Theoretical study on the molecular structures and electronic spectra of Al³⁺ detection fluorescent probes with the ESIPT mechanism

Based on the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods, the molecular structures and electronic spectra properties of 3-hydroxyflavone (3 HF) and its two cyano-and amino-substituted derivatives (3 HF-CY and 3 HF-AM) with the excited state intramolecular proton transfer (ESIPT) characteristic, as fluorescence probe molecules for the detection of Al³⁺ ions in the aqueous solution, were studied. The bond lengths, bond angles and potential energy curves related to the ESIPT process were calculated, and the absorption and fluorescence spectra of isolated molecules and toward Al³⁺ complexes were simulated. The results show that the introduction of cyano-or amino-group can inhibit the proton transfer of 3 HF in the ground-(S₀) or excited-state (S₁). It can be seen from the obtained absorption spectra that introducing the cyano-group into 3 HF can cause the red shift of absorption spectrum, while the introduction of amino-group results in the opposite phenomenon. In addition, compared with that of 3 HF-AM, the fluorescence spectrum 3 HF-AM@Al³⁺ complex exhibits a blue shift of 75.88 nm, so it was speculated that the fluorescence of 3 HF-AM would change from the green to the purple after it reacts with Al³⁺ ions in the aqueous solution, which indicates that 3 HF-AM could be used as an effective fluorescence probe to detect the Al³⁺ ions in the aqueous solution. (authors)