Rong-Huei Yi

Nature, Published online: 25 May 2022; doi:10.1038/s41586-022-04726-w

Nature, Published online: 25 May 2022; doi:10.1038/s41586-022-04634-z

Molecular design of donor–acceptor–donor triads for high reverse intersystem crossing (hRISC) is discussed considering the frontier molecular orbitals, the exciton coupling model, and high-throughput computational screening. The latter is shown to be optimal to identify hot exciton hRISC candidates from a diverse dataset of 234 systems. Analysis of their hybrid local and charge transfer character is performed.

Abstract

Hot exciton materials have the potential to improve the quantum efficiency of organic light-emitting diodes (OLEDs) by promoting high reversed intersystem crossing (hRISC) between a high-lying triplet (T _n , n ≥ 2) and a radiative singlet (S _m ). In recent years, donor–acceptor–donor (D–A–D) molecular systems have shown great promise in its ability to enhance the hRISC process under certain conditions. However, strategies to find appropriate D–A–D combinations beyond trial-and-error are still elusive. This work exposes the limited applicability of the current fragment-based design rules and proposes high-throughput screening as the optimal route to find candidates that fulfill the energy criteria for hRISC. The strategy consists of first establishing the thresholds for large triplet–triplet splitting and small singlet–triplet gap, then filtering combinations through rate comparison of competitive crossing pathways, and finally confirming hRISC with spin-orbital coupling evaluation. Based on a dataset of 234 compounds, this protocol identifies 31 candidates with potential for hRISC, 4 of which are reported in the literature. Remarkably, while most of the promising systems show prominent hybridized local and charge transfer character, several candidates do not fulfill this condition, indicating that different routes are possible to design efficient OLED materials.

Four ternary boron/nitrogen-based polycyclic heteroaromatic emitters via B-π-B and B-π-C, two design types to modulate the size of the π-core, show small ∆E _ST, high k _r and k _RISC values with FWHMs of 24, 27, 20, and 28 nm, respectively. The color-tuning organic light-emitting diodes display high external quantum efficiencies of 13.7%, 17.6%, 26.7%, and 21.8%, respectively, which cover from blue to yellow electroluminescence.

Abstract

The development of boron/nitrogen-based polycyclic heteroaromatic emitters with multiple-resonance thermally activated delayed fluorescence (MR-TADF) property can efficiently promote the advancement of high-efficiency organic light-emitting diodes (OLEDs) with narrowband emission. Herein, a simple strategy to achieve four ternary boron/nitrogen-based polycyclic heteroaromatic emitters (SBON, SBSN, DBON, and DBSN) from pure blue (463 nm) to yellow (553 nm) via tuning the coordination between B/N and heteroatom (O or S) is demonstrated, aiming to increase charge transfer delocalization of the polycyclic heteroaromatic emitters and adjust photo-physical properties. This strategy endows the four emitters with full width at half maximum (FWHM) of 24, 27, 20, and 28 nm, respectively. Additionally, double-boron type green and yellow narrowband emitters (DBON and DBSN) possess 98% photoluminescence efficiencies in doped films. Besides, considerable rate constants of reverse intersystem crossing (RISC) are achieved because of the small singlet–triplet excited state energy gap and large spin–orbit coupling values. Consequently, the OLEDs covering from blue to yellow based on these emitters show the maximum external quantum efficiency of 13.7%, 17.6%, 26.7%, and 21.8%, respectively, with low-efficiency roll-off. These results provide a feasible design strategy to construct boron/nitrogen-based polycyclic heteroaromatic MR-TADF emitters for efficient OLEDs with color-tuning electroluminescence.

Publication date: August 2022

Source: Dyes and Pigments, Volume 204

Author(s): Zhenhong Zhao, Guoliang Wang, Xin Luo, Xiangbin Tian, Daqing Zhang, Shiyan Guo, Haitao Zhou, Yanqin Miao, Jinhai Huang, Hua Wang

Publication date: August 2022

Source: Dyes and Pigments, Volume 204

Author(s): Chunlong Shi, Di Liu, Jiuyan Li, Zhaolong He, Kai Song, Botao Liu, Qi Wu, Min Xu

Publication date: August 2022

Source: Dyes and Pigments, Volume 204

Author(s): Ming-Xing Zhang, Xiaofei Yang, Fen Tan, Xiaowen Ou, Guoping Zeng, Didi Chen, Zhiqiang Xu, Sheng Hua Liu

A series of narrowband red emitters with superb photophysical properties is developed by way of novel B/N/O-based polycyclic multiple resonance structures. The red OLEDs incorporating these emitters sensitized by the phosphor display state-of-the-art performance with EQE exceeding 36%, ultralow efficiency roll-off, ultrahigh brightness, and good device lifetime.

