EFRC seminar 10/3/2013

3 Oct 2013

"Electronic Requirements of Water Activation for Artificial Photosynthesis: Time-resolved X-ray Spectroscopy Studies"

 

Yulia Pushkar

Department of Physics, Purdue University

 

 

Abstract

The light driven water splitting process accomplished by Photosystem II (PS II) provides a blueprint for future energy solutions based on artificial photosynthesis. The main challenge is the development of economically feasible catalysts for water splitting. Design of such optimal catalysts is pending the understanding of the mechanistic aspects of water activation by metal-oxo catalysts. PS II accomplishes the water splitting cycle (2H2O + 4hv = O2 + 4H+ + 4e) within a few milliseconds. The fast kinetic and complex protein environment of the PS II oxygen evolving complex (OEC) make identification of the molecular and electronic structure of the S-state (presumably S4) difficult. S3 to S0 transition has been currently accessible only to time resolved measurements with no reports on distinct intermediates. We probed S3 to S0 transition with TR-X-ray emission spectroscopy and preliminary interpretation of obtained results will be presented. Using first molecular catalysts reported for water splitting we succeeded in more detailed study of the intermediate reactive to water.

Combined O2 evolution, UV-vis stopped-flow kinetic measurements, EPR, Ru K-edge and L2,3-edges XANES, Ru K-edge EXAFS and resonance Raman (RR) were used to analyze the structure and electronic configurations of catalytic intermediates in the Ru based water oxidation catalyst – the blue dimer (BD). Both BD and PS II OEC are activated by proton-coupled electron transfer to reach a higher oxidation state. The highest oxidation state of the PS II OEC is not known with certainty, but for BD we have experimentally shown that the highest detectable oxidation state corresponds to BD [4,5]. Both PS II OEC and BD have terminal water molecules coordinated to the metal center as well as µm-oxo-bridges. For BD we showed that the oxygen of the Ru-O-Ru µm-oxo-bridge remains intact (inactive) in the water oxidation (EXAFS, RR and 16O/18O data). The main action happens at the terminal Ru-H2O water molecule which is converted to RuV = O. We determined RuV =O 1.7Å distance by EXAFS and confirmed this bond via 816 cm–1 vibrations frequency (16O/18O isotope shift 35 cm–1) and mapped spin density distribution. 17O hyperfine splitting of the RuV = O fragment were recorded in EPR: |Axx| = 60G, Ayy and Azz estimated at 25G (upper limit) from modeling. The RuV =O unit is the first to demonstrate large radicaloid character and an associated large 17O hfs which explains its increased reactivity, including reactivity to O-O bond formation.

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