption and fluorescence emission properties.249 A fluoride-anion-induced red shift in the absorption from 286 to 382 nm takes place in association with a color change from colorless to yellow that is visible to the naked eye. At low fluoride concentrations (
Fluorescence and Colorimetric Chemosensors for Fluoride-Ion Detection Ying Zhou,†,‡ Jun Feng Zhang,§ and Juyoung Yoon*,† †
Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea Key Laboratory of Medicinal Chemistry for Natural Resource, School of Chemical Science and Technology, Yunnan University, Kunming, 650091, P. R. China § College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming 650500, P. R. China ‡
Corresponding Author Notes Biographies Acknowledgments Abbreviations References
1. INTRODUCTION In recent years, the development of methods to recognize and sense biologically and environmentally important species has emerged as a significant target in the field of chemical sensors.1−12 Although anions are known to play important roles in a wide range of chemical and biological processes, they have become a target of investigations aimed at sensor development only in the past decade.13−28 Among these anions, fluoride, having the smallest ionic radius, highest charge density, and a hard Lewis basic nature, has arisen as an attractive target for sensor designs owing to its association with a diverse array of biological, medical, and technological processes. Consequently, sensing and recognition of fluoride ion has become a highly popular topic of interest to scientist whose research focuses on host/guest and supramolecular chemistry. Knowledge about the importance of fluoride ions in biological and medical processes has grown in recent years. It is now known that this anion plays a role in dental health29 and has potential use for the treatment of osteoporosis.30,31 Fluoride is easily absorbed by the body but is excreted slowly. As a result, overexposure to fluoride can lead to acute gastric and kidney problems.32 In several underdeveloped countries, excessive fluoride levels in drinking water have been linked to the debilitating bone disease fluorosis. Fluoride also plays an important role in a number of military applications. For example, the refinement of uranium in nuclear weapons manufacturing utilizes fluoride.33 GB, more commonly known as sarin (isopropyl methylphosphonofluoridate), is a nerve agent employed by the Aum Shinrikyu cult in terrorist attacks in Tokyo in 1995. Fluoride is a product of the hydrolysis of sarin (and soman, a related G-type agent), and consequently, the ability to monitor fluoride in victims and surrounding environments after a terrorist incident using this nerve gas is of great value.34,35
CONTENTS 1. Introduction 2. Sensors Based on the Interactions between Fluoride Ions and Lewis Acids 2.1. Sensors Based on F−-Promoted Cleavage of Silicon−Oxygen Bonds 2.2. Sensors Based on F−-Promoted Cleavage of Silicon−Carbon Bonds 2.3. Sensors Based on the Interactions between Fluoride Ions and Boronic Acid Moieties 2.4. Sensors Based on Displacement Ensembles Using Metal Complexes 3. Sensors Utilizing Hydrogen-Bond Interactions 3.1. Sensors Bearing Thiourea Binding Sites 3.2. Sensors Bearing Urea Binding Sites 3.3. Sensors Bearing Amides as Binding Groups 3.4. Sensors Bearing Sulfonamides as Binding Sites 3.5. Sensors Bearing Imidazoline or Imidazole Groups 3.6. Sensors Bearing Indole Groups 3.7. Sensors Bearing Pyrrole Groups 3.8. Sensors Bearing Schiff Base Groups 3.9. Sensors Bearing Other Groups 4. Reaction-Based Sensors for the Detection of Fluoride Ion 5. Fluoride-Ion Sensors Based on Polymers 6. Fluoride-Ion Sensors Based on Quantum Dots and Gold Nanoparticles 7. Fluoride-Ion Detectors Based on Mesoporous Silica or Silica Particles 8. Conclusions Author Information © 2014 American Chemical Society
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Received: January 23, 2013 Published: March 25, 2014 5511
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Figure 1. Cleavage of the SiO bond of 1 by F− and structures of 2−4.
Despite the growing interest, the development of fluoride probes and anion sensors in general has been particularly challenging. Specifically, the requirement that these detection systems exhibit anion selectivities is a hurdle that needs to be overcome. Among several widely used fluoride-detecting and -sensing techniques, including the electrode method,36 19F NMR analysis,37 and colorimetric (UV) and fluorescence sensing, electrochemical systems have become the most wellestablished. However, this approach has major disadvantages associated with the need for fragile instrumentation and timeconsuming manipulations.38 In addition, 19F NMR spectroscopy can be used reliably to detect only micromolar levels of fluoride. Moreover, neither the electrochemical nor the NMR approach can be miniaturized for use in studying biological processes in vivo. As a result of these deficiencies, colorimetric and fluorescence sensor systems have attracted the greatest attention because they have been shown to have high sensitivities, with detection limits as low as sub-parts-permillion, and the capability of being employed for intracellular fluoride monitoring. Over the past two decades, a large number of optical sensors have been developed for detecting fluoride. For the most part, the methods employed in these sensors rely on strong interactions taking place between Lewis-acidic boron and fluoride, as well as hydrogen bonding and other types of interactions involving fluoride as a participant or a disrupter. In addition, reaction-based chemodosimeters have been devised for the selective detection of fluorides. In this review, recent studies aimed at the development of fluorescence and colorimetric chemosensors for fluoride are discussed using a format in which sensor types are classified on the basis of their topological and structural classes as well as the chemistry leading to optical responses. The topics covered include reaction-based chemosensors that utilize F−-promoted cleavage reactions; boron−fluoride interaction-based chemosensors; hydrogen-bond and π−π interaction-based chemosensors containing thiourea/urea, amide/sulfonamide, imidazoline/ imidazole, indole, pyrrole, Schiff base, and other groups as binding units; chemosensors bearing metal ion binding sites; and finally chemosensors based on polymers, quantum dots,
gold nanoparticles, mesoporous silica, silica particles, and other types of materials.
2. SENSORS BASED ON THE INTERACTIONS BETWEEN FLUORIDE IONS AND LEWIS ACIDS One important approach to the design of fluoride-selective optical sensors relies on unique interactions that occur between fluoride and Lewis acids. One general strategy of this type employs familiar, fluoride-promoted SiO and SiC cleavage reactions, owing to the large differences among SiC, SiO, and SiF bond dissociation energies (69, 103, and 141 kcal/ mol,39 respectively). Reactions that form SiF bonds at the expense of cleavage of SO and SiC bonds take place both rapidly and nearly irreversibly. Another approach takes advantage of the high propensity to form fluoroborate anions through reactions of borates with the fluoride anion. In the design of these types of fluoride chemodosimeters, the SiF and BF bond-forming processes generate fluorophores or chromophores from nonoptically responsive precursors. Several thorough reviews describing fluoride chemosensors that are based on unique interactions between fluoride and boron have been written by Gabbai ̈ and co-workers.40 Because of the earlier coverage, this topic will receive only brief attention in the current review. In another family of sensors discussed in this section, interactions between various metal ions and fluoride are used to promote displacement reactions that lead to fluorescence or color responses. 2.1. Sensors Based on F−-Promoted Cleavage of Silicon−Oxygen Bonds
Recently, great effort has been directed toward the development of reaction-based chemodosimeters for the detection of metal ions, reactive anions, oxygen species, thiols, and other important substances. Even though they often participate in irreversible chemical processes, chemodosimeters typically display higher selectivities than do their chemosensor counterparts. Silyl groups, which are widely used as protection groups for hydroxyl-containing substances, have been used as key reactive elements in fluoride-selective chemodosimeters. In this section, recent examples of fluoride-promoted selective SiO cleavage-based chemodosimeters for fluorides are discussed. 5512
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The tert-butyldiphenylsilane (TBDPS) group has become a common electrofugal group in reaction pathways that serves as the basis of fluoride-sensing systems. In 2007, Kim and Hong described chemodosimeter 1 (see Figure 1), which releases resorufin through substitution by F− (TBA+ and Na+ salts) at the silicon center.41 As a consequence of this reaction, CH3CN/H2O (50:50, v/v) and acetonitrile solutions of 1 treated with fluoride undergo dramatic changes in their UV−vis absorption spectroscopic properties corresponding to a change in color from yellow to pink. Also, upon the addition of F− to a solution of 1 in CH3CN, the fluorescence emission intensity at 591 nm increases by 500-fold and then reaches a maximum value after addition of 1400 equiv of F−. In CH3CN/H2O, the fluorescence intensity at 589 nm promoted by reaction of 1 with fluoride is enhanced by 200-fold after addition of 3000 equiv of F−. In the sensing mechanism, the displacement reaction of fluoride at the silyl ether center takes place to produce a phenolate anion, which then undergoes fragmentation to form p-quinomethane and resorufin, the latter of which serves as the chromophore and fluorophore responsible for the light absorption and emission changes. Two years later, Kim et al. synthesized another fluoride sensor, 2, that contains a tertbutyldiphenylsiloxy group as a trigger and a coumarin as the optical signaling group. This fluoride-ion probe, whose fluorescence intensity at 461 nm in HEPES buffer (pH 7.4) increases with increasing fluoride concentrations, was used for fluorescence cell bioimaging42 and for the detection of NaF in A549 human epithelial lung cancer cells under physiological conditions. A near-infrared (NIR) fluorescence chemodosimeter for F− that incorporates a tert-butyldiphenylsiloxy group and a dicyanomethylene-4H-chromene moiety was developed by Cao et al.43 In DMSO/H2O (95:5, v/v) solution, F− triggers specific SiO cleavage of 3 and promotes a dramatic color change from light yellow to blue. In addition, a significant turnon fluorescence response takes place at 718 nm corresponding to a 1000-fold intensity enhancement upon addition of 400 equiv of F−. Hu et al. developed the closely related sensor 4, which has a dual-wavelength fluorescence sensing capability.44 Using 4, fluoride ions can be detected at the 100 ppb level by determining the disappearance of blue-violet fluorescence and the appearance of bright yellow fluorescence, changes that can be observed by the naked eye. Specifically, addition of F− to 4 results in SiO bond cleavage to generate a benzothiazole derivative that exhibits a bright yellow emission at 560 nm in water. This effort also demonstrated that this sensor system can be absorbed on a paper sheet that can be used as a convenient and reliable tool for detecting F−. The new ICT-based fluorescent chemodosimeter 5 for the detection of F−, which contains a protected form of benzothiazolium hemicyanine dye as a latent fluorophore, was developed by Zhang’s group (see Figure 2).45 Reaction of 5 with F− triggers cleavage of the SiO bond and releases the long-wavelength-fluorescing product 6. Gradual addition of F− to a solution of 5 in an ethanol/water (3:7, v/v) mixture containing PBS causes a 110-nm red shift in the absorption maximum and an associated color change from light yellow to orange. Concurrently, F− addition causes a change in the wavelength of the emission maximum from 500 to 558 nm. The probe, which has a detection limit of 0.08 mM, was used for bioimaging of F− in live RAW 264.7 macrophage cells.
Figure 2. Proposed reaction mechanism of 5 and F−.
Wei et al. developed another novel fluorescent probe, 7, for F− detection (TBA+ and Na+ salts; Figure 3) that operates on the basis of a desilylation-triggered chromogenic reaction in 100% water.46 Probe 7 exhibits an excellent selectivity for the F− ions with a detection limit of 10.5 μM. The SiO-bondcleaving reaction of fluoride with 7 to produce a quinone product is responsible for the optical response of this sensor. This reaction, employing fluoride at concentrations as low as 1.5 mg L−1, produces a color change that is visible to the naked eye. The sensor was utilized in the reported study for imaging F− in HepG2 cells. The triisopropylsilane (TIPS) group, in the form of silyl ethers, has also been employed as a trigger in F−-sensing systems. For example, Kim’s group developed the highly selective naphthalimide-based colorimetric and ratiometric fluorescence probe 8 (Figure 4). Importantly, this sensor displays both one- and two-photon ratiometric changes.47 At lower F− concentrations in MeCN, the intensity of the absorption band of 8 at 365 nm decreases, and a new band with a shoulder at 487 nm appears. As the F− concentration is increased, the intensity of the shoulder at 487 nm decreases, and a new absorption maximum at 425 nm is generated with a corresponding dramatic color change from colorless to jadegreen. In addition, when F− is added to a MeCN solution of 8, the intensity of the emission band at 449 nm decreases, and the band at 508 nm increases. The remarkable fluorescence behavior of 8 is associated with a 220-fold change in the ratio of the emission intensities at 449 and 508 nm that corresponds to a sigmoidal dependence on the F− concentration. Sokkalingam and Lee developed another coumarin-based probe to quantitatively determine F− concentrations in both acetonitrile and water solutions. The sensor, 9 (Figure 5), operates through “turn-on” chromogenic and fluorogenic dual modes that are triggered by fluoride-promoted cleavage of the SiO bond.48 Addition of increasing concentrations of F− anion (as its TBA salt) to an acetonitrile solution of 9 results in a dramatic color change from colorless to yellow along with the appearance of strong fluorescence emission at 500 nm. The estimated detection limit of this sensor for F− was found to be 50 nM. An easy-to-prepare, short-response-time test system, obtained by simply dipping paper into a solution of a TIPSprotected coumarin derivative, was used to detect F−. Bozdemir et al. fabricated two BODIPY derivatives, 10 and 11 (Figure 6), containing silyl-protected phenolic groups that serve as sensors for fluoride in solution and a poly(methylmethacrylate) matrix.49 Reaction of 10 with fluoride liberates the phenolate group, which serves as a quencher of BODIPY fluorescence at 506 nm. In contrast, fluoride reaction 5513
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Figure 3. Cleavage of the SiO bond in probe 7 by F−.
Figure 4. Mechanism of the reaction of compound 8 with F−.
with styryl-conjugated, silyl-protected BODIPY derivative 11 triggers strong ICT associated with a gradual decrease in the intensity of the absorption band at 560 nm, a concurrent increase in a band at 682 nm, and a purple-to-green color change. Cao et al. described the new coumarin−BODIPY derivative 12 to detect F−, which operates by a siloxy ether deprotection mechanism.50 In the presence of F− in DMSO, 12 undergoes loss of the TIPS group in conjunction with a 88-nm red shift in
Figure 5. Cleavage of the SiO bond of 9 by F−.
Figure 6. Structures of 10 and 11 and cleavage of the SiO bond of 12 by F−. 5514
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Figure 7. Structures of 13−16 and proposed mechanisms of the reactions of 13 and 16 with F−.
of 17 displayed a 3-fold fluorescence increase at 460 nm. The mechanism of fluorescence turn-on in this system is based exclusively on the SiO bond cleavage reactions of 17, which can be effected by F− under mild conditions. Finally, the tris(Nsalicylideneamine) derivative 17 was shown to be a sensitive F− chemosensor. Treatment of a CH2Cl2 solution of 17 (Figure 8) with fluoride was observed to produce a 3-fold increase in fluorescence intensity at 460 nm.
the absorption maximum and a dramatic ratiometric fluorescence response. Specifically, the color of the solution changes from pink to red, and the emission color changes from orange to blue. These optical changes were found to be in accord with the results of density functional theory and timedependent density functional theory calculations. Finally, the detection limit of 12 for F− was determined to be 0.12 × 10−6 M. A wide variety of other types of F− sensors have been developed based on the t-butyldimethylsilyl ether cleavage strategy (see Figure 7). For example, the colorimetric and ratiometric fluorescent chemosensor 13, with a detection limit below 1.0 μM, was prepared by Bai’s group.51 This sensor in THF undergoes a color change from colorless to yellowishgreen (from 313 to 363 and 410 nm), and the intensity of the emission band at 403 nm gradually decreases, being replaced by a new peak at 520 nm [blue fluorescence (λmax = 403 nm) changes to yellowish-green (λmax = 520 nm)]. In addition, naphthalimide derivative 14 has been developed as a colorimetric chemosensor for fluoride with a detection limit of 0.59 μM.52 Addition of fluoride ion to a CH3CN solution of 14 results in a color change from colorless to yellow associated with a decrease in the intensity of the absorption band at 362 nm and growth of a new band at 474 nm. Chemodosimetric gelation system 15, which is transformed into a fluorescent gel in the presence of F− in aqueous media, was developed by Park et al.53 Selective cleavage of a silyl ether protecting group in the sensor gives rise to a turn-on fluorescence at 405 nm and a substance that undergoes sol-to-gel transition in methanol/ HEPES buffer (1:1, v/v; pH 7.40). An intelligent reaction-based relay recognition strategy served as the basis for the design of F− sensor 16 by Dong et al.54 Treatment of this probe with F− generates a prominent decrease in the intensity of the fluorescence band at 360 nm and the growth of a new peak at 460 nm. A tris(Nsalicylideneamine)-derived dynamic fluorophore was developed by Jiang et al.55 Upon addition of F−, a CH2Cl2 solution sample
Figure 8. Structure of 17.
