Soft Functional Materials Induced by Fibrillar Networks of Small

pubs.acs.org/Langmuir © 2009 American Chemical Society

Soft Functional Materials Induced by Fibrillar Networks of Small Molecular Photochromic Gelators† Santanu Bhattacharya*,‡,§ and Suman K. Samanta‡ §

‡ Department of Organic Chemistry, Indian Institute of Science, Bangalore 560 012, India, and Chemical Biology Unit, Jawaharlal Nehru Centre of Advanced Scientific Research, Bangalore 560 012, India

Received March 23, 2009. Revised Manuscript Received April 11, 2009

Downloaded by American Chemical Society on July 30, 2009 Published on June 22, 2009 on http://pubs.acs.org | doi: 10.1021/la901017u

Low-molecular-mass organogelators (LMOGs) based on photochromic molecules aggregate in selected solvents to form gels through various spatio-temporal interactions. The factors that control the mode of aggregation of the chromophoric core in the LMOGs during gelation, gelation-induced changes in fluorescence, the formation of stacked superstructures of extended π-conjugated systems, and so forth are discussed with selected examples. Possible ways of generating various light-harvesting assemblies are proposed, and some unresolved questions, future challenges, and their possible solutions on this topic are presented.

Organogels formed by low-molecular-mass organogelators (LMOGs) have emerged as important soft materials over the years because of their wide range of applications in diverse fields.1 Accordingly, important structure-property relationships in this class of materials have been established.2 There is a large body of literature that indicates that the design and synthesis of organogel-forming systems are increasingly leaning toward the discovery of materials endowed with novel properties.3 These include organogels containing fluorescent molecules,4 photoresponsive systems,5 organic-inorganic hybrid systems,6 nanocomposites,7 multicomponent assemblies,8 liquid crystals,9 and mesogens and mesophases.10 Other phenomena (e.g., phase-selective gelation11 †

Part of the Molecular and Polymer Gels; Materials with Self-Assembled Fibrillar Networks special issue. *Corresponding author and J. C. Bose Fellow, DST, New Delhi; Phone: +(91)-080-2293-2664. Fax: +(91)-080-2360-0529. E-mail: [email protected]. ernet.in. (1) (a) Llusar, M.; Sanchez, C. Chem. Mater. 2008, 20, 782. (b) George, M.; Weiss, R. G. Acc. Chem. Res. 2006, 39, 489. (c) Mallia, V. A.; Tamaoki, N. Chem. Soc. Rev. 2004, 33, 76. (d) van Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew. Chem., Int. Ed. 2003, 42, 980. (e) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263. (2) (a) Carretti, E.; Dei, L.; Weiss, R. G. Soft Mater. 2005, 1, 17. (b) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (3) (a) Chen, K.; Tang, L.; Xia, Y.; Wang, Y. Langmuir 2008, 24, 13838. (b) Schenning, P. H. J.; Juerzo, A. D.; Desvergne, J. -P.; Meijer, E. W.; Maan, J. C. Langmuir 2005, 21, 2108. (c) Rizkov, D.; Gun, J.; Lev, O.; Sicsic, R.; Melman, A. Langmuir 2005, 21, 12130. (4) Sugiyasu, K.; Fujita, N.; Shinkai, S. Angew. Chem., Int. Ed. 2004, 43, 1229. (5) (a) Ishi-i, T.; Shinkai, S. Top. Curr. Chem. 2005, 258, 119. (b) Srivastava, A.; Ghorai, S.; Bhattacharjya, A.; Bhattacharya, S. J. Org. Chem. 2005, 70, 6574– 6582. (6) George, M.; Funkhouser, G. P.; Weiss, R. G. Langmuir 2008, 24, 3537–3544. (7) (a) Pal, A.; Chhikara, B. S.; Govindaraj, A.; Bhattacharya, S.; Rao, C. N. R. J. Mater. Chem. 2008, 18, 2593. (b) Fujigaya, T.; Morimoto, T.; Niidome, Y.; Nakashima, N. Adv. Mater. 2008, 20, 3610. (c) Bhattacharya, S.; Srivastava, A.; Pal, A. Angew. Chem., Int. Ed. 2006, 45, 2934. (d) Basit, H.; Pal, A.; Sen, S.; Bhattacharya, S. Chem. Eur. J. 2008, 14, 6534. (e) Pal, A.; Basit, H.; Sen, S.; Aswal, V. K.; Bhattacharya, S. J. Mater. Chem. 2009, 19, 4325. (8) Velazquez, D. G.; Diaz, D. D.; Ravelo, A. G.; Tellado, J. J. M. J. Am. Chem. Soc. 2008, 130, 7967. (9) Camerel, F.; Ziessel, R.; Donnio, B.; Bourgogne, C.; Guillon, D.; Schmutz, M.; Iacovita, C.; Bucher, J.-P. Angew. Chem., Int. Ed. 2007, 46, 2659. (10) Beginn, U. Prog. Polym. Sci. 2003, 28, 1049. (11) (a) Bhattacharya, S.; Pal, A. J. Phys. Chem. B 2008, 112, 4918–4927. (b) Khatua, D.; Dey, J. Langmuir 2005, 21, 109. (c) Bhattacharya, S.; KrishnanGhosh, Y. Chem. Commun. 2001, 185. (12) (a) Vintiloiu, A.; Leroux, J.-C. J. Controlled Release 2008, 125, 179. (b) Tokuyama, H.; Kato, Y. Colloids Surf., B 2008, 67, 92. (c) Lim, P. F. C.; Liu, X. Y.; Kang, L.; Ho, P. C. L.; Chan, S. Y. Int. J. Pharm. 2008, 358, 102. (d) Penzes, T.; Blazso, G.; Aigner, Z.; Falkay, G.; Eros, I. Int. J. Pharm. 2005, 298, 47.

