DOI: 10.1002/chem.201504432
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& Metal–Organic Frameworks
A Luminescent Metal–Organic Framework Thermometer with Intrinsic Dual Emission from Organic Lumophores Hao Zhang,[a, b] Chensheng Lin,[a] Tianlu Sheng,[a] Shengmin Hu,[a] Chao Zhuo,[a, b] Ruibiao Fu,[a] Yuehong Wen,[a] Haoran Li,[a, b] Shaodong Su,[a, b] and Xintao Wu*[a] Abstract: A new mixed-ligand metal–organic framework (MOF), ZnATZ-BTB, has been constructed as a luminescent ratiometric thermometer by making use of the intrinsic dual emission at cryogenic temperatures. Its twofold interpenetrated network promotes the Dexter energy transfer (DET) between the mixed organic lumophores. The temperaturedependent luminescent behavior arises from the thermal equilibrium between two separated excited states coupled
Introduction Cryogenic temperature detection plays a crucial role in scientific research and industrial applications such as in the energy industry, superconductivity, transportation, medical technology, aerospace, and so on. Several types of thermometers including thermocouples, thermistors, and optical fiber sensors have been explored for this field. These are nontoxic, easy to use, precise, have a wide analytical range, and signals that can be easily digitized.[1] However, their size—from tens of micrometers to several millimeters—as well as the nature of contact measurement is not suitable for temperature measurements of large-area gradient distributions, fast-moving objects, or at the submicron scale. An array of these sensors may be used to achieve these goals, but this has disadvantages because of complications, high cost, and low spatial resolution. In addition, they also require an electrical link, thus blocking their application in strong electromagnetic fields. Luminescent thermometry techniques, particularly self-calibrated ratiometric methods, provide a promising alternative for the special features of noninvasiveness, observability, high sensitivity, and large-scale imaging.[2]
[a] H. Zhang, Dr. C. Lin, Prof. T. Sheng, Dr. S. Hu, C. Zhuo, Dr. R. Fu, Dr. Y. Wen, H. Li, S. Su, Prof. X. Wu State Key Laboratory of Structure Chemistry Fujian Institute of Research on the Structure of Matter Chinese Academy of Sciences, Fuzhou, Fujian, 350002 (China) E-mail: [email protected] [b] H. Zhang, C. Zhuo, H. Li, S. Su Graduate School of the Chinese Academy of Sciences Beijing 100049 (China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201504432. Chem. Eur. J. 2016, 22, 4460 – 4468
by DET, which is confirmed by Boltzmann distribution fitting. The small excited-state energy gap allows ZnATZ-BTB to measure and visualize cryogenic temperatures (30–130 K) with significantly high relative sensitivity (up to 5.29 % K¢1 at 30 K). Moreover, it is the first example of a ratiometric MOF thermometer the dual emitting sources of which are widely applicable mixed organic ligands, opening up new opportunities for designing such devices.
A variety of luminescent materials have been developed as luminescent ratiometric thermometers, such as small molecules, polymers, metal–organic frameworks (MOFs), and nanoparticles or composites.[3–5] MOFs allow for judicious choice of metal centers and organic linkers of various geometries and functionalities. Therefore, they offer a fascinating platform for the development of solid-state luminescent materials, within which metal nodes, organic linkers, and guest molecules all can potentially generate luminescence. More importantly, their luminescence can be tuned by the mutual interplay/interaction among multiple luminescent centers to develop novel dual emission for ratiometric thermometers. To date, most of MOF thermometers are fabricated by means of a mixed-lanthanide MOF (M’Ln-MOF) approach, featuring different temperature-dependent luminescent behaviors from mixed lanthanide centers within the same material.[5] In these cases, this methodology is limited to the available lanthanide-based emission. The feed ratio of the mixed lanthanides also has a strong influence on the thermosensitivity so that it is an inevitable problem that tedious doping engineering and extra calibration is required before data collection.[5, 6] Moreover, most of M’Ln-MOFs are not suitable for the measurement of cryogenic temperatures at their higher Tm values, at which the relative sensitivity reaches a maximum (Table 1, see below). In addition to the lanthanide-based emission, organic lumophores within MOFs can be another excellent luminescent center, which often differ in their emission behavior upon interlumophore interaction.[7] Our group has qualitatively demonstrated luminescent temperature sensing by using one transition from the ligand-based emission.[8] However, the scarcity of intrinsic dual emitting MOFs might be attributed to Kasha’s rule, which permits only the lowest excited state to emit.[6a] The higher excited states usually undergo thermal relaxation through nonradiative decay pathways under ambient condi-
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Full Paper tions, while cryogenic temperatures can drastically weaken this negative thermodynamic effect to achieve the intrinsic photophysical property. In addition, cryogenic temperatures might be favorable to achieving the dynamic interplay between different lumophores in light of thermodynamic considerations. The intriguing properties of cryogenic temperatures allow us to construct a cryogenic ratiometric thermometer with intrinsic dual emission from organic lumophores. We should also be able to expand the emission range for the variety and complexity of the organic lumophores. Furthermore, a large sensitivity would be obtained on account of the fact that the energy gap between dual emissive systems is usually very small (less than 2000 cm¢1).[3f] The intrinsic dual emitting approach is very appealing to target novel solid-state luminescent thermometers of higher sensitivities.[5a] Herein, we present a novel cryogenic luminescent MOF thermometer in which thermosensitive dual emitting arises from the thermal equilibrium between two separated, competitive excited states of mixed ligands coupled by Dexter energy transfer (DET). As a proof of concept, we select two classic p-conjugated organic lumophores, 1,3,5-tris(4-carboxyphenyl)benzene (BTB) and 5-amino-1-H-tetrazolate (ATZ), as the mixed linkers in the MOF.[9, 10] As a small multidentate co-ligand, ATZ can bring the lumophores close together, enabling electronic interactions (e.g., ligand-to-ligand charge transfer (LLCT), DET) between the lumophores, which is important to obtain tunable emission colors and multiple charge transition for dual emission.[10] Because linker-based luminescence is usually observed in MOFs containing d10 transition metals,[7b] we choose ZnII as the metal center to synthesize a new highly emissive ZnII-MOF {[(CH3)2NH2][Zn(BTB)2/3(ATZ)]·H2O·DMF}n (ZnATZ-BTB). Singlecrystal X-ray crystallography analysis reveals that ZnATZ-BTB possesses a twofold interpenetrated three-dimensional (3D) network, which promotes the DET process between the mixed organic lumophores. It presents purple emission centered at 377 nm from intra-ligand charge transfer (ILCT) of BTB and an appended green-emitting branch around 510 nm mainly from LLCT between ATZ units at cryogenic temperatures. The Boltzmann distribution law can be fitted to their temperaturedependent luminescent emission relationship owing to the thermal equilibrium of mixed linkers-based emitting. More importantly, the luminescent intensity ratios also correlate linearly very well with the temperature in the cryogenic range of 30 K to 130 K with significantly high sensitivity and reproducibility.
