Effect of macrocyclic ring size on the liquid-crystalline properties of calamitic materials

Abstract

The effect on the mesomorphic behaviour of a series of macrocyclic calamitic liquid crystals has been investigated by altering the size and nature of a macrocyclic ring located within the central core region of the molecules. Calamitic liquid-crystalline materials have been produced by positioning either a diaza-crown ether macrocyclic ring or a diazatetrathiacyclooctadecane ring between two mesogenic groups which were kept constant in structure. The 4-[4-(4-octyloxybenzoyloxy)benzoyloxy]benzoyl unit was used as the mesogenic group and 1,10-diaza-18-crown-6, 1,7-diaza-15-crown-5, 1,7-diaza-12-crown-4, piperazine and 1,10-diaza-4,7,13,16-tetrathiacyclooctadecane were used as the macrocyclic core substrates. The phase behaviour of the liquid-crystalline products was found to be critically dependent on the size, flexibility and nature of the macrocyclic core. In general, a decrease in the size of the macrocyclic core was found to lead to an increase in transition temperatures and clearing points. A model has been developed which attributes this effect to a decrease in the flexibility of the central regions of the molecules as the ring size is reduced.