TDDFT study on aluminum and fluoride dual-sensing mechanism of a Schiff-Base sensor

The aluminum and fluoride dual-sensing mechanism of a previously reported sensor with a Schiff-base moiety (Spectrochim. Acta A, 2017, 183, 267-274) has been investigated by density functional theory (DFT) and time-dependent DFT (TDDFT) methods. The present calculations reproduce the photoproperties of the sensor as well as its aluminum and fluoride complexes, which illustrates that DFT and TDDFT constitute a reliable tool for uncovering detailed fluorescence-based sensing mechanisms in diverse electronic states. Theoretical results indicate that there are two OH⋯N hydrogen bonds in the sensor and two OH⋯F hydrogen bonds in its F¯ complex. Different degrees of coplanarity caused by these hydrogen bonds are responsible for their distinct absorption wavelengths. However, excited-state geometry optimization and a scan of the potential-energy surface show that there is twisted intramolecular charge transfer about the CN bond in the sensor molecule and an excited-state proton-transfer process from the fluoride anion to the neighboring N atom in the fluoride-sensor complex, whereby the fluorescence is quenched. A chelation-enhanced fluorescence effect associated with the aluminum-sensor complex shows a different excited-state process. The local excitation and emission occur exclusively within the planar fluorophore, and negligible structural change upon excitation of the aluminum-sensor complex leads to its strong fluorescence. Therefore, it is theoretically explained why the sensor may be successfully used to analyze the fluoride anion by absorption spectroscopy and the aluminum cation by emission spectroscopy.