Selective colorimetric and fluorometric sensing of Cu(II) by iminocoumarin derivative in aqueous buffer

A new iminocoumarin based receptor L (C27H26N4OS) is synthesized with pyridyl and benzothiazolyl functionality. Synthesis of L is easy and it is isolated in good yield. L shows a selective and distinct color change from yellow to orange with Cu2+ over Li+, Na+, Ca2+, Mg2+, Cr2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, and Ag+ whereas a slight change in color is also observed in the case of Hg2+ but L shows selective fluorescent quenching only in the presence of Cu2+ in aqueous HEPES buffer (pH 7.0). The naked eye detection limit of Cu2+ is determined at 2 mu M whereas an emission experiment shows a lower detection limit at 200 nM. Selectivity studies of L in presence of 50 equivalents of other ion(s) by emission experiment show no interference toward the detection of 1 equivalent of Cu2+. Both UV-Vis and fluorescence studies in the presence of Cu2+-salts of different counter anions with various sizes and shapes (Cl-, ClO4-, NO3-, CF3SO3-, SO42- and BF4-) show almost similar spectral output in buffer media irrespective of the nature of the counter anions. The detailed UV-Vis and fluorescence titration experiments suggest the existence of both 1:1 and 2:1 (L : Cu2+) complexation stoichiometry and EPR study shows d(x)(2)-(2)(y) ground state of the Cu(II) centre in the complex. Furthermore the formation of a mononuclear [Cu(L)(CH3CN)]center dot 2ClO(4) complex and the flexible conformation of L in the solid state are confirmed by the single-crystal X-ray structural study.