New Organogels Based on an Anthracene Derivative with One Urea

Jul 6, 2007 - New Organogels Based on an Anthracene Derivative with One Urea Group and Its Photodimer: Fluorescence Enhancement after Gelation...
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Langmuir 2007, 23, 9195-9200

9195

New Organogels Based on an Anthracene Derivative with One Urea Group and Its Photodimer: Fluorescence Enhancement after Gelation Cheng Wang,†,‡ Deqing Zhang,†* Junfeng Xiang,† and Daoben Zhu†* Beijing National Laboratory for Molecular Sciences, Organic Solids Laboratory, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China, and Graduate School of Chinese Academy of Sciences, Beijing 100080, China ReceiVed April 18, 2007. In Final Form: May 26, 2007 By coupling the features of anthracene and urea, a new low-molecular-weight gelator (LMWG, 1) with anthracene and urea moieties was designed and synthesized. A nontransparent gel of LMWG 1 in 1,2-dichloroethane was formed and characterized. Of particular interest is the observation of significant fluorescence enhancement after gelation, which is referred as to gelation-induced enhanced fluorescence emission. UV light irradiation of the THF solution of LMWG 1 yielded a photodimer with the h-t conformation. The photodimer can gel several organic solvents, including cyclohexane, n-hexane, and n-heptane. It should be mentioned that the gel based on the photodimer is rather stable. Our studies indicate that neither the gel phase based on LMWG 1 nor that based on the photodimer can be transformed to the solution by respective UV light irradiation or visible light irradiation/heating.

Introduction There is growing interest in assembling organic molecules into well-defined functional aggregates. Organogel, which is the result of the entrapment and adhesion of the liquid in the largesurface-area solid 3D matrix, has attracted much interest.1 Organogels are formed by assembling low-molecular-weight gelators (LMWGs) through weak intermolecular interactions such as H bonding and π-π stacking.2 The properties of organogels are largely influenced by the structures of LMWGs.3 The development of new LMWGs is important for advanced applications of organogels such as drug delivery and the sol-gel process.4 LMWGs with electroactive/photoresponsive/chemical reactive groups have received much attention in recent years because these LMWGs may lead to organogels that respond to external physical and chemical stimuli.5 For instance, several LMWGs with azo groups have been reported;6 the corresponding orga* Corresponding author. E-mail: [email protected]. † Beijing National Laboratory for Molecular Sciences, Chinese Academy of Sciences. ‡ Graduate School of Chinese Academy of Sciences. (1) (a) Terech, P.; Weiss, R. G. Chem. ReV. 1997, 97, 3133-3159. (b) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263-2266. (c) Sangeetha, N. M.; Maitra, U. Chem. Soc. ReV. 2005, 34, 821-836. (d) Fages, F. Angew. Chem., Int. Ed. 2006, 45, 1680-1682. (2) (a) Shirakawa, M.; Fujita, N.; Shinkai, S. J. Am. Chem. Soc. 2005, 127, 4164-4165. (b) George, S. J.; Ajayaghosh, A. Chem.sEur. J. 2005, 11, 32173227. (c) Ajayaghosh, A.; Varghese, R.; Praveen, V. K.; Mahesh, S. Angew. Chem., Int. Ed. 2006, 45, 3261-3264. (d) George, S. J.; Ajayaghosh, A.; Jonkheijm, P.; Schenning, A.; Meijer, E. W. Angew. Chem., Int. Ed. 2004, 43, 3422-3425. (e) Ajayaghosh, A.; George, S. J.; Schenning, A. Top. Curr. Chem. 2005, 258, 83-118. (f) Ajayaghosh, A.; George, S. J. J. Am. Chem. Soc. 2001, 123, 51485149. (3) (a) Kuroiwa, K.; Shibata, T.; Takada, A.; Nemoto, N.; Kimizuka, N. J. Am. Chem. Soc. 2004, 126, 2016-2021. (b) Naota, T.; Koori, H. J. Am. Chem. Soc. 2005, 127, 9324-9325. (c) Kishimura, A.; Yamashita, T.; Aida, T. J. Am. Chem. Soc. 2005, 127, 179-183. (d) Zhan, C. L.; Gao, P.; Liu, M. H. Chem. Commun. 2005, 462-464. (e) Ajayaghosh, A.; Varghese, R.; George, S. J.; Vijayakumar, C. Angew. Chem., Int. Ed. 2006, 45, 1141-1144. (f) Wang, C.; Zhang, D. Q.; Zhu, D. B. Langmuir 2007, 23, 1478-1482. (4) (a) Vemula, P. K.; Li, J.; John, G. J. Am. Chem. Soc. 2006, 128, 89328938. (b) Nagai, Y.; Unsworth, L. D.; Koutsopoulos, S.; Zhang, S. G. J. Controlled Release 2006, 115, 18-25. (c) Ulijn, R. V. J. Mat. Chem. 2006, 16, 2217-2225. (d) Tamaru, S.; Takeuchi, M.; Sano, M.; Shinkai, S. Angew. Chem., Int. Ed. 2002, 41, 853-856. (e) Kobayashi, S.; Hamasaki, N.; Suzuki, M.; Kimura, M.; Shirai, H.; Hanabusa, K. J. Am. Chem. Soc. 2002, 124, 6550-6551. (f) Moreau, J. J. E.; Vellutini, L.; Man, M. W. C.; Bied, C. J. Am. Chem. Soc. 2001, 123, 1509-1510.

