Low Quantum Yields of Electron-Transfer Reaction of Photoexcited Ru(bpydc)34-with Co(tpy)23+ and Methyl Viologen2+ (bpydc:2,2‘-Bipyridine-4,4‘-dicarboxylate and tpy:2,2‘:6‘,2”-Terpyridine)

Quenching reactions of the phosphorescent state of Ru(bpydc) 3 4- by a cationic ion, Co(bpy) 3 3+, Co(phen) 3 3+, Co(tpy) 2 3+, and methyl viologen (MV 2+ ) to yield ET products were examined for the aqueous solutions by means of transient absorption kinetic spectroscopy and time-correlated single photon counting technique. A molecular complex formed in the ground state, [Ru(bpydc) 3 4- ... Co(tpy) 2 3+ ], produces ET-products in the bulk with a small fraction of 0.3 on the photoexcitation. The fractions of ET product formation in the collisional quenching by Co(tpy) 2 3+ (0.43) is lower than the re-dissociation fraction (0.71) of an encounter complex, Ru(bpydc) 3 3- −Co(tpy) 2 2+, for the reverse ET reaction. A process to reduce the formation of a cage-complex of [Ru(bpydc) 3 3- ...Co(tpy) 2 2+ ], in the collisional quenching is the conversion of unrelaxed cage-complex to the original reactant pair, [Ru(bpydc) 3 4- ...Co(tpy) 2 3+ ] via a potential intersect between the potential energy surfaces (PES) (avoid crossing). Such rapid a deactivation process is suggested by a lower fraction of ET-product formation (0.06) in the bimolecular quenching of 3 Ru(bpydc) 3 4- by MV 2+ than the re-dissociation fraction (0.18) of the encounter complex for the reverse ET reaction. Both Co(bpy) 3 3+ and Co(phen) 3 3+, which do not form a molecular complex with Ru(bpydc) 3 4- in the ground state, produce ET-products in the collisional quenching of 3 Ru(bpydc) 3 4- and 3 Ru(bpy) 3 2+ with a high fraction (0.93−1.0). The re-dissociation fractions of the encounter complex for the reverse ET reaction are close to unity (0.88−0.99).