Intramolecular Photoinduced Electron-Transfer Processes in Tetrathienylethylene−Quaterthiophene−[60]Fullerene Triad in Solutions

Intramolecular photoinduced charge-separation and recombination processes in a covalently connected tetrathienylethylene-quaterthiophene-C60 (TTE−4T−C60) triad have been studied by time-resolved fluorescence and transient absorption spectral methods. The observed low fluorescence intensity and the short fluorescence lifetime of the C60 moiety of the triad in benzonitrile (PhCN) indicate that charge separation takes place via the singlet excited state of the C60 moiety in quite fast rate and high efficiency. The nanosecond transient absorption spectra in PhCN showed the broad absorption bands in the 600−1500 nm region, which were attributed to (TTE−4T)•+−C60 •-, in which the radical cation (hole) delocalizes both on the TTE and 4T moieties. The charge-separated state decays to the neutral triad with a lifetime of 18 ns in PhCN at room temperature. From temperature dependence of the charge-recombination rate constants, the reorganization energy was evaluated to be 0.74 eV, which indicates that the charge-recombination process is in the inverted region of the Marcus parabola. In toluene, TTE−4T−C60* predominantly descends to TTE−4T−3C60*, which decays to the ground state.