A time-dependent density function theory study on the substituent effect on excited-state intramolecular proton transfer of 4′-methoxy-3-hydroxyl flavone

Theoretical investigations on the excited-state intramolecular proton transfer (ESIPT) in the 4 '-methoxy-3-hydroxyl flavone (MHF) derivatives 4R-MHF (R: COOH, CF3, and CN) are performed based on time-dependent density function theory (TD-DFT). According to our calculations, the intramolecular H bondings in 4R-MHF (R: COOH, CF3, and CN) are weakened by the substituents, which would be unfavorable to ESIPT reaction. ESIPT processes in the 4R-MHF (R: COOH, CF3, and CN) are slightly harder than that in MHF because the ESIPT barriers of 4R-MHF (R: COOH, CF3, and CN) are on average 0.38 kcal/mol larger than that of MHF. The stronger the electron-withdrawing ability, the larger is the ESIPT potential barrier.