Photoinduced electron and energy transfer processes in rotaxanes containing zinc porphyrin as pendant and [60]fullerene and ferrocene as axle ends

Photoinduced electron-transfer processes of a newly synthesized rotaxane containing porphyrinatozinc (ZnP), [60]fullerene (C₆₀), and ferrocene (Fc) have been studied in terms of the time-resolved fluorescence and transient absorption measurements in polar and nonpolar solvents. In this rotaxane, (ZnP;C₆₀-Fc)_rotax, ZnP was chosen as a pendant of a crown-ether necklace, through which an axle with C₆₀ and Fc at both termini was penetrated. By the selective excitation of the ZnP moiety in a nonpolar solvent, energy transfer predominantly takes place to the C ₆₀ moiety of (ZnP;C₆₀-Fc)_rotax. In polar solvents, charge-separation process takes place via the excited singlet state of the ZnP moiety in addition to the energy-transfer process. From the nanosecond transient absorption spectra, a clear absorption band of the C₆₀ ^•- moiety was observed at 1000 nm as well as a broad absorption in the 600-800 nm region due to ZnP^•+, suggesting the generation of (ZnP^•+;C₆₀^•--Fc)_rotax in the first step. Afterward, the hole-transferring process from ZnP ^•+ to Fc is thermodynamically possible, although this process is not fast because of its through-space process character. The final lifetimes of the C₆₀^•- moiety were evaluated to be 290 and 370 ns in benzonitrile and DMF, respectively. The ratios of the charge-separation rates to charge-recombination rates were ca. 1000, indicating that (ZnP;C ₆₀-Fc)_rotax affords an efficient photosynthetic model.