RAMAN SCATTERING INDUCED BY UNDOPED AND DOPED POLYPARAPHENYLENE

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1 RAMAN SCATTERING INDUCED BY UNDOPED AND DOPED POLYPARAPHENYLENE S. Krichene, S. Lefrant, G. Froyer, F. Maurice, Y. Pelous To cite this version: S. Krichene, S. Lefrant, G. Froyer, F. Maurice, Y. Pelous. RAMAN SCATTERING INDUCED BY UNDOPED AND DOPED POLYPARAPHENYLENE. Journal de Physique Colloques, 1983, 44 (C3), pp.c3-733-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1983 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colioque C3, supplement au n a 6, Tome 44, juin 1983 page C3-733 RAMAN SCATTERING INDUCED BY UNDOPED AND DOPED POLYPARAPHENYLENE # * ** **,., ** S. Krichene, S. Lefrant, G. Froyer, F. Maurice and Y. Pelous *Laboratoire de Physique Cvistalline (E.R.A. n IS du CNRS), Bat. 490, Universite de Paris-Sud, Orsay, France **C.N.E.T. HOC/TIC, Lannion B, Lannion Cedex, France Résumé - Nous présentons les résultats de diffusion Raman obtenus sur des échantillons de polyparaphénylène préparés par deux méthodes différentes. Les meilleurs spectres sont ceux fournis pour des excitations laser U.V. et les bandes Raman observées sont à 1216 cm~l, 1276 cnt* et 1598 cm~l (pour X _ = 334,8 nm). Une comparaison avec les spectres Raman induits par les oligomèresnous permet de corréler l'intensité de la bande Raman à = 1220 cm - ''- avec le nombre de noyaux phënyl qui composent le matériau étudié. Abstract - Raman scattering results on polyparaphenylene samples prepared by two different methods are presented. The best spectra are obtained with an excitation wavelength in the U.V. range and the Raman bands are peaked at 1216 cm-1, 1276 cm -1 and 1598 cm"! for X L = nm. A comparison with Raman spectra induced by oligomers leads to a correlation between the intensity of the Raman band at = 1220 cm~l and the number of phenyl rings present in the studied material. Polyparaphenylene (hereafter referred to as PPP) is now being studied in details because of its applicability to electronic or battery devices when doped with appropriate dopants (1). Both methods used for the synthesis of the material lead to a powder which must be pressed into pellets for light scattering experiments. We used excitation lines ranging from U.V. (A[_ = nm) to deep red (X _ = nm) on pristine and ASF5 doped PPP. Raman features were found independant on the temperature, but we used liquid nitrogen whenever possible. I - EXPERIMENTAL RESULTS The samples have been prepared by two different methods (Kovacic and Yamamoto) and characterized, as described in details elsewhere (2). a/ Raman results on PPP "Kovacic" With laser excitations in the visible range (from nm to nm), a very intense fluorescence is observed. This fluorescence hides almost completely the Raman signal except for a few wavelengths in the blue-green range. For example, Figure 1 shows the Raman spectrum of a PPP pellet, handled in liquid nitrogen, after we have substracted most of the fluorescence. A very weak band is seen at = 1608 cm"l. If the excitation laser wavelength is tuned to A _ = nm (3), the fluorescence has disappeared and the Raman spectrum is much stronger. It must be noticed that the absorption maximum (presumably due to a IT * ir* transition) is found at 380 nm and we are therefore in resonance conditions. In this case, Raman bands are clearly observed at 1216, 1276 and 1598 cm" 1. b/ PPP "Yamamoto" As in the previous case, the fluorescence is very strong with any excitation in the visible range, particularly in the blue-green region. Nevertheless, it goes down appreciably in the deep red region and again it can be substracted. Article published online by EDP Sciences and available at

