SMECTIC MESOPHASE PROPERTIES OF DIMERIC COMPOUNDS .1. DIMERIC COMPOUNDS BASED ON THE MESOGENIC AZOBENZENE UNIT

We have studied the mesophase behaviour and structure of certain dimeric compounds, the alpha,omega-bis(4-n-alkoxyazobenzene-4'-carbonyloxy)alkanes, in which two azobenzene moieties with alkoxy tails are linked by a polymethylene spacer. The effect of the length of the (CH2)(n) spacer was examined by preparing four series of the dimers in which the carbon number of the spacer, n, is varied from 3 to 9 while the carbon number of the terminal chain, m, is held constant at either 2, 4, 8 or 12. A marked odd-even effect typical of main chain polymers was observed, such that the transition parameters and mesophase structures strongly depend on the odd-even parity of the carbon number of the spacer. The effect of the alkyl tail length was also examined in six series of the dimers with the other combination in which n is held constant at either 4, 5, 6, 7, 8 or 9. For the dimers with even n, a short alkyl tail tends to produce a nematic phase. Increase in the length stabilizes a smectic A phase, but a further increase again induces the nematic phase. In contrast, the dimers with odd n invariably formed a smectic CA phase irrespective of the terminal rail length. These effects of the spacer and terminal chain lengths are discussed in relation to the molecular shapes which are imposed in a liquid crystalline field.