Synthesis and photoinduced electron transfer processes of rotaxanes bearing [60]fullerene and zinc porphyrin: Effects of interlocked structure and length of axle with porphyrins

Three rotaxanes, with axles with two zinc porphyrins (ZnPs) at both ends penetrating into a necklace pending a C-60 moiety, were synthesized with varying interlocked structures and axle lengths. The intra-rotaxane photoinduced electron transfer processes between the spatially positioned C60 and ZnP in rotaxanes were investioated. Charge-separated (CS) states (ZnP.+, C-60(.-))(rotaxane) are formed via the excited singlet state of ZnP ((ZnP)-Zn-1*) to the C-60 moiety in solvents such as benzonitrile, THF, and toluene. The rate constants and quantum yields of charge separation via (ZnP)-Zn-1* decrease with axle length, but they are insensitive to solvent polarity. When the axle becomes long, charge separation takes place via the excited triplet state of ZnP ((ZnP)-Zn-3*). The lifetime of the CS state increases with axle length from 180 to 650 ns at room temperature. The small activation energies of charge recombination were evaluated by temperature dependence of electron-transfer rate constants, probably reflecting through-space electron transfer in the rotaxane structures.