Unlocking the Fluorescent Colour Code of Probe PBQ: Theoretical Study of the Magic Switch On/Off Mechanism for...

In this paper, we investigated the sensing mechanism of 3-hydroxy-2-(4-(pyrrolidin-1-yl) phenyl) benzo[g] quinolin-4(1H)-one (PBQ) for detecting phosgene (COCl₂), employing DFT and TD-DFT. Computational results showed that PBQ underwent the coupled process of TICT and ESIPT. It was further explained that the experimentally observed dual fluorescence phenomenon originated from the co-contribution of the enol* and keto* forms. Interestingly, although the fluorescent product (PBQ-1) had the same proton transfer site with PBQ, only a single fluorescence was observed in the experiment. We hypothesized that the monofluorescence phenomenon was caused by the fluorescence quenching of the keto* form. From the perspective of TICT, PET, and ISC analysis, we confirmed in reverse that if the PT state existed, then fluorescence in the keto* form must have also existed. In contrast, hole-electron and PES analysis revealed that PBQ-1 had a higher energy barrier and weaker ICT in the enol* state than PBQ, thereby necessarily prohibiting the ESIPT process. The results further confirmed that the fluorescence observed in PBQ-1 originates solely from the enol* state. This study provided a comprehensive understanding of the fluorescent probe photophysical mechanism for PBQ and guidance for designing novel ESIPT-based fluorescent probes for chemical sensing.