Artificial PhotosynthesisArtificial photosynthesis for hydrogen production from sun and water by direct photochemistry in multifunctional catalytic synthetic complexes is one of the targets for SOLAR-H. This is a quite recent field and artificial photosynthesis of this type has never been demonstrated in man-made molecular systems. The goal is consequently quite distant. The idea is to design and synthesize entirely synthetic compounds able to harvest solar energy and convert this to a fuel like H2 using water as raw material. We adopt a so-called bio-mimetic strategy based on principles from natural enzymes (Figures 1,2). Our knowledge about natural photosynthesis and H2 forming reactions has increased dramatically in the last 15 years and important break-throughs have been the determination of the three-dimensional structures of photosynthetic reaction centers (Nobel prize in 1988) and many hydrogenases (several by partner 2B). This sophisticated knowledge is key for the bio-mimetic approach applied by the synthetic chemists in SOLAR-H and strong, truly bio-inspired chemistry is more and more feasible. One attempt along this line is the Swedish Consortium for Artificial Photosynthesis that already links both fields.
Artificial photosynthesis - photochemical water splitting. Objectives of the project.The visionary idea in SOLAR-H is to design, synthesize and characterize large metal-organic catalytic complexes that shall be able to:
i) absorb light;
ii) use the light to form an energy rich so called charge separated state;
iii) use this stored energy to split water (oxidize water) into hydrogen and oxygen.
If efficient photo-catalytic chemistry with these properties could be accomplished and incorporated into a technical device this might become an important future energy supply.
Figure 1. A. Top. Natural photosynthesis uses solar energy to transfer electrons from water to energy-rich compounds (normally carbohydrates but sometimes hydrogen) and this is what will be explored in the project. A Bottom & B. Schematic comparison of artificial photosynthesis and natural photosynthesis. The ruthenium-center absorbs light energy. This results in electron transfer to the linked acceptor A that is catalytic and reduces protons to hydrogen. The photo-oxidized Ru-center is strongly oxidizing and extracts electrons from the donor system, D, which also is catalytic and able to oxidize water. The donor system shall, in analogy with the natural system be composed of 2-4 high valent Mn-ions coupled to the Ru-center via a redox active link. B shows the vision behind our bio-mimetic approach for the synthetic chemistry. We attempt to develop chemistry based on principles for key reactions in nature. The figure shows a Ru-Mn complex designed and synthesized in the Swedish consortium (Partner 1) drawn next to the Photosystem II center in natural photosynthesis. A bio-mimetic approach to achieve hydrogen formation is shown in Figure 2.
Synthetic chemistry of this kind has never been achieved before. Nature however, carries out these reactions very efficiently in different enzymes. To manage our chemistry, the researchers in SOLAR-H have adopted a bio-mimetic approach where we find inspiration and knowledge in the natural reactions.
In natural photosynthesis solar energy is used to oxidize water into oxygen, protons and electrons. The electrons are then used in a secondary processes to sustain life and to form all the valuable products formed by photosynthetic organisms. Thereby, plants (note that also cyanobacteria and algae carry out water oxidizing photosynthesis) provide the biosphere with reducing power from the endless resources water and solar energy. Thus, plants carry out light-driven water splitting that is one of the key steps for us in SOLAR-H to mimic. The reaction is catalyzed by Photosystem II in a complex with four Mn ions that is known as the Water Oxidizing Complex (WOC) or the Oxygen Evolving Complex (OEC).
However, efficient enzymes also catalyze reactions where protons are reduced to molecular hydrogen using electrons from different substrates (including photosynthesis in green algae and cyanobacteria). These enzymes are known as hydrogenases. There exist different hydrogenases from many organisms but in common they have a chain of components transporting the electrons to the active site and an active site composed of two metal ions. These form either a di-iron center or a nickel-iron center.
