📄 chemistry

Color-Purity narrow-band near-infrared (NIR) phosphors of [Ir(C^N)2(N^N)]-bis- heteroleptic and [Ir(C^N)(C’^N’)(N^N)]-tris-heteroleptic Ir(III)-complexes forward to high-performance (all)-solution-processed NIR-PLEDs

This study demonstrates that asymmetric tris-heteroleptic Ir(III) complexes, specifically [Ir(dpbq)(iqbt)(ftrpz)], achieve superior color-purity near-infrared emission and record-breaking external quantum efficiencies in all-solution-processed NIR-PLEDs by optimizing the trade-off between efficiency, emission peak, and spectral width.

Original authors: Baowen Wang, Youquan Chen, Zhiming Zhang, Yu Chen, Xiaohui Yang, Sicheng Yao, Guorui Fu, Xingqiang Lü, Wai-Yeung Wong

Published 2026-08-12

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CC BY 4.0

Original authors: Baowen Wang, Youquan Chen, Zhiming Zhang, Yu Chen, Xiaohui Yang, Sicheng Yao, Guorui Fu, Xingqiang Lü, Wai-Yeung Wong

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

Imagine a world where light doesn't just have to be bright or colorful, but can also be invisible to the human eye yet powerful enough to see in the dark, heal tissues, or carry secret messages. This is the realm of Near-Infrared (NIR) light, a special band of the electromagnetic spectrum that sits just beyond the red end of what our eyes can see. Scientists have long been trying to create "light bulbs" that glow exclusively in this invisible zone without leaking any visible red or orange light, which would ruin the effect. The challenge is like trying to tune a radio to a single, quiet station in a storm of static; usually, when you push the light to be deeper red (infrared), it gets dimmer and fuzzier, losing its sharpness and efficiency. To solve this, researchers use tiny, complex molecules called Iridium(III) complexes. Think of these molecules as microscopic factories where electricity is turned into light. The goal is to design these factories so they only produce that specific, invisible infrared light with high efficiency, and to build the devices that hold them using simple, liquid-based methods (like painting or printing) rather than expensive, high-vacuum machinery.

In this study, a team of researchers decided to redesign the blueprint of these molecular factories. They moved away from the standard design, which usually uses two identical "main" parts and one "helper" part, to a more unique, three-part "asymmetric" design. They created a new molecule, which they named Ir-2, by mixing three different types of ligands (the building blocks that hold the iridium atom together). One part was chosen to be the "color master," responsible for creating the deep infrared glow, while another was a "stronger" part designed to tighten the molecule's structure and boost its performance. The result was a molecule that acted like a precision laser compared to the broader, fuzzier glow of its predecessor. The Ir-2 molecule emitted light at a peak of 758 nanometers with a very narrow spread (a Full Width at Half Maximum, or FWHM, of just 46 nm), meaning the light was incredibly pure and didn't leak into the visible spectrum. In contrast, their older version, Ir-1, glowed at 786 nm but was much broader (62 nm) and less efficient.

The researchers didn't just stop at making the molecule; they tested how well it worked in actual light-emitting devices. They built two types of prototypes. The first type used a mix of liquid and vacuum steps (non-all-solution), where the Ir-2 molecule helped create a device with a record-breaking efficiency of 2.456%. But the real magic happened with the second type: a device made entirely using liquid processing (all-solution), which is cheaper and easier to scale up. Even with this simpler manufacturing method, the Ir-2-based device achieved a record-breaking efficiency of 2.418% for this specific type of all-solution, color-pure infrared light. The study suggests that this new "three-part" molecular design is a powerful platform, proving that you can get high efficiency and pure color simultaneously in these difficult-to-make infrared lights, paving the way for cheaper, better devices for night vision, medical therapy, and secure communications.

Technical Summary: Color-Purity Narrow-Band NIR Phosphors Based on Asymmetric Tris-Heteroleptic Ir(III) Complexes

Problem Statement
The development of high-efficiency, color-purity near-infrared (NIR) organic/polymer light-emitting diodes (OLEDs/PLEDs) remains a formidable challenge. While iridium(III) (Ir(III)) complexes are successful in visible-light applications, their application in the NIR region (emission onset $\lambda_{em} \ge 700$ nm) is hindered by two primary factors:

The Energy Gap Law: As emission wavelengths extend beyond 700 nm, exciton-vibration coupling increases, leading to rapid non-radiative deactivation and reduced photoluminescent quantum yields ( $\Phi_{PL}$ ).
Spectral Purity vs. Efficiency Trade-off: Many NIR emitters suffer from broad emission spectra (large Full Width at Half Maximum, FWHM) due to charge-transfer (CT) effects, resulting in significant visible-light residuals. True "color-purity" NIR emission (Type-III), defined as full emission beyond 700 nm with minimal visible components, is difficult to achieve without sacrificing efficiency. Furthermore, while solution-processing offers cost advantages for large-area manufacturing, solution-processed NIR-PLEDs typically lag behind vacuum-deposited counterparts in performance.