3 C3-734 JOURNAL DE PHYSIQUE - I I t I cm' Figure 1 - Raman spectra of PPP Figure 2 - Raman spectra of PPP "Kovacic" Yamamoto at T = 20 C ; a/ AL = nm, T = 20 C a/ XL = nm ; b/ XL = nm b/ XL = nm, T = 78 K C/ AL = nm Figure 2 shows the results obtained for three different wavelengths (bands at 1222, 1282 and 1600 cm-1). c/ PPP llkovacicl doped with AsF5 (Figure 3) Raman spectra observed on doped PPP reveal the following features : - the fluorescence has disappeared completely in the whole range as shown on Figure 3. The Raman band at 1598 cm-1 does not shift in going from nm to nm. On the contrary, the two other ban s are now peaked at 1236 and 1320 cm-1 except when XL = nm (1234 and 1284 cm- f ). For the U.V. Laser excitation, the spectrum is similar to that of the undoped polymer. I1 - COMPARISON WITH OLIGOMERS For a comparison purpose, we have recorded Raman spectra of oligomers from biphenyl to sexiphenyl for XL = nm. These spectra are well known and in agreement with those reported in the literature. The purpose is not to discuss in details these spectra, but we want to notice the fact that one mode increases in intensity ( cm-1) when the number of phenyl rings increases, or at least the ratio between the intensity of this mode with the 1277 cm-1 one. The PPP "Yamamoto" reveals such a ratio larger than for the sexiphenyl crystal. This ratio is even larger for the,kovacic polymer, although it may be somewhat speculative to compare this result obtained with an U.V. excitation wavelength.

4 Figure 3 - Raman spectra of AsF5 - doped PPP llkovacic" at T = 20 C ; a/ XL = nm ; b/ XL = nm ; c/ XL = nm ; d/ XL = 514 e/ XL = nm Figure 4 - Raman spectra of oligomers at T = 78 K, XL = nm. a/ biphenyl ; b/ terphenyl ; c/ quaterphenyl ; d/ sexiphenyl ; e/ PPP "Yamamoto" (T = 20 C) ; f/ Comparison with PPP Kovacic (XL = nm, T = 20 C).

5 C3-736 JOURNAL DE PHYSIQUE I11 - DISCUSSION AND CONCLUSION Both PPP samples studied,here show Raman bands at = 1220, 1280 and 1600 cm-l. These frequencies agree well with those previously reported (4). The assignment of these modes was given in this brief report by comparison with results from biphenyl crystals (5) and the displacement vectors for these vibrations are representated below : From the results reported by excitations in th whole range from nm to nm, it must be noticed that the 1600 cm- mode, which involves all the carbons between the phenyl rings, depends very little on the chain length of the polymer. Also, this mode does not shift in frequency when the sample is doped with AsF5. These results seem to exclude a pure quinoid structure of the PPP polymer. By comparison with oligomers of known length, we observe that the ratio between the two modes at 1220 and 1271 cm-i depends on the number of phenyl rings, showing that the PPP grown by the Yamamoto method is shorter than the PPP polymerized by the Kovacic method, in agreement with U.V. and I.R. absorption experiments reported by Froyer et al. (2). Upon doping, the very strong fluorescence disappears. This tells us that the fluorescent centers themselves are presumably either destroyed or transformed by the dopant. The shifts of the 1220 cm-1 (to 1236 cm-1) mode and 1277 cm-1 (to 1320 cm-l) of doped pellets are unexplained in the absence of a vibrational model. When the laser line is tuned in the absorption band, we gradually observe the modes of the undoped material and this phenomenon can be an effect of an inhomogeneous doping. Acknoled~ment.We are indebted to Dr.C.Fabre for provided the sexiphenyl crystal. REFERENCES (1) SHACKLETTE L.W., CHANCE R.R., IVORY D.N., MILLER G.G. and BAUGHMAN R., Synth. Metals, 1(1979)307 (2) FROYER G., GOBLOT J.Y., GUILBERT J.L., MAURICE F. and PELOUS Y., This journal, Proceedings poster no 28 D, in Press (1983) (3) LEFRANT S., FAULQUES E., KRICHENE S., and SAGON G., Submitted to Polymer Journal (1983) (4) TZINIS C.H., BAUGHMAN R.H. and RISEN W.M.,Jr., ONR Technical Report no TR (1981) (5) ZERBI G. and SANDRONI S., Spectrochim. Acat - 24 A(1968)511 f