Recognition mechanism of imidazo[1,5-α]pyridine-based fluorescence probe towards thiophenols with multi-mechanisms of PET and ESIPT

The fluorescent turn-on response mechanism of an imidazo [1,5-alpha]pyridine-based probe MIPY-DNP for the detection of thiophenols (PhSH) has been theoretically studied. The first excited singlet (S1) state of probe MIPY-DNP is a dark state accompanying with complete charge transfer from the imidazo [1,5-alpha]pyridine group to the 2, 4-dinitrophenyl moiety, which implies the occurrence of the nonradiative photo-induced electron transfer (PET) process and explains the fluorescence quenching. The injection of PhSH induces the thiolysis of 2,4-dini-trophenyl ether group in the probe MIPY-DNP, and then the enol form of product MIPY-OH is formed. Due to lower potential barrier (6.72 kcal/mol) obtained by potential energy curves (PECs) in S1 state, the enol form of product MIPY-OH is converted into the keto form through the excited-state intramolecular proton transfer (ESIPT) process. Subsequently, the keto form of MIPY-OH emits strong fluorescence at 484 nm, which is consistent with the fluorescence observed in the experiment (478 nm). The frontier molecular orbitals (FMOs) and hole-electron analysis indicate that the electron transfer and distribution properties of the excited state, which further provides a reasonable explanation for the fluorescence quenching of probe MIPY-DNP and strong fluorescence of product MIPY-OH. Theoretical calculations gives a detailed detection mechanism of the probe MIPY-DNP towards PhSH based on the multi-mechanisms of PET and ESIPT.