Results and Discussion Structural characterization The material ZnATZ-BTB is formed by reacting Zn(NO3)2·6 H2O, BTB, and ATZ (2:1:1 molar ratio) in a DMF/H2O solvent mixture heated at 85 8C for 3 days, which produces colorless rod-like crystals. The formula of ZnATZ-BTB was established based on single-crystal X-ray diffraction studies and elemental microanalysis, and the phase purity of the bulk material was independently confirmed by powder X-ray diffraction (PXRD), Chem. Eur. J. 2016, 22, 4460 – 4468
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thermogravimetric analysis (TGA), and infrared spectra (IR; Figures S1–S3 in the Supporting Information). Single-crystal X-ray crystallography analysis reveals that ZnATZ-BTB possesses a twofold interpenetrated three-dimensional (3D) network that crystallizes in the trigonal R3¯ space group. The asymmetric unit contains one independent ZnII center, two distinct one third deprotonated BTB ligands (separated by a threefold symmetry axis), one deprotonated ATZ ligand, one (CH3)2NH2 + anion, one free water molecule, and one guest DMF molecule (Figure S4, in the Supporting Information). The ZnII cation is four-coordinated to two carboxylic oxygen atoms from two separated BTB ligands and two nitrogen atoms from the same ATZ ligand. Three carboxylate groups of the organic lumophore BTB all take the same monodentate coordination mode, whereas the ATZ ligand takes a m2-1,4 bridging mode to bridge the ZnII atoms, generating one-dimensional (1D) zigzag chains with a neighboring Zn···Zn separation of 6.067(5) æ, which is stabilized by a N¢H···N hydrogen bond (2.824(4) æ and 2.881(4) æ; Figure 1 a and Tables S3 and S4, in the Supporting Information). Consequently, the triangular adjacent 1D zigzag chains are bridged by parallel symmetrically independent BTB linkers along the threefold axis direction, leading to a 3D extended honeycomb-like network (Figure 1 b). There exists a hexangular-shaped channel with approximate width of 21 æ and two distinctive cylinder-like cages (Cage A and Cage B) in the single framework (Figure S5, in the Supporting Information). Owing to the spatial nature of the single net and the planar aromatic nature of BTB, two independent nets, related by a crystallographic inversion symmetry, mutually interpenetrate to each other. Interestingly, the struts within the distinct nets are staggered, locating the centroid of the conjugated p-deficient benzene rings’ faces at a short distance between each other, ranging from 3.452 æ to 3.905 æ, thereby enabling intermolecular face-to-face p–p* stacking interactions along the threefold axis (Figure S6, in the Supporting Information).[11] As a result, the hexangular-shaped channels are filled in a close-packing manner, and the cages from the inter-frameworks form a similar “interlocking” mode (Figure 1 c and Figure S5 b, in the Supporting Information). More importantly, the stability of the whole structure is enhanced and the interlumophore interaction is further strengthened in the doubly interpenetrated framework. In a simplified view, the ZnII centers can be considered as tetrahedral four-connected nodes, whereas the BTB and ATZ ligands can be simplified as triangular three-connecting nodes and a two-connecting spacer, respectively. The (3,4)-connected dinodal net reveals a rare topology with the Schlfli symbol of (62·84)3(63)2, which has only been observed in one polyoxometalate-based compound to date.[12] In practice, cryogenic temperatures are very favorable for the stability of crystalline structures. However, cryogenic application will involve the condensation and localized phase transformation of water, which is, itself, considered to be a severe degrading agent to MOF materials.[13] Remarkably, the PXRD patterns reveal that ZnATZ-BTB retained its crystallinity after being immersed in water for more than 7 days, thus exhibiting an exceptional water-resistance ability that is of great impor-
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Figure 1. a) View of a 1D zigzag chain in ZnATZ-BTB. Dashed lines represent the N¢H···N hydrogen bonds. b) A space-filling version of the 3D single net of ZnATZ-BTB. c) A space-filling version that highlights the interpenetrating nature of the 3D network and the interesting “interlocking” cages. Atoms of different nets are shown in different colors. d) Topological representation of the framework in ZnATZ-BTB.
tance in the field of MOF materials. Further investigations indicate that the sample could also retain its crystallinity in aqueous solutions associated with a broad range of pH values (pH = 3–10). It is worth noting that the slight shifts in the PXRD peaks in the low-angle region might be due to the solvent’s influence.[14] Additionally, the samples reserved for stability investigations have been stable under normal conditions for more than 3 months. (Figure S2, in the Supporting Information) The high structural robustness may be attributed to hydrogen-bonding and p–p* stacking interactions between the ligands in the doubly interpenetrated architecture. All these results strongly suggest the feasibility of using ZnATZ-BTB under cryogenic working conditions, even in a broad variety of applications.
77 K), the above HE intensity decreases weakly, and several emission bands at 477, 510, and 544 nm (lower energy, LE) emerge; their intensity at 10 K is stronger than that at 77 K. This clear redshift (of more than 100 nm) between HE and LE emission exceeds the shift range of vibrational energy levels (less than 50 nm), which indicates that these two emission bands come from two different types of emission species. Figure S8 in the Supporting Information shows the decay photoluminescent (PL) spectra located in the HE emission band (410 nm) and the LE emission band (510 nm), which also give evidence of the intrinsic difference between these two emission bands. The measured decay lifetimes are both consistent with the nanosecond scale of the fluorescence associated with the singlet de-excitation S1!S0, which precludes the possibility that the two emission species come from the spin-forbidden triplet phosphorescence T1!S0. However, the LE decay shows a longer decay lifetime than the HE decay. We speculate that the nonradiative charge transition process populates the excited state and generates the observed rise time and the lengthening of the temporal decay. In some cases, the structure of a MOF is temperature dependent, which might cause the emission change.[15] Under a cold nitrogen gas stream from a cryostat (close to 77 K), the additional crystallographic data was collected, but this found to show no essential changes compared with that at room temperature (Tables S1–S4, in the Supporting Information). To further interpret the origin of the dual emission system, time-dependent (TD)-DFT calculations were performed with the optimized geometries of the asymmetric unit, in which uncoordinated sites were substituted by hydrogen at the level of B3LYP/LanL2DZ for Zn and 6-31G* for the other atoms. Figure 3 a shows the electron flow under 373 nm excitation and Figure 3 b corresponds to excitation at 453 nm. Based on the orbital analysis, the former transition could be attributed to ILCT of the BTB ligand, which is in very good accord with the experimental diffuse reflectance UV/Vis spectra (Figure S9, in the Supporting Information). Although we cannot obtain the corresponding experimental spectra at cryo-
Photophysical properties The most remarkable feature of ZnATZ-BTB is its temperaturedependent emission behavior under near-UV excitation (Figure 2). The solid-state emission spectrum at room temperature displays an intense emission band centered at 377 nm (higher energy, HE) with an excitation maximum at 330 nm. Whereas the free ATZ ligand presents a weak photoluminescence emission at 340 nm in accordance with previous reports,[10] the excitation and emission features of ZnATZ-BTB closely match that of the organic ligand BTB (Figure S7, in the Supporting Information). Thus, this peak coincides with an intra-ligand-based emission (ILCT) and may be attributed to the cascade p–p* interactions occurring between the aromatic rings of the BTB linker. The luminescent quantum yield was determined by means of an integrating sphere and was found to be 22.11 %. Interestingly, at lower temperatures (10 K and Chem. Eur. J. 2016, 22, 4460 – 4468
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Figure 2. Relative emission spectra of ZnATZ-BTB under excitation at 330 nm in the solid state at 10, 77, and 298 K. Inset: relative excitation spectra of ZnATZ-BTB (monitored at 377 nm).
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Figure 4. Excitation and emission spectra of ZnATZ-BTB at 10 K.
Figure 3. The deformation of charge densities of the asymmetric unit of ZnATZ-BTB in the predicted a) 373 nm, and b) 453 nm excitations. The purple surfaces indicate regions that have gained charge with respect to the ground state, and the blue surfaces indicate charge depletion. The isosurface value is 0.002 e æ¢3.
