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 ΔTcl=35.4−14.8 °C. An improved mesomorphic behavior observed in compounds 1b was attributed to the H-bonds and π–π interaction.