Carbon-atom wires (CAWs) are linear, finite-length molecules, whose extreme limit is the so-called “carbyne” a 1-D carbon system for which outstanding properties have been predicted . As other polyconjugated systems, they display structural, electronic and opticalproperties strongly dependent on the length and the type of end-groups and are thus attractive for both fundamental and appliedscience. CAW properties can be tuned to have insulator−semiconducting−metallic functionalities, thus CAWs offer an appealing opportunity for developing functional nanostructures. For instance, carbon wires bridging two graphene edges and the development of nanoscale sp−sp2 hybrid systems are a promising approach for the exploitation of CAWs. In this framework, a fundamental and open issue regards how and to what extentthe properties of CAWs can be effectively modulated by controlling π-electron conjugation.
In this contribution, we explored increasingly π-conjugated CAWs terminated by sp2-carbon groups, as model systems suitable for investigating hybrid sp−sp2 structures such asgraphynes or graphene−wire−graphene systems. A combined experimental and computational approach, including Raman and surface enhanced Raman spectroscopy (SERS) experiments and density functional theory (DFT) calculations is employed to study structural changes and spectroscopy properties of CAWs. In particular, we focus on the polyyne-to-cumulene transition as a function of chemical substitution and/or charge transfer effects. Indeed, while polyynes have semiconducting electronic properties, more equalized systems tend to a cumulene-like structure with a nearly metallic behavior. The effect of different sp2 end groups in driving a semiconductor-to-metal transition is investigated on a series of CAWs capped with different groups. We first analyzed CAWs terminated by two biphenyl groups and then generalized the discussion to highlight the role of aromatic end groups of increasing size (i.e., phenyl,biphenyl, naphthyl, and coronene, also with oxygen substituents). By means of DFT calculations we show that theincrease of sp2 conjugation alone is not enough to endow the full tunability of properties. We highlight how a wide range tunability can be obtained instead by controlling charge transfer effects by proper chemical design (i.e., oxygen substitution) . These results provide a guideline for the design of novel sp−sp2 hybrid carbon nanosystems, 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−sp2 systems appealing for a future all-carbon-based science and technology.
 C.S. Casari et al. Nanoscale 2016, 8, 4414
 A. Milani et al. J. Phys. Chem. C 2017, DOI: 10.1021/acs.jpcc.7b02246