**1. Introduction**

Photochromism is a unique physical-chemical phenomenon that is widely applied in various fields, such as information presentation [1], photodriven nanomachines [2], optical switching [3], optical memory devices and sensors [4–6], high-density optical-electron information storage [7], and molecular recognition or examination [8]. Since single-system photochromic materials exhibit poor photochromic properties, such as poor reversibility, low photochromic activity after coloring, narrow spectral response, fatigability, and color monotony, photochromic materials that can easily overcome these limitations have attracted scientists' attention. Koski et al. [9] synthesized a kind of inorganic/inorganic photochromic material by intercalating the zero-valent metals Sn and Co into

MoO3 nanoribbons, resulting in a change in the of color MoO3 from transparent white to dark blue and demonstrating the potential of this system in intelligent color windows and convenient color change sensors. Li et al. [10] fabricated a semiconductor/semiconductor-type MoO3@TiO2 crystalline-core amorphous-shell nanorod photochromic material and the unique structure and heterojunction formed between MoO3 and TiO2 enhanced the photochromic properties of MoO3. Dai et al. [11] synthesized a doped inorganic-type photochromic material by doping Pb2+ into LiEuMo2O8, which expanded and distorted the crystal cell of LiEuMo2O8, resulting in a 50% enhancement in the red light emission from LiEuMo2O8. Lu et al. [12] synthesized a new type of inorganic/organic hybrid photochromic material, As4Mo8O33/C3N2H5, which showed a remarkable photochromic e ffect, indicating that proton transfer from the imidazole cation to the polyanion plays a key role in the photochromic process.

Polyoxometalates (POMs) are widely applied photochromic chemicals with complex morphological characteristics and unique physical-chemical properties, leading researchers to focus on developing many types of photochromic composites [13]. Posphomolybdic acid (H3PMo12 O40, PMoA), a representative Keggin-type polyoxometalate and polyprotic acid, possesses many advantages characteristic of photochromic materials, such as a large molecular volume, high thermal stability, maintenance of the crystal structure regardless of whether the material exists in the solid or liquid state, strong electron and proton storage capacities, and biocompatibility with organisms and it forms "organic-polyacidic compound anions"-type electron donor and electron acceptor hybrid materials. Feng et al. synthesized a series of organic-polyacidic compound anion-type photochromic materials, such as PMoA/polyacrylamide (PAM), PMoA/polyvinyl pyrrolidone (PVP) [14], PMoA/polyvinyl alcohol resin (PVA) [15], and PMoA/polyethylene glycol (PEG) [16], which can easily combine with PMoA to form "organic-polyacidic compound anion" hybrid films for diverse uses. However, the poor photochromic properties of these materials because of their functional groups and poor electron conversion properties limit their application. In addition, Xiao et al. [17] fabricated the Ni/Na inorganic/organic hybrid supermolecule NiEDTA-PW12, which exhibited rapid and reversible photochromism and strong antifatigue properties. Li et al. [18] fabricated polyvinyl pyrrolidone (ppy)/hexatungstate (HTA) and applied it to filter paper, realizing rapid and reversible color conversion.

Polyaniline (PANI) is a kind of industrial conductive polymer with unique electrical properties [19], optical properties [20], and magnetic properties [21,22]. Furthermore, as a kind of conductive polymer, PANI also easily undergoes protonation in the presence of a protonic acid, which will change the electrical conductivity and improve its photochromic properties, thus increasing its application value [19,20,23]. In this work, PMoA was combined with PANI to form an inorganic-organic hybrid photochromic PMoA/PANI hybrid film. PMoA, a polyprotic acid, could easily have a protonation e ffect on PANI, which would significantly change the conductivity of PANI. We explored the protonation effect of PMoA on PANI and the physical-chemical change of the PMoA/PANI hybrid film during the photochromic process through atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), electrochemical studies, ultraviolet-visible (UV-vis) spectroscopy and density functional theory (DFT) calculations, and these techniques were also used to elucidate the photochromic mechanism of the PMoA/PANI hybrid film. This characterization demonstrated that the protonation e ffect of PMoA on PANI played a significant role in enhancing proton conversion and the high maximum visible light absorbance of this photochromic material.
