Novel bis(biphenyl)-capped polyynes have been synthesized to investigate the modulation of the electronic and optical properties of sp-hybridized carbon-atom wires (CAWs) capped with π-conjugated sp 2 endgroups. Raman and Surface Enhanced Raman spectroscopy (SERS) investigation of these systems and Density Functional Theory (DFT) calculations reveal structural changes from polyyne-like with alternating single-triple bonds towards 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 possibility to drive a semiconductor-to-metal transition has been investigated by systematic DFT calculations on a series of CAWs capped with different conjugated endgroups revealing that the modulation of the structural, electronic and vibrational properties of the sp-carbon chain towards the metallic wire cannot be simply obtained by using extended π-conjugated sp 2 carbon endgroups, but require a suitable chemical design of the endgroup and control of charge transfer. These results provide useful guidelines 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.KEYWORDS: sp-carbon, polyyne, cumulene, carbyne, Bond length alternation; Raman Spectroscopy; SERS; Density Functional Theory; electronic band gap Fullerenes, nanotubes and graphene represent a chief example of the versatility of carbon in producing nanostructures with different properties and dimensionality (zero, quasi-one and two, respectively).Such systems are mainly based on sp 2 hybridization, only one of the three different possibilities (sp, sp 2 and sp 3 ) available for carbon, two of which can also form stable allotropes occurring in nature (i.e., sp 2 and sp 3 for graphite and diamond, respectively). 1 The long quest for the still lacking "third carbon allotrope" based on sp-hybridization has led to the development of many interesting nanoscale/molecular systems in the form of carbon-atom wires (CAWs). [2][3][4][5] CAWs are linear systems with finite length trying to approach the ideal infinite wire (i.e., carbyne) representing the ultimate 1-D carbon system with intriguing properties, as evidenced by theoretical predictions [6][7][8] . The ideal carbyne can be in two different configurations: semiconducting wire with alternate single-triple bonds (i.e., polyyne) and metallic wire with all double bonds (i.e., cumulene). As finite systems, CAWs display structural electronic and optical properties strongly dependent on the length (i.e., number of carbon atoms) and the type of functional termination (i.e., endgroup) and are thus attract...

sp-Hybridized carbon atomic wires are appealing systems with large property tunability. In particular, their electronic properties are intimately related to length, structure, and type of functional end-groups as well as to other effects such as the intermolecular charge transfer with metal nanoparticles. Here, by a combined Raman, Surface Enhanced Raman Scattering (SERS) investigation and first principles calculations of different N,N-dimethylanilino-terminated polyynes, we suggest that, upon charge transfer interaction with silver nanoparticles, the function of sp-carbon atomic wire can change from electron donor to electron acceptor by increasing the wire length. In addition, the insertion into the wire of a strong electrophilic group (1,1,4,4-tetracyanobuta-1,3-diene-2,3-diyl) changes the electron-accepting molecular regions involved in this intermolecular charge transfer. Our results indicate that carbon atomic wires could display a tunable charge transfer between the sp-wire and the metal, and hold promise as active materials in organic optoelectronics and photovoltaics.