Abstract

High-color-purity blue and green organic light-emitting diodes (OLEDs) have been resolved thanks to the development of B/N-based polycyclic multiple resonance (MR) emitters. However, due to the derivatization limit of B/N polycyclic structures, the design of red MR emitters remains challenging. Herein, a series of novel red MR emitters is reported by para-positioning N–π–N, O–π–O, B–π–B pairs onto a benzene ring to construct an MR central core. These emitters can be facilely and modularly synthesized, allowing for easy fine-tuning of emission spectra by peripheral groups. Moreover, these red MR emitters display excellent photophysical properties such as near-unity photoluminescence quantum yield (PLQY), fast radiative decay rate (k _r) up to 7.4 × 10⁷ s⁻¹, and most importantly, narrowband emission with full-width at half-maximum (FWHM) of 32 nm. Incorporating these MR emitters, pure red OLEDs sensitized by phosphor realize state-of-the-art device performances with external quantum efficiency (EQE) exceeding 36%, ultralow efficiency roll-off (EQE remains as high as 25.1% at the brightness of 50 000 cd m⁻²), ultrahigh brightness over 130 000 cd m⁻², together with good device lifetime.

The authors present that the potential molecular room temperature phosphorescence (RTP) of carbazole derivatives can be effectively unlocked by dispersing into polymers at the molecular level through a thermoplasticizing strategy, challenging the existing experimental results and the prevailing RTP views. Moreover, colorful RTP thermoplastics as well as a red fluorescence afterglow emission with an ultralong photopattering memory effect are realized for the first time.

Abstract

The current prevailing views are that carbazole derivatives without isomers show inferior room temperature phosphorescence (RTP) and that N-aryl carbazole derivatives exhibit no ultralong RTP in dilute film states since the distorted structures easily dissipate the excitation energy. In the current work, we present that doping N-aryl carbazole derivatives with heteroatoms and/or heavy halogen into PMMA at the molecular level by thermoplasticizing solution-cast films can achieve ultra-long-lived (>1 s) and highly-efficient (>36%) molecular RTP without the need of crystallization and isomer doping. Unlike isomer-doped crystals that almost all emit yellow RTP, the RTP colors of doped PMMA films depend on the molecular structures of carbazole derivatives. Also, we can realize highly efficient and long-lived red fluorescence afterglow via persistent Förster resonance energy transfer from RTP emission to fluorescent emitter. These colorful afterglow polymer films exhibit an ultra-long (>20 h) photo-activation pattern memory effect. This work has challenged existing experimental results and relative RTP mechanisms, providing a reasonable processing strategy for manifesting and enhancing the deserved RTP properties of organic doped polymers.

Gold-centered carbene-metal-amide materials with nitrogen substitution at various positions of the carbazole donor allow systematic control and variation of the charge transfer and locally excited states. Aza-substitution in para-position of the carbazole is a reliable strategy to realize bright, fast, and deep-blue luminescent materials while maintaining the charge transfer character of the emission mechanism.

Abstract

A series of gold-centered carbene-metal-amide (CMA) complexes are synthesized with the carbazole donor ligand modified by substitution with nitrogen atoms in varying positions. The luminescence of new aza-CMA complexes shows a significant blueshift depending on the position of the N atom, to provide bright blue-green (500 nm), sky-blue (478 nm), blue (450 nm) and deep-blue (419 nm) light-emitters. The impact of the electron-withdrawing aza-group on the nature of the luminescence and the excited state energies of the locally excited (LE) or charge transfer (CT) states have been interpreted with the help of transient absorption, in-depth photoluminescence experiments and theoretical calculations. By considering the orbital characters of the lowest CT and LE states, we develop a new concept for simultaneous energy tuning for both of these states with a single aza-substitution, allowing for fast and blue CT emission. This concept allows the interference of ³LE phosphorescence to be avoided at room temperature. The approach is extended to two N substitutions at the optimal location in the 3- and 6-positions of the carbazole skeleton. These results suggest a practical molecular design towards the development of bright and deep-blue emitting CMA materials to tackle the stability problem of energy-efficient deep-blue OLEDs.

Efficient deep-blue luminescence via thermally activated delayed fluorescence is obtained with a dense manifold of excited-state energies. Fast reverse intersystem crossing rates enable small efficiency roll-off in organic light-emitting diodes (OLEDs). Hyperfluorescence OLEDs using ν-DABNA exhibit high efficiency and a color purity with CIE (Commission Internationale de l'Éclairage) coordinates (0.13,0.15).

Abstract

There is increasing interest in thermally activated delayed fluorescence (TADF) in materials, and to understand its mechanism in the excited state dynamics. Recent challenges include color purity, efficient deep-blue emission, fast exciton decay lifetimes, high reverse intersystem crossing rates (k _RISC), low-efficiency roll-off in organic light-emitting diodes (OLEDs), and long device lifetimes. Here, a series of compounds having benzonitrile and carbazole rings are examined, that provide a detailed understanding of the excited states, and a guideline for high-performance TADF. A dense alignment of the excited states with several different characters within a small energy range results in high k _RISC of >2 × 10⁶ s⁻¹, while maintaining radiative rate constants (k _r) >10⁷ s⁻¹. OLEDs based on the optimum compound exhibit a low-efficiency roll-off and a CIEy (y color coordinate of Commission Internationale de l'Éclairage) <0.4. TADF-assisted fluorescence (TAF) OLED exhibits a maximum external quantum efficiency of 22.4% with CIE coordinates (0.13,0.15). This work also provides insights for device engineering and molecular designs.

