A new turn-on fluorescent chemosensor based on sensitive Schiff base for Mn2+ ion

A Schiff-base fluorescent probe - N,N′-bis((6-(thiophen-3-yl)pyridine-2-yl) methylene) benzene-1,2-diamine (L) - was synthesized and evaluated as a chemoselective Mn2+ sensor. Upon treatment with Mn2+, the complexation of L with Mn2+ resulted in a red-shift with a pronounced enhancement in the fluorescence emission intensity in ethanol solution. Moreover, other common alkali, alkaline earth and transition metal ions failed to induce response or minimal spectral changes. Fluorescence studies on L and L-Mn2+ complex reveal that the quantum yield strongly increases upon coordination. The complex solution of L with Mn2+ ion exhibited reversibility with EDTA and regenerated free ligand for further Mn2+ sensing. The stoichiometric ratio and association constant were evaluated using Benesi-Hildebrand relation giving 1:1 stoichiometry. This further corroborated 1:1 complex formation based on Job's plot analyses. This chemosensor exhibits a very good fluorescence sensing ability to Mn2+ over a wide acidic pH range. This chemosensor can be used as an important application for detection of Mn2+ in real water samples. Both L and L-Mn2+ complex were optimized using density functional theory and vibrational frequency calculations confirmed that both are at local minima on the potential energy surfaces. The corresponding energy differences between HOMO and LUMO of L and L-Mn2+ complex are 2.210 eV and 0.550 eV, respectively which implies a low kinetic stability and high chemical reactivity.