Novel hybrid inorganic-organic film based on the tungstophosphate acid-polyacrylamide system: Photochromic behavior and mechanism

A novel photochromic complex composed of Keggin-type tungstophosphate acid (PW12) and polyacrylamide (PAM) was prepared. Atomic force microscopy (AFM), thermogravimetric study, Fourier transform infrared spectra (FTIR), ultraviolet-visible absorption spectra (UV-VIS), and electron resonance spectra (ESR) were used to investigate the surface topography, composition, and photochromic behavior and mechanism of the film. Under UV irradiation, the film was reduced photochemically to yield a blue species. AFM images indicated that surface topography of the polymer matrix changed after adding PW12. FTIR results showed that the Keggin geometry of PW12 was still preserved inside the matrix, and a charge-transfer bridge was built between PW12 and PAM by hydrogen bonding. The characteristic signal of W (V) and the formation of a radical signal in ESR spectra indicated that PAM was a hydrogen donor and the photoreduced process was in accordance with the radical mechanism. The diffusion of oxygen in the polymeric network plays an important role in the reversibility of photochromism.