See-saw Ni(ii) thiosemicarbazones: energy and photophysical properties

Abstract

Depending on the orientation and nature of the substituents, the properties of metal complexes decorated at the external periphery change. In this study, we present the synthesis, characterization, and emission properties of three thiosemicarbazone-based nickel complexes. The three ligands used in this study were (E)-2-(anthracen-9-ylmethylene)-N-(aryl)hydrazine-1-carbothioamide (aryl = 3-methoxyphenyl, 2,3-dichlorophenyl, and 2,4-dichlorophenyl), each of which forms a bis-chelated nickel complex. Depending on the substituent, the complexes had different orientations of the peripheral anthracenyl and aryl groups. In the case of aryl = 2,3- or 2,4-dichlorophenyl, the complexes had an in–in orientation of the substituent rings across the central core, and the 3-methoxyphenyl-derived complex had an in–out orientation of the two aryl groups across the central core of the complex. The methoxyphenyl-derived complex revealed N–H⋯S interactions, whereas in the dichlorophenyl group-containing complexes, the C–H⋯π, C–H⋯Cl and S⋯Cl interactions stabilized the corresponding forms in the solid state. Theoretically optimized energies of the in–in, in–out, and out–out isomers of each complex revealed two aspects: (a) the lowest energy isomers were not necessarily observed experimentally, and (b) the HOMO and LUMO of the complexes were delocalized over the metal and ligands. These observed isomers of the complexes were responsive to temperature, concentration, and excitation wavelength, as reflected in their emission spectra and proton NMR studies. The temperature-dependent NMR and emission of nickel complexes, as well as theoretically calculated HOMO–LUMO energy gaps, were reflected in the experimentally determined visible spectra and the dual emission peaks shown by the respective nickel complexes.