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-¹C₆₀* predominantly descends to TTE-4T-³C₆₀*, which decays to the ground state.