Sensing mechanism of a fluorescent probe for thiophenols: Invalidity of excited-state intramolecular proton transfer mechanism.

Simple and effective detection of thiophenols has attracted great attention. A fluorescent probe 1 with high selectivity and sensitivity is designed and synthesized based on the excited-state intramolecular proton transfer (ESIPT) in experiment. However, we conclude that the ESIPT process fails to happen actually based on the calculation results. In the present work, the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods are employed to investigate the real sensing mechanism. The calculated absorption and emission spectra agree well with the experimental results. By comparing the energy of enol and keto configurations and the constructed potential energy surfaces (PESs) in the ground (S) and excited (S) states of 3-(benzo[d]thiazol-2-yl)-10-butyl-10H-phenothiazin-2-ol (dye 2), the ESIPT process is confirmed impossible because of the relatively high keto form energy and potential energy barrier. Besides, the transition state of dye 2 is optimized to offer the accurate potential energy barrier. The results of calculated frontier molecular orbitals (FMOs) and spectra indicate that it is the photoinduced electron transfer (PET) process that results in the fluorescence quenching of probe 1. After adding thiophenols, the thiolysis of 2,4-dinitrophenyl ether bond is triggered and dye 2, which emits strong fluorescence because of the absence of PET process, is obtained. Consequently, our study has demonstrated that probe 1 can act as a fluorescent probe to detect thiophenols through the off-on fluorescence variation based on the PET mechanism but not the ESIPT process.