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

Traditionally, organic luminescent materials have been classified based on their ability to exhibit either fluorescence or phosphorescence, two fundamentally different types of light emission originating from distinct excited-state processes. Fluorescence involves the prompt emission of photons as excited electrons return to the ground state, typically within nanoseconds. In contrast, phosphorescence arises from the slower relaxation of electrons trapped in a triplet excited state, extending emission lifetimes into microseconds and beyond. The ability to simultaneously manipulate both these emissive pathways within a single material platform has long challenged scientists due to the conflicting time scales and spin multiplicities involved.

The research team addressed this challenge by designing an organic luminescent system that strategically incorporates multiple excited states, thereby enabling efficient intersystem crossing and reverse intersystem crossing mechanisms in tandem. By fine-tuning the molecular architecture, they successfully achieved a material where delayed fluorescence—a form of thermally activated delayed fluorescence (TADF)—and phosphorescence coexist. This dual emission process was demonstrated under ambient conditions, a critical criterion for practical applications.

Central to this revelation is the molecular engineering that balances singlet and triplet excited states, allowing the material to harness triplet excitons that traditionally remain non-radiative or contribute to phosphorescence only. In this system, the conversion of triplet excitons back to singlet states facilitates delayed fluorescence. Concurrently, a portion of the triplet population emits directly through phosphorescence. The concurrency of these radiative decay channels is meticulously controlled through quantum yield optimization and excited state energy alignment.

The implications of this discovery extend beyond fundamental photophysics to real-world applications. Organic light-emitting diodes (OLEDs), a technology increasingly pervasive in modern display and lighting systems, stand to benefit significantly. By leveraging both delayed fluorescence and phosphorescence, devices can attain higher internal quantum efficiencies without relying on heavy metal dopants, which are not only costly but pose environmental concerns. This all-organic approach promises more sustainable and efficient OLED designs.

Moreover, the ability to tune the emission via multiple excited states opens new possibilities for color purity and tunability in lighting applications. Traditional phosphorescent materials often suffer from spectral broadening or color instability, while pure fluorescence emitters may lack efficiency. The dual mechanism stabilizes emission profiles and enhances brightness, potentially enabling customizable, high-resolution displays and adaptive lighting systems responsive to environmental inputs.

In the realm of bioimaging, organic compounds exhibiting prolonged emission lifetimes, namely through delayed fluorescence and phosphorescence, can significantly improve imaging resolution and contrast by minimizing background fluorescence. This allows for time-gated imaging techniques that isolate the desired luminescent signals, an advantage this new material system could amplify, offering more sensitive diagnostic tools and real-time bio-probes.

The authors employed an array of spectroscopic techniques to unravel the material’s excited-state dynamics, including time-resolved photoluminescence and transient absorption measurements, validating the coexistence of delayed fluorescence and phosphorescence with distinct temporal profiles. Their rigorous characterization ensures that the observed dual emission is intrinsic to the molecular design rather than an artifact of environmental variations or impurities.

Additionally, computational studies using quantum chemical calculations provided insight into the energy landscape and spin-orbit coupling effects governing intersystem crossing rates. The simulations guided the rational design of molecular entities with appropriate singlet-triplet energy gaps, a critical parameter for efficient reverse intersystem crossing that underpins delayed fluorescence.

This research represents a paradigm shift in the understanding and utilization of organic luminescent materials. By demonstrating control over multiple excited states to enable concurrent delayed fluorescence and phosphorescence, it redefines the boundaries of organic optoelectronics. The ability to engineer materials with tailored emission kinetics and spectral properties unlocks synergies previously deemed incompatible within a single molecular platform.

Future directions proposed by the team include expanding the molecular library of such dual-emissive compounds and integrating these materials into functional devices to test performance under operational conditions. They highlight the promise of this approach not only in OLEDs but also in organic lasers, sensing devices, and luminescent solar concentrators, suggesting a broad technological impact.

Challenges remain, particularly in scaling synthesis, ensuring long-term stability, and optimizing emission efficiency across the visible spectrum. However, the foundational knowledge established in this study offers a research roadmap toward overcoming these hurdles. Collaborative efforts bridging chemistry, physics, and engineering will be pivotal in translating this molecular innovation into commercial products.

In essence, Dou and colleagues’ breakthrough underscores the power of rational molecular design combined with mechanistic insight to circumvent limitations inherent in organic luminescent materials. This work exemplifies how a nuanced understanding of excited-state multiplicities and their interplay can be leveraged to craft materials with unprecedented photophysical properties, influencing a spectrum of scientific and industrial fields.

As the demand for sustainable, efficient, and versatile lighting and display technologies intensifies, such advances underscore the critical role of fundamental science in driving innovation. The confluence of delayed fluorescence and phosphorescence within a singular organic emitter charts a new course for the next generation of luminescent materials, heralding a future where organic electronics can achieve previously unattainable levels of performance and functionality.

Subject of Research:
Organic luminescent materials exhibiting simultaneous delayed fluorescence and phosphorescence through multiple excited states.

Article Title:
Simultaneous delayed fluorescence and phosphorescence in organic luminescent material employing multiple excited states.

Article References:
Dou, D., Liu, W., Zhou, X. et al. Simultaneous delayed fluorescence and phosphorescence in organic luminescent material employing multiple excited states. Light Sci Appl 15, 4 (2026). https://doi.org/10.1038/s41377-025-02063-x

Image Credits:
AI Generated

DOI:
01 January 2026