Emission Enhancement and Chromism in a Salen-Based Gel System

Mar 18, 2009 - A new salicylideneaniline-based organogelator has been synthesized, and it can gelatinize organic solvents, including cyclohexane, tolu...
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Emission Enhancement and Chromism in a Salen-Based Gel System† Peng Chen, Ran Lu,* Pengchong Xue, Tinghua Xu, Guojun Chen, and Yingying Zhao State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, PR China Received October 27, 2008. Revised Manuscript Received January 18, 2009 A new salicylideneaniline-based organogelator has been synthesized, and it can gelatinize organic solvents, including cyclohexane, toluene, benzene, and some mixed solvents. SEM images show that it has self-assembled into 1-D nanofibers, which further cross-link to form 3-D network. On the basis of the results of small-angle XRD and the optimized molecular length by semiempirical quantum calculations, the gelators are supposed to pack into a unimolecular lamellar structure with a period of 3.01 nm. Significantly, reversible chromism is realized with respect of the tautomerism between the NH and OH forms during the sol-gel freezing repetition. Furthermore, the gel can emit intense green light, and the fluorescent quantum yield of the gel is approximately 600 times higher than that of the solution. The aggregation-induced emission enhancement is ascribed to the formation of J aggregation and the inhibition of intramolecular rotation in the gel state.

Recently, there has been increasing interest in the development of functional gel systems with π-conjugated moieties because of their potential applications in various fields, including new organic soft materials, enhanced charge transport, fluorescence, catalysis, and sensing abilities.1 Generally, the gelators can self-assemble into fibers, rods, and ribbons on the nanoscale through noncovalent forces such as H bonding, π-π stacking, electrostatic forces, and van der Waals interactions. As a conjugated system, salen (N, N0 -bis(salicylidene) ethylenediamine) has been proven to be a useful building block for the construction of supramolecular architectures,2 besides the catalytic activities of the metal-salen complex.3 *Corresponding author. E-mail: [email protected]. † Part of the Gels and Fibrillar Networks: Molecular and Polymer Gels and Materials with Self-Assembled Fibrillar Networks special issue. (1) (a) Yamanaka, M.; Miyake, Y.; Akita, S.; Nakano, K. Chem. Mater. 2008, 20, 2072. (b) Tritt-Goc, J.; Bielejewski, M.; Luboradzki, R.; Lapinski, A. Langmuir 2008, 24, 534. (c) Li, X.; Peng, J.; Kang, J.; Choy, J.; Steinhart, M.; Knoll, W.; Kim, D. Soft Matter 2008, 4, 515. (d) Ajayaghosh, A.; Chithra, P.; Varghese, R.; Divya, K. P. Chem. Commun. 2008, 969. (e) Palma, M.; Levin, J.; Debever, O.; Geerts, Y.; Lehmann, M.; Samori, P. Soft Matter 2008, 4, 303. (f) Yagai, S.; Ishii, M.; Karatsu, T.; Kitamura, A. Angew. Chem., Int. Ed. 2007, 46, 1. (g) Strybulevych, A.; Leroy, V.; Scanlon, M.; Page, J. H. Soft Matter 2007, 3, 1388. (h) Sada, K.; Takeuchi, M.; Fujita, N.; Numata, M.; Shinkai, S. Chem. Soc. Rev. 2007, 415. (i) Li, Y.; Wang, T.; Liu, M. Soft Matter 2007, 3, 1312. (j) Deind~ orfer, P.; Davis, R.; Zentel, R. Soft Matter 2007, 3, 1308. (k) Ma, C. T. L.; MacLachlan, M. J. Angew. Chem., Int. Ed. 2005, 44, 4178. (l) Wang, C.; Zhang, D.; Zhu, D. J. Am. Chem. Soc. 2005, 127, 16372. (m) Huebon, F. J. M.; Jonkheijim, P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. Rev. 2005, 105, 1491. (n) Resendiz, M. J. E.; Noveron, J. C.; Disteldorf, H.; Fischer, S.; Stang, P. J. Org. Lett. 2004, 6, 651. (o) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (2) (a) Frischmann, P. D.; Jiang, J.; Hui, J. K.-H.; Grzybowski, J. J.; MacLachlan, M. J. Org. Lett. 2008, 10, 1255. (b) Frischmann, P. D.; Gallant, A. J.; Chong, J. H.; MacLachlan, M. J. Inorg. Chem. 2008, 47, 101. (c) Curreli, S.; Escudero-Adan, E. C.; Benet-Buchholz, J.; Kleij, A. W. Eur. J. Inorg. Chem. 2008, 2863. (d) Kleij, A. W.; Kuil, M.; Tooke, D. M.; Lutz, M.; Spek, A. L.; Reek, J. N. H. Chem.;Eur. J. 2005, 11, 4743. (e) Ma, C.; Lo, A.; Abdolmaleki, A.; MacLachlan, M. J. Org. Lett. 2004, 6, 3841. (3) (a) Zang, Z.; Zhao, G.; Zhou, Z.; Tang, C. Eur. J. Org. Chem. 2008, 1615. (b) Wezenberg, S. J. A.; Kleij, W. Angew. Chem., Int. Ed. 2008, 47, 2354. (c) Laskin, J.; Yang, Z.; Chu, I. K. J. Am .Chem. Soc. 2008, 130, 3218. (d) Mazet, C.; Jacobsen, E. N. Angew. Chem., Int. Ed. 2005, 47, 1762. (e) Jacobsen, E. N. Acc. Chem. Res. 2000, 33, 421. (f) Jeon, Y.; Armatas, G. S.; Heo, J.; Kanatzidis, M. G; Mirkin, C. A. Adv. Mater. 2008, 20, 2105.

