Breakdown of Interference Rules in Azulene, a Nonalternant Hydrocarbon

Abstract

We have designed and synthesized five azulene derivatives containing goldbinding 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.

Searching the Hearts of Graphene-like Molecules for Simplicity, Sensitivity, and Logic

“…This molecule is a challenge, because well-known bond counting rules for predicting QI 45,46 have been shown to break down in azulene cores 38 …”

Tuning the thermal conductance of molecular junctions with interference effects

“…In particular, special attention has been devoted to the role of Fano resonances [28][29][30][31][32] and to the determination of general rules governing the appearance of quantum interference effects in molecules with extended π-electron systems [33][34][35][36] . Also different experimental reports in recent years have convincingly shown the influence of the quantum interference on electronic transport of molecular junctions, as illustrated by measurements of linear conductances and current-voltage characteristics [37][38][39][40][41][42][43][44][45][46][47][48] . When comparing electron and phonon transport, it needs to be kept in mind that for electrons the interference has to occur within some k B T around µ ≈ E F , with the Fermi energy E F , to get a measurable effect on the linear conductance.…”

Correlating Structure and Function in Organic Electronics: From Single Molecule Transport to Singlet Fission

“…54−57 Further complicating the situation, our own work exploring different conducting pathways in azulene has shown that the models used to predict quantum interference break down for nonalternant hydrocarbons. 58 It is also known that varying the connectivities of polymers, such as 2,7 vs 3,6-carbazole 59 and 1,3 vs 4,7 azulenes 60−62 can influence their optoelectronic properties to a large extent, but thus far, these differences have been attributed to steric effects. Solomon and co-workers extended quantum inteference theories to stacked phenyl dimers and trimers (shown in Figure 7) proposing that the efficiency of transport through the stacks depends on the orientation of the linkers.…”