Helicene columns could impart chiral properties to optoelectronic devices

Helicene columns could impart chiral properties to optoelectronic devices

Controlling the chirality of organic semiconductors could increase their efficiency and bring new functionalities to devices. However, the fabrication of materials with tunable chirality in large quantities remained a challenge—only a few examples of oriented monolayers have been reported so far. Now, a multidisciplinary team of researchers from the UK, Canada and Japan has developed a simple templating method to grow ordered chiral thin films for a variety of applications.

“Thanks to computational design and clever synthesis, we knew we had really exciting materials,” says lead author Jess Wade of Imperial College London, UK. “Organic semiconductors already show interesting properties, they are lightweight, inexpensive, biocompatible and highly tunable.” Adding chirality to the list could enhance the existing properties of optoelectronics used in displays, sensors, solar panels, transistors, and more. “In addition, we can use chirality to manipulate the spin of photons and electrons at room temperature,” she says. “But to achieve this in devices, we needed to control the chiral properties in bulk.”

To achieve this unprecedented control over chiral properties, researchers used a vacuum-based deposition technique that works in combination with organic and inorganic templates. “With his approach [we] ordered thin films of helicenes grow on template layers,” explains Wade. In contrast to previous approaches, which mostly resulted in monolayers just a few atoms thick, this new solution creates organized films that are 20 times thicker – up to 200 nm. “The limit is the amount of molecules that our amazing chemists can synthesize!” She adds. This technology is much more precise in terms of precisely controlling and monitoring the process.

The deposition is very simple: the researchers take the chiral helicenes in powder form, heat them up and sublimate them onto the stencils. “These particular helicenes assemble into supramolecular columns, and we found out [they either] lying flat or standing upright, which has a huge impact on their optical and electronic properties,” adds Wade. “Depending on our choice of template layer, the helices orient themselves in different directions.”

On the one hand, a template based on a polyaromatic organic molecule leads to the formation of helicene columns that are oriented perpendicular to the surface. On the other hand, an inorganic template such as copper iodide favors the formation of flat-lying columns, possibly due to interactions between helicenes and the negative charge on the surface. They each have different properties, including light absorption and X-ray diffraction.

To plan

“Helicene [are] known to show very strong chiroptical activity,” explains Jeanne Crassous, expert on chiral materials at the Institute for Chemical Sciences of Rennes in France. “This result is important because [it] controls the orientation of semiconductor materials… in a very simple way,” she adds. Crassous also emphasized the key role of collaboration in this work. ‘[The team] gathers a unique union of synthetic chemists, theorists and physicists, enabling them to concretize simple and original ideas.’

Ghislaine Vantomme, who studies chiral materials at Eindhoven University of Technology in the Netherlands, agrees. The discovery came about because “people from very different backgrounds were talking and collaborating.” In addition, Vantomme points to the importance of computer studies. “The templates start to crystallize, but identifying the right match between the template layer and the small chiral molecule takes hard work and lots of calculations.” The calculations helped to simulate the crystal structure of the helicene columns, which also contributed to the full characterization of the self-assembled structures. “Controlling the alignment of molecules is very difficult, but really important for the functionality of the devices,” she adds.

Both Crassous and Vantomme agree that this study is a stepping stone to many more chiral materials, devices, and applications. “Right now they’ve been trying a few different helicenes and templates,” explains Vantomme. “The types of structures [and] Interactions open the door to new possibilities,” she adds.

The main disadvantage, Wade explains, is the availability of enantiomerically pure helicene. “Sometimes the most incredible molecules are complicated to synthesize,” she says. In this case, “it is even more difficult to separate them into enantiomerically pure forms”. But once that is solved, “this method could actually become scalable and cheap, and thus promising for applied developments in devices,” explains Crassous. “The applications are numerous and promising, from lighting to power generation,” she adds.

Like liquid crystal technology, which started small and ended up in all our screens, this technology has huge potential, comments Vantomme. “For example, chiral materials could improve the efficiency of OLEDs, a multi-billion dollar market,” she adds.

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