Connection among Raman wavenumbers, bond length alternation and energy gap in polyynes

Abstract

A theoretical model based on Hueckel theory is presented and applied to the infinite linear carbon chain. The effect of Peierls distortion, pi-electron confinement and electron–phonon coupling are interpreted on the basis of measurable parameters such as bond length alternation, electronic gap and Raman wavenumbers. A diagram is presented where the correlation among these quantities is shown on the basis of the model adopted. The effect of pi-electron confinement on experimental data can be directly evaluated, and insights on the physical properties of these materials can be derived

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

“…The reason for this behaviour lies in the interplay between the activation of out-of-Γ normal modes and the molecular parameters governing the Raman activity. 71 This originates in cumulenes a detectable Raman signal for LO modes other than ECC, as shown in Fig. 12.…”

“…[66][67][68][69][70] In this field, Raman spectroscopy certainly plays a major role (see for instance ref. [71][72][73][74][75] and references therein). The Raman spectra of π-conjugated polymers are usually dominated by intense Raman lines, which are related to BLA oscillation.…”

“…A simple model based on Hückel theory (i.e., tight binding restricted to π-electrons) can be introduced to better quantify these effects, 55,62,71 leading to the following formulation of the matrix elements of the F matrix of CC stretching force constants:…”

“…53 The model also allows for introduction of the correlation that exists among the BLA, LO phonon wavenumber, and band gap in CAWs, which formally defines the connection between structural, electronic, and vibrational properties. 71 The infinite chain model provides a reliable interpretation of the experimental data and captures the main trends observed for the electronic and vibrational properties, thus offering a useful unified framework for the understanding of real systems.…”

“…The connection between the properties of finite-length CAWs and the ideal infinite polymers was characterized in different studies by using DFT calculations 54,73 or tightbinding models, 55,62,71,85 reconciling the approaches usually found in the Physics and Chemistry communities. The same methodologies were applied to the characterization of new synthetic series polyynes bearing suitable endgroups.…”