Strategy to Control the Chromism and Fluorescence Emission of a

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Strategy to Control the Chromism and Fluorescence Emission of a Perylene Dye in Composite Organogel Phases Oudjaniyobi Simalou,† Xiaogang Zhao,‡ Ran Lu,*,† Pengchong Xue,† Xinchun Yang,† and Xiaofei Zhang† †

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry and ‡ Alan G MacDiarmid Institute, Jilin University, Changchun 130012, PR China Received July 8, 2009. Revised Manuscript Received September 1, 2009

Composite organogels based on 1,3,5-tris(4-dodecyloxybenzoylamino)phenylbenzene (DBAPB), a known gelator, and N,N0 -di(octadecyl)-perylene-3,4,9,10-tetracarboxylic diimide (C18PTCDI), a nongelator dye, have been achieved, leading to controllable color and emitting color changes. SEM images and XRD patterns revealed that the packing of the DBAPB-based gelator could almost be maintained in the composite gels. The temperaturedependent UV-vis absorption and temperature-dependent fluorescence emission spectra illustrated that the color and emitting color of the composite gels could be controlled by the content of C18PTCDI as well as the temperature in the gel phases. When the content of C18PTCDI was 1 mol %, C18PTCDI could be isolated as unimolecules in the composite gel, which was yellow and gave bright greenish-yellow emission under 365 nm light. For the mixed systems containing 2-10 mol % C18PTCDI, the fresh gels, which were obtained after cooling the hot solutions for a short time, were yellow and produced greenish-yellow emission under 365 nm illumination. However, the corresponding stable composite gels, which were obtained via prolonging the cooling time, were red and emitted weak red emission excited by UV light as a result of the formation of C18PTCDI aggregates. The reversible color and emitting color changes could be realized in the gel phases over a narrow temperature range. Moreover, the excitation energy of DBAPB could be transferred to C18PTCDI in the composite gels, leading to obvious emission quenching of the former.

Introduction Within a very short time period, organogels have developed from a chemical and physical curiosity into a highly promising new area of research.1 Their well-defined structures, large interfacial area, soft nature, modulation of their optical and photophysical properties, and sensitivity to some chemical and physical triggers make organogels very attractive for template synthesis of ordered superstructures, membrane separation technology, catalysis, cosmetics, drug delivery, molecular electronics and related fields, and so forth.2-4 In particular, π gels have received much attention recently on account of their potential applications in

various optoelectronic fields, including enhanced charge transport, fluorescence, and sensing.4,5 The organic π-conjugated molecules play a crucial role in supramolecular devices because their optical and electronic properties can be modulated by intermolecular interactions.4d,e,6,7 Among organic π-conjugated systems, perylene derivatives have attracted a great deal of attention as a result of their unique optical and photophysical properties and high thermal and photochemical stability and have been used as building blocks for organic superconductors,8 solar cells,9 fluorescent solar collectors,10 and lasers.11 It is notable that some perylene derivatives have self-assembled into organogels,12 which have been applied in light-harvesting systems, photovoltaic cells, field-effect transistors, and light-emitting diodes.13 It is well

*Corresponding author. E-mail: [email protected]. (1) Esch, J. H. V.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263. (2) (a) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (b) Saiani, A.; Guenet, J. M. Macromolecules 1997, 30, 967. (c) Saiani, A.; Guenet, J. -M. Macromolecules 1999, 32, 659. (d) Beginn, U.; M€oller, M. In Supramolecular Materials and Technologies; Reinhoudt, D. N., Ed.; Wiley: Chichester, U.K., 1999; Vol. 89. (e) Subi, J. G.; Ajayaghosh, A. Chem.;Eur. J. 2005, 11, 3217. (f) Ajayaghosh, A.; Praveen, V. K. Acc. Chem. Res. 2007, 40, 644. (3) (a) Xue, P. C.; Lu, R.; Li, D. M.; Jin, M.; Bao, C. Y.; Zhao, Y. Y.; Wang, Z. M. Chem. Mater. 2004, 16, 3702. (b) Xue, P. C.; Lu, R.; Huang, Y.; Jin, M.; Tan, C. H.; Bao, C. Y.; Wang, Z. M.; Zhao, Y. Y. Langmuir 2004, 15, 6470. (c) Xue, P. C.; Lu, R.; Li, D. M.; Jin, M.; Tan, C. H.; Bao, C. Y.; Wang, Z. M.; Zhao, Y. Y. Langmuir 2004, 20, 11234. (d) Bao, C. Y.; Jin, M.; Lu, R.; Song, Z. G.; Yang, X. C.; Song, D. P.; Xu, T. H.; Liu, G. F.; Zhao, Y. Y. Tetrahedron 2007, 63, 7443. (4) (a) Schenning, A. P. J.; Meijer, E. W. Chem. Commun. 2005, 3245. (b) Grimsdale, A. C.; M€ullen, K. Angew. Chem., Int. Ed. 2005, 44, 5592. (c) Lloyd Carroll, R.; Gorman, C. B. Angew. Chem., Int. Ed. 2002, 41, 4378. (d) Liu, B.; Gaylord, B. S.; Wang, S.; Bazan, G. C. J. Am. Chem. Soc. 2003, 125, 6705. (e) Levvitus, M.; Schmieder, K.; Ricks, H.; Shimizu, k. D.; Bunz, U. H. F.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2001, 123, 4259. (5) (a) Xue, P. C.; Lu, R.; Chen, G. J.; Zhang, Y.; Nomoto, H.; Takafuji, M.; Ihara, H. Chem.;Eur. J. 2007, 13, 8231. (b) Bao, C. Y.; Lu, R.; Jin, M.; Xue, P. C.; Tan, C. H.; Xu, T. H.; Liu, G. F.; Zhao, Y. Y. Chem.;Eur. J. 2006, 12, 3287. (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.

