New insights into the excited state intramolecular proton transfer (ESIPT) competition mechanism for different intramolecular hydrogen bonds of Kaempferol and Quercetin in solution

In this work, the process of the Excited State Intramolecular Proton Transfer (ESIPT) competition of Kaempferol and Quercetin in methanol was studied by means of the Density Functional Theory (DFT) and the Time-Dependent Density Functional Theory (TDDFT) method. Two different ESIPT reaction pathways were analyzed by the variation trend of the 3D potential energy surfaces (PESs) with the change of the intramolecular hydroxyl bond length through the PESs of the two molecules of the ground state (S 0 ) and the first excited (S 1 ) state. The ESIPT process only occurred between O 25 –H 28 and O 22 –H 28 and was completed within 33.1fs by molecular excited state dynamics simulation. It is demonstrated that when there are two kinds of hydrogen bonds in the molecule, the ESIPT prefers the six-membered ring hydrogen bonds to the five-membered ring hydrogen bonds. • The ESIPT competitive priority of adjacent hydrogen bonds has been studied. • A 3D potential energy surface was constructed to clarify the the ESIPT process. • Excited state dynamics simulation verify the occurrence of ESIPT. • Kaempferol and Quercetin also have the potential to be used as sunscreens.