Photoinduced Electron-Transfer Reactions in a Chlorophyllide-Pheophorbide Cyclophane. A Model for Photosynthetic Reaction Centers

The fluorescence and the lowest excited triplet state yields for a chlorophyllide-pheophorbide cyclophane are strongly quenched as the dielectric constant of the solvent in which it is dissolved increases. The decrease in fluorescence yield is accompanied by a decrease in fluorescence lifetime. The radiative rate for fluorescence remains approximately constant. Fluorescence decays of the cyclophane are best fit with two exponentials. The short component is 250 ± 50 ps, while the long component is about 2-4 ns. Time resolved absorbance changes at 655 and 445 nm are used to follow the decay of the excited states back to the ground state. A residual absorbance having a lifetime in excess of 30 ns is found in all solvents and is ascribed to the lowest excited triplet state of the cyclophane. After correction for this contribution the decay of the absorbance changes is best fit with a single exponential. In dichloromethane and butyronitile the absorbance decay times, 1.6 ns and 435 ps, respectively, are longer than the 250-ps fluorescence emission component of the cyclophane in these solvents. This indicates that a nonemissive excited state is formed. The observed dependence of both the fluorescence and the absorption change decays on the solvent dielectric constant suggests that quenching of the lowest excited singlet state involves an electron-transfer process. The effect of solvent dielectric constant on the quenching is analyzed in terms of the energetics of the electron-transfer reaction. The electron-transfer rate is 2 × 10⁹ s^-1 while the rate of the back electron transfer yielding the cyclophane in its ground electronic state is 10⁹ s^-1. The forward electron-transfer rate is about 100 times slower than that of photosynthetic reaction centers despite the close proximity of the donor and the acceptor (<6 Å). The difference in electron-transfer rates between the cyclophane and photosynthetic reaction centers is rationalized in terms of the importance of donor-acceptor orientation.