Critical effects of alkyl chain length on fibril structures in benzene-trans(RR)- or (SS)-N,N′-alkanoyl-1,2-diaminocyclohexane gels

Abstract

Vibrational circular dichroism (VCD) spectra were recorded on benzene-d⁶ gels formed by chiral low molecular mass gelators (LMGs), trans(RR)- or trans(SS)-N,N′-alkanoyl-1,2-diaminocyclohexane (denoted by RR-C_n or SS-C_n, respectively; n = the number of carbon atoms in an introduced alkanoyl group). Attention was focused on the effects of alkyl chain length on the structures of the gels. When n was changed from 6 to 12, the signs of the coupled peaks around 1550 cm⁻¹ in the VCD spectra, which were assigned to the symmetric and asymmetric CO stretching vibrations from the higher to lower wavenumber, respectively, critically depended on the alkyl chain length. In the case of RR-C_n, for example, the signs of the couplet were plus and minus for n = 8, 9, 10 and 12, while the signs of the same couplet were reversed for n = 6 and 7. The conformations of LMGs in fibrils were determined by comparing the observed IR and VCD spectra with those calculated for a monomeric molecule. The observed reversal of signs in the CO couplet was rationalized in terms of the different modes of hydrogen bonding. In the case of C₈, C₉, C₁₀ and C₁₂, gelator molecules were stacked with their cyclohexyl rings in parallel, forming double anti-parallel chains of intermolecular hydrogen bonds using two pairs of >NH and >CO groups. In case of C₆ and C₇, gelator molecules were stacked through a single chain of intermolecular hydrogen bonds using a pair of >NH and >CO groups. The remaining pair of >NH and >CO groups formed an intramolecular hydrogen bond.