nogels respond to light irradiation to effect the solution-gel phase transition. LMWGs featuring other photoresponsive moieties (e.g., stilbene) have also been developed.7 We have recently reported a new LMWG with a tetrathiafulvalene moiety, and the corresponding solution-gel phase transition can be tuned by oxidation and reduction as well as by reactions with electron acceptors.8 Akutagawa et al.9 and Shinkai et al.10 separately described the organogels derived from tetrathiafulvalene derivatives. Amabilino et al.11 have very recently detailed another new LMWG containing a tetrathiafulvalene moiety. It is well known that anthracene and its derivatives can be transformed into photodimers after UV light irradiation and that the photodimers can be dissociated into the monomers after heating or further visible light irradiation.12 Therefore, an anthracene moiety can be incorporated into LMWGs to produce photoresponsive organogels. In fact, several anthracene derivatives with steroidal groups are found to be efficient LMWGs,13 although only a limited number of photodimers can be formed after exposure of these organogels to UV light irradiation.13,14 (5) (a) Kawano, S.; Fujita, N.; Shinkai, S. J. Am. Chem. Soc. 2004, 126, 85928593. (b) Kawano, S.; Fujita, N.; Shinkai, S. Chem.sEur. J. 2005, 11, 47354742. (c) Wang, S.; Shen, W.; Feng, Y. L.; Tian, H. Chem. Commun. 2006, 1497-1499. (d) de Jong, J. J. D.; Lucas, L. N.; Kellogg, R. M.; van Esch, J. H.; Feringa, B. L. Science 2004, 304, 278-281. (e) Yang, Z. M.; Ho, P. L.; Liang, G. L.; Chow, K. H.; Wang, Q. G.; Cao, Y.; Guo, Z. H.; Xu, B. J. Am. Chem. Soc. 2007, 129, 266-267. (6) (a) Yagai, S.; Iwashima, T.; Kishikawa, K.; Nakahara, S.; Karatsu, T.; Kitamura, A. Chem.sEur. J. 2006, 12, 3984-3994. (b) Yagai, S.; Nakajima, T.; Kishikawa, K.; Kohmoto, S.; Karatsu, T.; Kitamura, A. J. Am. Chem. Soc. 2005, 127, 11134-11139. (c) Koumura, N.; Kudo, M.; Tamaoki, N. Langmuir 2004, 20, 9897-9900. (d) van der Laan, S.; Feringa, B. L.; Kellogg, R. M.; van Esch, J. Langmuir 2002, 18, 7136-7140. (e) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 6664-6676. (7) (a) Wang, R.; Geiger, C.; Chen, L. H.; Swanson, B.; Whitten, D. G. J. Am. Chem. Soc. 2000, 122, 2399-2400. (b) Geiger, C.; Stanescu, M.; Chen, L. H.; Whitten, D. G. Langmuir 1999, 15, 2241-2245. (c) Eastoe, J.; Sanchez-Dominguez, M.; Wyatt, P.; Heenan, R. K. Chem. Commun. 2004, 2608-2609. (8) Wang, C.; Zhang, D. Q.; Zhu, D. B. J. Am. Chem. Soc. 2005, 127, 1637216373. (9) Akutagawa, T.; Kakiuchi, K.; Hasegawa, T.; Noro, S.; Nakamura, T.; Hasegawa, H.; Mashiko, S.; Becher, J. Angew. Chem., Int. Ed. 2005, 44, 72837287. (10) Kitahara, T.; Shirakawa, M.; Kawano, S.; Beginn, U.; Fujita, N.; Shinkai, S. J. Am. Chem. Soc. 2005, 127, 14980-14981. (11) Puigmarti-Luis, J.; Laukhin, V.; del Pino, A. P.; Vidal-Gancedo, J.; Rovira, C.; Laukhina, E.; Amabilino, D. B. Angew. Chem., Int. Ed. 2007, 46, 238-241.

