Solar energy conversion: From natural to artificial photosynthesis

Solar energy has a great potential as a clean, cheap, renewable and sustainable energy source, but it must be captured and transformed into useful forms of energy as plants do. An especially attractive approach is to store solar energy in the form of chemical bonds as performed in natural photosynthesis. Therefore, there is a challenge in the last decades to construct semi-artificial and artificial photosynthetic systems, which are able to efficiently capture and convert solar energy and then store it in the form of chemical bonds of solar fuels such as hydrogen or hydrogen peroxide, while at the time producing oxygen from water. Here, we review the molecular level details of the natural photosynthesis, particularly the mechanism of light dependent reactions in oxygen evolving organisms, absorption efficiency of solar energy and direct energy production. We then demonstrate the concept and examples of the semi-artificial photosynthesis in vitro. Finally we demonstrate the artificial photosynthesis, which is composed of light harvesting and charge-separation units together with catalytic units of water oxidation and reduction as well as CO₂ reduction. The reported photosynthetic molecular and supramolecular systems have been designed and examined in order to mimic functions of the antenna-reaction center of the natural process. The relations between structures and photochemical behaviors of these artificial photosynthetic systems are discussed in relation to the rates and efficiencies of charge-separation and charge-recombination processes by utilizing the laser flash photolysis technique, as well as other complementary techniques. Finally the photocatalytic production of hydrogen peroxide as a more promising solar fuel is discussed in relation with the natural photosynthesis, which also produces hydrogen peroxide in addition to NADPH.