An excited state intramolecular proton transfer-based fluorescent probe with a large stokes shift for the turn-on detection of cysteine: a detailed theoretical exploration

Abstract

DFT and TDDFT calculations are adopted to study the sensing mechanism of a turn-on-type cysteine fluorescent probe (2-(1-phenyl-imidazo[1,5-α]pyridine-3-yl)phenyl acrylate, denoted as MZC-AC). The photoinduced electron transfer (PET) process of MZC-AC and the excited state intramolecular proton transfer (ESIPT) process of MZC have been investigated in detail. We demonstrate that the fluorescence quenching of MZC-AC is ascribed to the PET mechanism and the large Stokes shift fluorescence emission of MZC is the result of the ESIPT mechanism. The results have been cross-validated by geometries, frontier molecular orbital analysis, and potential energy curve scanning. As a result, our calculations completely reproduce the experimental results and give powerful evidence for the sensing mechanism of MZC-AC for cysteine.