Pico- and nano-second laser flash photolysis study on photoinduced charge separation in oligothiophene-C60 dyad molecules

Abstract

Photoinduced charge separation (CS) and charge recombination (CR) processes of octathiophene-C60 and dodecathiophene-C60 dyad molecules (8T-C60 and 12T-C60, respectively) have been investigated by time-resolved absorption spectroscopy in the visible and near-IR regions. In toluene, 18T*-C60 and 112T*-C60 showed energy transfer to 1C*-moiety predominantly, while 60 contribution of CS was small. In various polar solvents, on the other hand, CS states were predominantly formed from both singlet-excited oligothiophene and 1C6*0-moiety because of lower CS level in polar environments. The CR process generating both the triplet 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 (∈s) > 7), CS states showed two components decay: Slow decay component showed lifetime in the hundred nanosecond-region, while fast component decayed within a few nanoseconds. For the mechanism of the long-living CS state in polar solvents (∈s > 7), equilibrium between the CS state and the triplet state was proposed. Furthermore, effects of length of oligothiophene on the CS and CR processes were discussed on the basis of the free energy changes.