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(6) (a) Nelson, J. C.; Saven, J. G.; Moore, J. S.; Wolyness, P. G. Science 1997, 277, 1793. (b) Moore, J. S. Acc. Chem. Res. 1997, 30, 402. (c) Prince, R. B.; Brunsveld, L.; Meijer, E. W.; Moore, J. S. Angew. Chem. 2000, 112, 234. Angew. Chem., Int. Ed. 2000, 39, 228. (d) Brunsveld, L.; Meijer, E. W.; Prince, R. B.; Moore, J. S. J. Am. Chem. Soc. 2001, 123, 7978. (7) (a) Mcquade, D. T.; Kim, J.; Swager, T. M. J. Am. Chem. Soc. 2000, 122, 5885. (b) Mcquade, D. T.; Hegedous, A. H.; Swager, T. M. J. Am. Chem. Soc. 2000, 122, 12389. (c) Bunz, U. H. F. Chem. Rev. 2000, 100, 1605. (d) Bunz, U. H. F. Acc. Chem. Res. 2001, 34, 998. (e) Kim, J.; Swager, T. M. Nature 2001, 411, 1030. (f) Arnt, L.; Tew, G. N. J. Am. Chem. Soc. 2002, 124, 7664. (g) Breitenkamp, R. B.; Tew, G. N. Macromolecules 2004, 37, 1163. (8) Inokuchi, H. Angew. Chem., Int. Ed. 1988, 27, 1747. (9) Hiramoto, H.; Kishigami, M.; Yokoyama, M. Chem. Lett. 1990, 119. (10) Seybold, G.; Wagenblast, G. Dyes Pigm. 1989, 11, 303. (11) (a) Gvishi, R.; Reisfeld, R.; Burshtein, Z. Chem. Phys. Lett. 1993, 213, 338. (b) Sadrai, M.; Hadel, L.; Sauers, R.; Husain, S.; Krogh-Jespersen, K.; Westbrook, J. D.; Bird, G. R. J. Phys. Chem. 1992, 96, 7988. (12) (a) W€urthner, F.; Hanke, B.; Lysetska, M.; Lambright, G.; Harms, G. S. Org. Lett. 2005, 7, 967. (b) Li, X.-Q.; Stepanenko, V.; Chen, Z.; Prins, P.; Siebbeles, L. D. A.; W€urthner, F. Chem. Commun. 2006, 3871. (c) Sugiyasu, K.; Fujita, N.; Shinkai, S. Angew. Chem., Int. Ed. 2004, 43, 1229. (13) (a) W€urthner, F. Chem. Commun. 2004, 1564. (b) W€urthner, F.; Thalacker, C.; Sautter, A.; Sch€artl, W.; Ibach, W.; Hollricher, O. Chem.;Eur. J. 2000, 6, 3871.

Published on Web 09/09/2009

DOI: 10.1021/la902457k

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Letter Scheme 1. Molecular Structures of DBAPB and C18PTCDI

known that perylene tetracarboxylic diimide can be used as a class of n-type semiconductor.14-17 In addition, its carrier mobility would be enhanced in well-defined 1D nanostructures based on semiconductors.18 In this work, we have constructed functional nanofibers based on a perylene diimide derivative (C18PTCDI) assisted by a known gelator, triphenylbenzene discotic molecule (DBAPB),5b which have been synthesized by our group previously (Scheme 1). It is interesting that C18PTCDI could be maintained in a monomolecular state either in a system containing less than 1 mol % C18PTCDI or in fresh composite gels with 2-10 mol % C18PTCDI, which looked yellow and produced strong greenish-yellow emission under UV illumination. However, in the aging composite gels with 2-10 mol % C18PTCDI, the aggregates of C18PTCDI emerged, accompanied by a color change from yellow to red. Meanwhile, the stable gel produces weak red emission. Notably, the color and the emitting color of the composite gels could be changed from red to yellow, and the reversible color change in the gel phases depend not only on the content of C18PTCDI but also on the gelation time or the temperature. Moreover, resonance energy transfer (RET) from DBAPB to C18PTCDI occurred in the composite gels.

