Manipulating Spectral Width and Emission Wavelength towards Highly Efficient Blue Asymmetric Carbazole Fused Multi-Resonance Emitters

Rational Design of Color-Pure Blue Organic Emitters by Poly-Heteroaromatic Omni-Delocalization

Current research on organic light emitters which utilize multiple resonance-induced thermally activated delayed fluorescence (MR-TADF) materials is gaining significant interest because of the materials' ability to efficiently generate color-pure blue emission. However, the underlying reasons for high color purity remain unclear. It is shown here that these emitters share a common electronic basis, which is deduced from resonance structure considerations following Clar's rule, and which is termed as "poly-heteroaromatic omni-delocalization" (PHOD). The simple and clear design rules derived from the PHOD concept allow extending the known chemical space by new structural motifs. Based on PHOD, a set of novel high-efficiency color-pure emitters with brilliant deep-blue hue is specifically designed.

Boosting External Quantum Efficiency to 12.0 % of an Ultraviolet OLED by Engineering the Horizontal Dipole Orientation of a Hot Exciton Emitter

Currently, much research effort has been devoted to improving the exciton utilization efficiency and narrowing the emission spectra of ultraviolet (UV) fluorophores for organic light-emitting diode (OLED) applications, while almost no attention has been paid to optimizing their light out-coupling efficiency. Here, we developed a linear donor-acceptor-donor (D-A-D) triad, namely CDFDB, which possesses high-lying reverse intersystem crossing (hRISC) property. Thanks to its integrated narrowband UV photoluminescence (PL) (λPL: 397 nm; FWHM: 48 nm), moderate PL quantum yield (ϕPL: 72 %, Tol), good triplet hot exciton (HE) conversion capability, and large horizontal dipole ratio (Θ//: 92 %), the OLEDs based on CDFDB not only can emit UV electroluminescence with relatively good color purity (λEL: 398 nm; CIEx,y: 0.161, 0.040), but also show a record maximum external quantum efficiency (EQEmax) of 12.0 %. This study highlights the important role of horizontal dipole orientation engineering in the molecular design of HE UV-OLED fluorophores.

Efficient Deep-Blue Multiple-Resonance Emitters Based on Azepine-Decorated ν-DABNA for CIEy below 0.06

Ultrapure deep-blue emitters are in high demand for organic light-emitting diodes (OLEDs). Although color coordinates serve as straightforward parameters for assessing color purity, precise control over the maximum wavelength and full-width at half-maximum is necessary to optimize OLED performance, including luminance efficiency and luminous efficacy. Multiple-resonance (MR) emitters are promising candidates for achieving ideal luminescence properties; consequently, a wide variety of MR frameworks have been developed. However, most of these emitters experience a wavelength displacement from the ideal color, which limits their practical applicability. Therefore, a molecular design that is compatible with MR emitters for modulating their energy levels and color output is particularly valuable. Here, it is demonstrated that the azepine donor unit induces an appropriate blue-shift in the emission maximum while maintaining efficient MR characteristics, including high photoluminescence quantum yield, narrow emission, and a fast reverse intersystem crossing rate. OLEDs using newly developed MR emitters based on the ν-DABNA framework simultaneously exhibit a high quantum efficiency of ≈30%, luminous efficacy of ≈20 lm W-1, exceptional color purity with Commission Internationale de l'Éclairage coordinates as low as (0.14, 0.06), and notably high operational stability. These results demonstrate unprecedentedly high levels compared with those observed in previously reported deep-blue emitters.

Diradical-Based Strategy in Designing Narrowband Thermally Activated Delayed Fluorescence Molecules with Tunable Emission Wavelengths

In the design of thermally activated delayed fluorescence (TADF) materials, narrow-band emission is of particular importance for the development of organic light-emitting diodes (OLEDs). In this work, we proposed a new strategy for designing TADF molecules utilizing degenerate nonbonding (NB) orbitals of diradical parent molecules, and these designed molecules are termed NB-TADF molecules. Based on this strategy, a series of NB-TADF molecules is finely designed and systematically studied by theoretical calculations. Taking advantage of the nonbonding properties, these NB-TADF molecules exhibit desirable narrowband emissions and high quantum yields. More importantly, the emission bands can be easily tuned from blue to near-infrared by changing the conjugate length of the parent group in the NB-TADF molecules. We hope that this new strategy can open a new door for the design of novel TADF materials.

