Photoinduced Energy and Electron Transfer Processes in Supramolecular Species. Tris(bipyridine) Complexes of Ru(II)/Os(II), Ru(II)/Ru(III), Os(II)/Os(ITI), and Ru(II)/Os(III) Separated by a Rigid Spacer

The bis(bipyridine) bridging ligand 1,4-bis[2-(2,2′-bipyridin-5-yl)ethenyl]bicyclo[2.2.2]octane(bpy-S-bpy), where S is a rigid spacer made of a bicyclooctane 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+ (RuII·A), (bpy)2Os(bpy-S-bpy)2+ (OsII·A), (bpy)2-Ru(bpy-S-bpy)Ru(bpy)24+ (RuII·A·RuII), (bpy)2Os(bpy-S-bpy)Os(bpy)24+ (OsII·AII·OsII), (bpy)2Ru(bpy-S-bpy)-Os(bpy)24+ (RuII·A·OsII) have been prepared as PF6- salts. The length of the rigid spacer S is 9 Å, and the center-to-center separation distance in the dinuclear complexes is 17 Å. In all these novel compounds, each Ru-based and Os-based unit displays its own absorption spectrum and electrochemical properties, regardless of the presence of a second metal-based unit. The homometallic dinuclear compounds exhibit the same luminescence properties as the corresponding mononuclear species, whereas in the heterometallic dinuclear RuII·A·OsII species 91% of the Ru-based luminescence intensity is quenched by energy transfer to the Os-based unit, whose luminescence is accordingly sensitized (acetonitrile solution, room temperature). The excited state lifetime of the Ru-based unit (209 ns) is reduced to 18 ns, and a comparable risetime is observed for the energy transfer sensitization of the Os-based luminescence. The energy transfer process occurs with rate constant 5.0 × 107 s−1, predominantly by an exchange mechanism. Partial oxidation of the binuclear species RuII·A·RuII, OsII·A·OsII, and RuII·A·OsII by Ce(I V) in acetonitrile-water solutions leads to mixed-valence MII·A·MIII species (M = Ru and/or Os) where the oxidized metal-based unit quenches the luminescent excited state of the unit that is not oxidized. For the RuII·A·OsIII 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 (kel = 8.7 × 109 s−1) with formation of the thermodynamically unstable RuIII·A·OsII valence isomer which then goes back (kb = 1.0 × 106 s−1) to RuII·A·OsIII. The RuII·A·RuIII and OsII·A·OsIII mixed-valence compounds can only be obtained in the presence of the corresponding MII·A·MII and MIII·A·MIII species, according to a statistical distribution. For both the homometallic mixed-valence compounds the quenching of the luminescence intensity of the nonoxidized unit by the oxidized one is larger than 90%. Lifetime measurements have shown that the quenching rate constant is 1.1 × 109 s−1 for RuII·A·RuIII and 5.0 × 109 s−1 for OsII·A·OsIII. 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 family of compounds are discussed in the light of current theories. It is shown that the electronic matrix element is ∼0.6 cm−1 for the energy transfer process in *RuII·A·OsII, ∼7–10 cm−1 for the electron transfer processes in *RuII·A·OsIII, *RuII·A·RuII, and *OsII·A·OsIII, and ∼ 1.0 cm−1 for the (back) electron transfer process in RuII·A·OsII.