A groundbreaking discovery in the realm of organic photonics heralds new possibilities for turning invisible light into vivid, visible emissions using a single organic crystal. Researchers from Japan, spearheaded by Professor Akiko Hori at Shibaura Institute of Technology (SIT), have engineered an innovative yellow organic crystal capable of emitting two distinct visible colors when excited by ultraviolet (UV) and near-infrared (NIR) light. This dual-mode emission is a remarkable feat, combining fundamentally different optical phenomena within one molecular system and opening avenues for advanced optical sensors and photonic devices.
Invisible light, encompassing wavelengths such as ultraviolet and near-infrared radiation, plays a pivotal role across science and technology—from telecommunications to biomedical imaging. Despite its significance, detecting and visualizing these wavelengths often necessitates bulky, sophisticated instrumentation, limiting their practical utility in compact or flexible devices. The ability to convert these non-visible wavelengths directly into visible signals through efficient materials is thus a coveted goal. Achieving this conversion not only simplifies detection apparatus but also provides deep insights into photophysical mechanisms important for future technology.
Organic luminescent materials stand out as promising candidates to address this challenge. Their intrinsic benefits—lightweight nature, ease of chemical customization, and structural versatility—make them alluring for tailored optical applications. However, a persistent hurdle has been their optical efficiency, often compromised by energy losses through molecular vibrations and nonradiative decay. Overcoming these limitations requires a refined molecular design that restricts internal motion and harnesses beneficial intermolecular interactions.
Focusing on these principles, the research team designed a rigid, π-conjugated organic compound embedding a 1,2,5-thiadiazole-substituted pyrazine moiety. This molecular architecture fosters tight crystal packing conducive to collective optical phenomena. Through careful synthesis and crystallization techniques, they produced high-quality single crystals exhibiting unanticipated but highly desirable luminescent properties. Although visually yellow under ambient light, these crystals reveal extraordinary optical behavior under specific light excitations.
When exposed to ultraviolet illumination, the crystal emits a striking red fluorescence characterized by an exceptionally large Stokes shift—signifying that emitted photons possess considerably lower energy than absorbed photons. This phenomenon results from the formation of excimers, excited-state dimers stabilized by close intermolecular contacts unique to the crystal lattice. Excimer emission is rarely observed in solution, highlighting the critical role of the solid-state molecular arrangement in enabling this red luminescence.
Equally astonishing is the crystal’s optical response upon near-infrared irradiation. Instead of fluorescence, it exhibits green visible light generated via second harmonic generation (SHG), a nonlinear optical process where two photons of lower energy combine to produce a single photon at twice the frequency. This effect underscores the crystal’s nonlinear optical properties, typically a hallmark of inorganic crystals used in sophisticated optical applications. Witnessing SHG in an organic molecular crystal broadens the understanding of organic material capabilities in photonics.
What elevates this discovery is the coexistence and independence of both optical responses within the same crystal matrix without detrimental interference. This dual emission modality—red fluorescence from excimer states under UV light and green SHG under NIR excitation—demonstrates a harmonious confluence of fundamentally distinct photophysical processes. Such an intricate balance demands precise molecular design and controlled crystal engineering that stabilizes and segregates these phenomena spatially or energetically.
Professor Akiko Hori emphasizes the novelty of observing two divergent yet concurrent mechanisms within a single organic crystal. Through deliberate control over molecular structure and spatial packing, the research team has realized a system that effectively visualizes different invisible light regimes by leveraging distinct optical routes. This breakthrough challenges traditional notions that organic crystals are limited in their nonlinear optical functionalities and expands material strategies for multipurpose photonic applications.
The genesis of this work stems from the researchers’ curiosity about the interplay between molecular arrangements and resultant optical behavior. Initial observations of a yellow crystal unexpectedly emitting red light prompted deeper inquiry into the molecular packing’s influence on luminescence. This curiosity-driven scientific approach underscores how fundamental observations can inspire innovation in material science, where tuning crystal structuring can engineer diverse and enhanced optical responses.
The implications of this dual-mode emission reach far into future technological landscapes. Organic crystals capable of converting UV and NIR light to visible signals have profound utility as elements in optical sensing, imaging technologies, and measurement instrumentation. Unlike traditional inorganic crystals, these organic counterparts offer advantages in weight, processability, and potential integration into flexible or wearable devices. They may also reduce manufacturing complexity and cost, favoring widespread adoption in practical applications.
Furthermore, this research pushes the boundaries of molecular crystal engineering by revealing untapped potentials within organic materials for nonlinear optical functions traditionally dominated by inorganic systems. By exploiting molecular design and crystal packing nuances, scientists can tailor multifunctional photonic materials that respond distinctly to diverse optical stimuli. This synergy might fuel innovative device architectures supporting multiplexed sensing and light manipulation in communication and diagnostic technologies.
In summary, the successful demonstration of red fluorescence and green second harmonic generation within a single 1,2,5-thiadiazole-substituted pyrazine organic crystal represents a landmark advancement in material photonics. The intimate coupling of distinct photophysical mechanisms controlled via molecular and crystal engineering heralds a new era of organic crystals designed for dual or multifunctional optical operation. This study not only broadens the fundamental understanding of organic photonic materials but also lays the groundwork for next-generation sensing and imaging platforms that capitalize on visualizing unseen spectrums of light.
Subject of Research: Not applicable
Article Title: Red-fluorescence under UV and green-SHG under NIR dual-mode emission in a yellow crystal of a 1,2,5-thiadiazole derivative
News Publication Date: 22-Jan-2026
References: DOI: 10.1039/D5CC05735C
Image Credits: Professor Akiko Hori from Shibaura Institute of Technology, Japan
Keywords
Photonics, Organic chemistry, Optics, Ultraviolet radiation, Electromagnetic radiation, Infrared radiation, Luminescence, Optical properties, Crystallography, Sensors
