Abstract: In order to enhance the photochromic effect of polyacid composites,   a protonated complex  film was prepared by combining polypyrrole with phosphomolybdic acid. The structure,  photochromic properties and discoloration mechanism were studied by atomic force microscopy,  Fourier transform infrared spectroscopy,  X-ray photoelectron spectroscopy and ultraviolet-visible absorption spectroscopy. The results show that phosphomolybdic acid maintains a stable Keggin structure in the complex film,  and the basic structures of phosphomolybdic acid and polypyrrole are not destroyed. The interaction between active group of polypyrrole and phosphomolybdic acid through non chemical bond lactam bond promotes the protonation,  improves the photochromic properties and broadens the light absorption range of the complex film. Under visible light irradiation,  the colorless and transparent polypyrrole and phosphomolybdic acid  complex film turns blue,  showing strong photochromic response. The discolored material is heated in air, showing good reversibility.  The appearance of MO⁵⁺ in X-ray photoelectron spectroscopy after discoloration shows that there is a photoreduction reaction between phosphomolybdic acid particles and polypyrrole matrix,  and the photochromic process proceeds according to the proton transfer mechanism.

Key words: polypyrrole,  , phosphomolybdic acid,  , protonation,  , visible light photochromism,  , complex film