Abstract
DFT and TD-DFT were used in this article to investigate the effects of different substitutions at multiple sites on the photophysical mechanism of bis-HBX in the gas phase. Four different substitution modes were selected, denoted as A(1) (X=Me, Y=S), A(2) (X=OMe, Y=S), B(1) (X=Me, Y=NH), and C(1) (X=Me, Y=O). The geometric parameters proved that the IHBs enhanced after photoexcitation, which was conducive to promote the ESIPT process. Combining the analysis of the PECs, it was revealed that the bis-HBX molecule underwent the ESIPT process, and the ease of the ESIPT process was in the order of A(1) > A(2)> B(1) > C(1). In particular, the TICT process in A(1) and B(1) promoted the occurrence of the ESIPT process. In addition, the IC process was identified, particularly in C(1). Meanwhile, the calculation of fluorescence lifetime and fluorescence rate further confirmed that A(1) was the most effective fluorescent probe molecule. This theoretical research provides an innovative theoretical reference for regulating ESIPT reactions and optimizing fluorescent probe molecules.