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However, their self-assembled properties remain unexplored in the field of gels, although 1-D nanostructures based on Zn (salphen) have been recently reported by MacLachlan et al.4 Salicylideneanilines and its derivates belong to a class of the most well known photochromic and/or thermochromic compounds,5 which may have intriguing consequences in the field of chemistry.6 The variation in the population of the OH and NH forms is considered to be the origin of the chromism, in accordance with proton tautomerization.7 It has been found that crystals of N,N0 -bis(salicylidene)-o-phenylenediamine (BSPD) are thermochromic, for example, nearly colorless at 77 K but orange at room temperature, and give strong fluorescence emission as a result of the formation of J aggregates in the crystals.8 We have previously reported a nonplanar salicylideneaniline-based organogel with a flexible 1,10 -bis(pinophenyl)methane linker that emits strong green light.9 Thus, a unique gel system with strong emission (4) Hui, J. K.-H.; Yu, Z.; MacLachlan, M. J. Angew. Chem., Int. Ed. 2007, 46, 1. (5) (a) Day, J. H. Chem. Rev. 1968, 68, 649. (b) Day, J. H. Chem. Rev. 1963, 63, 65. (6) (a) Harada, T.; Kawano, M.; Kojima, T.; Fujita, M. Angew. Chem., Int. Ed. 2007, 46, 6643. (b) Ohshima, A.; Momotake, A.; Arai, T. Bull. Chem. Soc. Jpn. 2006, 79, 305. (c) Ohshima, A.; Momotake, A.; Nagahata, R.; Arai, T. J. Phys. Chem. A 2006, 109, 9731. (d) Sliwa, M.; Letard, S.; Malfant, I. Chem. Mater. 2005, 17, 4727. (e) Taneda, M.; Amimoto, K.; Koyama, H.; Kawato, T. Org. Biomol. Chem. 2004, 2, 499. (f) Li, S.; He, L.; Xiong, F.; Li, Y.; Yang, G. J. Phys. Chem. B 2004, 108, 10887. (g) Ohshima, A.; Momotake, A.; Arai, T. J. Photochem. Photobiol., A 2004, 162, 473. (h) Hadjoudis, E.; Mavridis, I. M. Chem. Soc. Rev. 2004, 33, 579. (i) Ogawa, K.; Harada, J. J. Mol. Struct. 2003, 647, 211. (j) Ogawa, K.; Harada, J.; Fujiwara, T.; Yoshida, S. J. Phys. Chem. A 2001, 105, 3425. (k) Irie, M. Chem. Rev. 2000, 100, 1683. (l) Shen, M.; Zhao, L.; Goto, T.; Mordzinski, A. J. Chem. Phys. 2000, 112, 2490. (m) Sekikawa, T.; Takayoshi, K. J. Phys. Chem. B 1997, 101, 10645. (n) Ottolenghi, M.; McClure, S. D. J. Chem. Phys. 1967, 46, 4613. (7) (a) Harada, J.; Fujiwara, T.; Ogawa, K. J. Am. Chem. Soc. 2007, 129, 16216. (b) Miura, M.; Harada, T.; Ogawa, K. J. Phys. Chem. A 2007, 111, 9854. (c) Harada, J.; Uekusa, H.; Ohashi, Y. J. Am. Chem. Soc. 1999, 121, 5809. (d) Ogawa, K.; Kasahara, Y.; Ohtani, Y.; Harada, J. J. Am. Chem. Soc. 1998, 120, 7107. (8) Pahor, N. B.; Calligaris, M.; Delise, P.; Dodic, G.; Nardin, G.; Randaccio, L. J. Chem. Soc., Dalton Trans. 1976, 2478. (9) Xue, P.; Lu, R.; Chen, G.; Zhang, Y.; Nomoto, H.; Takafuji, M.; Ihara, H. Chem.;Eur. J. 2007, 13, 8231.

