Mechanisms of the energy and electron transfer during sensitization of soluble photoconducting polyimides

The effect of fluorescence quenching was observed for several sensitizing d yes in the films of soluble photoconducting polyimides. The efficiency of q uenching, as well as the efficiency of photogeneration sensitization, depen ds on the type and strength of the electron donor-acceptor (EDA) interactio n in these polymers. Pls with pronounced interchain EDA interaction and a s ufficiently large overlap between the fluorescence spectra of dyes (Coumari n 30, Rhodamine 6G) and the absorption spectra of interchain charge-transfe r CCT) complexes in PI, the quenching obeys a quadratic dependence on the c oncentration of these CT complexes, in accordance with the Ferster equation . Parameters of the nonradiative dipole-dipole energy transfer, including t he critical distance and the transfer rate constant, art: determined. For a low overlap between the fluorescence and absorption spectra(Oxazine 1), th e quenching follows the Stern-Volmer law and is determined by the formation of an exciplex between the excited dye molecule surd a do not (triphenylam ine) fragment of die PI chain (electron transfer mechanism). This is confir med by the effect of the applied magnetic held (H < 1 kOe; T = = 293 K) on the fluorescence from exciplexes related by mutual transitions to the radic al ion pairs whose thermal and field-induced dissociation leads to sensitiz ed photogeneration of charge carrier. For PIs with predominant formation of interchain CT complexes, the energy is transferred by an efficient nonradi ative mechanism from the dye (Oxazine 1) to the radical cations of tripheny lamine fragments in the polymer chains, stabilized in the field of counteri on (Poole-Frenkel pairs). These radicals absorb in the red and near-IR spec tral region, outside the fluorescence band of CT complexes. No effective no nradiative energy transfer was observed in PIs with dominating intrachain E DA interactions. The nonradiative energy transfer from dye molecules to int erchain CT complexes and stabilized radical cations, observed in this work, provides explanation for the mechanism of "universal" sensitization of the charge carrier photogeneration reported for same of these PIs.