Photoinduced electron-transfer processes between [C60]fullerene and triphenylamine moieties tethered by rotaxane structures. Through-space electron transfer via excited triplet states of [60]fullerene

Two rotaxanes tethering [60]fullerene (C(60)) and triphenylamine (TPA) moieties were synthesized in good yields by the urethane end-capping method using a crown ether-secondary amine motif. In these rotaxanes, the C(60) group serving as electron acceptor is attached to the crown ether wheel through which the axle with a TPA group acting as electron donor on its terminal penetrates. One rotaxane has an ammonium moiety, whereas the other has a neutral amide moiety in the center of the axle. The corresponding reference compounds without rotaxane structures were also prepared. The intra-rotaxane photoinduced electron-transfer processes of C(60) and TPA have been investigated by time-resolved transient absorption and fluorescence measurements with changing solvent polarity and temperature. Nanosecond transient absorption measurements of these rotaxanes demonstrated that the long-lived charge-separated state (C(60)(.-),TPA(.+))(rotaxane) is formed via the excited triplet state of C(60) ((3)C(60)*) in polar solvents. The rate constants for the charge separation process were in the range of (5-8) x 10(7) s(-1), while the rate constants of charge recombination were in the range of (3-6) x 10(6) s(-1), corresponding to the lifetimes of the charge-separated states of 170-300 ns. Both rate constants depended on rotaxane structure, solvent polarity, and temperature. The activation free energy changes of charge separation and recombination processes were evaluated to be 0.01-0.03 and 0.03-0.06 eV by temperature dependences, respectively. Such low activation energies may be related to through-space electron transfer in these rotaxanes. On the other hand, in a covalently connected TPA-C(60) dyad, fast charge separation from the excited singlet state and fast charge recombination were observed through bonds in polar solvents.