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

Mixed-Valent Diruthenium (II,III) Long-Chain Carboxylates. 1. Molecular Design of Columnar Liquid-Crystalline Order

“…A large part of the recent research in this field has been directed toward the investigation of mesomorphic properties (particularly the influence of the new molecular geometries arising from coordination chemistry) and, to a less extent, of physical properties − Rh(II), ,, Ru(II), ,,, Mo(II), and Cr(II) . It has been shown by several structural techniques 2,4,9 that these derivatives exhibit a columnar hexagonal mesophase above ca.…”

“…The unsolvated species present the same supramolecular organization, as found for example in the case of copper or rhodium derivatives, where dimeric species are coordinated in axial positions by oxygen atoms of neighboring molecules, thus giving rise to polymeric chains. This polymeric arrangement is retained in the liquid-crystalline phase, in which the peripheral aliphatic chains are melted, resulting in the apparition of a closely packed hexagonal 2D array of columns. ,, …”

Metal‐Containing Liquid Crystals

“…Rh-K-edge EXAFS measurements showed no change in the core structure passing from the solid state to the mesophase [48].…”

Simulation of mesogenic diruthenium tetracarboxylates: Development of a force field for coordination polymers of the MMX type

“…In this context, liquid-crystalline coordination polymers (CP) form a class of materials expected to exhibit the properties of both CP [1][2][3] and metal-containing liquid crystals (LC). [7][8][9][10][11] Among them, bimetallic carboxylates M 2 (O 2 CR eq ) 4 L ax [(L ax ¼ axial ligand; R eq ¼ equatorial ligand, Scheme 1a] have been studied thoroughly in the last years [10][11][12][13][14][15][16][17][18] with the purpose of obtaining macroscopically orientable, one-dimensional structures. [7][8][9][10][11] Among them, bimetallic carboxylates M 2 (O 2 CR eq ) 4 L ax [(L ax ¼ axial ligand; R eq ¼ equatorial ligand, Scheme 1a] have been studied thoroughly in the last years [10][11][12][13][14][15][16][17][18] with the purpose of obtaining macroscopically orientable, one-dimensional structures.…”

“…[10][11][12][13]17] Given the inherent difficulties to obtain direct experimental information at an atomic level in LC phases, the structural aspects of the columnar mesophases of these compounds have been studied by a combination of different techniques such as XRD in LC phase, local probes (RR, IR, SQUID, and EXAFS), volumetric studies, and single-crystal crystallography of nonmesogenic analogues. [10][11][12][13]17] Given the inherent difficulties to obtain direct experimental information at an atomic level in LC phases, the structural aspects of the columnar mesophases of these compounds have been studied by a combination of different techniques such as XRD in LC phase, local probes (RR, IR, SQUID, and EXAFS), volumetric studies, and single-crystal crystallography of nonmesogenic analogues.…”