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An acridinium-based sensor as a fluorescent photoinduced electron transfer probe for proton detection modulated by anionic micelles† Stefano Basili, Tiziana Del Giacco,* Fausto Elisei and Raimondo Germani A water-soluble fluorescent pH sensor of 9-amino-10-methylacridinium chromophore with the
Received 13th March 2014, Accepted 30th June 2014
2-(diethylamine)ethyl chain as a receptor shows an “off–on” response going from basic to acidic solution.
DOI: 10.1039/c4ob00559g
Photoinduced electron transfer has been directly demonstrated to be the quenching mechanism by the observation of the long-lived acridinyl radical. The interaction of the protonated sensor with anionic
www.rsc.org/obc
micelles causes a significant increase in the detection sensitivity of pH.
Introduction The design of fluorescent chemosensors is of great interest due to their wide application in analytical chemistry, cell biology, biomedicine and environment survey. Various chemical and biochemical analytes can be detected by these devices with high sensitivity and low detection limits. The most extensively used mode of fluorescence modulation in the recognition process of chemosensors is the photoinduced electron transfer (PET).1 Such controllable “on/off” characteristics of fluorescence emission by PET are of immediate relevance for the design of not only molecular sensors, but also molecular digital systems, which are promising candidates for the achievement of innovative materials for information technology.2 So far, an extensive number of fluorescence-based pH sensors have been reported.3 Most of them are “fluorophoreH+ amine acceptor” assemblies, separated by a spacer or not. Recent chemosensor research is aiming at tuning the sensor ability by a simple way rather than by synthesizing new water soluble sensor molecules. In this context, studies on the effect of micellar aggregates in improving the sensing ability of fluorescent sensors are of growing interest.4 In particular, it has been reported that neutral sensors for both protons and cations increase considerably their efficiency up to the critical micellar concentration (c.m.c.) because the micelles provide a hydrophobic local environment to solubilize the sensor that
Dipartimento di Chimica, Biologia e Biotecnologie and Centro di Eccellenza Materiali Innovativi Nanostrutturati (CEMIN), Università di Perugia, Via Elce di sotto 8, 06123 Perugia, Italy. E-mail: [email protected]; Fax: +39 0755855560; Tel: +39 0755855539 † Electronic supplementary information (ESI) available: 1H and 13C NMR spectra of compounds, HPLC-HRMS analysis of Acr+-A Cl−, absorption and emission spectra, relative fluorescence emission as a function of pH plots, laser flash photolysis experiments. See DOI: 10.1039/c4ob00559g
This journal is © The Royal Society of Chemistry 2014
promotes the complexation ability with cations, and the fluorescence enhancement is favored by the less polar region.5 Given the limited number of investigations of sensors with fluorophores of ionic nature reported in the literature,6 we have extended our investigation by using 9-amino-10-methylacridinium ion (Acr+) as a fluorescent chromophore for the construction of a pH sensor. The Acr+-unit confers to all the derivatives synthesized so far water solubility, contrary to many pH-indicators with uncharged fluorophore that generally show a poor water solubility. Moreover, the Acr+-unit shows particular advantages to be used in sensory systems, such as a high fluorescence quantum yield (ΦF = 0.96),7 which guarantees a strong emission signal and a long fluorescence lifetime (τF = 18.2 ns),7 which allows the detection of relatively slow quenching processes. A further advantage is the high reduction potential of the singlet excited state (E*(S1)red = 2.43 V),8 which makes the PET process, and therefore the fluorescence quenching, thermodynamically and kinetically more favorable. The attractive results presented in this paper suggest that the pH sensor 9-(2-(diethylamine)ethylamino)-10methylacridinium chloride (Acr+-A Cl−) operates in an “off–on” fashion, whereas in the “off” state (amine receptor free from H+) a PET mechanism has been unequivocally demonstrated. The further increase of fluorescence quantum yield in the “on” state (amine receptor linked to H+) observed in the presence of anionic surfactants has evidenced a newsworthy role of micelles.
Results and discussion Acr+-A Cl− was synthesized in good yield from reaction of N,N(diethylethane)-1,2 diamine and 9-chloro-10-methylacridinium chloride salts in turn prepared from the commercially avail-
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Scheme 2
Scheme 1
Chemical structures at various pH values of the sensor.