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Organogels based on the binary gelators composed of anthracene9-carboxylate and alkylammonium show light and temperature responsiveness; UV light irradiation induces the transition of the gels into solutions, and the gel phases can be regenerated by heating the solutions.14 Besides steroidal groups, urea moieties are also frequently employed as building blocks of LMWGs. This is because urea derivatives are able to form extended chains of intermolecular H bonds. A number of LMWGs with urea groups have been described.8,15 A new LMWG (1) with anthracene and urea moieties was designed to take advantage of the features of both. In this article, we will present studies of organic gels based on LMWG 1, in particular, the significant fluorescence enhancement observed for the solution of LMWG 1 after gelation of the organic solvents.16 We observed the formation of the photodimer (with h-t conformation) of LMWG 1 after UV light irradiation, which is also able to gel several organic solvents. Our studies also indicate that neither the gel phase based on LMWG 1 nor that based on the photodimer can be transformed to the solution by respective UV light irradiation or visible light irradiation/heating. Experimental Section Materials. Tetrahydrofuran (THF), anthrone, PPh3, and NaN3 were obtained from Beijing Chemical Company. Dodecylisocyanate was purchased from Acros (Belgium). All solvents were purified and dried following standard procedures, unless otherwise stated. 9-(2-Bromoethoxy)anthracene (2) was synthesized according to the literature.17 (Caution! NaN3 is potentially explosiVe and should be handed in small quantities.) Characterization Techniques. Melting points were measured with an XT4-100X apparatus and uncorrected. The 1H NMR and 13C NMR spectra were recorded with Bruker DMX 300 MHz, Avance 400, and Avance 600 MHz spectrometers. MS spectra were determined with BEFLEX III for TOF-MS and AEI-MS 50 for EI-MS. HRMS was carried out with FTICR-APEX II. Elemental analyses were performed on a Carlo-Erba-1106 instrument. Infrared spectra were obtained on a Perkin-Elmer System 2000 FT-IR spectrometer. XRD data were collected on a Rigaku D/max-2500 X-ray diffractometer with Cu KR radiation (λ ) 1.5406 