Liquid crystalline properties of dissymmetric molecules VI. The effect of alkyl chain length on molecular arrangement in the smectic A phase in three-aromatic-ring systems with two ester groups

The effect of alkyl and alkoxy chain lengths on the layer structures of sme ctic A and C phases has been examined by X-ray diffraction measurements on three isomeric systems: 4-alkoxyphenyl and 4-n-alkylphenyl 4-[(4-octyloxyph enyl)carbonyloxy]benzoates (1); 4-octyloxyphenyl 4-[4-(octyloxyphenyl)carbo nyloxy]benzoates (2); 4-octyloxyphenyl, 4-alkoxyphenyl and 4-n-alkylphenyl terephthalates (3); and p-phenylene 4-octyloxybenzoates 4-alkoxybenzoate an d 4-n-alkylbenzoate (4). Although all the derivatives exhibit smectic A and /or C phases having a monolayer arrangement of the molecules, the layer spa cings are considerably affected by alternation of the ester linkages. The l ayer spacings for the homologues of 1 are a little shorter than the calcula ted molecular lengths, while those for 2 agree with the calculated molecula r lengths. The layer spacings for 3 show a notable even-odd alternation in the higher homologues. The results are discussed in terms of a subtle chang e in the molecular structures due to replacement of the ester groups.