Examination of molecular packing in orthogonal smectic liquid crystal phases: a guide for molecular design of functional smectic phases

The reported layer spacings (d(smectic)) of six homologues of mesogens exhibiting orthogonal smectic phases (SmE, SmB, and SmA phases) are reexamined. The slopes of the linear dependences on chain length (n, the number of carbon atoms in the hydrocarbon chain) are clearly categorized into two groups: 1.9 angstrom (CH2)(-1) and 1.4 angstrom (CH2)(-1). It is clarified that in the former the molecules take a rod-like form (rod-form; category-I), whereas in the latter the molecules are bent around the connection between the core and chain moieties (bent-form; category-II). The average relative positions of adjacent molecules within the smectic structures are deduced from the intercept of the linear functions of dsmectic against n. The relation between and the features of molecules belonging to the two categories are discussed for molecular design of functional smectic liquid crystals.