The whole photosynthetic process can be divided into three distinct steps:
- initial light-harvesting process and local charge separation in PS I and PS II,
- proton-coupled electron transfers between redox cofactors along the photosynthetic chain, allowing further spatial charge separation and
- multi-electronic redox catalysis generating hydrogen and oxygen at remarkable enzymatic sites such as the dinuclear NiFe and FeFe clusters in hydrogenases or the oxygen-evolving CaMn4 center (OEC) of photosystem II.
As far as the energetic aspect is concerned, the photosynthetic process is a fascinating example of efficiency. Firstly, a highly ordered array of pigments (the so-called antenna system) absorbing a large range of the visible spectrum converts light into chemical energy at photosystem II. Secondly, charge recombination is prevented by the presence of an electron transport chain driving electrons to the photosystem I. Finally, a second light-harvesting process occurs at PS I thus providing additional energy to the electrons for their final purpose (either CO2 fixation through the Calvin cycle or hydrogen production at the hydrogenase). The application of concepts derived from natural photosynthesis is therefore highly attractive for the development of novel hydrogen production technologies. Understanding this biological process and exploiting this knowledge for designing original synthetic molecular systems achieving light-to-chemical energy conversion is the basis of a large field of research called "artificial photosynthesis". In this context, our group is currently developping :
Figure 2: Schematic representation of the photosynthetic chain in the oxygenic photosynthesis.
The development of a novel energetic scenario based on the use of two renewable resources, water and sunlight, is a challenging solution to the energy crisis the world is currently facing. Conversion of solar energy into chemical energy through the light-driven water splitting indeed generates the environmentally benign oxygen gas and hydrogen, a carbon-free fuel with the highest energy output relative to molecular weight. Thanks to the fuel cell technology, this energetic vector can be converted on request into electricity, with very high energy conversion efficiencies and without exhausting greenhouse gas. This approach would moreover provide an attractive solution for storing the tremendous amount of sunlight energy falling every year on earth. In that context, natural photosynthesis is a great source of inspiration for the scientific community. Light-to-chemical energetic transduction is indeed achieved by photosynthetic organisms and green plants. In such a process, light is used to extract electrons from water, which is oxidized to O2. Most organisms use these photogenerated electrons to reduce atmospheric carbon dioxide and produce carbohydrates, proteins or lipids as the main constituents of their biomass but some micro-organisms such as cyanobacteria or micro-algae are able, under very specific conditions, to photosynthesize hydrogen as well. These transformations are realized thanks to a fascinating biological machinery, presented schematically in Figure 2, consisting of two large protein complexes, the photosystem I (PS I) and the photosystem II (PS II), assisted by various redox cofactors, and a unique enzyme, the hydrogenase.
Axis II : Artificial Photosynthesis