Cumulenes are sometimes described as 'metallic' because an infinitely long cumulene would have the band structure of a metal. Here we report the single-molecule conductances of a series of cumulenes and cumulene analogues, where the number of consecutive C=C bonds in the core is n = 1, 2, 3 and 5. The [n]cumulenes with n = 3 and 5 have almost the same conductance, and they are both more conductive than the alkene (n = 1). This is remarkable because molecular conductance normally falls exponentially with length. The conductance of the allene (n = 2) is much lower, due to its twisted geometry. Computational simulations predict a similar trend to the experimental results and indicate that the low conductance of the allene is a general feature of [n]cumulenes where n is even. The lack of length-dependence in the conductances of [3] and [5]cumulenes is attributed to the strong decrease in HOMO-LUMO gap with increasing length.
