title: Light induced manganese oxidation and long-lived charge separation in a Mn2II,II-RuII(bpy)3-acceptor triad.
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 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 (Mn2II,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 micros 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 (lambda approximately 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 (Delta-G degrees = 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.
authors.affiliation: Department of Physical Chemistry, Uppsala University, P.O. Box 579, SE-751 23 Uppsala, Sweden.
authors.forename: Magnus
authors.initials: M
authors.lastname: Borgström
authors.forename: Nizamuddin
authors.initials: N
authors.lastname: Shaikh
authors.forename: Olof
authors.initials: O
authors.lastname: Johansson
authors.forename: Magnus F
authors.initials: MF
authors.lastname: Anderlund
authors.forename: Stenbjörn
authors.initials: S
authors.lastname: Styring
authors.forename: Björn
authors.initials: B
authors.lastname: Akermark
authors.forename: Ann
authors.initials: A
authors.lastname: Magnuson
authors.forename: Leif
authors.initials: L
authors.lastname: Hammarström