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

Abstract

AbstractIn 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 ∇2ρ(r) at the bond critical point using atoms‐in‐molecule theory, the intramolecular hydrogen bonds (O1‐H2⋯O5 and O3‐H4⋯O5) have been supported to be formed in the S0 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 S1 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 S1 state. Frontier molecular orbitals depict the nature of electronically excited state and support the ESIPT reaction. Our scanned potential energy curves according to variational O1‐H2 and O3‐H4 coordinates demonstrate that the proton transfer process should be more likely to occur in the S1 state via hydrogen bond wire O1‐H2⋯O5 rather than O3‐H4⋯O5 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.