Results and Discussion Self-Assembly Properties. 1,3,5-Tris(4-dodecyloxybenzoylamino)phenylbenzene (DBAPB) was synthesized according to ref 5b, and N,N0 -di(octadecyl)-perylene-3,4,9,10-tetracarboxylic diimide (C18PTCDI) was obtained by standard procedures.18 Composite gels containing DBAPB and C18PTCDI were prepared by cooling the hot toluene solutions naturally. The concentration of DBAPB was kept at 0.5 w/v % in the samples unless otherwise noted, and the content of C18PTCDI was assessed by its molar percent relative to the total concentration of DBAPB and C18PTCDI. (14) (a) Newman, C. R.; Frisbie, C. D.; da Silva Filho, D. A.; Bredas, J.-L.; Ewbank, P. C.; Mann, K. R. Chem. Mater. 2004, 16, 4436. (b) Xu, B. Q.; Xiao, X.; Yang, X.; Zang, L.; Tao, N. J. J. Am. Chem. Soc. 2005, 127, 2386. (c) Li, X.; Xu, B. Q.; Xiao, X.; Yang, X.; Zang, L.; Tao, N. J. Faraday Discuss. 2006, 131, 111. (d) Horowitz, G.; Kouki, F.; Spearman, P.; Fichou, D.; Nogues, C.; Pan, X.; Garnier, F. Adv. Mater. 1996, 8, 242. (15) Law, K.-Y. Chem. Rev. 1993, 93, 449. (16) (a) Gronheid, R.; Stefan, A.; Cotlet, M.; Hofkens, J.; Qu, J.; Muellen, K.; Van der Auweraer, M.; Verhoeven, J. W.; De Schryver, F. C. Angew. Chem., Int. Ed. 2003, 42, 4209. (17) (a) Liu, R.; Holman, M. W.; Zang, L.; Adams, D. M. J. Phys. Chem. A 2003, 107, 6522. (b) Holman, M. W.; Liu, R.; Zang, L.; Yan, P.; Dibenedetto, S. A.; Bowers, R. D.; Adams, D. M. J. Am. Chem. Soc. 2004, 126, 16126. (c) Sauer, M. Angew. Chem., Int. Ed. 2003, 42, 1790. (d) Grimsdale Andrew, C.; M€ullen, K. Angew. Chem., Int. Ed. 2005, 44, 5592. (18) Balakrishnan, K.; Datar, A.; Naddo, T.; Huang, J.; Oitker, R.; Yen, M.; Zhao, J.; Zang, L. J. Am. Chem. Soc. 2006, 128, 7390.

