Light Induced Manganese Oxidation and Long-Lived Charge Separation in a Mn2II,II−RuII(bpy)3−Acceptor Triad

• Published in: Journal of the American Chemical Society Vol. 127; no. 49; pp. 17504 - 17515
• Main Authors: BORGSTRöM, Magnus, SHAIKH, Nizamuddin, JOHANSSON, Olof, ANDERLUND, Magnus F., STYRING, Stenbjörn, ÅKERMARK, Björn, MAGNUSON, Ann, HAMMARSTRöM, Leif
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 14.12.2005
• Subjects: Chemistry; Exact sciences and technology; General and physical chemistry; Medicin och hälsovetenskap; Photochemistry; Physical chemistry of induced reactions (with radiations, particles and ultrasonics); Cationic complex; Reaction intermediate; Temperature dependence; Manganese Complexes; EPR spectrometry; Charge separation; Organic ligand; Electron transfer; Transition metal Complexes; Organic radical anion; Photochemical reaction; Experimental study; Transfer reaction; Chemical recombination; Complexes Mixed; Visible radiation; Dinuclear complex; Ruthenium Complexes; Platinoid Complexes; Triad; Acetic acid; Tetradentate ligand; Intramolecular interaction; Ultraviolet visible spectrometry
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/ja055243b

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.