Alkyl chain flexibility in discotic columnar mesophases

Abstract

A steric model with a single adjustable parameter is employed to compute configurationally averaged quadrupolar splittings of deuterium labeled alkyloxy chains in a homologous series of hexaalkyloxytriphenylenes (THEn). A comparison with new experimental data [Goldfarb et al., J. Chem. Phys. (in press)] permits inferences about the alkyl chain flexibility in discotic mesogens. The parameter, a control of the constraints on the chain mobility, is first optimized by fitting deuterium NMR data in a homolgous series of nematogens with labeled alkyloxy chains. The same optimum value of the parameter is subsequently used to model the quadrupolar splittings in the columnar phase of THEn discotigens and succeeds in qualitatively accounting for the experimental observations. This result indicates that alkyl chain mobility is quite comparable in these two classes of liquid crystals. The distinctly different features of the quadrupolar splitting patterns observed for nematogens and columnar discotigens are shown to originate from the different orientation of the molecular order tensor relative to the mean direction of chain propagation in the two types of mesogens.