Biomimetics. molecular energy conversion

Until a recent x-ray diffraction study (17) provided direct evidence of the arrangement of the pigment species in the reaction center of the photosynthetic bacterium Rhodopseudomonas Viridis, a considerable amount of all evidence pertaining to the internal molecular architecture of plant or bacterial reaction centers was inferred from the results of in vitro spectroscopic experiments and from work on model systems (5, 18, 19). Aside from their use as indirect probes of the structure and function of plant and bacterial reaction centers, model studies have also provided insights into the development of potential biomimetic solar energy conversion systems. In this regard, the work of Netzel and co-workers (20-22) is particularly noteworthy, and in addition, is quite relevant to the material discussed at this conference. 

Three key issues must be addressed in the development of effective biomimetic solar energy conversion systems. First, the molecular system should possess a large optical absorption cross-section in the desired spectral region. Second, the system should possess appropriate characteristics to insure formation of a sufficiently long-lived, low-lying state which can initiate the primary ET efficiently. And third, the system should be able to effect the ET process irreversibility, that is electron-hole recombination should be substantially inhibited. 

The first issue can be addressed in two ways a primary ET species which has a large optical absorption cross-section can be chosen or arrays of molecules with large optical absorption cross-sections can be used as "antennas" that will efficiently collect and transport the electronic excitation energy to the primary ET species, in direct analogy to photosynthetic systems. While in the latter case it should be possible to develop systems with more efficient solar photon collection, the number of primary ET species will have to be reduced due to the spatial limitations, which will also reduce the potential electric current that can be produced by the system. Thus, questions related to the detailed molecular architecture of biomimetic solar energy conversion devices will have to address this issue, and it is quite likely that a number of compromises will have to be made before optimal design characteristics are obtained. 

Thus, it appears that several systems have been developed which hold promise as prototypes of biomimetic solar energy conversion devices. And in conjunction with the advances being made in experimental and theoretical methods for investigating molecular excited-state processes, prospects for the development of practical biomimetic devices are now substantially better than they were only a decade ago. 

In Chap. 6, biological supermolecules are explained and classified by function. Artificial supramolecular systems that mimic biological ones are also described. Biomimetic chemistry, which mimics the essence of a biosystem and then develops an artificial system that is better than the biological one, is widely used in this field. Fimctional developments, such as molecular transport, information transmission and conversion, energy conversion and molecular conversion (enzymatic functionaUty) based on biomimetic chemistry are described. New methodologies such as combinatorial chemistry and in vitro selection mimic evolutionary processes in nature. We leave this topic until the end of the book because we want to show that there is still lots to do in supramolecular chemistry, and that supramolecular chemistry has huge future potential. 

Fig. 2 A biomimetic system for solar-energy conversion. A molecular triad embedded in the bilayer membrane allows photoexcited electrons to be transported across the membrane to acceptor species in the interior. This charge transport is accompanied by the transmembrane flow of hydrogen ions, leading to a decrease in pH inside the compartment. This proton-motive force is then used to drive ATP synthesis by the membrane protein ATP synthase, as it is in photosynthesis.

Sivakumar R, Thomas J, Yoon M (2012) Polyoxometalate-based molecular/nano composites advances in environmental remediation by photocatalysis and biomimetic approaches to solar energy conversion. J Photochem Photobiol C 13 277-298... 

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