On the illusory connection between cross-conjugation and quantum interference

TL;DR

This paper challenges the conventional belief that cross-conjugation always leads to destructive quantum interference in molecular conductance, showing that closed-loop structures can invert this expectation.

Contribution

It introduces a new understanding of quantum interference in conjugated molecules by considering closed-loop effects, expanding beyond traditional cross-conjugation rules.

Findings

Closed-loop conjugated molecules can exhibit destructive interference despite linear conjugation.

Cross-conjugated molecules can show constructive interference due to closed-loop effects.

Azulene's conductance behavior is explained by its closed-loop structure.

Abstract

Quantum interference, be it destructive or constructive, has a substantial influence on the magnitude of molecular conductance and consequently there is significant interest in predicting these effects. It is commonly thought that cross-conjugated paths result in suppressed conductance due to destructive quantum interference. Using H\"uckel theory and DFT calculations we investigate systems that break this cross-conjugation rule of thumb. We predict and rationalize how a class of conjugated molecules containing closed loops can exhibit destructive interference despite being linearly conjugated and exhibit constructive interference despite being cross-conjugated. The arguments build on the graphical rules derived by Markussen et al. [1] and the hitherto neglected effects of closed loops in the molecular structure. Finally, we identify the 1,3 connected Azulene molecule as belonging to the closed-loop class and argue that this explains recent measurements of its electrical conductance.