Turning the Tap: Conformational Control of Quantum Interference to Modulate Single‐Molecule Conductance

Abstract

Together with the more intuitive and commonly recognized conductance mechanisms of charge‐hopping and tunneling, quantum‐interference (QI) phenomena have been identified as important factors affecting charge transport through molecules. Consequently, establishing simple and flexible molecular‐design strategies to understand, control, and exploit QI in molecular junctions poses an exciting challenge. Here we demonstrate that destructive quantum interference (DQI) in meta‐substituted phenylene ethylene‐type oligomers (m‐OPE) can be tuned by changing the position and conformation of methoxy (OMe) substituents at the central phenylene ring. These substituents play the role of molecular‐scale taps, which can be switched on or off to control the current flow through a molecule. Our experimental results conclusively verify recently postulated magic‐ratio and orbital‐product rules, and highlight a novel chemical design strategy for tuning and gating DQI features to create single‐molecule devices with desirable electronic functions.

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

“…However, charge transport simulations and scanning tunnelling microscopy break junction (STM-BJ) experiments showed that o,p-MeO is substantially more conductive than m-OPE3 or m,m-MeO, which have very similar properties. 36 The results for o,o-MeO are insightful; experimentally its conductance is lower than m,m-MeO and m-OPE3, but it can be shown computationally that its conductance can be "switched on" by altering the conformation of the methoxy substituent to a sterically unfavourable position in which an oxygen lone pair can interact with the π-system. This geometry gives conductance comparable to o,p-MeO.…”

“…Jiang et al reported unusual QI behaviour in a combined experimental and computational study of meta-connected oligo( phenylene-ethynylene) (OPE) molecular wires substituted with methoxy groups at different positions on the central ring. 36 Here we will name their structures based on the relationship between the methoxy substituent and the two phenylene-ethynylene units bearing the sulfur-based anchoring groups; either ortho to one and para to the other (o,p-MeO), meta to both (m,m-MeO) or ortho to both (o,o-MeO) ( Fig. 5a).…”

“…In the above examples, the molecules that do not follow CARs contain oxygen or nitrogen heteroatoms. For the AQ species 61 the heteroatoms are located in electron-withdrawing carbonyl groups, whereas for the methoxy-substituted OPEs 36 the ether oxygen atoms are electron donating. In the pyridine series 10 the pyridine nitrogen is electron-withdrawing.…”

“…3,5,10,29,[36][37][38][39][40][41][42] A general conclusion is that heteroatom substitution has little or no effect on CQI [39][40][41][42] but can greatly influence DQI. 10,36,38 Charge-transport simulations of molecular conductance rely on the calculation of the transmission function of a molecular junction. 1 Transmission functions can also be deter-mined experimentally.…”

Constructive Quantum Interference in Single‐Molecule Benzodichalcogenophene Junctions

“…[1] Furthermore, unlessamolecule is electrostatically or electrochemically gated, the highest occupied andl owest unoccupiedm olecular orbitals (HOMOa nd LUMO,r espectively) levels adjust themselves such that the Fermie nergy of the electrodes lies in the vicinity of the middle of the HOMO-LUMO gap. [18,19] Consequently,i ft he core of the molecule is weakly coupled to the electrodes (e.g.,t hrough triple bonds, which link the core to an anchoring group), the electricalc onductancei sp roportional to (g ij ) 2 ,w here g ij is the amplitude of an electronic de Brogliew ave on atom j,d ue to electrons injected into the core at site i with energies close to the middle of the HOMO-LUMO gap. These concepts of weak coupling, connectivity and mid-gap transport have been utilized in as eries of papers to develop quantumc ircuit rules for materials discoverya nd to develop as implem agic ratio rule (MRR) [15] for describing the influence of connectivity on QI in heterocyclic PAHs.…”