Abstract
Controlling charge transport through molecular wires by utilizing quantum interference (QI) is a growing topic in single-molecular electronics. In this article, scanning tunneling microscopy-break junction techniques and density functional theory calculations are employed to investigate the single-molecule conductance properties of four molecules that have been specifically designed to test extended curly arrow rules (ECARs) for predicting QI in molecular junctions. Specifically, for two new isomeric 1-phenylpyrrole derivatives, the conductance pathway between the gold electrodes must pass through a nitrogen atom: this novel feature is designed to maximize the influence of the heteroatom on conductance properties and has not been the subject of prior investigations of QI. It is shown, experimentally and computationally, that the presence of a nitrogen atom in the conductance pathway increases the effect of changing the position of the anchoring group on the phenyl ring from para to meta, in comparison with biphenyl analogues. This effect is explained in terms of destructive QI (DQI) for the meta-connected pyrrole and shifted DQI for the para-connected isomer. These results demonstrate modulation of antiresonances by molecular design and verify the validity of ECARs as a simple “pen-and-paper” method for predicting QI behavior. The principles offer new fundamental insights into structure–property relationships in molecular junctions and can now be exploited in a range of different heterocycles for molecular electronic applications, such as switches based on external gating, or in thermoelectric devices.
Highlights
Single-molecule conductance values have been determined for a diverse array of molecular wires trapped between metal electrodes since the development of specialized measurement techniques in the late 1990s and early 2000s.1−3 These methods include mechanically controlled break junction[4] and scanning tunneling microscopy-break junction (STM-BJ)[5] experiments
By combining these techniques with the power of organic synthesis, it has been widely demonstrated that substantial variation in the conductance of molecular wires can be achieved by small structural modifications, such as structural isomerism and/or the presence of heteroatoms.[6−13] in the case of π-conjugated systems, much of this behavior can be attributed to quantum interference (QI) effects,[14−16] which are readily visualized in transmission functions derived from charge-transport simulations.[2,17−20]
This agrees with extended curly arrow rules (ECARs) which predict destructive QI (DQI) near EF for 2−4 and Shifted DQI (SDQI) for 1
Summary
Single-molecule conductance values have been determined for a diverse array of molecular wires trapped between metal electrodes since the development of specialized measurement techniques in the late 1990s and early 2000s.1−3 These methods include mechanically controlled break junction[4] and scanning tunneling microscopy-break junction (STM-BJ)[5] experiments By combining these techniques with the power of organic synthesis, it has been widely demonstrated that substantial variation in the conductance of molecular wires can be achieved by small structural modifications, such as structural isomerism and/or the presence of heteroatoms.[6−13] in the case of π-conjugated systems, much of this behavior can be attributed to quantum interference (QI) effects,[14−16] which are readily visualized in transmission functions derived from charge-transport simulations.[2,17−20]. A characteristic feature of DQI is a sharp antiresonance in the transmission curve where T(E) approaches zero
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