Photochromic behavior and mechanism of thin films containing tungstosilicate acid

Abstract

A novel reversible photochromic nanocomposite film based on a hybrid inorganic-organic matrix in which heteropolyacid H4SiW12O40 (SiWA) was entrapped in Si-O-Ti network was prepared. Structure, photochromic behaviors and mechanism of the film were investigated by means of infrared spectroscopy (IR), ultraviolet-visible (UV-Vis) absorption spectra and electron spin resonance (ESR). The results showed that heteropolyanion i.e. SiW12O 4− 40 (SiW12), maintained a Keggin structure in the film and there was a strong interaction between anion SiW12 and cation R-NH + 3 (R=link of hybrid composite). The photochromic properties of the composite film originated from reversible charge transfer between the anions and cations. Under UV irradiation, the anion would be reduced via one-electron step with simultaneous oxidation of the cation, accompanied by a color change from colorless to blue. Bleaching could occur when the film was in contact with ambient air or O2 in the dark.