Journal of Physical Chemistry B, Vol.110, No.13, 6516-6525, 2006
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 (C-60) and ferrocene (Fc) moieties (abbreviated as (C-60;Fc)(rotax+)) was synthesized in a good yield by the urethane end-capping of pseudorotaxane based on the crown ether-secondary amine motif. In (C-60;Fc)(rotax+), the C-60 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 C-60 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(center dot-);Fc(center dot+))(rotax+) is formed mainly via the excited triplet state of C-60 in polar solvents. The lifetime of (C60(center dot-);Fc(center dot+))(rotax+) was evaluated to be 20 ns in dimethylfortnamide (DMF) at room temperature. With lowing temperature, the lifetime of (C-60(center dot-);Fc(center dot+))(rotax+) extends to 270 ns in DMF at -65 degrees C, due to the structural changes leaving C-60(center dot-) and Fc(center dot+) at a relatively long distance in the low-temperature region.