Axle charge effects on photoinduced electron transfer processes in rotaxanes containing porphyrin and [60]fullerene.

Two rotaxanes containing zinc porphyrin (ZnP) with crown ether and [60]fullerene (C(60)) with cationic and neutral axles are synthesized. Optimized structures calculated by molecular orbital methods indicate that the rotaxane with an ammonium cation in the center of the axle has a shorter distance between the C(60) and ZnP moieties than that of the rotaxane with a neutral axle because of acylation of the ammonium cation, which draws away the C(60) and ZnP moieties by releasing the interaction with the crown ether. The charge-transfer interaction is revealed by absorption spectra for the rotaxane with a short distance, but not for the rotaxane with a long distance, which strongly affects the rates and efficiencies of photoinduced electron-transfer and energy-transfer processes via the excited singlet states of the ZnP and C(60) moieties and their triplet states, as revealed by the time-resolved fluorescence and absorption measurements. The rate of the charge recombination of the radical ion pair of the rotaxane with a neutral axle is slower than that of the rotaxane with a cationic axle, due to the loose structure of the former rotaxane, which results from the long distance between the C(60) anion radical and the ZnP cation radical.