Synthesis of Acr+-A Cl−.
Fig. 2 Fluorescence emission spectra of Acr+-A (1.5 × 10−5 M) in water recorded at different pH values, λexc = 445 nm. Inset: intensity recorded at 490 nm as a function of pH; the full line is the fitting curve obtained using eqn (1).
Fig. 1 Absorption spectra of Acr+-A (7.5 × 10−5 M) recorded in water at different pH values. Inset: plot of absorbance at 385 and 445 nm vs. pH.
able 10-methyl-9(10H)-acridone and SOCl2 (Scheme 1). The absorption spectra of Acr+-A are essentially unchanged at different pH (2–8) values in water as shown in Fig. 1. Thus, protonated (Acr+-AH+) and unprotonated (Acr+-A) tertiary amine species absorb at the same wavelength, as expected being the structural difference away from the chromophore. In more alkaline solutions ( pH > 8) the absorption bands shift to smaller wavelengths; in particular the visible band shifts hypsochromically by 40 nm (from 445 to 385 nm). This effect is evident from the inset of Fig. 1. The presence of isosbestic points (250, 280, 304, 336 and 400 nm) indicates the coexistence of two species in equilibrium. The hypsochromic shift is explainable on the basis of the less aromatic structure (Acr-H)-A, a derivative of 9-imine10-methylacridane, in equilibrium with Acr+-A (Scheme 2), formed by deprotonation of the 9-amine group in very basic solution, in line with the well-known strong electrophilicity of the 9-position of the acridinium ring.9 The (Acr-H)-A species was characterized by 1H-NMR spectroscopy (see the Experimental section). As the absorption spectra of Acr+-AH+ and Acr+-A are essentially unchanged with pH, the same is observable for the shape and position of the emission spectra, recorded exciting
6678 | Org. Biomol. Chem., 2014, 12, 6677–6683
Fig. 3 Fluorescence emission spectra of Acr+-A (1.5 × 10−5 M) in water recorded at pH 2.91 (solid line), 6.00 (dashed line), 8.84 (dashed and dotted line) and 11.11 (dotted line), λexc = 400 nm.
at 445 nm, where (Acr-H)-A does not absorb (Fig. 2). The excitation at 400 nm (isosbestic point), where all three species absorb (Scheme 2), produced conversely different emission spectra depending on the pH (Fig. 3). In acidic solution, as expected, the emission shape is identical to that observed at λexc = 445 nm, while at alkaline pH a spectral shift to smaller wavelengths and a band structured in two maxima at 430 and 450 nm, attributed to the emission of the less aromatic structure (Acr-H)-A, were observed. At intermediate pH (6.00) the spectrum is clearly resulting from the emission of all the species present in solution.
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In order to avoid the interference of (Acr-H)-A in the investigation of the pH-responsive fluorescence properties of Acr+-A Cl−, the photophysical characterization was performed exciting at 445 nm. Under these conditions, the fluorescence intensity shows an increase upon protonation (Fig. 2). A 17-fold enhancement in fluorescence quantum yield ΦF (from 0.017 to 0.30) was measured. It has to be noted that this value is close to that shown by N,N-diethylaminoethyl bonded to the aniline nitrogen of 4-aminonaphthalimide chromophore.10 The fluorescence intensity (IF) at the maximum wavelength (490 nm) is reported as function of pH ranged from 3 to 12 (inset in Fig. 2, the corresponding relative fluorescence emission plot is reported in Fig. S1†). The emission intensity–pH profile was accurately analyzed according to eqn (1),11 and the obtained acidity constant is pKa = 7.0. This value is significantly smaller than those of the analogous sensor 4-(2-(diethylamine)ethylamino)-naphthalimide ( pKa = 8.7)10 and the parent triethylamine ( pKa = 10.7). The higher basicity of triethylamine with respect to 2-(diethylamine)ethylamino sensors can be explained on the basis of the electron withdrawing aniline nitrogen in the more complex structure, IF ¼
I F ðAcrþ -AHþ Þ þ I F ðAcrþ -AÞ10ðpK a pHÞ 1 þ 10ðpK a pHÞ
ð1Þ
