Enhanced photochromic efficiency of transparent and flexible nanocomposite films based on PEO–PPO–PEO and tungstate hybridization

Nanocomposite hybrid films were prepared by depositing tungstate on PEO–PPO–PEO (EPE) templates under acidic conditions by a one-pot self-assembly sol–gel method. Uniform and transparent films based on this precursor were fabricated and easily manipulated using a facile casting method. Upon irradiation by sunlight, the film exhibits a rapid photochromic response that was reversible at room temperature. UV-Vis analysis revealed that folding of the –CH2–O– chains in the polymer and the larger tungstate clusters increased the red shift for the adsorption of visible light. The mechanism underlying the effect of hybridization of polyethylene oxide (PEO) on the enhanced photochromic effect was characterized by NMR, Raman and FTIR. The degree of folding of the –CH2–O– polymer chains was influenced by the presence of acid, which increased the oxygen coordination of WO6 and WO4 in the tungstate clusters. The characterized W–O bonding peaks shifted upon EPE incorporation, consistent with changes in oxygen coordination within the material. This weak coordination between tungstate and –CH2–O– improved the intervalence charge transfer (IVCT). Oxygen from EPE may enter the oxygen sites in the O–W–O clusters to produce an unbalanced electron state, resulting in oxygen defects, which has a critical effect on the enhancement of the photochromic response. In addition, this method provides protons via this weak coordination, thereby dramatically enhancing the efficiency of the hybrid film.