| Abstract
| - The photoinduced electron-transfer reactions in a Mn2II,II−RuII−NDI triad (1) ([Mn2(bpmp)(OAc)2]+,bpmp = 2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-methylphenolate and OAc = acetate, RuII = tris-bipyridine ruthenium(II), and NDI = naphthalenediimide) have been studied by time-resolved optical andEPR spectroscopy. Complex 1 is the first synthetically linked electron donor−sensitizer−acceptor triad inwhich a manganese complex plays the role of the donor. EPR spectroscopy was used to directly demonstratethe light induced formation of both products: the oxidized manganese dimer complex (Mn2II,III) and thereduced naphthalenediimide (NDI•-) acceptor moieties, while optical spectroscopy was used to follow thekinetic evolution of the [Ru(bpy)3]2+ intermediate states and the NDI•- radical in a wide temperature range.The average lifetime of the NDI•- radical is ca. 600 μs at room temperature, which is at least 2 orders ofmagnitude longer than that for previously reported triads based on a [Ru(bpy)3]2+ photosensitizer. At 140K, this intramolecular recombination was dramatically slowed, displaying a lifetime of 0.1−1 s, which iscomparable to many of the naturally occurring charge-separated states in photosynthetic reaction centra.It was found that the long recombination lifetime could be explained by an unusually large reorganizationenergy (λ ≈ 2.0 eV), due to a large inner reorganization of the manganese complex. This makes therecombination reaction strongly activated despite the large driving force (−ΔG° = 1.07 eV). Thus, theintrinsic properties of the manganese complex are favorable for creating a long-lived charge separation inthe “Marcus normal region” also when the charge separated state energy is high.
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