Three Distinct Torsion Profiles of Electronic Transmission through Linear Carbon Wires

Abstract

The one-dimensional carbon allotrope carbyne, a linear chain of sp-hybridized carbon atoms, is predicted to exist in a polyynic and a cumulenic structure. While molecular forms of carbyne have been extensively characterized, the structural nature is hard to determine for many linear carbon wires that are made in-situ during pulling experiments. Here, we show that cumulenes and polyynes have distinctively different low-bias conductance profiles under axial torsion. We analyze the change of the electronic structure, Landauer transmission, and ballistic current density of the three types of closed-shell molecular carbynes as a function of the torsion angle. Both polyynic, odd-carbon cumulenic, and even-carbon cumulenic carbon wires exhibit helical frontier molecular orbitals when the end-groups are not in a co-planar configuration. This helical conjugation effect gives rise to strong ring current patterns around the linear wires. Only the transmission of even-carbon polyynic wires follows the well-known cosine-squared law with axial torsion that is also seen in biphenyl-type systems. Notably, the transmission of even-carbon cumulenic carbon wires rises with axial torsion from co-planar towards perpendicular orientation of the end-groups. The three distinct transmission profiles of polyynes, odd-carbon cumulenes, and even-carbon cumulenes may allow for experimental identification of the structural nature of linear carbon wires. Their different electron transport properties under axial torsion furthermore underline that, in the molecular limit of carbyne, three different subclasses of linear carbon wires exist. File list (3)download file view on ChemRxiv torsion_chemrxiv3.pdf (6.17 MiB) download file view on ChemRxiv SI_chemrxiv3.pdf (9.24 MiB) download file view on ChemRxiv files.zip (21.38 MiB)

Are Current Discontinuities in Molecular Devices Experimentally Observable?

“…For example, if a fraction η of a charge q oscillating along the z-axis over a distance 2a lacks a corresponding current, the predicted maximum longitudinal field (along z) is E L,max = 2ηω 2 qa/(c 2 r). It is tempting to speculate that this kind of oscillations could happen in graphene-based nanojunctions or carbon nanowires [21,22,[44][45][46].…”

“…In a series of papers, starting in 2018 [18][19][20][21], Jensen, Garner and collaborators have analysed in depth the concept of density current in quantum transport, applying it to specific molecules and obtaining results compatible with the approach by Wang et al…”

“…In [20,21], linear carbon wires are considered. Detailed plots of the integrated local current density are given, as compared to the (constant) total current.…”

Helical Electronic Transitions of Spiroconjugated Molecules

“…This strain alters the energetics of the frontier MOs considerably by unwinding or overwinding the helices. 13,21 The chiroptical response is increased in S-1,4-butadiylspiropentadiene and the equivalently constrained allene (see Figure S6 and S7). This strained spiropentadiene species sees has a strong chiroptical response compared to the dimethylspiropentadiene.…”

“…Still, relations to molecular properties have been proposed. 12,[18][19][20][21][22][23] Given that the through-bond electron delocalization in spiroconjugated molecules is closely related to that in allene (Figure 1), an interesting question is whether different types of p-systems are also limited by the dual-helicity of the MOs?…”