Jean-Yves Mevellec

A spectroscopic study of the fluorination and defluorination reactions on single-walled carbon nanotubes

The manipulation of isolated single-wall carbon nanotubes (SWNTs), rather than bundles, has led to active research in the field of the functionalisation of such carbon compounds. Different ways exist today to obtain some new soluble macromolecules from SWNTs. Here we focus on the fluorination functionalisation. As the solubility properties depend essentially on the degree of functionalisation, it is important to develop reliable and simple methods to quantify this. The CnF stoichiometry of three different fluorinated SWNTs samples are determined with X-ray photoelectron spectroscopy (XPS). Then the evolution of the Raman spectra with the degree of fluorination of these samples is discussed. An atomic force microscopy (AFM) study highlights the good solvation properties of the most fluorinated sample with a majority of isolated nanotubes being observed. Then we take advantage of these good solvation properties, combined with the possibility of recovering the pristine non-fluorinated nanotubes, to carry out surface enhanced Raman spectroscopy (SERS) studies of well-dispersed SWNTs. These studies put in evidence the bundle effect, which is due to the agglomeration of SWNTs into bundles. This effect can be readily observed by Raman spectroscopy.

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.

Role of polyenes in the coloration of cultured freshwater pearls

The exact nature of pigments present in cultured freshwater pearls is still not well known. We examined 21 untreated cultured freshwater pearls from Hyriopsis of typical colors by diffuse reflectance UV-Vis-NIR and Raman scattering measurements, at high resolution. The objective was to establish the relation between color and the nature of the pigment mixture in pearls, using strictly non-destructive methods. All natural color samples show the two major Raman resonance features of unmethylated (unsubstituted) polyenes, not carotenoids. Their general chemical formulae are R-(-CH=CH) N -R′ with N = 6 to 14 and they give absorptions from violet to yellow–green. Each color is due to a mixture of pigments, not a single pigment. Different colors are explained by different mixtures. Each pigment identified by Raman spectroscopy can be related to a specific absorption with apparent maximum in the range 405–568 nm, thus absorbing from violet to yellow–green. This is the first study of the precise relation between Raman and diffuse reflectance spectra of cultured freshwater pearls.

SERS studies on single-walled carbon nanotubes submitted to chemical transformation with sulfuric acid

Abstract Surface-enhanced Raman scattering (SERS) at 676.44 nm and 1064 nm excitation wavelengths was used to investigate chemical transformation of single-walled carbon nanotubes (SWNTs) deposited on a gold support. Sulfuric acid was used as the chemical reagent. Special attention was paid to the changes in the Raman bands associated to radial and tangential vibration modes. Partial restoration of the Raman spectra by a subsequent alkaline treatment indicates a transformation with a certain degree of reversibility. The recovery reaction achieved with a 0.5 M KOH solution showed that the variations of tangential and radial band groups are not correlated. The intensity changes of the radial bands is a principal indicator for the chemical transformation of the SWNTs. Particular attention was paid to radial bands at 164 and 176 cm−1, observed with 1064 nm and 676.44 nm excitation wavelength, respectively, and their 14 cm−1 up-shifted replicas i.e. the bands at 178 and 190 cm−1. A different behavior of these bands in the anti-Stokes side was observed.

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.

Raman study of α-oligothiophenes and model compounds of poly(thienylene vinylene)

Abstract In this work, we present a complete vibrational study of a series of the α-coupled oligothiophenes. Both experimental and theoretical investigations have been carried out by using spectroscopic techniques such as Raman Scattering and infrared absorption and analysed as a function of the oligomer length. Detailed calculations lead to a clear assignment of the vibrational modes in all compounds and to determination of a main force constants associated with the different chemical bonds. In a similar way, Raman studies of model compounds of poly(thienylene vinylene) (PTV), such as 1–2 bis (thienylene) vinylene (2TV) and 2–5 bis (2-thienyl vinyl) thiophene (3T2V) have been carried out. The two modes associated with the CC stretching of the vinyl group and the thiophene ring respectively are found to slightly depend on the length of the compounds.

