Gelation of Organic Liquids by Some 5α-Cholestan-3β-yl <i>N</i>-(2-Aryl)carbamates and 3β-Cholesteryl 4-(2-Anthrylamino)butanoates. How Important Are H-Bonding Interactions in the Gel and Neat Assemblies of Aza Aromatic-Linker-Steroid Gelators?

Abstract

Thermally reversible organogels, comprised of a variety of organic liquids and low concentrations of relatively low molecular mass aza analogues of previously investigated ALS (i.e., aromatic linker steroid) molecules have been investigated. The aza-ALS gelators are derivatives of 3β-cholesteryl 4-(2-anthrylamino)butanoate (CAAB) and 5α-cholestan-3β-yl aryl- or alkylcarbamates. The relationship between gelator structure and efficiency of gelation is explored. The molecular packing arrangements of the gelators in the gel (strands) and other phases are investigated by several physical methods. The results provide insights into the factors that do and do not lead to efficient gelators of this type. For instance, UV−vis absorption and fluorescence spectra of the gel and crystalline phases of 5α-cholestan-3β-yl N -(2-anthryl)- N -methylcarbamate (CAMC) exhibit red-shifted bands that can be associated with J-type aggregates. From comparative infrared spectral investigations, it is concluded that there are no hydrogen-bonding interactions of 5α-cholestan-3β-yl N -(2-anthryl)carbamate (CAC) or 5α-cholestan-3β-yl N -(2-naphthyl)carbamate (CNC) molecules in 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-substituent of CAC that allows closer molecular packing in strands. The small transition 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 phases of CAC indicate that its aggregates are in macrochiral arrangements. More than one strand morph can be formed by changing the cooling protocol associated with sol ⇒ gel transitions of CAC/1-pentanol samples. Dichroism of 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 excellent “oxa” gelator, 3β-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 5α-cholestan-3β-yl N -(4- n -butylphenyl)carbamate (CBPC, a nongelator) provide no clear evidence for H-bonding to oxygen atoms in their crystalline 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.