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Organogels Resulting from Competing Self-Assembly Units in the Gelator: Structure, Dynamics, and Photophysical Behavior of Gels Formed from Cholesterol-Stilbene and Cholesterol-Squaraine Gelators Cristina Geiger, Marina Stanescu, Liaohai Chen, and David G. Whitten* Chemical Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, and Center for Photoinduced Charge Transfer, Department of Chemistry, University of Rochester, Rochester, New York 14627 Received October 5, 1998. In Final Form: December 31, 1998 Organogels formed from novel organic gelators containing a cholesterol tethered to squaraine dyes or trans-stilbene derivatives have been studied from several different perspectives. The two types of molecules are active toward several organic liquids, gelling in some cases at w/w percentages as low as 0.1. While relatively robust, macroscopically “dry” gels are formed in several cases, studies with a variety of probes indicate that much of the solvent may exist in domains that are essentially liquid-like in terms of their microenvironment. The gels have been imaged by atomic force microscopy and conventional and fluorescence microscopy, monitoring both the gelator fluorescence in the case of the stilbene-cholesterol gels and, the fluorescence of solutes dissolved in the solvent. Remarkably, our findings show that several of the gels are composed of similarly appearing fibrous structures visible at the nano-, micro-, and macroscale.

An area of increasing recent interest is the formation of semistable gels by the addition of small amounts of low-molecular-weight organic molecules (“gelators”) to relatively low-molecular-weight nonviscous organic liquids.1 While several different categories of gelators have been identified in recent studies,2-15 gel formation can usually be best understood as resulting from competition between tendencies for the gelator to dissolve in the solvent and tendencies for it to self-assemble and crystallize. Among the types of gelators found in different investigations thus far are single molecules or pairs which interact through hydrogen bonding, amphiphiles, substituted cholesterol derivatives, and various alkyl-aromatic compounds.1 Particularly interesting among the different classes of gelators studied thus far are a group of tethered “dyads” which contain two different groups, each having a strong tendency to self-assemble. Examples of these include several cholesterol-linked anthracene derivatives studied by Weiss and co-workers2 and by Weiss and Terech3,4,9 and cholesterol-linked azobenzenes studied by (1) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (2) Lin, Y.-C.; Kachar, B.; Weiss, R. G. J. Am. Chem. Soc. 1989, 111, 5542. (3) Terech, P.; Furman, I.; Weiss, R. G. J. Phys. Chem. 1995, 99, 9558. (4) Terech, P.; Ostuni, E.; Weiss, R. G. J. Phys. Chem. 1996, 100, 3759. (5) van Esch, J.; De Feyter, S.; Kellogg, R. M.; De Schryver, F. C.; Feringa, B. L.; Chem. Eur. J. 1997, 3, 1238. (6) Hanabusa, K.; Tanaka, R.; Suzuki, M.; Kimura, M.; Shirai, H. Adv. Mater. 1997, 9, 1095. (7) Crisp, G. T.; Gore, J. Synth. Commun. 1997, 27, 203. (8) Hanabusa, K.; Koto, C.; Kimura, M.; Shirai, H.; Kakehi, A. Chem. Lett. 1997, 191. (9) Terech, P.; Coutin, A.; Giroud-Godquin, A. M. J. Phys. Chem. B 1997, 101, 6810. (10) Hanabusa, K.; Okui, K.; Karaki, K.; Kimura, M. Shirai, H. J. Colloid Interface Sci. 1997, 195, 86. (11) Hafkamp, R. J. H.; Kokke, B. P. A.; Danke, F. M.; Geurts, H. P. M.; Rowan, A. E.; Feiters, M. C.; Nolte, R. J. M. J. Chem. Soc., Chem. Commun. 1997, 545. (12) 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. (13) Placin, F.; Colomes, M.; Desvergne, J.-P. Tetrahedron Lett. 1997, 38, 2665. (14) Inoue, K.; Ono, Y.; Kanekito, Y.; Ishi-i, T.; Yoshihara, K.; Shinkai, S. Tetrahedron Lett. 1998, 2981. (15) Pozzo, J.-L.; Clavier, G. M.; Colomes, M.; Bouas-Laurent, H. Tetrahedron 1997, 53, 6377.