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On the basis of our previous work, we found that DBAPB selfassembled into right- and left-handed fibers in the gel phases. As shown in Figure 1, the SEM images of the dried composite gels obtained from toluene also exhibit right- and left-handed fibers with a diameter of about 300 nm, which are similar to those in the neat DBAPB xerogel except for the larger diameters. We can also see that the helical fibers are built up from thinner helical ones with diameters of 30-70 nm.5b This illustrates that the morphology of the host DBAPB gel is almost maintained in the composite gels. To evaluate whether the molecular packing mode of DBAPB changed in the composite gels, small-angle X-ray diffraction measurements were made. The XRD patterns of xerogels containing e10 mol % C18PTCDI were similar to that of the neat DBAPB xerogel (Figure 1f). For example, two diffraction peaks corresponding to d spacings of 6.54 and 2.50 nm can be detected for the pure DBAPB xerogel, and the xerogel containing 2 mol % C18PTCDI also gave two noticeable peaks at d spacings of 6.27 and√ 2.62 nm. The periodicity of the two diffractions is closed to 1:1/ 6, which means that the molecules are probably packed into a hexagonal columnar structure in the composite gels.5b Therefore, the hexagonal columnar packing mode of DBAPB-based gelator molecules could be preserved in the composite gels containing e10 mol % C18PTCDI. However, when the concentration of C18PTCDI reached 33 mol %, the XRD pattern of the composite gel not only exhibited the peaks √ corresponding to d spacings of 6.45 and 2.61 nm, close to 1:1/ 6, but also exhibited peaks at d spacings of 3.30, 1.62, and 1.10 nm (Figure 1f), with periodicity close to 1:1/2:1/3 suggesting the coexistence of hexagonal columnar and layered aggregations.19 On the basis of the XRD pattern of C18PTCDI bulk material (Figure S1b), we could deduce that C18PTCDI molecules in such a composite gel might pack into a layered structure with a period of 3.30 nm. Therefore, the well-organized aggregations of C18PTCDI and helical fibers of DBAPB could be obtained in the composite gels. It can be further confirmed by the fluorescence microscopy images of the xerogels. As shown in Figure S2, the neat gel fibers based on DBAPB emit greenish-blue light under irradiation from 330 to 385 nm, and the xerogel containing 33 or 50 mol % C18PTCDI emits red light under irradiation from 330 to 385 and from 510 to 550 nm, depicting the formation of C18PTCDI-based 1D fibers. Chromism Properties. When cooling the hot solutions containing DBAPB and C18PTCDI in toluene naturally, we found that yellow or red composite gels could be formed and the color of the composite gels changed with the cooling time as well as with the concentration of C18PTCDI (Figure S3). For instance, the solution of the sample containing 1 mol % C18PTCDI in DBAPB was yellow, and it changed to a yellow gel after cooling the hot solution for 12 min. Then, after more than 12 h, the gel still remained yellow. However, the sample containing 5 mol % C18PTCDI gave a yellow gel within 6 min after cooling, and the gel became red 12 min later. Furthermore, the solution and the gel looked dark red for the sample containing more than 33 mol % C18PTCDI. We suggest that such color changes are related to the molecular aggregation of C18PTCDI. For example, for a low concentration of C18PTCDI (e1 mol %), C18PTCDI molecules existed as isolated monomers entrapped in a DBAPB gel scaffold, so the yellow gel could be observed. When the hot yellow solutions of the samples containing 2-10 mol % C18PTCDI were cooled to room temperature for a short period of time, the fresh gels were yellow on account of no aggregation of C18PTCDI. However, (19) (a) Jung, J. H.; Kobayashi, H.; Masuda, M.; Shimizu, T.; Shinkai, S. J. Am. Chem. Soc. 2001, 123, 8785. (b) Estroff, L. A.; Leiserrowitz, L.; Addadi, L.; Weiner, S.; Hamilton, A. D. Adv. Mater. 2003, 15, 38. (c) Esch, J. V.; Schoonbeek, F.; de Loos, M.; Kooijman, H.; spek, A. L.; Kellogg, R. M.; Feringa, B. L. Chem.;Eur. J. 1999, 5, 937.

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Figure 1. SEM images of the xerogel based on DBAPB (0.5 w/v %) (a) and composite xerogels containing 2 (b), 10 (c), 33 (d), and 50 mol % (e) C18PTCDI in DBAPB (0.5 w/v %) obtained from toluene gels. R and L show right- and left-handed helical fibers, respectively. Small-angle X-ray diffraction patterns of the xerogel based on DBAPB (0.5 w/v %) (f, curve i) and the composite xerogels containing 2 mol % (f, curve ii) and 33 mol % (f, curve iii).

with prolonged aging time C18PTCDI might aggregate gradually, and the yellow gels turned into red ones, which could be further confirmed by the temperature-dependent UV-vis absorption spectra (Figure 2). Because of the high optical density of DBAPB around 300 nm at 0.5 w/v %, only the absorbance over 375 nm has been depicted. As shown in Figure 2a, in the hot solution and in the gel of the sample containing 1 mol % C18PTCDI we can find three pronounced peaks in the range of 450-525 nm, which correspond to the 0-0, 0-1, and 0-2 electronic transitions of C18PTCDI, respectively,18 and the solution and the gel were yellow. Furthermore, the excitation spectra of this sample did not change during the sol-gel process as shown in Figure S4c, suggesting that C18PTCDI existed as monomers in the gel state, similar to the sol state. When the hot solution of the sample containing 5 mol % C18PTCDI was cooled, the absorption bands at 450-525 nm decreased gradually (a shoulder around 425 nm Langmuir 2009, 25(19), 11255–11260

