Reverse Bond-Length Alternation in Cumulenes: Candidates for Increasing Electronic Transmission with Length

Single-molecule conductance generally decays exponentially with the length of the molecule when the transport mechanism is a coherent tunneling process. However, it was recently found that this length dependence can be reversed in linear conjugated molecules if the bond-length alternation is reversed. In this work we show that even-carbon cumulenes show this behavior as the bond lengths are reversed for the dominant π-system compared to the equivalent polyenes and polyynes. We explore the electronic origins of the reversed bond-length alternation in cumulenes and its relation to the length dependence of the electronic transmission. Through density functional theory and nonequilibrium Greens function calculations we predict that cumulenic wires have reverse decay of transmission with length; that is, the decay constant β is found to be negative. As a direct consequence of the reversed bond-length alternation, the electronic transmission increases with length as the highest occupied molecular orbital–lowest unoccupied molecular orbital gap rapidly narrows. On the basis of recent progress in cumulene synthesis, we discuss substituent strategies that may increase reverse bond-length alternation. Cumulenes stand out as promising candidates for a series of molecules that may show reverse decay of single-molecule conductance with increasing length.