Resonant Raman characterization of polarons and bipolarons in alkali-metal-doped poly(p-phenyelenevinylene)

The resonant Raman spectra of lightly sodium-doped and heavily potassium-doped poly(p-phenyelenevinylene)'s (PPV's) have been studied. These spectra have been analyzed on the basis of the resonant Raman spectra of the radical anions and dianions of three model compounds CH₃(C₆H₄CH = CH)_nC₆H₄CH₃ (PVn, n = 1-3), which correspond, respectively, to negative polarons and bipolarons in PPV. The lightly sodium-doped PPV film contains a bipolaron which is localized in a region close to PV3, in addition to two kinds of polarons whose lengths are close to PV2 and PV3. The 1064-nm-excited resonant Raman spectrum of potassium-doped PPV is attributed to a bipolaron localized in a region close to PV3. The resonant Raman spectra of potassium-doped PPV excited at 514.5, 632.8 and 752.5 nm are explained by the existence of negative polarons. However, the bands in the 1271 - 1246 cm^-1 range, which are observed for sodium-doped PPV and assigned to polarons, are not observed for potassium-doped PPV. The reason for their absence is discussed.