An Ideal Molecular Construction Strategy for Ultra-Narrow-Band Deep-Blue Emitters: Balancing Bathochromic-Shift Emission, Spectral Narrowing, and Aggregation Suppression

Narrowband emissive multiple resonance (MR) emitters promise high efficiency and stability in deep-blue organic light-emitting diodes (OLEDs). However, the construction of ideal ultra-narrow-band deep-blue MR emitters still faces formidable challenges, especially in balancing bathochromic-shift emission, spectral narrowing, and aggregation suppression. Here, DICz is chosen, which possesses the smallest full-width-at-half-maximum (FWHM) in MR structures, as the core and solved the above issue by tuning its peripheral substitution sites. The 1-substituted molecule Cz-DICz is able to show a bright deep-blue emission with a peak at 457 nm, an extremely small FWHM of 14 nm, and a CIE coordinate of (0.14, 0.08) in solution. The corresponding OLEDs exhibit high maximum external quantum efficiencies of 22.1%-25.6% and identical small FWHMs of 18 nm over the practical mass-production concentration range (1-4 wt.%). To the best of the knowledge, 14 and 18 nm are currently the smallest FWHM values for deep-blue MR emitters with similar emission maxima under photoluminescence and electroluminescence conditions, respectively. These discoveries will help drive the development of high-performance narrowband deep-blue emitters and bring about a revolution in OLED industry.

Organoboron-based multiple-resonance emitters: synthesis, structure-property correlations, and prospects

Boron-based multiple-resonance (MR) emitters exhibit the advantages of narrowband emission, high absolute photoluminescence quantum yield, thermally activated delayed fluorescence (TADF), and sufficient stability during the operation of organic light-emitting diodes (OLEDs). Thus, such MR emitters have been widely applied as blue emitters in triplet-triplet-annihilation-driven fluorescent devices used in smartphones and televisions. Moreover, they hold great promise as TADF or terminal emitters in TADF-assisted fluorescence or phosphor-sensitised fluorescent OLEDs. Herein we comprehensively review organoboron-based MR emitters based on their synthetic strategies, clarify structure-photophysical property correlations, and provide design guidelines and future development prospects.

Tailored Design of π-Extended Multi-Resonance Organoboron using Indolo[3,2-b]Indole as a Multi-Nitrogen Bridge

Extending the π-skeletons of multi-resonance (MR) organoboron emitters can feasibly modulate their optoelectronic properties. Here, we first adopt the indolo[3,2-b]indole (32bID) segment as a multi-nitrogen bridge and develop a high-efficiency π-extended narrowband green emitter. This moiety establishes not only a high-yield one-shot multiple Bora-Friedel-Crafts reaction towards a π-extended MR skeleton, but a compact N-ethylene-N motif for a red-shifted narrowband emission. An emission peak at 524 nm, a small full width at half maximum of 25 nm and a high photoluminescence quantum yield of 96 % are concurrently obtained in dilute toluene. The extended molecular plane also results in a large horizontal emitting dipole orientation ratio of 87 %. A maximum external quantum efficiency (EQE) of 36.6 % and a maximum power efficiency of 135.2 lm/W are thereafter recorded for the corresponding device, also allowing a low efficiency roll-off with EQEs of 34.5 % and 28.1 % at luminance of 1,000 cd/m2 and 10,000 cd/m2 , respectively.

