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

Abstract

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.

Extended curly arrow rules to rationalise and predict structural effects on quantum interference in molecular junctions

“…[16][17][18] Many qualitative and quantitative methods to predict and explain QI behaviour have been developed. These include curly arrow rules (CARs), 19,20 graphical and topological rules, [21][22][23][24][25][26] orbital symmetry rules, 27,28 magic ratio rules (MRRs), 3,[29][30][31] QI maps 8 and charge transport simulations using molecular orbitals based on Hückel (tight-binding) theory and/or density functional theory (DFT). 1,23,24,[32][33][34] Short of full DFT-based charge transport simulations, it can be difficult to apply predictive methods to systems that are not based on a bipartite hydrocarbon lattice, such as species containing five-membered rings 32 or non-alternant hydrocarbons.…”

“…While this use of curly arrows is widely accepted in the literature, surprisingly few works set out how curly arrows work in this context. 19,20,52,53 In the interdisciplinary field of molecular electronics this can lead to a 'language barrier' between chemists, for whom curly arrows are instinctive, and physicists, who may ignore the simplicity of curly arrows and prefer to interpret QI phenomena in more mathematical terms, such as graphical rules or MRRs. This is despite the predictions of CARs and graphical rules typically agreeing for alternant hydrocarbons (see below).…”

Anomalous Effect of Quantum Interference in Organic Spin Filters

“…The dips in transmission spectra of m-Benzene associated with QI were characterized by ring currentreversal, while none of these features were observed for p-Benzene 43. These striking features of cyclic currents are also connected to signatures of QI in a recent report by Stuyver et al60 Local transport contributions in p-Pent and m-Pent associated with QI dips at Fermi level. The diameter of the arrow is proportional to the magnitude of local transmission contribution, which are depicted only if they are 10% of the maximum local transmission between two atoms.…”

The Interconnection of Two Positive Charges by Conjugation and Cross‐Conjugation in Bis‐Quinolinium Ethynyls

“…In the case of polyenes and related hydrocarbons, for example, cross‐conjugated dendralenes display remarkable inhibitions of the delocalization compared to linear polyenes so that they can be considered as sets of isolated butadiene units . Considerable effort is currently being directed toward studying the differences of linear conjugation in comparison to cross‐conjugation of organic molecules from the perspective of computational chemistry, materials chemistry of electrochromic molecules, heteroelement chemistry for which phenylene‐bridged 1,2,3‐trisilacyclopentadienes can be given as an example here, polymer chemistry, and even mathematics . Scheme shows 1,3,5‐hexatriene 1 as an example of Kekulé even alternant hydrocarbons, which generally have classical polyene structures, as well as its cross‐conjugated isomer 3‐methylenepenta‐1,4‐diene 2 .…”