2.2. Sensors Based on F−-Promoted Cleavage of Silicon−Carbon Bonds
In this section, chemosensors for fluoride-ion detection that are based on SiC bond cleavage reactions are discussed. It is known that anionic fluoride reacts selectively with compounds containing the CCSiMe3 group to generate the corresponding terminal acetylenes. This reaction occurs more rapidly than do SiO bond-cleavage processes. One example of a BODIPY dye-containing sensor of this type was developed by Rao et al. for the selective colorimetric and fluorescence detection of fluoride ion (see Figure 9).56 5515
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Figure 9. Structure of 18 and reaction-based chromogenic and fluorescence sensing of F− by 19.
Figure 10. Structure of 20 and proposed sensing mechanism for the interaction of 21 with F−.
Addition of F− ions to 18 in CH2Cl2 results in ratiometric changes corresponding to a decrease in the intensity of the absorption band at 571 nm and the appearance of a new band at 551 nm. This process also takes place concurrently with clear ratiometric changes in the fluorescence spectra involving transformation of the emission from bright orange to green and associated with a decrease in the intensity of the band at 584 nm. Fu et al. created the related BOIDPY dye-containing chemodosimeter 19 (see Figure 9),57 which operates on the basis of a fluoride-promoted desilylation of a silylacetylene and has a limit of detection of 67.4 nM. Addition of F− to an acetone solution of 19 leads to a decrease in the intensity of the absorption band at 555 nm and the appearance of a new peak at 538 nm, processes that occur concomitantly with a decrease in the fluorescence intensity at 571 nm and an increase in the emission intensity at 554 nm (orange to green). The substituted 1,8-naphthalene diimide 20 bearing trimethylsilylacetylene moieties serves as an effective colorimetric and fluorescence chemodosimeter for fluoride-anion detection (see Figure 10).58 An immediate and significant visible change from yellow to dark brown occurs upon addition of F− ions to a solution of this probe. Likewise, a gradual fluoride-promoted increase in the fluorescence intensity of 20 at 329 and 455 nm takes place concurrently. The pyrene derivative 21, containing four trimethylsilylethynyl substituents, was designed by Lu et al.59 as a chromogenic and fluorescence chemodosimeter for fluoride ion. Addition of F− to a THF solution of 21 results in the removal of the trimethylsilyl substituents and causes a blue shift in both the absorption and fluorescence maxima of the probe.
Specifically, a significant visible color change from light green (absorption bands at 436 and 410 nm) to colorless (417 and 392 nm) and a blue-to-purple emission change take place in this system. Moreover, a test paper for fluoride containing this probe is easily made by immersing filter paper into a THF solution of 21. 2.3. Sensors Based on the Interactions between Fluoride Ions and Boronic Acid Moieties
Addition of fluoride ion to triarylboranes results in the formation of fluoroborate anions. Furthermore, as shown in Figure 11, fluoride also readily adds to arylboronic acids.60
Figure 11. Mechanism for the binding of fluoride by a boronic acid moiety.
These unique reactions of fluoride ion with boron moieties coupled with changes in the electronic properties of the boron substituents have been cleverly adopted to develop optical sensors for fluoride by various groups.61 In 2002, six new fluorescence probes, 22−27 (Figure 12), that operate on the basis of interactions between fluoride and boronic acid groups were developed for fluoride sensing.62 All of these substances display spectral shifts and/or intensity changes in the presence of fluoride in water/methanol (2:1 v/ v) solutions, which enable them to be employed as wavelengthratiometric probes. 5516
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Figure 12. Structures of 22−27.
between fluoride and imidazolium groups. This stabilization causes the binding constant of F− with the imidazolium moiety of 29 to be (2.3 ± 0.2) × 106 M−1 in acetonitrile, much larger than that between fluoride and 28. Hydrogen bonding between imidazolium CH and F− in the complex promotes a blue shift of the emission maximum from 445 to 370 nm that is associated with a significant color change. Utilizing this strategy, Badugu et al. developed the new water-soluble quinolinium salt fluorescence probe 30 (Figure 13), which, in aqueous solution, displays ratiometric and colorimetric changes in the presence of fluoride.65 As the concentration of fluoride increases, the absorption band of 30 at 388 nm decreases, whereas the intensity of a new band at 342 nm increases. Similar changes are seen in the fluorescence emission of 30, where the intensity of the band at 546 nm decreases whereas the emission band at 450 nm increases as a function of added fluoride. These and other results demonstrate that the probe 30 responds well to fluoride even when high concentrations of other species are present in aqueous solutions, such as in a biological cocktail consisting of 50 mM glucose, 50 mM aqueous Cl−, and 5 mM fructose. The fluorescein boronic acid probe 31 (Figure 14) was investigated by Swamy et al. as a fluorescence chemosensor for
The fluoride sensor 28 (Figure 13), bearing boronic acid and imidazolium moieties and displaying selective and ratiometric
Figure 13. Structures of 28−30 and proposed mechanism for the reaction of 28 with F−.
fluorescence changes upon addition of fluoride ions, was developed by Xu et al.63 Addition of F− to a CH3CN solution of 28 causes a blue shift in the emission maximum from 440 to 372 nm, which leads to a significant emission color change that is visible to the naked eye. In CH3CN/HEPES (95:5, v/v), F− addition first promotes an increase in the intensity of the emission band of 28 at 440 nm, and then the behavior seen in pure acetonitrile is observed. These findings demonstrate that 28 displays a mutlistep mode of fluoride activation. Specifically, addition of two fluorides to the boron center occurs first and is followed by formation of a (CH)+···F−-type ionic hydrogenbonded complex with the imidazolium moiety. Further studies by Jun et al. further uncovered the new ratiometric fluoride-ion fluorescence probe 29, containing one boronic acid moiety and two imidazolium groups.64 The boron center participates in a cooperative manner with imidazolium to bind F−, with the initially formed BF complex stabilizing the interaction
Figure 14. Proposed mechanism for the reaction of 31 with F−.
F−.66 This sensor participates in an off−on-type fluorescence change that is promoted by blocking fluoresceinamine PET quenching of the singlet excited state induced by the reaction between fluoride and the boronic acid moiety. The apparent association constant for formation of the fluoride−31 complex was determined to be 9.2 × 1010 M−3. Koskela et al. fabricated the two crown-ether-based sensors 32 and 33 (Figure 15), which behave like an AND logic gate involving potassium and fluoride binding as inputs and 5517
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in association with the production of an absorption band with λmax at 571 nm. The formed BA−F− complex has a 1:1 stoichiometry and a binding constant of (1.93 ± 0.53) × 105 M−1. Galbraith et al. designed a new family of boronate blocked phenols including 36−39 (Figure 17) as colorimetric detectors for fluoride and other anions.70 Addition of an excess of TBAF to chloroform solutions of each of these substances promotes a dramatic color change from colorless to yellow (36, 38) or to purple (37, 39). In the cases of 37−39, addition of TBAF causes complete quenching of fluorescence, which is attributed to fluoride-mediated deprotonation. Finally, addition of chloride ion to 39 in a dichloromethane solution produces a selective decrease in the fluorescence intensity as a consequence of associative hydrogen bonding between chloride and two boronic acid groups. Rao and Ravikanth explored the fluoride-sensing properties of 40 (Figure 18), the B(OH)2 complex of the expanded coremodified porphyrin oxasmaragdyrin.71 The results of this effort showed that addition of fluoride to a CH2Cl2 solution of 40 results in a decrease in the intensity of the absorption bands at 446, 475, and 705 nm, concurrent with an increase in new bands at 455, 487, and 720 nm. Using the Benesi−Hildebrand equation, the binding constant for a 1:1 receptor/anion complex formation was determined to be 1.3 × 104 M−1. The addition of increasing amounts of TBAF to a solution of 40 results in quenching of the emission band at 713 nm. The novel fluoride sensor 41 (Figure 19), with a lower-rim boron-substituted calixarene structure, was devised by Arimori et al.72 Titration of a chloroform solution of this substance with Bu4NF results in a decrease of the fluorescence in association with the formation of a bidentate 1:1 “endo” complex, which has a large binding constant (log K1 = 6.3 ± 0.4). The aryl boronate 42 (Figure 19), having a 4-(N,N-methylamino)benzonitrile structure, was reported to be a selective F− sensor.73 In the presence of F−, the absorption bands in the range of 270−320 nm of the sensor undergo a concentrationdependent blue shift. Based on this absorption response, the binding constant of 42 with F− was determined to be (3.4 ±
Figure 15. Structures of 32 and 33.
fluorescence enhancement as the output.67 Cooperative complexation between the metal cation and fluoride anion enables these probes to display selective binding with potassium fluoride over other similar salts. The formation constants (Kobs) between KF and 32 and 33 were determined to be 1 × 10−4 and 320 M−1, respectively. Interestingly, fluorescence from the complex can be “switched off” by removal of the potassium cation from the benzocrown ether receptors of sensors 32 and 33 through addition of the more strongly potassium-cation-binding [2.2.2]-cryptand. The fluoride-selective chemosignaling system consisting of a mixture of the merocyanine dye 34 and phenylboronic acids (PBA−Rs) (Figure 16) was investigated by Cha et al.68 The mechanistic basis for the operation of this sensor involves fluoride-induced displacement of the dye from a complex formed between it and the PBA−Rs in association with the formation of a chromogenic signal. In CH3CN, the stabilities of the complexes of PBA−Rs with 34 were found to be in the following order: PBA−CHO > PBA−NO2 > PBA−CHO-4 > PBA−H > PBA−OCH3. Polymer-bound PBA complexed with 34 also exhibits a fluoride-selective UV−vis absorbance signal with a detection limit of 9 × 10−6 M. A similar displacementbased assay, involving the reaction of Brooker’s merocyanine (BM) and phenylboronic acid (BA), 35 (Figure 16), with fluoride was developed.69 BM, which is violet in acetonitrile solution, becomes colorless when it is present in the BA−BM complex. Only fluoride, among strongly nucleophilic anions, reacts with BA−BM to displace BM and generate a violet color
Figure 16. Proposed mechanism for the reactions of 34 and 35 with F−. 5518
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Figure 17. Structures of 36−39 and mechanism of fluoride-mediated deboronation (38 or 39) or deprotonation (36 or 37) followed by atmospheric oxidation to produce a colored radical.
around 402 nm, and a new emission band around 343 nm simultaneously appears. The source of these optical changes is proposed to be a reduction in ICT interactions with the boronate moiety upon formation of the negatively charged fluoroboronate product. The closely related substance 43 (Figure 19), containing the DMABN (4-N, N-dimethylaminobenzonitrile) structure, was designed as a new ratiometric fluorescence sensor for F− on the basis of expected changes in twisted-intramolecular-chargetransfer (TICT) quenching.74 The results show that 43 displays dual fluorescence with emission bands at 464 and 352 nm. Addition of F− to a solution of 43 causes a decrease in the intensity of fluorescence at 464 nm and an increase in the band at 352 nm. The corresponding binding constants of 43 with F− are estimated to be 1.47 × 104, 2.37 × 103, and 2.10 × 103 M−1 in DMF, THF, and CH2Cl2, respectively.
Figure 18. Structure of 40.
2.4. Sensors Based on Displacement Ensembles Using Metal Complexes
Another strategy for the design of fluoride-ion chemosensors utilizes a displacement approach in which dyes weakly bound to metal ions are released in conjunction with the generation of an optical signal. An example of this strategy is found in studies of the water-soluble fluoride-ion-signaling fluorescence probe 44 (Figure 20) by Sathish et al. in 2007.75 This probe operates through a mechanism involving the exchange of two Alizarin Red S (ARS) molecules coordinated to a central Al(III) ion by fluoride-ion-promoted substitution. The binary complex 44 serves as a sensitive signaling system by undergoing a decrease in its pale orange-colored fluorescence of 44 associated with emission at 575 nm (excitation at 435 nm) in the presence of fluoride ion in the concentration range from 5 × 10−6 to 3 × 10−4 M (detection limit of 0.1 mg L−1). In 2008, these workers described the related sensor 45 (Figure 20), comprising an Al(III) complex with 5-hydroxyflavone (5HF) and coordinated MeOH molecules.76 Successively increasing the fluoride-ion concentration results in a decrease in the intensities of the absorption bands of the complex at 289 and 397 nm and concurrent increases in bands associated with free 5HF at 269 and 336 nm. In addition, the subsequent ligand-exchange reaction of the complex with fluoride ion forms 46 in
Figure 19. Structures of 41−43 and schematic of F− sensor 43 based on TICT.
0.2) × 105 M−1 in THF. In addition, addition of F− also promotes a decrease in the intensity of the emission band 5519
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Figure 20. Structure of 44 and proposed mechanism for the detection of F− by 45.
Figure 21. Proposed binding mode for the detection of F− by 47 and “off−on” fluorescence sensory system 48 for the detection of F− in aqueous media.
conjunction with an increase in the fluorescence at 420. This system can be employed to detect F− at concentrations ranging from 5 × 10−5 to 7 × 10−4 M. A novel chemosensing ensemble probe, 47−Fe (Figure 21), for the detection of fluoride ions was developed by Fu et al.77 Addition of Fe3+ to the free anthracene derivative 47 in aqueous solution at pH 3.5 results in the generation of 47−Fe, a process that causes a decrease in the intensity of the absorption band of 47 at 259 nm and the formation of new red-shifted absorption bands at 350−420 nm. Reaction of the complex with fluoride
promotes regeneration of the absorption and fluorescence of 47, the latter of which is quenched in the complex 47−Fe. Lu et al. developed a simple and efficient method for the selective detection of fluoride anions in neutral aqueous media that is based on the reversible formation of a metal complex (Figure 21).78 This off−on-type sensor system is based on the specific affinity of a thiohydrazide fluorescent receptor for ferric ions associated with the formation of the complex 48−Fe(III). Generation of a 48−Fe3+ complex results in fluorescence quenching of 48 at 400 nm. Upon addition of fluoride, Fe3+ is 5520
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displays an intense blue fluorescence at 460 nm, which decreases upon addition of fluoride ions. This quenching effect is believed to be caused by a ligand-exchange process in which flavonol coordinated to Zr(IV) is replaced by fluoride ions.81 This signaling system can be used to detect F− in the concentration range from 1 × 10−3 to 3 × 10−6 M without any significant interference from other anions. Matsunaga et al. devised a related complex consisting of Zr−EDTA and 3hydroxy-2′-flavone (FS) (Figure 23).82 A solution of this complex in water displays an intense blue fluorescence at 460 nm, whose intensity decreases upon addition of F−. The mechanism for the fluoride-detection process involves ligand exchange of FS bound to Zr−EDTA. This sensor method is both rapid and selective, and it enables F− detection at concentrations ranging from 5 × 10−6 to 1 × 10−3 M. A closely related detection method takes advantage of the transformation of the blue color of PV−Zr−EDTA to orangered of the 1:1 complex in the presence of fluoride (Figure 24).83 A ligand-exchange reaction of PV coordinated to Zr(IV) with fluoride is responsible for this optical change, which can be used to determine fluoride concentrations ranging from 1.5 × 10−5 to 1.5× 10−4 M−1, with a detection limit of 4.5234 × 10−4 M−1. This chemosensor has been used to detect fluoride in industrial effluents. Another simple ligand-exchange-based method for selective fluorescence detection of fluoride ions utilizes a ternary complex containing Zr(IV), EDTA, and 8-hydroxyquinoline (Figure 25).84 The complex undergoes an exchange reaction with F− in the concentration range from 6 × 10−7 to 8 × 10−4 M (detection limit of 12 ppb) in association with a decrease in the intensity of the fluorescence band at 532 nm. A turn-on-type fluoride-selective fluorescence chemosensor relies on the ligand-exchange reaction between fluoride anion and a flavonol-coordinated Zr−EDTA complex (Figure 26).85 The fluorescence intensity at 554 nm of a solution of Zr− EDTA−51 increases by a factor of 6 upon addition of fluoride ion in aqueous solution at physiological pH values. Wang and co-workers prepared the red-emitting ligand 52 and studied its complex with Zr−EDTA (Figure 27), which serves as a highly selective and sensitive sensor for fluoride anions in ethanol/water solution at physiological pH values (6−8).86 The ratiometric signal used for fluoride-selective sensing is a consequence of inhibition of an ESIPT process caused by a ligand-exchange reaction occurring between fluoride anions and flavonol coordinated to Zr−EDTA. The intensity of the absorption peak at 510 nm gradually decreases, and a new peak at 460 nm emerges upon the addition of fluoride anions to the aqueous solution of equimolar amounts of Zr−EDTA and 52. Moreover, addition of F− to the solution of Zr−EDTA−52 causes a decrease in the intensity of emission
displaced from the in situ formed complex, a process that leads to recovery of the fluorescence of the receptor. A low detection limit of 140 μM and a high selectivity of F− over AcO− were reported for this system. Rochat and Severin developed the simple metal-chelationbased sensor 49 (Figure 22) for the detection of fluoride in
Figure 22. Structures of 49 and 50.
water (pH 7.0, MOPS buffer).79 Complexation with Ca2+ results in quenching of the fluorescence of 49 at 370 nm. Addition of fluoride to the complex leads to partial precipitation of CaF2 and an enhancement in fluorescence. The intensity of the emission was found to be dependent on the concentration of fluoride over the range of 0.5−5.0 mM with a detection limit of 0.30 mM. Importantly, this fluoride assay technique has been used for the analysis of dental health products, such as mouthwashes and toothpastes. A fluoride sensor based on 1,3-di-p-nitrophenylazocalix[4]arene-calix[4]pyrrole (50) was described by Thiampanya et al.80 In CH3CN, the color of a solution of 50 (Figure 22) changes from light orange to light sky blue, blue, light pink, and light yellow upon addition of F−, CH3CO2−, PhCO2−, and H2PO4− ions, respectively. When Ca2+ is added to a solution of the complex formed with fluoride, 50−F−, the F− ions are removed to form the CaF2 salt, which results in revival of the original absorption spectrum of 50. It has been demonstrated that Zr−EDTA binds fluoride ion and that this phenomenon can be employed to create a fluoride-ion ratiometric fluorescent sensor. An aqueous solution of [Zr(H2O)2EDTA]·2H2O and 3-hydroxyflavone (flavonol)
Figure 23. Proposed binding mode for the detection of F− using a flavonol-coordinated Z−EDTA complex. 5521
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Figure 24. Proposed binding mode with F− using PV−Zr−EDTA.
