Screening the Optimal Probe by Expounding the ESIPT Mechanism and Photophysical Properties in Bis-HBX with Multimodal Substitutions.

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 (X=Me, Y=S), A (X=OMe, Y=S), B (X=Me, Y=NH), and C (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 > A> B > C. In particular, the TICT process in A and B promoted the occurrence of the ESIPT process. In addition, the IC process was identified, particularly in C. Meanwhile, the calculation of fluorescence lifetime and fluorescence rate further confirmed that A was the most effective fluorescent probe molecule. This theoretical research provides an innovative theoretical reference for regulating ESIPT reactions and optimizing fluorescent probe molecules.