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, (1)H & (13)C 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 Cu(2+) 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 π-π* transition of the biphenyl moiety. L 1 exhibited high selectivity and sensitivity towards Cu(2+) in both the medium over other common metal ions like Ni(2+), Co(2+), Mn(2+), Mg(2+), Zn(2+), Pb(2+) and Hg(2+). The binding constant with Cu(2+) 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 Cu(2+) and quenches (turn-off) upon addition of Na2-EDTA.