Axle length effect on photoinduced electron transfer in triad rotaxane with porphyrin, [60]fullerene, and triphenylamine

Photoinduced multiple electron-transfer processes of a newly synthesized rotaxane with one acceptor and two donors are studied with the time-resolved fluorescence and absorption methods. In this rotaxane, zinc porphyrin (ZnP) with a crown-ether necklace is employed as a photosensitized electron donor; through the crown-ether, a short axle with C₆₀ and triphenylamine (TPA) at both terminals is penetrating as an electron acceptor and a hole-shift, respectively (abbreviated as (ZnP;C₆₀-(A_S)-TPA) _Rot). The time-resolved fluorescence and transient absorption measurements reveal that the through-space electron-transfer processes take place via the excited states of the ZnP unit to the spatially arranged C ₆₀ moiety, giving the radical ion pair (ZnP^̇+;C ₆₀^--(A_S)-TPA)_Rot in polar solvents. Consecutively, (ZnP;C₆₀^--(A_S)-TPA ^̇+)_Rot is also generated by the through-space hole-shift between ZnP and TPA, in addition to the through-bond charge separation via the excited state of the C₆₀ moiety. Both radial ion pairs have lifetimes of 320-420 ns, which are longer than those of the previously reported similar rotaxane with cationic longer axle (150-170 ns).