could be detected corresponding to the 0-3 electronic transition of C18PTCDI, which is not detectable in Figure 2a because of the overlap with the absorbance of DBAPB), and one new peak at ca. 580 nm appeared (Figure 2b), which was due to the formation of the aggregates of C18PTCDI.12,13a,14a,15,18,20,21 Moreover, the (20) (a) Herrikhuyzen, J.; Syamakumari, A.; Schenning, A. P. H. J.; Meijer, E. W. J. Am. Chem. Soc. 2004, 126, 10021. (b) Li, X.; Sinks, L. E.; Rybtchinski, B.; Wasielewski, M. R. J. Am. Chem. Soc. 2004, 126, 10810. (c) van der Boom, T.; Hayes, R. T.; Zhao, Y.; Bushard, P. J.; Weiss, E. A.; Wasielewski, M. R. J. Am. Chem. Soc. 2002, 124, 9582. (d) Ahrens, M. J.; Sinks, L. E.; Rybtchinski, B.; Liu, W.; Jones, B. A.; Giaimo, J. M.; Gusev, A. V.; Goshe, A. J.; Tiede, D. M.; Wasielewski, M. R. J. Am. Chem. Soc. 2004, 126, 8284. (e) Wang, W.; Li, L. S.; Helm, G.; Zhou, H. H.; Li, A. D. Q. J. Am. Chem. Soc. 2003, 125, 1120. (f) Schenning, A. P. H. J.; Herrikhuyzen, J. V.; Jonkheijm, P.; Chen, Z.; W€urthner, F.; Meijier, E. W. J. Am. Chem. Soc. 2002, 124, 10252. (21) (a) Kazmaier, P. M.; Hoffmann, R. J. Am. Chem. Soc. 1994, 116, 9684. (b) Balakrishnan, K.; Datar, A.; Oitker, R.; Chen, H.; Zuo, J.; Zang, L. J. Am. Chem. Soc. 2005, 127, 10496.

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Figure 2. Temperature-dependent UV-vis absorption spectra of the mixed sample containing 1 (a) and 5 mol % (b) C18PTCDI in DBAPB. (c) Plots of Tgel versus the content of C18PTCDI in composite gels based on DBAPB, from left to right: 0 mol %, 94 °C; 1 mol %, 79 °C; 2 mol %, 92 °C; 2.5 mol %, 98 °C; 5 mol %, 100 °C; 10 mol %, 103 °C; 20 mol %, 105 °C; 33 mol %, 106 °C; and 50 mol %, 106.5 °C. The inset shows the reversible thermochromism of the composite gel with 5 mol % C18PTCDI. The concentration of DBAPB was 0.5 w/v %.

obvious decrease in the intensity of the excitation bands ascribed to C18PTCDI with decreasing temperature also indicated the aggregation of C18PTCDI in the stable composite gel (Figure S4d). Therefore, C18PTCDI could be entrapped within the gel medium as isolated molecules at low concentration (such as 1 mol % C18PTCDI) or in the initial stage of gel formation at a concentration of 2-10 mol % C18PTCDI in the composite gels, which were yellow. However, the aggregates of C18PTCDI could be formed after the stable gels containing 2-10 mol % C18PTCDI were formed, so the yellow gels turned into the red ones. C18PTCDI preferred to aggregate even in hot solution if its content was higher than 33 mol %, so the color of the solutions and the gels containing more than 33 mol % C18PTCDI was red. In addition, we also find that a red gel could be changed into a yellow one when it is heated below Tgel (inset of Figure 2c). Figure 2c gives the plot of Tgel against the concentration of C18PTCDI, and we can see that Tgel of the composite gel containing 1 mol % C18PTCDI was lower than that of the pure DBAPB gel. Moreover, the higher the concentration of the dye, the higher the Tgel of the composite gels. It has been known that C18PTCDI existed as monomers in the composite gel containing 1 mol % C18PTCDI. In this case, the isolated C18PTCDI molecules might be entrapped in the pores of the gel networks, so they might have an effect on the aggregates of DBAPB, resulting in a decrease in Tgel compared with that of pure DBAPB. Meanwhile, the rheological experiments22 (Figure S7) revealed that the time constant (τ) representing the recovery speed of the pure DBAPB gel after being sheared at a constant shear rate of 2 s-1 for 600 s was 244 s, which was shorter than that of the composite gel containing 1 mol % C18PTCDI (428 s). This result agreed with the observation shown in Figure S3 that it would take longer for the formation of the gel from the mixed sample containing 1 mol % C18PTCDI in DBAPB than from neat DBAPB. In the stable composite gels with high C18PTCDI (22) Huang, X.; Raghavan, S. R.; Terech, P.; Weiss, R. G. J. Am. Chem. Soc. 2006, 128, 15341.