The key problem, and the most difficult part, is to achieve the oxidation of water (water splitting) with solar energy and we focus strong resources in the artificial photosynthesis part of SOLAR-H on this problem. Our idea is to develop bio-mimetic chemistry based on the principles in the natural Water Oxidizing Complex where a cluster of 4 high-valent manganese ions, together with a tyrosine amino-acid side chain, comprises the active site (Figure 1B). In the synthetic part of the project (partners 1, 3,& 4) we will couple 2-4 high valent Mn ions to a photoactive center. We have chosen to focus on Ru-centers (Figure 1), which we link to Mn-ensembles via redox active links. Early steps along this path have been developed by several of the participants (Partners 1,3, &4). We now join our forces to accelerate the research and achieve higher nuclearity of the Mn ensemble than earlier. We will also develop ligands and redox active links able to withstand the oxidizing power from the photo-oxidized Ru-center.
The second reaction to mimic is the reduction of protons to molecular hydrogen. Also here we follow a bio-mimetic path and we will mimic natural hydrogenases to accomplish this chemistry. Hydrogenases use reducing power from a variety of substrates. The electrons are transported to the active site in an electron transfer chain. The active site is composed of either a nickel-iron or a di-iron metal site where the metals have unusual ligands (Figure 2). The chemists in SOLAR-H will not mimic the electron transfer chain. Instead they (Partners 1, 3 &4) intend to use electrons provided from a photo-oxidized ruthenium center to directly feed electrons into the catalytic part of the molecule Figure 2). The latter will be built with the natural nickel-iron and/or di-iron centers as inspiration.
Figure 2. Presentation of how a di-iron center in a hydrogenase (left) is mimicked in a di-iron complex that also is linked to a Ru-center. In the di-iron complex the ligands are not too different from those in the hydrogenase di-iron center. The coupled ruthenium-di-iron complex on the right is synthesized by Partner 8. The idea is that photo-excitation of the Ru-center shall drive electron transfer to the coupled iron-dimer.
We follow a modular approach in our work and in the separate work packages we build different parts of the system. These are then to be joined in secondary synthetic steps.
To accomplish this advanced chemistry the synthetic chemists need continuous input from the advances made by biochemists and biophysicists who study the natural photosynthetic reaction centers and hydrogenases. The project contains groups devoted to spectroscopic studies of both Photosystem II (1, 2 & 5) and hydrogenases (partner 2 & 4). These provide input to the synthetic chemists with the latest knowledge and crucial ideas for how to improve the photoactive complexes. The synthetic groups and the biophysicists are linked in close collaborations and in some cases these are based on already existing personal and scientific links.
Artificial photochemical water splitting - state of the art.There exist very few man-made systems able to oxidize water at all and even fewer that have been unequivocally shown to do this with light energy. The bio-mimetic approach in SOLAR-H using Ru-Mn chemistry originates from SOLAR-H partners and was initiated by the Swedish Consortium for Artificial Photosynthesis (Partner 1) and Partner 4 a few years ago. The researchers in the Swedish Consortium have managed to take four electrons from Mn by photo-oxidation of the Ru-center. They have also coupled Ru and the Mn-complex with a bio-micking redox active ligand and the most advanced molecules use light to oxidize the manganese three reversible steps, while a forth step seems to be irreversible (four steps are needed to accomplish water oxidation). The redox potentials of the manganese ions are quite close to potentials in the natural system. Thus, the ruthenium manganese field is lead by researchers assembled in SOLAR-H and by combining our efforts in a collaborative manner it is highly likely that in the next few years we will achieve our goal: to split water.
The bio-mimetic chemistry involved in the reduction of protons to hydrogen is a developed field and is driven by the interest for this kind of catalysis in fuel cells. In the US and in England (groups outside SOLAR-H) several strong groups are active in this area. Interesting molecules (of similar character as the one in Figure 2) able to form hydrogen under strongly reducing conditions have been made. However, to our knowledge no one outside the SOLAR-H network has ever tried to link the di-iron centers to photoactive molecules and Figure 2 shows and early molecule from the Swedish consortium created in this vain. Partner 3 in SOLAR-H attempts to link hydrogenase mimicking nickel-iron and di-iron metal-centers to ruthenium to attempt photochemical hydrogen formation.