A research team led by Professor Akiko Hori of Shibaura Institute of Technology, in collaboration with Waseda University and Tokyo University of Science, has developed a new organic crystalline material that can visualize ultraviolet and near-infrared light, which are invisible to the human eye, as "color."
Invisible light, such as ultraviolet and near-infrared light, is important as an energy source and information medium, and there has been a need to develop materials that convert it into visible light so that it can be directly observed. While the conversion of low-energy to high-energy light has been studied in inorganic materials, organic molecules have faced challenges such as significant energy loss due to molecular motion and thermal vibrations, which tend to weaken luminescence and nonlinear optical response in the solid state.
The research team synthesized a pyrazine derivative containing 1,2,5-thiadiazole and investigated its single-crystal structure and photoresponse properties. The results showed that the crystal exhibited red emission under ultraviolet light irradiation, originating from intermolecular interactions (excimer formation), and displayed a very large wavelength difference (Stokes shift) between absorbed and emitted light exceeding 200 nm.
Furthermore, when the same crystal was irradiated with near-infrared light, second-harmonic generation occurred due to the crystal's non-centrosymmetric structure, and the conversion of near-infrared light to visible green light was confirmed.
In this study, two optical responses based on different physical mechanisms were simultaneously achieved in a single organic crystal: long-wavelength visible emission in response to ultraviolet light and short-wavelength light conversion via upconversion (second harmonic generation) in response to near-infrared light. This demonstrates a new approach to material design that allows switching optical responses through the design of organic molecules and control of crystal structure.
This is expected to lead to the development of organic materials whose emission color and optical response wavelength can be freely adjusted, and is anticipated to serve as a fundamental guideline for the effective utilization of invisible light, with potential applications in industrial materials.
Paper information: [Chemical Communications] Red-fluorescence under UV and green-SHG under NIR dual-mode emission in a yellow crystal of a 1,2,5-thiadiazole derivative
