Photoinduced electron-transfer reactions of [(tpy)Ru II (tpy-tpy)Co III (tpy)]5+ (tpy ) 2,2′:6′,2′′-terpyridine and tpy-tpy ) 6′,6′′-bis(2-pyridyl)-2,2′:4′,4′′:2′′,2′′′-quarterpyridyne), [(tpy)Ru II (tpy-ph-tpy)Co III (tpy)] 5+ (tpy-phtpy ) 1,4-bis[2,2′:6′,2′′-terpyridine-4′-yl]benzene), and [(bpy) 2 Ru II (tpphz)Co III (bpy) 2 ] 5+ (bpy ) 2,2′-bipyridine, and tpphz ) tetrapyrido[3,2-a:2′,3′-c:3′′,2′′-h::2′′′,3′′′-j]phenazine) were studied in the range 140-298 K by means of subpicosecond transient absorption spectroscopy. 3 MLCT(Ru) of [(tpy)Ru II (L-L)Co III (tpy)] 5+ (L-L: tpy-tpy and tpy-ph-tpy) underwent intramolecular electron-transfer (EET) reaction to form [ 2 Ru III (tpy)(L-L) 2 Co II (tpy)] 5+ in a shorter time than 10 ps. Low quantum yields of the EET products (0.53 for tpy-tpy and 0.41 for tpy-ph-tpy in butyronitrile at 298 K) measured before the fast return electron-transfer were found as in the intermolecular electron-transfer quenching of 3 MLCT(Ru) by [Co(tpy) 2 ] 3+ . The quantum yield of [(tpy)-Ru III (tpy-tpy)Co II (tpy)] 5+ (Φ) decreases to 0.38 in a slowly reorganizing solvent of propylene carbonate and that of [(tpy)Ru II (tpy-ph-tpy)Co III (tpy)] 5+ to 0.21 at 180 K. The reductions in the EET yields, 1 -Φ, can be ascribed to a fast transition of the nonrelaxed EET product to the lowest triplet d-d* state of [Co III (tpy) 2 ] moiety during the solvent reorganization. A tunneling transition of the nonrelaxed EET products to the lowest lying d-d excited-state of [Co III (tpy) 2 ] moiety takes place as a hole transfer, HT, from the dπ-orbital of Ru(III) to that of Co(II) with a configuration of dπ 6 dσ*. An electronic coupling of dπ(Ru)-dπ(Co) estimated from the intensity of inter-valence transition of [(tpy)Ru III (L-L)Ru II (tpy)] 5+ (Collin, J.-P.; Laine, P.; Launay, J.-P.; Sauvage, J.-P.; Sour, A. J. Chem. Soc., Chem. Commun. 1993, 434) is large enough to carry out the HT. The weak coupling of dπ(Ru)-dπ(Co) in the case of [(bpy) 2 Ru(tpphz)Co(bpy) 2 ] 5+ attenuates the HT rate to bring about a high yield of EET (0.8). The possibility that energy-transfer of 3 MLCT(Ru) to the Co(III) moiety via an intermediate super-exchange coupling between dπ(Ru) and dπ(Co) occurs in competition with the EET of [(tpy)Ru III (tpy-ph-tpy)Co II (tpy)] 5+ is also pursued.

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A series of mono- and dinuclear Ru(bpy)2 complexes (bpy = 2,2‘-bipyridine) containing 2,2‘-bis(benzimidazol-2-yl)-4,4‘-bipyridine (bbbpyH2) were prepared. The mononuclear complex [Ru(bpy)2(bbbpyH2)](ClO4)2·CH3OH·4H2O was characterized by an X-ray structure determination. Crystal data are as follows: triclinic, space group P1̄, a = 14.443(4) Å, b = 15.392(4) Å, c = 11.675(2)Å, α = 101.44(2)°, β = 107.85(2)°, γ = 96.36(2)°, V = 2380(1) Å3, Z = 2. The coordination geometry of the ruthenium(II) ion is approximately octahedral. The dihedral angle between the two pyridyl rings in bbbpyH2 is 9.4(3)°, which is close to coplanar, in the complex. Mono- and dinuclear complexes exhibit broad charge-transfer absorption bands at 420−520 nm and emission at 660−720 nm in CH3CN solution with lifetimes of 200−800 ns at room temperature. Transient difference absorption spectra and resonance Raman (rR) spectra were used to assign the charge-transfer bands in the 420−520 nm region and to identify the lowest excited states. Both absorption and emission spectra are sensitive to solvent and solution pH. Deprotonation of the dinuclear complex raises the energies of the π* orbitals of the bbbpyH2 ligand, so that they become closer in energy to the π* orbitals of bpy. The intervalence band of [(bpy)2Ru(bbbpyH2)Ru(bpy)2]5+ is observed at 1200 nm ( ε = 170 M-1 cm-1) in CH3CN. The value of the electronic coupling matrix element, H AB, was determined as 120 cm-1. Upon deprotonation, the IT band was not observed. It is therefore concluded that a superexchange pathway occurs predominantly via the Ru(II) dπ−bbbpyH2 π* interaction, since deprotonation decreases the interaction. The role of the intervening fragments in the bridging ligand is discussed from the viewpoint of orbital energies and their orbital mixing with Ru dπ orbitals.