Solvent polarity dependence of photoinduced charge separation in a tetrathiophene-C-60 dyad studied by pico- and nanosecond laser flash photolysis inthe near-IR region

Photoinduced charge separation and recombination processes in a tetrathioph ene-C-60 dyad molecule (4T-C-60) in various solvents were investigated by o bserving transient absorption spectra in the near-IR region. In polar solve nts 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 separ ation was not observed. The quantum yields for the charge separations were nearly unity in polar solvents. Charge recombination occurred in 100 ps - 1 0 ns: the shorter lifetimes occurred in the highly polar solvents. After re combination, the triplet excited state of C-60 was generated predominantly. In polar solvents such as benzonitrile. the charge-separated state was als o observed in the microsecond time region. To explain the unprecedented lon g lifetime of the second charge-separation step, an equilibrium between the charge-separated state and the triplet excited state was taken into consid eration.