Photosynthesis and Artificial Leaves

Of the many complex natural mechanisms for generating useful biochemical energy perhaps the most interesting is photosynthesis. The ability of plants to turn sunlight into energy has fascinated humans for as long as they have watched plants grow. More recently we have been able to unravel the biochemical pathways involved 

Similar reactions occur in red algae and cyanobacteria but because the full spectrum of sunlight does not always penetrate very far below the surface of the sea they make use of different pigments. These may be structurally similar to those used by plants, and are therefore classed as bacteriochlorophylls, or belong to the structurally distinct phycobilin class of compounds. Unlike chlorophyll, which has a cyclic tetrapyrrole structure containing magnesium, the phycobilins are acyclic tetrapyrroles with similarities to the breakdown product of the haem ring, bilirubin. 

Each of the four manganese ions cycles between Mn(II) and Mn(III) to generate the four electrons necessary. Had each pathway consisted of a single antenna and manganese centre the overall water cleaving reaction would take far longer and require a complex stepwise pathway consequently the entire photosynthetic 

One such example involves a ruthenium tris(bipyridine) complex, illustrated in Fig. 4.24, in which the metal can accept energy from photons and transfer it to the bipyridine ligands. The process of metal to ligand charge transfer is a well known phenomenon in coordination chemistry and the experimental conditions needed to form the complexes are fairly well understood. Added complexity is encountered when the bipyridine units have been modified as in the work of Hammarstrom [47], 

Having demonstrated that single electron transfer could be modelled successfully the next stage was to put several of the systems together so that enough manganese 

---