The effects of amino group meta- and para-substitution on ESIPT mechanisms of amino 2-(2’-hydroxyphenyl) benzazole derivatives

Abstract The promising amino 2-(2′-hydroxyphenyl) benzazole derivatives have taken extensively attractions in biochemical and photochemical fields because of their strong electron-donor -NH2 group, however little was known about its importance for the excited-state intramolecular proton transfer (ESIPT). In present work, the effects of different -NH2 group positions on the ESIPT mechanisms and its photophysical properties were investigated by using time-dependent DFT calculations. The optimized geometries and infrared (IR) vibration frequencies were analyzed to explore the excited-state intramolecular hydrogen bond (ESIHB) strengthening behaviors. Absorption and emission energies calculated in this work were agreement with the observed results in experiment, reproducing the photophysical phenomena well. To illustrate in detail the effects of different substituent positions on electronic structures properties, the intramolecular charge transfer (ICT) characters were analyzed by calculating frontier molecular orbitals (FMOs) and electron-hole distribution. The minimum energy pathways explicitly illuminated the complicated ESIPT mechanisms. The calculated Gibbs free energy barriers and reduce density gradient (RDG) scatter diagrams indicated that the order in which ESIPT occurred was consistent with ESIHB intensities. The conclusion was that para-substitution of amino group had a positive influence on the ESIPT reactions, since it brought the greater effect on ESIHB compared with meta-substitution of amino group.