Investigation on non-radioactive behavior of an acylhydrazone-based fluorescent probe: Coexistence of PET and TICT mechanisms.

A fluorescent probe (E)-((2,4-dihydroxybenzyl)diazenyl)(pyridin-2-yl)methanone (HL) to effectively and selectively detect Al was designed and synthesized in the experiment. Herein, we explained the excited state dynamics mechanism of HL by using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The potential energy surfaces (PESs) proved that the excited-state intramolecular proton transfer (ESIPT) process hardly occurs due to the high reaction barriers, so the fluorescence quenching behavior of HL was not based on ESIPT. The frontier molecular orbitals (FMOs) and spectral properties were analyzed to better understand the origination of fluorescence quenching. It was found that an electron on C = N in HL could be transferred to the fluorophore during excitation in the absence of Al, accompanied by the PET process. The excited state could undergo a twisted intramolecular charge transfer (TICT) process, releasing non-radiative decay. After binding to Al, the photo-induced electron transfer (PET) process has no longer occurred, and the TICT process is eliminated, resulting in a significant fluorescence enhancement. Therefore, the calculation results well explain the quenching and enhancement behaviors of fluorescence before and after the reaction with Al.