Theoretical exploration about the ESIPT mechanism and hydrogen bonding interaction for 2‐(3,5‐dichloro‐2‐hydroxy‐phenyl)‐benzoxazole‐6‐carboxylicacid

Abstract

AbstractExcited state hydrogen bonding interactions and the excited state dynamical behaviors are of paramount importance in the photochemical and photophysical fields. In the present work, based on density functional theory and time‐dependent density functional theory methods, we theoretically explore the excited state hydrogen bonds and excited state intramolecular proton transfer (ESIPT) mechanism for the novel 2‐(3,5‐dichloro‐2‐hydroxy‐phenyl)‐benzoxazole‐6‐carboxylicacid (DHPB) system. Firstly, comparing the non‐hydrogen bond DHPB‐O form with DHPB, we confirm the formation of hydrogen bond in DHPB molecule in the S0 state. Upon the investigations about the stable excited state structure (ie, geometrical parameter, infrared vibrational spectra, and simulated bond energy), we verify that intramolecular hydrogen bond OH···N should be strengthened in the first excited state. The simulated hydrogen bonding energy via constructing potential energy curves further confirms the strengthening phenomenon of OH···N for DHPB system. In view of photoexcitation, the charge redistribution around hydrogen bonding moieties reveals that the increased electronic densities facilitate attracting hydrogen proton. On the basis of B3LYP, Cam‐B3LYP, and PBE0 functionals, we further construct the potential energy surfaces along with ESIPT reaction path, which demonstrates that the ESIPT process is ultrafast because of the low potential barrier. It explains the reason about why the normal fluorescence cannot be observed in previous experimental phenomenon. This work fills vacancy of ESIPT mechanism for DHPB system and presents the unambiguous dynamical behavior legitimately.