Glutathione sensing mechanism of a fluorescent probe: Excited state intramolecular proton transfer and photoinduced electron transfer

In this work, the fluorescence turn-on mechanism of glutathione probe azido-substituted 2-(2'-hydroxyphenyl) benzoxazole derivative (AHBO) has been thoroughly studied based on the density functional theory and time-dependent density functional theory methods. The constructed potential energy curves demonstrate that the proton transfer (PT) processes of the probe AHBO and the final product AHBOG after the glutathione-azide reaction are more likely to occur in the first excited state than in the ground state. Results of frontier molecular orbital analyses show that the S-1 state of AHBO is a complete charge-separation state, and the non-radiative acceptor-excited photoinduced electron transfer (a-PET, fluorophore as the electron acceptor) from the excited azide group to the 2-(2'-hydroxyphenyl) benzoxazole (HBO) would take place upon photoexcitation, which is responsible for the fluorescence quenching of the probe AHBO. Whereas, without the electron-rich azide group, the product AHBOG undergoes the excited state intramolecular proton transfer (ESIPT) in conjunction with the weak intramolecular charge transfer (ICT) process in the S-1 state. The absence of the a-PET and the two processes mentioned above provide explanations for the fluorescent enhancement observed with the product AHBOG.