Photoinduced electron transfer processes in rotaxanes containing [60]fullerene and ferrocene: Effect of axle charge on light-induced molecular motion

Two rotaxanes containing [60]fullerene (C₆₀) as pendants on a crown-ether necklace, to which ferrocene (Fc) as axle stoppers were added, have been synthesized. One rotaxane has an ammonium cation in the centre of the axle (C₆₀;Fc)_Rot+ and the other has a neutral axle (C ₆₀;Fc)_Rot. Optimized structures, calculated using a molecular orbital method, suggest that in the ground states (C ₆₀;Fc)_Rot+ has a shorter distance between C₆₀ and Fc than that of (C₆₀;Fc)_Rot. In both rotaxanes, efficient intra-rotaxane photoinduced electron-transfer processes have been observed by the selective excitation of C₆₀ which acts as a photosensitized electron acceptor. The rates and efficiencies of the charge-separation and charge-recombination processes were evaluated by time-resolved fluorescence and transient absorption measurements with changing solvent polarity. From the different kinetic parameters between (C ₆₀;Fc)_Rot+ and (C₆₀;Fc)_Rot, the light-induced molecular motions of these rotaxanes in the excited states and charge-separated states were separately revealed.