Matteo Tommasini

Raman spectroscopy of polyconjugated molecules and materials: confinement effect in one and two dimensions

The effect of the confinement of π electrons in one– and two–dimensional domains is illustrated with several examples ranging from linear polyene chains to planar molecules with honeycomb structure. Theoretical computations and specific Raman experiments on molecular materials demonstrate that a molecular approach provides a unified key to the interpretation of the Raman response both of linear polyconjugated polymers (polyacetylene) and of nanostructured graphitic materials.

Relationship between infrared and Raman intensities in molecules with polarized π electrons

Abstract A model is presented which allows to obtain a linear relationship between infrared and Raman intensity parameters of the strongest vibrational bands of push–pull conjugated molecules. The results obtained clarify the origin of the exceptionally large values of the vibrational first hyperpolarizability shown by these molecules.

Evidence for solution-state nonlinearity of sp-carbon chains based on IR and Raman spectroscopy: violation of mutual exclusion.

Adamantyl-end-capped polyynes with chains of 4, 6, 8, 10, 12, 16, and 20 sp-hybridized carbons (C4-C20) have been synthesized and their IR and Raman spectra obtained. On the basis of violations of the mutual-exclusion principle between IR and Raman spectroscopy, spectral evidence demonstrates that these molecules possess a noncentrosymmetric molecular structure in both the solid and solution states. This premise is supported by X-ray crystallographic analysis of C12, which shows a bent, noncentrosymmetric structure in the solid state. Density functional theory (DFT) calculations for adamantyl-end-capped polyynes, in comparison with those for hydrogen-end-capped polyynes, show that the observed violation of mutual exclusion is independent of the end group of the polyyne chain (i.e., adamantyl versus H). The origin of these experimental spectroscopic observations is ascribed to the existence of dynamic contributions to molecular nonlinearity resulting from low-frequency skeletal bending vibrations of the chains and/or the existence of low-energy bent conformations of the polyyne chains, as DFT-optimized structures seem to suggest.

Resonance Raman contribution to the D band of carbon materials: Modeling defects with quantum chemistry

Polycyclic aromatic hydrocarbons (PAHs) are employed to model the Raman features that are generally associated with sp(2) nanostructures in carbon materials or with disorder and defects in graphitic materials. To this end molecular parameters (geometry changes upon electronic excitation, vibrational normal modes, and displacement parameters) are computed with semiempirical quantum-chemical methods for a series of PAHs ranging from 6 to 384 carbon atoms, and Raman intensities are evaluated according to Albrechts formalism restricted to the A term. The computed preresonance and resonance Raman intensities are compared with available experimental data for hexa-peri-hexabenzocoronene and for pyrene. For the latter compound, simulations carried out at semiempirical and ab initio levels of theory are shown to be of comparable quality. Finally, the collection of displacement parameters computed for the sample of conjugated molecules is used to model the effect of disorder and defects in the Raman response of a carbon material containing sp(2) islands. It is shown that the computed D-band frequency dispersion, with respect to excitation wavelength, reproduces closely the experimental data measured for sp(2) hybridized carbon materials.

Stabilization of linear carbon structures in a solid Ag nanoparticle assembly

Linear sp carbon nanostructures are gathering interest for the physical properties of one-dimensional systems. At present, the main obstacle to the synthesis and study of these systems is their instability. Here the authors present a simple method to obtain a solid system where linear sp chains (i.e., polyynes) in a silver nanoparticle assembly display a long term stability at ambient conditions. The presence and the behavior of linear carbon are investigated by surface enhanced Raman scattering exploiting the plasmon resonance of the silver nanoparticle assembly. This model system opens the possibility to investigate an intriguing form of carbon nanostructures.

Low-frequency modes in the Raman spectrum of sp-sp2 nanostructured carbon

Amorphous carbon films with

Raman and SERS recognition of β-carotene and haemoglobin fingerprints in human whole blood

sp\text{\ensuremath{-}}s{p}^{2}

Metal-filled carbon nanotubes as a novel class of photothermal nanomaterials.

hybridization, produced by supersonic cluster beam deposition, shows the presence of both polyynic and cumulenic species [L. Ravagnan et al., Phys. Rev. Lett. 98, 216103 (2007)]. Here, we present an in situ Raman characterization of the low-frequency vibrational region

A relationship between Raman and infrared spectra: the case of push-pull molecules

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Toward carbyne: Synthesis and stability of really long polyynes

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