Photosynthesis artificial

Photosynthesis, the process by which plants and other organisms use solar energy to convert water and carbon dioxide into complex energy-rich chemicals, the carbohydrates and oxygen, is essential to life on this planet. We now have a greater understanding of how this process works and hence we should be able to construct processes that mimic nature to our advantage. 

Research into artificial photosynthesis is being driven by the desire to understand, and then mimic, the following features of natural photosynthesis.  

Antennae for light harvesting Reaction centre for charge separation Membrane for physical separation 

The multi-step electron transfer process in namral photosynthesis has been utilized in the construction of various triads using porphyrin, metalloporphyrin, fullerene, and imide as basic components for harvesting solar energy as electrical energy and for photoreduction of water to get clean fuel hydrogen [6, 7]. Recently, tetrads, pentads and hexads have been constructed using porphyrin, fullerene, and a chromophoric unit as basic components for fast energy transfer process. 

As it has been remarked, the OEC operates at ambient conditions of temperature and pressure and at neutral and near-neutral pHs and uses Earth-abundant elements. This contrasts with the artificial photocatalytic water-splitting systems that often involve the use of rare elements [14-16]. 

At any rate, it would seem not reasonable to embark in an attempt to reproduce such a complex structure. A still formidable goal would be the production of a convenient energy vector, such as hydrogen (a clean fuel, oxidation product is only water) by photolysis of water, abundant on the planet, and under conditions inspired to nature, although much simplified. 

We have seen that, in photosynthetic bacteria, visible light is harvested by the antenna complexes, from which the collected energy is funnelled into the special pair in the reaction centre. A series of electron-transfer steps occurs, producing a charge-separated state across the photosynthetic membrane with a quantum efficiency approaching 100%. The nano-sized structure of this solar energy-conversion system has led researchers over the past two decades to try to imitate the effects that occur in nature. 

Synthetic chemistry enables us to mimic the energy- and electron-transfer processes by linking together donor and acceptor groups by means of covalent bonds or bridging groups, rather than using the protein matrix found in natural systems. 

Excitation of the chromophore is followed by photoinduced electron transfer to the electron acceptor, then electron transfer from the electron donor to the oxidised chromophore  

In artificial photosynthetic models, porphyrin building blocks are used as sensitisers and as electron donors while fullerenes are used as electron acceptors. Triads, tetrads, pentads and hexads containing porphyrins and Qo have been reported in the literature (see the Further Reading section). 

Furthermore, a pentad consisting of a caretenoid (C), zinc porphyrin (ZnP), porphyrin (H2P) and two quinines (QA and QB) has been produced, which aims to utilise components allied to those found in natural photosynthetic systems. The zinc porphyrin absorbs at 650 nm, a process that is followed initially by fast energy transfer  

The main reactions that occur within the reaction centres of illuminated photosynthetic bacteria are energy- and electron-transfer processes between remote but closely-spaced components. A series of light-harvesting complexes collect incident photons and, by a succession of rapid steps, transfer the photon 

Other reviews or updates have appeared during the past year. A comprehensive discussion of current electron-transfer theory, with emphasis on the more puzzling features, has been reported. Related aspects of bimolecular electron-transfer reactions have been discussed but in much less detail. Other authors have reviewed the special case in which light-induced electron transfer is followed by bond fragmentation or bond formation. In recent years there has been a virtual revolution in synthetic organic chemistry that has resulted in the construction of a wide range of exotic molecular architectures. Such systems have not 

1 Photoinduced Electron Transfer in Molecular Dyads and Higher-order Analogues - Light-induced electron transfer from a donor to an acceptor, followed by successive transfer of the redox equivalents across the membrane, has 

Light induced water splitting using semiconductor-based photoelec-trodes, nanoparticulate metal oxides, (oxy)nitrides and (oxy)sul-fides, and molecular catalysts that mimic aspects of the active sites in natural enzymes have been extensively investigated and a wealth of new materials and device architectures has been presented in recent years. In the following sections we will look in detail at examples of integrated systems 

1 Photoinduced Electron I ransfer in Molecular Dyads and Higher-order 

---

Artificial photosynthesis water cleavage into hydrogen and oxygen by visible light. M. Gratzel, Acc. Chem. Res., 1981,14, 376-384 (54). 