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enhancement as well as thermochromism properties could be expected via the self-assembly of the gelator molecules bearing the BSPD moiety, on account of its conjugated coplanarity and potential applications in metal selectivity.3f Herein, we describe a thermochromic gel generated from cholesterol-containing salen-based organogelator 1,10 which acts as an excellent organogelator in cyclohexane, benzene, toluene, and some mixed solvents. Notably, the nearly colorless hot solution turns into a yellow gel with the temperature decreasing to room temperature and the yellow gel becoming nearly colorless at 77 K in terms of the tautomerism between the OH and NH forms. Meanwhile, aggregation-induced emission (AIE) enhancement is observed during gel formation, and the fluorescence quantum yield of the gel is approximately 600 times greater than that in the solution ascribed to a combination of the inhibition of intramolecular rotation and the formation of the J aggregate. The gelation test of 1 was carried out in various solvents. It was found that 1 was readily soluble in chloroform, dichloromethane, and THF but insoluble in aliphatic hydrocarbon solvents and lower alcohols such as ether petroleum, nhexane, n-decane, and n-butanol at room temperature. However, it could gelatinize cyclohexane, benzene, toluene, and the mixed solvents of benzene and n-butanol, benzene and n-hexane, toluene and n-butanol, and toluene and n-hexane (Table S1). The minimum gelation concentration (MGC) necessary for gel formation in cyclohexane was smaller (0.4 wt %) than that in benzene and toluene (1.1 wt %). Remarkably, the MGC decreased with the addition of n-butanol or n-hexane, in which 1 has poor solublility, to the benzene or toluene gel systems. In addition, the sol-gel phase-transition temperature increased with the concentration increase in cyclohexane and benzene/n-hexane (1/4 v/v) systems (Figure S1). It was anticipated that the salen moiety would facilitate the formation of a 1-D superstructure via π-π interactions. To reveal the microstructure of organogel 1, the SEM image of the xerogel was obtained (Figure 1a). It demonstrated that the organogelator molecules in the gel phase were selfassembled into 1-D nanofibers with a 25-100 nm width, which further cross-linked to form 3-D networks. As shown in Figure 1b, the small-angle X-ray diffraction pattern (SAXD) of the xerogel obtained from the cyclohexane gel exhibited two reflection peaks of 3.01 and 1.49 nm (in a ratio of 1: 1/2), illustrating that the molecules were packed into the lamellar structure. The semiempirical quantum calculation (AM1 force field) was made to optimize the ground-state geometry of 1.9 It showed that 1 possessed a V-shaped conformation (Figure 1c), and the molecular length in the extended form was estimated to be 3.01 nm, which was in agreement with the long period based on SAXD (Figure 1d). Moreover, a red shift of 2 nm was observed in the UV-vis absorption spectrum of the gel (337 nm) compared with that in the dilute solution (335 nm) (Figure S2), indicating the formation of the J aggregate in the gel phase. Therefore, the 1-D molecular packing model was proposed in the gel phase as shown in Figure 1e, and the 1-D superstructures could self-assemble into thin fibers, which were further wound or laced to give nanofibers of 25-100 nm width as shown in Figure 1a. Interestingly, a remarkable phenomenon was detected in the process of gel formation. The nearly colorless hot solution (10) Zinic, M.; Vogtle, F.; Fages, F. Top. Curr. Chem. 2005, 256, 39.

