A new bis-quinolylimine ligand containing an azadiene moiety, 1,4-bis(2-quinolyI)-2,3-diaza-1,3-buthadiene (1), was synthesized by one-step facile condensation. This simple ligand, when dissolved in acetonitrile, shows a Cu(2+)-selective fluorescence enhancement. Coordination of 1 with Cu(2+) produces two kinds of complexes with 1:1 and 1:2 stoichiometries. The 1:2 complex shows a strong fluorescence (Phi(F) = 0.37), while the 1:1 complex does not (Phi(F) < 0.01). Ab initio molecular orbital calculation reveals that the 1:1 complex has a distorted structure, while the 1:2 complex has a planar structure. The planar configuration of the 1:2 complex, therefore, allows an extended pi-conjugation over the entire molecule and, hence, results in fluorescence enhancement. (C) 2010 Elsevier B.V. All rights reserved.