even more effectively by the peculiar positive charge of the acridinium chromophore. The pKa of 7.0 makes Acr+-A Cl− a good candidate as a biologically relevant probe to monitor small pH fluctuations near to neutral physiological values in aqueous solution and in vivo. The switching on/off of fluorescence is indicative of an intramolecular fluorescence quenching by PET. According to this mechanism, one electron of the most basic nitrogen (aliphatic nitrogen) is transferred to the excited amino-acridinium moiety. When the amine is protonated, the fluorescence is restored. The occurrence of an intramolecular PET process from tertiary amine to 1Acr+* has been unequivocally confirmed by flash photolysis investigation. Indeed, upon laser excitation (λexc = 355 nm) of N2-saturated aqueous solutions of Acr+-A Cl−, time-resolved absorption spectra were recorded at pH values of 3.0, 7.6, 9.0 and 13.12 The absorption spectra obtained 1.1 μs after the laser pulse are shown in Fig. 4. It is interesting to note that at pH 3.0, where fluorescence is the predominant deactivation channel of the S1 state of Acr+-AH+, no transient was recorded. At pH 7.6 an absorption centered at 520 nm, assigned to the amino-acridinyl radical moiety, was detected.13 This evidence endorses the PET hypothesis, although the absorption of the resulting amine radical cation was not detected, most likely because of the detectable wavelength region.14 The charge separation could be promoted both by anion-complexation15 and a polar solvent. At higher pH (9.0) an additional band was detected at 560 nm that is likely due to triplet–triplet absorption of (Acr-H)-A because: (i) it was formed within the laser pulse, (ii) it decayed by first order kinetics and (iii) its decay was accelerated by molecular
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Fig. 4 Absorption spectra of Acr+-A (5.5 × 10−4 M) in N2-saturated H2O recorded 1.1 μs after the laser pulse at pH 3.0 (□), 7.6 (●), 9.0 (△) and 13 (▾); λexc = 355 nm.
oxygen through an almost diffusional process (kox = 6.4 × 109 M−1 s−1). This species is the only one visible at very alkaline pH, confirming its formation from (Acr-H)-A. The spectrum at pH 8 recorded 1.1 μs after the laser pulse in a wider wavelength scale (Fig. S3†) shows a broad band from 650 to 900 nm that disappears at pH 12. This absorption can be reasonably attributed to the formation of a π-dimer between the electron transfer (ET) state, Acr•-A•+, and the Acr+-A ground state. A similar broad absorption band has already been observed by Fukuzumi and coworkers with both 9-(1-naphthyl)- and 9-mesityl-10-methylacridinium in MeCN,16 though shifted to greater wavelengths (880–1100 nm), maybe due to the more aromatic system.17 The decays recorded at 510 and 750 nm in N2-saturated solution at pH 8 obey first-order kinetics with similar rate constants (1.2 × 105 s−1 and 1.0 × 105 s−1, respectively), in line with the intramolecular back electron transfer of the ET state (insets of Fig. S3†). These results clearly show that a rapid equilibrium is established between a monomer and a dimer. The long-lived ET state suggests that the thermal back electron transfer (BET) from Acr• to A•+ moieties is slower than the forward photoinduced electron transfer, although both processes are strongly exergonic (the free energy change was determined to be −1.47 eV for the PET and −1.44 eV for the BET).18 This behavior is explainable assuming that the free energy change of BET is in the Marcus inverted region.20 On the basis of this model the BET rate decreases with increasing the thermodynamic driving force in the exergonic region. It should be noted that the PET process from the diethylamine to the 9-amino-10-methylacridinium fluorophore across the dimethylene spacer is rather more exergonic with respect to the system with the amine receptor bonded, by the same spacer, to the aniline nitrogen of 4-aminonaphthalimide (ΔGPET ≈ 0, with triethylamine taken as the model).10,21 This is in agreement with the lower oxidizing power of the latter fluorophore. Really, the difference in ΔGPET between Acr+-A Cl− and the corresponding 4-amino-1,8-naphthalimide-based chemosensor should be smaller, considering that the amino receptor
