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 (C₆₀) 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 C₆₀ and ZnP moieties than that of the rotaxane with a neutral axle because of acylation of the ammonium cation, which draws away the C₆₀ 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 C₆₀ 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 C₆₀ anion radical and the ZnP cation radical.