A solid-state NMR investigation of the odd–even effect in a series of liquid-crystal dimers

Abstract

The origin of the odd–even effect in the solid–nematic phase transition temperatures for a series of molecules is investigated by a variety of solid-state NMR experiments and powder XRD. The molecules consist of a flexible alkyl chain –(CH₂)_n–, with identical aromatic mesogenic groups at either end. It is found that the alkyl chain in the molecules exists in a range of different conformations in the solid state. The different conformations are in exchange with each other at room temperature and above. Molecules with n odd show a greater range of conformations at any given temperature than those with n even. The conformational distributions in the nematic phases is investigated by quenching samples from the nematic phase to low temperature. It is found that the range of conformations available to the alkyl chains in the nematic phase is similar to that in the solid state at room temperature. The molecular packing in the solid state is investigated by powder XRD and by studying the molecular dynamics of both the alkyl chains and the mesogenic groups by solid-state NMR. It is concluded that compounds with n odd have significantly different packing from those with n even. This in turn accounts for the different range of alkyl chain conformations seen in the n odd and n even compounds, and at least in part explains the odd–even effect in the solid–nematic phase transition temperatures seen along the series of molecules.