Put simply, non-alternant hydrocarbons usually feature at least one ring containing an odd number of carbon atoms, whereas alternant hydrocarbons have no such rings. Non-alternant hydrocarbons such as azulene have been largely ignored in single-molecule studies, which is surprising because they do have interesting photophysical properties and now, as we have discovered in this new work, single-molecule conductance, explains team leader Luis Campos of Columbia University in New York.

In their experiments, the researchers studied five azulene derivatives containing gold-binding groups. Each derivative consists of an azulene core, which is “wired” at different points on the core. These points allowed the team to probe different conductance pathways through the molecules.

“We carried out our conductance measurements using the scanning tunnelling microscope-based break junction technique,” team member Brian Capozzi told nanotechweb.org. “We repeatedly drove a gold STM tip into and out of contact with a gold substrate. As the tip retracts, the resulting junction thins down to a gold atom-gold atom contact, which breaks as it further elongates.”

Measuring conductance

This gap then becomes small enough to accommodate a gold binding molecule and once the molecule is in the junction, the researchers are able to determine its conductance by applying a voltage to the system and measuring the current passing through it. This process is repeated thousands of times for a given molecule of interest to obtain a statistically significant value of the conductance.

Better understanding the relationship between molecular connectivity and conductance properties in these molecules could help us design advanced systems for molecular electronics devices, says Campos. “The fact that azulene is a non-alternant hydrocarbon makes this a unique system, where basic models to predict conductance do not hold.”

The team, which includes researchers from the University of Copenhagen and Tel Aviv University, says that it will now be looking at azulene-based polymers for use in optoelectronics applications, where monomer connectivity could play an important role. “While this will be the main focus of future applications, we are also interested in how we can take advantage of the unique ‘quantum interference’ properties of azulene in single-molecule electronics,” added Campos. “We would also like to find out whether our studies could be extended to other prototypical non-alternant hydrocarbons.”

The present work is detailed in Nano Letters DOI: 10.1021/nl5010702.