Hydrogen-Bonding-Induced Supramolecular Liquid Crystals and Luminescent Properties of Europium-Substituted Polyoxometalate Hybrids

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Abstract
Eu-containing polyoxometalates, Na9EuW10O36, K11Eu(PW11O39)2, and K13Eu(SiW11O39)2, were electrostatically canned by a cationic surfactant, N-[12-(4-carboxylphenoxy)dodecyl]-N-dodecyl-N,N-dimethylammonium bromide, through the replacement of counterions, and the resulting surfactant-encapsulated polyoxometalate complexes were characterized in detail by elemental analysis as well as IR and NMR spectra. The carboxyls bearing in the complexes were confirmed existing in the dimer state through intermolecular hydrogen bonding, which leads to stable and reversible thermotropic liquid crystal properties of these complexes. The results of differential scanning calorimetry, polarized optical microscopy, and X-ray diffraction revealed that these complexes underwent smectic mesophases during the heating and cooling cycles. These complexes displayed intrinsic luminescence both in the amorphous powder states and in their mesophases. The photophysical properties showed the dependence on the existing states of samples, and the quantum yields of the complexes in the liquid crystalline structures are higher than the corresponding amorphous powders. The present investigation provides an example for developing hydrogen-bonding-induced polyoxometalate-containing hybrid liquid crystal materials with intrinsic luminescence.

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