Exploring Electrical Currents through Nanographenes: Visualization and Tuning of the through-Bond Transmission Paths

In this work, electrical currents through nanographenes, a class of alternant hydrocarbons also known as polycyclic aromatic hydrocarbons, in molecular junctions under small bias are explored. We illustrate that when the π-current dominates, that is, when no quantum interference takes place, the current prefers the direction of the shortest bond (the bond with the highest double bond character) upon entering the molecule from the contacts. As such, the idea of electrons propagating through double bonds from contact to contact, originating from the curly arrow drawings used in a previously established selection rule for transmission, seems to be more deeply rooted in the actual physical process of electron transport than previously anticipated. Furthermore, this work confirms that the σ-current behaves completely differently than the π-current. When this type of current becomes important, that is, when quantum interference takes place, the current generally prefers the shortest path from contact to contact, irrespective of the length of the bonds constituting this path, in accordance with the strong distance dependency of the σ-current. Finally, it is demonstrated that keto groups (and cross-conjugating groups in general) can be used to seal off parts of the molecule for the current. No current flows through the sealed off part of the molecule under small bias and it does not influence the transmission spectrum of the considered system.