A series of photochromic sol-gel films are prepared through entrapping tungsten heteropolyoxometallates (PW_(12)O_(40)~(3-), SiW_(12)O_(40)~(4-)) and molybdenum heteropolyoxometallate (PMo_(12)O_(40)~(3-)) 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 O_2 in the dark. The Keggin-type polyanions interact with R-NH_3~+ cations strongly, and thus disperse uniformly I 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.
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