3.1. Sensors Bearing Thiourea Binding Sites
It is known that sulfur forms weaker hydrogen bonds than does oxygen, a fact that makes it much easier to align the thiourea functionality for complex formation with fluoride ion in comparison than to its urea counterpart. Also, because thioureas have higher NH acidities than do ureas, they serve as better receptors for F−. These phenomena have been utilized advantageously in the design of novel fluoride-ion chemosensors. For example, the efficient colorimetric sensor 53 (Figure 28), containing a thiourea moiety as a binding unit and pnitrophenylhydrazine as a signaling unit, was developed by Shao et al.88 Addition of fluoride to a DMSO solution of 53 results in a decrease in the intensity of the absorption band at 372 nm and an increase in the band at 532 nm, in association with a color change from light to deep purple. The association constant for complex formation between 53 and fluoride was determined to be (7.90 ± 0.16) × 104 M−1. Similar spectral changes take place in DMSO/H2O (95:5, v/v) solutions. To demonstrate its potential practical applications, this sensor was employed to detect F− in toothpaste. Devaraj et al. designed the two thiourea-containing receptors 54 and 55, that were shown to exhibit selective recognition toward F− over other halide anions.89 The F− selectivity for of these probes was attributed mainly to the hydrogen-bond interaction. Addition of TBAF to CH3CN solutions of 54 and 55 promotes color changes from colorless to brown and yellow, respectively. The binding constant for F− complexation of the receptors 54 and 55 were estimated to be (1.2 ± 0.23) × 103 and (3.3 ± 0.02) × 103 M−1, respectively, the difference being attributed to the presence in the latter of the OH group, which serves as an extra binding moiety. Wu et al. synthesized the structurally simple, thioureacontaining colorimetric sensor 56 (Figure 29).90 Addition of F−
Figure 25. Proposed mechanism for the detection of F− using a Zr(IV)−EDTA−8-hydroxyquinoline complex.
at 470 nm, which is related to the inhibition of ESIPT, as well as a blue shift of the emission band at 610 to 570 nm.
3. SENSORS UTILIZING HYDROGEN-BOND INTERACTIONS Whereas reaction-based probes and unique boron fluoridebased probes have been actively investigated in recent years, receptors utilizing conventional hydrogen bonds have been extensively evaluated for many years. During the past decade, fluorescent or colorimetric receptors utilizing hydrogen bonding have been developed and studied for the selective recognition of fluorides. For example, the urea group contains two relatively acidic NH groups as a rigid binding site with fluoride. In some cases, the relatively strong base fluoride can deprotonate these acidic amidic NH groups, as the pKa of HF is reported to be 1.5.87 This section is categorized according to the various hydrogen-bond donors, such as thiourea, urea, amide, sulfonamide, imidazoliums/imidazole, indole, pyrrole, and Schiff base.
Figure 26. Proposed binding mode for the detection of F− using a flavonol-coordinated Zr−EDTA complex. 5522
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Figure 27. Proposed binding mode of 52 with F−.
CH3CO2− > H2PO4− > HSO4− > Cl− > Br−. The results of AM1 calculations suggest that the most stable configuration of 56 is one that has the Z−E conformation, in which a sixmembered ring is formed through intramolecular hydrogen bonding. Two chromogenic thiourea-based sensors, 57 and 58, containing a p-nitrophenyl group as the signaling unit were reported by Kumar et al.91 The color of DMSO solutions of these sensors changes from pale yellow to an intense red upon addition of both fluoride and cyanide ions in association with a decrease in the intensity of the absorption peak at 364 nm and a concurrent increase in a new band centered at 466 nm. Analysis of the absorption spectral changes indicates that the complexes formed between the sensors and fluoride have 1:2 stoichiometric compositions. Applications of 57 and 58 were demonstrated by their use in detecting fluoride ions in toothpaste and potassium cyanide in aqueous medium using the naked eye. A family of highly sensitive thiocarbonohydrazone anion chemosensors, including 59−62 (Figure 30), was prepared and studied by Han et al.92 The results showed that sensors 59 and 60 undergo two stepwise deprotonation reactions of the thioamide framework in the presence of F−. In contrast, sensor 61 undergoes only a single deprotonation of the two NH groups. Addition of 0.4 equiv of F− to a colorless solution of 59 causes development of reddish color, whereas addition of 20 equiv of F− changes the solution color to blue. In contrast, 100 equiv of F− is required to promote the development of a blue color for sensor 60. Finally, the color of a solution of 62 turns from light yellow to red in the presence of F−. The efficient colorimetric chemosensor 63 (Figure 31), containing a thiourea binding site and 2-amino-6-nitro-
Figure 28. Structures of 53−55.
Figure 29. Structures of 56−58.
to a solution of 56 in acetonitrile causes a color change from colorless to yellow. The fluoride-ion selectivity of this probe is revealed in the association constants of formation of anionic complexes with 56 that lie in the following order: F− ≫
Figure 30. Structures of 59−62. 5523
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Calixarenes are ideal macrocyclic platforms for the creation of host molecules with ion-sensing properties. The new thiacalix[4]arene-based fluorescence sensor 69 (Figure 33),
Figure 31. Proposed mechanism for the reaction of 63 with F−.
benothiazole group, was investigated by Misra et al.93 This chemosensor, which displays a naked-eye-detectable change, selectively recognizes fluoride and acetate anions in dry DMSO solutions. Addition of AcO− or F− results in the disappearance of the ICT band of 63 at 392 nm and the formation of a new absorption peak in the yellow region of the spectra at 457 nm. In association with these spectral changes, the color of the solution changes from light yellow-green to red. The association constants for 63 and these anions were found to be on the order of 1 × 107. The thiourea- and urea-functionalized 4-amino-1,8-naphthalimides 64−66 (Figure 32), whose design is based on the
Figure 33. Sensing mechanism of 69 for the detection of F−.
bearing two naphthylthiourea groups, was explored by Kumar et al.96 In this probe, intermolecular binding interactions between F− ions and the NH protons of thiourea moieties trigger intramolecular π−π interactions of naphthyl groups, which leads to excimer formation and emission. The addition of increasing amounts of F− in DMSO induces a decrease in the absorption band of 69 at 257 nm and the formation of new redshifted absorption band at 360 nm concurrent with a significant increase in the intensity of a fluorescence emission band at 385 nm with a shoulder at 412 nm. Fluoroionophore 69 displays a high selectivity for F− with a detection limit of 2.6 × 10−7 M. Raposo et al. developed the family of heterocyclic thiosemicarbazone dyes 70−78 (Figure 34), which serve as fluoride-anion sensors.97 Titration of receptors 70−78 with fluoride brings about a small bathochromic shift of their absorption band as a result of coordination of the anion with the thiourea group. This is followed the growth of new redshifted bands associated with a color change from yellow or pale yellow to orange-red owing to deprotonation as the fluoride-ion concentration is further increased. In general, receptors 70−75 display enhanced fluorescence emission upon addition of moderate amounts of fluoride anion and quenching of the emission band at higher anion concentrations simultaneous with the growth of a new band at longer wavelengths. Two new chemosensors, 79 and 80 (Figure 35), were designed based on the photochromic dithienylcyclopentene moiety by Tian’s group.98 These probes, which operate through a thioamide proton deprotonation mechanism, respond to F− by undergoing decreases and red shifts of their absorption bands at about 350 nm. The fluorescence intensities of 79 and 80 also decrease upon addition of F−. Specifically, when F− is added to solutions of 79 and 80, the intensities of their emission bands at about 583 nm increase significantly and undergo a bathochromic shift of about 18 nm. The unique supramolecular sensing system 81 (Figure 36) for fluoride ion was constructed from new bispyrenyl thioureas linked by poly(ethylene glycol) (PEG) chains and methoxy benzene pyrene thiourea.99 Upon binding with F− in CHCl3, 81 exhibits a strong excimer emission band (IE) at 500 nm and a weak monomer emission band (IM) at 398 nm. Analysis of a Job plot shows that 1:1 and 2:2 complex formations take place between 81 and F−. The IE/IM ratios of the 81·F− complex are dependent on the concentration of 81, implying that
Figure 32. Structures of 64−68.
fluorophore−spacer−receptor principle, were reported by Duke and Gunnlaugsson94 to be selective fluoride detectors in DMSO. Emission from 65 and 66 is quenched by fluoride as a consequence of F−-binding-enhanced SET from the receptor to the excited state of the fluorophore. In addition, red shifts take place in the absorption maxima of these sensors upon addition of fluoride. Two anion sensors, 67 and 68 (Figure 32), which contain 4amino-1,8-naphthalimide fluorophores and diarylthiourea receptors, connected through a methylene spacer to a naphthalimide group, were described by Veale and Gunnlaugsson.95 Additions of acetate, phosphate, and fluoride were shown to quench the fluorescence of these sensors to different extents. When treated with F−, very effective quenching (ca. 90%) of the fluorescence band of 67 occurs at 528 nm, but no change in the absorption band takes place. For 68, titration with F− also causes quenching of the excited-state emission band at 533 nm, but to a lesser extent than for 67 (ca. 50%). Significant changes occur in the case of 67 at high concentrations of F−, where a long-wavelength absorption band rises at 529 nm along with a second transition band at 332 nm. These results demonstrate that bidirectional SET quenching is responsible for sensing by these naphthalimide-based anion probes. 5524
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Figure 34. Structures of 70−78.
Figure 35. Sensing mechanism of 79 and 80 for the detection of F−.
Figure 36. Possible structure of the 81−F− complex.
intermolecular excimer formation takes place. Probe 81 exhibits an extremely large binding constant (1 × 104) and sensitivity (detection limit as low as 46.2 g/L) toward F−. New ferrocene-based receptors for F− were developed by Devaraj et al. (Figure 37).100 The absorption peak of one of these, 82, in CH3CN at 480 nm is blue-shifted by 15 nm upon addition of F−. Simultaneously, a significant increase in the intensity of fluorescence emission at 438 nm occurs. Similar observations were made in studies with the related sensor 83, which displays a higher fluoride binding constant [(24.0 ± 0.12) × 103 M−1] than does 82 because of the presence of the nitro group. The results of electrochemical titrations of 82 and 83 in CH3CN reveal that the addition of F− anion to 82 alters its oxidation potential cathodically from 690 to 600 mV. Receptor 83 shows a similar response to fluoride, experiencing a cathodic shift in oxidation potential from 790 to 690 mV. The cathodic shifts are a result of the fact that the oxidation process becomes easier owing to electrostatic stabilization by the negatively charged ions present in the fluoride complexes. Jun et al. devised the new anthracene-linked thiourea-based fluoride chemosensors 84−86 (Figure 37).101 The X-raycrystallography-characterized thiourea derivatives in DMSO
Figure 37. Structures of 82−87.
display charge-transfer absorption peaks at around 430 nm in the presence of fluoride. In addition, new peaks at ca. 568 nm arise in their fluorescence spectra upon addition of F−, corresponding to a change from blue to orange fluorescence. 5525
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Figure 38. Structures of 88−91.
two ions, F− and HSO4−. The remarkable color change from colorless to yellow and fluorescence quenching upon fluoride addition can be used to discriminate F− from other anions using 91.
The novel bisenaminone-containing thiourea 87 (Figure 37) serves as an optical signaling agent for F−, AcO−, and H2PO4−.102 Addition of F− beyond 0.4 mM to an acetonitrile solution of 87 results in a color change from colorless to an intense green that is visible to the naked eye. This change takes place concurrently with the rise of a new absorption peak at 460 nm and an emission peak at 500 nm. Lee et al. devised the anion sensor 88 (Figure 38), which contains a biaryl−thiourea structure and exhibits a fluorescence emission enhancement upon hydrogen-bond-mediated complexation with F−.103 Addition of increasing amounts of F− concentration up to 2.5 equiv results in a 2.4-fold fluorescence enhancement of 88. Addition of further amounts of F− causes a reversal of the emission increase, suggesting that both 1:1 and 1:2 host/guest complexes are formed. The association constants of the 1:1 and 1:2 complexes were calculated to be 1.08 × 104 M−1 and 2.28 × 107 M−2, respectively. The chromogenic sensor 89 (Figure 38) for F−, which does not appear to contain any defined chromophore, was studied by Vázquez et al.104 Addition of F− to a solution of 89 results in a decrease in the intensity of the absorption band at 324 nm and the appearance of a new band at 412 nm in association with the development of a yellow color. These changes are likely consequences of strong interactions between F− and the internal NH fragment of the thiourea group, which enhances π delocalization over the receptor’s framework and causes the π−π* transition to shift from the UV to the visible region. A new binaphthol thiourea derivative 90 (Figure 38) was prepared and demonstrated to be a fluoride-ion sensor by Tang et al.105 Upon addition of F−, the color of a CH3CN solution of 90 changes from colorless to yellow, and the absorption maximum red shifts from 340 to 414 nm. Concurrently, a gradual increase in the intensity of fluorescence at 338 nm takes place. The binding constant for formation of a 2:1 complex between fluoride and 90 was determined to be 3.02 × 104 M−2. Lu et al. recently explored the simple sensor 91 (Figure 38), which displays a highly fluoride-anion-selective absorption and fluorescence response in DMSO.106 Because the fluorideinduced chromogenic changes can be completely reversed by addition of HSO4−, receptor 91 operates as a reversible colorimetric and fluorescence switch with complementary “IMPLICATION/INHIBIT” logic function by inputting these
3.2. Sensors Bearing Urea Binding Sites
In 2002, Kim and Yoon described the bisurea anthracene 92 (Figure 39), whose fluorescence in acetonitrile/DMSO (9:1, v/
Figure 39. Structures of 92−94.
v) is selectively quenched by fluoride ions through an SET process.107 For 92, the anthracene moiety in this probe acts not only as a fluorophore but also as a template for introducing binding selectivity. The association constant of the 92−F− complex was calculated to be 71270 M−1. In 2004, Kim’s group also developed the related anthracene derivatives 93 and 94 (Figure 39), which serve as a colorimetric sensor and fluorescence chemosensor, respectively, for fluoride ion.108 Probe 93 displays an absorption peak at 485 nm that is a redshifted by 129 nm upon addition of fluoride or pyrophosphate ions, which causes a color change from pale yellow to red. Upon the addition of F− to 94 in DMSO, fluorescence quenching takes place. The results of this study demonstrated that the anthracene 9-H functions as a CH hydrogenbonding donor in addition to the traditional urea NH hydrogen-bonding donor. In 2003, Cho et al. developed chemosensor 95 (Figure 40), which contains two phenylurea groups at the 1,8-positions of a naphthalene moiety, and demonstrated that it interacts with the fluoride anion through strong hydrogen bonding.109 In CH3CN/DMSO (9:1, v/v), 95 forms a complex with F− in 5526
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Figure 40. Structures of 95−100.
