The enhanced π-π Interactions in metallomesogens

Two new series of mesogenic palladium complexes 1a-b derived from salicylaldimines containing phenyl or naphenyl moiety were prepared, characterized and their mesomorphic properties investigated. Two single crystallographic structures of mesogenic 1a (n=4) and nonmesogenic 1b (n=3) were determined by X-ray analysis; both crystals crystallize in a triclinic space group P1. The geometry at both palladium atoms was nearly square planar. All Schiff bases 2a-b and palladium complexes 1a-b exhibited mesomorphic properties. However, the formation of mesophases was sensitive to terminal naphthalyl and phenyl group. Compounds 2a formed N or/and SmC phases and compounds 2b exhibited N or/and SmC phases. In contrast, palladium complexes 1a formed N or N/SmC phases and complexes 1b exhibited N, N or/and SmC phases. Compounds 1b have a higher clearing temperature than those of 1a by a ΔT_cl=35.4-14.8 °C. An improved mesomorphic behavior observed in compounds 1b was attributed to the H-bonds and π-π interaction.