Abstract's Details

Toward Visible Light Driven Water Oxidation in Nanoporous Silicates Using Grafted Mn Complexes
Abstract ID	MAT-25
Presenter	Heinz Frei
Presentation Type	Poster
Full Author List	W.W. Weare (1), Y. Pushkar (1), V.K. Yachandra (1), H. Frei (1)
Affiliations	(1) Physical Biosciences Division, LBNL
Category	Materials Science
Abstract	This poster describes the structure and reactivity of potential water oxidation catalysts isolated within nanoporous silica structures (MCM-41), with the ultimate goal to oxidize water utilizing light-driven electron pumps. In nature, this demanding reaction is accomplished by four separate single-photon excitation events within the protein complex PSII, oxidizing water at an isolated Mn4 active site. We have successfully grafted a simple model of a portion of this site (bpy4Mn2O2) within MCM-41. To demonstrate that the Mn2O2 diamond core remains intact upon grafting we utilize a number of techniques, including EPR, FT-Raman, and XANES/EXAFS. These techniques are particularly amenable to observing Mn2III/IVO2 units, which are conclusively identified with the 16-line EPR signal characteristic of Mn2III/IVO2 and a Raman resonance at 702 cm-1 assigned to the Mn-O stretch of [bpy4Mn2III/IVO2]. A second nearest neighbor (Mn-Mn) configuration is observed in EXAFS, independently confirming intact Mn2O2 within the pores. If the MCM-41 framework contains a redox active metal (such as CrVI), we observe redox coupling of the Mn core with the Cr center; EPR, Raman, and Cr and Mn K-edge XANES show partial oxidation of Mn2III,IV, with concomitant reduction of CrVI. In the presence of a chromophore (CrVI) visible light induced photo-oxidation of Mn2III,IV to Mn¬2IV,IV is observed, the first step toward photo-driven water oxidation in such systems.
Footnotes
Funding Acknowledgement	This work was supported by the Director, Office of Science of the US Dept. of Energy under Contract No. DE-AC03/76SF00098