genic temperatures, the excitation spectra monitored at 510 nm at 10 K validates that the new LE emission bands at cryogenic temperature should be assigned to LLCT related to the ATZ ligands in the doubly interpenetrated framework (Figure 4).[10] As illustrated in the preliminary study (Figure S10, in the Supporting Information), the LE emission becomes progressively more clear only when the temperature decreases below 130 K. Considering that the thermodynamic effect becomes less efficient at low temperature, the thermal activation of nonradiative decay pathways may be responsible for the extinction of the LE emission at more than 130 K. Thus, we investigated the temperature-dependent PL properties ranging from 10 K to 130 K in terms of both intensity and lifetime to establish its potential as a luminescent thermometer. The integrated intensities in the specific spectral ranges (350–460 nm and 460– 650 nm for the HE and LE emission, respectively) as a function of temperature are presented in Figure 5 a. The absorption band is not temperature sensitive and only shows very few changes over the whole temperature range (Figure 5 b). In contrast, the HE emission shows weak enhancement, whereas the Chem. Eur. J. 2016, 22, 4460 – 4468
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intensity of the LE emission decreases gradually as the temperature increases. Additionally, the measured decay lifetimes do not depend on the temperature in the range 10–130 K, showing some interesting and atypical properties compared with reported M’Ln-MOFs.[5] As shown in Figure 4, the overlap between the LE excitation spectra and the HE emission spectra suggests that the intensive HE emission could be re-absorbed by the LE ground state. Furthermore, the interpenetrating behavior in ZnATZ-BTB can shorten the distance between organic lumophores (less than 5 æ) to favor the overlap of the electron cloud, which indicates that the excited donor and ground-state accepter should be close enough so that electron exchange could happen. The short-ranged electron exchange between dual emissive systems is usually described by the Dexter energy transfer (DET) mechanism (Figure 6 a).[16] The DET mechanism can be also confirmed by the temperature-independent decay lifetimes. Usually, the fluorescent decay time mainly depends on the energy transfer and the thermal activation of nonradiative decay pathways.[5g] However, both fluorescence and DET processes generally happen fast within the timescale of nanoseconds, which means an exchanged electron would immediately fluoresce before it proceeds to thermal activation. Hence, the temperature has no effect on the PL delay time of either the HE and LE emission under cryogenic conditions. Additionally, the longer delay of LE emission can be interpreted by the fact that it spends more time occupying the excited state for the nonradiative DET process. Similar to the reported dual emissive systems, the luminescent thermochromism may originate from the dynamic interplay between the dual emissive sources.[3–5] There exists an effective electron exchange (DET) between two separate excited states of dual emission. In other words, the separated excited states of HE and LE emission are coupled by the DET process. Thus, we may attribute temperature-dependent luminescent behavior to the thermal equilibrium between the two separated excited states of dual emission, the relative population of which is governed by the Boltzmann distribution law.
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Figure 5. a) Emission, and b) excitation spectra of ZnATZ-BTB recorded between 10 and 130 K excited at 330 nm. Inset: the temperature-dependent integrated intensity of the HE and LE transition for ZnATZ-BTB. c) The temperature-independent decay lifetimes of the HE and LE transitions for ZnATZ-BTB.
The intensity ratio–temperature relationship is given by Equation (1):[5a, 17] IHE DE ¼ B exp ¢ ILE kB T
ð1Þ
where IHE and ILE are the two luminescence intensities of the dual emission, B is a constant, DE is the effective energy gap between the two separate excited states, kB is the Boltzmann constant, and T is the absolute temperature. Figure 6 b shows that the numerical fitting gives the relationship between the intensity ratio and the absolute temperature is as follows [Eq. (2)]: IHE 88:261 ¼ 20:420 exp ¢ ILE T
ð2Þ
This suggests that the energy gap is calculated to be DE(cal) = 88.261 kB = 61 cm¢1, which is within the thermal coupling range (less than 2000 cm¢1).[3f] If we assume that the irradiative transition of the fluorescent emission is from the separated exChem. Eur. J. 2016, 22, 4460 – 4468
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cited states to the same ground state, we can calculate the wavelength of LE emission (lLE) to be: lLE ðcalÞ ¼ lHE ðobsÞ þ
hc ¼ 540 nm DEðcalÞ
ð3Þ
where lHE(obs) is the observed wavelength of HE emission, h is the Planck constant, and c is the speed of light. The calculated result is in good agreement with the experimental value lLE(obs) = 544 nm (Figure 2). The observed vibrational character of the LE emission band can be understood by the different ground states of LLCT related to ATZ ligands. Therefore, all the results verify our speculation of the thermal equilibrium between two separated excited states of dual emissions coupled by the DET process. The simulated line fits well with the experimental data in the temperature range 40–130 K. However, it should be noted that there is a clear deviance between the measured data and the fitted curve line when the temperature is lower than 40 K. The reason for this deviance is the spectral overlap between the HE emission band and the LE emission band originating from the small energy gap. Generally, the separation of two
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Figure 6. a) The Dexter energy transfer (DET) mechanism in a dual emissive system. b) The temperature-dependent intensity ratios of the HE and LE transitions for ZnATZ-BTB.
separated excited states requires more than 200 cm¢1 to avoid substantial overlap of the two fluorescent wavelengths.[3f, 17a] It is clear that the LE broad band become closer and closer to the HE band from the rapid growth of its intensity with decreasing temperature. Thus, the degree of spectral overlap in the lower temperature range from 40 K to 10 K cannot be neglected for the constituent mixed in the HE band makes the intensity of LE emission increase slower than would be the case if the overlap did not exist. In other words, the existence of spectral overlap makes the fitted value of the Boltzmann distribution less reliable (correlation coefficient R2 = 0.9652) in the lower temperature range (10–40 K).
Ratiometric thermometer To further improve the potential use of ratiometric luminescent MOF thermometers, we find that there is a very good linear relationship between the intensity ratio and temperature from 30 K to 130 K, which can be linearly fitted as a function of T [Eq. (4)]: I T ¼ 10:80 HE þ 11:52 ILE Chem. Eur. J. 2016, 22, 4460 – 4468
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Figure 7. a) The linear-fitted line for the temperature-dependent intensity ratio of the HE and LE transitions in ZnATZ-BTB. b) CIE chromaticity diagram showing the temperature-dependent luminescence color change of ZnATZBTB in the cryogenic temperature range (10–130 K).
with a correlation coefficient R2 = 0.9956 (Figure 7 a). This suggests that the linear fitting method is not only more reliable than the above Boltzmann fitting method, but also extends the applicable temperature range. In addition, the linear fitting method, without complex calculation, is more convenient for practical applications. The changing rate of intensity ratio versus temperature, known as sensitivity, is of great importance in thermal sensing applications. Generally, the absolute sensitivity (Sab) of luminescent thermometers can be defined as the intensity ratio change with temperature by Equation (5): Sab ¼
@ ðIHE =ILE Þ @T
ð5Þ
According to this equation, the absolute sensitivity clearly depends on the magnitude of the temperature-dependent luminescent variations. Thus, the thermometer that shows large
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Full Paper intensity ratio changes with small changes in temperature will possess a high absolute sensitivity. However, it is meaningless to quantitatively compare the absolute sensitivity among thermometers that operate by different mechanisms or that are based on different material systems.[3g] To compare the performances of the different luminescent thermometers, the relative sensitivity (Sre) is usually utilized and is defined as in Equation (6):[5a] Sre ¼
@ðIHE =ILE Þ=@T IHE =ILE
ð6Þ
Following these definitions, the absolute sensitivity (Sab) of ZnATZ-BTB is 0.0926 K¢1, whereas the maximum relative sensitivity (Sm) is determined to be 5.29 % K¢1 at 30 K.[5a] Additionally, both the sensitivity of ZnATZ-BTB and that of other reported types of ratiometric luminescent MOF thermometers are compared in Table 1, which indicates a significantly enhanced sensitivity compared with the other M’Ln-MOF thermometers. Therefore, selecting organic lumophores as ratiometric dual emissive sources is an efficient strategy to obtain higher sensitivity. In addition, the temperature (Tm) at which the relative sensitivity reaches its maximum is located in the cryogenic temperature region (30 K), indicating its high availability for the cryogenic temperature range. Most M’Ln-MOFs exhibit higher Tm values, which makes them insensitive to cryogenic temperatures. Although this effective measuring range of ZnATZ-BTB is narrow, only available in the cryogenic temperature range (30–130 K), such high sensitivity has rarely been reported. In fact, the relative sensitivity is inversely related to the temperature sensing range, and thus significantly high sensitivity results in a thermometer with a poor temperature range.[5a] In addition, the HE and LE emission intensities at different temperatures remain almost unchanged for each cycle, demonstrating the excellent reversibility of this system (Figure S11, in the Supporting Information). Above all, ZnATZ-BTB is an excellent luminescent thermometer that is especially applicable in the cryogenic temperature range (30–130 K). Although the ratiometric method of thermosensitive dual emission is sufficiently sensitive for practical temperature detection, the broad emission band of the organic lumophores as well as the spectral overlap means there are some difficulties in accurately identifying the emission intensity. To facilitate the application of this system, the temperature-dependent spectra have been transformed into the Commission Internationale de L’Eclairage (CIE) 1931 coordinates. Figure 7 b shows the color change of the luminescence from (0.2369, 0.3827) at 10 K to (0.2201, 0.1699) at 130 K. This temperature-dependent emission color range is due to the variation of the population of the two separated excited states. Compared with the prebuilt temperature-dependent CIE chromaticity standard diagram, the luminescence color change allows ZnATZ-BTB to act as a sensitive luminescent colorimetric thermometer for in situ visualization of the temperature change instantly and straightforwardly in the cryogenic temperature range. This scope of the temperature range from 30 K to 130 K makes ZnATZ-BTB useful for temperature detection in areas such as superconducChem. Eur. J. 2016, 22, 4460 – 4468
Table 1. Comparison of sensitivities of other reported ratiometric luminescent MOF thermometers with ours, including working ranges (Range, K), absolute sensitivity (Sab, % K¢1), maximum relative sensitivity values (Sm, % K¢1), and the temperature at which Sm is maximum (Tm, K).
