📄 chemistry

Ultraviolet Persistent Luminescence in Purely Organic Systems

This study reports a "hot-exciton" strategy utilizing indolocarbazole derivatives to achieve room-temperature ultraviolet persistent luminescence in purely organic systems by enabling reverse intersystem crossing through a high-lying triplet state, thereby overcoming the spectral limitations of conventional metal-free emitters.

Original authors: Yujian Zhang, Qing Luo, Xuan Zhang, Cong Liu, Yu Lang, Huiwen Zeng, Jiadong Zhou, Guang Shi, Bingjia Xu

Published 2026-08-25

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CC BY 4.0

Original authors: Yujian Zhang, Qing Luo, Xuan Zhang, Cong Liu, Yu Lang, Huiwen Zeng, Jiadong Zhou, Guang Shi, Bingjia Xu

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Light that glows in the dark is a familiar wonder, from fireflies to glow-in-the-dark stars on a child's ceiling. In the scientific world, this phenomenon is called persistent luminescence, where a material absorbs energy from a light source and then slowly releases it as a visible glow long after the source is removed. For decades, scientists have relied on inorganic materials, often containing metals, to create these long-lasting glows. More recently, researchers have developed purely organic versions, made from metal-free molecules, which are flexible and easier to manufacture. However, a significant hurdle has remained: while these organic materials can glow in visible colors like green or red, or even in the near-infrared spectrum, they have stubbornly refused to glow in the ultraviolet range. This missing piece of the puzzle is crucial because ultraviolet light has unique applications, such as sterilizing surfaces or aiding in medical treatments, but creating a metal-free organic material that glows in this specific, invisible spectrum has been considered nearly impossible.

The challenge lies in how these molecules handle energy. When a molecule absorbs light, its electrons jump to a higher energy state. To glow, these electrons must eventually fall back down, releasing that energy as light. In many organic materials, the electrons get stuck in a "triplet" state, a specific energetic configuration that usually leads to a glow in the red or blue part of the spectrum, but not the high-energy ultraviolet range. To get ultraviolet light, scientists typically need the electrons to jump back to a higher energy level before falling, but the energy gaps between these levels in organic molecules are often too wide to allow this to happen efficiently at room temperature. It is a delicate balancing act where the molecular structure must be rigid enough to prevent energy loss but flexible enough to allow the necessary energy jumps.

A team of researchers at Zhejiang Normal University and South China Normal University has now found a way to bypass this limitation using a strategy they call the "hot exciton" approach. Instead of trying to force the electrons through the usual, difficult path, they designed a specific organic molecule, a derivative of indolocarbazole named ICzO2, that allows the electrons to take a different, higher-energy route. In this new pathway, the electrons move from a higher triplet state directly to a singlet state, a process that is usually too slow to be useful but becomes viable in this specific molecular design. The researchers synthesized this molecule and embedded it into a film made of polyvinyl alcohol, a common plastic used in things like glue and packaging. When they shined ultraviolet light on the film, the material absorbed the energy and, remarkably, began to emit its own ultraviolet light that persisted for over a second after the light source was turned off.

The glow was not just a fleeting flash; it lasted for 1.01 seconds, a duration long enough to be seen clearly by the human eye under normal conditions. The light appeared at two specific wavelengths, 390 and 370 nanometers, placing it firmly in the ultraviolet and deep blue regions of the spectrum. To understand exactly how this was happening, the team conducted a series of rigorous tests. They measured how the glow changed with temperature and time, and they used computer simulations to map out the energy levels of the molecule. The results confirmed that the molecule's design created a specific energy gap that was just right to allow the electrons to cross from the higher triplet state to the singlet state. This "hot" pathway was the key, as the traditional route was blocked by a much larger energy gap that would have prevented the glow. The researchers also ruled out the possibility that the glow was caused by impurities or accidental mixtures, proving that the effect came directly from the ICzO2 molecules themselves.

This discovery opens a new door for creating ultraviolet light sources that are purely organic. Because the material is based on simple carbon-based molecules, it can be easily processed into large, flexible films. The researchers demonstrated this potential by creating a film that could be "written" on with ultraviolet light. By placing a stencil with a peony pattern over the film and exposing it to light, they created a glowing image that could be read out later. This image could even be erased by heating the film, allowing the process to be repeated with new patterns. This rewritable capability suggests that these materials could be used for secure printing or temporary displays. Furthermore, because the material glows in the ultraviolet and blue regions, it can act as a power source to excite other dyes, potentially leading to complex, multi-colored glowing systems. The work provides a clear, practical method for achieving ultraviolet persistent luminescence without the need for heavy metals, solving a problem that has limited the development of organic light-emitting technologies for years.

