Theoretical analysis of mesogenic properties of benzalazine and benzopyran derivatives

Mesogenic behaviors of 4,4′-di-n-alkoxybenzalazine (1) and 3-phenyl-4H-4-[one]-benzopyran (2) derivatives are analyzed using theoretical models. A general phase diagram including smectic and nematic phases is explained by McMillan's model which is the extension of the model of Maier and Saupe. Some of the fundamental features of 1 and 2 are reproduced by the model, while there are also some discrepancies between the experimental and theoretical results. This may be ascribed to the lack of ordering caused by rigid-body repulsive interaction in the model. In order to investigate the nematic phases in more detail, Kimura's model is applied. The electronic properties of 1 and 2 are calculated by a semiempirical molecular orbital method (AM1). It has turned out that 1 has high polarizability and small permanent dipole moments, while 2 has low polarizability and large permanent dipole moments. The higher temperature for nematic-isotropic transition (T_NI) of 2 than 1 must be due to more flexibility of 1 or larger permanent dipole moment of 2.