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gradually turned into a yellow gel with the temperature decreasing to room temperature. As shown in Figure S2, a new weak absorption at around 480 nm, which originated from the E-NH form in previous reports,6g,7c,11 emerged in the gel state. To exclude the occurrence of the decomposition or the irreversible reaction, the gel was reheated to over the transition temperature, and the temperature-dependent UV-vis spectra of 1 in cyclohexane were recorded during cooling of the hot solution to room temperature (Figure 2a). As a result, the intensity at 297 and 334 nm gradually decreased, and a new peak at 480 nm appeared as the temperature was decreased. It should be emphasized that such a spectral transformation could be repeatable several times (inset in Figure 2a). Therefore, it was deduced that the absorption spectral change could result from the molecular aggregation. In other words, aggregation-induced chromism from colorless to yellow could be realized because of the tautomerism from the OH to NH form of the salen derivative. Although no visible vibrational absorption of the R,β-unsaturated ketone of the NH form in the IR spectrum was detected in the gel because of the small quantity of tautomers and the peak overlap (Figure S3),12,13 the fluorescence excitation spectra of 1 was similar to that of the NH form of thermochromic or solvatochromic salicylideneaniline derivatives,6g illustrating the appearance of the NH form. As shown in Figure 2b, in the dilute solution of 1, the excitation spectrum was similar to the absorption spectrum except for weak peaks in the range of 400-500 nm. Furthermore, the peak at ca. 450 nm ascribed to the NH form emerged significantly in the gel state. It indicated that the OH and NH forms of 1 coexisted in the solution and gel phases7d and the dominant OH form in the solution could partially transfer to the NH form in the gel as a result of the molecular aggregation. As proposed by Ogawa and Arai,6i,6g the energy-level difference between the ground states of the OH and NH forms in solution is too large to induce the transformation of OH to the NH form, so the colorless OH form existed predominantly in solution (Scheme S1). However, the stabilization of the NH form in the aggregate is enhanced, so the energy-level difference between the OH and NH forms in the gel is less than that in the solution, which favors the proton tautomerization leading to the formation of the yellow NH form. Thus, the aggregation-induced chromism in the gel system could be observed. Extensive studies have revealed that the populations of the NH and OH forms at different temperatures mainly depend on the molecular structures in the thermochromic salicylideneaniline crystal; for example, the NH form of N-(5-chloro-2hydroxybenzylidene)-4-hydroxyaniline is dominant, and the OH form of N-(5-chloro-2-hydroxybenzylidene)-aniline is excessive at low temperature.7d Herein, when gel 1 was cooled to 77 K, the decoloration of the gel from yellow to nearly colorless was detected. Figure 2c showed the time-dependent absorption spectra upon warming the frozen gel from 77 to 298 K. It was found that absorption in the range of 450-500 nm (E-NH form) became stronger with increasing (11) (a) Bayrakc-eken, F.; Sevinc- , P. C. Spectrochim. Acta, Part A 2007, 66, 184. (b) Ogawa, K.; Harada, J. J. Mol. Struct. 2003, 647, 211. (12) (a) Fabian, W. M. F.; Antonov, L.; Nedeltcheva, D.; Kamounah, F. S.; Taylor, P. J. J. Phys. Chem. A 2004, 108, 7603. (b) Manal, A.; Koll, A.; Filarowski, A.; Majumder, D.; Mukherjee, S. Spectrochim. Acta, Part A 1999, 55, 2861. (13) (a) Bao, C.; Lu, R.; Jin, M.; Xue, P.; Tan, C.; Liu, G.; Zhao, Y. Org. Biomol. Chem. 2005, 3, 2508. (b) Sagawa, T.; Fukugawa, S.; Yamada, T.; Ihara, H. Langmuir 2002, 18, 7223.