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Luminescent MOF[a]
Range [K]
Sab [% K¢1]
Sm [% K¢1]
Tm [K]
ZnATZ-BTB Eu0.0069Tb0.9931-DMBDC Eu0.0046Tb0.9954-DMBDC Eu0.0011Tb0.9989-DMBDC Tb0.9Eu0.1PIA Tb0.99Eu0.01PIA Tb0.95Eu0.05PIA Tb0.50Eu0.50PIA Tb0.98Eu0.02-OA-DSTP Tb0.98Eu0.02-BDC-DSTP Tb0.957Eu0.043-cpda Eu0.7Tb0.3-cam-Himdc Eu0.02Gd0.98-DSB Tb0.99Eu0.01(BDC)1.5(H2O)2 Eu0.0616Tb0.9382pcdmb Eu0.5Tb99.5@In(OH)bpydc ZJU-88perylene
30–130 50–200 50–200 50–200 100–300 100–300 100–250 75–275 77–275 77–225 40–300 100–450 20–300 290–320 25–200 283–333 293–353
0.0926 0.0038 – – 0.0353 0.2 0.06 – 0.0047 0.007 0.0037 0.0008 – 0.0014 0.0056 0.0497 0.0128
5.29 1.15 0.61 0.52 3.27 2.75 2.48 2.02 2.40 2.75 16.0 0.11 4.75 0.31 0.34 2.53 1.28
30 200 200 200 300 300 250 275 275 225 300 450 20 318 200 333 293
[a] See reference [5a] and references therein.
tivity, space exploration, materials research, and other fields that need accurate cryogenic temperature detection.
Conclusions We have proposed a conceptually different approach for luminescent MOF thermometers by making use of the widely applicable mixed-ligands method. The intrinsic photophysical property of cryogenic temperature can be taken advantage of to fabricate a dual emissive system arising from the DET process between mixed organic lumophores. The intensity ratio of the dual emission serves as a self-calibrated signal and can be fitted by Boltzmann distribution, indicating that the temperature-dependent luminescent behavior is the consequence of a thermal equilibrium between two separated excited states coupled by the DET process. Owing to the excellent linear correlation between temperature and luminescence intensity ratio from 30 to 130 K, this luminescent MOF thermometer provides new insights into the fabrication of cryogenic temperature sensors. The intrinsic dual emission can circumvent effectively tedious doping engineering and sensitivity variation problems that exist with mixed Ln-MOFs. In addition, the novel MOF thermometer shows significantly higher sensitivity than the previously developed MOF ratiometric thermometers. Furthermore, the remarkable structural variety and tunability of organic lumophores not only afford the potential of more diverse emission ranges, but can also be used to adjust and optimize the energy gap to obtain higher sensitivity and robustness. It should be noted that achieving control over the spatial localization of the self-assembly sites is a challenging task for device fabrication, which remains a major scientific goal for the development of MOF-based technology.[18]
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Full Paper Experimental Section Materials and methods All reagents and solvents for the syntheses were purchased from commercial sources and used as received. All measurements except for the elemental analyses were carried out in air at room temperature. Elemental analyses (C, H, and N) were performed on an Elementar Vario MICRO CHNOS elemental analyzer. The infrared spectra (IR) with KBr pellets were recorded in the range 4000– 400 cm¢1 on a PerkinElmer Spectrum One FTIR spectrometer. Thermogravimetric analyses (TGA) were performed on a TGA/DSC 1 STARe system from room temperature to 1000 8C with a heating rate of 10 K min¢1 under nitrogen. Powder X-ray diffraction (PXRD) data were collected on a Rigaku Desktop MiniFlexII diffractometer using CuKa radiation (l = 1.54056 æ) powered at 30 kV and 15 mA. The simulated pattern was produced by using the Mercury Version 1.4 software and single-crystal reflection diffraction data. Solidstate UV/Vis spectra were measured on a PerkinElmer Lambda9500 UV/Vis/NIR spectrophotometer equipped with an integrating sphere, and by using dry BaSO4 as a ‘background’ matrix. The solid-state luminescence emission/excitation spectra were recorded on an Edinburgh Instrument FLS920 fluorescence spectrophotometer equipped with a continuous Xe-900 xenon lamp and a F900 microsecond flash lamp. For low-temperature measurements, microcrystal samples were mounted on a closed cycle helium cryostat. The temperature-dependent emission spectra were recorded by the above FLS920 fluorescence spectrometer with a PolyScience Temperature Control Solution PD07R-20. The luminescence decay curve was measured by using a 397 nm OPO laser as the light source. The overall quantum yields of the solid-state samples were determined by an absolute method using an integrating sphere (150 mm diameter, BaSO4 coating) on the above FLS920 spectrometer.
mation. Select bonds and angles are list in Table S2, and those of the hydrogen bonds are listed in Tables S3 and S4 (in the Supporting Information). CCDC 1423556 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.
Theoretical calculations All DFT calculations were performed with the Gaussian 03 program package. The TD-DFT electronic excitation calculations were performed by using the optimized geometries of the asymmetric unit in which uncoordinated sites were substituted by hydrogen at the level of B3LYP/LanL2DZ for Zn and 6-31G* for other atoms.[20]
Acknowledgments The authors gratefully acknowledge financial support from the 973 Program (2012CB821702, 2014CB845603), the National Natural Science Foundation of China (21233009 and 21203194), and Fujian Province (2013J05039). Keywords: cryogenic temperatures · energy fluorescence · metal–organic frameworks · thermometer
Synthesis of {[(CH3)2NH2][Zn(BTB)2/3(ATZ)]·H2O·DMF}n (ZnATZ-BTB) A mixture of Zn(NO3)2·6 H2O (150 mg, 0.5 mmol), H3BTB (109 mg, 0.25 mmol), HATZ (21 mg, 0.25 mmol), and 16.5 mL DMF/H2O (DMF = N,N-dimethylformamide, v/v = 10:1) was sealed in a screw cap vial. Ten drops of 50 % HBF4 were added, then the solution was heated and kept at 85 8C for 3 days. It was slowly cooled to 30 8C at about 5 8C h¢1 to give colorless rod-like crystals, which were obtained by filtration and washed with DMF before they were used for the X-ray diffraction determination. Yield: 64 % based on HATZ; elemental analysis calcd (%) for C24H29N7O6Zn (576.91): C 49.96, H 5.07, N 16.99; found: C 49.89, H 5.14, N 16.80.
