Molecular Alignment Behavior and Second-Order Nonlinear Optical Property of Termotropic Liquid-Crystalline Organic Cations without a Rigid Core

Abstract

Thermotropic liquid-crystalline behavior of various organic cations without rigid cores was evaluated by differential scanning calorimetry, polarizing optical microscopy and X-ray diffractometry. The phosphonium salts and the metal-containing ammonium complexes were found to show smectic A phase in which a homeotropic structure was formed spontaneously. It is significant that these compounds possess advantageous feature as thermotropic liquid crystals and show a stable liquid-crystalline phase and a simple phase transition behavior in comparison with commonly available ammonium analogs. Furthermore, to explore the microstructure of the organic cations in a solid state, the second harmonic generation (SHG) was measured in transmission by means of the Maker fringe method. The liquid-crystalline phosphonium salts and metal-containing ammonium complexes were found to be active for SHG. The SHG activity was strongly affected by the molecular alignment in a solid state.