Gelation of organic liquids by some 5 alpha-cholestan-3 beta-yl N-(2-aryl)carbamates and 3 beta-cholesteryl 4-(2-anthrylamino)butanoates. How important are H-bonding interactions in the gel and neat assemblies of aza aromatic-linker-steroid gelators?

Thermally reversible organogels, comprised of a variety of organic liquids and low concentrations of relatively low molecular mass aza analogues of pr eviously investigated ALS (i.e., aromatic linker steroid) molecules have be en investigated. The aza ALS gelators are derivatives of 3 beta-cholesteryl 4-(2-anthrylamino)-butanoate (CAAB) and 5 alpha-cholestan-3 beta-yl aryl- or alkylcarbamates. The relationship between gelator structure and efficien cy of gelation is explored. The molecular packing arrangements of the gelat ors in the gel (strands) and other phases are investigated by several physi cal methods. The results provide insights into the factors that do and do n ot lead to efficient gelators of this type. For instance, UV-vis absorption and fluorescence spectra of the gel and crystalline phases of 5 alpha-chol estan-3 beta-yl N-(2-anthryl)-N-methylcarbamate (CAMC) exhibit red-shifted bands that can be associated with J-type aggregates. From comparative infra red spectral investigations, it is concluded that there are no hydrogen-bon ding interactions of 5 alpha-cholestan-3 beta-yl N-(2-anthryl)carbamate (CA C) or 5 alpha-cholestan-3 beta-yl N-(2-naphthyl)carbamate (CNC) molecules i n the gels; the larger interactions of CAC molecules than of CAMC molecules in the gels appear to be a consequence of the smaller size of the N-substi tuent of CAC that allows closer molecular packing in strands. The small tra nsition dipoles of naphthyl rings of CNC molecules are apparently too weak to promote detectable exciton coupling of aggregates (if present) in their gel and solution phases. Strong extrinsic circular dichroism of the gel pha ses of CAC indicate that its aggregates are in macrochiral arrangements. Mo re than one strand morph can be formed by changing the cooling protocol ass ociated with sol --> gel transitions of CAC/1-pentanol samples. Dichroism o f the gel phases of CAMC and CNC is large but significantly weaker and less well defined than that of CAC. Although CAAB, the amino analogue of the ex cellent "oxa" gelator, 3 beta-cholesteryl 4-(2-anthryloxy)butanoate, gelled none of several liquids examined, its amide derivatives did. As with CAC, IR spectra of CAAB and single-crystal-X ray diffraction and IR data for 5 a lpha-cholestan-3 beta-yl N-(4-n-butylphenyl)carbamate (CBPC, a nongelator) provide no clear evidence for H-bonding to oxygen atoms in their crystallin e phases. Thus, hydrogen bonding, a critical factor in many other aggregate geometries, including those of Various gelator strands, is subordinated to other van der Waals forces.