Figure 41. Structures of 101−104.
fluoride ion. The origin of the color changes in these probes is attributed to charge-transfer interactions that take place between the electron-rich donor units and the electrondeficient p-nitrophenyl and azophenyl moieties. The bisurea-functionalized naphthalene organogelator 100 (Figure 40), which operates through cooperative hydrogen bonding and π−π stacking interactions, was developed by Yang et al.113 This sensor undergoes a strong red shift in its emission when it is in its gel state compared to when it is in solution. Both the gel−sol transition and the fluorescent emission change are reversibly controlled by temperature changes or addition of F− and protons. The effect of fluoride anions on the fluorescence and gel−sol processes are a consequence of the dissociation of intermolecular hydrogen bonds by bonding of
association with the disappearance of an absorption peak at 345 nm and the formation of a red-shifted peak at 408 nm. This study also demonstrated that the fluoride selectivity of the probe is a result of the fact that fluoride binds much closer to the amide protons than do the other halides.110 The same probe was also reported to be a fluoride-selective receptor by Xu and Tarr at nearly the same time.111 Likewise, a series of very closely related naphthalene−urea derivatives was also described by Nam’s group in 2005.112 Distinct color changes take place when 96−98 are treated with fluoride ions. For example, the pale yellow solution of 96 in DMSO becomes red when fluoride ions are added. A similar red color was observed to arise when fluoride ions are added to azophenyl derivative 97, whereas a blue color is generated for 98 in the presence of 5527
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fluoride anions to the urea groups of the gelator. Meanwhile, the sol form can be returned to the gel state by addition of trifluoroacetic acid. Anthraquinone derivatives have also been utilized as both templates and signaling groups in novel F− sensors. For example, Cho et al. devised chromogenic anthraquinone 102 (Figure 41), which contain tetra-urea moieties as anion binding sites.114 Addition of fluoride to 102 (blue solution becomes green) in DMSO causes a UV−vis spectral change similar to that of 101 (blue solution becomes dark green), suggesting that fluoride binding occurs equally well at the amino site as at the urea moiety. Related receptors for selective fluoride-ion sensing, containing anthraquinone chromogenic signaling subunits and urea/thiourea binding sites, were previously reported.115 In the presence of fluoride ion, a DMSO/CH3CN (1:9, v/v) solution of 103 (Figure 41) does not undergo a color change even when allowed to stand at room temperature for more than 24 h. However, the color of the solution changes to pale red when the temperature of the mixture is raised to 60 °C. The results of titration experiments using a DMSO/CH3CN (1:9, v/v) solution show that the related sensor 104 undergoes an immediate color change upon addition of fluoride at room temperature. The results of calculations show that intramolecular NH···O hydrogen bonding in 103 is strong and it prevents fluoride ion from complexing with the NH donor atoms. Because sulfur forms weaker hydrogen bonds, 104 acts as a better receptor for F− at room temperature. Upon complexation with F−, a color change from yellow to pale red for 103 and to purple for 104 takes place, concurrently with red shifts of 108 and 102 nm, respectively, in their absorption bands. The association constants with F− were calculated to be (4.4 ± 0.2) × 105 and (8.2 ± 0.5) × 105 M−1 for receptors 103 and 104, respectively. Calix[4]arenes have been employed as unique supramolecular receptors for anion recognition. The chemosensor 105 (Figure 42), containing a urea binding site on the lower rim of a 1,3-alternate calix[4]arene structure was synthesized by Nam and co-workers.116 These workers observed that addition of Pb2+ ion to a solution of 105 in acetone-CHCl3 (v/v = 7:3)
promotes quenching of the monomer and excimer bands owing to reverse SET from the pyrene units to the urea groups. The quenching is reversed by addition of F− to the Pb2+-105 complex. These results demonstrate that fluoride ions bind to the amide protons in 105, thereby disrupting metal cation complexation. A related, colorimetric anion sensor 106, containing four amidourea receptors in the lower rim of a calix[4]arene structure, has been described.117 This probe contains a highly symmetrical and preorganized cavity for anion recognition through hydrogen bonding. Anions such as F− and AcO− undergo strong 1:2 binding with this probe, with F− recognition being most likely the results of formation of HF2− upon deprotonation of the urea moiety. Because the color changes from colorless to red as a consequence of the disturbance in the ICT nature of the anion receptor, probe 106 can be employed as a naked eye sensor for F−. The nitronaphthalene urea derivative 107 (Figure 43), synthesized by simple reaction of 1-naphthylamine and its corresponding isocyanate, was described by Nam and coworkers.118 A pale yellow DMSO solution of this probe becomes red when fluoride ions are added. In contrast, addition of acetate and benzoate to a solution of this sensor causes a color change to brown-yellow. Two related anion receptors, thiourea 108 and urea 109 (Figure 43), were synthesized by Kim et al.119 Addition of TBAF to DMSO solutions of receptors 108 and 109 causes dramatic color changes from colorless to yellow, as a result of fluoride-ion-induced hydrogen bonding and deprotonation of the NH acidic protons. In 2004, Boiocchi et al. reported that F− forms a strong Hbond with the NH hydrogens of the urea subunit in 110 (Figure 43).120 Moreover, proton transfer takes place from the urea group in the presence of a second F− ion, resulting in the formation of HF2−. The chemical process occurs in conjunction with the appearance of a charge-transfer absorption band of 110 in the visible region and a dramatic color change. In 2005, Esteban-Gómez et al. evaluated 110 along with the related substance 111 (Figure 43) more thoroughly.121 In the presence of fluoride, urea-based receptor 111, which contains electronwithdrawing chromogenic substituents, undergoes stepwise deprotonation of the two NH protons in a DMSO solution. Upon addition of an excess of TBAF, the color of the DMSO solution of 110 turns from yellow to red, and then a blue color develops at higher fluoride concentrations. Receptor 111 undergoes a similar two-step deprotonation process that results in pale yellow-to-orange and orange-to-violet color changes. Two colorimetric chemosensor, 112 and 113 (Figure 44), macrocyclic amino ether frameworks linked to three nitrophenylurea groups, were described by Aldrey et al.122 Treatment of 112 and 113 with TBAF in DMSO solutions leads to the formation of a new absorption band centered at ca. 480 nm and a decrease in the intensity of the band at 348 nm for 112. The colors of solutions of both receptors change from light yellow to brown in the presence of F−, CN−, and H2PO4−. The stability constants for the interactions of chemosensors 112 and 113 with anions are in the order F− > OH− > CN− > H2PO4−. The selectivity is a consequence of the size of the macrocyclic cavity and the flexibility of the pendant urea arms. The reversible assembly and disassembly processes of a 2ureido-4[1H]-pyrimidinone quadruple hydrogen-bonded AADD supramolecule has been exploited as the foundation of a new F− detector (Figure 45).123 In the absence of fluoride ions, the fluorescence spectrum of a CH2Cl2 solution of the hydrogen-bonded complex exhibits three typical emission
Figure 42. Structures of 105 and 106. 5528
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Figure 43. Structures of 107−111.
Figure 44. Structures of 112 and 113.
Figure 46. Structures of 115−117.
Figure 45. Proposed mechanism for the reaction of 114 with F−.
units spaced by a flexible diethylenetriamine fragment.125 In both DMSO and CH3CN, the fluorescence emission of 117 in the visible range (400 nm) was found to be perturbed by the presence of anions. The emission of 117 is quenched by acetate, chloride, and pyrophosphate, whereas it is enhanced by fluoride. It is presumed that 117 forms a stronger H-bond with F−, making the nitrogen lone-pair electrons more available for conjugation with the coumarin ring, thus favoring formation of an ICT emissive state. In contrast, weaker H-bond interactions with acetate, chloride, and pyrophosphate increase formation of the flexible, less emissive TICT while hindering the more rigid and sterically hindered ICT state. Ren et al. evaluated the fluoride-sensing properties of two dimers, 118 and 119 (Figure 47), that contain perylene fluorophores linked by urea bridges.126 Both the absorption and fluorescence spectra of 118 and 119 in THF solution are nearly the same, indicating that the two dimers have nearly identical configurations. Addition of F− to these solutions results in quenching of the perylene emission, as a likely consequence of F− coordination, which decreases the oxidation potential of the urea donors, causing SET of the singlet excited perylene groups to become more efficient. The F− association constants were
bands associated with the anthracene fluorophore at 397, 420, and 445 nm. Introduction of fluoride ion into the solution leads to the generation of monomeric species 114 and a dramatic reduction in the fluorescence intensity but not the positions of the emission maxima. The association constant for 114 and F− was calculated to be 13200 M−1. Wu et al. developed a new type of fluorescent anion sensor that is based on inhibition of an excited-state intramolecular proton process.124 The ESIPT process in which 115 (Figure 46) participates can be readily blocked by deprotonation of the sulfonamide unit by the addition of basic anions such as F−, CH3COO−, and H2PO4−. For 116, which is a good hydrogenbond donor, the ESIPT process is inhibited by either the fluoride-induced deprotonation of the urea unit or the formation of a strong CH3COO−−urea intermolecular hydrogen-bond complex. These two types of inhibition mechanisms result in different ratiometric emission responses, where the peak of 116 at 554 nm shifts to 510 nm upon F− binding and to 414 nm after CH3COO− binding. The anion fluorescence chemosensor 117 (Figure 46), described by Ambrosi et al., contains two coumarin−urea 5529
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corresponding to a color change from light yellow to red are clearly visible to the naked eye. 3.3. Sensors Bearing Amides as Binding Groups
A number of sensors bearing amide moieties as binding groups and the calixarene framework have been explored. In 2005, Kim et al. described the calixarene-based fluorescent chemosensor 122 (Figure 48), which contains two fluorogenic pyrene units conjugated to amide groups as guest recognition sites.129 Complexation of F− to 122 causes a red shift in its absorption band to 400 nm in conjunction with an enhancement and blue shift of the excimer emission to 470 nm, which are attributed to ground-state pyrene dimer-like formation. The association constant of 122 with F− was calculated to be 2.5 × 102 M−1. Based on the above findings, calix[4]arene derivatives 123 and 124 (Figure 48) bearing 4-nitrophenylazo and pyrene moieties were prepared by Kim’s group.130 F− was shown to bind to the amide groups (NH) of 123, resulting in a change in the characteristic excimer emission peak at 480 nm and the formation of a new emission peak at 460 nm. In contrast, the addition of F− to a solution of 124 promotes a change in the characteristic absorption spectrum corresponding to a color change from yellow to blue owing to complex formation with the hydroxyl protons (OH). In 2006, Kim’s group described the results of studies with the calix[4]arene-based anion sensor 125 (Figure 49), which contains two coumarin units attached through amide groups that serve as binding sites.131 Titration of a solution of 125 with F− causes a decrease in intensity and the formation of a redshifted absorption band at 349 nm and new band at 408 nm. The emission band of the probe at 420 nm decreases upon addition of F− and is eventually quenched, and a weak band arises at 508 nm. The association constant (Ka) of 125 for F− was determined to be 1.08 × 104 M−1. Kim’s group extended this study to the chemosensor 126 (Figure 49), which bears two amidoanthraquinone groups (1-AAQ) at the lower rim of p-tert-butylcalix[4]arene.132 A red-shifted absorption band centered at 510 nm arises along with a concomitant decrease in the band at 405 nm of 126 in CH3CN as a function of F− concentration. This change promotes a color change from pale yellow to deep yellow. New bands at 527 and 560 nm appear in the emission spectrum of the probe, associated with a fluorescence color change from colorless to green, upon
Figure 47. Structures of 118−121.
calculated to be 1.03 × 105 M−1 for 118 and 1.017 × 105 M−1 for 119. The phthalimide−urea chemosensor 120 (Figure 47) was shown to display a selective fluorescence response to F−.127 An acetonitrile solution of 120 displays an intense emission band at 430 nm, which is attributed to the aminophthalimide fluorophore. The addition of F − promotes significant quenching of the fluorescence at 430 nm and the appearance of a weak red-shifted emission band in the 520−550-nm range. These findings indicate that the complex between 120 and F− forms through hydrogen-bonding interactions rather than receptor deprotonation. The 1,8-naphthalimide urea sensor 121 (Figure 47) was devised as a colorimetric and fluorescence sensor for anions.128 The 3-position of the naphthalimide ring contains a trifluoromethyl-substituted phenylurea moiety as the anion recognition site. Upon addition of 25 equiv of F− to a DMSO solution of 121, large changes take place in the absorption spectrum at 495 nm. In addition, the intensity of the absorption band of the probe at 327 nm is enhanced dramatically, accompanied by a large decrease in the absorbance band centered at 272 nm. These changes in the absorption spectrum
Figure 48. Structures of 122−124. 5530
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Figure 49. Structures of 125 and 126 and proposed mechanism for the inhibition of ESIPT in 126 induced by F−.
Figure 50. Proposed mechanism for the reaction of 127 with F−.
addition of F− ion. These results demonstrate that the ESIPT process of 126 is inhibited by fluoride-induced H-bonding, followed by deprotonation of NH of the 1-AAQ group. In 2007, the novel calix[4]arene 127 (Figure 50) bearing one 2,3-naphthocrown-6 and two coumarin amide units at the lower rim in the partial-cone conformation was developed by Kim and co-workers as a colorimetric and FRET-based fluorometric sensor for F− and Cs+ ions.133 Intramolecular FRET arising from naphthalene emission and coumarin absorption results in highly selective fluorescence response toward F−. The addition of F− induces a significant red shift in the coumarin absorption from 344 to 435 nm. Moreover, the addition of a large excess of F− (for example, 1000 equiv) results in coumarin emission quenching at 536 nm, which is presumably a result of single
electron transfer (SET) from F− to the excited coumarin fluorophore. Calixarene-based fluorescence receptors 128 and 129 (Figure 51) displaying excimer emission were designed to incorporate pyrene as the fluorogenic unit.134 The ionophoric groups in these substances strongly interact with only F−, and this interaction results in a new absorption band around 430 nm and an observable color change from colorless to yellow. The new band is a consequence of ICT promoted by deprotonation of OH groups. In acetonitrile/chloroform (9:1), the emission intensities of 128 and 129 are quenched upon addition of increasing amounts of F−. The new C-linked peptidocalix[4]arene 130 (Figure 51), functionalized with four L-alanine and dansyl units at the upper rim, was investigated previously.135 A decrease in the fluorescence intensity of this probe upon the 5531
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Figure 51. Structures of 128−131.
addition of F− was observed. The results of fluorescence titration experiments show that the association constant for 130 binding F− is 29500 M−1. The new pyrene-appended probe based on a thiacalix[4]arene in a 1,3-alternate conformation (131) was prepared by Kumar et al.136 Addition of anion F− to THF solution of this sensor causes quenching of monomer emission at 386 nm and slightly increases excimer emission at 475 nm. These phenomena are associated with anion coordination with the NH amides through hydrogenbonding interactions, which induces the SET process, resulting in a fluorescence quenching effect. Receptor 131 was also used in a displacement assay for the fluorimetric recognition of F−. In this assay, addition of Fe3+ to a CH3CH2OH solution of 131 induces quenching of pyrene monomer emission as a result of the formation of the 131−Fe3+ complex. The subsequent addition of anion F− promotes recovery of the pyrene fluorescence caused by the release of free 131. Hu et al. developed the new turn-on fluorescence chemosensor 132 (Figure 52) as an anthracene diamine derivative incorporating two indole units.137 Upon mixing 132 with F− in DMSO/CH2Cl2 (1:9, v/v), an increase in the intensity of fluorescence at 430 nm takes place. The results of a Job plot indicate that a 1:1 complex between 132 and F− forms with an association constant of 7.3 × 104 M−1. These results indicate that fluoride anion participates in strong hydrogen-bonding interactions not only with the amide NH groups but also with the indole NH and the 9-H of anthracene. Rajamalli and Prasad described the low-molecular-weight gel (LMWG) 133 (Figure 52), which consists of an anthracene chromophore attached to a polybenzyloxy arene group through
Figure 52. Proposed binding modes of 132 and 133 with F−.
an acylhydrazone linkage.138 A remarkable 590-fold fluorescence enhancement, referred to as a gel-induced enhanced emission (GIEE), occurs upon TBAF-promoted gelation of this probe. The presence of TBAF not only changes the color of the probe but also disrupts the preformed gel through slow diffusion of the anion, a change that can be observed using the naked eye. Analysis of a Job plot suggests that a complex with a 5532
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Figure 53. Structures of 134−137.
