Structure and photochromism of polyoxometalates nanoparticles in cross-linked polymer networks

Cross-linked polymer (P(VP-MB)) based on N-vinylpyrrolidone (VP) and N, N'-methylene-bis-acrylamide (MB) was used as polymer matrix to construct photochromic nanocomposite thin films with entrapping the Keggin-type polyoxometalates (POM). The microstructure, photochromic behavior, and mechanism of the films were studied with transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible spectra (UV-vis), and electron resonance spectra (ESR). The transparent films changed from colorless to blue under UV irradiation. The films showed good reversible photochromism and could recover the colorless state gradually in the air, where oxygen plays an important role during the bleaching process. The heteropolyanions dispersed in composite films uniformly and exhibited strong coulombic interaction with cross-linked polymer. Composite films contained molybdenum had higher photochromic efficiency and slower bleaching reaction than films contained tungsten. According to ESR, the photochromic mechanism was followed by charge-transfer happened between POM and cross-linked polymer.