Deuteron magnetic resonance comparison of mesogen molecules with octyl and octyloxy end-chains

Abstract

The presence of molecular order in thermotropic liquid crystals BOBOA and CBOOA with perdeuterated octyl and octyloxy end-chain respectively is investigated through deuteron magnetic resonance experiments (DMR). In both cases methylene and methyl groups are perfectly resolved in the DMR spectra because of different motional averaging along the entire chain. Although both compounds have similar chain lengths, the quadrupolar splittings of their methyl deuterons exhibit very different temperature dependences in both the smectic A and nematic phases. This does not come from different dynamical behaviour due to the presence of the oxygen atom on the octyloxy chain, but arises from a geometrical parity effect related to the average orientation of the methyl axis with respect to the long molecular axis. This interpretation results from an analysis of the molecular motions, and their corresponding averaging effects, in two stages : first, internal deformations and overall rotation of the molecule, second collective orientational fluctuations of the long molecular axes. Under these conditions the observed discrepancy in the variations in splitting can be understood as follows : the DMR of the methyl group of the octyl chain is dominated by the intramolecular dynamics whereas that of the methyl group of the octyloxy chain and of all the methylene groups is controlled by the collective dynamics. Finally, our experimental results are compared with a mean field theory concerning the end-chain ordering in thermotropic systems