Key Takeaways & Executive Findings
- •• • Novel MR-TADF skeleton: Breaking the fused ring with a linear conjugated diene linker enables simultaneous introduction of three MR units in one step, achieving high yield and reduced synthetic cost compared to traditional fused-ring extensions. • • Pure-red emission: Both NF-CON1 and NF-CON2 emit at 620 nm with FWHM <35 nm in toluene, meeting the stringent color purity requirements for wide-color-gamut displays. • • Efficient RISC: Singlet-triplet energy gaps of ~0.2 eV and RISC rates of ~3.0×10^5 s^-1 ensure rapid upconversion, supporting high internal quantum efficiency. • • Solution-processability: Low reorganization energies (0.209 eV for NF-CON1, 0.194 eV for NF-CON2) and short delayed lifetimes (31–40 μs) mitigate exciton quenching, enabling efficient solution-processed OLEDs.
Abstract
Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters are pivotal for achieving high color purity and 100% internal quantum efficiency in organic light-emitting diodes (OLEDs). However, extending emission to the red region (>600 nm) remains challenging due to limited MR cores and the synthetic complexity of fused-ring extensions. Here, we report a novel strategy that breaks the fused ring by employing a linear conjugated diene linker between nitrogen and carbonyl moieties, enabling simultaneous introduction of three MR units in one step with high yield. Two emitters, NF-CON1 and NF-CON2, exhibit pure-red emission at 620 nm with narrow full-width at half-maximum (FWHM) below 35 nm in dilute toluene, small Stokes shifts of 26 nm, and weak solvatochromic shifts (~25 nm from toluene to ethanol), confirming the MR-characteristic short-range charge transfer. The emitters show low reorganization energies (0.209 eV for NF-CON1 and 0.194 eV for NF-CON2), singlet-triplet energy gaps of ~0.2 eV, and high reverse intersystem crossing rates of ~3.0×10^5 s^-1. In doped films with m-MTDATA, delayed lifetimes of 31–40 μs are achieved. These properties, combined with high oscillator strengths, ensure efficient RISC and high IQE. The facile synthesis and solution-processability of this MR-TADF skeleton address the bottlenecks of cost and scalability, offering a promising route for practical pure-red OLEDs.
1. Introduction
Conventional red emitters in OLEDs rely on noble-metal phosphorescent complexes or donor-acceptor TADF molecules, but achieving both high efficiency and color purity remains elusive. Multi-resonance TADF (MR-TADF) offers narrow emission and 100% IQE, yet extending emission to the red region (>600 nm) typically requires extending fused aromatic rings, which increases synthetic complexity and cost, and exacerbates aggregation-caused quenching. Moreover, the large planar structures hinder solution processing, a key requirement for low-cost, large-area manufacturing.
This work introduces a novel strategy that breaks the fused ring by linking nitrogen and carbonyl moieties via a linear conjugated diene. This design extends conjugation to lower the energy gap while preserving the MR effect, as evidenced by alternating charge distribution and narrow emission. The approach allows one-step introduction of three MR units, significantly simplifying synthesis. The resulting emitters, NF-CON1 and NF-CON2, exhibit pure-red emission at 620 nm with FWHM below 35 nm, low reorganization energies, and high RISC rates, demonstrating a viable path to solution-processed pure-red OLEDs with high color purity and efficiency.
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SHEN Xu, YE Tao, CHEN Runfeng (2026). Breaking the Fused Ring: A Novel MR-TADF Skeleton for Solution-Processed Pure-Red OLEDs. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3861-4
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Frequently Asked Questions
What is the synthetic yield and scalability of the proposed one-step introduction of three MR units?
The research text states that three MR units can be introduced simultaneously in one step with high yield, but specific yield percentages are not provided. The high yield and facile preparation are highlighted as advantages for practical production, suggesting scalability is favorable compared to traditional multi-step fused-ring syntheses.
How do the photophysical properties of NF-CON1 and NF-CON2 compare to state-of-the-art red MR-TADF emitters in terms of FWHM and efficiency?
NF-CON1 and NF-CON2 exhibit FWHM <35 nm at 620 nm, which is narrower than typical nitrogen/carbonyl MR-TADF emitters that often exceed 50 nm. Their small Stokes shifts (26 nm) and weak solvatochromism (~25 nm) confirm MR characteristics. While efficiency data are not fully detailed, the high RISC rates (~3.0×10^5 s^-1) and low reorganization energies (0.209/0.194 eV) suggest competitive performance.
What are the operational stability and degradation mechanisms of these emitters under continuous electrical stress in OLED devices?
The research text does not provide device lifetime or stability data. However, the short delayed lifetimes (31–40 μs) and low reorganization energies may reduce exciton-induced degradation. Further studies are needed to assess long-term operational stability.
How does the solution-processability of these emitters compare to vacuum-deposited counterparts, and what are the implications for manufacturing cost?
The emitters are designed for solution processing, which is advantageous for low-cost, large-area fabrication. The research highlights that the large planar structures of traditional fused-ring MR-TADF emitters hinder solution processing, whereas the novel skeleton with a linear diene linker likely improves solubility and film formation. This could reduce manufacturing costs compared to vacuum deposition.
What is the external quantum efficiency (EQE) achieved in the OLED devices, and how does it correlate with the photophysical properties?
The research text mentions that a series of OLED devices were fabricated, but specific EQE values are not provided in the excerpt. The high oscillator strengths and efficient RISC suggest potential for high IQE, but actual EQE depends on device architecture and outcoupling. Further details are needed.
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