Ionic liquid crystals with hemicyanine chromophores

(E)-N-alkyl-4-(2-(4-dimethylaminophenyl)ethenyl) pyridinium bromides (hemicyanine bromides) with C-14, C-16 and C-18 straight alkyl chains are described. The thermal behaviour of these compounds was investigated by differential scanning calorimetry (DSC) and by optical microscopy. The textures observed in polarised light revealed that these compounds exhibit a smectic A mesophase. A focal-conic fan texture was observed, together with regions showing homeotropic alignment. The phase behaviour of the compound with the C(1)6 chain can be summarized as: Cr-1. 99 . Cr-2. 249 . SmA . 268 . I (dec.). The transition temperatures are rather independent of the length of the alkyl chain. The compounds have a strong tendency to supercool to a glassy mesophase. A structural model for the mesophase is proposed. The bromide anion of the hemicyanine bromides can be replaced by the bulky tetrakis(beta-diketonato)lanthanide(III) anion. As the beta-diketone, 2,4-pentanedione (acetylacetone, acac) and 1,3-diphenyl-1,3-propanedione (dibenzoylmethane, DBM) are chosen. None of the lanthanide complexes containing the hemicyanine chromophore shows a mesophase. The complexes either melt directly to an isotropic liquid, or decompose without melting.