Columnar mesomorphism in molecular alloys based on Halogen-Bridged PtII/PtIV mixed valence complexes. The key role of the PtII/PtIV ratio

Abstract

In this paper, we studied the thermotropic behavior and phase diagrams of blends of two platinum-based met- allomesogens with different oxidation states and ligands, which are prone to self-association through the for- mation of a mixed-valence system via PtII⋅⋅⋅Cl-PtIV interactions. The constituent molecules are cis-[PtCl2(CNL)2] (CNL = 2-(6-(4-isocyanophenoxy)hexyloxy)-3,6,7,10,11-pentakisdodecyloxytriphenylene) and [PtCl4(Bipy)] (Bipy = didodecyl 2,2′-bipyridyl-4,4′-dicarboxylate), which display an organic/inorganic segregated multi- columnar mesophase and a lamellar mesomorphism, respectively. Firstly, the phase diagram was constructed using polarized optical microscopy, differential scanning calorimetry and X-ray scattering data. The results indicated that compositions exceeding 50 % of [PtCl2(CNL)2] led to columnar mesophases very similar to the one of pure [PtCl2(CNL)2], where triphenylene cores and platinum moieties are segregated into different columnar units. Then, employing tight-binding quantum chemical methods, we studied the supramolecular organization in the columnar mesophases, as well as the electronic structure properties of mixtures with compositions larger than 50 % of [PtCl2(CNL)2]. The results revealed that the formation of the metal–organic columns relies on a fine balance between PtII⋅⋅⋅PtII and PtII⋅⋅⋅Cl-PtIV intermolecular interactions, which strongly depend on the compo- sition of the system. A deep analysis of the density of states showed that lower concentrations of [PtCl4(Bipy)] result in a hole-doped system, while at higher [PtCl4(Bipy)] concentrations the intervalence charge transfer between the metal center in different oxidation states significantly changes the electronic structure.