Solvent polarity dependence of photoinduced charge separation in a tetrathiophene-C60 dyad studied by pico- and nanosecond laser flash photolysis in the near-IR region

Photoinduced charge separation and recombination processes in a tetrathiophene-C-60 dyad molecule (4T-C-60) in various solvents were investigated by observing transient absorption spectra in the near-IR region. In polar solvents such as tetrahydrofuran and benzonitrile, charge separation occurred at a rate on the order of 10(10) s(-1). which decreased down to the order of 10(9) s(-1) in moderately polar solvents. In nonpolar toluene, charge separation was not observed. The quantum yields for the charge separations were nearly unity in polar solvents. Charge recombination occurred in 100 ps - 10 ns: the shorter lifetimes occurred in the highly polar solvents. After recombination, the triplet excited state of C-60 was generated predominantly. In polar solvents such as benzonitrile. the charge-separated state was also observed in the microsecond time region. To explain the unprecedented long lifetime of the second charge-separation step, an equilibrium between the charge-separated state and the triplet excited state was taken into consideration.