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and drug delivery using gels12) have also been achieved. Some of these aspects have been reviewed.1,2 A subfield of organogels has shifted increasingly toward the development of luminescent organogels. These LMOGs manifest interesting photophysical characteristics, especially upon aggregation. Herein we focus our attention on some recent examples of organogels and LMOGs possessing appropriate optical and luminescent properties. Several photochromic systems form self-assembled fibrillar networks (SAFINs) that lead to organogels, and their fluorescence properties vary depending on the state of their aggregation in the gel phase. A recent review on the use of organogels involving excitation-energy transfer, light-harvesting systems, and photoresponsiveness has appeared.13 This perspective addresses issues involving various chromophoric or fluorophoric core groups in an assortment of organogelators (Chart 1), the aggregation of which allows the modulation of various photophysical properties of the chromophores through their self-assembly. Luminescent gels are useful for applications in light-emitting diodes (LEDs), fluorescence labeling, light harvesting, photonic devices, and so forth. Several LMOGs are known with chromo/ fluorogenic units, such as naphthyl, phenanthryl, perylyl, pyrenyl, carbazolyl, porphyrin groups, and oligo phenylene vinylenes (OPVs) as cores. What are the interactions among such photochromic molecules and with the solvents in which gelation takes place? How do the various chromophores assemble in the gel system to bring about such an interesting photophysical property? Clearly, the correlation of molecular features with their aggregation properties holds important clues to answer these questions. Specific Modes of Aggregation during Gelation. Recently, a cholesteryl-appended merocyanine (1) was shown to induce gelation in organic solvents of different polarities, where the van der Waals interactions between the cholesteryl units serve as a gelation auxiliary. Minor structural variations (R = CH3 to n-C4H11) triggered a morphological transition from kinetically formed nanofibers to thermodynamically stable globules and the concurrent disappearance of intermolecular charge transfer (ICT) excitonic interactions between the chromophoric building blocks. (13) Ajayaghosh, A.; Praveen, V. K.; Vijayakumar, C. Chem. Soc. Rev. 2008, 37, 109.

