Bis-Benzimidazolyl Diamide Based Fluorescent Probe for Copper(II): Synthesis, Structural and Fluorescence Studies

A new fluorescent probe based on a bis-benzimidazole diamide N (2),N (2')-bis[(1-ethyl-benzimidazol-2-yl)methyl]biphenyl-2,2'-dicarboxamide ligand L (1) with a biphenyl spacer group and a Copper(II) trinuclear metallacycle has been synthesized and characterized by X-ray single crystallography, elemental and spectral (FT-IR, H-1 & C-13 NMR, UV-Visible) analysis. The fluorescence spectra of L (1) in MeOH show an emission band centered at 300 nm. This band arises due to benzimidazolyl moiety in the ligating system. The diamide L (1) in the presence of Cu2+ show the simultaneous 'quenching' of (300 nm) and 'enhancement' of (375 nm) emission band. Similar fluorescence behavior was found in water-methanol mixture (9:1). The new emission band at 375 nm is attributed to intra ligand pi-pi* transition of the biphenyl moiety. L (1) exhibited high selectivity and sensitivity towards Cu2+ in both the medium over other common metal ions like Ni2+, Co2+, Mn2+, Mg2+, Zn2+, Pb2+ and Hg2+. The binding constant with Cu2+ was calculated by the Benesi-Hildebrand equation. Selective "off-on-off" behavior of L (1) in methanol has also been studied. The fluorescent intensity of 375 nm bands in L (1) enhances (turns-on) upon addition of Cu2+ and quenches (turn-off) upon addition of Na-2-EDTA.