Light induced manganese oxidation and long-lived charge separation in a Mn₂^II,II-Ru^II(bpy)₃-acceptor triad

The photoinduced electron-transfer reactions in a Mn₂ ^II,II-Ru^II-NDI triad (1) ([Mn₂(bpmp)(OAc) ₂]⁺, bpmp = 2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4- methylphenolate and OAc = acetate, Ru^II = tris-bipyridine ruthenium(II), and NDI = naphthalenediimide) have been studied by time-resolved optical and EPR spectroscopy. Complex 1 is the first synthetically linked electron donor-sensitizer-acceptor triad in which a manganese complex plays the role of the donor. EPR spectroscopy was used to directly demonstrate the light induced formation of both products: the oxidized manganese dimer complex (Mn₂^II,III) and the reduced naphthalenediimide (NDI ^.-) acceptor moieties, while optical spectroscopy was used to follow the kinetic 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 of magnitude longer than that for previously reported triads based on a [Ru(bpy)₃]²⁺ photosensitizer. At 140 K, this intramolecular recombination was dramatically slowed, displaying a lifetime of 0.1-1 s, which is comparable 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 reorganization energy (λ ≈ 2.0 eV), due to a large inner reorganization of the manganese complex. This makes the recombination reaction strongly activated despite the large driving force (-ΔG° = 1.07 eV). Thus, the intrinsic properties of the manganese complex are favorable for creating a long-lived charge separation in the "Marcus normal region" also when the charge separated state energy is high.