Technical Summary: Ultraviolet Persistent Luminescence in Purely Organic Systems

Problem Statement
While purely organic persistent luminescence (PersL) materials have advanced significantly, achieving ultraviolet (UV) emission (315–400 nm) from metal-free organic molecules remains a formidable challenge. Conventional strategies face inherent trade-offs:

Phosphorescence: Typically relies on low-lying triplet states ( $T_1$ ) in fused-ring molecules. However, these states often possess low energy levels that result in visible or near-infrared emission rather than UV. Furthermore, the large singlet-triplet energy gap ( $\Delta E_{ST}$ ) in short-conjugated, UV-emitting molecules hinders efficient intersystem crossing (ISC).
Thermally Activated Delayed Fluorescence (TADF): Requires a small $\Delta E_{ST}$ to facilitate reverse intersystem crossing (RISC) from $T_1$ to $S_1$ . Achieving this small gap usually necessitates charge-transfer (CT) character, which induces a red shift in emission, pushing the spectrum out of the UV region and into the visible range.

Consequently, there is a lack of purely organic materials capable of emitting UV PersL at room temperature.

Methodology
The authors propose a "hot-exciton" strategy to bypass the limitations of conventional $T_1 \to S_1$ RISC pathways. This approach utilizes high-lying triplet states ( $T_n, n \ge 2$ ) to facilitate RISC to the $S_1$ state (hRISC).

Molecular Design: A new indolocarbazole derivative, ICzO2 (6,7-dihydro-diindolo[1,2,3-de:3',2',1'-ij]quinoxaline), was synthesized via N-alkylation. A control molecule, ICzC2 (featuring a cleaved alkyl unit), was also synthesized to rule out impurity effects.
Matrix Engineering: The molecules were doped into rigid polymer matrices, specifically Poly(vinyl alcohol) (PVA) and melamine-formaldehyde (MF), to suppress non-radiative decay and stabilize triplet excitons.
Characterization: Comprehensive photophysical analysis was conducted, including steady-state and time-resolved photoluminescence (PL) spectroscopy, temperature-dependent delayed PL measurements (260–320 K), and density functional theory (DFT) calculations (M062X/6-31G(d,p) and B3LYP-D3(BJ)/6-311g(d) levels) to map energy levels and spin-orbit coupling.

Key Results

Photophysical Properties: The ICzO2 molecule exhibits localized exciton (LE) characteristics with a small charge-transfer (CT) component (excited-state dipole moment $\mu_e = 6.4$ D). In a 0.1 wt% doped PVA film, ICzO2 achieved a photoluminescence quantum yield (PLQY) of 41.6%.
UV Persistent Luminescence: Upon photoactivation, the doped film emitted bright UV PersL visible to the naked eye for approximately 4 seconds. The emission spectrum featured two distinct peaks at 390 nm and 370 nm. The lifetime of this delayed emission was measured at 1.01 s, a significant increase from the initial 34–50 ms observed before oxygen depletion in the matrix.
Mechanism Validation:
- Energy Gaps: Theoretical calculations revealed a large $S_1-T_1$ energy gap of 0.60 eV (experimental: 0.63 eV), which effectively suppresses conventional RISC from $T_1$ .
- Hot-Exciton Pathway: Conversely, the energy gap between the second triplet state ( $T_2$ ) and $S_1$ was calculated to be a moderate 0.42 eV. This facilitates a slow, thermally activated RISC process from $T_2$ to $S_1$ (hRISC).
- Rate Constants: The calculated rate constant for $T_2 \to S_1$ RISC ( $k_{RISC} \approx 10.7 s^{-1}$ ) is orders of magnitude higher than that for $T_1 \to S_1$ ( $k_{RISC} \approx 6.0 \times 10^{-5} s^{-1}$ ), confirming the dominance of the hot-exciton pathway.
Control Experiments: The control molecule ICzC2 showed a red-shifted emission and longer lifetimes, confirming that the specific cyclic structure of ICzO2 is critical for the observed UV PersL. The independence of emission from excitation wavelength and the lack of PersL in crystals at room temperature ruled out isomeric impurities as the source of the signal.

Significance and Applications
The paper claims to report the first instance of ultraviolet PersL in purely organic materials achieved through the manipulation of hot excitons. This strategy offers a new avenue for designing metal-free organic UV emitters that do not rely on the trade-off between small energy gaps and red-shifted emission.

Demonstrated applications include:

Rewritable Photoprinting: Large-area, flexible PVA films doped with ICzO2 were used to create patterns (e.g., peony shapes) via UV exposure. These patterns could be "read" via UV re-irradiation and "erased" by heating, demonstrating potential for reusable data storage or security printing.
Energy Transfer: The broad UV/blue emission of ICzO2 allows it to function as an efficient energy donor for phosphorescence-fluorescence resonance energy transfer (PhFRET). When paired with acceptors like sodium fluorescein or rhodamine B, the system produced colored long-afterglow patterns, suggesting utility in multifunctional optical devices.

In summary, this work establishes that "hot-exciton" materials with hybridized local and charge-transfer (HLCT) characteristics can overcome the energetic constraints of traditional organic phosphors, enabling efficient, room-temperature UV persistent luminescence.

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