Excited state intramolecular proton transfer via different size of hydrogen bond ring: a theoretical insight

Abstract

4′-methoxy-3-hydroxyflavone (5-HB), 2-(5-carboxyphenyl)-2-hydroxyphenyl) benzothiazole (6-HB) and o-LHBDI (7-HB), which have five-, six- and seven-membered intramolecular H-bonding ring between proton donor and proton acceptor, respectively, were chosen to investigate excited state intramolecular proton transfer (ESIPT) process in the gas phase by using density functional theory and time-dependent density functional theory methods. The intramolecular H-bond is strengthened in the excited state on account of the structural parameters and IR vibrational frequencies of the related group. The enhanced intramolecular H-bond is favorable of ESIPT process to convert enol form into keto form. 7-HB has a high chemical activity and low kinetic stability by analyzing the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. The calculated absorption and fluorescence spectra are in agreement with the experimental values. The potential energy curves (PECs) of 5-HB, 6-HB and 7-HB in the S0 and S1 states are scanned by altering O1–H2 distance in increment of 0.05 A. Our PECs results indicate that ESIPT happens easily in the S1 state with a very small barrier. The rate of ESIPT process follows this order: 6-HB~7-HB > 5-HB.