Self-association and columnar liquid crystalline phase of cationic alkyl-substituted-bipyridine benzenedithiolato gold(iii) complexes

Abstract

The introduction of a Au(III) ion into a mesogenic core, [M(Bdt)(Cnbpy)]⁺ (Bdt = 1,2-benzenedithiolato and Cnbpy = 4,4′-di-alkyl-2,2′-bipyridine (n = 13 (4,4′-di-tridecyl-2,2′-bipyridine (C13bpy)) and 8,10 (4,4′-di-(3-octyltridecyl)-2,2′-bipyridine (C8,10bpy)))), leads to the formation of ionic molecular assemblies in crystalline and mesophases. Successive syntheses of precursor complexes, [AuCl₂(Cnbpy)]PF₆ (n = 13 (1) and 8,10 (2)), followed by the target complexes, [Au(Bdt)(Cnbpy)]PF₆ (n = 13 (3) and 8,10 (4)), were achieved. The crystallographic analysis of 3 revealed that the central cationic cores form a characteristic one-dimensional columnar structure, which is an essential property for the formation of columnar structures in the liquid crystalline phase. The central cores of the [Au(Bdt)(C13bpy)]⁺ cations in 3 stack alternatively so as to cancel out their dipole moments and the counteranions lie between the cationic columns. Furthermore, the introduction of the branched alkyl tails in 4 induces the formation of a rectangular columnar liquid crystalline phase (Col_r) with C2/m symmetry, and it melts to an isotropic liquid at 69 °C. The Col_r phase of 4 is partially similar in its liquid crystalline structure to the neutral [Pt(Bdt)(C8,10bpy)] liquid crystal in a hexagonal columnar phase reported previously, while complex 4 shows a lower clearing point than that of the Pt analogue. Based on these results, the present study demonstrates the designability of the liquid crystalline structures of the [M(Bdt)(C8,10bpy)] skeleton together with their assembled structures and physico-chemical properties.