Luminescence Properties of Platinum (II) Dithiooxamide Compounds

Tight contact ion pairs of general formula {Pt(H2−R2−dto)22+,(X-)2} have been prepared, and their absorption spectra and luminescence properties (at room temperature in dichloromethane fluid solution and at 77 K in butyronitrile rigid matrix) have been studied (dto = dithiooxamide; R = methyl, X = Cl (1); R = butyl, X = Cl (2); R = benzyl, X = Cl (3); R = cyclohexyl, X = Cl (4); R = cyclohexyl, X = Br (5); R = cyclohexyl, X = I (6)). The absorption spectra of all the compounds are dominated by moderately strong Pt(dπ)/S(p) to dithiooxamide (π*) charge transfer (Pt/S → dto CT) bands in the visible region (ε in the 104−105 M-1 cm-1 range). Absorption features are also present at higher energies, due to π−π* transitions centered in the dto ligands (ligand centered, LC). All the compounds exhibit a unstructured luminescence band in fluid solution at room temperature, with the maximum centered in the 700−730 nm range. The luminescence bands are blue-shifted about 4000 cm-1 on passing to the rigid matrix at 77 K. Luminescence lifetimes are on the 10-8−10-7 s time scale at room temperature and 1 order of magnitude longer at 77 K. Luminescence is assigned to triplet Pt/S → dto CT excited states in all cases. Compounds 3−6 also exhibit a second higher-energy luminescence band at room temperature, centered at about 610 nm, attributed to a LC excited state. Charge transfer interactions between halides and dto ligands destabilize dto-centered orbitals, affecting the energy of Pt/S → dto CT transitions and states. The X counterions and X → dto CT levels are proposed to play a role in promoting excited state conversion between LC and Pt/S → dto CT levels. The R substituents on the nitrogen atoms of the dto ligands influence the absorption and photophysical properties of the compounds, by affecting proximity of the ion pairs. The possibility to functionalize the R substituents may open the way to interface these luminescent compounds with desired substrates and to construct supramolecular assemblies.