Evidence for Two Separate One-Electron Transfer Events in Excited Fulleropyrrolidine Dyads Containing Tetrathiafulvalene (TTF)

Abstract

1,3-Dipolar cycloadditions of TTF-azomethine ylides (TTF = tetrathiafulvalene) to C60 have been used to synthesize a series of novel donor−bridge−acceptor dyads. In these dyads the pyrrolidine[3‘,4‘:1,2][60]fullerene is covalently attached to the electron donor TTF either directly (5) or alternatively 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 reversible oxidation steps. The former are associated with the reduction of the C60 core, whereas the latter correspond to the formation of the radical cation and dication of the TTF moiety, respectively. Semiempirical PM3 calculations reveal donor−acceptor distances of 4.8 Å (5), 7.6 Å (2a), and 10.5 Å (7), and a deviation from planarity between the TTF fragment and the vinylogous spacer. In relation to an N-methylfulleropyrrolidine, the emission of the fullerene singlet excited state in dyads 2a, 5, and 7 is substantially reduced. Furthermore, the fluorescence quantum yield correlates well with the solvent dielectric constant and also with the spatial separation of the donor and acceptor moieties in the dyads. These correlation suggest that intramolecular electron-transfer processes evolving from the fullerene singlet excited state generate the (C60•-)−(TTF•+) pair. Pico- and nanosecond-resolved transient spectroscopy further substantiate a rapid transformation of the initially formed singlet excited state into the charge-separated radical pair with intramolecular rates ranging between 1.17 × 1010 s-1 and 1.47 × 109 s-1. In all cases, the product of back electron transfer is the triplet excited state, which is generated with markedly high quantum yields (0.61−0.97). The latter is, in addition to the rapid primary intramolecular electron transfer, subject to a slower, secondary intermolecular electron transfer with rate constants of 7 × 108 M-1s-1 (5) in benzonitrile and 1.6 × 109 M-1 s-1 (5) in toluene.