Resolving the Photophysics of Nitrogen-Embedded Multiple Resonance Emitters: Origin of Color Purity and Emitting Efficiency

The emerging nitrogen-embedded multiple resonance (MR) emitters with an indolo[3,2,1-jk] carbazole (ICz) unit have exhibited promising performance for high-resolution organic light-emitting diode (OLED) devices, while the underlying photophysics has been rarely reported. In this work, the optical spectra, color purity, and emitting efficiency of ICz-based MR emitters were investigated by using electronic structure and thermal vibration correlation function (TVCF) calculations. Unlike B-N MR emitters, the high color purity of investigated ICz-based MR emitters was mainly contributed by considerable structural rigidity, which also greatly affects the radiative decay rate and fluorescence quantum yield of the S1 state. For the majority of investigated emitters, potential reverse intersystem crossing (RISC) channels (T1 → S1 and T2 → S1) are limited by thermally inaccessible ΔEST* or insufficient spin-orbital coupling (SOC), which can be distinguished by the calculated temperature-dependent RISC rate pattern. We provided a systematic photophysical picture for ICz-based MR emitters that might be interesting for the OLED design and application community.

Achieving narrowband emissions with tunable colors for multiple resonance-thermally activated delayed fluorescence materials: effect of boron/nitrogen number and position

The boron/nitrogen (B/N)-based multiple resonance-thermally activated delayed fluorescence (MR-TADF) materials with tunable colors have attracted widespread attention owing to their great potential in next-generation display, white lighting, and imaging applications. Numerous MR-TADF emitters with different B/N number and position have been reported to realize full-color narrowband emissions. To gain a better understanding of the effect of B/N number and position on the photo-electronic properties, geometric and electronic properties, Huang-Rhys factors and reorganization energies, charger transfer and absorption/emission properties were analyzed in detail to determine the structure-property relationship for the investigated molecules. The calculated results show that the molecules with para-atoms having the same electronic characteristics (para-B-π-B/para-N-π-N) exhibited smaller structural relaxations upon excitation, and the molecules with increased B/N atoms showed more obvious short-range charge transfer (SRCT) properties. Besides, the para-B-π-N and para-B-π-B/para-N-π-N substructures could reduce and enhance the donor and acceptor strengths, respectively, leading to tunable HOMO-LUMO gaps and emission colors. Such theoretical insights well rationalize the experimental results, revealing that the small reorganization energy and dominant SRCT property should be two key factors in realizing narrowband emissions of MR-TADF materials. These findings and understandings could give an in-depth insight into the structure-property relationship, providing molecular design strategies for the exploration of narrowband MR-TADF materials with tunable emission colors.

RGB Thermally Activated Delayed Fluorescence Emitters for Organic Light-Emitting Diodes toward Realizing the BT.2020 Standard

With the surging demand for ultra-high-resolution displays, the International Telecommunication Union (ITU) announce the next-generation color gamut standard, named ITU-R Recommendation BT.2020, which not only sets a seductive but challenging milestone for display technologies but also urges researchers to recognize the importance of color coordinates. Organic light-emitting diodes (OLEDs) are an important display technology in current daily life, but they face challenges in approaching the BT.2020 standard. Thermally activated delayed fluorescence (TADF) emitters have bright prospects in OLEDs because they possess 100% theoretical exciton utilization. Thus, the development of TADF emitters emitting primary red (R), green (R), and blue (B) emission is of great significance. Here, a comprehensive overview of the latest advancements in TADF emitters that exhibit Commission Internationale de l'Éclairage (CIE) coordinates surpassing the National Television System Committee (NTSC) and approaching BT.2020 standards is presented. Rational strategies for molecular designs, as well as the resulting photophysical properties and OLED performances, are discussed to elucidate the underlying mechanisms for shifting the CIE coordinates of both donor-acceptor and multiple resonance (MR) typed TADF emitters toward the BT.2020 standard. Finally, the challenges in realization of the wide-color-gamut BT.2020 standard and the prospects for this research area are provided.

Fine-tuning benzazole-based probe for the ultrasensitive detection of Hg2+ in water samples and seaweed samples

Developing potentially toxic metal ion probes is significant for environment and food safety. Although Hg²⁺ probes have been extensively studied, small molecule fluorophores that can integrate two applications of visual detection and separation into one unit remain challenging to access. Herein, by incorporating triphenylamine (TPA) into tridentate skeleton with an acetylene bridge, 2,6-bisbenzimidazolpyridine-TPA (4a), 2,6-bisbenzothiazolylpyridine-TPA (4b) and 2,6-bisbenzothiazolylpyridine-TPA (4c) were first constructed, expectably showing distinct solvatochromism and dual-state emission properties. Since the diverse emission properties, the fluorescence detection of 4a-4b can be achieved with an ultrasensitive response (LOD = 10^-11 M) and efficient removal of Hg²⁺. More interestingly, 4a-4b can not only be developed into paper/film sensing platform, but also reliably detect Hg²⁺ in real water and seaweed samples, with recoveries ranging from 97.3% to 107.8% and a relative standard deviation of less than 5%, indicating that they have excellent application potential in the field of environmental and food chemistry.