Published on Web 06/22/2009

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Bhattacharya and Samanta

Perspective


Chart 1. Molecular Structures of Photochromic LMOGs

Thus, a minor modification of the barbituric acid part controls the morphology and the ICT interactions during gelation.14 Dumbbell-shaped dendritic molecules with a para-terphenylene core (2) form organogels through a cooperative effect of various noncovalent forces. In the gels, H-aggregate formation resulted in a blue shift of the UV absorption band and an ∼200 fold increase in the fluorescence intensity compared to that of solutions. This allowed the tuning of emission by controlling the degree of aggregation, which might be useful in the design of fluorescent labels and optical sensors.15 A dichalcone-substituted carbazole (3) was shown to form organogels through hydrogen bonding and π-π interactions. The existence of J-type aggregates in this system provides a red shift in the fluorescence spectrum depending on the degree of aggregation. This provides a simple way of tuning the emission band by controlling the extent of aggregation by varying the concentration of the LMOGs.16 Porphyrins are known to self-assemble via π-π stacking interactions. The UV-vis spectra of 4 show the splitting of the Soret band (λmax = 417-399 and 428 nm) upon increasing the gelator (14) Yagai, S.; Ishii, M.; Karatsu, T.; Kitamura, A. Angew. Chem., Int. Ed. 2007, 46, 8005. (15) Chen, Y.; Lv, Y.; Han, Y.; Zhu, B.; Zhang, F.; Bo, Z.; Liu, C.-Y. Langmuir 2009, 25, 8548. (16) Su, L.; Bao, C.; Lu, R.; Chen, Y.; Xu, T.; Song, D.; Tan, C.; Shi, T.; Zhao, Y. Org. Biomol. Chem. 2006, 4, 2591.

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concentration from 0.1 to 0.2 mM where the solution becomes a “weak” gel. Chirally self-assembled nanofibrillar networks induce gel formation as a consequence of both R-chiral, H-type aggregation and S-chiral, J-type aggregation among the porphyrin moieties.17 Gelation-Induced Changes in Fluorescence. Salicylideneaniline-based organogels (5) show an interesting reversible thermochromism phenomenon that has been interpreted to occur through tautomerism between the NH and OH forms during sol-gel freezing. The gel can emit intense green light, and the observed aggregation-induced enhanced emission (AIEE) effect was ascribed to a combination of intramolecular rotation and the formation of J aggregates.18 An anthracene derivative (6) was found to form a gel in 1, 2-dichloroethane as a result of hydrogen-bonding interactions between the urea moieties and π-π stacking between the anthracene units. A significant fluorescence enhancement was observed upon gel formation. Tunable emission can be observed by UV irradiation of the solution of 6 in THF, which showed a decrease in the fluorescence intensity, yielding a photodimer that also retained its property as an efficient gelator.19 (17) Jintoku, H.; Sagawa, T.; Sawada, T.; Takafuji, M.; Hachisako, H.; Ihara, H. Tetrahedron Lett. 2008, 49, 3987. (18) Chen, P.; Lu, R.; Xue, P.; Xu, T.; Chen, G.; Zhao, Y. Langmuir 2009, 25, 8395. (19) Wang, C.; Zhang, D.; Xiang, J.; Zhu, D. Langmuir 2007, 23, 9195.