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content (such as 10 mol %), most C18PTCDI molecules should exist as aggregates that might be adsorbed on or be adjacent to the DBAPB gel fibers. The interactions between the aggregates of C18PTCDI and DBAPB gel fibers as well as between the aggregates of C18PTCDI themselves should favor the stability of the composite gel phases, so Tgel increased with increasing concentration of C18PTCDI. Notably, as shown in the inset of Figure 2c, the gel containing 5 mol % C18PTCDI is red at 50 °C and became yellow at 60 °C; meanwhile, the yellow gel could also be changed into a red one under cooling from 60 to 50 °C. This transformation should be due to the formation of aggregates of C18PTCDI at lower temperature and the disaggregation of aggregates at higher temperature. On one hand, we found that C18PTCDI can be easily dissolved in toluene at 60 °C under the same concentration and the solution is yellow. On the other hand, the composite gels cannot be destroyed at 60 °C, which is lower than Tgel. Therefore, the isolated C18PTCDI molecules should be dispersed and entrapped in the pores of DBAPB fibrous networks at 60 °C, which gives a yellow gel. When the temperature decreased to 50 °C, C18PTCDI molecules should aggregate either in the pores of the networks or along the DBAPB-based gel fibers, resulting in the appearance of the red gel. Moreover, such a red gel could be turned into a yellow one on account of the disaggregation of the self-assemblies of C18PTCDI under heating. Therefore, reversible thermochromism could be realized in a narrow temperature range in the gel phases, which allowed these composite gels to be used as thermosensitive materials. It must be noted that the concentrations of DBAPB and the gelator also play a role in the chromism process of the composite gels. For instance, we found that the time required for the color change from yellow to red (or from red to yellow) was longer in the composite gel with 1.0 w/v % DBAPB than in the composite gel with 0.5 w/v % DBAPB when the content of C18PTCDI was maintained at 5 mol %. It was understandable that the gel networks were much tighter under a higher concentration of gelator, so that the aggregation and disaggregation of C18PTCDI became more difficult, resulting in Langmuir 2009, 25(19), 11255–11260

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Figure 3. Time-dependent emission spectra of the samples containing 1 mol % (a, b) and 5 mol % (c, d) C18PTCDI in DBAPB (0.5 w/v %) upon cooling the hot toluene solutions from 108 to 20 °C; the excitation wavelengths are 300 nm (a, c) and 525 nm (b, d). (b) The inset shows the stable composite gel with 1 mol % C18PTCDI under 365 nm light, and the insets in plot d show the composite gel containing 5 mol % C18PTCDI after cooling the solution for 6 and 12 min under 365 nm light. The gray lines represent the hot yellow solutions, the black solid lines represent the yellow gels, and the light-gray lines bounded by two black dotted lines belong to the red gels.

a slower chromism process. Furthermore, no stable composite gels with a high content of C18PTCDI could be generated when the concentration of DBAPB was lower than 0.4 w/v %. To get stable composite gels with fast chromism, 0.5 w/v % DBAPB was selected in this work. Controllable Fluorescence Emission and RET. The fluorescence emission of the composite gels based on DBAPB and C18PTCDI can also be controlled by the concentration of C18PTCDI and the temperature. For instance, from the inset of Figure 3b, it is clear that the stable composite gel containing 1 mol % C18PTCDI exhibits strong greenish-yellow emission under 365 nm light. However, after we cooled the hot solution of the sample containing 5 mol % C18PTCDI for 6 min, the obtained fresh yellow gel emitted greenish-yellow light under 365 nm illumination whereas the corresponding aging composite gel emitted weak red light after the sample was cooled for 12 min (inset of Figure 3d). In addition, the gel exhibiting weak red emission can be changed into the one emitting strong greenishyellow emission under 365 nm light via heating below Tgel. Therefore, the fluorescence emission of the composite gels can be reversibly controlled by the temperature, opening the way to the development of novel thermosensitive fluorescent soft materials. The temperature-dependent fluorescence spectra given in Figure 3 illustrate such fluorescence change behaviors. When the sample containing 1 mol % C18PTCDI was excited at 300 nm, the emission intensity at 420 nm ascribed to DBAPB increased significantly with decreasing temperature, which could be explained by aggregation-induced emission.5b,23 The emission band at 540 nm ascribed to C18PTCDI increased slightly once the gel was formed because of partial excitation-energy transfer from DBAPB to monomers of C18PTCDI, which will be discussed (23) (a) Babu, S. S.; Praveen, V. K.; Prasanthkumar, S.; Ajayaghosh, A. Chem.;Eur. J. 2008, 14, 9577. (b) Ryu, S. Y.; Kim, S.; Seo, J.; Kim, Y. W.; Kwon, O. H.; Jang, D. J.; Park, S. Y. Chem. Commun. 2004, 70. (c) An, B.-K.; Kwon, S.-K.; Jung, S.-D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410.

