Semiconductor-to-Metal Transition in Carbon-Atom Wires Driven by sp2 Conjugated End Groups

Bis(biphenyl)-capped polyynes are investigated to unveil the modulation of the electronic and optical properties of sp-hybridized carbon atom wires (CAWs) capped with pi-conjugated sp(2) end groups. Raman and surface enhanced Raman spectroscopy experiments and density functional theory (DFT) calculations reveal structural changes from polyyne-like with alternating single-triple bonds toward cumulene-like with more equalized bonds as a consequence of the charge transfer occurring when wires interact with metallic nanoparticles. While polyynes have semiconducting electronic properties, a more equalized system tends to a cumulene-like structure characterized by a nearly metallic behavior. The effect of different sp(2) end groups in driving a semiconductor-to-metal transition is investigated by DFT calculations on a series of CAWs capped with different terminations. We discuss how the modulation of the structural, electronic, and vibrational properties of the sp-carbon chain toward the metallic wire is not trivial and requires a suitable chemical design of the end group and control of charge transfer. These results provide a guideline for the design of novel sp-sp(2) hybrid carbon nanosystems with tunable properties, where graphene-like and polyyne-like domains are closely interconnected. The capability to tune the final electronic or optical response of the material makes these hybrid sp-sp(2) systems appealing for a future all-carbon-based science and technology.