The new sensor for naked-eye detection of F− 137 (Figure 53), which is based on a perylene diimide platform, was explored recently.142 Addition of fluoride to a DCM solution of 137 results in an observable color change from red to blackishgreen as a consequence of a 151-nm red shift in the absorption maximum that corresponds to a detection limit of 0.14 mM. The mechanism for the process leading to this change involves proton transfer from a NH hydrogen of the sensor to fluoride anion. Two biphenyl-like colorimetric anion sensors, 138 and 139 (Figure 54), were developed by Costero et al.143 Selective color
1:1 stoichiometric ratio between the gelator and anion is formed and has an association constant of 2.9 × 104 M−1. In 2005, Liu and Tian devised 4-benzoylamido-N-butyl-1,8naphthalimide, 134 (Figure 53), to be both a colorimetric and fluorescence selective chemosensor for fluoride ion in CH3CN.139 Using this probe, fluoride can be detected at concentrations ranging from 20 to 100 μM. An observable colorless-to-yellow color change and blue-to-orange emission color change takes place upon addition of fluoride to a solution of 134, changes that are attributed to deprotonation of the 4amide moiety of the naphthalimide fluorophore. In 2006, Tian and co-workers also described a multiple switching process involving photographic images of the naphthalimide derivative 135 under the cooperative effects of light, thermal, fluoride ions, and protons.140 Upon addition of F− to a THF solution of this probe, the intensity of the absorption band around 330 nm progressively increases, and a new band with a maximum at around 495 nm forms. One possible explanation for this phenomenon involves deprotonation of the amino moiety by F−, which results in the development of negative charge density on the amide nitrogen. A fluoride-ion-triggered dual-fluorescence molecular switch based on the naphthalimide-winged zinc porphyrin 136 (Figure 53) was also developed by Li et al.141 When fluoride is added to a THF solution of this sensor, fluorescence arising from the zinc porphyrin unit of 136 can be regulated in an on−off manner by selective excitation at 365 nm and in an off−on manner by selective excitation at 504 nm. The obvious color change from pale to dark orange, induced by intermolecular proton transfer in the NH fragments, is clearly visible to the naked eye.
Figure 54. Structures of 138 and 139.
changes from colorless to orange take place when these probes are in the presence of fluoride. These changes are associated with the appearance of a broad absorption band centered at ca. 450 and 479 nm for 138 and 139, respectively. Two different types of products are generated by reaction of fluoride with these sensors. One species is a coordination complex, and the other is a salt generated by NH deprotonation, the latter of which induces a conformational change that gives rise to color development. 5533
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sensor has a layered structure as a result of π−π stacking and the CH···π interactions between the ferrocene moieties and naphthalene rings. In the presence of F− and H2PO4−, the emission intensity of 143 at 490 nm is enhanced significantly. The results of 1H NMR titration experiments demonstrate that addition of F− induces the deprotonation of the NH groups in 143. The closely related ferrocene-based receptor 144 (Figure 56) was designed to undergo hydrogen-bonding interactions with anions.146 Observations made in UV−vis spectral titration experiments showed that 144 forms a 1:1 complexes with F− in DMSO. Upon addition of F−, the absorption maximum of the probe at 394 nm undergoes a large red shift to 520 nm in association with a dramatic change in color from brown to purple. This change is easily observed by the naked eye and is selective for F− as compared to other anions. Zhao et al. devised the fluorescence sensor 145 (Figure 57), which can be used to detect fluoride ion through an excitedstate intermolecular proton transfer (ESIPT) mechanism.147 Addition of fluoride to a DMSO solution of 145 results in a decrease in the intensities of the absorption bands at 330 and 370 nm and the appearance of new bands at 440, 465, and 495 nm. In addition, a large enhancement in the intensity of a redshifted emission band at 520 nm takes place. The association constant for the formation of the 1:1 complex between fluoride and 145 was determined to be given by log Kassoc = 3.78 ± 0.2. The pyridinium amide receptor 146 (Figure 57), containing two anthracene moieties, was demonstrated to be a fluorescence chemosensor for H2PO4−, F−, and AcO−.148 The intensity of the monomer emission from this probe is quenched significantly by F− and AcO−. Quenching of the excited state of 146 by F− is attributed to the generation of strong interactions in the open cleft initially through NH···F− and CH···F− hydrogen-bond formation. The new gelator 147 (Figure 57) was reported to form a thermally reversible gel in DMSO/H2O and an ultrasoundstimulated gel in DMSO.149 Formation of these gels is attributed to cooperative intermolecular hydrogen bonding and π−π stacking and cyano interactions. Addition of fluoride not only changes the color of the probe from colorless to yellow but also disrupts the preformed gel into a solution. Moreover, the reversible gel−sol transition and noticeable color change can be controlled by addition of fluoride and protons.
The fluoride-sensing properties of the new chromogenic receptors 140−142 (Figure 55), containing 2-nitrophenyl and
Figure 55. Structures of 140−142.
3,5-dinitrophenyl groups appended to amide and in secondary amine positions, have been investigated.144 Upon addition of fluoride to solutions of 140 and 141 in acetonitrile, a yellow color develops. In contrast, the color of an acetonitrile solution of 142 turns intense purple color in the presence of fluoride, a change that can be detected by the naked eye even at parts-permillion concentrations of fluoride. The binding constants (Ka) of 140−142 with fluoride were determined to be 6.8 × 103, 8.1 × 103, and 1.9 × 104 M−1, respectively. The color changes taking place with these probes are most likely a result of the formation of hydrogen-bond complexes between the amide groups and F−. Zhang et al. described the host/guest complexation of anions with the ferrocenenylphalene dyad 143 (Figure 56).145 This
Figure 56. Structures of 143 and 144.
Figure 57. Structures of 145−147. 5534
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A solution of the naphthol amide sensor 151 (Figure 59) in acetonitrile was observed to undergo a red shift in its fluorescence emission upon addition of anions such as F−, AcO−, and H2PO4−.153 The magnitude of the shift is dependent on the basicity of the anion. A new, red-shifted emission band appears at 515 nm in the presence of F− as a consequence of hydrogen-bonding interactions involving the amide NH and phenol OH groups. The novel turn-on fluorescence colorimetric sensor for fluoride 152 (Figure 59) was described by Li et al.154 Addition of F− to a DMSO solution of 152 results in enhancements in the fluorescence intensities at 338 and 352 nm that can be detected by the naked eye. Similarly, AcO− and H2PO4− also enhance the fluorescence of the probe but to a much lower extent than does F−. The color of the solution of 152 turns to red from yellow upon addition of fluoride. These optical changes are attributed to a fluoride-anion-induced increase of the charge density and rigidity of 152. The new pyrrole 2,5-diamide 153 (Figure 60) contains 3,5dinitrophenyl groups appended to the amide positions and an anion hydrogen-bonding cleft.155 Fluoride addition to an acetonitrile solution of this sensor results in the development of a deep blue color, which is attributed to a deprotonation process in which fluoride acts as a base. The subsequent chargetransfer interaction occurring between the deprotonated pyrrole and the nitroaromatic groups causes the optical changes. The chemosensor 154 (Figure 60), with a pyreneamide bipyridine structure, was studied by Jeon et al.156 In acetonitrile, 154 undergoes turn-on fluorescence at 420 nm in the presence of fluoride ion, and the colorless solution becomes orange. The association constant (K) of the complex formed between 154 and F− was determined to be 2.0 × 104 M−1. Xiong et al. demonstrated that the tetrathiafulvalene (TTF) derivative 155 (Figure 60), containing a boron dipyrromethene (BODIPY) moiety, was observed to display selective optical and electrochemical changes in the presence of fluoride ion. 157 Intermolecular NH···F and CH···F hydrogen bonding plays a key role in the fluoride-anion-recognition process. A significant decrease in the fluorescence intensity of 155 at 527 nm takes place upon addition of increasing amounts of F−, whereas weak quenching occurs in the presence of CH3COO− and H2PO4− under the same conditions. Addition of F− leads to a color change from orange to amaranth (rose-red), accompanied by the formation of a new broad absorption band centered at 580 nm and a decrease in the BODIPYcharacteristic peak. Piatek and Jurczak developed the amide-based macrocycle 156 (Figure 61) for the colorimetric detection of F−, AcO−,
Džolić et al. evaluated the sensor properties of the new anthraquinone-derived oxalamide gelator 148 (Figure 58).150
Figure 58. Structures of 148−150.
This substance displays an excellent gelation ability toward aromatic solvents and alcohols at very low concentrations. The gel phase 148−EtOH is responsive to fluoride, which promotes a color change from yellow to orange. In addition, fluoride prevents the gelation of 148 in p-xylene. Interestingly, simple contact between fluoride and the 148−p-xylene gel leads to a gel-to-sol transition. The flavone derivative 149 (Figure 58) was designed for the recognition of fluoride ions.151 Upon progressive addition of TBAF to a solution of this sensor, the intensity of the 317-nm absorption band decreases slightly, and a new band arises at 408 nm, which was accompanied by a color change from colorless to yellow. Meanwhile, a significant decrease in the intensity of the fluorescence band occurs, accompanied by the emergence of a new emission band centered at 590 nm. The selectivity of the probe for fluoride over other halides is attributed to the higher basicity of fluoride, and the spectral changes are attributed to proton abstraction rather than the more common hydrogen-bonding mechanism. Lin et al. explored the interactions of halide and benzoate anions with the 4,5-diacetamidoacridine-9(10H)-one 150 (Figure 58).152 Whereas chloride and bromide exhibit simple association with this substance to form 1:1 complexes, both fluoride and benzoate also form complexes but then deprotonate the acidic acridinone NH proton of 150 using a second equivalent. Upon addition of F− to a solution of the probe, the initial absorption peak at 435 nm disappears and is replaced by new peaks at about 465 and 495 nm, along with a much less intense peak near 530 nm.
Figure 59. Structure of 151 and proposed binding mode of 152 with F−. 5535
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Figure 60. Structures of 153−155.
Figure 61. Structures of 156−158.
and H2PO4− ions in both DMSO and acetonitrile solutions.158 In DMSO, the initially colorless solution of this probe turns yellow when exposed to any of these ions. However, addition of fluoride, acetate, and dihydrogen phosphate ions to acetonitrile solutions of 156 results in the development of turquoise (λ = 408, 700 nm), yellow (λ = 378 nm), and purple (λ = 537, 393 nm) colors, respectively. These visual changes are completely consistent with the association constants of 156 with these
anions in acetonitrile, which are in the following order: F− ≫ H2PO4− ≫AcO− > Cl− ≈ HSO4− ≫ Br−. The new dendritic gelator 157 (Figure 61), consisting of carbazole building blocks, was developed by Xu et al. This sensor forms a supergel in DMSO and in some DMSOcontaining mixed solvents.159 The organogel of 157 enables naked-eye recognition of F−, because it is destroyed in the presence of 4 equiv of F−, a process that is accompanied by an enhancement in the emission at 420 nm. This phenomenon 5536
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Figure 62. Structures of 158−160.
Figure 63. Structures of 161−164.
Figure 64. Structures of 165−167.
results from the interaction of F− with 157 through H-bonding, which leads to an increase in molecular coplanarity. The selectivity of 157 toward F− is a consequence of the size of the ion and the fact that larger anions fail to interact with the amide groups in 157 because of steric hindrance.
hydroxy-2-methylquinoline, which is nonfluorescent because of efficient radiationless relaxation to the ground state through intra- and intermolecular ESPT. However, a new hydrogen bond is formed between the OH group and F−, resulting in color and fluorescence changes caused by a new charge-transfer interaction between the F-bound OH and the electron-deficient dibrominated quinoline. The anion receptor 159 (Figure 62), containing anthracene and TTF units, was prepared and studied by Lu et al.161 This substance, which displays a unique selectivity for fluoride relative to other anions, undergoes a dramatic fluorescence enhancement upon addition of fluoride under neutral conditions. The 159−F− complex in CH2Cl2 was found to
3.4. Sensors Bearing Sulfonamides as Binding Sites
The chemosensor 158 (Figure 62), consisting of four sulfonamide groups linked to two dansyl moieties, was developed by Bao et al.160 Addition of 2 equiv of fluoride to a CH3CN solution of this sensor promotes a ca. 10-nm blue shift and an increase in the fluorescence band. Upon addition of F−, 158 undergoes deprotection to give 5,7-dibromo-85537
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anions were found to be in the following orders: for 166, CN− > BzO− > AcO− > F−; for 167, F− > AcO− > CN− > BzO−.
have a 1:2 stoichiometry and an association constant of 1.51 × 108 M−2. Two fluoride ions participate in effective hydrogenbonding interactions not only with the protons of the amide but also with the 9-H of anthracene. This bonding rigidifies the receptor, which leads to a reduction in the efficiency of SET quenching. Bhalla et al. developed the linked thiacalix[4]arene−dansyl moiety fluorescence sensor 160 (Figure 62), which is highly selective for Cu2+ and F−.162 In CH3CN/CH2Cl2 (1:1, v/v), the presence of Cu2+ induces the formation of a 1:1 metal−160 complex that exhibits intense 71-nm-blue-shifted emission at 433 nm. The fluorescence enhancement by Cu2+ is a result of the reduction in the electron-donating ability of the two nitrogens. The fluorescence emission at 504 nm is quenched in the presence of F− because of deprotonation of the amide moiety to produce a better donor for transferring electron quenching of the dansyl moiety. This was the first example of a thiacalix[4]arene-1,3-diamine scaffold combined with a dansyl moiety to produce a fluorescent sensor for copper and fluoride ions. Interactions of anions with the sulfonamide-containing Ru(II)- and Co(II)-based receptors 161−164 (Figure 63) were studied by Shang et al.163 Addition of AcO− and F− to a solution of the ruthenium complex 161 results in the formation of three new absorption bands at 380, 475, and 500 nm, which is associated with a color change from orange-red to red. The results of solution studies demonstrated that 1:1 complexes between 161 and the two anions are generated as a result of hydrogen-bonding interactions. Nearly the same response takes place with 162. The anion-binding abilities of 161 and 162 are in the order AcO− > F− > H2PO4− > OH− > Cl− > Br− > I−. Additionally, the cobalt complex 163 in DMSO displays an absorption band at 471 nm, whose intensity increases upon the addition of AcO− and F− anions. The binding abilities of 163 and 164 were found be in the order F− > AcO− > H2PO4 > − OH > Cl− > Br− > I−. The core-substituted naphthalene diimide sensor 165 (Figure 64), bearing a bis-sulfonamide group, was described by Bhosale et al.164 In CHCl3, about 97% quenching of the fluorescence of this probe at 635 and 696 nm takes place upon the addition of TBAF. The stoichiometry of the complex formed between the probe and fluoride was determined to be 2:1, and the association constant was determined to be 4.1 × 105 M−2. In DMSO, a significant bathochromic shift of about 100 nm in the absorption band of 165 occurs upon complexation with fluoride, corresponding to the development of a green color. Analysis of a Job plot shows that a 1:1 complex is formed in DMSO with an association constant of (1.2 ± 0.5) × 106 M−1. The two novel quinoxalinophenanthrophenazine-based anion sensors 166 and 167 (Figure 64) were found to be effective fluorescence sensors for several anions.165 Addition of anions to a solution of 166 causes a decrease in the absorbance at 280 nm and an enhancement of the absorbance bands at 355 and 450 nm. Titrations of this sensor with acetate, benzoate, cyanide, and fluoride anions result in an increase in the emission intensity along with a bathochromic shift of the maximum to 550 nm. When acetate, benzoate, cyanide, and fluoride are added to solutions of 167, the intensity of emission at 450 nm decreases. Theoretical modeling of 1:1 anion complexes with 166 and 1:2 anion complexes with 167 was carried out. The binding constants of 166 and 167 to various
3.5. Sensors Bearing Imidazoline or Imidazole Groups
Cui et al. developed the new type of anion sensor 168 (Figure 65), which consists of a Ru(II)-bpy moiety as a chromophore
Figure 65. Proposed mechanism for the detection of F− and OAc− by 168.
and the 2,2′-biimidazole (H2biim) as the anion receptor.166 In CH3CN solution, sensor 168 serves as a donor for hydrogen bonding with Cl−, Br−, I−, NO3−, HSO4−, H2PO4−, and OAc− anions, resulting in a color change from yellow to orangebrown. Fluoride ion reacts to deprotonate the NH group of the H2biim ligand owing to the formation of the highly stable HF2− anion. The stepwise deprotonations of the two NH fragments induce a vivid color change from yellow to orangebrown and then to violet. Two new imidazo[4,5-f ]-1,10-phenanthroline-containing Ru(II) (169) and Re(I) (170) complexes (Figure 66), which serve as colorimetric and fluorescence chemosensors for F−, were recently investigated.167 A solution of probe 170 in DMSO undergoes an immediate naked-eye-visible change from yellow to pink when fluoride anion is added. In contrast, the emission intensity of 169 in DMSO at 512 nm is strongly enhanced upon the addition of F−. The effective anion sensor 171 (Figure 66) was devised for selective recognition of F− and OAc− by Kundu et al.168 Sensing by this substance is based on the oxidative generation of 171+, which undergoes selective colorimetric changes in the presence of F− and OAc− anions. The MLCT band of 171+ at 498 nm is red-shifted to 538 nm with a distinct color change from reddish-orange to pink upon addition of these anions. In addition, the emission band of 171+ at 716 nm is quenched in the presence of 1 equiv of F− or OAc−. In CH3CN, the binding constants of 171+ with F− and OAc− were determined to be log K = 7.61 and log K = 7.88, respectively. The binol-based receptor 172 (Figure 67) can be employed to detect F− anion through color changes and F− and AcO− through fluorescence changes.169 In CH3CN/DMSO (9:1 v/v), the color of 172 solution changes from colorless to yellow upon the addition of 50 equiv of F− and to pale yellow upon the addition of 50 equiv of AcO−. The distinct chromogenic response of 172 is attributed to deprotonation of the imidazolium moieties by F−. In contrast, AcO− interacts with the imidazolium moieties only through the formation of hydrogen bonds. Probe 172 displays an emission band at 370 nm in acetonitrile that undergoes a bathochromic shift to 474 and 454 nm upon addition of F− and AcO−, respectively. Receptors 173 and 174 (Figure 67), which participate in highly efficient intramolecular ESPT, can be used for the highly selective detection of fluoride ions.170 Upon addition of fluoride to a solution of 173 in CH3CN, a new absorption band appears between 400 and 440 nm with a concurrent color change from 5538
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Figure 66. Structures of 169−171.