X-ray crystallographic study Data collection for ZnATZ-BTB was performed on a Rigaku Saturn 724 HG CCD diffractometer equipped with graphite-monochromated MoKa radiation (l = 0.71073 æ) at 77 K and 293 K. The intensity was corrected for Lorentz and polarization effects as well as for empirical absorption based on the multiscan technique. The structures were solved by direct methods and refined by full-matrix least-squares on F2 by using the SHELX-97 program.[19] All non-hydrogen atoms were refined anisotropically. Hydrogen atoms bound to carbon atoms were generated geometrically and were fixed at calculated positions and refined by using a riding mode, and those bound to oxygen and nitrogen atoms were determined by difference Fourier maps. The crystallographic data and structure refinement details are summarized in Table S1 in the Supporting InforChem. Eur. J. 2016, 22, 4460 – 4468
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Received: November 4, 2015 Published online on February 11, 2016
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& Metal–Organic Frameworks
A Luminescent Metal–Organic Framework Thermometer with Intrinsic Dual Emission from Organic Lumophores Hao Zhang,[a, b] Chensheng Lin,[a] Tianlu Sheng,[a] Shengmin Hu,[a] Chao Zhuo,[a, b] Ruibiao Fu,[a] Yuehong Wen,[a] Haoran Li,[a, b] Shaodong Su,[a, b] and Xintao Wu*[a] Abstract: A new mixed-ligand metal–organic framework (MOF), ZnATZ-BTB, has been constructed as a luminescent ratiometric thermometer by making use of the intrinsic dual emission at cryogenic temperatures. Its twofold interpenetrated network promotes the Dexter energy transfer (DET) between the mixed organic lumophores. The temperaturedependent luminescent behavior arises from the thermal equilibrium between two separated excited states coupled
Introduction Cryogenic temperature detection plays a crucial role in scientific research and industrial applications such as in the energy industry, superconductivity, transportation, medical technology, aerospace, and so on. Several types of thermometers including thermocouples, thermistors, and optical fiber sensors have been explored for this field. These are nontoxic, easy to use, precise, have a wide analytical range, and signals that can be easily digitized.[1] However, their size—from tens of micrometers to several millimeters—as well as the nature of contact measurement is not suitable for temperature measurements of large-area gradient distributions, fast-moving objects, or at the submicron scale. An array of these sensors may be used to achieve these goals, but this has disadvantages because of complications, high cost, and low spatial resolution. In addition, they also require an electrical link, thus blocking their application in strong electromagnetic fields. Luminescent thermometry techniques, particularly self-calibrated ratiometric methods, provide a promising alternative for the special features of noninvasiveness, observability, high sensitivity, and large-scale imaging.[2]
[a] H. Zhang, Dr. C. Lin, Prof. T. Sheng, Dr. S. Hu, C. Zhuo, Dr. R. Fu, Dr. Y. Wen, H. Li, S. Su, Prof. X. Wu State Key Laboratory of Structure Chemistry Fujian Institute of Research on the Structure of Matter Chinese Academy of Sciences, Fuzhou, Fujian, 350002 (China) E-mail: [email protected] [b] H. Zhang, C. Zhuo, H. Li, S. Su Graduate School of the Chinese Academy of Sciences Beijing 100049 (China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201504432. Chem. Eur. J. 2016, 22, 4460 – 4468
by DET, which is confirmed by Boltzmann distribution fitting. The small excited-state energy gap allows ZnATZ-BTB to measure and visualize cryogenic temperatures (30–130 K) with significantly high relative sensitivity (up to 5.29 % K¢1 at 30 K). Moreover, it is the first example of a ratiometric MOF thermometer the dual emitting sources of which are widely applicable mixed organic ligands, opening up new opportunities for designing such devices.
A variety of luminescent materials have been developed as luminescent ratiometric thermometers, such as small molecules, polymers, metal–organic frameworks (MOFs), and nanoparticles or composites.[3–5] MOFs allow for judicious choice of metal centers and organic linkers of various geometries and functionalities. Therefore, they offer a fascinating platform for the development of solid-state luminescent materials, within which metal nodes, organic linkers, and guest molecules all can potentially generate luminescence. More importantly, their luminescence can be tuned by the mutual interplay/interaction among multiple luminescent centers to develop novel dual emission for ratiometric thermometers. To date, most of MOF thermometers are fabricated by means of a mixed-lanthanide MOF (M’Ln-MOF) approach, featuring different temperature-dependent luminescent behaviors from mixed lanthanide centers within the same material.[5] In these cases, this methodology is limited to the available lanthanide-based emission. The feed ratio of the mixed lanthanides also has a strong influence on the thermosensitivity so that it is an inevitable problem that tedious doping engineering and extra calibration is required before data collection.[5, 6] Moreover, most of M’Ln-MOFs are not suitable for the measurement of cryogenic temperatures at their higher Tm values, at which the relative sensitivity reaches a maximum (Table 1, see below). In addition to the lanthanide-based emission, organic lumophores within MOFs can be another excellent luminescent center, which often differ in their emission behavior upon interlumophore interaction.[7] Our group has qualitatively demonstrated luminescent temperature sensing by using one transition from the ligand-based emission.[8] However, the scarcity of intrinsic dual emitting MOFs might be attributed to Kasha’s rule, which permits only the lowest excited state to emit.[6a] The higher excited states usually undergo thermal relaxation through nonradiative decay pathways under ambient condi-
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Full Paper tions, while cryogenic temperatures can drastically weaken this negative thermodynamic effect to achieve the intrinsic photophysical property. In addition, cryogenic temperatures might be favorable to achieving the dynamic interplay between different lumophores in light of thermodynamic considerations. The intriguing properties of cryogenic temperatures allow us to construct a cryogenic ratiometric thermometer with intrinsic dual emission from organic lumophores. We should also be able to expand the emission range for the variety and complexity of the organic lumophores. Furthermore, a large sensitivity would be obtained on account of the fact that the energy gap between dual emissive systems is usually very small (less than 2000 cm¢1).[3f] The intrinsic dual emitting approach is very appealing to target novel solid-state luminescent thermometers of higher sensitivities.[5a] Herein, we present a novel cryogenic luminescent MOF thermometer in which thermosensitive dual emitting arises from the thermal equilibrium between two separated, competitive excited states of mixed ligands coupled by Dexter energy transfer (DET). As a proof of concept, we select two classic p-conjugated organic lumophores, 1,3,5-tris(4-carboxyphenyl)benzene (BTB) and 5-amino-1-H-tetrazolate (ATZ), as the mixed linkers in the MOF.[9, 10] As a small multidentate co-ligand, ATZ can bring the lumophores close together, enabling electronic interactions (e.g., ligand-to-ligand charge transfer (LLCT), DET) between the lumophores, which is important to obtain tunable emission colors and multiple charge transition for dual emission.[10] Because linker-based luminescence is usually observed in MOFs containing d10 transition metals,[7b] we choose ZnII as the metal center to synthesize a new highly emissive ZnII-MOF {[(CH3)2NH2][Zn(BTB)2/3(ATZ)]·H2O·DMF}n (ZnATZ-BTB). Singlecrystal X-ray crystallography analysis reveals that ZnATZ-BTB possesses a twofold interpenetrated three-dimensional (3D) network, which promotes the DET process between the mixed organic lumophores. It presents purple emission centered at 377 nm from intra-ligand charge transfer (ILCT) of BTB and an appended green-emitting branch around 510 nm mainly from LLCT between ATZ units at cryogenic temperatures. The Boltzmann distribution law can be fitted to their temperaturedependent luminescent emission relationship owing to the thermal equilibrium of mixed linkers-based emitting. More importantly, the luminescent intensity ratios also correlate linearly very well with the temperature in the cryogenic range of 30 K to 130 K with significantly high sensitivity and reproducibility.
