Magnetic exchange effects in nematogenic Schiff's base Cu(II) complexes. An EPR study

Abstract

Two new nematogens, copper complexes derived from Schiff's bases, bis(N-n-pentyl-4-[(4′-decyloxy)benzoyloxy]salicylaldiminate) copper(II) (labelled as Cu5) and bis(N-(4″-n-pentoxyphenyl)-4-[(4′-decyloxy)benzoyloxy]salicylaldiminate) copper(II) (CuO5) have been studied by electron paramagnetic resonance spectroscopy at different temperatures in their different mesophases. Both compounds show a nematic phase and CuO5 also presents a smectic C mesophase at lower temperatures. The copper coordination geometry in frozen solutions in toluene and in concentrated samples is square planar, while in solutions in their analogous zinc complexes, a twist of the N-Cu-O coordination planes is found. In the stable solid phases, the spectra reveal the existence of intermolecular magnetic exchange coupling. The fluid phases of Cu5 can be frozen forming different structures that depend on the freezing process rate. In Cu5 the exchange interaction is strongly reduced in the nematic phase because of the loss of positional correlation of the molecules. The EPR spectra indicate differences in the local arrangement of this mesophase compared to the nematic phases of cylindrically symmetric molecules.