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in the latter should be more easily oxidizable on the basis of pKa values (see above). Cationic, anionic and nonionic surfactants, such as CTAB (cetyltrimethylammonium bromide), SDS (sodium dodecyl sulfate) and SOBS (sodium octyl benzene sulfonate), SB3-14 (N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate), respectively, above the c.m.c. (8.62 × 10−4, 8.30 × 10−3, 1.42 × 10−2, and 2.88 × 10−4 M for CTAB, SDS, SOBS and SB3-14, respectively)22 have been explored in order to maximize the pH sensing efficiency of Acr+-A Cl−. No significant effect was observed on the spectral properties of the sensor in the presence of cationic and zwitterionic surfactants (Fig. S4 and S5†). Such a behavior is expected as the CTAB headgroups repel both protons and the positive sensor, while the zwitterionic SB3-14 produces low electrostatic affinity. Instead, the addition of SDS and SOBS, though it did not induce appreciable shift of the absorption (Fig. S6 and S7†) and emission maxima, resulted in a substantial enhancement of fluorescence emission in the pH detection as the solutions went from basic to acidic (Fig. 5 and 6).23 From the data of fluorescence emission intensity at 490 nm as a function of pH,
Fig. 5 Fluorescence emission spectra of Acr+-A (1.5 × 10−5 M) in water in the presence of SDS (0.01 M) at different pH; λexc = 445 nm. Inset: intensity recorded at 490 nm as a function of pH; the full line is the fitting curve obtained using eqn (1).
Fig. 6 Fluorescence emission spectra of Acr+-A (1.5 × 10−5 M) in water in the presence of SOBS (0.02 M) at different pH; λexc = 445 nm. Inset: intensity recorded at 490 nm as a function of pH; the full line is the fitting curve obtained using eqn (1).
6680 | Org. Biomol. Chem., 2014, 12, 6677–6683
Organic & Biomolecular Chemistry
a profile similar to that in only water is obtained (insets in Fig. 5 and 6, the corresponding relative fluorescence emission plots are reported in Fig. S8 and S9†), but with a shift of the “off–on” response window to the right along the pH axis. The data correlation by eqn (1) produced pKa of 9.2 and 9.4 for SDS and SOBS,24 respectively, values quite higher than that determined in water (7.0). This apparent increase of the basic strength of the amine moiety reveals that the positive sensor structure, localized on the negatively charged micellar surface in place of the counterion (Na+), is exposed to higher H+ concentration in the micellar interface, producing a “proton sponge effect”. Being the effective pH on the interface lower than that measured in the bulk, there is no evidence of the hypsochromic shift in the absorption spectra in basic solution (Fig. S5 and S6†) due to the deprotonation of the 9-amine group (see above).24 SDS and SOBS micelles not only move the position of the pH response window, but also affect strongly the fluorescence enhancement (68 and 79 with SDS and SOBS, respectively, against 17 determined in water), in terms of quantum yields (ΦF from 0.0088 to 0.60 and 0.0076 to 0.60, respectively), thus enhancing the sensor sensitivity. The almost fourfold enhancement in ΦF value has benefited from the fact that the double protonated sensor (Acr+-AH+) is less free to rotate and vibrate, due to the electrostatic interaction with the negatively charged surface of micelles, causing an increase in fluorescence at the expense of internal conversion.25 Another factor that favors the ΦF increase is the prevention of the otherwise efficient quenching by water, that is, a water-associated non-radiative decay. The interaction of the protonated sensor (Acr+-AH+) with SDS and SOBS in aqueous solution has also been investigated in pre-micellar regions. The two systems, Acr+-AH+/SDS and Acr+-AH+/SOBS, were studied by emission spectroscopy by varying the surfactant concentration. As shown in Fig. 7, the fluorescence intensity decreases with surfactant concentrations up to 2.5 mM and 5 mM for SDS and SOBS, respectively, while it increases up to plateau values when SDS and SOBS concentrations approach the c.m.c. values. Accordingly, the interaction of the sensor with the
Fig. 7 Fluorescence emission intensity of Acr+-A (1.5 × 10−5 M) recorded at 490 nm as a function of [SDS] (□) and [SOBS] (■) at pH 2.8; λexc = 445 nm.
