Photoinduced electron transfer in phytochlorin- [60]fullerene dyads

Novel molecular electron donor-acceptor (DA) dyads, composed of a phytochlorin donor and a [60]fullerene acceptor, have been photochemically characterized. In these dyads, a pyrrolidine spacer group links the chlorin and the C-60 moieties covalently, forming a rigid dyad with a short and almost constant D-A distance. The photochemical behavior of the metal-free dyads and the corresponding Zn complexes was studied by means of fluorescence and absorption spectroscopies with femto- and picosecond time resolutions in polar benzonitrile and nonpolar toluene solutions. In consistence with the previous studies on porphyrin-fullerene dyads, the novel chlorin-fullerene dyads underwent a fast intramolecular photoinduced electron transfer in a benzonitrile solution, The recombination rates of the charge-transfer (CT) slates were 4.8 x 10(10) s(-1) for the Zn dyads and ca. 1.5 x 10(10) s(-1) for the metal-free compounds. The CT state was preceded by at least three intermediate states in the time domain from 200 fs to 100 ps. Two of the states were identified as singlet excited states of either the phytochlorin or the fullerene moiety. The third state was attributed to an intramolecular exciplex, which was transformed to the CT state. In the frame of this model, the formation rate constant of the CT state was estimated to be 1.6 x 10(11) s(-1) for the Zn dyads and 0.5 x 10(11) s(-1) for the metal-free compounds. The formation of the exciplex was also observed in nonpolar solvents, e.g., toluene. In contrast to the behavior in polar solvents, the exciplex relaxed in toluene directly to the ground state, without the formation of the CT stale. The lifetime of the exciplex was 140 ps for the Zn dyads and 1-2 ns for the metal-free compounds in toluene.