STABILIZATION OF NONDISCOID COLUMNAR LIQUID-CRYSTALS - STUDIES OF UNSYMMETRICAL COPPER BIS-BETA-DIKETONATES

A series of copper(bis-beta-diketonate) complexes are reported which d isplay columnar liquid-crystal phases with a hexagonal disordered stru cture (D-hd). The complexes do not have the disc-shape characteristic of most D-hd materials, but produce a disc shape by forming dimers wit h 90 degrees rotations between nearest neighbors. In the liquid-crysta lline state this dimerized nature produces short-range rotational corr elations. Three side-chain copper bis-beta-diketonates with a single p henyl substituent, 2, are not liquid crystalline, and it was found tha t an extension of the mesogenic core is necessary to introduce liquid crystallinity. The simple phenyl analogues, 3a, are monotropic, and th e 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 n ot effective at stabilizing the mesophase. Substitution of the complex with thiophene groups rather than phenyls, 4, produces stable mesopha ses with greatly lowered melting and clearing points. This latter resu lt indicates that thiophene substitution provides dispersive forces wh ich destabilize the crystal phase. Thiophene substitution may provide a general method for reducing transition temperatures in metallomesoge ns.