Theoretical study on ESIPT mechanism of 2-acetylindan-1,3-dione in hexane and acetonitrile solvents

In the present work, using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, we investigated the compared excited-state intramolecular proton transfer (ESIPT) mechanism of 2-acetylindan-1,3-dione (AID) in both non-polar (hexane) and polar (acetonitrile) solvents theoretically. Based on the calculation of electron density ρ ( r ) and Laplacian ∇ 2 ρ ( r ) at the bond critical point using Atoms-In-Molecule (AIM) theory, the intramolecular hydrogen bond (O–H∙∙∙O) has been proved to be existent in the S 0 state. Comparing the prime structural variations of AID involved in the intramolecular hydrogen bond, we can conclude that O–H∙∙∙O should be strengthened in the S 1 state, which may facilitate the ESIPT process. Concomitantly, infrared vibrational spectra analysis further verify the stability of hydrogen bond. In good agreement with previous experimental results, AID reveals two kinds of excited-state structures (AID-enol* and AID - keto*). In addition, the role of charge transfer interaction has been addressed under the frontier molecular orbitals (MOs), which depicts the nature of electronical excited state and supports the ESIPT reaction. Our scanned potential energy curves according to variational O–H coordinate demonstrates that the proton transfer process should be more likely to occur in the S 1 state due to the inappreciable potential energy barriers. In addition, due to the minute differences of potential energy barriers contrasting hexane and acetonitrile solvents in the S 1 state, we believe that solvent effect could play roles in controlling excited state behaviors of AID system.