Controlling Molecular Electronics with Rigid Ladder-Like Molecules

As electronic devices become smaller, physical size limitations are beginning to disrupt the trend of doubling transistor density on silicon-based electronic chips about every two years, according to Moore’s Law. Molecular electronics, which involves using individual molecules as building blocks for electronic components, offers a potential path for the continued miniaturization of small-scale electronic devices. Devices that use molecular electronics require precise control of the flow of electrical current.

However, the dynamic nature of these unique molecular components affects device performance and impacts reproducibility.

Researchers at the University of Illinois at Urbana-Champaign have presented a unique strategy for controlling molecular conductance using molecules with rigid backbones, such as ladder-like molecules, known for their shape persistence. In addition, they demonstrated a simple “all-in-one” method for synthesizing such molecules. The principles were then applied to the synthesis of a butterfly-shaped molecule, showing the generality of the strategy for controlling molecular conductance.

This new research, led by Charles Schroeder, the James Economy Professor of Materials Science and Engineering and professor of chemical and biomolecular engineering, along with postdoctoral fellow Xiaolin Liu and graduate student Hao Yang, appears in the journal Chemistry of nature.

“In molecular electronics, you have to consider the flexibility and motion of molecules and how that affects the functional properties,” Schroeder says. “And it turns out that this plays an important role in the electronic properties of molecules. To overcome this challenge and achieve consistent conductivity regardless of conformation, our solution was to prepare molecules with rigid backbones.”

One of the main challenges in molecular electronics is that many organic molecules are flexible and exhibit multiple molecular conformations (the arrangement of atoms due to bond rotation), with each conformation potentially resulting in a different electrical conductance.

Liu explains: “For a molecule with multiple conformations, the conductance variation is very large, sometimes 1,000 times different. We decided to use ladder-type molecules, which have a persistent shape, and they showed a stable set of rigid conformations so that we could obtain a stable and robust molecular junction conductance.”

Ladder-type molecules are a class of molecules that contain an uninterrupted sequence of chemical rings with at least two shared atoms between the rings, which “locks” the molecule into a certain conformation. Such a structure ensures shape persistence and limits the rotational motion of the molecule, which also minimizes conductance variation.

Constant conductance is especially important when the ultimate goal of molecular electronics is to be used in a functional device. This means that billions of components must have the same electronic properties.

“Conductance variation is one of the problems that has prevented the successful commercialization of molecular electronic devices. It is very difficult to manufacture the large number of identical components needed and to control molecular conductance in the junctions of individual molecules,” Yang says. “If we can do this precisely, it could help accelerate commercialization and make electronic devices very small.”

To control the molecular conductance of shape-persistent molecules, the team used a unique one-pot ladder synthesis strategy that produced charged and chemically diverse ladder molecules. Traditional synthesis methods use expensive starting materials and are typically two-component reactions, which limits product diversity. Using the one-pot multicomponent strategy, also called modular synthesis, the starting materials are much simpler and commercially available.

“We can use many different combinations of these raw materials and create a wide variety of product molecules suitable for molecular electronics,” Liu says.

Liu and Yang then applied the rules they learned from ladder-like molecules and demonstrated the broad applicability of shape persistence by designing, synthesizing, and characterizing the electronic properties of a butterfly-like molecule. These molecules have two “wings” of chemical rings, and like ladder-like molecules, butterfly-like molecules exhibit a locked backbone structure and constrained rotation. This will pave the way for the design of other functional materials and, ultimately, more reliable and efficient devices.