Photosynthesis exciton transfer

The study of exciton transfer and primary charge separation in photosynthesis requires excitation by picosecond flashes. If a significant fraction of the reaction centers (RCs) shall be closed by a flash, the excitation density must be chosen such high that more than one exciton resides at the same time in the pool of antenna pigments. Then excitons can be lost by singlet-singlet annihilation before they are trapped by the photochemistry in the RC. Annihilation leads to an apparent acceleration of all other reactions connected with the exciton dynamics. The quantitative treatment of this competitive deactivation path allows the bimolecular rate constant of exciton-exciton annihilation to be determined that characterizes a given antenna bed. 

The approach for this system is the mimicry of the highly efficient photosynthesis process in biological systems, by which an antenna device collects the light energy before a series of exciton, energy, and electron transfers, which lead to the synthesis of the plant s fuel.70-73... 

Knox R. Exciton energy transfer and migration theoretical considerations. In Bioenergetics of Photosynthesis. Govindjee ed. 1975. Academic Press, New York. pp. 183-221. 

Energy Transfer and Excitons are, as we have already mentioned, perhaps the most interesting and in any case the most characteristic photophysical processes in molecular crystals. The investigation of these processes began in 1934, when A. Winterstein, U. Schon and H. Vetter [5] were able to explain the green fluorescence radiation from anthracene crystals, which had been described as due to the effect of an unknown chrysogen , in terms of sensitised fluorescence. This fluorescence is emitted by tetracene molecules which are present at very low concentration in the anthracene. Pure anthracene fluoresces in the crystalline phase just as in solution with a blue-violet colour. This observation set off a large number of spectroscopic studies of the sensitised emission from mixed crystals. Very soon, J. Franck and E. Teller [6] pointed out in a summary report of this field that there was an important cormection here to the primary processes of photosynthesis and other biophysical processes. 

T p y+l)/k0j, where k j is the RC trapping rate and V is the ratio of the probabilities of finding the exciton in the antenna system and RC. It can be shown that from nine experimental observables (fluorescence and phosphorescence intensities and quantum yields, 7) qj) only one is independent. In all the transfer regimes, the observables depend only on V which is in general a function of time, intramolecular rate constants, size of the photosynthetic unit and initial conditions. Therefore, V (t) is the maximum information obtainable from the observables. These and further results representing general theoretical answers to problems l)-5) were illustrated on the case of the bacterial photosynthesis (Rhodopseudomonas viridis) where they are valid for the whole range of the physically acceptable values of the Forster radius. 

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