Shinkai and co-workers.12 Several of these compounds exhibit the ability to gel a diverse array of solvents at levels of 1% (weight/weight) or below; under these conditions one molecule of the gelator may (on the macroscopic scale) immobilize several thousand solvent molecules. The structures of organogels have been studied by a variety of different techniques including neutron scattering and X-ray scattering techniques,3,4 various forms of imaging, and spectroscopic and calorimetric investigations.12 Although much has been learned in a number of recent studies regarding the structure and properties of these organogels, many questions remain. Some of these include the relationship between gelator structure in the gel and the crystal structure of pure gelator, the specific interaction between the solvent and gelator, the state or states of the solvent, and the factors that determine what solvents a given gelator will gel and how the material properties of the gel on a macroscopic scale will be influenced by the nano- and mesostructures of the gel. In several recent investigations we have examined the ability of aromatic derivatized amphiphiles to self-assemble in thin films at the air-water interface,16-18 aqueous dispersions,17 and mixed aqueous-organic solutions.19 In particular, we have found that aromatic chromophores such as trans-stilbene derivatives and substituted squaraines show a strong tendency to associate into “pinwheel” clusters or extended glide or herringbone arrays in which there are prominent edge-face interactions.17,20,21 These attractive noncovalent interactions are strong enough to modify the “normal” self-assembly tendencies of the corresponding amphiphiles (not containing the chromophores) such that in several cases quite (16) Chen, H.; Herkstroeter, W. G.; Perlstein, J.; Law, K. Y.; Whitten, D. G. J. Phys. Chem. 1994, 98, 5138. (17) Song, S.; Geiger, C.; Leinhos, U.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1994, 116, 10340. (18) Song, S.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1997, 119, 9144. (19) Chen, H.; Farahat, M. S.; Law, K.-Y.; Whitten, D. G. J. Am. Chem. Soc. 1996, 118, 2584. (20) Chen, H.; Law, K.-Y.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1995, 117, 7257. (21) Song, X.; Geiger, C.; Farahat, M.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1997, 119, 12481.

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Chart 1. Cholesterol-Stilbene and Cholesterol-Squaraine Gelators and Fluorescent Probes

Figure 1. Normalized absorbance and fluorescence spectra of trans-4-octyloxy-4′carboxystilbene in cyclohexane (- - -) and in a Langmuir-Blodgett multilayer assembly (s) and the spectra of a gel of 1a in 1% w/w octanol (- - -).

different structures result.21,22 Since the aromaticaromatic self-assembly of the stilbenes and squaraines is marked by characteristic spectroscopic and photophysical “signatures”, we felt that the examination of the gelation properties of simple gelators incorporating one of these chromophores and a second self-assembling unit might be fruitful in providing additional insights into the formation, structure, and resulting properties of organogels. Accordingly, we have synthesized a number of cholesterol-stilbene and cholesterol-squaraine derivatives and examined the properties of the resultant gels formed when these reagents are added to a variety of organic solvents. We find that these molecules act as efficient gelators toward a variety of solvents and that both self-assembly elements may play important roles in the gelation process. While we find that relatively robust macroscopically “dry” gels are formed in several cases, studies with a variety of probes indicate that much of the solvent may exist in domains that are essentially liquidlike in terms of their microenvironment. Most remarkably, our findings with some of these new gelators show the formation of qualitatively similar fibrous structures on the nano-, meso-, and macroscale. The structures of the gelators used in this study are shown in Chart 1; their synthesis and purification will be reported elsewhere.23,24 Although several different transstilbene-derivatized amphiphiles could be tethered to cholesterol (either the natural structure or its C-3 epimer) via ester linkages through the cholesterol OH, the p-alkoxy-p′-carboxy-stilbene derivatives (1) and p-alkyl-p′-carboxystilbenes were found to be effective, gelling at gelator (22) Song, X.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1997, 119, 9144. (23) Geiger, C.; Whitten, D. G. Manuscript in preparation. (24) Stanescu, M.; Whitten, D. G. Manuscript in preparation.