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below. However, when we cooled the solution containing 5 mol % C18PTCDI, the emission intensity at 420 nm increased because of the aggregation of DBAPB, whereas the emission in the range of 525-650 nm ascribed to C18PTCDI increased first in the initial stage of gel formation and the corresponding gel exhibited greenish-yellow emission under 365 nm illumination (Figure 3c). It is notable that the emission intensity at 420 nm for the composite gel containing 5 mol % C18PTCDI was much weaker than that containing 1 mol % C18PTCDI, although the concentration of DBAPB was the same and the PL test conditions were similar for both samples. The reason should be that the energy-transfer efficiency in the composite gel with 5 mol % C18PTCDI was higher than that with 1 mol % C18PTCDI, which could be confirmed later. When the temperature decreased further, the emission intensity for those bands between 525 and 650 nm decreased gradually, and the corresponding gel exhibited weak red emission under 365 nm illumination. Similar fluorescence changes were observed for the sample containing 10 mol % C18PTCDI (Figure S5). When the mixed sample containing 1 mol % C18PTCDI was excited at 525 nm, the emission spectra for the gel state were very similar (Figure 3b, only a slight red shift). However, when the sample containing 5 mol % C18PTCDI was excited at 525 nm during the sol-gel process, we can see that the emission in the gel state exhibited two remarkable steps: the black solid lines in Figure 3d represent the emission spectra of the fresh yellow gel with a development similar to that of the composite gel containing 1 mol % C18PTCDI under 525 nm excitation (Figure 3b), supporting the nonaggregation of C18PTCDI in this stage, and the lightgray solid lines bounded by two black dotted lines shown in Figure 3d represent the emission spectra of the red gel, whose emission intensity decreased rapidly along with prolonging the cooling time on account of the formation of C18PTCDI aggregates. Moreover, it could be deduced that energy transfer should occur in the composite gels on the basis of the increase in emission DOI: 10.1021/la902457k

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Letter

Simalou et al.

intensity of 525-650 nm bands (ascribed to C18PTCDI) in the initial stage of gel formation under excitation at 300 nm. As depicted in Supporting Information (Figure S6a), there is excellent overlap between the emission of the DBAPB gel and the absorption of C18PTCDI, so the RET (resonance energy transfer) from DBAPB to C18PTCDI should happen.24,25 Figure S6b gives the emission spectra of the stable composite gels containing different contents of C18PTCDI excited at 300 nm. It is clear that the emission from DBAPB obviously decreased with increasing amounts of C18PTCDI, which suggests that the excitation energy of DBAPB may transfer to the C18PTCDI acceptor.24-27 Because C18PTCDI is known to be less emissive in the aggregate state,11a,17,20 the composite gel containing 1 mol % C18PTCDI gave the strongest emission at 525-650 nm as a result of nonaggregation of C18PTCDI. To obtain further information on the RET, fluorescence lifetime decay profiles for the samples containing different amounts of C18PTCDI monitored at 420 nm (the emission for neat DBAPB gel) are monitored. The neat gel of DBAPB exhibited a monoexponential decay with a time constant of τ = 1.6 ns, which decreased to 1.5 ns for the composite gels containing 2.5, 10, and 33 mol % C18PTCDI (Figure S6c-f). Therefore, RET from DBAPB to C18PTCDI happened in the composite gel systems.27 The energy-transfer efficiency24c,27 was assessed to be 4, 85, 89, and 97% for the composite gels containing 1, 5, 10, and 50 mol % C18PTCDI, respectively, with a rate constant of 2.05  1010 s-1.24c

Conclusions We have illustrated a rational approach to adjusting the color and the emitting color of N,N0 -di(octadecyl)-perylene-3,4,9,10-tetracarboxylic diimide (C18PTCDI) in an organogel phase built from 1,3,5-tris(4-dodecyloxybenzoylamino)phenylbenzene (DBAPB). It is interesting that the color and the emitting color of the gels could be controlled by varying the content of C18PTCDI as well as the temperature in the gel phases. When the concentration of C18PTCDI was low (such as 1 mol %), C18PTCDI could be dispersed as unimolecules in the composite gel, so the gel was yellow and produced strong greenish-yellow emission under 365 nm light. As to the mixed systems containing 2-10 mol % C18PTCDI, the fresh gels obtained via cooling the hot solutions for a short time were yellow, exhibiting strong greenish-yellow emission under 365 nm light, but the stable composite gels were red, emitting weak red emission under UV illumination as a result of the formation of C18PTCDI aggregates. Notably, the color and the emitting color (24) (a) Del Guerzo, A.; Olive, A. G. L.; Reichwagen, J.; Hopf, H.; Desvergne, J.-P. J. Am. Chem. Soc. 2005, 127, 17984. (b) Schenning, A. P. H. J.; Peeters, E.; Meijer, E. W. J. Am. Chem. Soc. 2000, 122, 4489. (c) Ajayaghosh, A.; Vijayakumar, C.; Praven, V. K.; Babu, S. S.; Varghese, R. J. Am. Chem. Soc. 2006, 128, 7174. (d) Ajayaghosh, A.; Praveen, V. K.; Vijayakumar, C.; George, S. J. Angew. Chem., Int. Ed. 2007, 46, 6260. (25) Ajayaghosh, A.; Praveen, V. K.; Vijayakumar, C. Chem. Soc. Rev. 2008, 37, 109. (26) (a) Bong, D. T.; Clark, T. D.; Granja, J. R.; Ghadiri, M. R. Angew. Chem. 2001, 113, 1016. Angew. Chem., Int. Ed. 2001, 40, 988. (b) Stupp, S. I.; Jebonheur, V; Walker, K.; Li, L. S.; Huggins, K. E.; Keser, M.; Amstutz, A. Science 1997, 276, 384. (c) Zubarev, E. R.; Pralle, M. U.; Li, L.; Stupp, S. I. Science 1999, 283, 523. (d) Zubarev, E. R.; Pralle, M. U.; Sone, E. D.; Stupp, S. I. J. Am. Chem. Soc. 2001, 123, 4105. (27) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.; Springer: New York, 2006.