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surfactant is operating in the pre-micellar region. These data can be rationalized as already reported for the 9-aminoacridinium/SDS system.26 At low surfactant concentrations ([SDS]
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PAPER
Cite this: Org. Biomol. Chem., 2014, 12, 6677
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An acridinium-based sensor as a fluorescent photoinduced electron transfer probe for proton detection modulated by anionic micelles† Stefano Basili, Tiziana Del Giacco,* Fausto Elisei and Raimondo Germani A water-soluble fluorescent pH sensor of 9-amino-10-methylacridinium chromophore with the
Received 13th March 2014, Accepted 30th June 2014
2-(diethylamine)ethyl chain as a receptor shows an “off–on” response going from basic to acidic solution.
DOI: 10.1039/c4ob00559g
Photoinduced electron transfer has been directly demonstrated to be the quenching mechanism by the observation of the long-lived acridinyl radical. The interaction of the protonated sensor with anionic
www.rsc.org/obc
micelles causes a significant increase in the detection sensitivity of pH.
Introduction The design of fluorescent chemosensors is of great interest due to their wide application in analytical chemistry, cell biology, biomedicine and environment survey. Various chemical and biochemical analytes can be detected by these devices with high sensitivity and low detection limits. The most extensively used mode of fluorescence modulation in the recognition process of chemosensors is the photoinduced electron transfer (PET).1 Such controllable “on/off” characteristics of fluorescence emission by PET are of immediate relevance for the design of not only molecular sensors, but also molecular digital systems, which are promising candidates for the achievement of innovative materials for information technology.2 So far, an extensive number of fluorescence-based pH sensors have been reported.3 Most of them are “fluorophoreH+ amine acceptor” assemblies, separated by a spacer or not. Recent chemosensor research is aiming at tuning the sensor ability by a simple way rather than by synthesizing new water soluble sensor molecules. In this context, studies on the effect of micellar aggregates in improving the sensing ability of fluorescent sensors are of growing interest.4 In particular, it has been reported that neutral sensors for both protons and cations increase considerably their efficiency up to the critical micellar concentration (c.m.c.) because the micelles provide a hydrophobic local environment to solubilize the sensor that
Dipartimento di Chimica, Biologia e Biotecnologie and Centro di Eccellenza Materiali Innovativi Nanostrutturati (CEMIN), Università di Perugia, Via Elce di sotto 8, 06123 Perugia, Italy. E-mail: [email protected]; Fax: +39 0755855560; Tel: +39 0755855539 † Electronic supplementary information (ESI) available: 1H and 13C NMR spectra of compounds, HPLC-HRMS analysis of Acr+-A Cl−, absorption and emission spectra, relative fluorescence emission as a function of pH plots, laser flash photolysis experiments. See DOI: 10.1039/c4ob00559g
This journal is © The Royal Society of Chemistry 2014
promotes the complexation ability with cations, and the fluorescence enhancement is favored by the less polar region.5 Given the limited number of investigations of sensors with fluorophores of ionic nature reported in the literature,6 we have extended our investigation by using 9-amino-10-methylacridinium ion (Acr+) as a fluorescent chromophore for the construction of a pH sensor. The Acr+-unit confers to all the derivatives synthesized so far water solubility, contrary to many pH-indicators with uncharged fluorophore that generally show a poor water solubility. Moreover, the Acr+-unit shows particular advantages to be used in sensory systems, such as a high fluorescence quantum yield (ΦF = 0.96),7 which guarantees a strong emission signal and a long fluorescence lifetime (τF = 18.2 ns),7 which allows the detection of relatively slow quenching processes. A further advantage is the high reduction potential of the singlet excited state (E*(S1)red = 2.43 V),8 which makes the PET process, and therefore the fluorescence quenching, thermodynamically and kinetically more favorable. The attractive results presented in this paper suggest that the pH sensor 9-(2-(diethylamine)ethylamino)-10methylacridinium chloride (Acr+-A Cl−) operates in an “off–on” fashion, whereas in the “off” state (amine receptor free from H+) a PET mechanism has been unequivocally demonstrated. The further increase of fluorescence quantum yield in the “on” state (amine receptor linked to H+) observed in the presence of anionic surfactants has evidenced a newsworthy role of micelles.
Results and discussion Acr+-A Cl− was synthesized in good yield from reaction of N,N(diethylethane)-1,2 diamine and 9-chloro-10-methylacridinium chloride salts in turn prepared from the commercially avail-
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Scheme 2
Scheme 1
Chemical structures at various pH values of the sensor.
