The fixation ofCO2 in organic matter is achieved on earth by plants and photosynthetic microorganisms through the fascinating mechanism of photosynthesis. Solar photons are absorbed by molecular photosensitizers present in photosystems I and II. This absorption of energy leads to a separation of negative and positive charges. The positive holes created serve to oxidize water to oxygen, while the negative excited electrons serve to reduce NAD to NADH, the biological reductant that enters the Calvin cycle to provide the reducing equivalents needed forCO2 fixation. This is howCO2 is converted into organic biomass.
Chemists have long been trying to reproduce such a process of converting solar energy, water andCO2 into carbonaceous molecules, and this latest lecture shows how this can be put into practice as simply and efficiently as possible, through the presentation of recent results in this field.
An artificial (synthetic) photosynthetic system must first contain an efficient photon collector, a photosensitizer, which absorbs a broad spectrum of visible sunlight. A great deal of work has gone into the development of semiconductor materials or molecular photosensitizers, whose general properties and modes of operation are discussed. These compounds must have optimized properties for charge conduction, stabilization of excited states, charge separation and coupling with catalysts. Indeed, the other important element of such a device is the catalyst that will efficiently use the excited electrons to bind them to theCO2 molecule. These catalysts are essentially the same as those described in Lecture 4. This lecture therefore focuses on semiconductor/photosensitizer assemblies using the most efficient catalysts available today, enabling a mixture ofCO2, water and an electron donor to be converted into energy-rich carbon molecules by simple illumination.
