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 III ( t )] = [ 2 Co II (0)](1 − [ f α exp(− k α t ) + f β exp(− k β t )]) 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 α, f α, k β, and f β in a range of 180−300 K, are interpreted in terms of RET and ISC reactions among three states: [(tpy) 2 Ru III (tpy-ph-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 (tpy-ph-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 of 0.044 eV and 2.8 × 10 10 s -1 at of −0.01 eV. Meanwhile, the rate constant of the quartet−doublet ISC, k DQ (∼1.6 × 10 10 s -1 at 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 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 of −0.01 eV.
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Yoshimura et al. (2004) studied this question.