Synthesis of Acr+-A Cl−.
Fig. 2 Fluorescence emission spectra of Acr+-A (1.5 × 10−5 M) in water recorded at different pH values, λexc = 445 nm. Inset: intensity recorded at 490 nm as a function of pH; the full line is the fitting curve obtained using eqn (1).
Fig. 1 Absorption spectra of Acr+-A (7.5 × 10−5 M) recorded in water at different pH values. Inset: plot of absorbance at 385 and 445 nm vs. pH.
able 10-methyl-9(10H)-acridone and SOCl2 (Scheme 1). The absorption spectra of Acr+-A are essentially unchanged at different pH (2–8) values in water as shown in Fig. 1. Thus, protonated (Acr+-AH+) and unprotonated (Acr+-A) tertiary amine species absorb at the same wavelength, as expected being the structural difference away from the chromophore. In more alkaline solutions ( pH > 8) the absorption bands shift to smaller wavelengths; in particular the visible band shifts hypsochromically by 40 nm (from 445 to 385 nm). This effect is evident from the inset of Fig. 1. The presence of isosbestic points (250, 280, 304, 336 and 400 nm) indicates the coexistence of two species in equilibrium. The hypsochromic shift is explainable on the basis of the less aromatic structure (Acr-H)-A, a derivative of 9-imine10-methylacridane, in equilibrium with Acr+-A (Scheme 2), formed by deprotonation of the 9-amine group in very basic solution, in line with the well-known strong electrophilicity of the 9-position of the acridinium ring.9 The (Acr-H)-A species was characterized by 1H-NMR spectroscopy (see the Experimental section). As the absorption spectra of Acr+-AH+ and Acr+-A are essentially unchanged with pH, the same is observable for the shape and position of the emission spectra, recorded exciting
6678 | Org. Biomol. Chem., 2014, 12, 6677–6683
Fig. 3 Fluorescence emission spectra of Acr+-A (1.5 × 10−5 M) in water recorded at pH 2.91 (solid line), 6.00 (dashed line), 8.84 (dashed and dotted line) and 11.11 (dotted line), λexc = 400 nm.
at 445 nm, where (Acr-H)-A does not absorb (Fig. 2). The excitation at 400 nm (isosbestic point), where all three species absorb (Scheme 2), produced conversely different emission spectra depending on the pH (Fig. 3). In acidic solution, as expected, the emission shape is identical to that observed at λexc = 445 nm, while at alkaline pH a spectral shift to smaller wavelengths and a band structured in two maxima at 430 and 450 nm, attributed to the emission of the less aromatic structure (Acr-H)-A, were observed. At intermediate pH (6.00) the spectrum is clearly resulting from the emission of all the species present in solution.
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In order to avoid the interference of (Acr-H)-A in the investigation of the pH-responsive fluorescence properties of Acr+-A Cl−, the photophysical characterization was performed exciting at 445 nm. Under these conditions, the fluorescence intensity shows an increase upon protonation (Fig. 2). A 17-fold enhancement in fluorescence quantum yield ΦF (from 0.017 to 0.30) was measured. It has to be noted that this value is close to that shown by N,N-diethylaminoethyl bonded to the aniline nitrogen of 4-aminonaphthalimide chromophore.10 The fluorescence intensity (IF) at the maximum wavelength (490 nm) is reported as function of pH ranged from 3 to 12 (inset in Fig. 2, the corresponding relative fluorescence emission plot is reported in Fig. S1†). The emission intensity–pH profile was accurately analyzed according to eqn (1),11 and the obtained acidity constant is pKa = 7.0. This value is significantly smaller than those of the analogous sensor 4-(2-(diethylamine)ethylamino)-naphthalimide ( pKa = 8.7)10 and the parent triethylamine ( pKa = 10.7). The higher basicity of triethylamine with respect to 2-(diethylamine)ethylamino sensors can be explained on the basis of the electron withdrawing aniline nitrogen in the more complex structure, IF ¼
I F ðAcrþ -AHþ Þ þ I F ðAcrþ -AÞ10ðpK a pHÞ 1 þ 10ðpK a pHÞ
ð1Þ
