ortho-Palladated metallomesogens: the effects of structural modifications on their liquid crystalline behaviour

Abstract

The influence of structural changes such as the length, nature or number of terminal chains and the nature of the intermetallic bridge, on the liquid crystalline properties of palladium complexes [Pd(µ-X)Lj]2 (X^–= CH₃COO^–, Cl^–, Br^–) derived from imines is reported. Acetato bridges are unfavourable for liquid crystal behaviour, but both halogen bridges clearly promote mesophase formation; S_A and S_C where X is Cl, and S_A and N where X is Br. As far as the terminal tails are concerned, three main conclusions can be drawn: (i) the longer the terminal chains, the lower the melting points and the more ordered the mesophases, (ii) alkoxy tails increase the melting points but stabilize more ordered phases in comparison with alkyl tails, and (iii) the presence of a terminal substituent in the freely rotating anilinic ring has a more favourable effect on mesophase formation than when it is in the fixed (ortho-palladated) aldehydic ring. X-Ray experiments in the mesophase have revealed that molecular interdigitation is only present for the complexes containing two tails.