π-Conjugation and End Group Effects in Long Cumulenes: Raman Spectroscopy and DFT Calculations

We have investigated the structure and spectroscopic properties of cumulenic carbon chains, focusing on the peculiar pi-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 line (longitudinal CC stretching modes), and BLA. While the low 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 pi-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(2)-carbon species, which could play a key role in nanostructured sp/p(2)-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.