Photoinduced Electron Transfer in Covalently Linked Ruthenium Tris(bipyridyl)-Viologen Molecules: Observation of Back Electron Transfer in the Marcus Inverted Region

The rates of photoinduced electron transfer (ET) in a series of donor-acceptor molecules (4,4′-R₂-2,2′-bipyridine)₂Ru(1-(4-CH₃-2,2′,-bipyridine-4′,-yl-(CH₂)_n)-4,4′-bipyridinediium-1′-R′)⁴⁺ (R = H, CH₃; R′ = CH₃, CH₂CN; = 1, 2) were studied by picosecond flash photolysis/transient absorbance techniques. The rate of intramolecular forward ET (MLCT quenching) in acetonitrile varied with -ΔG° according to classical Marcus theory for n = 2, but not for n = 1. The back ET reactions for both n = 1 and n = 2 showed an inverted rate behavior described quantitatively by semiclassical Marcus theory. For the n = 2 compounds, the value of |V(r)|² (V(r) ≡ electron-exchange matrix element) was approximately 1 order of magnitude higher for the forward than for the back ET reactions. This difference was rationalized in terms of a larger distance over which the back reaction (viologen-to-metal) occurs relative to the forward reaction (bipyridine-to-viologen). Cage-escape efficiencies for a series of Ru(bpy)₃²⁺ electron donors and viologen acceptors did not show any apparent relationship to driving force for the back ET reaction. In this case, geometric factors are more important than energetic factors in controlling the rates of back reaction and cage escape.