percentages (w/w) of 1% or less. Gel formation was generally obtained by heating a mixture of gelator and solvent in either a capped vial or a capillary or by spreading a heated solution (above the gelling temperature) on surfaces such as graphite (suitable for AFM experiments) or glass or quartz rendered hydrophobic by pretreatment with octadecyl tricholormethylsilane (OTS) (for conventional microscopy) and subsequent cooling either to room temperature or below. Solvents gelled by these cholesterolstilbenes include benzene, nitrobenzene, toluene, haloalkanes, dioxane, acetonitrile, pyridine, and several alcohols. Although both cholesterol epimers of 1 gel several of the same solvents at comparable levels, the “unnatural” isomer (1a) generally gave better gels (transparent, more stable) than the “natural “ isomer (1b). While a number of squaraine derivatives could gel different organic solvents, of those studied to date 2b has been found by far to be the most effective, and interestingly for the squaraines, it is the isomer with the natural configuration of the cholesterol that is the more effective gelator for several solvents. This interesting situation where the “natural” isomer is the more effective gelator for some cholesterol derivatives but the reverse is true for a different series has been noted in previous studies.12,25 Thus we find that 2b gels several alcohols at levels of 1% (weight/weight) or less. We note that, for 1a and acetonitrile, where gelation occurs at levels as low as 0.1% (and effectively the melting temperature of acetonitrile is raised from -48 to 50 °C; a 2% gel melts at 74 °C), gelation is occurring at levels of one gelator molecule for more than 15 000 solvent molecules (similar, although not quite so dramatic ratios are observed for 1a, 2b, and several other gelators in this series). The discussion which follows will focus on the gels formed from 1a and 2b in a few typical solvents (1-butanol, 1-octanol, and acetonitrile) that are optically transparent throughout the region where the gelators absorb and fluoresce. Gel formation could be detected readily by failure of the gelled mixture to flow. In addition formation of gels from 1a and 2b was marked by significant changes in optical properties. For 1a formation of gel was marked by small changes (mostly broadening) of the absorption spectrum corresponding to the stilbene transitions while the fluorescence showed a small shift to longer wavelengths (Figure 1) and a 16-fold increase in intensity, compared to the isotropic phase.26 The changes in absorption and (25) Mukkamala, R.; Weiss, R. G. J. Chem. Soc., Chem. Commun. 1995, 375.

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Figure 2. Absorption spectra of a 0.3% gel of 2b in butanol in isotropic solution (a), a fresh gel (b), and a stable gel (c).

fluorescence are much smaller than those observed in Langmuir-Blodgett films of the corresponding stilbene amphiphiles17 and thus suggest that any stilbene aggregation in the gels is probably much weaker and perhaps characterized by a lower aggregation number than that for films or vesicles of the stilbene amphiphiles. There is a strong “excitonic” induced circular dichroism observed for 1a in the gels corresponding to the stilbene transitions, suggesting a dimer (or larger aggregate)17,21,22 which disappears immediately as the gel is melted to produce the corresponding isotropic phase. In contrast to the cholesterol-stilbene gelators, the cholesterol-squaraine derivative shows similarities to the corresponding squaraine amphiphiles in films and vesicles. Thus 2b in 1-butanol or 1-propanol exhibits a pronounced blue shift upon going from the isotropic phase to gel, indicating strong association and which corresponds quite closely (Figure 2) to the spectral shifts observed when the corresponding squaraine amphiphiles form either a tetramer or an extended aggregate.19 The corresponding squaraine dimer absorbs intermediate in wavelength between the monomer and the aggregate and is easily distinguished from the latter.16 The formation of gels with 2b was marked by a loss of fluorescence and the appearance of a strong induced “excitonic” circular dichroism; similar changes have been observed for the aggregation of squaraine amphiphiles in films and vesicles. Thus, for both the stilbene cholesterol and squaraine cholesterols there is evidence that aggregation of the aromatic chromophores accompanies gel formation. For the gels formed from 1a and 2b, it is observed that an increase in gelator concentration results in an increase in the “melting point” (Tg) for the gel. As has been found by Shinkai and co-workers for azobenzene derivatives,12 there is a reasonable linear relationship when Tg-1 (K-1) is plotted versus ln [gelator]; from the slopes of these plots, ∆H for the gelation may be estimated. Values obtained for 1a in 1-butanol and acetonitrile are 118 and 112 kJ/mol while for 2b in 1-butanol a value of (26) The excitation wavelength for both gels and sols is 320 nm where there is similar optical density for both forms; fluorescence intensities are compared at their respective maxima (Figure 1).

