The reaction of chlorophyll a+ (Chl a+) with either Chl a- or pheophytin a- (Pheo a-) in addition to the reaction of Pheo a+ with Pheo a- was studied in butyronitrile (BCN), BCN-1% THF, THF, and DMF. The electrochemically produced radical ion pairs Chl a+-Chl a- and Pheo a+-Pheo a- react in each solvent to produce a 10-7-10-6 yield of luminescent states on the basis of the initial number of radical pairs. The Chl a+-Pheo a- reaction produces no observable luminescence in any of the solvents examined. The luminescence maximum for the Pheo a+-Pheo a- reaction occurs at 730 nm in each solvent and is strongly red-shifted relative to the fluorescence maxima for optically excited Pheo a in these solvents. A similar result is obtained for the Chl a+-Chl a- reaction in BCN. However, emission from the Chl a+-Chl a- reaction in the other three solvents occurs at 680 nm and corresponds more closely to normal fluorescence from optically excited Chl a. The red-shifted spectra are consistent with the formation of excimers. AC voltammetry of Chl a in BCN provides evidence that Chl a is aggregated in the ground state in this solvent. Chl a reduction shows four waves in BCN and two waves in the other three solvents. Thus, the Chl a+-Chl a- reaction in BCN does not form a true excimer, whereas the Pheo a+-Pheo a- reactions in each solvent do. The luminescence efficiencies of these charge-transfer neutralization reactions are discussed in terms of the geometric constraints on electron-transfer reactions in photosynthetic reaction centers.
|Number of pages||6|
|Journal||Journal of the American Chemical Society|
|Publication status||Published - 1980|
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