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.

A study of laser kinetic spectroscopy on [RuII(tpy)(BL)CoIII(tpy)]5+ (BL:  1,4-bis[2,2‘:6‘,2‘ ‘-terpyridine-4‘-yl]benzene (btbz) and 6‘,6‘ ‘-bis(2-pyridyl)- 2,2‘:4‘,4‘ ‘:2‘ ‘,2‘ ‘‘-quarter-pyridine (bpqp)) revealed rise-and-decay of a triplet Ru-to-ligand charge-transfer state, 3CT(Ru), and an electron-transfer product of [(tpy)RuIII(BL)CoII(tpy)]5+. 3CT(Ru) underwent electron-transfer with a rate constant of 0.3 × 1012 and 1 × 1012 s-1 to produce [(tpy)RuIII(BL)CoII(tpy)]5+ with a doublet electronic configuration d π 6d σ * of cobalt(II). The return electron transfer (RET) of [RuIII(tpy)(BL)CoII(tpy)]5+ (BL:btbz and bpqp) in acetonitrile occurred with a rate constant of 1 × 1010 and 6 × 1010 s-1 at 298 K, respectively, which are much faster than intramolecular RET of [(bpy)2Ru(BL)Co(bpy)2]5+ from cobalt(II) with a quartet electronic configuration of d π 5d σ * 2. The magnitude of reorganization energy (0.95 ± 0.15 eV) and electronic coupling matrix element (0.8 and 2 meV) are evaluated from the independence of RET rate on the temperature by using a small entropy change of RET. The small intramolecular reorganization energy (0−0.3 eV) for 2Co(II)/Co(III) redox process of [(tpy)RuII(BL)CoIII(tpy)]5+ (BL: btbz and bpqp) are responsible for the fast RET. The magnitude of the intramolecular reorganization energy is in agreement with those (0.27−0.29 eV) calculated by using a density functional theory.

Return electron transfer (RET) and intersystem crossing (ISC) of [(tpy)Ru III (tpy-ph-tpy) 2,4 Co II (tpy)] 5+ (tpy ) 2,2′:6′,2′′-terpyridine and tpy-ph-tpy ) 1,4-bis[2,2′:6′,2′′-terpyridine-4′-yl]benzene) and [(tpy)Ru II (tpy-tpy)-Co III (tpy)] 5+ (tpy-tpy ) 6′,6′′-bis(2-pyridyl)-2,2′:4′,4′′:2′′,2′′′-quarter-pyridyne) produced on the subpicosecond laser excitation have been investigated in a wide temperature range. A biexponential recovery process of [(tpy)Ru II (tpy-ph-tpy)Co III (tpy)] 5+ and [Co) occurs immediately after the production of [ 2 Ru III (tpy)(tpy-pph-tpy) 2 Co II (tpy)] 5+ in the excited-state electron-transfer quenching of [(tpy)Ru II (tpy-ph-tpy)Co III (tpy)] 5+ . Four parameters of biexponential recovery, k R , f R , k , and f in a range of 180-300 K, are interpreted in terms of RET and ISC reactions among three states: [(tpy) 2 Ru III (tpyph-tpy) 2 Co II (tpy)] 5+ , [(tpy) 2 Ru III (tpy-tpy) 4 Co II (tpy)] 5+ , and [(tpy)Ru II (tpy-ph-tpy)Co III (tpy)] 5+ . RET of [(tpy) 2 Ru III (tpyph-tpy) 2 Co II (tpy)] 5+ regenerating [(tpy)Ru II (tpy-ph-tpy)Co III (tpy)] 5+ with k RET(D) of ∼4 × 10 10 s -1 competes with doublet-quartet ISC with k QD of 0.20 × 10 10 s -1 at ∆G°Q D of 0.044 eV and 2.8 × 10 10 s -1 at ∆G°Q D of -0.01 eV. Meanwhile, the rate constant of the quartet-doublet ISC, k DQ (∼1.6 × 10 10 s -1 at ∆G°D Q of -0.01 eV), is much larger than RET of [(tpy) 2 Ru III (tpy-ph-tpy) 4 Co II (tpy)] 5+ . Dependences of k QD on ∆G°Q D give rise to a rough estimation of reorganizational free energy (0.03 eV). As for [(tpy)Ru II tpy-tpy)Co III (tpy)] 5+ , k RET(D) , k DQ , and k QD are evaluated to be 2.3 × 10 11 s -1 , 1.5 × 10 11 s -1 and 1.5 × 10 11 s -1 at ∆G°Q D of -0.01 eV.