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109 kJ/mol is obtained. Studies with 1a in several solvents show little variation and in general give values of ∆H very similar to those obtained by Shinkai12 for closely corresponding trans-azobenzene cholesterol gelators. The similarity of ∆H for a range of different solvents has been suggested by Shinkai to indicate that the structure of the gel may not be influenced appreciably by solvent and that the “scaffolding” responsible for the gelation consists largely if not exclusively of gelator molecules. It has also been suggested in other studies that microcrystal formation by the gelator is responsible for the gelation.27 For the gels formed from 1a and 2b, it appears that the scaffolding may not be formed from simple microcrystals of the gelator. Thus we find that gels made from 1a and 1b are subject to photoisomerization of the stilbene from trans to cis (with concomitant “melting” of the gelssee below) whereas crystals of 1a and other stilbene amphiphiles are photostable to isomerization. The absorption and fluorescence of the stilbene chromophore in the gels are also different from those of the similarly substituted stilbene derivatives in pure crystals.28 One of the most interesting questions concerns the nature of the interaction between the solvent and the gelator and the structure of the gel on the nanoscale. To determine more about the structure of these gels at this level, we have used atomic force microscopy (AFM) and scanning electron microscopy (SEM). While previous studies have generally employed SEM and related techniques, we have found AFM (operating in the “tapping mode”) very convenient for obtaining structural information on the nanoscale.29 When we examine gels from 1a and 2b in the different solvents using AFM27 or SEM, images and structures very similar to those observed by others for quite different organogels1,30 are seen (Figure 3). Thus we observe by nanometer to micrometer resolution a fibrous network in which smaller fibers can be seen with structures of diameter approximately 14 nm or less that apparently weave into larger bundles of fiber that have a distinct helical pitch. The structures observed for different solvents appear to be qualitatively similar but with some significant differences (density, length of fibers, etc). When the same gels are examined by conventional microscopy (much lower magnification (ca. 400-fold)), fibrous structures are also clearly observable. In this case, the gels formed from the stilbene gelators are particularly interesting, since the trans-stilbene chromophore of the gelator exhibits strong fluorescence. Figure 4 compares a gel from 1a in 1-octanol observed by conventional and fluorescence microscopy with illumination by visible light (Figure 4a) and by wavelengths selectively absorbed by the stilbene gelator (Figure 4b) and by a coumarin dye (3) (Figure 4c) which has been dissolved in the solvent. The three images obtained correspond to the physical profile of the gel, the fluorescence of the gelator, and the coumarin fluorescence. The patterns observed are superimposable and clearly establish that gel, gelator, and solvent all occupy the same region of space, namely the fiber network. Qualitatively similar structures are observed for 1-butanol and aceto(27) Bujanowski, V. J.; Katsoulis, D. E.; Ziemelis, M. J. J. Mater. Chem. 1994, 4, 1181. (28) Vaday, S.; Geiger, H. C.; Cleary, B.; Perlstein, J.; Whitten, D. G. J. Phys. Chem. B 1997, 101, 321. (29) The highly oriented pyrolitic graphite (HOPG) substrate was spotted with hot gelator solution and allowed to cool in a sealed chamber for 45 min. Under these conditions minimal evaporation was observed. Both pure solvent and the solvent-gelator solution completely “wet” this substrate. We estimate the film thickness to be 5-10 mM; under these conditions we presume that dewetting plays little role in the structure formation observed. The images were obtained in the tapping mode. (30) Brotin, T.; Desvergne, J. P.; Fages, F.; Utermohlen, R.; Bonneau, R.; Bouas-Laurent, H. Photochem. Photobiol. 1992, 55, 349.

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Figure 3. AFM image of a 0.5% gel of 2b in butanol spread on HOPG graphite.

nitrile, but different (solvent-specific) fiber sizes are observed under both the AFM and fluorescence microscopy resolution. The formation of gel and its destruction by photoisomerization are shown under similar illumination in Figure 5. The clear fibrous network observed for the gel under conventional microscopy is very similar to the images obtained by AFM or SEM; however, the size of the fibers observed by the former is 100-1000 times those seen on the nanoscale by AFM. Most interestingly, we find for freshly formed gels of 1a (0.5-1.0% 1a in 1-butanol) the gel may be drawn from the vial or tube to form fibers (highly and uniformly fluorescent) which may be g 2 cm in length and 0.5-1 mm in diameter (Figure 6c).31 Thus we note (Figure 6) a remarkable tendency for these organogels to self-assemble to generate qualitatively similar structures on the nano-, meso-, and macroscale. To examine the state of the solvent in the gel, we have applied a number of different techniques including NMR and solvatochromic and fluorescent probes dissolved in the solvent prior to addition of the gelator. We find, for example, that in gels formed from perdeutero-1-butanol and 2b there is no detectable difference in the NMR chemical shifts of the butanol above and below the temperature of gelation. The T1 relaxation times of 4-(tert-butyl)phenol dissolved in 1-butanol and 2b were found to exhibit no change above and below Tg. Similar results have been obtained looking at the fluorescence of a viscosity-sensitive probe (4) and the fluorescent lifetimes of a merocyanine dye (5), whose lifetime normally shows a strong increase with increase in viscosity.32 No fluorescence spectral shifts were observed for 4 on cooling from the isotropic to a “robust”gel phase while in a control experiment for a solvent exhibiting melting in the same temperature range a clear shift is observed as the liquid converts to solid. Although our knowledge of gel structure is far from complete, the current results suggest several remarkable (31) The concentration of the gelator is critical for obtaining these “fibers”; at lower concentrations the gels are not sufficiently stable, while at higher concentrations it is difficult to “pull” out the strings. The fibers are relatively elastic and easily distorted by pressure. (32) In the case of 4 we actually observe a small decrease in fluorescent lifetime upon gelation, which may suggest an increase in self- or impurity-quenching which might be attributed to an increase in concentration upon gelation as pure solvent is incorporated into the “scaffolding” while the remaining “solution” is enriched in solute.

