Controlling molecular electronics with rigid, ladder-like molecules

Molecular electronics, which uses single molecules as the building blocks for electronic components, offers a promising solution for further miniaturisation. However, controlling the flow of electrical current in such devices is challenging due to the dynamic nature of these single-molecule components, which affects performance and consistency.

Researchers at the University of Illinois Urbana-Champaign have introduced a novel approach to overcome these obstacles by utilising molecules with rigid backbones, such as ladder-type molecules that are ‘shape-persistent’. In addition, they developed a simple ‘one-pot’ method for synthesising these molecules. This approach was further demonstrated by synthesising a butterfly-shaped molecule, showcasing the versatility of the strategy for controlling molecular conductance.

The research, led by Charles Schroeder, the James Economy Professor of Materials Science and Engineering and Professor of Chemical and Biomolecular Engineering, along with postdoctoral researcher Xiaolin Liu and graduate student Hao Yang, was recently published in Nature Chemistry.

Schroeder explained the significance of molecular rigidity in electronics: “In the field of molecular electronics, you have to consider the flexibility and motion of the molecules and how that affects the functional properties. And it turns out that plays a significant role in the electronic properties of molecules. To overcome this challenge and achieve constant conductivity regardless of the conformation, our solution was to prepare molecules with rigid backbones.”

One major challenge in molecular electronics is that organic molecules often exhibit flexibility and multiple conformations, which can vary depending on bond rotations. These varying conformations result in different electrical conductance levels. Liu elaborated on the issue: “For a molecule with multiple conformations, the variation in conductance is very large, sometimes 1000 times different. We decided to use ladder-type molecules, which are shape-persistent, and they showed a stable set of rigid conformations so that we can achieve stable and robust molecular junction conductance.”

Artistic representation of a ladder molecule acting as a component in molecular electronics (Source: University of Illinois Urbana-Champaign)

Ladder-type molecules are a class of compounds characterised by a continuous sequence of chemical rings, with at least two atoms shared between adjacent rings. This structure locks the molecule into a specific shape, minimising rotational movement and reducing fluctuations in conductance.

Consistent conductance is critical for the eventual use of molecular electronics in functional devices, which would require billions of components with identical electronic properties. Yang highlighted the challenges of scaling up: “The variation in conductance is one of the issues that has prevented the successful commercialisation of molecular electronic devices. It is very difficult to fabricate the large number of identical components necessary and control the molecular conductance in single molecule junctions. If we are able to precisely do this, that can help push the commercialisation and make electronic devices very small.”

To achieve this control, the research team developed a unique ‘one-pot’ ladderisation synthesis method to create chemically diverse, charged ladder molecules. Traditional synthesis methods are often limited, relying on expensive starting materials and two-component reactions, which restrict the diversity of the resulting products. The one-pot approach, also known as modular synthesis, uses simpler, commercially available starting materials, allowing for a broader range of product molecules suitable for molecular electronics. Liu commented on the flexibility of the new method: “We can use many different combinations of those starting materials and make a rich diversity of product molecules suitable for molecular electronics.”

Building on the insights gained from ladder-type molecules, Liu and Yang expanded their work by designing and synthesising a butterfly-like molecule, further demonstrating the broad applicability of shape-persistent molecules. These butterfly molecules have a structure featuring two ‘wings’ of chemical rings and, like ladder molecules, are locked into a rigid conformation with constrained rotation. This paves the way for the design of other functional materials and contributes to the development of more reliable and efficient molecular electronic devices.