Prolongation of the lifetime of the charge-separated state at low temperatures in a photoinduced electron-transfer system of [60]fullerene and ferrocene moieties tethered by rotaxane structures

A rotaxane tethering both fullerene (C₆₀) and ferrocene (Fc) moieties (abbreviated as (C₆₀;Fc)_rotax+) was synthesized in a good yield by the urethane end-capping of pseudorotaxane based on the crown ether -secondary amine motif. In (C₆₀;Fc)_rotax+, the C₆₀ group serving as an electron acceptor is attached to the crown ether wheel, through which the axle with a Fc group acting as an electron donor on its end penetrates. The intrarotaxane photoinduced energy-transfer and electron-transfer processes between C₆₀ and Fc in (C ₆₀;Fc)_rotax+ have been investigated by time-resolved transient absorption and fluorescence measurements with changing solvent polarity. Nanosecond transient absorption measurements of the rotaxane demonstrated that the charge-separated state (C₆₀^-Fc ⁺)_rotax+ is formed mainly via the excited triplet state of C₆₀ in polar solvents. The lifetime of (C₆₀ ^-;Fc⁺)_rotax+ was evaluated to be 20 ns in dimethylformamide (DMF) at room temperature. With lowing temperature, the lifetime of (C₆₀^-;Fc⁺)_rotax+ extends to 270 ns in DMF at - 65°C, due to the structural changes leaving C ₆₀^- and Fc⁺ at a relatively long distance in the low-temperature region.