Results and Discussion Structural characterization The material ZnATZ-BTB is formed by reacting Zn(NO3)2·6 H2O, BTB, and ATZ (2:1:1 molar ratio) in a DMF/H2O solvent mixture heated at 85 8C for 3 days, which produces colorless rod-like crystals. The formula of ZnATZ-BTB was established based on single-crystal X-ray diffraction studies and elemental microanalysis, and the phase purity of the bulk material was independently confirmed by powder X-ray diffraction (PXRD), Chem. Eur. J. 2016, 22, 4460 – 4468
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thermogravimetric analysis (TGA), and infrared spectra (IR; Figures S1–S3 in the Supporting Information). Single-crystal X-ray crystallography analysis reveals that ZnATZ-BTB possesses a twofold interpenetrated three-dimensional (3D) network that crystallizes in the trigonal R3¯ space group. The asymmetric unit contains one independent ZnII center, two distinct one third deprotonated BTB ligands (separated by a threefold symmetry axis), one deprotonated ATZ ligand, one (CH3)2NH2 + anion, one free water molecule, and one guest DMF molecule (Figure S4, in the Supporting Information). The ZnII cation is four-coordinated to two carboxylic oxygen atoms from two separated BTB ligands and two nitrogen atoms from the same ATZ ligand. Three carboxylate groups of the organic lumophore BTB all take the same monodentate coordination mode, whereas the ATZ ligand takes a m2-1,4 bridging mode to bridge the ZnII atoms, generating one-dimensional (1D) zigzag chains with a neighboring Zn···Zn separation of 6.067(5) æ, which is stabilized by a N¢H···N hydrogen bond (2.824(4) æ and 2.881(4) æ; Figure 1 a and Tables S3 and S4, in the Supporting Information). Consequently, the triangular adjacent 1D zigzag chains are bridged by parallel symmetrically independent BTB linkers along the threefold axis direction, leading to a 3D extended honeycomb-like network (Figure 1 b). There exists a hexangular-shaped channel with approximate width of 21 æ and two distinctive cylinder-like cages (Cage A and Cage B) in the single framework (Figure S5, in the Supporting Information). Owing to the spatial nature of the single net and the planar aromatic nature of BTB, two independent nets, related by a crystallographic inversion symmetry, mutually interpenetrate to each other. Interestingly, the struts within the distinct nets are staggered, locating the centroid of the conjugated p-deficient benzene rings’ faces at a short distance between each other, ranging from 3.452 æ to 3.905 æ, thereby enabling intermolecular face-to-face p–p* stacking interactions along the threefold axis (Figure S6, in the Supporting Information).[11] As a result, the hexangular-shaped channels are filled in a close-packing manner, and the cages from the inter-frameworks form a similar “interlocking” mode (Figure 1 c and Figure S5 b, in the Supporting Information). More importantly, the stability of the whole structure is enhanced and the interlumophore interaction is further strengthened in the doubly interpenetrated framework. In a simplified view, the ZnII centers can be considered as tetrahedral four-connected nodes, whereas the BTB and ATZ ligands can be simplified as triangular three-connecting nodes and a two-connecting spacer, respectively. The (3,4)-connected dinodal net reveals a rare topology with the Schlfli symbol of (62·84)3(63)2, which has only been observed in one polyoxometalate-based compound to date.[12] In practice, cryogenic temperatures are very favorable for the stability of crystalline structures. However, cryogenic application will involve the condensation and localized phase transformation of water, which is, itself, considered to be a severe degrading agent to MOF materials.[13] Remarkably, the PXRD patterns reveal that ZnATZ-BTB retained its crystallinity after being immersed in water for more than 7 days, thus exhibiting an exceptional water-resistance ability that is of great impor-
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Figure 1. a) View of a 1D zigzag chain in ZnATZ-BTB. Dashed lines represent the N¢H···N hydrogen bonds. b) A space-filling version of the 3D single net of ZnATZ-BTB. c) A space-filling version that highlights the interpenetrating nature of the 3D network and the interesting “interlocking” cages. Atoms of different nets are shown in different colors. d) Topological representation of the framework in ZnATZ-BTB.
tance in the field of MOF materials. Further investigations indicate that the sample could also retain its crystallinity in aqueous solutions associated with a broad range of pH values (pH = 3–10). It is worth noting that the slight shifts in the PXRD peaks in the low-angle region might be due to the solvent’s influence.[14] Additionally, the samples reserved for stability investigations have been stable under normal conditions for more than 3 months. (Figure S2, in the Supporting Information) The high structural robustness may be attributed to hydrogen-bonding and p–p* stacking interactions between the ligands in the doubly interpenetrated architecture. All these results strongly suggest the feasibility of using ZnATZ-BTB under cryogenic working conditions, even in a broad variety of applications.
77 K), the above HE intensity decreases weakly, and several emission bands at 477, 510, and 544 nm (lower energy, LE) emerge; their intensity at 10 K is stronger than that at 77 K. This clear redshift (of more than 100 nm) between HE and LE emission exceeds the shift range of vibrational energy levels (less than 50 nm), which indicates that these two emission bands come from two different types of emission species. Figure S8 in the Supporting Information shows the decay photoluminescent (PL) spectra located in the HE emission band (410 nm) and the LE emission band (510 nm), which also give evidence of the intrinsic difference between these two emission bands. The measured decay lifetimes are both consistent with the nanosecond scale of the fluorescence associated with the singlet de-excitation S1!S0, which precludes the possibility that the two emission species come from the spin-forbidden triplet phosphorescence T1!S0. However, the LE decay shows a longer decay lifetime than the HE decay. We speculate that the nonradiative charge transition process populates the excited state and generates the observed rise time and the lengthening of the temporal decay. In some cases, the structure of a MOF is temperature dependent, which might cause the emission change.[15] Under a cold nitrogen gas stream from a cryostat (close to 77 K), the additional crystallographic data was collected, but this found to show no essential changes compared with that at room temperature (Tables S1–S4, in the Supporting Information). To further interpret the origin of the dual emission system, time-dependent (TD)-DFT calculations were performed with the optimized geometries of the asymmetric unit, in which uncoordinated sites were substituted by hydrogen at the level of B3LYP/LanL2DZ for Zn and 6-31G* for the other atoms. Figure 3 a shows the electron flow under 373 nm excitation and Figure 3 b corresponds to excitation at 453 nm. Based on the orbital analysis, the former transition could be attributed to ILCT of the BTB ligand, which is in very good accord with the experimental diffuse reflectance UV/Vis spectra (Figure S9, in the Supporting Information). Although we cannot obtain the corresponding experimental spectra at cryo-
Photophysical properties The most remarkable feature of ZnATZ-BTB is its temperaturedependent emission behavior under near-UV excitation (Figure 2). The solid-state emission spectrum at room temperature displays an intense emission band centered at 377 nm (higher energy, HE) with an excitation maximum at 330 nm. Whereas the free ATZ ligand presents a weak photoluminescence emission at 340 nm in accordance with previous reports,[10] the excitation and emission features of ZnATZ-BTB closely match that of the organic ligand BTB (Figure S7, in the Supporting Information). Thus, this peak coincides with an intra-ligand-based emission (ILCT) and may be attributed to the cascade p–p* interactions occurring between the aromatic rings of the BTB linker. The luminescent quantum yield was determined by means of an integrating sphere and was found to be 22.11 %. Interestingly, at lower temperatures (10 K and Chem. Eur. J. 2016, 22, 4460 – 4468
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Figure 2. Relative emission spectra of ZnATZ-BTB under excitation at 330 nm in the solid state at 10, 77, and 298 K. Inset: relative excitation spectra of ZnATZ-BTB (monitored at 377 nm).
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Figure 4. Excitation and emission spectra of ZnATZ-BTB at 10 K.
Figure 3. The deformation of charge densities of the asymmetric unit of ZnATZ-BTB in the predicted a) 373 nm, and b) 453 nm excitations. The purple surfaces indicate regions that have gained charge with respect to the ground state, and the blue surfaces indicate charge depletion. The isosurface value is 0.002 e æ¢3.
