The influence of lateral fluorination and cyanation on the mesomorphism of polycatenar mesogens and the nature of the SmC phase therein

Abstract

Several series of tetracatenar mesogens having different lateral substituents (–F, –CN, –CF₃) on the central phenylene ring of the molecule have been synthesised and their mesomorphic behaviour investigated by polarising microscopy, differential scanning calorimetry and small-angle X-ray scattering methods. The study offers an insight into the effect of different lateral polar functions on the mesomorphism of tetracatenar mesogens and starts to try to delineate polar and steric contributions. Thus –CF₃ exerts a strong steric effect that destabilises more organised mesophases (SmC, ribbon) and reduces transition temperatures. Use of a central tetrafluorophenyl unit also destabilises organised phases in common with observations made in calamitic materials, but the use of only one or two lateral fluorines has relatively little effect on transition temperatures and mesomorphism. The more polar cyano function is different in that addition of one such group reduces transition temperatures and destabilises the nematic and crystal phases, whereas the more symmetric derivative with two cyano functions shows more stable crystal and mesophases. A detailed geometrical analysis supported by X-ray diffraction data (and a dilatometry data base) leads to the proposal of a model for the SmC-to-Col_r phase transformation triggered by chain-induced layer undulations in the smectic phase.