| Abstract
| - Two rotaxanes tethering [60]fullerene (C60) and triphenylamine (TPA) moieties were synthesized in goodyields by the urethane end-capping method using a crown ether−secondary amine motif. In these rotaxanes,the C60 group serving as electron acceptor is attached to the crown ether wheel through which the axle witha 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 compoundswithout rotaxane structures were also prepared. The intra-rotaxane photoinduced electron-transfer processesof C60 and TPA have been investigated by time-resolved transient absorption and fluorescence measurementswith changing solvent polarity and temperature. Nanosecond transient absorption measurements of theserotaxanes demonstrated that the long-lived charge-separated state (C60•-,TPA•+)rotaxane is formed via the excitedtriplet state of C60 (3C60*) in polar solvents. The rate constants for the charge separation process were in therange of (5−8) × 107 s-1, while the rate constants of charge recombination were in the range of (3−6) × 106s-1, corresponding to the lifetimes of the charge-separated states of 170−300 ns. Both rate constants dependedon rotaxane structure, solvent polarity, and temperature. The activation free energy changes of charge separationand 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−C60 dyad, fast charge separation from the excited singletstate and fast charge recombination were observed through bonds in polar solvents.
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