genic temperatures, the excitation spectra monitored at 510 nm at 10 K validates that the new LE emission bands at cryogenic temperature should be assigned to LLCT related to the ATZ ligands in the doubly interpenetrated framework (Figure 4).[10] As illustrated in the preliminary study (Figure S10, in the Supporting Information), the LE emission becomes progressively more clear only when the temperature decreases below 130 K. Considering that the thermodynamic effect becomes less efficient at low temperature, the thermal activation of nonradiative decay pathways may be responsible for the extinction of the LE emission at more than 130 K. Thus, we investigated the temperature-dependent PL properties ranging from 10 K to 130 K in terms of both intensity and lifetime to establish its potential as a luminescent thermometer. The integrated intensities in the specific spectral ranges (350–460 nm and 460– 650 nm for the HE and LE emission, respectively) as a function of temperature are presented in Figure 5 a. The absorption band is not temperature sensitive and only shows very few changes over the whole temperature range (Figure 5 b). In contrast, the HE emission shows weak enhancement, whereas the Chem. Eur. J. 2016, 22, 4460 – 4468
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intensity of the LE emission decreases gradually as the temperature increases. Additionally, the measured decay lifetimes do not depend on the temperature in the range 10–130 K, showing some interesting and atypical properties compared with reported M’Ln-MOFs.[5] As shown in Figure 4, the overlap between the LE excitation spectra and the HE emission spectra suggests that the intensive HE emission could be re-absorbed by the LE ground state. Furthermore, the interpenetrating behavior in ZnATZ-BTB can shorten the distance between organic lumophores (less than 5 æ) to favor the overlap of the electron cloud, which indicates that the excited donor and ground-state accepter should be close enough so that electron exchange could happen. The short-ranged electron exchange between dual emissive systems is usually described by the Dexter energy transfer (DET) mechanism (Figure 6 a).[16] The DET mechanism can be also confirmed by the temperature-independent decay lifetimes. Usually, the fluorescent decay time mainly depends on the energy transfer and the thermal activation of nonradiative decay pathways.[5g] However, both fluorescence and DET processes generally happen fast within the timescale of nanoseconds, which means an exchanged electron would immediately fluoresce before it proceeds to thermal activation. Hence, the temperature has no effect on the PL delay time of either the HE and LE emission under cryogenic conditions. Additionally, the longer delay of LE emission can be interpreted by the fact that it spends more time occupying the excited state for the nonradiative DET process. Similar to the reported dual emissive systems, the luminescent thermochromism may originate from the dynamic interplay between the dual emissive sources.[3–5] There exists an effective electron exchange (DET) between two separate excited states of dual emission. In other words, the separated excited states of HE and LE emission are coupled by the DET process. Thus, we may attribute temperature-dependent luminescent behavior to the thermal equilibrium between the two separated excited states of dual emission, the relative population of which is governed by the Boltzmann distribution law.
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Figure 5. a) Emission, and b) excitation spectra of ZnATZ-BTB recorded between 10 and 130 K excited at 330 nm. Inset: the temperature-dependent integrated intensity of the HE and LE transition for ZnATZ-BTB. c) The temperature-independent decay lifetimes of the HE and LE transitions for ZnATZ-BTB.
The intensity ratio–temperature relationship is given by Equation (1):[5a, 17] IHE DE ¼ B exp ¢ ILE kB T
ð1Þ
where IHE and ILE are the two luminescence intensities of the dual emission, B is a constant, DE is the effective energy gap between the two separate excited states, kB is the Boltzmann constant, and T is the absolute temperature. Figure 6 b shows that the numerical fitting gives the relationship between the intensity ratio and the absolute temperature is as follows [Eq. (2)]: IHE 88:261 ¼ 20:420 exp ¢ ILE T
ð2Þ
This suggests that the energy gap is calculated to be DE(cal) = 88.261 kB = 61 cm¢1, which is within the thermal coupling range (less than 2000 cm¢1).[3f] If we assume that the irradiative transition of the fluorescent emission is from the separated exChem. Eur. J. 2016, 22, 4460 – 4468
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cited states to the same ground state, we can calculate the wavelength of LE emission (lLE) to be: lLE ðcalÞ ¼ lHE ðobsÞ þ
hc ¼ 540 nm DEðcalÞ
ð3Þ
where lHE(obs) is the observed wavelength of HE emission, h is the Planck constant, and c is the speed of light. The calculated result is in good agreement with the experimental value lLE(obs) = 544 nm (Figure 2). The observed vibrational character of the LE emission band can be understood by the different ground states of LLCT related to ATZ ligands. Therefore, all the results verify our speculation of the thermal equilibrium between two separated excited states of dual emissions coupled by the DET process. The simulated line fits well with the experimental data in the temperature range 40–130 K. However, it should be noted that there is a clear deviance between the measured data and the fitted curve line when the temperature is lower than 40 K. The reason for this deviance is the spectral overlap between the HE emission band and the LE emission band originating from the small energy gap. Generally, the separation of two
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Figure 6. a) The Dexter energy transfer (DET) mechanism in a dual emissive system. b) The temperature-dependent intensity ratios of the HE and LE transitions for ZnATZ-BTB.
separated excited states requires more than 200 cm¢1 to avoid substantial overlap of the two fluorescent wavelengths.[3f, 17a] It is clear that the LE broad band become closer and closer to the HE band from the rapid growth of its intensity with decreasing temperature. Thus, the degree of spectral overlap in the lower temperature range from 40 K to 10 K cannot be neglected for the constituent mixed in the HE band makes the intensity of LE emission increase slower than would be the case if the overlap did not exist. In other words, the existence of spectral overlap makes the fitted value of the Boltzmann distribution less reliable (correlation coefficient R2 = 0.9652) in the lower temperature range (10–40 K).
Ratiometric thermometer To further improve the potential use of ratiometric luminescent MOF thermometers, we find that there is a very good linear relationship between the intensity ratio and temperature from 30 K to 130 K, which can be linearly fitted as a function of T [Eq. (4)]: I T ¼ 10:80 HE þ 11:52 ILE Chem. Eur. J. 2016, 22, 4460 – 4468
ð4Þ www.chemeurj.org
Figure 7. a) The linear-fitted line for the temperature-dependent intensity ratio of the HE and LE transitions in ZnATZ-BTB. b) CIE chromaticity diagram showing the temperature-dependent luminescence color change of ZnATZBTB in the cryogenic temperature range (10–130 K).
with a correlation coefficient R2 = 0.9956 (Figure 7 a). This suggests that the linear fitting method is not only more reliable than the above Boltzmann fitting method, but also extends the applicable temperature range. In addition, the linear fitting method, without complex calculation, is more convenient for practical applications. The changing rate of intensity ratio versus temperature, known as sensitivity, is of great importance in thermal sensing applications. Generally, the absolute sensitivity (Sab) of luminescent thermometers can be defined as the intensity ratio change with temperature by Equation (5): Sab ¼
@ ðIHE =ILE Þ @T
ð5Þ
According to this equation, the absolute sensitivity clearly depends on the magnitude of the temperature-dependent luminescent variations. Thus, the thermometer that shows large
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Full Paper intensity ratio changes with small changes in temperature will possess a high absolute sensitivity. However, it is meaningless to quantitatively compare the absolute sensitivity among thermometers that operate by different mechanisms or that are based on different material systems.[3g] To compare the performances of the different luminescent thermometers, the relative sensitivity (Sre) is usually utilized and is defined as in Equation (6):[5a] Sre ¼
@ðIHE =ILE Þ=@T IHE =ILE
ð6Þ
Following these definitions, the absolute sensitivity (Sab) of ZnATZ-BTB is 0.0926 K¢1, whereas the maximum relative sensitivity (Sm) is determined to be 5.29 % K¢1 at 30 K.[5a] Additionally, both the sensitivity of ZnATZ-BTB and that of other reported types of ratiometric luminescent MOF thermometers are compared in Table 1, which indicates a significantly enhanced sensitivity compared with the other M’Ln-MOF thermometers. Therefore, selecting organic lumophores as ratiometric dual emissive sources is an efficient strategy to obtain higher sensitivity. In addition, the temperature (Tm) at which the relative sensitivity reaches its maximum is located in the cryogenic temperature region (30 K), indicating its high availability for the cryogenic temperature range. Most M’Ln-MOFs exhibit higher Tm values, which makes them insensitive to cryogenic temperatures. Although this effective measuring range of ZnATZ-BTB is narrow, only available in the cryogenic temperature range (30–130 K), such high sensitivity has rarely been reported. In fact, the relative sensitivity is inversely related to the temperature sensing range, and thus significantly high sensitivity results in a thermometer with a poor temperature range.[5a] In addition, the HE and LE emission intensities at different temperatures remain almost unchanged for each cycle, demonstrating the excellent reversibility of this system (Figure S11, in the Supporting Information). Above all, ZnATZ-BTB is an excellent luminescent thermometer that is especially applicable in the cryogenic temperature range (30–130 K). Although the ratiometric method of thermosensitive dual emission is sufficiently sensitive for practical temperature detection, the broad emission band of the organic lumophores as well as the spectral overlap means there are some difficulties in accurately identifying the emission intensity. To facilitate the application of this system, the temperature-dependent spectra have been transformed into the Commission Internationale de L’Eclairage (CIE) 1931 coordinates. Figure 7 b shows the color change of the luminescence from (0.2369, 0.3827) at 10 K to (0.2201, 0.1699) at 130 K. This temperature-dependent emission color range is due to the variation of the population of the two separated excited states. Compared with the prebuilt temperature-dependent CIE chromaticity standard diagram, the luminescence color change allows ZnATZ-BTB to act as a sensitive luminescent colorimetric thermometer for in situ visualization of the temperature change instantly and straightforwardly in the cryogenic temperature range. This scope of the temperature range from 30 K to 130 K makes ZnATZ-BTB useful for temperature detection in areas such as superconducChem. Eur. J. 2016, 22, 4460 – 4468
Table 1. Comparison of sensitivities of other reported ratiometric luminescent MOF thermometers with ours, including working ranges (Range, K), absolute sensitivity (Sab, % K¢1), maximum relative sensitivity values (Sm, % K¢1), and the temperature at which Sm is maximum (Tm, K).
