title: Visible light-induced electron transfer from di-mu-oxo-bridged dinuclear Mn complexes to Cr centers in silica nanopores.
abstract: The compound (bpy) 2Mn (III)(mu-O) 2Mn (IV)(bpy) 2, a structural model relevant for the photosynthetic water oxidation complex, was coupled to single Cr (VI) charge-transfer chromophores in the channels of the nanoporous oxide AlMCM-41. Mn K-edge EXAFS spectroscopy confirmed that the di-mu-oxo dinuclear Mn core of the complex is unaffected when loaded into the nanoscale pores. Observation of the 16-line EPR signal characteristic of Mn (III)(mu-O) 2Mn (IV) demonstrates that the majority of the loaded complexes retained their nascent oxidation state in the presence or absence of Cr (VI) centers. The FT-Raman spectrum upon visible light excitation of the Cr (VI)-O (II) --> Cr (V)-O (I) ligand-to-metal charge transfer reveals electron transfer from Mn (III)(mu-O) 2Mn (IV) (Mn-O stretch at 700 cm (-1)) to Cr (VI), resulting in the formation of Cr (V) and Mn (IV)(mu-O) 2Mn (IV) (Mn-O stretch at 645 cm (-1)). All initial and final states are directly observed by FT-Raman or EPR spectroscopy, and the assignments are corroborated by X-ray absorption spectroscopy measurements. The endoergic charge separation products (Delta E o = -0.6 V) remain after several minutes, which points to spatial separation of Cr (V) and Mn (IV)(mu-O) 2Mn (IV) as a consequence of hole (O (I)) hopping as a major contributing mechanism. This is the first observation of visible light-induced oxidation of a potential water oxidation complex by a metal charge-transfer pump in a nanoporous environment. These findings will allow for the assembly and photochemical characterization of well-defined transition metal molecular units, with the ultimate goal of performing endothermic, multielectron transformations that are coupled to visible light electron pumps in nanostructured scaffolds.
authors.affiliation: Physical Biosciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720, USA.
authors.forename: Walter W
authors.initials: WW
authors.lastname: Weare
authors.forename: Yulia
authors.initials: Y
authors.lastname: Pushkar
authors.forename: Vittal K
authors.initials: VK
authors.lastname: Yachandra
authors.forename: Heinz
authors.initials: H
authors.lastname: Frei
mesh_terms: D015082:2,2'-Dipyridyl / Q000031:analogs & derivatives; D002857:Chromium / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D008027:Light; D008345:Manganese / Q000737:chemistry*; D008954:Models, Biological; D049329:Nanostructures / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry* / Q000378:metabolism; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry / Q000378:metabolism; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000378:metabolism; D016062:Porosity; D012822:Silicon Dioxide / Q000737:chemistry*; D013059:Spectrum Analysis, Raman; D014867:Water / Q000737:chemistry
chemical_list: D009942:Organometallic Compounds; D045332:Photosystem II Protein Complex; D014867:Water; D002857:Chromium; D008345:Manganese; D015082:2,2'-Dipyridyl; D012822:Silicon Dioxide; D010100:Oxygen