Fabrication of heteropolyoxometalate-based photochromic inorganic–organic nanocomposites

A series of photochromic sol-gel films are prepared through entrapping tungsten heteropolyoxometallates (PW₁₂O₄₀³⁻, SiW₁₂O₄₀⁴⁻) and molybdenum heteropolyoxometallate (PMo₁₂O₄₀³⁻) into a kind of inorganic–organic matrix cohydrolyzed from tetraethylorthosilicate and 3-aminopropyltriethoxysilane. The films show reversible photochromicity. Irradiated with ultraviolet light, the transparent films change from colorless to blue. Then, bleaching occurs when the films are in contact with air or O₂ in the dark. The Keggin-type polyanions interact with R–NH₃⁺ cations strongly, and thus disperse uniformly in the sol-gel matrix, as proved by Fourier transform infrared spectra and x-ray diffraction. The molybdenum heteropolyoxometallate sol-gel film has higher photochromic efficiency and much slower bleaching than its counterparts of tungsten heteropolyoxometallate. A charge-transfer model which is supported by electron spin resonance and related literature [T. Yamase, Chem. Rev. 98, 307, (1998)] is put forth to explain the above experimental results.