Å). Absorption spectra were recorded with a Hitachi (model U-3010) spectrophotometer. Fluorescence measurements were carried out with a Hitachi (model F-4500) spectrophotometer. For all scanning (12) (a) Bouas-Laurent, H.; Castellan, A.; Desvergne, J. P.; Lapouyade, R. Chem. Soc. ReV. 2001, 30, 248-263. (b) Bouas-Laurent, H.; Castellan, A.; Desvergne, J. P.; Lapouyade, R. Chem. Soc. ReV. 2000, 29, 43-55. (c) BouasLaurent, H.; Castellan, A.; Desvergne, J. P.; Lapouyade, R. Pure Appl. Chem. 1980, 52, 2633-2648. (d) Bailey, D.; Williams, V. E. J. Org. Chem. 2006, 71, 5778-5780. (e) Zhang, G. X.; Zhang, D. Q.; Zhao, X. H.; Ai, X. C.; Zhang, J. P.; Zhu, D. B. Chem.sEur. J. 2006, 12, 1067-1073. (13) Lin, Y.-C.; Kachar, B.; Weiss, R. G. J. Am. Chem. Soc. 1989, 111, 55425551. (14) Ayabe, M.; Kishida, T.; Fujita, N.; Sada, K.; Shinkai, S. Org. Biomol. Chem. 2003, 1, 2744-2747. (15) (a) George, M.; Tan, G.; John, V. T.; Weiss, R. G. Chem.sEur. J. 2005, 11, 3243-3254. (b) Wu¨rthner, F.; Hanke, B.; Lysetska, M.; Lambright, G.; Harms, G. S. Org. Lett. 2005, 7, 967-970. (c) Schoonbeek, F. S.; van Esch, J. H.; Hulst, R.; Kellogg, R. M.; Feringa, B. L. Chem.sEur. J. 2000, 6, 2633-2643. (d) van Esch, J.; Schoonbeek, F.; de Loos, M.; Kooijman, H.; Spek, A. L.; Kellogg, R. M.; Feringa, B. L. Chem.sEur. J. 1999, 5, 937-950. (e) Dautel, O. J.; Robitzer, M.; Lere-Porte, J. P.; Serein-Spirau, F.; Moreau, J. J. E. J. Am. Chem. Soc. 2006, 128, 16213-16223. (f) Varghese, R.; George, S. J.; Ajayaghosh, A. Chem. Commun. 2005, 593-595. (16) There are also some examples of organogels showing strong fluorescent enhancement: (a) Ryu, S. Y.; Kim, S.; Seo, J.; Kim, Y. W.; Kwon, O. H.; Jang, D. J.; Park, S. Y. Chem. Commun. 2004, 70-71. (b) An, B. K.; Lee, D. S.; Lee, J. S.; Park, Y. S.; Song, H. S.; Park, S. Y. J. Am. Chem. Soc. 2004, 126, 1023210233. (c) Bao, C. Y.; Lu, R.; Jin, M.; Xue, P. C.; Tan, C. H.; Liu, G. F.; Zhao, Y. Y. Org. Biomol. Chem. 2005, 3, 2508-2512. (d) Manna, S.; Saha, A.; Nandi, A. K. Chem. Commun. 2006, 4285-4287. (17) Pirkle, W. H.; Finn, J. M. J. Org. Chem. 1983, 48, 2779-2780. (18) The fluorescence spectra of LMWG 1 (20 mg/mL) in solution and the gel phase in 1,2-dichloroethane with an excitation wavelength of 372 nm are provided in Supporting Information.

