Photoexcitation spectroscopy of conducting-polymer–C60composites: Photoinduced electron transfer

We present a comparative study of the steady-state photoinduced absorption and photoinduced electron-spin-resonance (ESR) spectra of conducting polymers mixed with the fullerene ${\mathrm{C}}_{60}$. For conjugated polymers with nondegenerate ground states as donors, electron transfer takes place prior to either radiative or nonradiative recombination of π-electron photoexcitations on the conducting polymer. In the case of a conjugated polymer with a degenerate ground state as donor, the structural relaxation associated with the formation of charged solitons is faster; and no indications of photoinduced charge transfer are observed. Thus, composites using a derivative of poly(1,6-heptadyene) as donor do not exhibit long-lived charge separation, whereas charge transfer and charge separation are observed in composites using poly(p-phenylene vinylene) or polythiophene derivatives as donors. The relaxation (as a function of temperature) of the charge separated state is studied through photoinduced absorption spectroscopy (excitation spectroscopy) and photoinduced ESR. The results are discussed in terms of designing suitable donor-acceptor pairs for photoinduced electron transfer using conducting polymers and ${\mathrm{C}}_{60}$ as donor and acceptor, respectively.