Scientists have made an extraordinary step forward in synthetic chemistry by mastering the trick of defying the physical principles of symmetry to develop a whole new category of materials that emit light. The research team led by Prof. Mitsuhiko Shionoya of the Tokyo University of Science (TUS) has developed a new method called asymmetric alloying.
The breakthrough is published in the journal, Nature Communications, and enables scientists to engineer sub-nanometer clusters of metal atoms with atomic precision. The team has intentionally built the geometry to be irregular in highly symmetrical metallic structures, thereby opening up a completely new route to the design of ultra-efficient luminescent nanomaterials.
The Symmetric Chemistry Revision
We need to understand the importance of this huge deal through the understanding of how molecules are usually constructed. Metals cluster molecules are extremely small molecules consisting of several metal atoms. Of course the clusters like to form into very regular, balanced, symmetrical shapes, such as perfect molecular dice.
They are very stable and symmetric structures, but because of the uniformity of their interior, their interaction with light is restricted. It has long been known that breaking this structural symmetry could reveal other electronic and optical properties of the material but it has been a remarkable challenge to do it on purpose.
The addition of other metals to create an alloy is a good method of modifying a material’s properties. Typical alloying typically occurs randomly, however. The new technique developed by the TUS team, called asymmetry alloying, enables them to introduce a foreign metal atom into the cluster in a non-equivalent position, that is, deliberately disrupt the symmetry of the cluster, to make a “chiral” functional material.
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The Recipe: Gold, Silver & Micro Etching
To test their theory, Professor Shionoya and his group began with a carbon-centered, six-membered cluster of gold atoms (CAu$\text{I}_6$), which is extremely symmetric in its chemical structure. This cluster has a clean octahedral structure which serves as a stable, well-balanced structure.
The magic is when you add silver to the mix. The researchers combined the triphenylphosphine protected gold cluster with the silver trifluoroacetate. A very precise chemical reaction took place:
- The silver ions actively targeted the gold structure.
- The silver was used to remove precisely two gold atoms from the middle by a process called atomic-level etching.
- The silver atoms settled in the empty space changing the molecular structure from a regular octahedron to a more complex, asymmetric polyhedron called a bicapped square antiprism.
Unlocking Red and Near-InfraRed Photoluminescence
This cluster is physically reshaped, and it changed the way it processes energy. The internal electronic structure changed drastically by selectively forcing the gold and silver atoms to form an asymmetric arrangement.
The immediate result of this structural destruction led to a dramatic increase of luminescent performance. The new alloy clusters that were created showed a very unique and high intensity red to near infrared (NIR) phosphorescence when stimulated with energy.
The team achieved such control by using special optically active, homochiral carboxylate ligands during the synthesis. They are able to generate both lefty and rightey structural variants of the cluster with their own distinct chiroptical properties such as Circularly Polarized Luminescence (CPL).
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Why the Near-Infrared Spectrum Matters?
It has real-world implications because of the stable, high-yield, near-infrared light that can be produced from the sub-nanometer alloy. NIR light sits in a sweet zone of the electromagnetic spectrum, and is able to penetrate dark environments and organic tissue far greater than regular visible light.
With this atomic-level molecular editing technique perfected, the doors are opening to some next-level applications in the real world:
- Near-infrared light can penetrate skin and tissue without harming cells, so these bright, asymmetric clusters can be attached to cells that are the targets of interest, allowing them to track changes that occur in deep tissue or track the delivery of drugs inside the human body with remarkable clarity.
- The materials are highly effective at testing the purity of pharmaceuticals since they respond to molecules moving in different directions, depending on the chemical’s chirality, which occurs naturally.
- Advanced Optoelectronics: Symmetry breaking offers a way to more efficient display technologies, night vision systems and secure optical communications networks.
The success of the project provided a new approach to controlling the structure of metal ion clusters, Professor Shionoya said. The technique showed that scientists could tailor a metal core atom-by-atom to create new atomic structures, opening a new materials design paradigm for a cleaner and brighter future in functional optics.
