Uncovering the effect of atom substitution on ESIPT direction and Luminescent property of the asymmetric two proton acceptor compound: A TD-DFT study

Abstract

For the sake of systematically probing the atomic substitution effect on luminescent properties and excited state intramolecular proton transfer (ESIPT) behaviors, based on 3-(benzo[d]thiazol-2-yl)-2‑hydroxy-5‑methoxy-benzaldehyde (BTHMB), three new compounds BIHMB, TPHBM1 and TPHMB2 were designed by atomic substitution at different positions. For BIHMB, TPHBM1 and TPHMB2, there are two possible ESIPT pathways, and there are two isomers generated and represented by –N or –O. The absorption and fluorescence spectra of BIHMB, TPHBM1 and TPHMB2 were simulated, and their frontier molecular orbitals (FMOs) were analyzed. The atomic substitution can influence their electronic spectra. After investigating the structures, infrared vibrational frequencies and electron densities, we found that the intramolecular hydrogen bonds (IHBs) in the studied compounds are enhanced in the excited (S1) state. For BIHMB, TPHBM1 and TPHMB2, the EISPT process can occur in two directions since the ESIPT barriers of two directions are not much different. The ESIPT process along the IHB of O1H2…O3 is faster in the order of BIHMB-O > TPHMB2-O > TPHMB1-O, and the ESIPT process along the IHB of O1H2…N4 is slower in the order of BIHMB-N < TPHMB2-N < TPHMB1-N. The stronger the IHB strength, the lower the ESIPT barrier, which has been proved by the potential energy curves.