Researchers at Kyushu University in Japan have developed a new class of organic molecules that emit circularly polarized light deep in the near-infrared spectrum, solving a trio of longstanding problems that had limited the usefulness of this kind of material in real-world technologies, according to a study published in Angewandte Chemie International Edition.
Circularly polarized light has properties that make it valuable across a growing range of applications, from next-generation 3D displays to bioimaging tools capable of detecting signals deep inside living tissues. Producing it efficiently with small organic molecules has been difficult because researchers have repeatedly run into tradeoffs: a molecule might emit light efficiently, but degrade quickly, or maintain good stability while producing weak output.
The Kyushu team, led by Associate Professor Ken Albrecht of the Institute for Materials Chemistry and Engineering, took a known family of radicals called tris(2,4,6-trichlorophenyl)methyl, or TTM, as their starting point. TTM-based radicals are naturally chiral, meaning they exist in mirror-image forms that cannot be perfectly superimposed on each other, which makes them candidates for producing circularly polarized light. In practice, however, they had performed poorly on efficiency, stability, and durability simultaneously.
To address those problems, Albrecht's team, which included doctoral student Kazuhiro Nakamura, started with a bromine-containing derivative of TTM called TTBrM and incorporated a nitrogen-containing organic compound known as carbazole. This produced three new molecules: CzTTBrM, 2CzTTBrM, and 3CzTTBrM. Adding the carbazole units fundamentally changed how the molecules emitted light. Instead of a simple localized electronic transition, emission occurred through a charge-transfer process between the carbazole donor and the TTBrM acceptor, shifting the output into the red to near-infrared range of 650 to 800 nanometers.
The performance gains were substantial. Measurements of photoluminescence quantum yield, a metric that describes how efficiently a molecule converts absorbed energy into emitted light, showed values approximately 30 times greater in the best-performing compound compared to conventional chiral luminescent radicals. Photostability improved by roughly 100-fold: the new radicals survived more than 1,300 seconds of continuous laser irradiation, while TTBrM lasted only 19 seconds under the same conditions.
The chirality of all three new compounds also held stable, with high barriers to racemization, the process by which an optically active compound loses its optical properties by converting to an inactive form. That stability matters for practical applications, where a molecule needs to maintain its light-emitting characteristics over time and under repeated use.
Near-infrared circularly polarized light is particularly useful in biomedical imaging because tissue absorbs and scatters less light at those wavelengths, allowing signals to penetrate more deeply into the body. Materials that combine high efficiency, durability, and stable chirality in that spectral range have been difficult to produce, and the Kyushu results represent a meaningful step toward usable devices.
The study was published in Angewandte Chemie International Edition, one of the leading journals in chemistry research.
