ORIGINAL PAPER A new dioxotetraamine ligand derived from binicotinic acid: synthesis, coordination, and fluorescence behaviour towards divalent transition metal ions Bikram Kishore Kanungo • Minati Baral • Rati Kanta Bera • Suban Kumar Sahoo Received: 17 September 2008 / Accepted: 27 November 2009 / Published online: 9 January 2010 Ó Springer-Verlag 2009 Abstract A novel bipyridyl-based fluorescent system, N,N 0 -bis(2-aminoethyl)-2,2 0 -bipyridine-3,3 0 -dicarboxamide (BABD), with two different coordination sites, bidentate bipyridyl and tetradentate dioxotetraamine, has been syn- thesized, and characterized by elemental analysis and spectroscopic (UV–Vis, IR, 1 H NMR, and 13 C NMR) methods. The lowest energy molecular geometry of BABD was obtained by empirical then quantum mechanical treatment. The binding ability of BABD with H ? , Co(II), Ni(II), Cu(II), and Zn(II) ions was investigated in aqueous 0.1 M KCl at 25 ± 1 °C by potentiometric methods. Four protonation constants were determined for BABD, and were used as input data to evaluate the formation constants of the metal complexes. The coordination behaviour of BABD in solution indicated that at low pH the bipyridyl unit coordinates to all metal ions, but at higher pH ([7.0) coordination of the dioxotetraamine unit occurs with the Cu(II) and Ni(II) ions only. The 3D-model structure of the metal complex was predicted by semi-empirical calculation using the AM1/d Hamiltonian. Fluorimetric titrations indicate that at pH 9 BABD exhibits fluorescence enhancement with increasing concentration of Cu(II) and Ni(II) ions, but at pH 7.2 fluorescence enhancement is observed only in the presence of Cu(II) ions. No remark- able effect on the fluorescence of BABD was observed in the presence of other biologically relevant metal ions, for example Ni(II), Fe(II), Mn(II), Co(II), Zn(II), Mg(II), Ca(II), and Hg(II). Keywords Semi-empirical calculations  Sensor  Metal complexes  Copper  Stability constants Introduction In recent years there has been burgeoning interest in the design of chelating agents which can selectively bind metal ions of interest [1–4]. The design of such chelates received widespread stimulation in the late 1970s [5, 6] after the development of supramolecular chemistry and the pio- neering work of Pedersen, Cram, and Lehn. Whenever such a chelate is suitably linked to a light-emitting group (fluorophore) through a spacer, it produces a distinct fluo- rescence signal on chelation with the metal ion (analyte). This signal is helpful for both qualitative and quantitative estimation of the target metal ion for which the chelate is designed. During analyte detection, the fluorosensor undergoes two processes, i.e. molecular recognition and signal transduction, which can be understood schematically from Scheme 1 representing an ON–OFF fluorosensor. In an ON–OFF fluorosensor, complete fluorescence of the fluorophore is observed when no analyte is bound to the receptor. Once the analyte is recognized by the receptor, fluorescence is quenched, presumably because of either electron-transfer (eT) or energy-transfer (ET) processes [7, 8]. Besides quenching, fluorescence enhancement of the fluorophore is also implemented in the design of fluoro- sensors [9–14]. B. K. Kanungo (&)  R. K. Bera Department of Chemistry, Sant Longowal Institute of Engineering and Technology, Longowal 148 106, Punjab, India e-mail: b.kanungo@vsnl.com M. Baral Department of Chemistry, National Institute of Technology, Kurukshetra 136 119, Haryana, India S. K. Sahoo Department of Chemistry, S V National Institute of Technology, Surat, India 123 Monatsh Chem (2010) 141:157–168 DOI 10.1007/s00706-009-0235-2