Novel thermotropic liquid crystals without a rigid core formed by amphiphiles having phosphonium ions

Thermotropic liquid-crystalline behavior of phosphonium salts, which are structurally simple amphiphiles with positively-charged phosphorus atoms and without rigid cores, was evaluated by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffractometry. The phosphonium salts were found to show a smectic A (S-A) phase in which a homeotropic structure was formed spontaneously. It is significant that the phosphonium salts possess an 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. For example, dimethylditetradecylphosphonium chloride showed the highest S-A to isotropic (I) phase transition temperature (T-SI, 187 degrees C) and the S-A phase in the widest temperature range (119 degrees C on heating) among the cationic amphiphiles examined in the present study, while dimethylditetradecylammonium chloride with the same structure except the cationic part showed the lowest T-SI (70 degrees C) and the narrowest liquid-crystalline temperature range (22 degrees C on heating).