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Perspective

Apart from the above-mentioned chromophores, there are several others that have been developed and reported recently. For example, a highly fluorescent, chiral, transparent organogel was formed as a consequence of chirality transfer through the hydrogen-bonding-mediated assembly of the nonfluorescent, achiral gelator (7) and chiral L-TA (L-tartaric acid) or D-TA. The AIEE was observed as a result of the strong intermolecular π-π interactions between the aromatic moieties.20 Novel fluorescent organogelators with π-conjugated phenyl ethynyl frameworks, featuring long-chain carboxamides (8), have been made as well. AIEE has been observed in these organogels, with an order of magnitude higher fluorescence quantum yield than in the corresponding solutions. In addition, some of the molecules exhibit liquid-crystalline properties.21 OPV-based chromophores (e.g., 9) can act as fluorescent probes in the gel matrices. Their fluorescence may be useful in probing self-assembly and sol-gel phenomena. The preparation of such gelators with enriched hydrogen-bonding sites at the termini and with extended chromophore lengths are particularly advantageous gelators. The 3D assemblies formed by strong electronic interaction among the chromophores may allow the modulation of optical properties that could be useful in the emerging field of supramolecular electronics.22 Multicomponent Systems. One- or two-component photochromic gels are of interest because of the tunable red-greenblue emission that has been obtained from naphthalimides (10-12). Electron transfer plays a crucial role in providing the tunable colors in the mixed-gel system. These gels are ideal scaffolds for the design of supramolecularly assembled lightharvesting systems and could be developed in display technologies using soft materials with tunable optical properties.23 Stacked Superstructures. The naphthalene diimide (NDI) chromophore has been used in the design of the near-infrared (NIR) chiroptical switching system (13) in electrochromic organogels. The redox-active, helically stacked chiral selfassembly showed large (but reversible) changes in the absorption and circular dichroism spectra in the visible and NIR regions (e.g., 1310-1550 nm).24 An extension of the NDI chromophore was obtained in the perylene bisimides (PBI), (14) and (15), which exhibit interpenetrating networks leading to strong gelation abilities. Accordingly, a system for visible light harvesting has been designed where energy transfer takes place through the 1D alignment of chromophores. On the basis of the high n-type charge carrier mobility of PBI and its wide absorption range (from 400 to 600 nm), this gelator may provide a route to the development of organic solar cells when it is mixred with a p-type organic semiconductor such as pentacene or polythiophene.25 Organogel formation was also found in a benzocoronenebased chromophore that provides a wide π surface on which to induce strong stacking interactions. Hexa-peri-hexabenzocoronene derivatives (HBCs) (16) that have hydrogen-bonding groups (either amido or ureido) adjacent to the aromatic cores were (20) (a) Seo, J.; Chung, J. W.; Jo, E.-H.; Park, S. Y. Chem. Commun. 2008, 2794. (b) Chung, J. W.; An, B.-K.; Park, S. Y. Chem. Mater. 2008, 20, 6750. (21) Shen, Y.-T.; Li, C.-H.; Chang, K.-C.; Chin, S.-Y.; Lin, H.-A.; Liu, Y.-M.; Hung, C.-Y.; Hsu, H.-F.; Sun, S.-S. Langmuir 2009, 25, 8714. (22) Samanta, S. K.; Pal, A.; Bhattacharya, S. Langmuir 2009, 25, 8567. (23) Shu, T.; Wu, J.; Lu, M.; Chen, L.; Yi, T.; Li, F.; Huang, C. J. Mater. Chem. 2008, 18, 886. (24) Zheng, J.; Qiao, W.; Wan, X.; Gao, J. P.; Wang, Z. Y. Chem. Mater. 2008, 20, 6163. (25) (a) Sugiyasu, K.; Kawano, S-i.; Fujita, N.; Shinkai, S. Chem. Mater. 2008, 20, 2863. (b) Li, X.-Q.; Stepanenko, V.; Chen, Z.; Prins, P.; Siebbeles, L. D. A.; Wurthner, F. Chem. Commun. 2006, 3871. (c) Sugiyasu, K.; Fujita, N.; Shinkai, S. Angew. Chem., Int. Ed. 2004, 43, 1229.

