J.P. Buisson

Theoretical and experimental vibrational study of emeraldine in salt form. Part II

We present a complete study of the conductive form of the polyaniline emeraldine salt. The experimental spectra of emeraldine salt doped in the classical way and also by the action of camphorsulfonic acid–m-cresol are presented. Vibrational calculations on emeraldine salt are presented, considering the non-planar geometry of the polymer. A study of the influence of the torsion angle between cycles is proposed. Finally, a complete assignment of the Raman vibrational modes of emeraldine salt is discussed. Copyright

Vibrational spectroscopic studies of the isotope effects in polyaniline

Comparative Raman spectroscopy studies of nondeuterated polyaniline and polyaniline deuterated on the ring and on the nitrogen enabled us to confirm the assignments of principal Raman modes. Spectroelectrochemical studies carried out for these two isotopically different polymers indicate the creation of two types of structures upon electrochemical doping, namely polaron lattice and bipolaronic structure.

Low-frequency Raman studies of multiwalled carbon nanotubes: Experiments and theory

In this paper, we investigate the low frequency Raman spectra of multiwalled carbon nanotubes (MWNTs) prepared by the electric arc method. Low frequency Raman modes are unambiguously identified on purified samples thanks to the small internal diameter of the MWNTs. We propose a model to describe these modes. They originate from the radial breathing vibrations of the individual walls coupled through the van der Waals interaction between adjacent concentric walls. The intensity of the modes is described in the framework of bond polarization theory. Using this model and the structural characteristics of the nanotubes obtained from transmission electron microscopy, we simulated the experimental low frequency Raman spectra with excellent agreement. Therefore Raman spectroscopy can be as useful in the characterization of MWNTs as it is for single-wall nanotubes.

Comparison of the vibrational properties of polythiophene and polyalkylthiophenes

Abstract In this paper, we present a study of vibrational properties of conducting polymers containing thiophene rings : polythiophene and polyalkylthiophenes. The spectrocopic studies include Resonance Raman Scattering (RRS) and Fourier Transformed InfraRed absorption (FTIR), that we applied to the polymers mentionned above and to some thiophene oligomers. Results are interpreted by performing dynamical calculations based on a valence force field model. With the oligomer approach, the force constants are less numerous than the experimental frequencies and the results for our assignment much more reliable. The calculations provide a better understanding of Raman and infrared modes, leading to a clear comparison of the electronic and vibrational properties of neutral polythiophene, polymethylthiophene and polyoctylthiophene.

Vibrational studies of polyparaphenylene-vinylene (PPV)

Abstract In this paper, we present a complete vibrational study of polyparaphenylene-vinylene (PPV) using Resonance Raman Scattering (RRS) and Infrared spectroscopy. A dynamical model based on valence force-field calculations has been built in order to assign the observed Raman and IR vibrational modes, and trans-stilbene served as a model compound to establish a good set of force constants for the neutral polymer. Upon doping with FeCl 3 , the Raman spectra exhibit rather strong modifications. In order to obtain a good agreement with experimental data of doped PPV, a new set of force constants is needed which supports the existence of a quinoid-like structure along the PPV chains.

Interpretation of vibrational studies in phenyl compounds: From oligophenyls to polyparaphenylene

Abstract Among the conducting polymers extensively studied, those presenting a non-degenerated ground state such as polyparaphenylene, polypyrrole, polythiophene… have attracted a real interest. Although they cannot accommodate soliton-like defects, they present transport properties as interesting as in polyacetylene since an n- or p-doping can increase the conductivity by several orders of magnitude. Experimental Raman and infrared results have been recently presented for polyparaphenylene (PPP) polymerized using different procedures such as those developed by Kovacic or Yamamoto. In particular, a comparison between oligophenyls and PPP can be made. The Raman spectra of these samples are essentially composed of a strong band at ≅ 1600 cm −1 and weaker ones at 1220 and 1280 cm −1 whose frequencies are practically independent on the chain length. On the contrary, relative intensities can vary from one sample to the other since, for example, the ratio between the integrated intensity of the 1220 and 1280 cm −1 Raman modes increases with the number of phenyl rings. Also, from biphenyl to PPP, relative intensities between modes around 1600 cm −1 are observed to vary. In this paper, we present a vibrational model able to interpret these experimental data. The model is based on the use of a number of force constants defined in terms of internal co-ordinates. Starting from force constants used to fit Raman and IR modes in benzene and by adding an appropriate number of parameters, a reasonable agreement has been obtained for all the series from biphenyl to PPP. The reasonant behavior of the Raman spectra of AsF 5 -doped PPP allows the assumption that shifted modes could be associated with a quinoid character on the polymer chain. This interpretation is in agreement with the band gap calculations performed by Bredas.

