Investigating the Influence of Electronic Effects of Functional Groups on the Fluorescence Mechanism of Probes in Water Samples.

This study investigates the fluorescence quenching mechanism of formaldehyde detection probe Naph1 and its contrast probe Naph3 in water samples and discussed the effect of the electron-donating group and electron-withdrawing group on fluorescence characteristics based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT). We optimized the structures of the four probes Naph1, Naph1-S, Naph3, and Naph3-S (Scheme 1) and calculated the absorption and emission spectra, which were in good agreement with the experiment. Frontier molecular orbitals (FMOs) were used to analyze the charge arrangement in the excited state. To investigate the intramolecular proton transfer (ESIPT) phenomenon, a potential energy curve was constructed. The amount of fragment charge transfer was analyzed by the IFCT method, and then it was determined whether there was an intramolecular charge transfer (ICT) process. It was found that there was an ICT process in Naph3. The electronic effect of the functional groups did not determine the ICT characteristics and the fluorescence characteristics of the substance. Furthermore, the spin-orbit coupling  (SOC) constant based on the intersystem crossing (ISC) was supplemented, which showed that the fluorescence quenching of Naph1 and Naph3 was caused by the ISC and the corresponding quenching of Naph3-S was caused by charge transfer (CT) in the excited state.