Syntheses, Electronic Structures, and Dichroic Behavior of Dinuclear Cyclopalladated Complexes of Push−Pull Azobenzenes

Eight new chloride-/acetate-bridged dimeric Pd^II complexes have been synthesized by using 4-heptoxyazobenzenes 4′-substituted with groups of varying electron-withdrawing strength (NO₂, CN, F, or CO₂Et). The effects of charge density distribution and the bridging ligand on the relative proportions of structural and geometrical isomers produced is explored by using NMR spectroscopy. UV−vis spectroscopy reveals a single low-energy absorption for each proligand but three distinct bands for each of the complexes. The origins of the observed absorptions are inferred from the effects of changing the bridging ligands and corroborated by time-dependent density functional theory (TD-DFT) calculations. Dichroic ratio (DR) measurements in two different liquid-crystal (LC) hosts show decreased performance upon complexation, although the results are notably wavelength dependent. The lowest energy absorption band, associated with intraligand charge-transfer transitions, displays superior dichroism with respect to the adjacent band at higher energy. The transition dipole moment directions calculated via TD-DFT correlate with the observed DR results, implying a decrease in alignment within the LC hosts for the complexes with respect to their proligands.