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Figure 1. (a) SEM image and (b) SAXD pattern of xerogel 1 obtained from cyclohexane. (c) Molecular structure of 1. (d) Unimolecular stacking with a long period of 3.01 nm in the gel. (e) Proposed molecular packing model along the growing direction of the gel fiber.

Figure 2. (a) Temperature-dependent UV-vis spectra of 1 in cyclohexane (3.0  10-3 mol/L) during the cooling of the hot solution from 333 to 303 K with an interval of 2 K; the inset shows the reversibility of the sol-gel process monitored at 523 nm. (b) Excitation spectra of 1 in cyclohexane solution at (a) 1.0  10-5 mol/L and (b) for the cyclohexane gel at 3.0  10-3 mol/L. (c) Time-dependent UV-vis spectra of cyclohexane gel 1 at 3.0  10-3 mol/L while warming the frozen gel from 77 to 298 K naturally. (d) Images of the solution at 353 K and the gel at 298 and 77 K.

temperature. Meanwhile, the color of the sample was altered from nearly colorless to yellow. However, the fluorescence spectra at lower temperature (80-260 K) also illustrated the occurrence of tautomerization (Figure S4), which means that the tautomeric equilibrium markedly shifted to the E-OH Langmuir 2009, 25(15), 8395–8399

form at 77 K because of its higher stability at lower temperature (Scheme S1). It is worth noting that such a spectral transformation is reversible. Therefore, the reversible thermochromism through sol-gel-freezing circulation was achieved on the basis of salen derivative 1 (Figure 2d). DOI: 10.1021/la8035727

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Figure 3. Absorption spectral change of cyclohexane gel 1 with irradiation time (0, 2, 5, 10, 15, 20, and 30 min) exposed to 365 nm light from a high-pressure mercury lamp.

