Lowering Melting Points in Asymmetrically Substituted Salen-copper(II) Complexes Exhibiting Mesomorphic Behavior. Structure of the Mesogen Cu(5-hexyloxySalen)

In comparison with their symmetrical analogues, unsymmetrically substituted Cu−Salen complexes show mesophases with lowered melting points. For terminally substituted complexes, symmetrical ones (R1 = R2) have only an SA phase, while for unsymmetrical alkoxy substitution a monotropic SE phase occurs and the melting temperature decreases with no loss in mesophase stability. Lateral substitution, when it is symmetrical (R3 = R4), lowers mesophase stability but not melting temperature, and when it is unsymmetrical, it greatly lowers both mesophase stability and melting temperature compared with the parent compound. Substitution at the imine carbon (R5, R6) also lowers chemical stability (decomposition) of the compounds. The structure of the 5-hexyloxy complex (R1 = R2 = OC6H13, R3 = R4 = R5 = H) shows the pre-mesophasic arrangement likely adopted after melting