Raman Studies on the Self-localized Excitations in Lightly and Heavily Dopedtrans-Polyacetylene with Sodium

Abstract

The difference of Raman spectra between lightly and heavily Na-doped trans-polyacetylene (trans-PA) was observed with the 1320 nm laser line, and these were discussed on the basis of the Raman spectra of the negatively charged anions Ph(CH) n Ph - [DP n -, n = 3, 5, 7, 9, and 13] and the radical-anion species Ph(CH) n Ph . - [DP n . - , n = 4, 6, 8, and 10] and 19,19',20,20'-tetranor-β,β'-carotene [TNBC . - , n = 22] of the model compounds, which correspond respectively to negatively charged soliton and negative polarons in trans-PA. The observed wavenumbers of the v 1 and v 4 bands (C=C and C-C stretches) of lightly doped trans-PA are lower than those of the corresponding bands of pristine trans-PA, whereas the contrary is the case for heavily doped trans-PA. Raman spectra of the negatively charged soliton models (DP n -) and the negative polaron models (DP n . - and TNBC . - ) were very similar to their spectrum patterns as a whole, but their wavenumbers and relative intensities are mutually different to some extent. In particular, the Raman wavenumbers of heavily doped trans-PA are very similar with those of the polaron models. The dispersion of the C=C stretching band (v 1 ) in doped PA with the various wavelengths of excitation laser line were explained as the presence of charged domains having various lengths of electron localization.