Different positions of cyano substitution controlled directionality of ESIPT processes with two asymmetric proton acceptors system: A TD-DFT study

Abstract

In order to systematically investigate the effect of substitution position on excited state intramolecular proton transfer (ESIPT) process and photophysical properties, three 3-(benzo[d]thiazol-2-yl)-2-hydroxy-5-methoxy-benzaldehyde (BTHMB) derivatives (BTHMB-1, BTHMB-2 and BTHMB-3) were designed by introducing the strong electron-withdrawing group (–CN) at different positions. Based on the analyses of configurations, electron densities and infrared vibrational frequencies, the intramolecular hydrogen bond (IHB) of BTHMB derivatives are all enhanced in the excited state, which would promote the ESIPT process. All the BTHMB-1, BTHMB-2 and BTHMB-3 compounds can undergo ESIPT process via two possible pathways. The potential energy curves (PECs) showed that the ESIPT barriers decrease along the order of BTHMB-2-O > BTHMB-3-O > BTHMB-1-O, and BTHMB-3-N > BTHMB-2-N > BTHMB-1-N. Due to the lower energy barrier, the ESIPT process in BTHMB-2 tends to happen to the direction of proton acceptor O atom, while the ESIPT processes in BTHMB-1 and BTHMB-3 tend to happen to the direction of proton acceptor N atom. No matter where the substituted position was, the introduction of –CN group resulted in the obvious red-shift of absorption and fluorescence peaks. For BTHMB-1-O/BTHMB-2-O/BTHMB-3-O and BTHMB-1-N/ BTHMB-2-N/BTHMB-3-N, the red-shift values of absorption and fluorescence peaks in BTHMB-1-O and BTHMB-3-N are the largest, respectively. The more red-shift the absorption and emission peaks show, the smaller the S0 and S1 energy gap between HOMO and LUMO are.