THERMAL AND LIGHT CONTROL OF THE SOL-GEL PHASE-TRANSITION IN CHOLESTEROL-BASED ORGANIC GELS - NOVEL HELICAL AGGREGATION MODES AS DETECTED BY CIRCULAR-DICHROISM AND ELECTRON-MICROSCOPIC OBSERVATION

Nineteen cholesterol derivatives containing a variety of azobenzene moieties coupled to C-3 of a steroidal moiety through an ester linkage were synthesized. We employed two different esterification methods by which cholesterol derivatives with the natural (S)-configuration at C-3 and those with the inverted (R)-configuration at C-3 were obtained (the latter derivatives are indicated by a prime). Among them, cholesterol derivatives bearing a p-alkoxyazobenzene moiety (2R and 2R') acted as excellent thermally-reversible gelators of various organic fluids, but the gelation ability is fairly different between 2R and 2R': 2R could gelatinize hydrocarbons such as n-hexane, n-octane, and toluene, halogen solvents such as 1,2-dichloroethane and dichloromethane, ether solvents such as diethyl ether and THF, and alcohols such as ethanol and I-butanol whereas 2R' could gelatinize ketones, methanol, and polysiloxanes. In general, the solubility of 2R' in apolar solvents is superior to that of 2R, so 2R is useful for gelation of apolar solvents whereas 2R' is useful for gelation of polar solvents. We found that the sol-gel phase transition is sensitively ''read-out'' by a change in the circular dichroism (CD) spectrum: the gel phase is CD-active whereas the sol phase is totally CD-silent. For example, the 2Me-1-butanol gel gave a positive exciton coupling band with (R)-chirality whereas the 2Et'-methanol gel gave a negative exciton coupling band with (S)-chirality. These results mean that dipoles in the azobenzene moiety are oriented in a clockwise (in (R)-chirality) or anticlockwise (in (S)-chirality) direction when they interact in the excited state. Strangely, we accidentally found that the CD sign of the gels prepared from 2Pr, 2Bu, and 3Me' (azobenzene-linked cholesterol derivative with p-NMe(2)) is frequently inverted. After careful examination of the gel preparation conditions, we found that inversion takes place only when the cooling speed is fast. The scanning electron microscopic studies established that gelators form three-dimensional networks with helical fibrils. Interestingly, we found that in the 3Me' gel prepared from cyclohexane the gel with (R)-chirality in CD possesses a right- handed helix whereas the gel with (S)-chirality in the CD possesses a left-handed helix. The sol-gel phase transition was also induced by photoresponsive cis-trans isomerism of the azobenzene moiety: the gel formed from the trans-isomer was efficiently converted to the sol when trans-to-cis isomerization was photochemically induced, and this process can be repeated reversibly. The photoinduced sol-gel phase transition was also ''read-out'' as a change in CD spectroscopy.