Wang et al. Scheme 1 Chemical Structure of LMWG 1 and the Synthetic Approacha

a (a) 2-Bromoethanol, benzene, reflux for 2 days. (b) NaN , DMF, 3 80 °C. (c) PPh3, THF, 45 °C. (d) Dodecylisocyanate, THF, ambient temperature.

electron microscopy (SEM) experiments, a JEOL JSM 6700F field emission scanning electron microscope was used with a sample, which was sputtered with platinum. Slow evaporation of solvents at low temperature under reduced pressure for 24 h led to xerogels. The xerogels were put onto the clean Si substrate for SEM measurements. Fluorescence lifetimes were obtained using a timecorrelated single-photon-counting spectrometer (model Horiba NAES-1100). For the ultraviolet light irradiation experiment, a 500 W high-pressure mercury lamp (365 nm) was used. Synthesis of LMWG 1. A solution of dodecylisocyanate (0.33 g, 1.56 mmol, 0.38 mL) in dry THF (10 mL) was slowly added to a solution of compound 4 (0.37 g, 1.56 mmol) in dry THF (30 mL) under argon. The reaction mixture was stirred overnight at room temperature. The solvents were removed in vacuum, and the residue was crystallized from CH3OH to afford LMWG 1 (yellow minicrystals, 0.30 g) in 42.9% yield, mp 145-147 °C. 1H NMR (400 MHz, CDCl3): δ 8.26 (m, 3H), 8.02 (m, 2H), 7.49 (m, 4H), 5.06 (s, 1H), 4.71 (s, 1H), 4.30 (br, 2H), 3.84 (br, 2H), 3.24 (t, J ) 5.4 Hz, 2H), 1.57-1.51 (m, 2H), 1.27-1.17 (m, 18H), 0.90 (t, J ) 5.6 Hz, 3H). 13C NMR (150 MHz, CDCl3): δ 158.5, 150.3, 132.4, 128.5, 125.5, 125.4, 124.5, 122.5, 121.9, 75.3, 41.3, 40.7, 31.9, 30.21, 29.6, 29.6, 29.5, 29.3, 26.9, 22.7, 14.1. MS (EI): 448 (M+). IR (KBr, cm-1): 1622, 1577. Anal. calcd for C29H40N2O2: C, 77.64; H, 8.99; N, 6.24. Found: C, 77.30; H, 8.99; N, 6.14. Synthesis of the Photodimer. A solution of LMWG 1 (100 mg) in dry THF (10 mL) was irradiated under a 500 W high-pressure mercury lamp in a N2 atmosphere. After irradiation for 160 min, some white precipitate was formed. The precipitate was washed with ether (3 × 10 mL). After being dried in vacuum, the photodimer was obtained as a white precipitate (60 mg) in 60% yield. 1H NMR (400 MHz, CDCl3): δ 7.01-6.97 (m, 8H), 6.88-6.83 (m, 8H), 4.81 (s, 2H), 4.72 (s, 2H), 4.43 (s, 2H), 3.69 (m, 4H), 3.60 (m, 4H), 3.27 (m, 4H), 1.62-1.55 (m, 4H), 1.38-1.20 (m, 36H), 0.88 (t, J ) 5.7 Hz, 6H). 13C NMR (150 MHz, CDCl3): δ 158.5, 141.4, 140.6, 127.5, 126.1, 125.5, 125.5, 89.2, 64.5, 63.7, 41.3, 40.9, 31.9, 30.4, 29.7, 29.6, 29.4, 29.3, 27.0, 22.7, 14.1. IR (KBr, cm-1): 1632, 1575. HRMS: Anal. calcd for C58H80N4O4: 896.6180. Found: 897.6237 (M + H)+. Gel Formation. In a typical gelation experiment, a weighed amount of gelator 1 and 1.0 mL of the solvent were placed in a test tube, which was sealed and then heated until the compound was dissolved. The solution was then cooled to room temperature.

Results and Discussion Synthesis of LMWG 1 and Gel Formation and Characterization. The synthesis of LMWG 1 is shown in Scheme 1. The reaction of anthrone with 2-bromoethanol led to 2, which was reacted with NaN3 to yield 3 in good yield. The reduction

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Figure 1. Illustration of gel formation in 1,2-dichloroethane containing LMWG 1.

Figure 4. 1H NMR spectra of LMWG 1 in CDCl3 at different concentrations recorded at 298 K.

Figure 2. XRD pattern of the xerogel based on LMWG 1 in 1,2dichloroethane.

Figure 5. Fluorescence spectra of LMWG 1 (20 mg/mL) in solution (black), the gel phase in 1,2-dichloroethane (red), and the solution formed after heating the gel phase (green). The inset shows the reversible variation of the emission intensity at 500 nm during the solution-gel phase transition; λex ) 410 nm.18

Figure 3. SEM image of the xerogel based on LMWG 1 (A) and fluorescence microscopy image of the xerogel (B) in 1,2-dichloroethane.

of 3 yielded 4, which was transformed to LMWG 1 after reaction with dodecylisocyanate. The chemical structure of LMWG 1 was characterized by MS and NMR spectroscopic data, and its purity was checked by elemental analysis.

