Switching from columnar to calamitic mesophases in a new class of rod-like thienoviologens

Abstract

Here we report the synthesis and the liquid crystalline properties of a new class of thienoviologen salts of the dication series 4,4′-(2,2′-bithiophene-5,5′-diyl)bis(1-alkylpyridinium)⁺⁺ (mV⁺⁺s) having different alkyl-chain lengths (m = 9–11). As determined by DSC, polarized optical microscopy and X-ray diffraction analyses, the anion of the salt plays a crucial role in disclosing the liquid crystalline nature of the mV⁺⁺s. It is shown that the bis(triflimide) salts, mV(NTf₂)₂s, exhibit a unique bimesomorphic behaviour, self-segregating into columnar rectangular and/or smectic phases. Despite the rod-like shape of these π-extended viologen dications, columnar phases are obtained by the self-assembly of disk-like structures formed by the association of dications into dimers through bridging interacting anions. Interestingly, the bimesomorphism allows “switching” from a columnar to a calamitic phase and vice versa by changing the temperature and/or the alkyl-chain length. In particular, as the length of the alkyl chains increases from m = 9, where a stable rectangular columnar mesophase is observed, to m = 10, an incipient calamitic mesomorphism takes place which manifests itself with the formation of a lamello-columnar mesophase. This ultimately evolves toward a smectic A phase for m = 11.