Frustrated liquid crystals: Synthesis and mesomorphic behavior of unsymmetrical dimers possessing chiral and fluorescent entities

A novel approach to designing liquid crystal dimers with high chirality is described. The synthesis, and liquid crystal properties of several new optically active unsymmetrical fluorescent dimers belonging to four homologous series are presented. These new oligomesogens possess pro-mesogenic cholesterol as a chiral moiety covalently linked to a fluorescent bent aromatic (chalcone) core through a trimethylene (4-oxybutanoyl)/tetramethylene (5-oxypentanoyl)/pentamethylene (6-oxyhexanoyl)/heptamethylene (8-oxyoctanoyl) central spacer giving rise to four series of chiral dimers. In each series, the chalcone entity is substituted with ethoxy, n-butyloxy to n-dodecyloxy tails, with the aim of learning the relation between the molecular structure and thermotropic behavior. The molecular structures have been characterized by routine characterization techniques. These compounds are fluorescent, as evidenced by recording the emission spectra for two different excitation maxima for some representative compounds. The mesomorphic properties of these dimers have been evaluated by polarizing microscopic and calorimetric studies; for some representative samples, the X-ray diffraction studies has also been carried additionally. Except one, all other compounds exhibit liquid crystal behavior. Within the homologous series comprising trimethylene, pentamethylene, and heptamethylene central spacer, compounds with short hydrocarbon tails exhibit chiral nematic phase and blue phase, whereas the members with intermediate spacer length (but for few exceptions) show the twist grain boundary and/or smectic A phase/s in addition. The thermal behavior of the higher members of these series seems to be dependent on the length of the central spacer. A similar trend was observed for lower and higher homologues of dimers possessing tetramethylene (5-oxypentanoyl, C5, i.e., odd spacer) spacer. Interestingly, the higher homologues of this series were found to exhibit two twist grain boundary phases consecutively, occurring over a relatively wide thermal range with a new phase sequence. Thus, the occurrence of a fluid frustrated phase, in particular, the blue phase in the majority of the dimers, indicates that chirality of these mesogens is high enough such that the helical pitch of the chiral nematic phase is sufficiently short. This behavior can be attributed to the presence of the bent-core chalcone entity in the dimer structure, which enhances the biaxiality and chirality of the system.