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Luminescent MOF[a]
Range [K]
Sab [% K¢1]
Sm [% K¢1]
Tm [K]
ZnATZ-BTB Eu0.0069Tb0.9931-DMBDC Eu0.0046Tb0.9954-DMBDC Eu0.0011Tb0.9989-DMBDC Tb0.9Eu0.1PIA Tb0.99Eu0.01PIA Tb0.95Eu0.05PIA Tb0.50Eu0.50PIA Tb0.98Eu0.02-OA-DSTP Tb0.98Eu0.02-BDC-DSTP Tb0.957Eu0.043-cpda Eu0.7Tb0.3-cam-Himdc Eu0.02Gd0.98-DSB Tb0.99Eu0.01(BDC)1.5(H2O)2 Eu0.0616Tb0.9382pcdmb Eu0.5Tb99.5@In(OH)bpydc ZJU-88perylene
30–130 50–200 50–200 50–200 100–300 100–300 100–250 75–275 77–275 77–225 40–300 100–450 20–300 290–320 25–200 283–333 293–353
0.0926 0.0038 – – 0.0353 0.2 0.06 – 0.0047 0.007 0.0037 0.0008 – 0.0014 0.0056 0.0497 0.0128
5.29 1.15 0.61 0.52 3.27 2.75 2.48 2.02 2.40 2.75 16.0 0.11 4.75 0.31 0.34 2.53 1.28
30 200 200 200 300 300 250 275 275 225 300 450 20 318 200 333 293
[a] See reference [5a] and references therein.
tivity, space exploration, materials research, and other fields that need accurate cryogenic temperature detection.
Conclusions We have proposed a conceptually different approach for luminescent MOF thermometers by making use of the widely applicable mixed-ligands method. The intrinsic photophysical property of cryogenic temperature can be taken advantage of to fabricate a dual emissive system arising from the DET process between mixed organic lumophores. The intensity ratio of the dual emission serves as a self-calibrated signal and can be fitted by Boltzmann distribution, indicating that the temperature-dependent luminescent behavior is the consequence of a thermal equilibrium between two separated excited states coupled by the DET process. Owing to the excellent linear correlation between temperature and luminescence intensity ratio from 30 to 130 K, this luminescent MOF thermometer provides new insights into the fabrication of cryogenic temperature sensors. The intrinsic dual emission can circumvent effectively tedious doping engineering and sensitivity variation problems that exist with mixed Ln-MOFs. In addition, the novel MOF thermometer shows significantly higher sensitivity than the previously developed MOF ratiometric thermometers. Furthermore, the remarkable structural variety and tunability of organic lumophores not only afford the potential of more diverse emission ranges, but can also be used to adjust and optimize the energy gap to obtain higher sensitivity and robustness. It should be noted that achieving control over the spatial localization of the self-assembly sites is a challenging task for device fabrication, which remains a major scientific goal for the development of MOF-based technology.[18]
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Full Paper Experimental Section Materials and methods All reagents and solvents for the syntheses were purchased from commercial sources and used as received. All measurements except for the elemental analyses were carried out in air at room temperature. Elemental analyses (C, H, and N) were performed on an Elementar Vario MICRO CHNOS elemental analyzer. The infrared spectra (IR) with KBr pellets were recorded in the range 4000– 400 cm¢1 on a PerkinElmer Spectrum One FTIR spectrometer. Thermogravimetric analyses (TGA) were performed on a TGA/DSC 1 STARe system from room temperature to 1000 8C with a heating rate of 10 K min¢1 under nitrogen. Powder X-ray diffraction (PXRD) data were collected on a Rigaku Desktop MiniFlexII diffractometer using CuKa radiation (l = 1.54056 æ) powered at 30 kV and 15 mA. The simulated pattern was produced by using the Mercury Version 1.4 software and single-crystal reflection diffraction data. Solidstate UV/Vis spectra were measured on a PerkinElmer Lambda9500 UV/Vis/NIR spectrophotometer equipped with an integrating sphere, and by using dry BaSO4 as a ‘background’ matrix. The solid-state luminescence emission/excitation spectra were recorded on an Edinburgh Instrument FLS920 fluorescence spectrophotometer equipped with a continuous Xe-900 xenon lamp and a F900 microsecond flash lamp. For low-temperature measurements, microcrystal samples were mounted on a closed cycle helium cryostat. The temperature-dependent emission spectra were recorded by the above FLS920 fluorescence spectrometer with a PolyScience Temperature Control Solution PD07R-20. The luminescence decay curve was measured by using a 397 nm OPO laser as the light source. The overall quantum yields of the solid-state samples were determined by an absolute method using an integrating sphere (150 mm diameter, BaSO4 coating) on the above FLS920 spectrometer.
mation. Select bonds and angles are list in Table S2, and those of the hydrogen bonds are listed in Tables S3 and S4 (in the Supporting Information). CCDC 1423556 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.
Theoretical calculations All DFT calculations were performed with the Gaussian 03 program package. The TD-DFT electronic excitation calculations were performed by using the optimized geometries of the asymmetric unit in which uncoordinated sites were substituted by hydrogen at the level of B3LYP/LanL2DZ for Zn and 6-31G* for other atoms.[20]
Acknowledgments The authors gratefully acknowledge financial support from the 973 Program (2012CB821702, 2014CB845603), the National Natural Science Foundation of China (21233009 and 21203194), and Fujian Province (2013J05039). Keywords: cryogenic temperatures · energy fluorescence · metal–organic frameworks · thermometer
Synthesis of {[(CH3)2NH2][Zn(BTB)2/3(ATZ)]·H2O·DMF}n (ZnATZ-BTB) A mixture of Zn(NO3)2·6 H2O (150 mg, 0.5 mmol), H3BTB (109 mg, 0.25 mmol), HATZ (21 mg, 0.25 mmol), and 16.5 mL DMF/H2O (DMF = N,N-dimethylformamide, v/v = 10:1) was sealed in a screw cap vial. Ten drops of 50 % HBF4 were added, then the solution was heated and kept at 85 8C for 3 days. It was slowly cooled to 30 8C at about 5 8C h¢1 to give colorless rod-like crystals, which were obtained by filtration and washed with DMF before they were used for the X-ray diffraction determination. Yield: 64 % based on HATZ; elemental analysis calcd (%) for C24H29N7O6Zn (576.91): C 49.96, H 5.07, N 16.99; found: C 49.89, H 5.14, N 16.80.
X-ray crystallographic study Data collection for ZnATZ-BTB was performed on a Rigaku Saturn 724 HG CCD diffractometer equipped with graphite-monochromated MoKa radiation (l = 0.71073 æ) at 77 K and 293 K. The intensity was corrected for Lorentz and polarization effects as well as for empirical absorption based on the multiscan technique. The structures were solved by direct methods and refined by full-matrix least-squares on F2 by using the SHELX-97 program.[19] All non-hydrogen atoms were refined anisotropically. Hydrogen atoms bound to carbon atoms were generated geometrically and were fixed at calculated positions and refined by using a riding mode, and those bound to oxygen and nitrogen atoms were determined by difference Fourier maps. The crystallographic data and structure refinement details are summarized in Table S1 in the Supporting InforChem. Eur. J. 2016, 22, 4460 – 4468
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Received: November 4, 2015 Published online on February 11, 2016
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