A theoretical assignment on excited‐state intramolecular proton transfer mechanism for quercetin

In the present work, we investigate a new chromophore (ie, quercetin) (Simkovitch et al J Phys Chem B 119 [2015] 10244) about its complex excited‐state intramolecular proton transfer (ESIPT) process based on density functional theory and time‐dependent density functional theory methods. On the basis of the calculation of electron density ρ ( r ) and Laplacian ∇ 2 ρ ( r ) at the bond critical point using atoms‐in‐molecule theory, the intramolecular hydrogen bonds (O 1 ‐H 2 ⋯O 5 and O 3 ‐H 4 ⋯O 5 ) have been supported to be formed in the S 0 state. Comparing the prime structural variations of quercetin involved in its 2 intramolecular hydrogen bonds, we find that these 2 hydrogen bonds should be strengthened in the S 1 state, which is a fundamental precondition for facilitating the ESIPT process. Concomitantly, infrared vibrational spectra analysis further verifies this viewpoint. In good agreement with previous experimental spectra results, we find that quercetin reveals 2 kinds of excited‐state structures (quercetin* and quercetin‐PT1*) in the S 1 state. Frontier molecular orbitals depict the nature of electronically excited state and support the ESIPT reaction. Our scanned potential energy curves according to variational O 1 ‐H 2 and O 3 ‐H 4 coordinates demonstrate that the proton transfer process should be more likely to occur in the S 1 state via hydrogen bond wire O 1 ‐H 2 ⋯O 5 rather than O 3 ‐H 4 ⋯O 5 because of the lower potential energy barrier 2.3 kcal/mol. Our present work explains previous experimental result and makes up the deficiency of mechanism in previous experiment. In the end, we make a reasonable assignment for ESIPT process of quercetin.