Syntheses of [60]Fullerene and N,N-Bis(4-biphenyl)aniline-Tethered Rotaxane: Photoinduced Electron-Transfer Processes via Singlet and Triplet States of [60]Fullerene

Abstract

Abstract A rotaxane containing [60]fullerene (C60) and N,N-bis(4-biphenyl)aniline (BBA) moieties was synthesized. In this structure, C60 acting as an electron acceptor, is attached to the crown-ether ring through which the axle with terminal BBA moieties acting as electron donors on both ends is penetrating. This rotaxane had a neutral amide moiety in the center of the axle in which two BBA moieties act as stoppers. The intra-rotaxane photoinduced electron-transfer processes of the C60 and BBA moieties were investigated by time-resolved transient absorption and fluorescence measurements while changing solvent polarity and temperature. Time-resolved transient absorption measurements of the rotaxanes confirmed that the long-lived charge-separated state (C60•−; BBA•+)rotaxane was formed via both the excited singlet and triplet states of C60 (1C60* and 3C60*, respectively) in polar solvents. The rate constants for charge-separation process were evaluated to be in the range of (3.6–3.7) × 108 s−1 via 1C60* and (5.1–5.6) × 107 s−1 via 3C60* in the ratio of (0.36–0.38):(0.43–0.51). The rate constants of charge recombination were 2.5 × 106 s−1 and 4.4 × 106 s−1, corresponding to the lifetimes of the charge-separated states of 400 ns and 230 ns in THF and benzonitrile, respectively. By the temperature dependences, the activation free-energy changes of charge-separation process via 3C60* were evaluated to be 0.10 eV, while those of the charge-recombination process were estimated to be 0.03 eV in THF and benzonitrile. These low activation energies are one of the characteristics of through-space electron transfer in the rotaxanes.