Methodology
The authors employed a molecular design strategy focusing on asymmetric tris-heteroleptic Ir(III) complexes of the general formula $[Ir(C^N)(C'^N')(N^N)]$ .

Molecular Design: The study targeted the complex $[Ir(dpbq)(iqbt)(ftrpz)]$ (designated Ir-2). This design features:
- One color-responsible $Hdpbq $($ HC^N$) main ligand to ensure NIR emission.
- One $Hiqbt $($ HC'^N'$) ligand to modulate the electronic structure.
- One $Hftrpz $($ HN^N$) ancillary ligand with strong ligand-field strength to stabilize the complex and tune the emission.
- This was compared against a bis-heteroleptic counterpart, $[Ir(dpbq)_2(ftrpz)]$ (Ir-1), to evaluate the impact of the third distinct ligand.
Synthesis: The complexes were synthesized via a one-pot reaction involving $\mu$ -chloro-bridged dimer intermediates and subsequent purification via flash column chromatography.
Characterization: Comprehensive analysis included elemental analysis, FT-IR, $^1$ H NMR, ESI-MS, X-ray single-crystal diffraction, thermogravimetric analysis (TGA), and cyclic voltammetry (CV).
Device Fabrication:
- Non-all-solution-processed devices: Fabricated with a vacuum-deposited electron transport layer (TmPyPB) to benchmark performance (NIR-PLED-1A and NIR-PLED-2A).
- All-solution-processed devices: Fabricated using a multi-layer orthogonal solvent system strategy. This utilized a polymeric electron transport layer (Poly(vinyl-PBD)) to replace vacuum-deposited layers, enabling a fully solution-processed device (NIR-PLED-2B). The host matrix was PVK:OXD7 (60:35 wt%) with a 5% doping ratio.

Key Results

Structural and Thermal Properties: X-ray diffraction confirmed the asymmetric, distorted octahedral geometry of the tris-heteroleptic Ir-2 complex. Thermal stability analysis showed 5% weight-loss temperatures of 223°C (Ir-1) and 237°C (Ir-2), sufficient for solution-processing.
Photophysical Properties:
- Ir-2 exhibited a peak emission at 758 nm with a narrow FWHM of 46 nm and a photoluminescence quantum yield ( $\Phi_{PL}$ ) of 4.9%.
- Ir-1 emitted at 786 nm with a broader FWHM of 62 nm and a lower $\Phi_{PL}$ of 3.6%.
- Theoretical calculations (TD-DFT) indicated that the superior performance of Ir-2 arises from a "trade-off" between efficiency and spectral purity. The introduction of the $iqbt$ ligand increased the $^3$ MLCT character (6.89% vs. 5.80% in Ir-1), enhancing the radiative decay rate ( $k_r$ ), while the specific ligand combination maintained a narrow emission band dominated by $^3$ ILCT character.
Device Performance:
- Non-all-solution-processed (NIR-PLED-2A): Achieved a maximum external quantum efficiency ( $\eta_{EQE}$ ) of 2.456% at 758 nm, with a radiance of 970.3 mW/(sr·m²).
- All-solution-processed (NIR-PLED-2B): Using the polymeric ETL strategy, the device achieved a record $\eta_{EQE}$ of 2.418% at 758 nm. This performance surpassed previously reported Ir(III)-complex-based color-purity all-solution-processed NIR-PLEDs.
- Both devices demonstrated "Type-III" color purity, with nearly 100% of the emission spectrum occurring beyond 700 nm.

Significance and Claims
The paper claims that the asymmetric tris-heteroleptic design strategy successfully addresses the efficiency-purity trade-off in NIR emitters. By utilizing a specific combination of ligands (one color-responsible, one structural modulator, and one strong-field ancillary), the authors achieved a "compromise" that optimizes the balance between emission wavelength, spectral width, and quantum efficiency.

The primary significance of this work is the demonstration that high-performance, color-purity NIR-PLEDs can be realized via all-solution processing. The study establishes $[Ir(C^N)(C'^N')(N^N)]$ complexes, specifically Ir-2, as a viable platform for efficient, low-cost, large-area NIR optoelectronic devices, overcoming the historical performance gap between solution-processed and vacuum-deposited NIR OLEDs.

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