EPR Studies of Ruthenium-Manganese Complexes as Biomimetic Models for Photosystem II - Approaching Artificial Photosynthesis

Sammanfattning: In natural photosynthesis, solar energy is converted to chemical energy by photosynthetic reaction centers. In green plants and algae, Photosystem II (PSII) and Photosystem I (PSI) absorb light and utilize the charge separation reactions to convert solar energy to chemical energy. The energy is later stored in form of energy- rich substances formed in reductive biosynthetic reactions. The electrons needed are extracted from water, which is oxidized to molecular oxygen by the Water Oxidizing Complex (WOC) in PSII. In artificial photosynthesis we attempt to mimic key reactions in natural photosynthesis to produce an energy-rich fuel like hydrogen using water as substrate. We have designed a molecular module D (electron donor)-S (photosensitizer)-A (electron acceptor)-system for this purpose. A series of model complexes containing Mn-Ru constitute the D-S part. In essential aspects this mimics the donor side of PSII. The Mn complexes have been studied with Electron Paramagnetic Resonance (EPR) spectroscopy. In the model complexes, Mn is linked to the Ru-center via a tyrosyl moiety. This reduces the quenching of the excited Ru'(II) state caused by the Mn-dimers. It also mediates electron transfer between Mn and photo-generated Ru(III). These functional aspects mimic the function of tyrosine-Z that is a redox intermediate in water oxidation in PSII. In the model complexes phenolic radicals generated by photo-induced Ru(III) were observed. The electron transfer rate from the phenolic moiety to the Ru(III) center was found to correlate to the strength of H-bond(s) in the system. The Mn-complexes can be oxidized several steps by photo-generated Ru(III). In the photo-oxidized Mn- complexes, high oxidation states such as Mn2(III,IV) and Mn2(IV,IV) have been reached. Upon extensive photo-oxidation, an EPR signal from a ligand-originated phenolate radical was detected in one model complex. The radical was proposed to be magnetically coupled to the close-lying Mn2(IV,IV) center. We have demonstrated that an increased O/N ratio in the Mn-coordination sphere stabilizes higher Mn-oxidation states. We also showed that water plays a crucial role in reaching high Mn-valence states. This probably involves bridging-mode modifications due to water entering into the Mn coordination sphere, thereby fine-tuning the redox potential of high-valent Mn-systems. Aspects concerning the mechanism for water oxidation that involve the O/N ratio and Mn bridging-mode modifications in the WOC are discussed. Potential improvements of the model-complexes are also discussed.