Furthermore, the photochromism of cyclohexane gel 1 irradiated by UV light of 365 nm was investigated, and the absorption spectral change of the gel with the variable irradiation time was shown in Figure 3. We could find that the band in the range of 450-500 nm decreased and the bands in the range of 260-400 nm increased gradually when the irradiation time was prolonged. This suggested that gel 1 also exhibited a significant photochromic property. Figure 4 showed the fluorescence emission spectra of gelator 1 in solution and in the gel phases. When the solution in cyclohexane was excited at 334 nm, we could find two emission peaks at 462 and 523 nm whose intensities increased with the increase in concentration.14 Significantly, it was worth noting that the increasing rate of the emission intensity at 523 nm was faster than that at 462 nm. Moreover, only one emission peak at 523 nm was observed in the cyclohexane gel (3.0  10-3 mol/L) with a large Stokes shift of ∼186 nm, whereas the peak at 462 nm disappeared. It is well known that once gelator 1 in the E-OH form in the ground state absorbed one photon then the excited-state intramolecular rapid proton-transfer process from the oxygen atom to the nitrogen atom would happen and yield the excited-state Z-NH form. Moreover, the Z-NH form could undergo a transition from the excited state to its ground state, accompanied by a longer emission wavelength, and then return to the E-OH ground state rapidly.12 Thus, this could lead to a large Stokes shift comparing with the absorption peak of the E-OH form. Therefore, the two peaks at 462 and 523 nm might have originated from the E-OH form in different species. To distinguish the origin of the two peaks at 462 and 523 nm, a small amount of ethanol (1/1000 v/v ethanol/ cyclohexane) was added to the cyclohexane gel of 1 so as to break the molecular aggregation; meanwhile, the cyclohexane gel turned into a clear solution.13 Although ethanol was (14) (a) Ohshima, A.; Momotake, A.; Nagahata, R.; Arai, T. J. Phys. Chem. A 2005, 109, 9731. (b) Hadjoudis, E.; Mavridis, I. M. Chem. Soc. Rev. 2004, 33, 579. (c) Li, S.; He, L.; Xiong, F.; Yang, G. J. Phys. Chem. B 2004, 108, 10887. (d) Ogawa, K.; Harada, J.; Fujiwara, T.; Yoshida, S. J. Phys. Chem. A 2001, 105, 3425. (e) Nakatani, K.; Delaire, J. A. Chem. Mater. 1997, 9, 2682. (f) Sekikawa, T.; Kobayashi, T. J. Phys. Chem. A 1997, 101 644.

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Figure 4. Fluorescence emission spectra of 1 in cyclohexane gel at 3.0  10-3 mol/L (a), in cyclohexane solution at 1.0  10-5 mol/L (b), 2.0  10-5 mol/L (c), 4.0  10-5 mol/L (d), 6.0  10-5 mol/L (e), 8.0  10-5 mol/L (f), and 1.0  10-4 mol/L (g), and in a solution of cyclohexane/ethanol (1000/1 v/v) at 3.0  10-3 mol/L (h). All experiments are carried out at 293 K. a poor solvent for 1, such a hydrogen bonding donor was usually able to disrupt the gelation as reported by Smith.15 Herein, ethanol molecules might interact with CdN or OH groups in 1 via hydrogen bonding, which might weaken the π-π interaction between BSPD units as a result of the decreasing coplanarity and destroy the gel, forming a solution. It was found that the emission at 523 nm disappeared and a new peak at 460 nm similar to those in the dilute solution appeared in the ethanol/cyclohexane system (Figure 4). It could be deduced that the emission bands at 523 and 462 nm originated from the aggregation and the monomer states of the E-OH form, respectively. Molecules in the two species might coexist in the cyclohexane solution. Figure S5 showed the temperature-dependent fluorescence spectra of 1 in cyclohexane excited at 334 nm upon cooling the hot solution to room temperature gradually. For the hot solution, only one wide emission peak at 450-470 nm appeared in accordance with that of the monomer state. However, a peak at 523 nm emerged after the hot solution was allowed to cool for 2 min, and its intensity became stronger and stronger with the decreasing temperature, accompanying the kinetic descent of the peak at 450-470 nm. Such a fluorescence spectral change might have originated from the molecular aggregation during gel formation, which could also be confirmed by the disappearance of emission at 523 nm when a little ethanol was added to the gel so as to break the aggregation (Figure 4). The fluorescence quantum yield (ΦF) of the solution of 1 in ethanol/cyclohexane (1/1000 v/v) was 1.24  10-4. The inset in Figure S5 revealed that the gel can emit strong green light with ΦF = 0.0768, whereas the solution had a faint blue emission. The fluorescence quantum yield of the gel was approximately 600 times more than that of the solution. It is well known that aggregation usually quenched emission, but several mechanisms had previously been proposed for the AIE phenomena in the solid state, compared (15) Smith, D. K. Chem. Commun. 2006, 34.

