LUMINESCENCE PROPERTIES OF PLATINUM(II) DITHIOOXAMIDE COMPOUNDS

Tight contact ion pairs of general formula {Pt(H-2-R(2)-dto)(2)(2+),(X (-))(2)} have been prepared, and their absorption spectra and luminesc ence properties (at room temperature in dichloromethane fluid solution and at 77 K in butyronitrile rigid matrix) have been studied (dto = d ithiooxamide; R = methyl, X = Cl (1); R = butyl, X = Cl (2); R = benzy l, X = Cl (3); R = cyclohexyl, X = Cl (4); R = cyclohexyl, X = Br (5); R = cyclohexyl, X = I (6)) The absorption spectra of all the compound s are dominated by moderately strong Pt(d pi)/S(p) to dithiooxamide (p i) charge transfer (Pt/S --> dto CT) bands in the visible region (eps ilon in the 10(4)-10(5) M(-1) cm(-1) range). Absorption features are a lso present at higher energies, due to pi-pi transitions centered in the dto ligands (ligand centered, LC). All the compounds exhibit a uns tructured luminescence band in fluid solution at room temperature, wit h the maximum centered in the 700-730 nm range. The luminescence bands are blueshifted about 4000 cm(-1) on passing to the rigid matrix at 7 7 K. Luminescence lifetimes are on the 10(-8)-10(-7) s time scale at r oom temperature and 1 order of magnitude longer at 77 K. Luminescence is assigned to triplet Pt/S --> dto CT excited states in all cases. Co mpounds 3-6 also exhibit a second higher-energy luminescence band at r oom temperature, centered at about 610 nm, attributed to a LC excited state. Charge transfer interactions between halides and dto ligands de stabilize dto-centered orbitals. affecting the energy of Pt/S --> dto CT transitions and states. The X counterions and X --> dto CT levels a re proposed to play a role in promoting excited state conversion betwe en LC and Pt/S --> dto CT levels. The R substituents on the nitrogen a toms of the dto ligands influence the absorption and photophysical pro perties of the compounds, by affecting proximity of the ion pairs. The possibility to functionalize the R substituents may open the way to i nterface these luminescent compounds with desired substrates and to co nstruct supramolecular assemblies.