A deuterium N.M.R. study of chain ordering in the liquid crystals 4,4′-di-n-heptyloxyazoxybenzene and 4-n-octyl-4′-cyanobiphenyl

Abstract

Deuterium N.M.R. spectra are described and analysed for two liquid crystals containing eight-membered alkyl or alkyloxy end chains, which are partially or fully deuteriated. The spectra yield quadrupolar and dipolar couplings which are assigned unambiguously to segment positions and which are then used to test certain theories of orientational ordering. We examine, in particular, the idea that a single ordering matrix S can describe the orientational order for non-rigid mesogens. It is shown that plots of ratios of quadrupole splittings, Δv _i/Δv _j against Δv _k/Δv _j, where i, j, k are different segment numbers are linear when j refers to the terminal methyl groups, but markedly non-linear otherwise. This constrasts with previous work on 4,4′-di-n-heptyloxyazoxybenzene by Bos et al. (1977, Molec. Crystals liq. Crystals, 40, 59) and hence is in disagreement with their view that such ratio plots can be used to obtain the biaxiallity in S. Calculations of segmental order parameters have been carried out which assume a single ordering matrix and which evaluate averages over internal conformations by invoking the rotameric state model to describe segmental energetics in the alkyl chains. The calculated order parameters are in agreement with the general features observed experimentally, but fail to give exact agreement with the variation of ordering along the chains. It is proposed that this failure is a consequence of assuming that a single S matrix can be used and that the N.M.R. data reveal the need to describe each conformation adopted by a molecule with a separate ordering matrix.