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

Here we report the synthesis and the liquid crystalline properties of a new class of thienoviologen salts ofthe dication series 4,40-(2,20-bithiophene-5,50-diyl)bis(1-alkylpyridinium)++ (mV++s) having different alkylchainlengths (m ¼ 9–11). As determined by DSC, polarized optical microscopy and X-ray diffractionanalyses, 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, selfsegregatinginto columnar rectangular and/or smectic phases. Despite the rod-like shape of these pextendedviologen dications, columnar phases are obtained by the self-assembly of disk-like structuresformed 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 bychanging the temperature and/or the alkyl-chain length. In particular, as the length of the alkyl chainsincreases from m ¼ 9, where a stable rectangular columnar mesophase is observed, to m ¼ 10, anincipient calamitic mesomorphism takes place which manifests itself with the formation of a lamellocolumnar mesophase. This ultimately evolves toward a smectic A phase for m ¼ 11.