Physical gelation of organic fluids by anthraquinone-steroid-based molecules. Structural features influencing the properties of gels

Ten new molecules containing 2-anthraquinonyl, steroidal groups, and various linking groups have been synthesized. Several structural properties of the new molecules have been found to influence gelation abilities: (1) gelation is facilitated by beta stereochemistry at C-3 of the steroidal moiety; (2) minor modifications of the C-17 alkyl chain of the steroidal moiety do not destroy gelling ability; and (3) the length and functionality of the linker group can affect gelation ability significantly, although the number of examples studied is not sufficient to discern a clear correlation. Gelation temperatures (T-g) were measured for several gels by the ''inverse-flow'' and ''ball-drop'' methods. Since gels composed of cholesteryl 4-(2-anthraquinonyloxy)-butanoate (1) and D-, L-, or DL-2-octanol have virtually the same T(g)s over a range of gelator concentrations, specific liquid-gelator interactions do not appear to be a sensitive variable, at least when the liquid molecules being compared are similar in shape. Liquid polarity, on the other hand, has a significant effect on T-g. Gels of 1/alcohol exhibit T-g values that are much lower than those of 1/alkane gels; the opposite trend was observed for cholesteryl 4-(2-anthryloxy)butanoate gels. Circular dichroism spectral studies give rise to extrinsic circular dichroism that is not observable in isotropic phases. Again, liquid chirality has no apparent effect on the chirality of the gelator aggregates, indicating that specific diastereomeric liquid-gelator interactions do not dictate the mode of molecular packing of the gelators.