Figure 67. Structure of 172 and proposed binding modes of 173 and 174 with F−.
colorless to yellow. Addition of increasing concentrations of fluoride to the solution of 173 promotes generation of two new emission bands at 425 and 500 nm. In the case of 174, the intensity of the fluorescence peak at 586 nm gradually decreases in concert with the appearance of a new blue-shifted emission band at 515 nm. Because of its on−off−on behavior, 174 can be used for the ratiometric determination of F−. The new benzoimidazole naphthalimide 175 (Figure 68) was devised to sense F−.171 Upon addition of fluoride anions to a THF solution of this substance, the absorption band at 378 nm progressively decreases concurrent with the appearance of a new band at 476 nm. These changes result in an observable color change from colorless to magenta that can be seen by the
Figure 68. Structures of 175 and 176.
naked eye. In addition, a significant decrease takes place in the emission band at 478 nm, and the emission maximum is redshifted to 603 nm. These absorption and fluorescence changes are a result of deprotonation by fluoride anions of the amino moiety located in the benzoimidazole group. 5539
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Figure 69. Proposed mechanism for the detection of F− by 177 and structures of 178 and 179.
Figure 70. Proposed mechanism for the interaction of 180 with F−.
The new fluorescence sensor 176 (Figure 68), designed for the detection of fluoride by Wang et al.,172 contains a similar Nimidazolyl-1,8-naphthalimide moiety. A solution of this substance in CH2Cl2 displays strong fluorescence owing to the formation of a coplanar geometry as a result of intramolecular H-bonding between the naphthalimide and imidazole moieties. Addition of F− disrupts this H-bonding, which results in significant fluorescence quenching at 450 nm. In addition, the results of nonlinear regression analysis indicate that the stoichiometry of the 176−F− complex is 1:1 with an association constant of log Ka = 5.31 ± 0.07. Xu et al. provided the first concrete theoretical and experimental evidence for the existence of anion−π interactions between electron-rich alkylbenzene rings and fluoride ion in CH3CN.173 Probe 177 (Figure 69), which contains a cyclophane cavity bridged by three naphthoimidazolium groups, selectively interacts with fluoride ion through a combination of anion−π interactions and (CH)+···F−-type ionic hydrogen bonding in the form of a sandwich complex. Upon addition of 1 equiv of F−, a strong increase in the fluorescence emission band of the probe centered at 385 nm occurs, and the band at 474 nm decreases. The results of fluorescence titration experiments showed that the ratio of the emission intensities at 375 and 474 nm are dependent on the concentration of F−.
The analogous bisbenzimidazolium cyclophane receptors 178 and 179 (Figure 69) were described by Amendola et al.174 F− in CH3CN interacts with the three coplanar imidazolium CH hydrogens in each probe to give stable complexes. In both cases, upon the addition of F−, the absorbance at 240 nm increases continuously, and a broad defined band centered at 300 nm is formed. Probe 178 is a completely specific receptor for fluoride. Other anions, such as Br− and NO3−, cannot be accommodated in a coplanar fashion in the triangular structure of 178. Although the cage complex [178···F]2+ is stable in the presence of excess F−, the related tripodal complex [179···F]2+ decomposes as a consequence of deprotonation of the imidazolium CH protons and the formation of very stable [HF2]− species. Zhang et al. devised the novel Y-shaped imidazole derivative 180 (Figure 70) to be a single- and two-photon-excited fluorescence chemosensor for fluoride anions.175 Upon the addition of F− to a CH3CN solution of this sensor, the emission color changes from green to brown, as a result of deprotonation of the imidazole core. Upon complexation with fluoride anion, the maximum wavelength of the two-photon-excited fluorescence (TPEF) peak shifts from 490 to 565 nm, indicating that 180 is an excellent ratiometric TPEF sensor for fluoride anion, with a K value of 7.3 × 103 M−1. 5540
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Figure 71. Structures of 181−184.
The N-arylimidazolium-based chromofluorescent probe 181 (Figure 71) was developed as a selective, ratiometric sensor for F−.176 Upon gradual addition of TBAF to a solution of 181 in CH3CN/DMSO (20:1, v/v), the absorbance at 405 nm undergoes a gradual decrease in intensity with a concomitant increase at 480 nm and an isosbestic point at 440 nm in association with a color change from light yellow to orange. Upon addition of F−, the fluorescence intensity of the probe at 480 nm disappears, a new red-shifted fluorescence emission band at 580 nm appears, and the emission color changes from green to red. Peng et al. designed the two ratiometric fluorescent chemosensors 182 and 183 for fluoride.177 The spectral responses of these probes to fluoride in acetonitrile includes an approximately 100-nm red shift in the absorption and fluorescence emission bands and significant ratiometric fluorescent responses (Rmax/Rmin) of 88 for sensor 182 and 548 for sensor 183. The selectivity for F− can be tuned by changing the electronic push−pull properties of substituents on the phenyl para position of the sensor. As a result, 182 displays a higher selectivity than does 183. 2,6-Bis(2-benzimidazolyl)pyridine 184 (Figure 71) was developed as a highly selective and sensitive (concentrations as low as 10−7 M) chemosensor for the detection of fluoride ions with high selectivity.178 Upon the addition of fluoride to a CH3CN solution of the probe, the absorption band at 327 nm progressively decreases and broadens, and a new peak at 311 nm arises. The association constant (Ka) was calculated to be 439.49 M−1, suggesting that strong hydrogen bonding takes place between 184 and fluoride. Significant quenching of the 375-nm emission band and the development of a new band at 434 nm occur upon the addition of TBAF, indicating that a hydrogen-bonded complex between TBAF and 184 is formed. Unlike previously described optical methods for CO2 sensing, which depend primarily on the acidic properties of this gas, a new approach has been developed recently in which anions are used as activators.179 The sensor 185 (Figure 72) consists of a benzobisimidazolium group that, in the absence of exogenous base, undergoes a deprotonation reaction with fluoride. Among other anions, F− reaction with 185 in acetonitrile induces significant bathochromic shifts of the absorption bands at 290 to 344 nm, which is ascribed to the formation of strong (CH)+···F− bonds. In addition, fluorescence quenching takes place upon addition of F−. When the fluoride-treated sensor is exposed to CO2 gas, fluorescence is recovered, and a hypochromic shift in absorption peak back to 290 nm takes place. These observations illustrate how this novel fluoride-activated system senses CO2 in the form of turn-on fluorescence and a colorimetric response.
Figure 72. Proposed reactions that occur when 185 is treated first with F− and then with either CO2 or CS2.
3.6. Sensors Bearing Indole Groups
Receptors containing maleimide dyes as a signaling subunits and indoles (186 and 187, Figure 73) and pyrroles (188, Figure 73) as binding sites have been developed by Lin et al.180 For example, the colors of solutions of 186 and 187 change from yellow (absorption band at 474 nm) to orange (505 nm) in the presence of F− and H2PO4−, suggesting that the two indolyl NH groups chelate the anions through hydrogen-bonding interactions. The colors of dichloromethane solutions of 188 change from red (absorption band at 516 nm) to blue (580 nm) upon the addition of F− and CN−. Also, the emission band of 188 at 646 nm is gradually quenched and shifted to 705 nm upon addition of these anions as a consequence of deprotonation of one of the pyrrole groups. In 2006, Shao’s group described the new substance 189 (Figure 73), containing an acidic H-bond-donor moiety and a basic H-bond-acceptor moiety, which acts as a selective colorimetric sensor for either F− or HSO4−.181 In CH3CN, F− induces a color change of this probe from yellow to red, associated with the disappearance of the absorption band at 423 nm and the formation of a new red-shifted band at 517 nm. In aqueous solvent systems (CH3CN/H2O), a significant color change takes place only upon addition of HSO4−. A deprotonation/protonation reaction of 189 is responsible for the dramatic color changes promoted by both F− and HSO4−. 5541
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Figure 73. Structures of 186−193.
and two new peaks at 540 and 570 nm appear. In addition, fluoride induces an increase in the intensity of the fluorescence emission of 194, together with a red shift from 410 to 435 nm. The association constant between 194 and F− was determined to be 1.02 × 104 M−1. Indole derivatives 195−197 (Figure 75) were designed by Pereira et al.185 for the fluorescence recognition of F−. In
In 2011, the tris(indolyl)methene receptors 190−192 (Figure 73), containing conjugated bisindole skeletons, were reported by Shao’s group.182 These sensors selectively detect F− using a two-stage proton-transfer pathway that induces dramatic color changes. The first step in the double-deprotonation process promoted by F− has a higher dissociation constant than does the second step, which is attributed to a decrease in the NH proton acidity following monodeprotonation. In the presence of F−, the color of a solution of 190 gradually changes from yellow to light pink and then to orange. In the case of 191, two distinct color changes occur from orange to light pink and then to yellow in the presence of F−. Owing to the increase in the acidity of indole NH groups caused by the presence of electronwithdrawing groups, the nitro-substituted derivative 192 display a much higher binding affinity for F− than do 190 and 191. In 2012, Mallick et al. described the fluoride chemosensor 193 (Figure 73), which has a structure that is closely related to that of 189.183 Upon addition of increasing concentrations of fluoride in CH3CN, the absorption band of this probe at 530 nm blue shifts by about 10 nm, and the band at 425 nm increases. These changes enable the sensor to be used for the detection of fluoride even at submicromolar levels and as high as 2 mM. Based on their previous studies, Shao’s group developed the indole hydrazone-based colorimetric sensor 194 (Figure 74), which displays a colorimetric response and turn-on fluorescence in the presence of fluoride anions.184 Upon addition of F−, the band of the probe at 425 nm progressively decreases,
Figure 75. Structures of 195−197.
CH3CN solutions, the sensors display emission bands in the 340−420-nm region. Upon addition of F− but not Cl−, Br−, AcO−, or HSO4−, quenching of the emission of the probes in the 340−420-nm range takes place, along with the appearance of a red-shifted band at 465 nm for 195, 515 nm for 196, and 510 nm for 197. The new red-shifted emissions are ascribed to the deprotonated form of the receptors generated by reaction with basic F−. In an investigation of the interesting indole-linked quinolone derivative 198 (Figure 76), Panzella et al. observed that this material can be employed as a turn-on fluorescent sensor for fluoride.186 Upon addition of increasing concentrations of F− to a CH3CN solution of this substance, a colorless-to-yellow color change takes place in association with the appearance of an absorption band at 414 nm. In CH3CN, addition of F− results in the appearance of an emission band at 489 nm. The stoichiometry of the fluoride−198 interaction was determined to be 2:1 based on analysis of a Job plot. The bisindole diazine 199 (Figure 77) has been identified as a novel fluoride sensor.187 Addition of F− to a DMSO solution of 199 leads to the disappearance of the absorption band at 352 nm and the formation of a new band at 409 nm. This red shift induces a color change from colorless to yellow. Upon addition of increasing quantities of F− anion, a new emission band at 455 nm appears in association with blue-colored fluorescence.
Figure 74. Proposed binding mode of receptor 194 with F−. 5542
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Figure 76. Proposed binding mode of 198 with F−.
These studies indicate that a 1:2 host/guest complex forms as a result of hydrogen-bonding interactions between the NH protons of the indole groups and the anion. Kim et al. devised chemosensor 200 (Figure 77) in which the indole moiety acts as a proton-donor unit and the furan moiety acts as an acceptor.188 Upon addition of fluoride to this sensor in acetonitrile, the absorption band at 523 nm progressively shifts to 545 nm. and the color changes from orange-red to purple-pink as a consequence NH deprotonation. In conjunction with the color change, fluorescence of the probe at 568 nm is dramatically quenched. Lozano et al. developed organogelator 201 (Figure 77), which contains asparagine and tryptophan residues in the outer core and a central piperazine with two 1,3,5-triazine units in the inner core.189 In acetonitrile (2%, w/v), the 201 gel undergoes a rapid nakedeye-observable gel-to-sol transition upon addition of TBAF. Concurrently, quenching of the emission band at 340 nm takes place. The gel−sol transition was rationalized in terms of the dissociation of intermolecular hydrogen bonds caused by bonding of fluoride anions to the asparagine amide groups of the gel form. The ICT-based chemosensor 202 (Figure 77), which contains a π-rich indole as the H-donor and the cyanoquinazolinone ring as the π-acceptor, was recently developed.190 Incremental addition of TBAF causes the absorbance band of the probe at 412 nm to decrease gradually as a red-shifted maximum at 524 nm appears. This change enables naked-eye detection of a color change from yellow to red. In DMSO,
Figure 77. Structures of 199−203.
Figure 78. Structures of 204−208. 5543
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was shown that F− can displace BM through the formation of a complex with 211, concurrent with the development of a violet color of the solution. Addition of HSO4− transforms the solution to almost colorless because it deprotonates the receptor site in 211 and protonates the dye. These changes enable this sensor system to be used for visual detection of F− and HSO4−. The dipyrrolemethane-containing compounds 212−215 (Figure 81) were shown to be colorimetric fluoride sensors in CH3CN.197 A new absorption peak appears at 490 nm when 212 forms a complex with F−, and the absorption peak of 213 at 330 nm is shifted to 510 nm when fluoride ions are added to compound 213. For compound 214, the absorption peak at 413 nm is shifted to 501 nm in the presence of this anion. Both 212 and 215 display dramatic color changes from colorless to yellow-red upon addition of fluoride, and 213 and 214 also undergo distinct color changes from light yellow to deep red when F− is added. Electrostatic interactions between the pyrrole NH groups of the dipyrrolemethane moiety and fluoride anion have been shown to be responsible for these changes. The efficient colorimetric fluoride sensors 216 and 217 (Figure 81), incorporating various numbers of hemiquinone and pyrrole moieties, were described by Wang et al.198 Addition of only fluoride to a solution of 216 in DMSO induces a gradual decrease in the absorption peak at 496 nm and the appearance of a new peak at 568 nm. In addition, a color change occurs from orange to blue as a result of deprotonation of the NH moiety. Based on the unique bis-deprotonation process of 217, addition of F− results in a decrease of the absorption bands at 459 and 571 nm and the increase of a new band at 740 nm in conjunction with a color change from winered to gray. The detection limits for F− in DMSO were determined to be 2 × 10−4 and 2 × 10−5 M for 216 and 217, respectively. The colorimetric sensor 218 (Figure 82) containing a dipyrromethane (donor)−7,7′,8,8′-tetracyanoquinodimethane (acceptor) charge-transfer dyad displays excellent selectivity for the detection of F−.199 The sensing mechanism was ascribed to the interruption of charge transfer between the donor and acceptor in the presence of F−. Upon addition of F− to a MeCN/H2O (1:1, v/v) solution of the probe, the intensity of the CT band at 612 nm decreases, and a new band appears at 510 nm. In addition, a distinct colorimetric change from intense blue to light pink takes place. The pyrazole-based fluorescent sensor 219 (Figure 83) was studied as a fluoride-anion detector in organic (DMSO) and aqueous [DMSO/H2O (95:5, v/v)] solutions.200 Sensor 219 displays changes in both its UV−vis absorption and fluorescence spectra upon addition of F−. Addition of F− induces a red shift of the main absorption band of 219 from 342 to 368 nm in association with a dramatic, 607-fold enhancement of the intensity of blue emission at 424 nm. The stoichiometry of the complex formed between 219 and F was determined to be 1:2, and the stability constants of the complex were determined to be log K1 = 4.67 ± 0.16 and log K2 = 4.68 ± 0.05. The two efficient colorimetric anion sensors 220 and 221 (Figure 83), with a pyrrole NH moiety as the binding site and a nitro group as the signaling unit, have been explored.201 The receptors undergo color changes to yellow and permanganate upon the addition of F− and OH−, respectively. The nitro group in receptor 221 causes it to serve as a superior sensor in
emission of the probe at 489 nm is replaced by a new redshifted emission band at 551 nm in a manner that enables fluorescence ratiometric detection. The detection limit of 202, determined using the colorimetric response, was found to be 1.096 × 10−6 M. The BODIPY−indole conjugate 203 (Figure 77) serves as a colorimetric and fluorometric probe for selective and sensitive detection of F−.191 This sensor in CH3CN interacts with F− in a 1:1 stoichiometric manner through a hydrogen-bonding interaction between the indole NH and F−. The addition of F− to 203 leads to a decrease in the 599-nm absorption band along with an increase in a 718-nm band, leading to a clear color change from blue to green. In addition, the orange fluorescence of 203 at 624 nm is quenched by fluoride. The association constant between 203 and F− was determined to be 7.8 × 102 M−1. 3.7. Sensors Bearing Pyrrole Groups
In 2002, the cobalt(III) complex 204 (Figure 78) was described by Mizuno et al.192 When a solution of 204 is treated with increasing quantities of TBAF, the absorption peaks at 323 and 525 nm decrease, and a new peak at 652 nm appears, in conjunction with a color change from red-pink to pale purple. Chemosensors bearing dipyrrolyl motifs as recognition sites and tetrathiafulvalene redox-active moieties have also been investigated.193 For example, the receptor 205 (Figure 78) shows specific optical signaling for fluoride by undergoing significant changes in its absorption spectrum and dramatic enhancement in the intensity of emission. The electrooptical sensors 206−208 (Figure 78), containing conjugated chromophores, display colorimetric changes as well as redox activities in response to anions.194 A unique anion-specific response was observed for fluoride, pyrophosphate, and acetate. Distinct colorimetric changes from yellow to blue (for 206), brown (for 207), and purple (for 208) and changes in electrochemical properties occur for these sensors in CH3CN upon addition of F−. Pyreno[2,1-b]pyrrole 209 and its dimeric derivative 210 (Figure 79) are reported to display excellent selectivity and
Figure 79. Structures of 209 and 210.
sensitivity for the detection of fluoride ions.195 Reversible hydrogen bonding with fluoride ion produces obvious visible colorimetric and fluorescent changes. In DMSO, the colorless solution of 209 turns grass-green upon addition of F−. In DMSO solution, 210 also undergoes a dramatic color change from greenish-yellow to bright red upon fluoride addition. In acetonitrile, growth of a new absorption band of 209 at 450 nm takes place only when F− ion is added in excess amounts. The association constants in CH3CN were determined to be Kassoc = 2 × 103 M−1 and Kassoc = 2 × 104 M−1 for 209 and 210, respectively. The formation of a 211−BM species (Figure 80) that acts as an anionic chromogenic sensor has been described.196 Upon addition of the cyclic polypyrrole 211 to a violet solution of the zwitterion BM in acetonitrile, the color changes to orange. It 5544
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Figure 80. Proposed binding mode of receptor 211 with anions.
