Ionic liquid crystals with hemicyanine chromophores

(E)-N-alkyl-4-(2-(4-dimethylaminophenyl)ethenyl) pyridinium bromides (hemic yanine bromides) with C-14, C-16 and C-18 straight alkyl chains are describ ed. The thermal behaviour of these compounds was investigated by differenti al scanning calorimetry (DSC) and by optical microscopy. The textures obser ved in polarised light revealed that these compounds exhibit a smectic A me sophase. A focal-conic fan texture was observed, together with regions show ing 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 al kyl chain. The compounds have a strong tendency to supercool to a glassy me sophase. A structural model for the mesophase is proposed. The bromide anio n of the hemicyanine bromides can be replaced by the bulky tetrakis(beta-di ketonato)lanthanide(III) anion. As the beta-diketone, 2,4-pentanedione (ace tylacetone, acac) and 1,3-diphenyl-1,3-propanedione (dibenzoylmethane, DBM) are chosen. None of the lanthanide complexes containing the hemicyanine ch romophore shows a mesophase. The complexes either melt directly to an isotr opic liquid, or decompose without melting.