The excited state intramolecular double proton transfer (ESIDPT) is a complex process. Systematic research on it is conducive to understanding the mechanism of molecular tautomerism and developing new probe molecules. In this work, the density functional theory (DFT) and the time-dependent density functional theory (TD-DFT) methods are used to reveal the ESIDPT mechanism of compound named probe1, derived from 2-Aminothiophenol, 4-aminosalicylic acid and salicylaldehyde. The potential energy surfaces (PESs) analysis meticulously incorporates potential energy curves (PECs) analysis and energy barrier values to confirm the path of the ESIDPT process. Furthermore, the path of the ESIDPT process is verified by the reduced density gradient (RDG) analysis, topological parameters, the core-valence bifurcation (CVB) index and hydrogen bonding energies. In addition, the electron transfer and charge redistribution under photoexcitation, resulting from the electron density of the O1 and O4 atoms decreased, while that N3 and N6 atoms increased, leading to the distinct different of H proton transfer for intramolecular hydrogen bond 1 (IHB1) and intramolecular hydrogen bond 2 (IHB2). Our work provides deep insights in multi-step proton transfer mechanism through various means and offers theoretical basis for the development of new fluorescent probe molecules.
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