The conformations adopted by alkyl chains in α,ω-bis(aryl)alkanes in liquid-crystalline phases

Abstract

The conformations adopted by the alkyloxy chains in the α,ω-bis(4-phenyloxy)alkanes (PONOP) with 2, 3 and 4 methylene groups in the alkyl chain have been investigated for an isotropic solution in acetone for the compound with N = 2, and for samples dissolved in three nematic solvents for all three compounds. The energy difference, E ^cc _tg, between gauche and trans conformers produced by rotation about the C-C bond in PO2OP dissolved in acetone was derived from the scalar coupling constants obtained by analysing the proton spectrum from molecules containing a ¹³C atom at one C_α position. This gave E ^cc _tg in the range-0·58 to 0·49 kJ mol⁻¹. The deuterium NMR spectra of fully deuteriated samples of the PONOP dissolved in the nematic solvents were used to obtain local order parameters from the quadrupolar splittings. These were compared with values calculated by the mean field method of Emsley, Luckhurst and Stockley (ELS) to give values of E ^cc _tg in the liquid-crystalline phases. The values obtained for E ^cc _tg change from being negative for PO2OP, through zero for PO3OP to 2·5 kJ mol⁻¹ for PO4OP. The deuterium NMR data obtained previously for the mesogenic molecules α,ω-bis(4-cyanobiphenyloxy)alkanes in their nematic phases have been re-interpreted and it is shown that E ^cc _tg in these compounds is also strongly dependent on chain length.