The gelation ability of 1 was tested in several organic solvents. Among the solvents tested, LMWG 1 was able to gel only 1,2dichloroethane (Table S1 in Supporting Information) when the concentration of LMWG 1 reached 16 mg/mL. An opaque gel was formed when the hot solution of LMWG 1 in 1,2dichloroethane was slowly cooled to room temperature (Figure 1). The gel was transformed to the solution when the gel was heated to 50 °C. This sol-gel interconversion was fully reversible by several cycles of heating and cooling. The xerogel was subjected to XRD analysis, and a sharp peak around 3.2° was observed (Figure 2), corresponding to d ) 2.8 nm, which was slightly longer than the “end-end” length of LMWG 1 (2.3 nm) (Figure S7 in Supporting Information). Figure 3 shows the SEM image of the xerogel of LMWG 1. Interestingly, platelike structures (rather than thin solid fibers) with widths of tens of micrometers and lengths of up to hundreds of micrometerss were observed, and the thin plates were further interconnected to generate an entangled network. The 1H NMR spectra of LMWG 1 recorded in CDCl3 at different concentrations imply that intermolecular H bonds due to urea moieties may be responsible for the gel formation of LMWG 1 in 1,2-dichloroethane. By increasing the concentration of LMWG 1, the corresponding signals at 5.06 and 4.71 ppm due to urea moieties are gradually downfield shifted as shown in Figure 4. For instance, the signals at 5.06 and 4.71 ppm were shifted to 5.31 and 4.91 ppm, respectively, by changing the concentration of LMWG 1 from 2 to 20 mg/mL. Moreover, the

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Scheme 2 Photodimerization of LMWG 1 upon UV Light (365 nm) Irradiation

1H NMR spectra of LMWG 1 were also recorded at different temperatures (Figure S5 of Supporting Information). By increasing the temperature, the corresponding signals due to urea moieties were gradually upfield shifted. These results indicate that intermolecular H bonds are formed between the urea moieties of neighboring molecules of 1. The π-π stacking of anthracene moieties of LMWG 1 may also contribute to gel formation. Gelation-Induced Enhanced Fluorescence Emission. The fluorescence spectra of LMWG 1 in 1,2-dichloroethane (20 mg/ mL) before and after gel formation were recorded. As shown in Figure 5, LMWG 1 exhibits rather weak fluorescence in the solution phase. This is likely due to the concentration quenching effect. However, large fluorescence enhancement is observed after gel formation. Although the gel is not transparent, the fluorescence intensity is enhanced about 10 times in the gel phase compared to that in the solution phase. Moreover, the fluorescence spectra maximum is red shifted by ca. 60 nm after gel formation (Figure 5). We refer to this kind of fluorescence enhancement after gelation as gelation-induced enhanced fluorescence emission. In agreement with the fluorescence spectrum of the gel, the xerogel emits green light as shown in Figure 3B, where the fluorescence microscopy image of the xerogel is displayed. Interestingly, such fluorescence variation observed during the solution-gel phase transition can reversibly proceed for more than 10 cycles as shown in the inset of Figure 5. From the application perspective, this organogel system can be used as a potential thermal sensor and a thermally driven fluorescence molecular switch. The fluorescence decay at 500 nm for the gel of LMWG 1 in 1,2-dichloroethane (20 mg/mL) was measured, and the corresponding fluorescence lifetimes were estimated to be 10.92 and 71.63 ns. The measurement of the fluorescence decay for the corresponding solution of LMWG 1 in 1,2-dichloroethane (20 mg/mL) was not carried out because of the rather weak fluorescence intensity of the solution. However, the fluorescence decay at 418 nm for the solution of LMWG 1 in 1,2-dichloroethane (1.0 × 10-5 M) was recorded, and the fluorescence lifetime was estimated to be 7.62 ns. Thus, it may be concluded that the respective emission states of the solution and gel phases of LMWG 1 in 1,2-dichloroethane are different. We assume that the strong green fluorescence is due to the intermolecular anthracene excimer emission. The strong fluorescence of the gel phase compared to that of the corresponding solution of LMWG 1 may be attributed to the following reasons: (1) It is probable that the intermolecular