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with lower photoluminescence in solution.16 In the case of 1, the formation of J aggregates in the gel phase was observed, which could promote the radiative transition from the excited to the ground state. At the same time, the restriction of the intramolecular bond rotation should also be responsible for enhanced emission because of the steric hindrance in the occurrence of aggregation. Accordingly, the AIE of the gel could be ascribed to the formation of the J aggregate and the rotational restriction of single bonds of the chromophores. Consequently, both color and fluorescence switches could be achieved by sol-gel phase transformation.

Conclusion In summary, a new salen-based oragnogelator, which could self-assemble into gels in cyclohexane, benzene, (16) (a) Yang, X.; Lu, R.; Xu, T.; Xue, P.; Liu, X.; Zhao, Y. Chem. :: Commun. 2008, 453. (b) Tong, H.; Hong, Y.; Dong, Y.; Ran, Y.; Haussler, M.; Lam, J. W. Y.; Wong, K.; Tang, B. J. Phys. Chem. B 2007, 111, 2000. (c) Qian, Y.; Li, S.; Zhang, G.; Wang, Q.; Wang, S.; Xu, H.; Li, C.; Li, Y.; Yang, G. J. Phys. Chem. B 2007, 111, 5861. (d) Tong, H.; Dong, Y.; Hong, Y.; :: Haussler, M.; Lam, J. W. Y.; Sung, H. H. Y.; Yu, X.; Sun, J.; Williams, I. D.; Kwok, H. S.; Tang, B. J. Phys. Chem. C 2007, 111, 2287. (e) Sun, Y.; Liao, J.; Fang, J.; Chou, P.; Shen, C.; Hsu, C.; Chen, L. Org. Lett. 2006, 8, 3713. (f) Itami, K.; Yoshida, J. Bull. Chem. Soc. Jpn. 2006, 79, 811. (g) Han, M. R.; Hirayama, Y.; Hara, M. Chem. Mater. 2006, 18, 2784. (h) Bao, C.; Lu, R.; Jin, M.; Xue, P.; Tan, C.; Xu, T.; Liu, G.; Zhao, Y. Chem.;Eur. J. 2006, 12, 3287. (i) Xie, Z.; Yang, B.; Xie, W.; Liu, L.; Shen, F.; Wang, H.; Yang, X.; Wang, Z.; Li, Y.; Hanif, M.; Yang, G.; Ye, L.; Ma, Y. J. Phys. Chem. B 2006, 110, 20993. (j) Bhongale, C. J.; Chang, C.; Lee, C.; Diau, E. W. G.; Hsu, C. S. J. Phys. Chem. B 2005, 109, 13472. (k) Toal, S. J.; Jones, K. A.; Magde, D.; Trogler, W. C. J. Am. Chem. Soc. 2005, 127, 11661. (l) Wang, Z.; Shao, H.; Ye, J.; Tang, L.; Lu, P. J. Phys. Chem. B 2005, 109, 19627.

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toluene, and some mixed solvents, has been synthesized. It is found that it self-assembled into nanofibers through unimolecular layer packing in cyclohexane, which further cross-linked to form a 3-D network on the basis of the results of SEM, XRD, and the molecular length in the extended form by semiempirical quantum calculations. During the formation of the gel, aggregation-induced emission enhancement is observed, which is ascribed to a combination of the inhibition of intramolecular rotation and the formation of J aggregates. Significantly, reversible chromism circulation is realized with respect to the tautomerism between the NH and OH forms with the variation of temperature. Therefore, potential applications of such a functional soft material in various switches might be expected. Acknowledgment. This work was supported by the National Natural Science Foundation of China (NNSFC, nos. 20574027 and 20874034), the 973 Program (2009CB939701), and the Program for New Century Excellent Talents in University (NCET). Supporting Information Available: Synthesis and gelation properties of 1, UV-vis absorption and IR spectra, timedependent excitation and fluorescence emission spectra, SEM images, plots of gelator concentration versus gelation temperature, and schematic potential energy levels of the OH and NH forms. This material is available free of charge via the Internet at http://pubs.acs.org.

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