even more effectively by the peculiar positive charge of the acridinium chromophore. The pKa of 7.0 makes Acr+-A Cl− a good candidate as a biologically relevant probe to monitor small pH fluctuations near to neutral physiological values in aqueous solution and in vivo. The switching on/off of fluorescence is indicative of an intramolecular fluorescence quenching by PET. According to this mechanism, one electron of the most basic nitrogen (aliphatic nitrogen) is transferred to the excited amino-acridinium moiety. When the amine is protonated, the fluorescence is restored. The occurrence of an intramolecular PET process from tertiary amine to 1Acr+* has been unequivocally confirmed by flash photolysis investigation. Indeed, upon laser excitation (λexc = 355 nm) of N2-saturated aqueous solutions of Acr+-A Cl−, time-resolved absorption spectra were recorded at pH values of 3.0, 7.6, 9.0 and 13.12 The absorption spectra obtained 1.1 μs after the laser pulse are shown in Fig. 4. It is interesting to note that at pH 3.0, where fluorescence is the predominant deactivation channel of the S1 state of Acr+-AH+, no transient was recorded. At pH 7.6 an absorption centered at 520 nm, assigned to the amino-acridinyl radical moiety, was detected.13 This evidence endorses the PET hypothesis, although the absorption of the resulting amine radical cation was not detected, most likely because of the detectable wavelength region.14 The charge separation could be promoted both by anion-complexation15 and a polar solvent. At higher pH (9.0) an additional band was detected at 560 nm that is likely due to triplet–triplet absorption of (Acr-H)-A because: (i) it was formed within the laser pulse, (ii) it decayed by first order kinetics and (iii) its decay was accelerated by molecular
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Fig. 4 Absorption spectra of Acr+-A (5.5 × 10−4 M) in N2-saturated H2O recorded 1.1 μs after the laser pulse at pH 3.0 (□), 7.6 (●), 9.0 (△) and 13 (▾); λexc = 355 nm.
oxygen through an almost diffusional process (kox = 6.4 × 109 M−1 s−1). This species is the only one visible at very alkaline pH, confirming its formation from (Acr-H)-A. The spectrum at pH 8 recorded 1.1 μs after the laser pulse in a wider wavelength scale (Fig. S3†) shows a broad band from 650 to 900 nm that disappears at pH 12. This absorption can be reasonably attributed to the formation of a π-dimer between the electron transfer (ET) state, Acr•-A•+, and the Acr+-A ground state. A similar broad absorption band has already been observed by Fukuzumi and coworkers with both 9-(1-naphthyl)- and 9-mesityl-10-methylacridinium in MeCN,16 though shifted to greater wavelengths (880–1100 nm), maybe due to the more aromatic system.17 The decays recorded at 510 and 750 nm in N2-saturated solution at pH 8 obey first-order kinetics with similar rate constants (1.2 × 105 s−1 and 1.0 × 105 s−1, respectively), in line with the intramolecular back electron transfer of the ET state (insets of Fig. S3†). These results clearly show that a rapid equilibrium is established between a monomer and a dimer. The long-lived ET state suggests that the thermal back electron transfer (BET) from Acr• to A•+ moieties is slower than the forward photoinduced electron transfer, although both processes are strongly exergonic (the free energy change was determined to be −1.47 eV for the PET and −1.44 eV for the BET).18 This behavior is explainable assuming that the free energy change of BET is in the Marcus inverted region.20 On the basis of this model the BET rate decreases with increasing the thermodynamic driving force in the exergonic region. It should be noted that the PET process from the diethylamine to the 9-amino-10-methylacridinium fluorophore across the dimethylene spacer is rather more exergonic with respect to the system with the amine receptor bonded, by the same spacer, to the aniline nitrogen of 4-aminonaphthalimide (ΔGPET ≈ 0, with triethylamine taken as the model).10,21 This is in agreement with the lower oxidizing power of the latter fluorophore. Really, the difference in ΔGPET between Acr+-A Cl− and the corresponding 4-amino-1,8-naphthalimide-based chemosensor should be smaller, considering that the amino receptor