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synthesized to study the effects of intracolumnar hydrogen bonds on the self-assembly behavior of HBCs. The hydrogen bonds effectively increased the aggregation tendency of these compounds in solution. In the bulk state, the typical columnar supramolecular arrangement of HBCs was either stabilized substantially or suppressed by dominant hydrogen-bonding interactions. The combined effect of the hydrogen bonds and π-stacking of the aromatic moieties led to the formation of fluorescent organogels. Owing to the intrinsic semiconducting nature of the stacked HBCs, such gels are promising candidates for 1D energy transfer in aggregates, enhanced charge transport based on organic semiconducting layers, and sensor devices.26 The electronic and self-assembling properties of novel oligoazaacenes (17, 18) have significant potential in molecular electronics and dye chemistry. They aggregate in solution, producing rolled-up sheets, foams, and fibrous structures reminiscent of SAFINs. The importance of the stacking interactions in these compounds on their solid-state photophysical properties has been elucidated with an emphasis on the role of heteroatom position and acene chromophore structure.27 The cited examples demonstrate how several forces are responsible for the gelation of various photochromes in organic media involving spatiotemporal interactions. Precise balance among these associative forces leads to efficient gel formation with tunable optical and photophysical properties for several LMOGs. Above all, the core chromophores play a crucial role in the modification of the supramolecular organization and the bulk properties. Toward Energy Transfer. An important aspect of the application of such types of gels is their light-harvesting properties that may be enhanced from a physical mixture of two or more gelator components. This is accomplished when one molecule acts as an electron/energy donor and the other acts as an acceptor.25c,28 Figure 1 shows the approximate absorption and emission wavelength ranges of the chromophores present in several gelator molecules. It provides a blueprint for how to choose a chromophore and its “complementary” chromophore to achieve efficient energy transfer upon self-assembly. The complementary molecule may have a modified structure in which the chromophore, chromophoric length, or solvent system is changed, for example. Photochromic organic gelators include a diverse range of molecular features: from steroids to polyaromatic systems (e.g., porphyrin, perylene, coronene, and naphthalimide), conjugated systems such as OPV, and so forth. A combination of noncovalent interactions, including hydrogen bonding, van der Waals forces, π-stacking, hydrophobic- and hydrophilic-induced microseparations, and dipolar forces, are mainly responsible for the selfassembly of LMOGs. Important parameters that emerge as control elements for the design of soft materials with potential photophysical applications include the mode of aggregation (J- or H-aggregate formation, stacking of molecular assemblies, or helical chiral superstructures). They can be exploited to induce enhanced emission and donor-acceptor band overlaps either by physical mixing or by precise molecular design. Thus, introducing a photochromic core within the 3D network of a soft gel assembly offers an effective (26) Dou, X.; Pisula, W.; Wu, J.; Bodwell, G. J.; Mullen, K. Chem.;Eur. J. 2008, 14, 240. (27) Richards, G. J.; Hill, J. P.; Okamoto, K.; Shundo, A.; Akada, M.; Elsegood, M. R. J.; Mori, T.; Ariga, K. Langmuir 2009, 25, 8408. (28) (a) Guerzo, A. D.; Olive, A. G. L.; Reichwagen, J.; Hopf, H.; Desvergne, J.P. J. Am. Chem. Soc. 2005, 127, 17984. (b) Sagawa, T.; Fukugawa, S.; Yamada, T.; Ihara, H. Langmuir 2002, 18, 7223.

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Perspective

Figure 1. Range of absorption (blue line) and emission (red line) maxima in solutions of the chromophores shown.

route toward adaptable nanodimensioned entities, the properties of which could also be modulated by light absorption. In only a few examples, an understanding has been achieved to decipher the factors that lead to specific types of aggregates and to the growth of aggregates leading to the formation of fibrillar networks. There is still no generally applicable correlation between thermodynamic and flow properties of such gels and their molecular structures. Until this is accomplished, no predictable photophysical properties of a gel will be possible, and therefore applications will not be based on specific molecular designs. Although various intermolecular interactions and their effects on optical spectra have been elucidated, the precise mode of aggregation (J or H) that occurs with a given molecular system and its effect on gel formation cannot be predicted. Future challenges lie in addressing several unresolved questions: (1) How can one predict which photochromic molecule will form gel with a specific liquid? (2) Is there a relation between supramolecular chirality in a fibrillar aggregate and the molecular structure of the constituent gelator molecules irrespective of whether the latter are chiral? (3) What is the effect of supramolecular chirality on a chromophore in a fibrillar aggregate? (4) Is chirality important for enhancing light-harvesting effects? (5) Can one modify the chromophore in a gelator to widen the range of the absorption band while narrowing the range of the emission band?

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Many approaches are conceivable. One can physically dope donor (D) and acceptor (A) photochromes into a gel matrix so that energy transfer will be increased when they are neighbors. However, if D and A confine themselves in isolated domains of the supramolecular gel, no such effects will be seen. A key, therefore, is to achieve control of the distribution of dopants in the gel fibers. An alternative option is to promote energy transfer intramolecularly in gelators that contain both D and A groups. This strategy could be useful for the design of the next generation of light-harvesting materials. Basic design principles for energy transfer are that the emission spectrum of the donor should overlap strongly with the absorption spectrum of the acceptor, and it is important that the separation between the donor and acceptor be small. Achieving controlled manipulation of these properties will lead to gels with applications in many areas. Acknowledgment. S.K.S. thanks CSIR for a senior research fellowship. Note Added after Print Publication. This perspective was released ASAP on June 22, 2009. The page numbers and some of the author names have been updated in references 15, 18, 21, 22, and 27. The correct version was posted on July 30, 2009.

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Soft Functional Materials Induced by Fibrillar Networks of Small

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