Unraveling the dual sensing mechanism of hydrogen peroxide probe: Photoinduced electron transfer and unusual local excited state intramolecular proton transfer

An understanding of hydrogen peroxide's (H2O2) sensing process is crucial for the development of effective fluorescent probes. In this investigation, we unraveled the sensing mechanism of the H2O2 fluorescent probe (BBD) derived from phthalimide-borate. The fluorescence of probe BBD is quenching and our findings establish this phenomenon can be attributed to a nonclassical photoinduced electron transfer (PET) process, wherein a crossing of energy curves between the local excited and charge transfer states is observed. The reaction between BBD and H2O2 causes the phenylboronic acid ester to oxidize and release a hydroxyl group, which turns off the PET process and causes the product HMD to enhance the fluorescence significantly. In addition, the results show that the generated HMD-N likewise undergoes fluorescence quenching, and the fluorescence band seen in the experiment originates from the HMD-T created by the unusual local excited state intramolecular proton transfer (ESIPT) process. In conclusion, our theoretical findings provide a comprehensive explanation for the fluorescence characteristics exhibited by BBD and HMD with an unusual local ESIPT combined PET process.