Thermotropic Liquid Crystals of a Non-Mesogenic Group Bearing Surfactant-Encapsulated Polyoxometalate Complexes

A tri(ethylene oxide)octadecyldimethylammonium p-toluenesulfonate (C18NEO3 center dot Ts) amphiphile was employed to encapsulate Keggin-type polyoxometalates by ion metathesis reactions with phosphotungstic acid, silicotungstic acid, pentapotassium dodecatungstoborate(III), and phosphomolybdic acid, giving the surfactant-encapsulated polyoxometalates (SEPs) in SEP-P, SEP-Si, SEP-B, and SEP-PMo, respectively. Meanwhile, a C18NEO3 center dot PF6 amphiphile was prepared by substituting the p-toluenesulfonate of C18NEO3 center dot Ts with hexafluorophosphate. The chemical composition of all SEPs and amphiphiles was characterized through H-1 NMR, infrared spectroscopy, mass spectroscopy, elemental analysis, and thermogravimetric analysis. The thermal properties of the SEPs and the amphiphiles were investigated by differential scanning calorimetry, polarized optical microscopy, and variable-temperature X-ray diffraction. The types of anions have a remarkable influence on the thermal properties of the prepared compounds or complexes. C18NEO3 center dot Ts displays a smectic A phase with low transition temperatures, and C18NEO3 center dot PF6 is a near-room-temperature ionic liquid. In contrast to the fact that SEP-Si and SEP-PMo decompose upon just reaching or before reaching the isotropic liquid state, both SEP-P and SEP-B reveal smectic B phase structures. The present results indicate that the combination of proper cationic amphiphiles and polyoxometalates can impart typical thermotropic liquid-crystalline behavior to SEPs, although no mesogenic groups are introduced into the complex systems.