Hydrazone molecular switches have significant application value in supramolecular chemistry, and new types of hydrazone molecular switches named isatin N2-diphenylhydrazones have been synthesized. Due to its cis-trans isomerization characteristics under visible light excitation, easy synthesis of derivatives, and sensitivity to external stimuli, it has important application value in the field of biochemistry. Due to its forward and backward visible light excitation characteristics, it is considered a class of compounds that are very suitable for molecular switches, and it has extremely wide application value in fields such as biotechnology. In addition, the derivatives compound exhibits strong interactions with negative ions, which enhances its functionality as a molecular switch, making it a four-state molecular switch that can be achieved by a single molecule. However, the photo-induced isomerization mechanism of these new molecular switches is not yet clear, and it is also unknown whether there are novel phenomena in the isomerization process. This article adopts a semi empirical OM2/MRCI based trajectory surface hopping dynamics method to systematically study a photo induced isomerization mechanism based on the E-Z isomerization process of the isatin N2-diphenylhydrazones molecular switch (Fig. 1). Optimization configuration (Fig.2) and the average lifetime (Fig.3) of the first excited S1 state based on the semi-empirical OM2/MRCI method of molecular switch were obtained. It has been found that the average lifetime of the S1 excited state of the E-configuration molecular switch is about 107 fs, and the quantum yield of E-Z isomerization of the molecular switch is 16.01%. By calculating the photo induced isomerization process of the molecular switch, two different isomerization mechanisms of the molecular switch were identified; In addition to the traditional molecular switch isomerization mechanism revolving around the C=N bond (Fig.4(c, d)、Fig5(c, d)), a new isomerization mechanism has been elucidated - the face-to-face twisting of the molecular switch rotor part (Fig.4(a, b)、Fig5(a, b)); By calculating the time-resolved fluorescence radiation spectrum, it was predicted that there would be an extremely fast fluorescence quenching phenomenon occurred in about 75 fs during the isomerization process (Fig 6 (a)), slightly faster than the S1 average decay events (107 fs). The wavelength-resolved decay information in different time were also calculated, reflecting the ultrafast fluorescence quenching process accompanied by the occurrence of fluorescence red shift (Fig.6 (b)), ranging from 2.1 × 10<sup>4</sup> cm<sup>-1</sup> to 3.4 × 10<sup>4</sup> cm<sup>-1</sup>; By comparing the calculated fluorescence spectra and the average lifetime of excited states, the existence of "dark states" was proposed, and possible explanations for the existence of "dark states" were provided, and those “dark state” may have some relation with the lower quantum yield. The research results can provide theoretical guidance for the design and application of new molecular switches. The ease of synthesis and sensitivity to external stimuli of its derivatives make those compounds extremely valuable in molecular switching and light measurement applications.
