RESONANCE RAMAN CHARACTERIZATION OF POLARONS AND BIPOLARONS IN SODIUM-DOPED POLY(PARA-PHENYLENEVINYLENE)

The resonance Raman spectra of sodium-doped poly(p-phenylenevinylene) (PPV) and the radical anions and dianions of three model compounds CH3(C6H4CH = CH)(n)C6H4CH3 (PVn, n = 1-3) have been studied. The Raman spectra of sodium-doped PPV show marked changes with laser wavelengths used for Raman excitation. These spectra have been analyzed on the basis of the resonance Raman spectra of the radical anions and dianions of the model compounds, which correspond, respectively, to negative polarons and bipolarons in PPV. Three kinds of negative-polarons whose lengths are close to PV1, PV2, and PV3, and a bipolaron which is localized in a region close to PV3, exist in a sodium-doped PPV film. Upon prolonged heat treatment of the sodium-doped PPV (290-degrees-C, 12 h), the shortest polaron corresponding to the radical anion of PV1 disappears, probably because it combines with another polaron to form a bipolaron. These results indicate that resonance Raman spectroscopy is a powerful tool for characterizing polarons and bipolarons in conducting polymers.