Electrochemical production of chlorophyll a and pheophytin a excited states

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.