Electronic spectra and photophysics of platinum(II) complexes with α-diimine ligands. Mixed complexes with halide ligands

Abstract

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 (Δν ∼ 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 (Δν ∼ 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 dσ*(dz2) level, yielding a dσ* → π*(bpy) state as the lowest energy excited state, while for the latter, the strongly electron-withdrawing substituents have stabilized a bpy π* level, yielding a dxz,yz → π*(bpy) state as the lowest energy excited state. The relative energies of the various types of excited states, including ligand 3ππ* states, are discussed in detail. The crystal structures of Pt(5,5′-Me2bpy)Cl2 (monoclinic Cc, Z = 4, a = 13.413(7) Å, b = 9.063(4) Å, c = 12.261(9) Å, β = 121.71(6)°) and Pt(3,3′-(CH3OCO)2bpy)Cl2 (triclinic P1, Z = 2, a = 7.288(2) Å, b = 9.932(3) Å, c = 11.881(5) Å, α = 98.04(3)°, β = 103.56(3)°, γ = 106.54(3)°) are reported. © 1993 American Chemical Society.