Heteroatom substitution controlled luminescent property and excited state intramolecular proton transfer (ESIPT) process of novel benzothiazole-based fluorophore: A TD-DFT investigation

Abstract

By using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods, we have systematically explored the spectral characteristics and excited state intramolecular proton transfer (ESIPT) processes of 3-(benzo[d]thiazol-2-yl)-2-hydroxy-5-methylbenzaldehyde (BTFM-OH) and its derivatives. The sulfur (S) atom in the thiazole ring of BTFM-OH was replaced by O/Se atom (denoted to BOFM-OH/BSeFM-OH) to probe the impact of heteroatom substitution on the luminescent property and ESIPT process. The simulated maximum absorption and fluorescence wavelengths of BTFM-OH are well consistent with the experimental values. From BOFM-OH to BTFM-OH and BSeFM-OH, the red-shifts of absorption and fluorescence (at normal and tautomer forms) wavelengths are found, which is in good accordance with the decreasing order of ground state and excited state HOMO-LUMO gaps of BOFM-OH < BTFM-OH < BSeFM-OH. According to the data obtained from structure, infrared vibrational frequency and electron density, it can be found that the intramolecular hydrogen bonds (IHBs) are enhanced in the S1 state, which is favorable to ESIPT process. From BOFM-OH to BTFM-OH and BSeFM-OH, the atomic electron-withdrawing ability of heteroatom is decreasing from O to S and Se, the IHB strengths become stronger in the order of BOFM-OH < BTFM-OH < BSeFM-OH, which makes the ESIPT process occur much easier along this order.