Columnar liquid crystalline triphenylene-bis(dithiolene)nickel complexes. Soft photothermal materials

Abstract

This work reports new soft photothermal materials based on liquid crystalline nickel bis(dithiolene) complexes bearing pentakis(dodecyloxy)triphenylene units in which the triphenylene core and the metal complex are linked through –(CH₂)_n– (n = 2, 4, 10) connectors. The mesomorphic properties of these materials can be modulated by the length of the linker. All the complexes, except the derivative with the longest linker, show columnar mesomorphism, characterized by polarized optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray scattering studies. The structure of the mesophases contains segregated organic and inorganic columns supported respectively by π-stacking of the triphenylene discs and weak intermolecular nickel–sulfur interactions. The photothermal activity was studied on the complex with n = 2. Under laser irradiation with a power density of 0.098 W cm⁻² for just over a minute an increase in temperature of ΔT = 50 °C was achieved. This produced the melting of the crystalline solid to give rise to the columnar mesophase, where, interestingly, the photothermal effect was enhanced. Quantum chemical calculations have also been performed to gain insight into the supramolecular self-assembled columnar structure at the molecular level, as well as into the photothermal behavior.