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.