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, p-conjugated aromatics, the p system usually dominates the transport. Herein, we investigate situations where model calculations including only the p system do not capture all of the subtleties of the transport properties. Including both the s and p 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 p system are insufficient where the transport is dominated by quantum interference and cases where geometric changes modulate the coupling between different regions of the p 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.