ELECTRONIC-SPECTRA AND PHOTOPHYSICS OF PLATINUM(II) COMPLEXES WITH ALPHA-DIIMINE LIGANDS - MIXED COMPLEXES WITH HALIDE LIGANDS

Emission properties have been studied for a series of compounds of the formula (L2)PtCl2, where L2 is N,N,N',N'-tetramethylethylenediamine, 2,2'-bipyridine (bpy), 4,4'-Me2bpy, 5,5'-Me2bpy, 4,4'-(t-Bu)2bpy, 3,3'-(CH3OCO)2bpy, and 1,10-phenanthroline, and also for the compound Pt(bpy)I2. Most of them exhibit orange to red luminescence from a triplet ligand-field (3LF) state, both as solids and in glassy solution. These emissions are very broad (fwhm 2300-3400 cm-1 at 10 K) and structureless and are strongly Stokes-shifted from absorption. The two exceptions are the solid ''red'' form of Pt(bpy)Cl2, which exhibits a relatively narrow (fwhm 1050 cm-1 at 10 K), vibronically structured (DELTAnu approximately 1500 cm-1) red emission, and Pt(3,3'-(CH3OCO)2bpy)Cl2, which exhibits a broad (fwhm 2500 cm-1 at 10 K) but structured (DELTAnu approximately 1300 cm-1) orange emission. Both of these emissions are assigned to triplet metal-to-ligand charge-transfer (3MLCT) excited states. For the former compound, a linear-chain structure has destabilized a dsigma*(d(z)2) level, yielding a dsigma* --> pi*(bpy) state as the lowest energy excited state, while for the latter, the strongly electron-withdrawing substituents have stabilized a bpy pi* level, yielding a d(xy,yz) --> pi*(bpy) state as the lowest energy excited state. The relative energies of the various types of excited states, including ligand 3(pipi), states, are discussed in detail. The crystal structures of Pt(5,5'-Me2bpY)Cl2 (monoclinic Cc, Z = 4, a = 13.413(7) angstrom, b = 9,063(4) angstrom, c = 12.261(9) angstrom, beta = 121.71(6)degrees) and Pt(3,3'-(CH3OCO)2bpy)Cl2 (triclinic P1BAR, Z = 2, a = 7.288(2) angstrom, b = 9.932(3) angstrom, c = 11.881(5) angstrom, a = 98.04(3)degrees, beta = 103.56(3)degrees, gamma = 106.54(3)degrees) are reported.