Crystal Structures of Two Isomeric Series of Mesogens, 4-Cyanophenyl 4-n-Alkoxybenzoates and 4-n-Alkoxyphenyl 4-Cyanobenzoates

Single crystal X-ray analysis has been done for two isomeric series, 4-cyanophenyl 4-n-alkoxybenzoates (CPnOB, n = 6, 7, and 8) with a phase sequence of crystal–nematic–isotropic and 4-n-alkoxyphenyl 4-cyanobenzoates (nOPCB, n = 6, 7, and 8), which have the smectic A phase as well as the nematic phase for n = 7 and 8. In the crystals of CP6OB and CP8OB, close arrangements between CN groups of a pair of molecules are observed, though in the latter crystal two carbonyloxy groups between another pair of molecules are also closely arranged. For CP7OB and 6OPCB, CN and carbonyloxy groups come close to each other between a pair of molecules. These pairs of molecules have half-and-half overlappings (imbricate structures), being closely related to the nematic structure. On the other hand, the crystal of 7OPCB has a distinct smectic-like layer structure with infinite networks of close arrangement of the CN groups, while in the crystal of 8OPCB dimers formed by association of the CN and carbonyloxy groups are arranged in an alternate stacking of core moieties and chains, also closely related to a smectic-like layer structure. The different features of the two series are discussed in terms of the different magnitudes of the overall dipole moments along the molecular long axes caused by opposite directions of the carbonyloxy groups between the two series.