Symmetry is a property that is repeated throughout the natural world. Chiral symmetry is a phenomenon where two objects seem identical, but they are actually mirror images, much like the left and right hand. While they seem identical, it would be impossible to superimpose one on top of the other. This gives rise to the name of chiral variants in chemistry as either left-handed or right-handed. Chiral symmetry is present in all natural and life sciences, from molluscs to peptides, small molecules to spiral galaxies.
Just as molecules can be either left-handed or right-handed, light can also possess chirality. The circular polarisation of light can be either left-handed or right-handed, depending on the sense of rotation of the electric field vector with respect to the direction of propagation. The ability to manipulate (chiral) circularly polarised light (CPL) has been intensively studied for decades. Materials that could selectively emit and absorb CPL hold the key to new display technologies, quantum computers and spintronic devices.
Unfortunately, the benefits of chiroptical properties in electronic devices has not yet been realised. Researchers used Diamond’s Synchrotron Radiation Circular Dichroism beamline (B23) and the Advanced Light Source’s Resonant soft X-ray scattering (RSoXS) instrument in California to study chiroptical effects of polymer thin films. They discovered that the way we thought about chirality in these polymer films needs to change. Prior to this study, it was thought that chiroptical properties arose from long range structural chirality in the molecular structure of the material. However, researchers found that in their experiments, chirality came from magneto-electric coupling, which is also a natural optical activity.
The combination of Diamond’s Circular Dichroism and the Advanced Light Source’s beamlines enabled the research team to investigate the emergence of chiroptical phenomena in achiral polymers blended with a chiral small-molecule additive (1-aza helicene) and intrinsically chiral-sidechain polymers using a combination of spectroscopic methods and structural probes. They also tested films that were aligned compared to films that were not aligned which is how they discovered that optical chirality did not come from long range structural chirality in the material itself. RSoXS and atomic force microscopy (AFM) suggest that the natural optical activity may arise due to the assembly of twisted polymer fibrils into a double twist cylinder type blue phase.
This study presents a new way of looking at chirality in thin polymer films that are important for electronics. The discovery that magneto-electric coupling is responsible for the high dis-symmetry of non-aligned chiral polymers —and not longer-range structural chirality—will allow the rational design of polymers for a range of device applications. More importantly, experiments at B23 were carried out under conditions that are useful in electronic fabrication at layer thicknesses that allow for the production of highly efficient electronics. Taken together, these findings present a roadmap for introducing chiroptical properties into more electronic devices in the future. Modern technology is built around the fact that we can encode information in electromagnetic radiation, from radio waves to light travelling from a screen. Imagine a world where we could encode even more information into light because our electronics can read how light is polarised. This innovation has the potential to fundamentally change the technology landscape and give rise to a new generation of devices, computers and screens.
Jessica Wade et al. Natural optical activity as the origin of the large chiroptical properties in π-conjugated polymer thin films. Nature Communications 11 6137 (2020).
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