Thermotropic mesomorphic behavior of surfactant-encapsulated polyoxometalate hybrids

We investigate in detail novel organic-inorganic hybrid liquid crystalline materials, the complexes of surfactant-encapsulated polyoxometalate clusters (SECs), using thermal, X-ray diffraction, and FT-IR spectroscopic analyses. The differential scanning calorimetry measurements reveal four phase transitions under heating processes. We employ FT-IR spectroscopy to understand these phase behaviors. On the basis of vibration spectral assignments, the evidence suggests that the first two phase transitions are associated with the increase of gauche conformers and the disruption of alkyl chains packing in the heating run; the third phase transition is due to the full conformational disorder of alkyl chains covered on the polyoxometalates (PMs); no significant C-H stretching or wagging vibrations are observed with the fourth transition. We find that the fourth endothermic peak is sensitive to the charges of the PMs, and the transition temperature decreases from 185, 177, to 164 degrees C with decreasing PM charges from 13, 11, to 9, respectively. Interestingly, the temperatures of the first three phase transitions of SECs are essentially independent of the PM charges.