Effect of push–Pull functional group on the ESIPT behaviour and photophysical property of thiolflavone-based compound: a TD-DFT investigation

Abstract

ABSTRACT The excited-state intramolecular proton transfer (ESIPT) mechanism and photophysical properties of three –CN-substituted (2-(4-(diethylamino)phenyl)- 3-mercapto-4H-chromen-4-one (3-NTF) compounds (3NTF-1, 3NTF-2, 3NTF-3) have been studied systematically based on density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. We analysed the structural parameters, infrared frequencies and electron densities of the studied compounds, and found that the intramolecular hydrogen bonds (IHBs) at tautomer form are weakened in the excited (S1) state. The weaker IHB at tautomer form is beneficial to ESIPT process. The potential energy curves (PECs) were established and showed that the ESIPT processes of 3NTF-1 and 3NTF-2 are barrierless. The ESIPT process of 3NTF-3 needs to overcome a very low energy barrier. The electron-withdrawing group (–CN) increases the energy barrier of ground state intramolecular proton transfer (GSIPT) process in 3NTF. The order of GSIPT barrier is consistent with the order of their IHB strengths of normal forms. When the electron-withdrawing group (–CN) was introduced, the absorption and fluorescence peaks red-shifted. Two –CN groups resulted in the fluorescence peak of 3NTF red-shift more than one –CN group did.