Electron Transport through Conjugated Molecules: When the π System Only Tells Part of the Story

In molecular transport junctions, current is monitored as a function of the applied voltage for a single molecule assembled between two leads. The transport is modulated by the electronic states of the molecule. For the prototypical delocalized systems, namely, π-conjugated aromatics, the π system usually dominates the transport. Herein, we investigate situations where model calculations including only the π system do not capture all of the subtleties of the transport properties. Including both the σ and π contributions to charge transport allows us to demonstrate that while there is generally good agreement, there are discrepancies between the methods. We find that model calculations with only the π system are insufficient where the transport is dominated by quantum interference and cases where geometric changes modulate the coupling between different regions of the π system. We examine two specific molecular test cases to model these geometric changes: the angle dependence of coupling in (firstly) a biphenyl and (secondly) a nitro substituent of a cross-conjugated unit.