PHOTOEXCITATION SPECTROSCOPY OF CONDUCTING-POLYMER-C(60) COMPOSITES - 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 C60. For conjugated polymers with nondegenerate ground states as donors, electron transfer takes place prior to either radiative or nonradiative recombination of pi-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 C60 as donor and acceptor, respectively.