Modification of electro-optical properties of an orthoconic chiral biphenyl smectogen with its isostructural carborane analogue

Abstract

The effect of isostructural carborane additive 2F on the phase structure, transition temperatures, and electrooptical properties of orthoconic biphenyl mesogen 1F was investigated with thermal, electro-optical, dielectric, and XRD methods. Dielectric and optical data demonstrated that addition of 10 mol% of 2F to 1F maintains the orthoconic properties, while 20 mol% of 2F changes the tilt angle to 40°. In both mixtures the helical pitch p_o is significantly shorter than in the host, which results in lower dielectric permittivity. Spontaneous polarization P_s remains practically unchanged upon addition of 2F, while the viscosity is systematically lowered and, consequently, switching time is shortened by up to 25%. Comprehensive analysis of the three materials (the host and the two solutions) revealed that the carborane dopant affects primarily the interlayer intermolecular interactions, which manifests itself in tighter pitch and consequently in higher relaxation frequency f_r and elastic constant K_φ.