Time-resolved spectroscopic study on photoinduced electron-transfer processes in...

Femtosecond time-resolved transient absorption studies have been performed to investigate the photoinduced energy and electron-transfer processes in Zn(II)porphyrin-Zn(II)chlorin-fullerene triad in which energy and oxidation potential gradients are directed along the donor-acceptor-linked arrays. Fast energy transfer (approximately 450 fs) from photoexcited Zn(II)porphyrin to Zn(II)chlorin was observed upon selective photoexcitation of Zn(II)porphyrin unit in the triad. In a nonpolar solvent such as toluene, the energy transfer from the excited singlet state of Zn(II)chlorin to fullerene occurs and is followed by the formation of an intermediate state with a time constant of nanoseconds, which was attributed to the intramolecular exciplex between Zn(II)chlorin and fullerene. In benzonitrile, on the other hand, the photoexcitation of the triad results in the fast electron transfer (< 1 ps) from photoexcited Zn(II)chlorin to fullerene. The generated charge-separated species recombine with a time constant of approximately 12 ps. The relatively fast charge separation and charge recombination rates imply that the strong electronic coupling between Zn(II)chlorin and fullerene moieties is probably induced by the folded conformation between Zn(II)chlorin and fullerene moieties which enhances direct through-space interaction between the approximately contacted pi systems.