Columnar liquid crystals derived from long-chain carboxylates of transition metal ions

Abstract

Copper 11 complexes of the fatty acids exhibit a thermotropic liquid crystalline phase of the columnar type. X-ray diffraction studies of this mesophase indicate that the spine of a column is made of regularly stacked dicopper tetracarboxylate cores, which are surrounded by disordered, liquid-like alkyl chains. In turn, the column axes occupy the nodes of a two-dimensional hexagonal lattice. Examination of the local environment of the copper atoms by Cu Ka 2 EXAFS spectroscopy reveals insignificant changes in bond lengths within the binuclear core upon the transition from the lamellar crystalline phase to the columnar mesophase. However, a slight, but sharp, decrease of the magnetic moment of the dicopper unit is observed at the solid-mesophase transition temperature (A/4eff « —0.04 ■ The columnar mesophase, and the crystalline phase alike, obey a Bleaney-Bowers relation. The singlet-triplet gap is higher in the former ( -2J = 310-330 cm -1 against 290-300 cm -1 ). Dirhodium 11.11 complexes of selected fatty acids were prepared, and their thermal behaviour was investigated. They exhibit the same thermotropic columnar mesophase as their Cu 11 congeners, reflecting the presence of isostructural dimetallic cores in both series. The metal-metal single bond that is present in these dirhodium 11,11 mesogens is Raman active (iv Rh _ Rh - 350 cm -1 ) ; it is a potential probe of the change in molecular and supramolecular structures of this class of compounds at their phase transition. Diruthenium 11.11 complexes of fatty acids were obtained by Cr 11 reduction of the mixed-valence chlorodiruthenium 11.111 tetrabutyrate, followed by ligand exchange with the appropriate carboxylic acid. Magnetic susceptibility studies indicate that these diruthenium 11.11 carboxylates have an M 5 = 0 ground state and a thermally accessible M 8 = ± 1 excited state, suggesting a zero-field split ( D = 300 cm -1 } a 2 7i 4 5 2 (5*7t*) 4 configuration. The formation of a columnar liquid-crystalline phase is reflected by a sharp increase in the magnetic moment of these complexes at the phase transition (A ju,ef{% +0