Structural variety, fluorescence and photocatalytic activity of dissymmetric thiosemicarbazone complexes

Abstract

A new dissymmetric bis(thiosemicarbazone) ligand, H2L2, containing a 4-(1-naphthyl)-3-thiosemicarbazone branch has been easily prepared and characterized. The reactivity of this ligand and the hybrid isopropylthiosemicarbazone/naphtoylhydrazone, H2L1, with zinc(II), copper(II) and nickel(II) nitrates in the presence of different amounts of lithium hydroxide has been explored. The results show that although both ligands present two different acidic protons, the degree of deprotonation of the ligand cannot be selectively controlled, since in almost all the reactions carried out without base the ligand is at least singly deprotonated. Nevertheless, an interesting structural diversity is found, also influenced by the metal structural preferences, leading to the formation of monomeric (1–5, 8 and 9), dimeric (6) and polymeric (7) species. The different degree of deprotonation of the ligands, together with the presence or the absence of nitrate ions and solvent molecules, lead to the formation of different supramolecular architectures based on different hydrogen bond arrays. Fluorescence spectroscopy in the solid state for the ligands and the complexes was performed and the results show that whereas H2L1 is fluorescent, for H2L2 no fluorescence emission was detected. Complexation of H2L1 leads, in general, to a quench of the fluorescence emission, except for complex [ZnL1(EtOH)] 2, which is much more fluorescent than the free ligand. Conversely, complexation of H2L2 with zinc(II) causes fluorescence emission. Photocatalytic experiments with the nickel and copper complexes shows that the four complexes can effectively degrade methyl orange under UV–vis irradiation.