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

A tri(ethylene oxide)octadecyldimethylammonium p-toluenesulfonate (C(18)NEO(3) x 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 C(18)NEO(3) x PF(6) amphiphile was prepared by substituting the p-toluenesulfonate of C(18)NEO(3) x Ts with hexafluorophosphate. The chemical composition of all SEPs and amphiphiles was characterized through (1)H 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. C(18)NEO(3) x Ts displays a smectic A phase with low transition temperatures, and C(18)NEO(3) x PF(6) 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.