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 CnesterNi and CnamideNi, all exhibit columnar phases over large temperature ranges including room temperature. CnesterNi complexes form columnar mesophases of rectangular (pseudo-hexagonal) and hexagonal symmetry whereas the amide linkage in CnamideNi complexes strongly stabilizes hexagonal columnar mesophases far below room temperature, as illustrated by C12amideNi which displays a Colh mesophase from −18 up to +177 °C. The role of hydrogen bonding between the amide functions in the stabilisation of the mesophases in CnamideNi 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.