11260 DOI: 10.1021/la902457k

changes were reversible in the gel phases, which could find applications in thermosensitive soft materials and photonic devices.

Experimental Section General Information. The solvents were dried using conventional methods. All other materials were used as received. 1H NMR spectra were recorded on a Mercury Plus 500 MHz spectrometer using CDCl3 as the solvent. Mass spectra were recorded on an Agilent 1100 series MS and an Axima CFR MALDI/TOF (matrix-assisted laser desorption ionization/time of flight) MS (Compact). UV-vis absorption spectra were recorded on a Shimadzu UV-1601PC spectrophotometer in a cell with 1 mm width. Fluorescence spectra were recorded on a Shimadzu RF-5301 luminescence spectrometer in a standard cell with 5 mm width (right-angle observation). Fluorescence lifetimes were measured using the time-correlated single-photon-counting technique with an FL920 fluorescence lifetime spectrometer. The excitation source was an nF900 ns flashlamp. Lifetimes were obtained by deconvolution of the decay curves. The time resolution of the setup is lower than 1 ns. Fluorescence microscopy images were taken on a fluorescence microscope (Olympus Reflected Fluorescence System BX51, Olympus, Japan). FTIR spectra were recorded using a Nicolet-360 FTIR spectrometer by incorporating samples into KBr pellets. X-ray diffraction (XRD) patterns were recorded on a Japan Rigaku D/max-γA instrument. The XRD instrument was equipped with graphite-monochromatized Cu KR radiation (λ) 1.5418 A˚, employing a scanning rate of 0.02 s-1 in the 2θ range from 0.7 to 10. Scanning electron microscopy (SEM) observations were carried out on a Japan Hitachi model X-650 San electron microscope. The samples for these measurements were prepared by casting an organogel on silicon wafers and drying at room temperature and then coating with gold. Rheology data were recorded on a TA Instruments AR 2000 using parallel plates (25 mm diameter). Synthesis. 1,3,5-Tris(4-dodecyloxybenzoylamino)phenylbenzene (DBAPB) was synthesized and characterized as reported earlier.5b N,N0 -Di(octadecyl)-perylene-3,4,9,10-tetracarboxylic diimide (C18PTCDI) was synthesized according to ref 18. 1H NMR (CDCl3): δ 0.89 (t, 6H, 2CH3), 1.40 (m, 60H, 30CH2), 1.79 (m, 4H, 2β-CH2), 4.23 (m, 4H, 2R-CH2), 8.65 (m, 4H, perylene), 8.72 (m, 4H, perylene). Gel-to-Sol Phase-Transition Temperature (Tgel). A tube containing the gel is immersed in a high-accuracy thermoregulated oil bath, and the temperature at which, on tube inversion, the gel no longer remains rigid is monitored. Acknowledgment. This work is financially supported by the National Natural Science Foundation of China (NNSFC, no. 20874034), the 973 Program (2009CB939701), and the Open Project of State Key Laboratory of Supramolecular Structure and Materials (SKLSSM200703). Supporting Information Available: XRD patterns of composite xerogels and C18PTCDI powder. Fluorescence microscopy images of neat DBAPB, neat C18PTCDI, and the composite gels. Pictures taken during the gelation process with chromism changes. UV-vis absorption and excitation spectra. Fluorescence emission spectra. Fluorescence emission decay profile intensity versus time. Recovery of the storage modulus (G0 ). This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2009, 25(19), 11255–11260