Mesophases of Achiral Bent Molecules

A systematic structural modification is carried out to change the molecular shapes of stilbene derivatives from rodlike through hockey stick then to bananalike by tuning the length of the functional group attached to the lateral direction of the rodlike skeletons. The mesophases and the corresponding properties are monitored by optical polarizing microscopy, differential scanning calorimetry, electric field effect, and X-ray diffraction. The existence of mesophase is strongly affected by the molecular conformation and intermolecular forces inherited in the moiety employed. Nematic, smectic C (SmC), and anticlinic smectic C (SmCa) phases are formed by the hockey stick molecules. Defects of 2- and 4-brushes are observed for the SmC and SmCa phases and indicate the existence of anticlinic arrangements of neighboring layers. Anticlinic layer structures constitute the intermediate phases between the calamitic and banana mesophases. Electric-field-switched texture changes are evident for the nematic and SmC phases and are different from those of known calamitic mesogens. These behaviors are ascribed to the bent conformation of these hockey stick molecules. Antiferroelectric B 2 banana phases are formed by hockey stick molecules with two arms carrying moieties of large enough polarizability.