PHOTOINDUCED ENERGY AND ELECTRON-TRANSFER PROCESSES IN SUPRAMOLECULARSPECIES - TRIS(BIPYRIDINE) COMPLEXES OF RU(II) OS(II), RU(II)/RU(III), OS(II)/OS(III), AND RU(II)/OS(III) SEPARATED BY A RIGID SPACER/

The bis(bipyridine) bridging ligand -(2,2'-bipyridin-5-yl)ethenyl]bicy clo[2.2.2]octane (bpy-S-bpy), where S is a rigid spacer made of a bicy clooctane unit symmetrically linked to two ethylene-type units in a E, E-configuration, has been synthesized and its complexes (bpy)2Ru(bpy-S -bpy)2+ (Ru(II).A), (bpy)2Os(bpy-S-bpy)2+ (Os(II).A), (bpy)2-Ru(bpy-S- bpy)Ru(bpy)2(4+) (Ru(II).A.Ru(II)), (bpy)2Os(bpy-S-bpy)Os(bpy)2(4+) (O s(II).A.Os(II)), (bpy)2Ru(bpy-S-bpy)-Os(bpy)2(4+) (Ru(II).A.Os(II)) ha ve been prepared as PF6- salts. The length of the rigid spacer S is 9 angstrom, and the center-to-center separation distance in the dinuclea r complexes is 17 angstrom. In all these novel compounds, each Ru-base d and Os-based unit displays its own absorption spectrum and electroch emical properties, regardless of the presence of a second metal-based unit. The homometallic dinuclear compounds exhibit the same luminescen ce properties as the corresponding mononuclear species, whereas in the heterometallic dinuclear Ru(II).A.Os(II) species 91% of the Ru-based luminescence intensity is quenched by energy transfer to the Os-based unit, whose luminescence is accordingly sensitized (acetonitrile solut ion, room temperature). The excited state lifetime of the Ru-based uni t (209 ns) is reduced to 18 ns, and a comparable risetime is observed for the energy transfer sensitization of the Os-based luminescence. Th e energy transfer process occurs with rate constant 5.0 X 10(7) s-1, p redominantly by an exchange mechanism. Partial oxidation of the binucl ear species Ru(II).A.Ru(II), Os(II).A.Os(II) and Ru(II).A.Os(II) by Ce (IV) in acetonitrile-water solutions leads to mixed-valence M(II).A.M( III) species (M = Ru and/or Os) where the oxidized metal-based unit qu enches the luminescent excited state of the unit that is not oxidized. For the Ru(II).A.Os(III) compound, the residual luminescent intensity of the Ru-based unit is < 1.5% and its excited state lifetime is 115 ps. The quenching occurs by electron transfer (k(el) = 8.7 X 10(9) s-1 ) with formation of the thermodynamically unstable Ru(III).A.Os(II) va lence isomer which then goes back (k(b) = 1.0 x 10(6) s-1) to Ru(II).A .Os(III). The Ru(II).A.Ru(III) and Os(II).A.Os(III) mixed-valence comp ounds can only be obtained in the presence of the corresponding M(II). A.M(II) and M(III).A.M(III) species, according to a statistical distri bution. For both the homometallic mixed-valence compounds the quenchin g of the luminescence intensity of the nonoxidized unit by the oxidize d one is larger than 90%. Lifetime measurements have shown that the qu enching rate constant is 1.1 x 10(9) s-1 for Ru(II).A.Ru(III) and 5.0 x 10(9) s-1 for Os(II).A.Os(III). The quenching process takes place by an electron transfer mechanism. The parameters which govern the rates of the energy and electron transfer processes in this homogeneous fam ily of compounds are discussed in the light of current theories. It is shown that the electronic matrix element is approximately 0.6 cm-1 fo r the energy transfer process in Ru(II).A.Os(II), approximately 7-10 cm-1 for the electron transfer processes in Ru(II).A.Os(III), *Ru(II) .A.Ru(III), and approximately 1.0 cm-1 for the (back) electron transfe r process in Ru(III).A.Os(II).