Structure and molecular packing in smectic BCr and Ad phases of Schiff base liquid crystal compounds through the analyses of layer spacing, entropy and crystal structure

Single-crystal structural analyses and heat capacity measurements were performed on two Schiff base liquid crystal compounds, 5CBAA (4-chlorobenzylidene-4′-pentyloxyaniline) and 5ABCA (4-pentyloxybenzylidene-4′-chloroaniline). The alkyloxy-chain of a 5CBAA molecule was conformationally ordered in the crystal at room temperature. While that of 5ABCA was partially disordered in the room temperature phase but ordered in a low-temperature phase at 100 K. The structural phase transition involving the disordering of the conformation was observed at 107 K in the heat capacity of 5ABCA. Both compounds showed two liquid crystalline phases, SmBCr and SmAd. The net entropy change associated with the chain disordering was essentially the same in them despite the difference in the orientation of their central –CHN– moiety. The layer-spacings of SmBCr and SmAd phases were analyzed for their chain-length dependence in both series of mesogens (nCBAA and nABCA), as well as in the case of nBBAA (4-bromobenzylidene-4′-alkyloxyaniline). The results reveal that these smectic structures are composed of alternately stacked core- and chain-layers with an antiparallel arrangement of cores and a bent-form of molecules.