Javelin-, hockey stick-, and boomerang-shaped liquid crystals. Structural variations on p-quinquephenyl

The ramifications of changing molecular geometry in a series of all-aromati c liquid crystals derived from p-quinquephenyl are reported. Substituting h eterocyclic rings such as thiophene, oxadiazole, oxazole, or 1,3-phenylene into the p-quinquephenylene core affects molecular shape changes via the su bstituent's exocyclic bond angle. In general, we found that introducing non linearity into molecules depresses the melting transition temperature. The symmetric (boomerang-shaped) molecules, 2,5-bisbiphenyl-4-yl-1,3,4-oxadiazo le, 2,5-bisbiphenyl-4-yl-oxazole, and 1,3-bisbiphenyl-4-yl-benzene, melt in to isotropic phases showing small monotropic mesophases. By contrast, the a symmetric (hockey stick-shaped) mesogens, 2-terphenyl-4-yl-5-phenyl thiophe ne and 2-terphenyl-4-yl-5-phenyl-1,3,4-oxadiazole, exhibit more stable enan tiotropic liquid crystalline phases. The hockey stick-shaped mesogens exhib it a smectic phase as well as a nematic phase. High-temperature X-ray deter mination of the smectic layer spacing gives an unambiguous picture of inter digitated, bilayerlike supramolecular architecture in the smectic phase. Th ere are associated changes in the mesogen's electrostatic profile when a he terocycle is introduced into the quinquiphenylene framework (e.g., conjugat ion is perturbed). Our findings suggest that steric packing considerations dominate the phase preferences (nematic versus smectic phases), However, el ectronic considerations (conjugation) appear to control the range of mesomo rphism in this new family of nonlinear liquid crystals.