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in the latter should be more easily oxidizable on the basis of pKa values (see above). Cationic, anionic and nonionic surfactants, such as CTAB (cetyltrimethylammonium bromide), SDS (sodium dodecyl sulfate) and SOBS (sodium octyl benzene sulfonate), SB3-14 (N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate), respectively, above the c.m.c. (8.62 × 10−4, 8.30 × 10−3, 1.42 × 10−2, and 2.88 × 10−4 M for CTAB, SDS, SOBS and SB3-14, respectively)22 have been explored in order to maximize the pH sensing efficiency of Acr+-A Cl−. No significant effect was observed on the spectral properties of the sensor in the presence of cationic and zwitterionic surfactants (Fig. S4 and S5†). Such a behavior is expected as the CTAB headgroups repel both protons and the positive sensor, while the zwitterionic SB3-14 produces low electrostatic affinity. Instead, the addition of SDS and SOBS, though it did not induce appreciable shift of the absorption (Fig. S6 and S7†) and emission maxima, resulted in a substantial enhancement of fluorescence emission in the pH detection as the solutions went from basic to acidic (Fig. 5 and 6).23 From the data of fluorescence emission intensity at 490 nm as a function of pH,
Fig. 5 Fluorescence emission spectra of Acr+-A (1.5 × 10−5 M) in water in the presence of SDS (0.01 M) at different pH; λexc = 445 nm. Inset: intensity recorded at 490 nm as a function of pH; the full line is the fitting curve obtained using eqn (1).
Fig. 6 Fluorescence emission spectra of Acr+-A (1.5 × 10−5 M) in water in the presence of SOBS (0.02 M) at different pH; λexc = 445 nm. Inset: intensity recorded at 490 nm as a function of pH; the full line is the fitting curve obtained using eqn (1).
6680 | Org. Biomol. Chem., 2014, 12, 6677–6683
Organic & Biomolecular Chemistry
a profile similar to that in only water is obtained (insets in Fig. 5 and 6, the corresponding relative fluorescence emission plots are reported in Fig. S8 and S9†), but with a shift of the “off–on” response window to the right along the pH axis. The data correlation by eqn (1) produced pKa of 9.2 and 9.4 for SDS and SOBS,24 respectively, values quite higher than that determined in water (7.0). This apparent increase of the basic strength of the amine moiety reveals that the positive sensor structure, localized on the negatively charged micellar surface in place of the counterion (Na+), is exposed to higher H+ concentration in the micellar interface, producing a “proton sponge effect”. Being the effective pH on the interface lower than that measured in the bulk, there is no evidence of the hypsochromic shift in the absorption spectra in basic solution (Fig. S5 and S6†) due to the deprotonation of the 9-amine group (see above).24 SDS and SOBS micelles not only move the position of the pH response window, but also affect strongly the fluorescence enhancement (68 and 79 with SDS and SOBS, respectively, against 17 determined in water), in terms of quantum yields (ΦF from 0.0088 to 0.60 and 0.0076 to 0.60, respectively), thus enhancing the sensor sensitivity. The almost fourfold enhancement in ΦF value has benefited from the fact that the double protonated sensor (Acr+-AH+) is less free to rotate and vibrate, due to the electrostatic interaction with the negatively charged surface of micelles, causing an increase in fluorescence at the expense of internal conversion.25 Another factor that favors the ΦF increase is the prevention of the otherwise efficient quenching by water, that is, a water-associated non-radiative decay. The interaction of the protonated sensor (Acr+-AH+) with SDS and SOBS in aqueous solution has also been investigated in pre-micellar regions. The two systems, Acr+-AH+/SDS and Acr+-AH+/SOBS, were studied by emission spectroscopy by varying the surfactant concentration. As shown in Fig. 7, the fluorescence intensity decreases with surfactant concentrations up to 2.5 mM and 5 mM for SDS and SOBS, respectively, while it increases up to plateau values when SDS and SOBS concentrations approach the c.m.c. values. Accordingly, the interaction of the sensor with the
Fig. 7 Fluorescence emission intensity of Acr+-A (1.5 × 10−5 M) recorded at 490 nm as a function of [SDS] (□) and [SOBS] (■) at pH 2.8; λexc = 445 nm.
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Organic & Biomolecular Chemistry
surfactant is operating in the pre-micellar region. These data can be rationalized as already reported for the 9-aminoacridinium/SDS system.26 At low surfactant concentrations ([SDS]