Improving the stability and color purity of a BT.2020 blue multiresonance emitter by alleviating hydrogen repulsion

Stable deep blue multiresonance emitters with small full width at half maximum (FWHM) are attractive for wide color-gamut organic light-emitting diodes (OLEDs). However, the steric repulsion from the spatially close hydrogens would twist the multiresonance skeletons, causing spectral broadening and molecular instability issues. Here, we strategically introduce a mesitylboron locking unit into a carbazole-embedded multiresonance model emitter, alleviating the hydrogen repulsions and also strengthening the para-positioned weak carbon-nitrogen bond in anionic states. An emission peaking at 452 nm with an FWHM of merely 14 nm and nearly BT.2020 blue chromaticity coordinates are obtained in toluene, affording a high maximum external quantum efficiency of 33.9% in a sensitizing device. Moreover, an impressive LT97 (time to decay to 97% of the initial luminance) of 178 hours at a constant current density of 12 mA/cm² was achieved in a stable device with a small y coordinate of 0.057, nearly 20 times longer than the model emitter with even a substantially red-shifted emission.

Intramolecular Cyclization: A Convenient Strategy to Realize Efficient BT.2020 Blue Multi-Resonance Emitter for Organic Light-Emitting Diodes

Rationally tuning the emission position and narrowing the full width at half-maximum (FWHM) of an emitter is of great importance for many applications. By synergistically improving rigidity, strengthening the resonant strength, inhibiting molecular bending and rocking, and destabilizing the HOMO energy level, a deep-blue emitter (CZ2CO) with a peak wavelength of 440 nm and an ultranarrow spectral FWHM of 16 nm (0.10 eV) was developed via intramolecular cyclization in a carbonyl/N resonant core (QAO). The dominant υ_0-0 transition character of CZ2CO gives a Commission Internationale de I'Éclairage coordinates (CIE) of (0.144, 0.042), nicely complying with the BT.2020 standard. Moreover, a hyper-fluorescent device based on CZ2CO shows a high maximum external quantum efficiency (EQE_max ) of 25.6 % and maintains an EQE of 22.4 % at a practical brightness of 1000 cd m^-2 .

Rational Molecular Design Strategy for High-Efficiency Ultrapure Blue TADF Emitters: Symmetrical and Rigid Sulfur-Bridged Boron-Based Acceptors

Developing highly efficient blue thermally activated delayed fluorescence (TADF) emitters with a narrowband emission is still a challenge. Here, novel ultrapure blue TADF emitters of TSBA-Cz and TSBA-PhCz were designed and synthesized for organic light-emitting diodes (OLEDs). Photophysical and time-dependent density functional theory calculation results simultaneously show the similar intramolecular charge-transfer character of MR-type TADF emitters. Benefiting from the symmetrical and rigid molecular configuration, compounds TSBA-Cz and TSBA-PhCz emit a pure blue emission peak at 463 and 470 nm, a narrow full width at half-maximum (FWHM) of 30 and 36 nm, and a small singlet-triplet energy gap (ΔE_ST) of 0.21 and 0.18 eV, respectively, facilitating their excellent TADF behavior in doped films. Furthermore, highly efficient TADF-OLED devices using the TSBA-Cz and TSBA-PhCz with external quantum efficiencies of 23.4 and 21.3% emit ultrapure blue electroluminescence (EL) at 464 and 472 nm with a narrow FWHM of about 35 nm and CIE color coordinates of (0.14, 0.11) and (0.12, 0.18). This work provides novel TADF emitters for blue OLEDs with narrowband EL.