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) × 10 7 s -1, while the rate constants of charge recombination were in the range of (3−6) × 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.