Syntheses, structures and photophysical properties of tetranuclear Cd–Ln coordination complexes

Abstract

Four novel hetero-tetranuclear d–f coordination complexes, [Ln₂Cd₂(p-toluylate)₁₀(phen)₂] (Nd, 1; Pr, 2; Sm, 3) and [Ho₂Cd₂(p-chloro-benzoate)₁₀(phen)₂](4) (phen = 1, 10-phenanthroline), are synthesized by the hydrothermal method, and their structures are studied by single-crystal X-ray diffraction. Structures 1–3 are isomorphous and structure 4 has a similar molecular structure. In the near-infrared (NIR) region or in the visible region, complexes 1–4 show the characteristic emission bands of Ln(III) ions, which are attributed to the sensitization from the ligands and d-block (cadmium–ligand section) to f-block (lanthanide–ligand section). Moreover, in the Cd–Ln complexes, the Cd⋯Ln separation is very close, so the d-orbital of the Cd(II) ion and the f-orbital of the Ln(III) ion may influence each other, which probably cause the intra-levels to be adjusted. This can have a significant effect on the emission bands in the NIR region, hence the corresponding emission bands of complexes 1, 2 and 4 exhibit shift or splitting compared with isolated Ln(III) ions. At the same time in the UV-Vis-NIR absorption spectra of the complexes, the corresponding absorption bands also show shift or splitting. The shift or splitting of bands in the NIR emission spectra and in the absorption spectra can evidence each other.