The Relation between Structure and Quantum Interference in Single Molecule Junctions

Abstract

Quantum interference (QI) of electron pathways has recently attracted increased interest as an enabling tool for single-molecule electronic devices. Although various molecular systems have been shown to exhibit QI effects and a number of methods have been proposed for its analysis, simple guidelines linking the molecular structure to QI effects in the phase-coherent transport regime have until now been lacking. In the present work we demonstrate that QI in aromatic molecules is intimately related to the topology of the molecule's π system and establish a simple graphical scheme to predict the existence of QI-induced transmission antiresonances. The generality of the scheme, which is exact for a certain class of tight-binding models, is proved by a comparison to first-principles transport calculations for 10 different configurations of anthraquinone as well as a set of cross-conjugated molecular wires.

Breakdown of Interference Rules in Azulene, a Nonalternant Hydrocarbon

“…naphthalene obeys fundamental rules: the 2,7Np and 1,3Np derivatives do not conduct while the 2,6Np and 1,4Np show clear conductance signatures. 14,22 These observations for naphthalene are consistent with the atom-counting model by Markussen et al, 21 which predicts QI for the 2,7Np and 1,3Np derivatives. Interestingly, the same model would predict that 1,3Az, and 5,7Az should exhibit QI (see Figure 1).…”

“…4,14,15 While conductance and interference patterns in alternant hydrocarbons have been the main focus, [14][15][16][17][18][19] little is known about how substitution patterns affect the single-molecule conductance properties of nonalternant hydrocarbons, 20 where quantum interference (QI) effects may not be predicted based on simple bond-counting models. 21,22 The optical properties and chemistry of alternant and non-alternant hydrocarbons vary widely with respect to each other. 20 A classical example is the comparison of naphthalene to azulene.…”

Electrochemical Control of Single-Molecule Conductance by Fermi-Level Tuning and Conjugation Switching

“…[26][27][28][29][30][31] Destructive QI leads to very low conductance -much lower than anticipated from a simple "Lorentzian" model treating each molecular level as an independent transport channel. It occurs as a result of a (nearly) complete cancellation of transmission probability due to interference between different electron pathways, and is predicted to take place in organic molecules whenever the path connecting the left and right electrodes via the molecule is cross conjugated.…”

“…It occurs as a result of a (nearly) complete cancellation of transmission probability due to interference between different electron pathways, and is predicted to take place in organic molecules whenever the path connecting the left and right electrodes via the molecule is cross conjugated. 26,32 For example, independent measurements have shown that the conductance of a cross-conjugated anthraquinone (AQ) is ~100 times lower than that of a linearly-conjugated anthracene [31][32][33] even though the molecular length, electronic energy levels, and optical properties of the two molecules are very similar.…”

A quantum circuit rule for interference effects in single-molecule electrical junctions

“…Owing to their small size (on the scale of Angstroms) and the large energy gaps (on the scale of eV), transport through single molecules can remain phase coherent even at room temperature, and constructive or destructive quantum interference (QI) can be utilized to manipulate their room temperature electrical [10][11][12][13] and thermoelectrical 14,15 properties. In previous studies, it was reported theoretically and experimentally that the conductance of a phenyl ring with meta (m) connectivity is lower than the isomer with para (p) connectivity by several orders of magnitude [16][17][18][19][20][21][22][23][24][25] . This arises because partial de Broglie waves traversing different paths through the ring are perfectly out of phase leading to destructive QI in the case of meta coupling, while for para or ortho coupling they are perfectly in phase and exhibit constructive QI.…”