MAGNETIC EXCHANGE EFFECTS IN NEMATOGENIC SCHIFFS BASE CU(II) COMPLEXES - AN EPR STUDY

Two new nematogens, copper complexes derived from Schiff's bases, enty l-4[(4'-decyloxy)benzoyloxy]salicylaldiminate) copper(II) (labelled as Cu5) and bis(N-(4 ''-n-pentoxyphenyl)-4- [(4'-decyloxy)benzoyloxy]sal icylaldiminate) copper(II) (CuO5) have been studied by electron parama gnetic resonance spectroscopy at different temperatures in their diffe rent mesophases. Both compounds show a nematic phase and CuO5 also pre sents a smectic C mesophase at lower temperatures. The copper coordina tion geometry in frozen solutions in toluene and in concentrated sampl es is square planar, while in solutions in their analogous zinc comple xes, a twist of the N-Cu-O coordination planes is found. In the stable solid phases, the spectra reveal the existence of intermolecular magn etic 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 beca use of the loss of positional correlation of the molecules. The EPR sp ectra indicate differences in the local arrangement of this mesophase compared to the nematic phases of cylindrically symmetric molecules.