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 rho(r) and Laplacian del(2)rho(r) at the bond critical point using atoms-in-molecule theory, the intramolecular hydrogen bonds (O-1-H2 center dot center dot center dot O5 and O-3-H4 center dot center dot center dot O5) 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-H2 center dot center dot center dot O5 rather than O-3-H4 center dot center dot center dot O5 because of the lower potential energy barrier 2.3kcal/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.