Structure and chain polarization of long polyynes investigated with infrared and Raman spectroscopy

Abstract

Symmetrically and unsymmetrically endcapped polyynes (triisopropylsilyl - TIPS - and super trityl endcapping groups) have been studied with density functional theory calculations and Raman and infrared spectroscopy. This spectroscopic investigation provides general guidelines for studying the vibrational and structural features of sp-carbon-based materials, which nowadays are at the center of growing interest for the potential applications of nanostructured carbon materials in molecular electronics. On the basis of the description of CC stretching modes of polyynes in terms of longitudinal optical phonons of the parent infinite sp-carbon chain, we have carried out the complete assignment of the observed infrared and Raman features in the 2300-1900 cm-1 range, for chain lengths in the range 4-20 triple bonds. This allows demonstration of polarization effects induced by an unsymmetric structure and deviations from chain linearity in long polyynes

Carbon-atom wires: 1-D systems with tunable properties

“…Hence the nuclear displacements associated with the LO phonon can be regarded as a collective BLA oscillation corresponding to the ECC mode. The frequency associated with this phonon can be analytically determined in terms of bond stretching force constants, 54,55,57,76 effectively demonstrating the role of long-range interactions in determining the marked frequency decrease in the phonon dispersion (at q = 0) as BLA approaches zero. This calculation can be conveniently carried out by adopting the infinite chain model with a unit cell of length c containing two carbon atoms and by following Wilson's GF method 72 generalized to periodic systems:…”

“…76 The strong Raman line (ECC mode) that is found on the LO dispersion corresponding to j = 1 for long wires is essentially equivalent to the q ≈ 0 phonon. 54,73,76 In finite polyynes, IR modes are observed for the values j > 1, and these can be simply considered as higher "harmonics" of the Raman active j = 1 mode (see Fig. 11).…”

“…However, the use of periodic boundary conditions along the chain axis has a non-negligible influence on the spectroscopic selection rules: deviations of the chain structure from linearity, 75 endgroup effects, 61,76 and Peierls distortion (cumulene vs. polyyne) in long C n wires require relaxing the infinite chain model, as described in the next section.…”

“…76 The assignment of these IR and Raman features by DFT calculations allows for drawing the corresponding phonon dispersion relationship. 76 This shows that the relationships qualitatively follow theoretical expectations, 54,73 and it exhibits the behaviour represented in Fig. 7 (left panel).…”

“…11). 75,76 The dependence of the vibrational properties on BLA makes the interpretation of the Raman spectra a non-trivial issue, requiring the support of numerical simulations. The Fig.…”