Average orientation of benzyl-d(5) alkyl ethers in cationic nematic lyomesophases

Abstract

Deuterium quadrupole splittings from the aromatic ring of a series of linear benzyl-d(5) alkyl ethers, with alkyl chain length from 1 to 15 carbon atoms, have been measured using H-2-NMR spectroscopy. These molecules, 20% pentadeuteriated in the aromatic ring, were incorporated into type II cationic nematic lyotropic liquid crystals, made from tetradecyltrimethylammonium bromide (TDTMABr) and hexadecylpyridinium chloride (HDPyCl). Using these H-2 quadrupole splittings, the two order parameters that completely describe the average orientation of the aromatic ring with respect to the direction of the magnetic field were estimated. Addition of these molecules did not have any appreciable effect on the integrity of either mesophase. An increase in the mobility of the Y molecular symmetry axis of the ring, on going from C-1 to C-2, is observed in both mesophases. This observation suggest that the aromatic ring plane of benzyl methyl ether is oriented nearly parallel to the bilayer surface, although in HDPyCl it seems to be more mobile. As the alkyl chain length increases, progressive incorporation of the aromatic ring into the superstructure is observed. The opposite tendency of the quadrupole splitting of water between TDTMABr and HDPyCl is explained in terms of the location and orientation of the aromatic ring of the guest molecules and the symmetry and solvation capabilities of the head groups.