Charge Transfer and Vibrational Structure of sp-Hybridized Carbon Atomic Wires Probed by Surface Enhanced Raman Spectroscopy

One-dimensional carbon atomic wires displaying sp hybridization have an appealing electronic and vibrational structure which profoundly affects their optical and transport properties. Here we investigated charge transfer in alternating triple single bond carbon atomic wires (polyynes) terminated by phenyl rings and its effects on the structure of the system. The occurrence of a charge transfer between carbon wires and metal nanoparticles (both in liquids and supported on surfaces) is evidenced by Raman and surface enhanced Raman scattering (SERS) as a softening of the vibrational stretching modes. This is interpreted, with the support of density functional theory (DFT) calculations of the Raman modes, as a modification of the bond length alternation of carbon atoms in the wire. As a consequence of the charge transfer, carbon wires rearrange their structure toward a more equalized geometry which corresponds to a tendency toward a cumulenic structure (i.e., all double bonds). These observations open potential perspectives for developing carbon-based atomic devices with tunable electronic properties.