This work proposes the circularly polarized luminescence molecule based on a hybridized local and charge-transfer (HLCT) chromophore through chiral perturbation, achieving excellent device performances of high exciton utilization and low-efficiency roll-off with a thermally activated delayed fluorescence sensitizer, which could pave the way to develop the novel CP-HLCT materials and highly efficient circularly polarized organic light-emitting diodes.

Abstract

This work describes the first hot exciton fluorescent material based on benzo[c][1,2,5]thiadiazole and chiral binaphthol enabling circularly polarized luminescence (CPL) through a chiral perturbation strategy. The new molecular architecture displays CPL, hybridized local and charge transfer (HLCT) properties concurrently. Utilizing it as the emitter, circularly polarized organic light-emitting diodes (CP-OLEDs) achieve an external quantum efficiency (EQE) of 7.2% with a good exciton utilization (36%) and a moderate circularly polarized electroluminescence (CPEL) dissymmetry factor (g _EL, 2.1 × 10⁻³). In addition, the CP-HLCT molecule is sensitized by a thermally activated delayed fluorescence material, significantly ameliorating the efficiency of HLCT fluorescent CP-OLEDs. Excellent performances of twofold maximum EQE (EQE_max) of 15.3% and 82% exciton utilization are obtained in the sensitized device, regarding an extremely low-efficiency roll-off of 2.6% at 1000 cd m⁻² as well as CPEL with a g _EL value of 2.0 × 10⁻³.

Publication date: August 2022

Source: Dyes and Pigments, Volume 204

Author(s): Junqing Shi, Zhiyu Ran, Fuwei Peng

Publication date: August 2022

Source: Dyes and Pigments, Volume 204

Author(s): Hao Ren, Yongjun Song, Renyou Yu, Mingxing Tian, Lei He

Publication date: August 2022

Source: Dyes and Pigments, Volume 204

Author(s): Puttavva Meti, Hwa-Sung Lee, Young-Dae Gong

An accessible, fast procedure is shown to measure light-emitting diode (LED) emission zones with current density, different device structures, and ageing. The emission zone in an organic LED is shown to be controlled by emitter doping and to have a strong relationship with device degradation and lifetime. Using the resulting insights, record stability for a thermally activated delayed fluorescence (TADF) organic LED is shown.

Abstract

Device optimization of light-emitting diodes (LEDs) targets the most efficient conversion of electrically injected charges into emitted light. The emission zone in an LED is where charges recombine and light is emitted from. It is believed that the emission zone is strongly linked to device efficiency and lifetime. However, the emission zone size is below the optical diffraction limit, so it is difficult to measure. An accessible method based on a single emission spectrum that enables emission zone measurements with sub-second time resolution is shown. A procedure is introduced to study and control the emission zone of an LED system and correlate it with device performance. A thermally activated delayed fluorescence organic LED emission zone is experimentally measured over all luminescing current densities, while varying the device structure and while ageing. The emission zone is shown to be finely controlled by emitter doping because electron transport via the emitter is the charge-transport bottleneck of the system. Suspected quenching/degradation mechanisms are linked with the emission zone changes, device structure variation, and ageing. Using these findings, a device with an ultralong 4500 h T ₉₅ lifetime at 1000 cd m⁻² with 20% external quantum efficiency is shown.

The change of donors in CT-based molecules significantly influences the charge carrier generation and follow-up reactions that affect the TADF emitter‘s stability. The TADF emitters can be easily electropolymerised, forming stable conjugated polymers. Electrochemically active TADF emitters possess significant electrochromic properties and can be used as both electrochromic and electroluminescence materials. The dual behaviour of the TADF emitters allows use in both OLED and Electrochromic Windows applications.

Abstract

Previous work has reported the synthesis of donor–acceptor–donor molecules based on dibenzophenazine acceptor group, presenting thermally activated delayed fluorescent (TADF) properties and their application in the assembly of highly efficient electroluminescent devices. Herein, we focus on the characterisation of charge carrier species through UV-Vis-NIR spectroelectrochemical and potentiostatic EPR techniques, in addition to the investigation of electropolymerisation properties of some compounds depicted in this study. The promising electrochromic features of both small molecules and conjugated polymers led to the assembly and investigation of electrochromic devices, evidencing the materials’ versatility, applied in such different approaches as electrochromic windows and electroluminescent devices. Furthermore, the assembled OLEDs provided high efficiencies, with small roll-off, EQEs up to 20.5 % and luminance values up to 85 000 cd/m².

Light: Science & Applications, Published online: 17 May 2022; doi:10.1038/s41377-022-00826-4