Vibrational analysis of the reduced form of polyaniline: the leucoemeraldine base

Abstract Extensive studies have been carried out on the different forms of polyaniline. In this paper, we present a study of the vibrational properties of the leucoemeraldine base, the reduced form of polyaniline. Infrared absorption and resonant Raman scattering (RRS) spectra are presented for the polymer, together with those obtained in model compounds such as diphenylamine and N , N ′-diphenyl-1,4-phenylenediamine. The Raman spectrum of the leucoemeraldine base exhibits mainly bands at 1181, 1219, 1597 and 1618 cm −1 . By the use of a valence-force-field model, we have assigned these vibrational modes, including those of the model compounds. Calculations are in rather good agreement with previous assignments and the main force constants, which are determined from the fit of the experimental IR and Raman bands, are physically reasonable compared to what is found in other polymers.

Tuning the Raman resonance behavior of single-walled carbon nanotubes via covalent functionalization.

We present a systematic Raman study over a range of excitation energies of arc discharge single-walled carbon nanotubes (SWCNTs) covalently functionalized according to two processes, esterification and reductive alkylation. The SWCNTs are characterized by resonance Raman spectroscopy at each step of the functionalization process, showing changes in radial breathing mode frequencies and transition energies for both semiconducting and metallic tubes. Particular attention is given to a family of tubes clearly identified in the Kataura plot for which we continuously tune the excitation energy from 704 to 752 nm. This allows us to quantify the energy shift occurring in the spacing of the van Hove singularities. We demonstrate that, independently of the functionalization technique, the type of chain covalently bound to the tubes plays an important role, notably when oxygen atoms lie close to the tubes, inducing a larger shift in transition energy as compared to that of other carbonaceous chains. The study shows the complexity of interpreting Raman data and suggests many interpretations in the literature may need to be revisited.

In situ resonant Raman and optical investigations of p-doped poly (p-phenylene vinylene)

Abstract In this paper, we present spectroscopy studies of poly (p-phenylene vinylene) p-doped by chemical or electrochemical ways with SO3− and BF4−, respectively. Since very few experimental data are available for in situ Raman studies of electrochemically doped samples, we have essentially used this technique in different resonance conditions (resonance Raman scattering (RRS)) to carry out our investigations using different excitation wavelengths. Several modifications in the Raman spectra of doped PPV compared to neutral PPV are analyzed and interpreted by the coexistence of polaronic and bipolaronic species in the sample. These results are illustrated in particular by gradual shifts in the frequency of carbon–carbon stretching vibrations, explained in terms of a quinoid character growing at the expense of the benzenoid one when the doping level is increased.

Vibrational studies of poly(paraphenylene): comparison with oligophenyls

Abstract Experimental Raman and infrared results are recalled for poly( pera -phenylene) (PPP) polymerized using different procedures. A comparison is made between oligophenyls and PPP. Raman spectra of these samples are essentially composed of a strong band at 1600 cm −1 and weaker ones at 1220 and 1280 cm −1 . These frequencies are practically independent of the chain length, but the relative intensity between the 1220 and 1280 cm −1 modes is found to depend on the number of phenyl rings, on the excitation wavelength and on the concentration level in the case of doped systems. We present a vibrational model able to interpret these experimental data, based on the use of a number of force constants defined in terms of internal coordinates. Starting from force constants used to fit Raman and i.r. modes in benzene and by adding an appropriate number of parameters, a reasonble agreement has been obtained for all the series from biphenyl to PPP. In view of the theoretical results, arguments are given concerning the β-electron localization along the chains.