**4. Conclusions**

A photochromic organic/inorganic PMoA/PANI hybrid thin film was easily synthesized by the protonation effect and demonstrated grea<sup>t</sup> photochromic properties, especially high light absorption of 3.46 in the UV-vis spectrum. AFM presented an even distribution of PMoA/PANI hybrid thin film root mean square roughness with 2.34 nm before illumination and 3.43 nm after illumination. All the characterizations and mechanistic analyses showed that protonation played a key role in the transformations of the quinone structure of eigenstate PANI and the single-polarized benzene structure of PANI observed by FTIR and XPS, which induced absorption by PMoA, enhancing the electrons transfer property verified by electrochemistry and DFT studies and providing electrons for PMoA to alter the valence state in order to achieve exceptional photochromic performance. All these results provided a new possibility for the application of PMoA and PANI, especially in the region of laser ink-free printing and optical control switches.

**Author Contributions:** Q.Z. contributed to overall organizing of all experiments, and writing-original draft preparation. S.G. and Y.S. contributed to photochromic properties experiments, data analysis and visualization. Z.L. contributed to sample preparation and characteration. W.F. contributed to providing ideas for all the experiments, methodology, and supervision. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was funded by the National Natural Science Foundation of China, gran<sup>t</sup> number 61774073, the Open Project of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, gran<sup>t</sup> number 2016-25), and the Science and Technology Development Program of Jilin Province, gran<sup>t</sup> number 20170101086JC. And the APC was funded by the Science and Technology Development Program of Jilin Province, gran<sup>t</sup> number 20170101086JC.

**Conflicts of Interest:** The authors declare no conflict of interest.