H bonds of urea moieties would induce the anthracene moieties of neighboring molecules of LMWG 1 to have close face-to-face contact, which would favor excimer formation. (2) The intermolecular H bonds would probably reduce the bond rotation within LMWG 1 and thus prohibit the nonradiative transitions to some extent.19 (3) According to the previous report, the intermolecular H bonds may also reduce the ISC efficiency for the anthracene moiety of LMWG 1.16a Photodimerization. As mentioned above, the anthracene derivatives can be transformed into photodimers by UV light irradiation (Scheme 2). Figure 6 shows the variation of the absorption spectrum of LMWG 1 in THF after UV light irradiation for different periods. The absorption intensities around 369 nm were reduced gradually, indicating that photodimerization of LMWG 1 occurs according to previous studies.12 The fluorescence of LMWG 1 in THF also became gradually weak after UV light irradiation as shown in Figure 7. Such fluorescence variation also supports the photodimerization of LMWG 1. After the solution of LMWG 1 in THF (10 mg/mL) was exposed to UV light from a 500 W high-pressure Hg lamp for 160 min, a white suspension was obtained. After centrifugation and careful washing with ether, the pure photodimer of LMWG 1 was yielded. There are two possible isomers for the photodimer, h-h and h-t (Scheme 2). As far as the intermolecular H bonding of urea moieties is concerned,20 the h-h photodimer may be produced, but the 1H and 13C NMR spectroscopic studies (Figure S1 of Supporting Information) indicate that only the h-t photodimer was generated. This is probably due to steric hindrance and, in particular, the electronic repulsion due to the oxygen atoms linked to the anthracene ring. Organogel Based on the Photodimer. The photodimer may be a potential LMWG because it contains two urea moieties. As listed in Table 1, the gel-formation ability of the photodimer was tested in several organic solvents. Transparent gels based on the photodimer were formed separately in cyclohexane, n-hexane, and n-heptane. These organogels were characterized by XRD (19) (a) Yu, G.; Yin, S. W.; Liu, Y. Q.; Chen, J. S.; Xu, X. J.; Sun, X. B.; Ma, D. G.; Zhan, X. W.; Peng, Q.; Shuai, Z. G.; Tang, B. Z.; Zhu, D. B.; Fang, W. H.; Luo, Y. J. Am. Chem. Soc. 2005, 127, 6335-6346. (b) Chen, J. W.; Law, C. C. W.; Lam, J. W. Y.; Dong, Y. P.; Lo, S. M. F.; Williams, I. D.; Zhu, D. B.; Tang, B. Z. Chem. Mater. 2003, 15, 1535-1546. (c) Luo, J. D.; Xie, Z. L.; Lam, J. W. Y.; Cheng, L.; Chen, H. Y.; Qiu, C. F.; Kwok, H. S.; Zhan, X. W.; Liu, Y. Q.; Zhu, D. B.; Tang, B. Z. Chem. Commun. 2001, 1740-1741. (20) Because THF is a polar solvent, the intermolecular H bonding due to urea moieties may be weak in THF.

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Figure 6. Absorption spectra of LMWG 1 after UV light (365 nm) irradiation for different periods. The concentration for UV light irradiation experiments was 10 mg/mL in THF, whereas for absorption spectral measurements the solution was diluted to 5.0 × 10-4 M in THF.

Figure 8. XRD pattern of the xerogel based on the photodimer in cyclohexane (A) and the SEM image of the xerogel (B).

Figure 7. Fluorescence spectra of LMWG 1 after UV light (365 nm) irradiation for different periods; λex ) 372 nm. The concentration for UV light irradiation experiments was 10 mg/mL in THF, whereas for fluorescence spectral measurements the solution was diluted to 1.0 × 10-5 M in THF. Table 1. Gelation Experimental Results for the Photodimera

a

solvent

gelation

n-hexane heptane cyclohexane toluene chloroform 1,2-dichloroethane tetrahydrofuran

6 (mg/mL) 6 (mg/mL) 6 (mg/mL) P P P P

Gel, G; solution, S; precipitation, P.

