This article covers a breakthrough from researchers at Shibaura Institute of Technology, Waseda University, and the Institute of Science Tokyo. They created an organic single crystal that can turn invisible UV and near-infrared (NIR) light into visible red and green beams, respectively.
By combining smart molecular design with careful crystal packing, the team managed to get two very different optical responses inside the same material. This opens up new options for compact, tunable photonic devices—think sensing, imaging, and measurement tools.
Dual-mode wavelength conversion in a single crystal
The researchers engineered a rigid, π-conjugated molecule with a 1,2,5-thiadiazole-substituted pyrazine unit. This setup let them grow high-quality single crystals where the molecules line up in a well-defined lattice.
Within this structure, two complementary optical pathways emerge: you get red emission under UV light and green emission under NIR light. So, one organic material pulls off two wavelength-conversion tricks without the processes interfering with each other.
They leaned on two main design principles. First, a stiff molecular backbone cuts down on nonradiative decay from molecular motion. Second, precise crystal packing encourages specific intermolecular interactions, letting the two optical effects—red emission and green emission—work independently in the same lattice.
Molecular design and crystal packing
- Rigid π-conjugated framework: cuts vibrational losses and supports coherent optical processes.
- 1,2,5-thiadiazole-substituted pyrazine motif: helps with electronic structure and intermolecular interactions.
- High-quality single-crystal growth: gives uniform packing and consistent optical behavior throughout the material.
- Controlled crystal packing: encourages specific excitonic interactions, making the dual responses possible without them tripping over each other.
- Independent channels: red and green pathways exist side by side but stay decoupled, so each emission process keeps its integrity.
Optical mechanisms: UV-to-red and NIR-to-green
When you hit the crystal with ultraviolet light, it emits visible red with a big Stokes shift. Spectroscopic analysis shows this red fluorescence comes from an excimer state—basically, an excited dimer formed by close contacts between molecules in the lattice.
The excitonic coupling in the crystal stabilizes this excimer, which allows efficient red emission at a wavelength you’d struggle to reach with just a single molecule. Flip the switch to near-infrared light, and the same crystal spits out green light through second harmonic generation (SHG).
SHG is a nonlinear optical process. Two lower-energy photons merge into a single higher-energy photon. Here, the crystal’s symmetry and electronic structure make SHG strong, converting NIR photons to green within the same lattice—without messing with the red excimer pathway.
Red excimer fluorescence under UV
The red output stands out for its large Stokes shift, which cuts down on reabsorption and gives a clearer signal for imaging and sensing. The excimer state gets stabilized by those close, orderly intermolecular contacts set up by the crystal packing.
This really shows how supramolecular interactions can shape emission pathways in organic crystals, doesn’t it?
Green SHG under NIR
The second harmonic signal taps into the nonlinear optical properties of the crystalline framework. By taking advantage of the cooperative alignment of transition dipoles and the rigid lattice, the material can coherently turn two NIR photons into one green photon.
It’s a neat complementary approach to wavelength conversion, right alongside the red excimer route.
Coexistence and independence of the two modes
Red excimer fluorescence and green SHG both work at the same time in the crystal but don’t interfere with each other. This decoupled coexistence shows that molecular design plus smart crystal engineering can give you multiple, distinct optical responses in a single organic material.
Impact on organic photonics and future prospects
Organic luminescent materials really stand out for their light weight, chemical tunability, and processing flexibility. Still, they usually lose efficiency because of molecular motion and nonradiative decay.
The researchers tackled these issues by using rigid frameworks and smart crystal packing. This approach helped them achieve sustained, multi-channel optical responses.
It’s pretty exciting—organic crystals might actually compete with inorganic materials for wavelength-conversion jobs in sensors, imaging, and measurement devices. They also bring perks in manufacturability and life-cycle processing that you just don’t get with inorganics.
The study, published in Chemical Communications, highlights something bigger happening in photonics. Scientists can now engineer molecular crystals to visualize invisible light through different optical pathways—think red via excimer fluorescence and green via SHG.
As more researchers dig into the design rules for dual-mode or even multi-mode crystals, the idea of compact, multi-functional photonic components starts to feel genuinely within reach. Real-world applications could see a real boost from this kind of flexibility.
Here is the source article for this story: Japan group achieves dual-mode visible light generation from UV and NIR sources