Ah&act-Application of 1064-urn-excited Fourier transform (FT)-Raman spectroscopy to the characterixation of conducting polymers is described. 1064-nm-excited FT-Raman spectra with high signal-to-noise ratios are obtained from polyacetylene (PA), poly(l,Cphenylene) (PPP), poly(l+phenylene vinylene) (PPV) and poly(2,5-thienylene vinylene) (PTV) in their neutral (insulating) state. The resonant Raman spectra of acceptor-or donor-doped (conducting) PA and PPV are also obtained wih 1064-nm excitation. The resonant Raman spectra of Na-doped PA change in two stages with increasing dopant concentration, the first change corresponding to the increase in electrical conductivity and the second to the appearance of a Pauli susceptibility. The 1064-nmexcited FT-Raman spectrum of Na-doped PPV indicates existence of negative bipolarons which are equivalent to divalent anions extending over a few repeating units in the polymer chains. IN-I-R~DUCTION NEAR-IN FRARBD (NIR) Fourier transform (FT)-Raman spectroscopy is now the most widely used method for overcoming the fluorescence problem in Raman spectroscopy. It should be noted that this method has another advantage of obtaining resonant Raman spectra of materials having NIR absorptions, such as conducting polymers, chargetransfer complexes, biological pigments, etc. It is the purpose of this paper to demonstrate the potential of 1064-rim-excited Raman spectroscopy in the characterization of conducting polymers.
Since organic polymers are typical insulators, their inability to carry electricity has been utilized in their various applications. However, a new class of organic polymers with the ability to conduct electricity has been developed for the past 15 years. These conducting polymers have conjugated n-electrons in common. They show high electrical conductivity (1 -lo5 S cm-', S=S2-') when doped with electron acceptors such as iodine, AsFs, H.$O.,, etc. or electron donors such as alkali metals, although they are insulators or semiconductors in the undoped (neutral) state [l]. The maximum conductivity reported so far for organic polymers at room temperature is more than lti S cm-' [2,3], which is comparable with the conductivity of copper (6 x 10s S cm-').
When electrons are removed from (or added to) n-conjugated polymer chains, structural changes supposedly extending over several repeating units occur. These doped-domain structures are classified into polarons, bipolarons, and solitons according to their types, and have been extensively discussed in relation to their roles in electric conduction [4]. Electronic absorptions associated with the doped domains often appear in the region from visible to NIR. Accordingly, NIR resonant Raman spectroscopy may give a clue to structural studies of the doped domains.
In this paper, we report and discuss the 1064-nm-excited IT-Raman spectra of trunspolyacetylene (PA), poly( 1 ,Cphenylene) (PPP) , poly(1 ,Cphenylene vinylene) (PPV) and poly(2,5_thienylene vinylene) (PTV) in both the neutral (insulating) and doped (conducting) states. Their structures are shown in Fig. 1. Samples EXPERIMENTAL &Rich PA films were synthesized according to SHIRAKAWA'S method [S] at -78ยฐC and thermally isomerized to truns-PA at 180ยฐC for 60 min in vacua. PPP was prepared by polycondensation of the Grignard reagent from l&dibromobenzene with an NiClz catalyst 163. PPV and PTV were synthesized via soluble precursors according to the methods in the literature [7,8].