Ab initio investigation of excited state dual hydrogen bonding interactions and proton transfer mechanism for novel oxazoline compound

Owing to the importance of excited state dynamical relaxation, the excited state intramolecular proton transfer (ESIPT) mechanism for a novel compound containing dual hydrogen bond (abbreviated as “1-enol”) is studied in this work. Using density functional theory (DFT) and time-dependent density functional theory (TDDFT) method, the experimental electronic spectra can be reproduced for 1-enol compound. We first verify the formation of dual intramolecular hydrogen bonds, and then confirm that the dual hydrogen bond should be strengthened in the first excited state. The photo-excitation process is analyzed by using frontier molecular orbital (HOMO and LUMO) for 1-enol compound. The obvious intramolecular charge transfer (ICT) provides the driving force to effectively facilitate the ESIPT process in the S₁ state. Exploration of the constructed S₀-state and S₁-state potential energy surface (PES) reveals that only the excited state intramolecular single proton transfer occurs for 1-enol system, which makes up for the deficiencies in previous experiment.