Evidence for two separate one-electron transfer events in excited fulleropyrrolidine dyads containing tetrathiafulvalene (TTF)

1,3-Dipolar cycloadditions of TTF-azomethine ylides (TTF = tetrathiafulvale ne) to C-60 have been used to synthesize a series of novel donor-bridge-acc eptor dyads. In these dyads the pyrrolidine[3',4':1,2][60]fullerene is cova lently attached to the electron donor TTF either directly (5) or alternativ ely through one (2a) or two (7) vinyl groups. In the ground state, dyads 2a , 5, and 7 undergo Four quasireversible one-electron reductions and two rev ersible oxidation steps. The former are associated with the reduction of th e C-60 core, whereas the latter correspond to the formation of the radical cation and dication of the TTF moiety, respectively. Semiempirical PM3 calc ulations reveal donor-acceptor distances of 4.8 Angstrom (5), 7.6 Angstrom (2a), and 10.5 Angstrom (7), and a deviation from planarity between the TTF fragment and the vinylogous spacer. In relation to an N-methylfulleropyrro lidine, the emission of the fullerene singlet excited state in dyads 2a, 5, and 7 is substantially reduced. Furthermore, the fluorescence quantum yiel d correlates well with the solvent dielectric constant and also with the sp atial separation of the donor and acceptor moieties in the dyads. These cor relation suggest that intramolecular electron-transfer processes evolving f rom the fullerene singlet excited state generate the (C-60(.-))-(TTF.+) pai r. Pico- and nanosecond-resolved transient spectroscopy further substantiat e a rapid transformation of the initially formed singlet excited state into the charge-separated radical pair with intramolecular rates ranging betwee n 1.17 x 10(10) s(-1) and 1.47 x 10(9) s(-1). In all cases, the product of back electron transfer is the triplet excited state, which is generated wit h markedly high quantum yields (0.61-0.97). The latter is, in addition to t he rapid primary intramolecular electron transfer, subject to a slower, sec ondary intermolecular electron transfer with rate constants of 7 x 10(8) M- 1 s(-1) (5) in benzonitrile and 1.6 x 10(9) M-1 s(-1) (5) in toluene.