Density Functional Study of Intramolecular Proton Transfer Mechanisms in Dithienylethylene-based Molecular Switches

Dithienylethylene-based photochromic dyes, featuring excellent thermal stability, fatigue resistance, and rapid response, have drawn significant interest. The function of the dithiophene ethylene dye molecular switch was experimentally found to be realized by visible-light phototropic intramolecular proton transfer reactions rather than conventional UV excited-state intramolecular proton transfer reactions. This paper used density functional theory to investigate the theoretical calculation of the-molecular proton transfer reaction driven by dithienene ethylene dye. The Frank-Condon excitation wavelength of the OH configuration / NH configuration agrees well with that reported in the experimental literature, indicating that the method used for theoretical chemical calculations in this study is reasonable. Under light action, the S0→S1 transition makes the distance of C1-C2 extended from 1.55Å to 1.58Å without destroying the bonding between C1 and C2 atoms of OH configuration. The study of the intramolecular proton transfer reaction shows that the energy barrier of the intramolecular proton transfer process of S0 passing through the TS0 transition state to generate S'0 and the intramolecular proton transfer process of S1 passing through the TS1 transition state to generate S'1 is low, and the reaction is completed by only red light of about 800 nm or infrared light with a longer wavelength, whereas, if the decisive speed step of S0→S*0→S1→TS1→S'1→S'0 is passed through S0→S*0 excitation process, need UV light 367nm to complete the whole process reaction, explaining the experiment of dithiophene ethylene visible light excitation by intramolecular proton transfer reaction to complete the molecular switching function of the C1-C2 bond opening and UV excitation of non-excited state intramolecular proton transfer results. This study provides a theoretical foundation for designing fully visible-light-driven dithienylethylene systems and developing visible-light-responsive fluorescent probes for detecting water pollutants and enabling intelligent environmental sensing.