Benniston AC, Haniman A (2008) Artificial photosynthesis. Materials Today 11 26-34 Inoue T, Fujishima A, Konishi S, Honda K (1979) Photoelectrocatalytic reduction of carbon dioxide in aqueous suspensions of semiconductor powders. Nature 277 637-638 Halmann M (1978) Photoelectrochemical reduction of aqueous carbon dioxide on p-type gallium phosphide in liquid junction solar cells. Nature 275 115-116 Heminger JC, Carr R, Somorjai GA (1987) The photoassisted reaction of gaseous water and carbon dioxide adsorbed on the SrH03 (111) crystal face to form methane. Chem Phys Lett 57 100-104... 

Bard AJ, Fox MA (1995) Artificial photosynthesis solar splitting of water to hydrogen and oxygen. Acc Chem Res 28 141-145... 

Approaches to the fundamental need to shift from fossil to renewable feedstocks for chemicals production wiU range from modifications to, and developments of, traditional chemical, engineering and biotechnological methods (that maybe implemented on a relatively short timescale, say, 10-15 years) to much more radical processes (such as direct capture of solar energy, through artificial photosynthesis), requiring longer time to implement (say 15-30 years). 

Chemical use artificial photosynthesis and chemical conversion to high added value products and fuels. 

Because C02 is a practically inert molecule, artificial photosynthesis of C02 involves the use of large amounts of energy so it must use a clean source of energy (such as solar radiation).Therefore, the use of catalytic agent to facilitate the process allowing even take place at ambient temperature and pressure is necessary. In this case, it is also called as photocatalysis or photoreduction. 

This is the present state in the development of chemical systems for artificial photosynthesis. For solar energy conversion and... 

Figure 4.17 Schematic representation of an ideal system for artificial photosynthesis. The fundamental elements are present a light harvesting system, a triad for charge separation (D—P—A, Donor—Primary acceptor—Acceptor), a...

Bard, A.J. and Fox, M.A. (1995) Artificial photosynthesis solar splitting of water to hydrogen and oxygen. Accounts of Chemical Research, 28 (3), 141—145. Imahori, H., Mori, Y., and Matano, Y. (2003) Nanostructured artificial photosynthesis. Journal of Photochemistry and Photobiology C Photochemistry Reviews, 4 (1), 51-83. 

Fig. 14.6). A key is that in many cases solution processing can lead to new structures that are difficult or impossible to attain by other means. This can include, for example, nanofiber arrays, core-shell structures, nanopods, and nanoribbons.30 32 These structures can lead to a variety of new functionalities—from 3D prototyping, to third-generation PV structures, to electronic paper, to a new class of non linear optics, to the ability to order nanostructures at very small length scales and maybe even to the holy grail of the energy field, artificial photosynthesis. Below we briefly discuss how some of these concepts are beginning to be realized.

Figure 14.15. Schematic for Artificial photosynthesis.74 (http //www.fotomol.uu.se/ Forskning/Biomimetics/fotosyntes/iudex.shtm) [Figure kindly provided by Prof. Stenbjorn Styring, Uppsala University, Sweden.]...

Ishida, H. 2002. From artificial photosynthesis to nano-science. Kokagaku 32 136-143. 

Ganguly, T. Pal, S. K. 2006. Photoinduced electron transfer research to build model compounds of artificial photosynthesis and solar energy conversion. J. Chinese Chem. Soc. 53 219-226. 

Much interest has recently been shown in artificial photosynthesis. Photosynthesis is a system for conversion or accumulation of energy. It is also interesting that some reactions occur simultaneously and continuously. Fujishima et al. [338] pointed out that a photocatalytic system resembles the process of photosynthesis in green plants. They described that there are three important parts of the overall process of photosynthesis (1) oxygen generation by the photolysis of water, (2) photophosphorylation, which accumulates energy, and (3) the Calvin cycle, which takes in and reduces carbon dioxide. The two reactions, reduction of C02 and generation of 02 from water, can occur simultaneously and continuously by a sonophotocatalytic reaction. 

Beer, R., G. Calzaferri, J. Li, and B. Waldeck (1991), "Towards Artificial Photosynthesis Experiments with Silver Zeolites", Part 2, Coord. Chem. Rev., in press. 

Describe the principles of the use of Rufbpy) as a sensitiser in artificial photosynthesis systems. 

For more details regarding artificial photosynthesis systems, the reader is referred to Section 7.5. 

Figure 12.12 Building blocks of an artificial photosynthesis system for hydrogen production using a chromophore (C), an electron acceptor (A) and a sacrificial donor(D)...

---