Liquid crystalline calixarenes

Bowl-shaped (bowlic) liquid crystals are reviewed and new bowlic materials containing rigid tungsten-oxo calix[4]arene based cores are discussed. Tungsten-oxo calix[4]arenes with 8 and 12 dodecyloxy sidechains have been investigated and exhibit bowlic columnar phases which are stable over approximately a 200° temperature range. The uncomplexed tetra-phenol ligands display only a transient liquid crystallinity on the first heating, and the conformational rigidity provided through tungsten-oxo complexation is necessary for well behaved mesomorphism. For the 8 sidechain analog the clearing point is at 320°C and the addition of four more sidechains results in a lower clearing point at 267°C. Polarized optical microscopy and DSC indicate that the 12 sidechain analog displays a phase with the columns packed in a hexagonal lattice which is conductive to the formation of polar phases. Both complexes exhibit a pronounced tendency to bind Lewis base guests in their cavities, and DMF forms very strong complexes which were spectroscopically characterized. The DMF guest produces large effects on the phase behavior by suppressing mesomorphism and lowering the isotropic points by 115°C and 84°C for the 8 and 12 sidechain compounds respectively. This extreme sensitivity to the DMF guest is conclusive proof that bowlic tungsten-oxo calix[4]arene liquid crystals organize in head-to-tail structures.This paper is dedicated to the commemorative issue on the 50th anniversary of calixarenes.