Mimicking Photosystem II with Manganese Model Complexes to Approach Artificial Photosynthesis

Abstract: The work in thesis aims for artificial photosyntesis, which could and will mimic natural photosynthesis, the process that uses light to create energy rich compounds from H2O and CO2. Water is oxidized in Photosystem II (PSII) by the Water Oxidizing Complex (WOC), which is a catalytic site on the lumenal side of the thylakoid membrane. The chlorophyll complex P680 absorbs light and electrons are transferred to the acceptor side of PSII. The electron-hole on P680 is filled by electrons from the Mn-cluster, which in turn oxidizes water. This can be viewed as a D (donor)-PS (photosensitizer)-A (acceptor) system. Artificial photosynthesis builds on the same principles, and in this work we have been focusing on redox reactions in dinuclear manganese complexes, using RuII(bpy)3 as photosensitizer.WOC in PSII is built up by four manganeses in a cluster, with ?-oxo bridges between the manganeses. On at least one of the manganeses, there is an open site for water binding.In this thesis I have investigated oxidation reactions of three different dinuclear manganese complexes, denoted complex 1, 2, and 3. 1 is a complex with N3O3 ligands coordination to each manganese, and is synthesised in Mn2II/II valance state. We have showed that it is possible to oxidize this complex three times to Mn2III/IV, which means that it forms four stable oxidation states. 2 has a N2O4 ligands coordinating each manganese, and is synthesised in the Mn2III/III valence state. This complex can be oxidized to what we think is Mn2IV/IV and Mn2IV/IVL?. 2 has five stable oxidation states. From electro chemistry we know that 2 is stable in Mn2II/II and Mn2II/III, and this means that 2 can be oxidized by photosensitizer three steps. 3 is an unymmetric manganese complex, with a mixture of the ligands in 1 and 2. On one side the manganese is coordinated to N3O3 ligands, and the other manganese is coordinated to N2O4 ligands. 3 is synthesised in the Mn2II/III valence state, and it is possible to oxidize this complex to, what we think is a Mn2IV/IV and a Mn2IV/IVL? oxidation state. By electro chemistry we can reduce 3 to Mn2II/II. This means that 3 can be oxidized four times with the photosensitizer and have five stable oxidation states. All three manganese complexes have acetate groups as bridging ligands to the manganeses. EXAFS measurements and from electro chemistry, indicate that the acetate groups can detach from the manganese, so that water can access the site and be converted into ligands to the manganese. From the X-ray absorption spectroscopy (EXAFS and K-edge), we could se that there was a shortening in the Mn-Mn distance of 0.5 Å for 1 in the Mn2III/III and at Mn2III/III for 2, which is an indication that a ?-oxo or a ?-hydroxo bond is formed. This means that the manganese cluster changes its conformation during oxidation.To oxidize our model complexes beyond Mn2III/III, we use CoIII(NH3)5Cl as sacrificial electron acceptor. When CoIII is reduced by Ru'(bpy)3 to CoII, an EPR signal appears in the g=5 region which belongs to CoII(H2O)6. After some time of illumination a new EPR signal appears in the same region, which is narrower and more symmetric in its shape, resulting from a photoreduction process in CoIII(NH3)5Cl. It is known that UV light can photoreduce CoIII(NH3)5Cl to CoII(NH3)4 and Cl?, but we have found that the same reaction occurs at 532 nm. We have interpreted this signal as an intermediate, when cobalt changes the ligands from NH3 and Cl- to H2O.