Nano Lett. 14, 2941–2945 (2014)

The conduction of electrons through π-conjugated molecules can, in some cases, be suppressed due to destructive quantum interference effects, and graphical rules based on an atom-counting model can predict the existence of these effects in single-molecule junctions. Luis Campos, Latha Venkataraman and colleagues now show that atom-counting rules may not apply to non-alternant hydrocarbons — conjugated systems with at least one odd-numbered ring.

The researchers — who are based in the US, Denmark and Israel — used azulene, a hydrocarbon made from fused 5- and 7-member rings, as a prototypical non-alternant molecule and placed it between two gold electrodes using gold-binding substituents. The conductance of four azulene molecules substituted in different positions was then measured against a small applied voltage. Azulenes substituted at opposite ends of the molecule and at opposite ends of the 7-member ring did not show quantum interference, as predicted by the atom-counting model. However, those substituted at alternate carbons on the 5- and 7-member rings (positions 1,3 and 5,7, respectively) were found to conduct even though these molecules should show destructive interference according to the model.

Campos and colleagues explain this breakdown of the quantum interference rules in azulene — which could be common to other non-alternant molecules — by comparing their experiments with calculations. The calculated transmission curve for 1,3 and 5,7-substituted azulenes indeed show destructive interference features. However, charge-transfer suppression occurs at higher bias than expected from simple atom-counting rules. The researchers attribute this effect to the asymmetric energy-level alignment of the frontier orbitals of azulene.