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 (.^-configuration at C-3 and those with the inverted (/^-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 1-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-l-butanol gel gave a positive exciton coupling band with (R)-chirality whereas the 2EFmethanol gel gave a negative exciton coupling band with (5)-chirality. These results mean that dipoles in the azobenzene moiety are oriented in a clockwise (in (R)-chirality) or anticlockwise (in (5")-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-NMe2) 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 franr-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.

As part of a more comprehensive investigation of following crystallization of polyethylene isobarically and isothermally at preselected portions of the pressure (P ) and temperature (T ) phase diagram within the P range of 2-5 kbar and supercoolings (AT ) up to 10 "C, the present work is centered on formation, lateral growth (including measurement of growth rates), and melting of crystals. In the course of it the salient observation was made that, within the above specified P and AT range at least, all crystal growth occurs in the hexagonal phase, and only in this phase, irrespective of whether in the hexagonal (h) or orthorhombic (0) stability regime of the P-T phase diagram. In the latter case the h crystals represent a metastable form and transform into the stable o phase at some stage of growth, when, as now observed, all growth stops. The observed lower melting temperature of the h phase in the appropriate portion of the P-T phase diagram ("below" the triplet point) is consistent with all the above and introduces a newly recognized "nongrowth" region in the phase diagram. The lateral growth measurement of h crystal (the only crystals which are seen to grow) is readily interpretable by an activated growth mechanism when referred to the supercooling in the h form (as opposed to the o form, even when the latter is the stable one). Here the barrier attributable to nucleation is largely uneffected by P, but the preexponential (including transport) is retarded by increasing pressure. The implications of these findings for polymer crystallization are discussed, at this stage, in a provisional manner.

A compound tentatively denoted as ZnzTi308 is determined to be a low-temperature form of ZnTi03. At a heating rate of lO"C.min-' the low-temperature form crystallizes at 600" to 765°C from an amorphous material prepared by the simultaneous hydrolysis of zinc acetylacetonate and titanium isopropoxide. I t has a cubic unit cell with a =0.8408 nm. The cubic-to-hexagonal transformation occurs slowly above 820 "C; during transformation ZnTi03 decomposes into Zn2Ti04 and TiOz (rutile) at 965 " to 1010°C. A single phase of the hexagonal form can be prepared by heating for 5 h at 900°C. The structure of both forms consists of octahedral Tie, groups.

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