Pico- and nano-second laser flash photolysis study on photoinduced charge separation in oligothiophene-C-60 dyad molecules

Photoinduced charge separation ICS) and charge recombination (CR) processes of octathiophene-C-60 and dodecathiophene-C-60 dyad molecules (8T-C-60 and 12T-C-60, respectively) have been investigated by time-resolved absorption spectroscopy in the visible and near-IR regions. In toluene, (1)8T*-C-60 a nd (1)12T*-C-60 showed energy transfer to C-1(60)*-moiety predominantly, wh ile contribution of CS was small. In various polar solvents, on the other h and, CS states were predominantly formed from both singlet-excited oligothi ophene and C-1(60)*-moiety because of lower CS level in polar environments. The CR process generating both the tripler state of oligothiophene and the ground state was confirmed in anisole and anisole/toluene mixture within a few nanoseconds, in more polar solvents (dielectric constant (epsilon (s)) > 7), CS states showed two components decay: Slow decay component showed l ifetime in the hundred nanosecond-region, while fast component decayed with in a few nanoseconds. For the mechanism of the long-living CS state in pola r solvents (epsilon (s) > 7), equilibrium between the CS state and the trip let state was proposed. Furthermore, effects of length of oligothiophene on the CS and CR processes were discussed on the basis of the free energy cha nges.