Room-temperature columnar mesophases of nickel-bis(dithiolene) metallomesogens

Abstract

Discotic nickel dithiolene complexes are prepared by sulfuration of original benzil proligands appended with gallate fragments carrying long carbon chains (n = 8, 12, 16), separated from the dithiolene core by ester or amide linkers. The electronic properties of these functional nickel dithiolene complexes were studied by cyclic voltammetry and UV-vis spectroscopy. The thermotropic properties were investigated by a combination of POM observations, DSC analysis and X-ray diffraction experiments. The neutral dithiolene complexes, denoted C_nesterNi and C_namideNi, all exhibit columnar phases over large temperature ranges including room temperature. C_nesterNi complexes form columnar mesophases of rectangular (pseudo-hexagonal) and hexagonal symmetry whereas the amide linkage in C_namideNi complexes strongly stabilizes hexagonal columnar mesophases far below room temperature, as illustrated by C₁₂amideNi which displays a Col_h mesophase from −18 up to +177 °C. The role of hydrogen bonding between the amide functions in the stabilisation of the mesophases in C_namideNi complexes and their benzil precursors was confirmed by IR spectroscopy. Moreover, the C₈amideNi compound exhibits a cubic phase, stable over ∼50 °C, from 155 to 203 °C. Models for the arrangements within the different columnar mesophases are proposed and discussed.