Theoretical analysis of the Raman spectra of PTV and PFV

Abstract In this work, we report experimental Raman and IR results obtained on two different polymers composed of heteroaromatic rings separated by a vinyl group, namely poly(thienylene vinylene) (PTV) and poly(furylene vinylene) (PFV). PTV and PFV exhibit a rather similar behaviour and, in particular, in both cases the observed Raman bands undergo a slight shift when the excitation wavelength is changed from 676.7 to 457.9 nm. By using a dynamical model based on valence force field calculations, we have assigned all IR and Raman modes. In order to obtain a good set of force constants, we started calculations by considering those of polythiophene and, also, we used a model compound such as trans -bis(2-thienyl)ethylene. It was found that the numerical values given for these force constants are consistent with the electronic structure usually assigned for these polymers.

Photochromic Organic Nanoparticles as Innovative Platforms for Plasmonic Nanoassemblies

The fabrication of hybrid core-shell nanoassemblies involving a nondoped azo photochromic core coated with a dense shell of gold nanoparticles is reported to investigate the influence of localized plasmons onto the azo core photoisomerization. Photochromic organic nanoparticles, regarded as a novel class of high-density photoswitchable nanomaterials, are first elaborated upon precipitation in water of push-pull azo molecules, containing sulfur-terminated units to chelate gold nanoparticles. Photoisomerization studies of the azo nanoparticles reveal significantly higher E → Z photoconversion yields and Z → E thermal back relaxation rate constants compared to those of dyes processed as thin films and in solution, respectively. These unexpected results are ascribed to the large surface-to-volume ratio and cooperative effects encountered in nanoparticles that deform without disassembling under polarized illumination as a result of the weak change in the azo dipole moment. UV-vis spectroscopy and Raman microscopy of the hybrid nanoassemblies show strong optical coupling between both photoactive constituents, confirming that gold nanoparticles are tightly positioned on the azo core surface. Such coupling causes partial quenching of the azo photoisomerization but does not impact the thermal back relaxation. Longer sulfur-terminated chains provide reduced quenching of the photoreaction by the localized plasmons, thereby opening perspectives toward plasmon-mediated deformation of nano-objects for light-controlled nanomechanics.

Comparative study of the vibrational properties of polymers with phenyl ring

Abstract We present and compare experimental Raman and infrared spectra of polyparaphenylene (PPP), polyphenylene-vinylene (PPV), polyphenylene-sulfide (PPS) and polyphenylene-oxide (PPO). To interpret these results, we have performed dynamical calculations based on a valence force field model, in order to fit successively : i) benzene; ii) benzene, biphenyl and PPP; iii) benzene, trans-stilbene and PPV. We obtain a good fit for all modes, except for the Raman one at 1220 cm −1 in PPP, for which no dispersion from one compound to other is calculated, contrary to the experimental results. The force constants are consistent with chemical structures of the compounds, and in particular those of the phenyl rings are found very close for all materials.

Experimental and theoretical investigation of the vibrational properties of poly(thienylene vinylene) (PTV) and poly(furylene vinylene) (PFV)

Abstract In this work, a complete assignment of Raman and IR modes is reported for both poly(thienylene vinylene) and poly(furylene vinylene) whose chains are composed of an alternation of aromatic rings and vinyl groups. In order to achieve these attributions, we have developped a valence force field model. The vibrations of an aromatic ring show some similarities in a series of compounds with the same cycle. So the same set of parameters is used to calculate vibrational modes for both polymers and model compounds such as trans-bis(2-thienyl)ethylene, a molecule composed of two thiophene rings separated by a vinyl group. Therefore, we present an experimental comparison between Raman spectra of poly(thienylene vinylene), thiophene and trans-bis(2-thienyl)ethylene, and the assignment of the vibrational modes in the latter molecule is also discussed.