Breakdown of Interference Rules in Azulene, a Non-Alternant Hydrocarbon

TL;DR

This study investigates how the unique non-alternant hydrocarbon azulene's quantum interference effects influence its electrical conductance, revealing limitations of simple predictive models and emphasizing the importance of advanced calculations.

Contribution

It demonstrates that simple interference models fail for azulene derivatives, and provides experimental evidence supporting GW calculations for predicting conductance.

Findings

Changing connectivity affects optical and conductive properties.

Simple models cannot reliably predict interference in azulene.

GW calculations align with experimental conductance trends.

Abstract

We have designed and synthesized five azulene derivatives containing gold- binding groups at different points of connectivity within the azulene core to probe the effects of quantum interference through single-molecule conductance measurements. We compare conducting paths through the 5-membered ring, 7-membered ring, and across the long axis of azulene. We find that changing the points of connectivity in the azulene impacts the optical properties (as determined from UV-Vis absorption spectra) and the conductivity. Importantly, we show here that simple models cannot be used to predict quantum interference characteristics of non-alternant hydrocarbons. As an exemplary case, we show that azulene derivatives that are predicted to exhibit destructive interference based on widely accepted atom-counting models show a significant conductance at low biases. Although simple models to predict the low-bias conductance do not hold with all azulene derivatives, we show that the measured conductance trend for all molecules studied actually agrees with predictions based on the more complete GW calculations for model systems.