Figure 83. Structures of 219−221.
Figure 81. Structures of 212−217.
that it exhibits stronger binding toward F− and OH−. The emission intensity at 398 nm for 221 gradually increases with an increase in F− concentration, which is caused by the formation of an intermolecular hydrogen bond (NHF). 3.8. Sensors Bearing Schiff Base Groups
Two new chromogenic receptors containing a nitro group as a signaling unit and OH and NH groups as binding sites were described by Saravanakumar et al.202 Both receptors 222 and 223 (Figure 84) display colorimetric responses to fluoride ions in CH3CN. Addition of F− to 222 gradually results in the
Figure 84. Structures of 222 and 223 and proposed mechanism for the F−-sensing process by 224.
Figure 82. Proposed mechanism for the interaction of 218 with F−. 5545
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Figure 85. Structures of 225−228 and possible structure of the complex formed between 229 and F−.
concentrations of these anions increase, the peak at 342 nm of the free receptor gradually disappears and is replaced by a new absorption peak at 450 nm, with a corresponding color change from light yellow to deep yellow that is visible to the naked eye. This change is likely caused by the development of a chargetransfer state upon interaction between the proton of the amine or phenolic group and the acceptor groups. The detection limits of this probe were determined to be 1.20 × 10−6, 1.23 × 10−6, and 3.20 × 10−6 M for fluoride, acetate, and dihydrogen phosphate, respectively. In 2007, Wu et al. described the results of studies with receptors 230−232 (Figure 86) containing p-nitrophenyl
development of a light orange color associated with generation of new absorption peaks at 369 and 457 nm. Upon further addition of F−, the orange color becomes more intense. For receptor 223, addition of fluoride causes the intensities of the absorption peaks at 233, 301, and 362 nm to decrease and the color of the solution to turn fluorescent yellow in association with the development of new peaks at 417 and 362 nm. The simple salicylaldehyde derivative 224 (Figure 84) was designed for fluoride sensing in DMSO and DMSO/H2O (9:1, v/v).203 In DMSO/H2O (9:1, v/v), addition of fluoride promotes a decrease in the intensity of the absorption band at 430 nm and a concomitant increase in a peak at 502 nm, which is attributed to charge transfer (CT) between the anionbound NH and OH units and the electron-deficient NO2 moiety. This process leads to a color change from yellow to red that can be seen with the naked eye. The colorimetric receptors 225−227 (Figure 85) based on anthraquinone fluorophores were synthesized by Devaraj et al.204 In DMSO, the presence of TBAF induces dramatic color changes of 225−227 from light pink to dark pink, brown, and golden yellow, respectively. The binding constants (Ka) for 225−227 with fluoride were determined to be 9.2 × 103, 5.78 × 103, and 1.90 × 104 M−1, respectively. Addition of fluoride ions to acetonitrile solutions of these sensors promotes a change in the color of the solutions from pale red to dark red (for 225 and 226) and reddish-brown (for 227). In CHCl3, the yellow solutions of these substances turn pink (for 225) and reddishbrown (for 226 and 227) in the presence of F−. These color changes are likely a consequence of the formation of hydrogen bonds between the OH groups and fluoride ions. The coumarin-based hydrazone receptor 228 (Figure 85) was designed to be a selective, naked-eye-detectable sensor for fluoride ion in aqueous DMSO (5:95, v/v).205 This compound can also be used to detect fluoride in a toothpaste sample using the naked eye. UV−vis spectral measurements showed that the absorption maximum of 228 undergoes a bathochromic shift from 455 to 514 and 484 nm in the presence of fluoride and acetate, respectively. Upon addition of F−, 228 develops a red color, whereas the addition of acetate produces a faint pink color. Nonlinear fits of the UV−vis titration data were used to determine that the association constant of a 1:1 complex of 228 with fluoride is (3.60 ± 0.47) × 105 M−1. Hijji and co-workers described the Schiff base 229 (Figure 85), which undergoes a color change upon addition of fluoride, acetate, and dihydrogen phosphate (DHP) ions.206 As the
Figure 86. Structures of 230−232 and proposed host/guest binding mode of 233 with F− in DMSO.
signaling subunits linked to phenol and hydrazone groups.207 In DMSO solutions, the three receptors display color changes from orange (absorption band at 440 nm) to magenta upon the addition of F−, AcO−, and H2PO4− anions. However, in DMSO/water (9:1, v/v), only 230 and 231 produce color changes in the presence of F− and AcO−, whereas only H2PO4− is able to induce a color changes in the case of 230. Finally, a selective response to AcO− is displayed by only 230 and 231 in 5546
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Figure 87. Structures of 234−236 and proposed mechanism for F− sensing by 234 and 235.
DMSO/water (7.5:2.5, v/v). Two years later, the artificial tweezer 233 (Figure 86), consisting of a symmetric 1,3phenylenedi(carbonylhydrazone)-based structure, was developed as a sensor by Wei’s group.208 The ability of this substance to recognize anions in DMSO and DMSO/H2O (85:15, v/v) was investigated by UV−vis spectroscopy. The results showed that 233 in both solvent systems undergoes a marked color change from colorless to yellow upon addition of AcO−, F−, and H2PO4−. In pure DMSO, the binding constant of 233 for F− (4.65 × 105 M−1) was found to be larger than those for AcO− and H2PO4−. The phenolic hydroxyl-based sensors 234−236 (Figure 87), bearing Schiff base groups, were described by Huang et al.209 These probes undergo naked-eye-detectable changes in color from light yellow to orange-red (234), intense purple (235), and yellow (236) in the presence of F− in DMSO. Sensor 234 takes part in a stepwise deprotonation of the two OH fragments when it interacts with excess fluoride. Upon addition of F− to a solution of 234, the intensity of the absorption peak at 332 nm decreases, and a new absorption peak at 440 nm appears. In the case of 236, addition of an excess of F− promotes a blue shift in the absorption band at 440 to 420 nm. The selectivity of anion-induced color changes for 234− 236 was determined to be F− > AcO− ≫ H2PO4− > Cl−, Br−, I−. Two salicylaldehyde-based colorimetric and turn-on fluorescence chemosensors, 237 and 238 (Figure 88), were developed by Shao’s group.210 Both receptors show a unique selectivity for F− over other anions in DMSO solutions. Titration of 237 with fluoride anions results in a bathochromic shift of the absorption-band maximum at 461 nm. Upon addition of fluoride, the two absorption bands at 269 and 331 nm of 238 in DMSO disappear, and a new band at 435 nm arises in association with a color change from colorless to yellow. The association constants of receptors 237 and 238 with fluoride anions were determined to be 4.2 × 103 and 5.5 × 103 M−1, respectively, by Benesi−Hildebrand analysis. The cleft-like receptor 239 (Figure 88) was shown to selectively recognize F− through both color changes and an increase in emission intensity.211 Addition of F− to a solution of this sensor in DMSO induces a moderate decrease in the intensity of the absorption band at 326 nm (colorless) and the appearance of a new band at 425 nm (yellow-green). This process is ascribed to the formation of a 1:1 host/guest complex through multiple hydrogen-bonding interactions of F− with the amide and hydroxyl groups. Similar spectral changes occur upon the addition of AcO− to a DMSO solution of 239,
Figure 88. Proposed binding modes of receptor 237 and 238 with F− and structure of 239.
but the color change is less intense. In addition, upon addition of F− and AcO−, the emission band of 239 at 463 nm shifts to 475 nm, and its intensity is enhanced. These emission enhancements are associated with blocking of the excited intramolecular proton-transfer process. The tripodal receptor 240 (Figure 89), containing three catechol subunits, exhibits a selective chromogenic response to the F− anion.212 In DMSO, 240 displays an absorption band centered at 274 nm and two shoulders at 306 and 353 nm. Addition of F− induces a slight hypsochromic shift of the highest-energy band, whereas the shoulder at 353 nm disappears and a new band appears at 433 nm. These changes promote a color change from pale to deep yellow. Deprotonation of the hydroxyl moieties of the catechol was found to be responsible for the color change. The negative charge generated in the deprotonation process results in an increase in the dipole moment and stabilizes the excited state, causing a significant red shift of the visible band. Mahapatra et al. synthesized the new tripodal receptors 241− 245 (Figure 89), which display distinct color changes only in the presence of fluoride ions in CH3CN solutions.213 For tripodal receptors 241 and 243−245, dramatic color changes 5547
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Figure 89. Structures of 240−245 and proposed host/guest binding mode of 246−248 with F−.
249 at 450 nm decreases upon the addition of fluoride anion, a new peak appears at 610 nm, and the color changes from yellow to green. The interaction between 250 and F− was found to be a consequence of initial hydrogen-bond formation, which is then followed by NH deprotonation and the formation of a hydrogen bond between the F− ion and the nearest CH group in the Schiff base. Interaction between receptor 249 and F−, AcO−, and H2PO4− is accompanied by color changes from yellow to violet or red. The binding abilities of receptors 249 and 250 with various anions are in the order AcO− > F− > H2PO4− ≥ Cl−, Br−, I−. Probes that contain multiple hydrogen-bonding donors, including hydrazine, hydrazone, and hydroxyl groups, as potential anion-interacting sites were reported by Chen et al.216 DMSO solutions of both 251 and 252 (Figure 91) are
take place from light pale yellow to orange, whereas 242 displays a noticeable deep color change from yellow to deep purple in the presence of 5 equiv of F−. Among these probes, 242 has the largest Kassoc value (3.3311 × 104 M−1) for fluoride and a detection limit of 1.00 × 10−5 M. The results of this study showed that deprotonation rather than H-bonding is the key factor for triggering the observed chromogenic effects for these receptors. Salicylaldimine-based Schiff base receptors 246−248 (Figure 89), containing different para-N-aryl substituents, were described by Sivakumar et al.214 Addition of F− to acetonitrile solutions of these probes promotes a dramatic color change from colorless to pale yellow (246), yellow (247), and orange (248). Generation of the new absorption peaks above 400 nm for the receptors 246 and 247 is attributed to the formation of the keto form of the receptors, which results from hydrogenbonding interactions with fluoride anion. The binding constants (Ka) of fluoride complexes of 246−248 were determined to be 2.787 × 103, 5.684 × 103, and 2.53 × 104 M−1, respectively. Finally, addition of fluoride to solutions of these receptors markedly enhances the intensity of emission at around 490 nm. Among the three, the receptor containing the NO 2 substituent (248) displays the highest fluorescence enhancement in the presence of fluoride. The hydrazone derivatives 249 and 250 (Figure 90), which contain anthracene fluorophores, were designed as fluoride sensors by Shang and Xu.215 In DMSO, the absorption band of
Figure 91. Structures of 251−253.
orange and display fairly weak fluorescence at room temperature. However, these probes undergo noticeable colorimetric and fluorescent responses only to F−, OAc−, and H2PO4−. The general trend of the sensitivity to these anions is in the order F− > OAc− > H2PO4− > Cl− > Br− ≈ I−. A 1:2 binding stoichiometry was found to exist for 251 with OAc− and H2PO4− and for 252 with F−, OAc−, and H2PO4−. The formation of an aggregated supramolecular complex upon addition of fluoride was found to be responsible for the observed optical responses.
Figure 90. Structures of 249−250 and proposed interaction of 249 with F−. 5548
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Figure 92. Structures of 254−263.
between AcO− and F−. In contrast, 266 and 267 exhibit high selectivity toward fluoride ions, and 266 undergoes a blue-toorange fluoride-anion-induced color change. Discrimination between fluoride and other anions by 266 and 267 is a consequence of their abilities to form 1:2 host/guest complexes. Zhao et al. designed the 2,2′-bispyridine (bipy) ruthenium complex 268 (Figure 94), which also contains a 3-hydroxyl-2naphthoylhydrazine recognition moiety.220 The OH group in the naphthalene fluorophore leads to a turn-on ESIPT signaling transduction. Addition of F− to a solution of this probe causes a dramatic change in its UV−vis spectrum, with the band at 345 nm decreasing progressively as the band at 440 nm increases. The association constant (log K) for the fluoride complex with 268 was determined to be 5.76 ± 0.07. A solution of 268 in CH3CN containing fluoride and excited at 355 nm displays a new emission band at about 530 nm. The sensing properties of the related fluoride sensor 269 (Figure 94), which contains a Ru-bipy fluorophore and a 2,4-dinitrophenylhydrazone chromophore, have also been explored.221 Addition of F− to a solution of this probe promotes the appearance of a new intense absorption band centered at about 580 nm and an accompanying dramatic change in color from yellow to magenta. The association constant (log K) of the complex formed in this case was determined to be 6.71 ± 0.04. Upon the addition of F−, the intensity of the emission of 269 is significantly enhanced. An easy-to-prepare test paper containing this sensor was used to detect F− in aqueous media, demonstrating the potential use of this system for detecting F− in natural aqueous environments without the need for sophisticated instrumentation. Lin et al. developed a naked-eye detection system consisting of 270 (Figure 94) for fluoride ions.222 Addition of F− to a solution of this substance in CH3CN causes a dramatic change in the color from orange to blue-violet, which is accompanied by an increase in a new intense absorption band at about 580 nm. The formation of a complex with 1:1 stoichiometry occurs in this case with an association constant of log K = 6.23 ± 0.03. The presence of F− also induces a significant increase in the luminescence intensities of Ru-based receptor 270 at 630 nm. Also, a test paper loaded with this probe exhibits color changes that depend on the fluoride concentration at pH ∼7 in water with a low detection limit of about 10 ppm (10 mg L−1). The two ruthenium(II) polypyridyl complexes 271 and 272 (Figure 95) with pendant phenol/catechol groups were shown to function as colorimetric sensors for fluoride ions.223 Addition of F− to CH3CN solutions of 271 and 272 causes a decrease in
The fluorescent bischromophoric dye 253 (Figure 91), combining anthracene and naphthalic anhydride units, has been investigated as a fluoride sensor.217 This substance in DMSO displays a FRET-mediated emission signal at 535 nm with a high energy-transfer efficiency (ET = 93%) and quantum yield (Φ = 0.155). Upon addition of F− and Hg2+ ions to this solution, the FRET signal decreases along with simultaneous enhancement of the anthracene emission band at 417−437 nm. In addition, the yellow-green color of the solution changes to dark red. A Job plot analysis based on fluorescence titration data suggests that a 1:1 complex is formed with an association constant of 2.24 × 104 M−1. Bose et al. prepared the series of new symmetrically functionalized guanidinium chlorides 254−263 (Figure 92) and evaluated their ability to sense anions in MeCN/DMF (24:1, v/v).218 Addition of only fluoride causes vivid color changes from colorless to yellow (254, λmax = 454 nm), red (255, λmax = 510 nm), reddish-orange (261, λmax = 468 nm), and finally blue (256, λmax = 600 nm; 262, λmax = 642 nm) and pink (263, λmax = 932 nm). Probe 262 can be used to sense both F− and AcO− colorimetrically with a detection limit of ∼6.25 × 10−5 M, whereas the indole-bearing derivative 263 displays a selective color change in the presence of F− that results in NIR absorption at about 930 nm. 3.9. Sensors Bearing Other Groups
In fluoride sensors 264−267 (Figure 93), designed by Fillaut et al.,219 alkynyl ruthenium complexes bearing terminal hydrogenbonding receptors are employed as luminophore recognition groups. Sensors 264 and 265, which contain only one recognition site, proved to be inefficient for distinguishing
Figure 93. Structures of 264−267. 5549
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Figure 94. Structures of 268 and 269 and proposed binding mode of receptor 270 with F−.
