Stabilization of Nondiscoid Columnar Liquid Crystals: Studies of Unsymmetrical Copper Bis-β-diketonates

Abstract

A series of copper(bis-β-diketonate) complexes are reported which display columnar liquid-crystal phases with a hexagonal disordered structure (Dhd). The complexes do not have the disc-shape characteristic of most Dhd materials, but produce a disc shape by forming dimers with 90o rotations between nearest neighbors. In the liquid-crystalline state this dimerized nature produces short-range rotational correlations. Three side-chain copper bis-β-diketonates with a single phenyl substituent, 2, are not liquid crystalline, and it was found that an extension of the mesogenic core is necessary to introduce liquid crystallinity. The simple phenyl analogues, 3a, are monotropic, and the addition of electron-withdrawing substituents to the phenyl moiety, 3b−d, results in a stabilization of the mesophase. These substituents produce favorable dipolar interactions which stabilize the mesophase. Consistent with this explanation, electron-donating substituents are not effective at stabilizing the mesophase. Substitution of the complex with thiophene groups rather than phenyls, 4, produces stable mesophases with greatly lowered melting and clearing points. This latter result indicates that thiophene substitution provides dispersive forces which destabilize the crystal phase. Thiophene substitution may provide a general method for reducing transition temperatures in metallomesogens.