π‑Conjugation
and End Group Effects in Long Cumulenes: Raman Spectroscopy and DFT
Calculations
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Abstract
We
have investigated the structure and spectroscopic properties of cumulenic
carbon chains, focusing on the peculiar π-conjugation properties
and end-group effects that influence their behavior. With support
from Density Functional Theory (DFT) calculations, we have analyzed
the IR and Raman spectra of cumulenes characterized by different end-capping
groups and we have related them to the bond length alternation (BLA)
pattern and local spectroscopic parameters associated with the CC
bonds along the sp-carbon chain. For cumulenes we observe a breakdown
of the correlation existing in polyynes among frequencies, Raman intensities
of the R line (longitudinal
CC stretching modes), and BLA. While the low R line
frequency and equalized CC bonds would indicate the “metallic”
character of cumulenic species, we obtain an unusually strong Raman
intensity, which is typical of bond-alternated (semiconductive) structures.
DFT calculations reveal that this is a consequence of π-electron
conjugation, which markedly extends from the sp-carbon chain to the
aryl rings belonging to the end groups. These findings suggest the
existence of a strong electronic, vibrational and structural coupling
between sp-carbon chains and sp<sup>2</sup>-carbon species, which
could play a key role in nanostructured sp/sp<sup>2</sup>-hybrid carbon
materials (e.g., linear carbon chains coupled to graphene domains).
Within this context, Raman spectroscopy is a valuable tool for the
detailed characterization of the molecular properties of this kind
of materials