Figure 4. Conventional (top) and fluorescence microscopic images of a gel of 1a in octanol on OTS-derivatized quartz. The middle image is obtained with UV activation of the stilbene chromophore of 1a, while the bottom image is obtained by activation of coumarin dye (3).

inferences may be made. If we assume that the cholesterol units in the current gelators stack in much the same way they form liquid crystals,1,12,33-35 we can imagine that the gelator “scaffolding” consists of stacks or columns which may have as their primary self-assembly element the cholesterol with the appended aromatics occurring moreor-less like “spokes” projecting from the columns. Interactions between stilbene or squaraines within the column may be difficult, but interactions between aromatic units in adjacent columns should be possible and may account for the weak indications of aggregation that are observed, especially for the stilbene gelators. For the squaraines, where the tether is longer and more flexible, the degree (33) Sawzik, P.; Craven, B. M. Acta Crystallogr. 1979, B35, 895. (34) Terech, P. Liq. Cryst. 1991, 9, 59. (35) Shinkai, S.; Nishi, T.; Shimamoto, K.; Manabe, O. Isr. J. Chem. 1992, 32, 121.

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Figure 6. Comparison of AFM and conventional microscopic and direct photographys of fibers formed from 1a in butanol. Figure 5. Conventional (top and bottom) and fluorescence microscopic images of 1a in butanol as gel formation commences (top), when gel is fully formed (middle), and after prolonged irradiation with UV light to “melt” the gel via photoisomerization.

of interaction between squaraines may be such that both intra- and intercolumnar interactions may take place. The observation that photoisomerization can occur for the gels of 1a and 1b and that it reversibly destroys the gels supports the idea of a structure in which weak selfassociation of the trans-stilbene units is essential for gel formation but that the self-association is quite different from the crystal structure of gelator alone. These results are in agreement with those of other studies that indicate much of the solvent in the gel exists as a nonviscous fluid in pools or channels that are sufficiently large such that solutes dissolved in the solvent experience an environment very similar to that in the fluid alone. This does not preclude the likelihood that some of the solvent is associated with the gelator and that it may be an important component of the gelator scaffold. The fact that these gels organize into fibrous structures on the nano-, meso-, and macroscopic levels supports a concept wherein gelator columns (or a small bundle of columns) may be the unit element of gel formation and that there may be similar interactions

occurring initially between columns, leading to the smallest fibrous strands entrapping solvents and to subsequent interactions between strands as progressively larger fibers are formed. It seems most reasonable that these proposed interactions may be largely due to self-assembly of the aromatic units or to a composite self-assembly process involving both the aromatic groups and some associated solvent molecules. The remarkable ability of the gels to exhibit, more-or-less simultaneously, glassy, fibrous, solidlike, and liquid-like behavior suggests their potential utility in a variety of advanced materials applications. Acknowledgment. We are grateful to the Center for Materials Research at Los Alamos National Laboratory for support of this work. We also thank Dr. Ariane Eberhardt and Ms. Rebecca Nyquist for help with the AFM measurements and the “Complex Adaptive Matter Team” for stimulating discussions and helpful suggestions. We thank the U.S. National Science Foundation (Grant CHE-9521048) and the NSF Center for Photoinduced Charge Transfer for support of the portion of the work carried out at the University of Rochester. LA981386I