Photoinduced Electron Transfer Processes in Rotaxanes Containing [60]Fullerene and Ferrocene: Effect of Axle Charge on Light-Induced Molecular Motion

Abstract

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 (C60;Fc)Rot. Optimized structures, calculated using a molecular orbital method, suggest that in the ground states (C60;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 (C60;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.