Theoretical study on ESIPT mechanism for 5'-amino-2-(2'-hydroxyphenyl) benzimidazole probe in detecting phosgene

Recently, the new fluorescent probe 5'-amino-2-(2'-hydroxyphenyl) benzimidazole (P1) utilised in the ultrasensitive detection of phosgene based on excited-state intramolecular proton transfer (ESIPT) was synthesised experimentally (Z. J. Li et al., RSC Adv., 2021, 11, 10836). In this paper, the ESIPT process of the P1 molecule was theoretically investigated in detail by density functional theory and time-dependent density functional theory methods. The molecular structures in the ground state (S0) and the first excited state (S1) were optimised, and the infrared vibrational spectra, the frontier molecular orbitals, the potential energy curves were discussed. The calculated electronic spectra data is consistent with experimental observation. Also studied in detail is the mechanism of the ESIPT reaction, which confirms that the hydrogen bond can be enhanced in the S1 state. It will provide significant theoretical reference for future fluorescent probe experiments. Additionally, the react site is verified by calculating the electrostatic potential. Delta G < 0 indicates that the reaction of P1 with COCl2 is exothermic and occurring spontaneously. These results fully confirm that P1 is a good sensor for detecting phosgene. {GRAPHICAL ABSTRACT}