Mesomorphic structures of protonated surfactant-encapsulated polyoxometalate complexes

Keggin-type heteropolyanions, H3PW12O40 (HPW), Na3PW12O40 (NaPW), H4SiW12O40 (HSiW`) and K4SiW12O40 (KSiW), were encapsulated by a cationic surfactant, di[12-(4'-octyloxy-4-azophenyl)dodecyloxy]dimethylam monium, bromide (L), through the replacement of counterions. The resulting surfactant-encapsulated polyoxometalate complexes were characterized by UV-vis, Raman, and NMR spectra in detail. The measurement results indicated that some azobenzene groups of the surfactant were protonated in the complexes HL/HPW (HL is the abbreviation of the protonated surfactant), HL/NaPW, and HL/HSiW during the process of encapsulation, whereas the protonation was not observed in L/KSiW. The thermotropic liquid crystal properties of these complexes were investigated by differential scanning calorimetry, polarized optical microscopy and variable-temperature X-ray diffraction. Interestingly, different smectic mesophases were observed between the protonated HL/HSiW and the non-protonated L/KSiW, which suggests that the protonation of azobenzene groups in HL/HSiW plays a key role in the liquid crystalline organization. However, protonated HL/HPW and HL/NaPW exhibit a similar smectic B phase to that of the de-protonated one, L/HPW. A competitive balance between the phase separation and the volume minimization of surfactants was proposed to explain the self-organized liquid crystal structures of these protonated and non-protonated complexes. To the best of our knowledge, the present investigation provides a specific example for protonated hybrid materials with stable liquid crystal properties.