Cumulene Wires Display Increasing Conductance with Increasing Length

Abstract

One-dimensional sp-hybridized carbon wires, including cumulenes and polyynes, can be regarded as finite versions of carbynes. They are likely to be good candidates for molecular-scale conducting wires as they are predicted to have a high-conductance. In this study, we first characterize the singlemolecule conductance of a series of cumulenes and polyynes with a backbone ranging in length from 4 to 8 carbon atoms, including [7]cumulene, the longest cumulenic carbon wire studied to date for molecular electronics. We observe different length dependence of conductance when comparing these two forms of carbon wires. Polyynes exhibit conductance decays with increasing molecular length, while cumulenes show a conductance increase with increasing molecular length. Their distinct conducting behaviors are attributed to their different bond length alternation, which is supported by theoretical calculations. This study confirms the long-standing theoretical predictions on sp-hybridized carbon wires and demonstrates that cumulenes can form highly conducting molecular wires.

A review of functional linear carbon chains (oligoynes, polyynes, cumulenes) and their applications as molecular wires in molecular electronics and optoelectronics

“…14C). 130 This length dependence of conductance of cumulenes is in marked contrast to comparable oligoynes 35 which have a positive b value (Fig. 14C), 130 as was expected from previous studies on oligoynes 6-10.…”

“…130 This length dependence of conductance of cumulenes is in marked contrast to comparable oligoynes 35 which have a positive b value (Fig. 14C), 130 as was expected from previous studies on oligoynes 6-10. 96 It is, therefore, firmly established theoretically and experimentally that cumulenes are excellent wires for molecular electronics applications.…”

A review of oligo(arylene ethynylene) derivatives in molecular junctions

“…However, unusual negative values of β were reported for porphyrin tapes 105 and cumulenes. 104 Related attenuation factors are observed for electron transfer in D-B-A systems and magnetic exchange coupling in biradical systems. 78 However, these values are not directly comparable because different exchange interactions and tunnelling energy barriers will exist in the backbones under the very different experimental conditions.…”

“…As the field of molecular electronics has developed, many different conductive conjugated backbones have been utilised in molecular wires. These include oligo(arylene vinylene)s 92 (most commonly oligo( phenylene vinylene) (OPV) [93][94][95] ), oligoarylenes 96 (including oligophenylenes (OP) 33,54,97,98 and oligothiophenes (OT) [99][100][101] ), oligoynes (OY), 102,103 cumulenes (CM) 104 and porphyrin tapes (PT), 105 for which generic examples are shown in Fig. 8.…”

“…Oligoynes and cumulenes are rigid systems but beyond relatively short molecular lengths they are unstable in the absence of significant steric shielding. 102,104 OAEs benefit from the rigidity of alkyne bonds but do not suffer from the same stability concerns.…”