Artificial photosynthetic systems

From a fundamental viewpoint, carbon dioxide reduction is a model reaction which can help us to understand better the mechanism of natural photosynthesis.11 Development of artificial photosynthetic systems, by mimicking functions of green plants, is one of... 

Tazuke and Kitamura162 reported the first example of an artificial photosynthetic system based on electron transport sensitization, although the product was not a hydrocarbon, but rather formic acid. Their system is shown schematically in Fig. 17. In this system, the photochemically generated singlet excited state of an aromatic hydrocarbon, such as pyren (Py) or perylene (Pe), was... 

Figure 17. Schematic representation of an artificial photosynthetic system.162...

Fig.4.30 Immobilization ofthe bacterial photosynthetic reaction center on tailored three-dimensional wormlike mesoporous W03-Ti02 films for artificial photosynthetic systems (A) procedure of film coating (B) proposed scheme of photoelectric conversion. Reprinted with permission from [229], Y. Lu et at., Langmuir 2005, 21, 4071. 2005, American Chemical Society.

Another important area is the use of photochemistry—chemistry that results from light absorption—to perform transformations that are not otherwise possible. The practical applications of photosynthesis were based on fundamental work to learn the new pathways that light absorption makes possible, but the work on these synthetic methods has also added to our basic understanding of the reaction mechanisms. The important natural process of photosynthesis also inspires some work in photochemistry, where the challenge is one of producing artificial photosynthetic systems that could use sunlight to drive the formation of energetic materials. 

Explosive research activity is going on in micellar photochemistry. This is related to the development of artificial photosynthetic systems, and the anisotropic nature of globular micelles and bilayer membranes is used for conservation of excitation energy. The subject has been recently reviewed (Kalyanasundaram, 1978). 

Explain how efforts have been made to develop artificial photosynthetic systems. Understand the difficulties and potential benefits of such systems. 

Cuendet P, Gratzel M (1982) Artificial photosynthetic systems. Cellular and Molecular Life Sciences 38 223-228... 

Recently, the interest in Re (I) complexes has been increased due to their potential utility for the activation and reduction of C02 into CO and C032 in a purpose of construction of artificial photosynthetic systems [11-13]. Rhenium Complexes such as ReX(CO)3(bpy) (X=C1, Br) and Re(CO)2(bpy)[P(OEt)3]2 have been used as photocatalysts for C02 reduction to CO in solvent mixture of triethanolamine/dimethylformamide [12,13]. Most of the research on photochemical activation and reduction of C02 using Re(I) complexes have focused on the homogeneous solution systems. There are few reports concerned about the encapsulation of rhenium complexes into molecular sieves and their photochemical application to the photochemical reduction of C02. 

Sensitization of Ti02 powders and films for water photolysis is still an attractive and as yet unsolved problem in the construction of an artificial photosynthetic system for creating energy sources from solar energy and water. 

Based on photosynthesis and the energy cycie on the earth, an artificial photosynthetic system was proposed (Fig. 19.2) tc create fuels from solar energy and water by utilizing photocatalytic reactions.95 This scheme takes minimum requirements to achieve photochemical energy conversion. 

The first commercial application of photocatalysts has started to clean our environment by Ti02 powders and films. In order to utilize photocatalysts for solar energy conversion, sensitization of large bandgap semiconductors is important. The most difficult task for an artificial photosynthetic system is to establish visible light-induced charge separation with minimum back charge recombination. 

The design of such artificial photosynthetic systems suffers from some basic limitations a) The recombination of the photoproducts A and S+ or D+ is a thermodynamically favoured process. These degra-dative pathways prevent effective utilization of the photoproducts in chemical routes, b) The processes outlined in eq. 2-4 are multi electron transfer reactions, while the photochemical reactions are single electron transformations. Thus, the design of catalysts acting as charge relays is crucial for the accomplishment of subsequent chemical fixation processes. 

Significant progress in the development of such artificial photosynthetic systems, particularly aimed at the photolysis of water, has been reported in recent years. Several approaches to resolve the problems involved in controlling the photoinduced electron transfer process as well as the development of catalysts for multi-electron fixation processes will be discussed in this paper. 

Different aspects involved in the design of artificial photosynthetic systems have been discussed. Charged colloids and water-oil microemulsions provide effective organized media for controlling photosensitized electron transfer processes. Development of catalysts capable of utilizing the photoproducts in chemical routes, particularly in multi-electron fixation processes is of major... 

Recent progress in understanding the theoretical basis of electron transfer has been rapid. Theoretical aspects of electron transfer are addressed in detail in other contributions to this series, and authoriative, up-to-date reviews are available [9-14], For our purposes, it will be sufficient to review some very basic electron transfer theory which will serve as a framework for the discussion of artificial photosynthetic systems which follows. 

Hydrogen Evolution and Hydrogenation Processes in Artificial Photosynthetic Systems.179... 

Although the thermodynamic feasibility to design artificial photosynthetic processes is obvious, practical assembly of such systems confronts substantial difficulties. Thermodynamic limitations accompanying artificial photosynthetic systems have been considered theoretically [40-42], and the recent progress in the subject has been reviewed [43, 44] in several articles and monographs [45, 46]. 

Intrinsic limitations of an artificial photosynthetic system include the thermodynamically favoured back electron transfer reactions of the intermediate photoproducts [47, 48]. For an oxidative ET quenching process the destructive back electron reactions are given by Eq. (9) and (10). The fraction of usable photo-... 

Fig. 11. Photolysis of water and C02-fixation in an artificial photosynthetic system...

---