Binding site-driven sensing properties of a quinazoline derivative with metal cations

Abstract

A compound, 6-furyl-5,6-dihydrobenzoimidazo[1,2-c]quinazoline (L) was synthesized and characterized. L showed weak fluorescence emission at 416 nm in CH₃OH when excited by 383 nm. Upon addition of Al³⁺, Cr³⁺ or Fe³⁺ ions, enhanced fluorescence emission and spectral shift (55 nm) could be observed. Job's plot, fluorescence titration, ¹H NMR and ESI-MS data for binding of L with Al³⁺, Cr³⁺ and Fe³⁺ ions showed 1 : 1 metal to L complexation with the transformation of the quinazoline compound (L) to the Schiff base ligand [2-(1H-benzoimidazol-2-yl)-phenyl]-furan-2-ylmethyleneamine (L¹). Cr³⁺ can be differentiated from Al³⁺ and Fe³⁺ by time-dependent fluorescence spectra, while Al³⁺ can be distinguished from Fe³⁺ by the emission at 430 nm when excited by 300 nm in the aqueous medium CH₃OH–H₂O (1/9, v/v). The detection limits were on the order of 10⁻⁶ M for Al³⁺, Cr³⁺ and Fe³⁺ ions. L is insensitive to Cd(II) ions. The crystal structures of Cd(II) complexes, [CdL₂(OAc)₂] (1) and [CdL₂Cl₂] (2) indicate the binding site of L with Cd(II) is one imidazole nitrogen atom. UV-vis spectra and fluorescence spectra of L, 1 and 2 in CH₃OH showed no significant difference. Also ¹H NMR signals of 1 were almost the same as those of L. These observations indicated that the coordination of L with Cd(II) has little effect on the spectroscopic properties of L. The response mechanism of L to Al³⁺, Cr³⁺ and Fe³⁺ ions was discussed.