and SEM analyses. For instance, the xerogel of the photodimer in cyclohexane exhibits an XRD pattern with a broad diffraction peak (Figure 8A), indicating that molecules of the photodimer are not arranged in an orderly manner in the xerogel. As displayed in Figure 8B, where the SEM image of the xerogel of the photodimer is shown, it seems that molecules of the photodimer are assembled into thin fibers that are further interconnected to form a 3D entangled network. The 1H NMR spectra of the photodimer were also recorded in CDCl3 at different concentrations. By increasing the concentration, the corresponding signals at 4.82 and 4.71 ppm due

Figure 9. 1H NMR spectra of the photodimer in CDCl3 at different concentrations recorded at 298 K.

to urea moieties were gradually downfield shifted as shown in Figure 9. Similarly, the 1H NMR spectra of the photodimer were also measured at different temperatures (Figure S6 of Supporting Information). By increasing the temperature, the corresponding signals due to urea moieties were gradually upfield shifted. These results imply that the intermolecular H bonds of urea moieties may be responsible for the gel formation. Compared to those observed for LMWG 1 by increasing the concentration, the downfield chemical shifts detected for the photodimer were rather small. This is probably due to the fact that the central core of the photodimer was not planar and, as a result, the neighboring molecules of the photodimer may not be able to be closely associated. Of note is that the gel phase based on the photodimer is very stable. For example, only a rather small portion of the gel phase

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of the photodimer in n-heptane (bp 97-99 °C) was transformed to solution by keeping the gel phase at 96 °C for 1 h.21 Similarly, direct visible light irradiation22 of the gel phase based on the photodimer induced the transition of only a rather small portion of the gel phase into solution. In comparison, the solution-gel phase transition for LMWG 1 in 1,2-dichloroethane can be reversibly carried out by alternating heating and cooling as discussed above. However, the gel phase of LMWG 1 was kept almost unchanged after direct UV light irradiation for several hours. The 1H NMR spectral analysis showed that the amount of LMWG 1 that was converted to the photodimer after UV light irradiation of the gel phase for 9 h could be neglected. This may be interpreted by considering the fact that the gel of LMWG 1 in 1,2-dichloroethane shows strong anthracene excimer emission and accordingly the efficiency of the photodimerization of anthracene moieties would be reduced.12c

Summary New LMWG 1 with anthracene and urea moieties was designed and synthesized. A nontransparent gel of LMWG 1 in 1,2dichloroethane was formed and characterized. The solution-gel (21) It was found that decomposition of the photodimer into the monomer (LMWG 1) occurred by refluxing the toluene solution of the photodimer, but the gel of the photodimer in toluene could not be formed. (22) The light irradiation experiments were performed by exposure of the gel in a quartz cell to a 500 W tungsten lamp, which was held about ca. 1 cm above the quartz cell.

Wang et al.

phase transition can be reversibly carried out by alternating heating and cooling, but UV light irradiation cannot induce the transition of the gel based on LMWG 1 into solution. Of particular interest is the observation of significant fluorescence enhancement after gelation, which is referred as to gelation-induced enhanced fluorescence emission. UV light irradiation of the THF solution of LMWG 1 yielded a photodimer with the h-t conformation. The photodimer can gel several organic solvents, including cyclohexane, n-hexane, and n-heptane. It should be mentioned that the gel based on the photodimer is rather stable. Only a rather small portion of the gel phase based on the photodimer is transformed into solution after either heating or visible light irradiation. Acknowledgment. This research was financially supported by the NSFC, the Chinese Academy of Sciences, and the State Key Basic Research Program. D.Z. thanks the National Science Fund for Distinguished Young Scholars. Supporting Information Available: Synthesis of 3 and 4, gel formation test for LMWG 1, structural characterization of the photodimer, fluorescence spectra of LMWG 1 in the solution and gel phases with an excitation wavelength of 372 nm, fluorescence lifetimes of LMWG 1 in dilute solution and in the gel state, fluorescence spectra of the xerogel suspended in water and the thin film of the xerogel, 1H NMR spectra of LMWG 1 and the photodimer in CDCl3 at different temperatures, and the energy-minimized structure of LMWG 1. This material is available free of charge via the Internet at http://pubs.acs.org. LA701142D