Figure 95. Structures of 271−274.
assigned to the naphthalimide π−π transition, appears. F− recognition was also naked-eye-visible in the form of a clear color change from yellow to red. The absorption spectral changes can be reversed by addition of competitive hydrogenbonding solvents such as EtOH and water. The sensing mechanism in this case is a result of the initial hydrogen bonding between the anion and the amine, which results in deprotonation of the acidic 4-amino-1,8-naphthalimide moiety. The trinuclear star-shaped organometallic host 274 (Figure 95), containing a π-acidic interior cavity, serves as a colorimetric sensor for F−.225 Acetonitrile solutions of receptor 274 display intense ligand-localized π−π* transitions at 250−
the emission intensity at 605 nm for 271 and 600 nm for 272. Detectable changes in colors from yellow to dark red for 271 and to red for 272 also occur when fluoride is added. The corresponding equilibrium constants for complexes of these probes with F− are 6.56 × 103 M−1 for 271 and 5.16 × 105 M−1 for 272. Selective sensing of fluoride was achieved by detecting fluorescence quenching of the metal-to-ligand charge transfer (MLCT) emission of the 1,8-naphthalimide ruthenium conjugate 273 (Figure 95).224 Upon addition of F− to a solution of this substance, the long-wavelength absorption band at 432 nm is red-shifted to 460 nm, and a shoulder at 340 nm, 5550
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Figure 96. Structures of 275−279 and proposed binding mode and deprotonation of 280 with fluoride and hydroxide ions.
Wang and co-workers synthesized the oxacalix[2]arene[2]triazine azacrown 281 (Figure 97) bearing an azacrown moiety and an electron-deficient V-shaped cleft formed by two triazine rings that serves as a receptor to interact with ion pairs.228 The fluorescence spectrum of 281 undergoes changes upon addition of TBAF with an enhancement occurring in the fluorescence band at 425 nm. The association constant of the complex between 281 and fluoride was determined to be 6.59 × 103 M−1. Observations made in ion-pair recognition tests showed that binding of [281·Pb]2+ with fluoride is comparable to that of 281 (Ka = 6.59 × 103 M−1) and that a more than 23-fold increase in the binding constant from 6.59 × 103 M−1 [281·F]− to 1.53 × 105 M−1 [Zn·281·F]+ takes place. Wang and coworkers also reported that tetraoxacalix[2]arene[2]triazine azacrowns 282 and 283 (Figure 97) also interact with fluoride anions, leading to cavity changes in the host molecules.229 For the syn forms, fluoride serves as a base to deprotonate two amino groups, resulting in the formation of an intramolecular hydrogen bond between the unprotonated and deprotonated amino groups. This interaction causes the two benzoate ester rings in the tetraoxacalix[2]arene[2]triazine azacrown backbone to move farther apart, thus promoting a decrease in the fluorescence intensity at 340 nm. With an increase in the concentrations of added n-Bu4NF and n-Bu4NOH, the emission at 477 nm decreases and eventually disappears, in concert with an increase of the emission band in the range of 378−396 nm. The new naphthalimide calix[4]arene 284 (Figure 97) was devised by Xu et al.230 as a highly selective fluorescent chemosensor for Cu2+ and F−. In acetonitrile, addition of Cu2+
310 nm and a weak MLCT band at 320 nm. Addition of increasing quantities of F− induces the appearance of an absorption band at 470 nm and an associated color change from yellow to orange. The color changes were ascribed to binding of F− with the π-electron-deficient triazine moiety, which perturbs the electronic properties of 274. In addition, the emission intensity of 274 at 574 nm is selectively quenched in the presence of F−. The azocalix[4]arene derivatives 275−279 (Figure 96), containing one or two azophenol chromophores and different binding sites, were designed by Chen and Chen.226 These substances in CH3CN recognize anions by means of hydrogenbonding interactions. Upon addition of F−, a solution of 275 undergoes a significant color change from light yellow to blue. Probe 276 also displays a noticeable color change from light yellow to blue upon the addition of F− and to violet-blue when the same amount of AcO− is added. Owing to their colorimetric properties, 277−279 can be used for the detection of F−, AcO−, and H2PO4−. The neutral calix[4]arene receptor 280 (Figure 96) with hydrazone moieties located on the lower rim exhibits a prominent naked-eye-visible color change and significant bathochromic shifts when F− is present.227 Interaction of 280 with F− promotes a bathochromic shift of 143 nm and the appearance of a new absorption peak at 549 nm, associated with a distinct color change from yellow to dark purple. Receptor 280 selectively recognizes fluoride ions through initial H-bonding interactions and subsequent deprotonation with a Ka value of 9.9 × 106 M−1. 5551
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Figure 97. Structures of 281−284.
or F− induces selective quenching of fluorescence. Analysis of a Job plot of data for binding between 284 with F− shows that a 1:1 complex is formed with an association constant of 1.1 × 104 M−1. In the complex, four NH and OH hydrogens in the rigid binding pocket of 284 participate in favorable hydrogenbonding interactions with F−. On the other hand, the two nitrogens in the naphthalimide moiety and the oxygens in the calixarene ring provide a binding pocket for Cu2+. A colorimetric displacement assay for F− and H2PO4− in DMSO/H2O (1:1, v/v) consisting of a complex between 2,6dichlorophenol indoo cresol sodium salt (DIC, Figure 98) and 285 was evaluated.231 The complex has an absorption peak at 625 nm. Addition of fluoride causes this peak and the blue color to disappear as a consequence of a process in which DIC is displaced from the complex. The colorimetric sensor for fluoride ions 286 (Figure 99), which contains an isomerizable enol−keto moiety as the recognition site and a phenothiazine group as the chromogenic center, was described by Han and Zhao.232 In the presence of F− in aprotic solvents such as CHCl3 and MeCN, the absorption band at 499 nm decreases with a concurrent increase in the band at 360 nm. These changes promote a color
change from red to light yellow that is visible to the naked eye. Fluoride-binding-governed enol−keto tautomerization is suggested to be responsible for the anion-sensing ability of 286. The fluorophore−spacer−receptor−spacer−fluorophore dyad 287 (Figure 100) was devised by Pischel et al.233 to serve as a fluoride sensor. Fluorescence of the probe occurs from the tertiary amine donor moiety in its unprotonated form. Quenching is blocked by protonation of the amine, which results in enhanced singlet−singlet energy transfer between the 1,8-naphthalimide and 4-amino-1,8-naphthalimide groups. In acetonitrile, addition of fluoride induces deprotonation of the 4amino group on the naphthalimide moiety, leading to very efficient (ca. 93%) fluorescence quenching. By using fluoride anions as degenerate inputs, a ternary NOR logic gate is established for 287. Fluoride receptor 288 (Figure 101) containing anthracene was described by Huang et al.234 This probe was shown to selectively recognize fluoride in DMSO and a DMSO/H2O (95:5, v/v), undergoing a decrease in the intensity of the absorption band at 482 nm and an increase in the band at 672 nm. The process can be observed by the naked eye in the form of a sharp color change from light red to dark brown. The 5552
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Figure 98. Proposed mechanism for the sensing of F− by 285.
Figure 99. Proposed F−-sensing mechanism of 286.
Figure 100. Proposed F−-sensing mechanism of 287.
fluorescence emission of 288 is quenched effectively by F− with a detection limit of 2 × 10−6 M. Moreover, receptor 288 can be employed to detect fluoride in toothpaste even at low F− concentrations.
The fluoride-sensing properties of the new colorimetric chemosensor 289 (Figure 102), consisting of a coumarin moiety linked to a p-nitrophenylhydrazone group, were investigated by Zhuang et al.235 Upon addition of F− to an 5553
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The binol derivative 296 (Figure 105) was found by Chen and Leung to be highly fluorescent in the visible region.238 Among the TBA salts tested, only that with F− was found to be an effective fluorescence quencher of this substance. Quantitative analysis of the fluorescence titration data revealed that the response of 296 to anions can be classified into two categories: F− and Cl form 2:1 complexes with 296, whereas the oxo anions OAc−, H2PO−, and TsO− produce 1:1 complexes with this substance. Upon addition of F− to a solution of 296, the absorption band at 375 nm increases and reaches a maximum after 2 equiv has been added, indicating again that a (296·F2)2− complex is formed. Mallick et al. observed that norharmane (NHM) serves as a ratiometric sensor for F− and HSO4− (see Figure 106).239 In aqueous solution, NHM exists in four different protonated forms. In the presence of strong bases such as F−, hydrogenbond formation occurs with acidic hydrogens in this substance, and the relatively strong acid (HSO4−) can easily protonate the basic sites of this substance. Both processes lead to optical changes. Addition of TBAF to an acetonitrile/water (5:1, v/v) mixture of this sensor leads to formation of a new absorption band around 375−425 nm and a fluorescence band around 425−475 nm. The unusually structured, cage-type bisenaminoimine 297 (Figure 107) was designed by Dudek et al. for recognition of F−.240 This substance undergoes a distinct color change and luminescence intensity increase upon the addition of F− in DMSO. The color change from orange to red and orange luminescence at 576 nm arises in the presence of fluoride. It was demonstrated that deprotonation of the bisenaminoimine group in the macrocycle takes place to cause these changes, a conclusion that was supported by the results of DFT calculations. An unidentified, irreversible reaction between the BODIPY dye 298 (Figure 108) and TBAF in acetone solution leads to the disappearance of the absorption band at 599 nm and the emission peak at 686 nm, concurrent with an increase in the absorption peak intensities at 452 and 482 nm.241 These spectral changes correspond to a color change of the solution from light blue to colorless and a fluorescence color change from red to blue. Another BODIPY dye, 299 (Figure 108), bearing a phenolic OH in the meso-phenyl moiety was designed and studied by Wang’s group for the colorimetric and fluorescence detection of F− ions by means of OH···F hydrogen-bonding interactions.242 Upon addition of F−, the absorbance of 299 at 502 nm decreases, and a gradual increase in the absorption band at 488 nm takes place with an isosbestic point at 493 nm. In addition, in response to F−, the
Figure 101. Proposed binding mode of 288 with F−.
acetonitrile solution of this sensor, the absorption peak at 475 nm decreases, and new absorption peaks at 620 and 650 nm are generated, in association with an observable color change from yellow to blue. In the proposed mechanism for this process, F− forms a hydrogen bond with 289, which results in [FH F]−-induced deprotonation of the hydrazine moiety. Tian’s group developed three new diketopyrrolopyrrole (DPP) derivatives, 290−292 (Figure 103), as fluorescent sensors for fluoride anions.236 The F−-recognition mechanism for these substances is assumed to involve intermolecular proton transfer of a lactam hydrogen in the DPP moiety to fluoride anion. For receptors 290−292 in DCM, addition of fluoride results in an orange-to-red color change concurrent with a large bathochromic shift (80 nm) and a yellow-to-red change in the emission color. Guha and Saha employed supramolecular interactions [anion−π and charge/electron transfer (CT/ET)] of fluoride ions with π-electron-deficient colorless naphthalene diimide (NDI) receptors to design fluoride sensors 293−295 (Figure 104).237 Strong electronic interactions between lone-pair electrons of F− and π* orbitals of the NDI unit lead to a F− → NDI ET event that produces the orange NDI•− radical anion. As a result, a colorless solution of 293 turns orange upon addition of F− (≤5 equiv) and pink at higher F− concentrations (>5 equiv), associated with formation of the NDI2− dianion. The color change exhibited by 293 is attributed to further fluoride-promoted reduction of NDI•− to produce NDI2− dianion. The preorganized NDI receptor 294 displays a similar two-step spectroscopic change in the presence of F−. In addition to forming an NDI/F−/NDI sandwich complex, 294 and 295 bind to a second F− ion in the amide cavities through H-bonding interactions (Figure 104). This event significantly improves their selectivity and sensitivity for F−, allowing F− to be detected at nanomolar concentrations in DMSO/H2O (85:15, v/v).
Figure 102. Proposed mechanism for the interaction of 289 with F−. 5554
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Figure 103. Structures of 290−292.
Figure 104. (Top) Structures of 293−295. (Bottom) Graphical illustrations of (a) anion−π and CT interactions between F− and NDI receptor generating a fluorochromogenic response through an F− → NDI ET event and (b) stepwise F− recognition by preorganized receptors (PRs) through (i) π−anion−π and (ii) H-bonding interactions.
fluorescence intensity of 299 decreases and undergoes a minor
intramolecular SET from the meso-phenyl moiety to the excited BODIPY chromophore. In 2008, Kumar’s group synthesized chemosensors 300 and 301 (Figure 108), which contain a quaternary ammonium cation and a NH group as recognition sites and an
blue shift. The spectroscopic changes are likely a consequence an OH···F hydrogen-bonding interaction that enhances the electron-donating ability of phenolic OH and facilitates 5555
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Figure 105. Difluoride complexation of 296.
Figure 106. Different acid−base equilibria for norharmane: CN, cation−neutral; NA, neutral−anion; ZA, zwitterion−anion; and CZ, cation−zwitterion.
anthracene-9,10-dione as both a chromogenic and fluorescent moiety.243 This substance can be used for the detection of fluoride ions at concentrations ranging from 500 to 1200 μM. A change in the color of the solution of 300 in CH3CN/DMSO (9:1, v/v) from yellow to pink occurs upon addition of F−, and a similar change takes place with 300. The dual emissions at 580 nm for free 300 and 301 and at 510 and 540 nm for 300 and 301, respectively, with F− enable these sensors to be used for ratiometric analysis of fluoride ions. The structurally similar F− probe 302 (Figure 108), bearing a pyridinium unit conjugated to an anthrapyridone moiety, was described by the same group in 2010.244 In a CH3CN/DMSO (20:1, v/v) solution, this probe undergoes a dual color change in the presence of fluoride. Specifically, an absorbance develops at 490 nm, generating an orange color, at F− ion concentrations between 5 and 75 μM, and a band at 630 nm associated with a green color arises when the concentration of fluoride is between 75 and 600 μM. Fluoride ions also induce an increase
Figure 108. Structures of 298−302.
in the intensity of the fluorescence by a factor of 25 at 510 nm, enabling selective determination of fluoride concentrations between 0.5 and 5 μM, which is a desirable range for assessment of the maximum contaminant level of F− allowed in drinking water (4 ppm). The unmodified fluorescein 303 (Figure 109) behaves as a fluorescence chemosensor for F− detection.245 The addition of F− to a solution of this substance results in the appearance of a strong absorption peak at 514 nm, along with a clear color change from colorless to yellow-green. An emission enhancement at 532 nm of about 2100-fold also occurs. These
Figure 107. Proposed interaction between 297 and F− and formation of zwitterionic species. 5556
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Figure 109. Structures of 303−305 and binding mode of 305 with F−.
Figure 110. Structures of 306−311.
and 1:1 in 1−6 M water/acetonitrile mixtures.248 Addition of F− and CH3COO− to acetonitrile solutions of this probe induces the generation of new absorbance bands at 443 nm in concert with a shift of the peak at 259 to 275 nm. In addition, an increase in the emission intensity takes place that is reflective of an increase in a broad emission band between 470 and 740 nm with an isoemissive point. Studies of the effect of the water content in acetonitrile on F− binding showed that the extent of tautomerization is significantly diminished when the concentration of water is increased to 2 M. This phenomenon is a consequence of the fact that enrichment of the solvate shell in water decreases the binding power of F− and lessens its effect on the electron density of the heterocyclic ring. Oxadiazole 307 (Figure 110) is able to sense fluoride anions in DMF solution through changes in both its absorption and fluorescence emission properties.249 A fluoride-anion-induced red shift in the absorption from 286 to 382 nm takes place in association with a color change from colorless to yellow that is visible to the naked eye. At low fluoride concentrations (
