Multi Resonance Research Articles

Carbonyl-nitrogen multi-resonance emitters for efficient OLEDs with high color purity

Multi-resonance (MR) materials hold an intriguing feature of narrow emission spectra and have attracted considerable attention in the manufacture of high-definition organic light-emitting diodes (OLEDs). However, the majority of MR materials are composed by a boron-nitrogen skeleton, which is unfavorable for expanding the scope of luminescent materials with narrow emission spectra to meet various application demands. In this work, we wish to report a new carbonyl-nitrogen (C = O/N) skeleton of 5,12-dihydroquinolino[2,3-b]acridine-7,14-dione (QA), and three tailored C = O/N MR molecules are synthesized and fully characterized by crystallography, thermal measurement, cyclic voltammetry, steady-state and transient spectroscopy and theoretical calculation. They show efficient green emissions with narrow full width at half maximum (FWHM) of about 27 nm and high photoluminescence quantum yields of up to 93% in doped films. Efficient hyperfluorescence OLEDs are fabricated using these materials as emitters, providing pure green lights with electroluminescence peaks at 526‒538 nm, narrow FWHMs of 29‒33 nm, excellent external quantum efficiencies of up to 29.48% and small efficiency roll-offs. These results reveal that QA could be a potential skeleton for exploring efficient C = O/N MR molecules.

Multi‐Resonance 1,4‐BN‐Heteroarene for Filterless Narrowband Photodetector

As an emerging class of optoelectronic materials, multi‐resonance (MR) 1,4‐BN‐heteroarenes have been extensively employed as narrowband electroluminescence materials, whereas their absorption feature has largely been neglected. Here we construct the first MR‐molecule‐based phototransistor for filterless narrowband photodetectors (NBPDs) by anchoring narrowband absorption MR molecules on the high‐mobility semiconductor indium‐zinc‐oxide (IZO) film. The resulting device exhibits high‐performance photodetection with a small full‐width at half‐maximum (FWHM) of 33 nm, which represents a new record for NBPDs based on intrinsic narrowband absorbing materials. These results demonstrate the great potential of MR materials as a new molecular platform for developing high‐performance NBPDs.

Multi-Resonance 1,4-BN-Heteroarene for Filterless Narrowband Photodetector.

As an emerging class of optoelectronic materials, multi-resonance (MR) 1,4-BN-heteroarenes have been extensively employed as narrowband electroluminescence materials, whereas their absorption feature has largely been neglected. Here we construct the first MR-molecule-based phototransistor for filterless narrowband photodetectors (NBPDs) by anchoring narrowband absorption MR molecules on the high-mobility semiconductor indium-zinc-oxide (IZO) film. The resulting device exhibits high-performance photodetection with a small full-width at half-maximum (FWHM) of 33 nm, which represents a new record for NBPDs based on intrinsic narrowband absorbing materials. These results demonstrate the great potential of MR materials as a new molecular platform for developing high-performance NBPDs.

Advancing efficiency in deep-blue OLEDs: Exploring a machine learning-driven multiresonance TADF molecular design.

The pursuit of boron-based organic compounds with multiresonance (MR)-induced thermally activated delayed fluorescence (TADF) is propelled by their potential as narrowband blue emitters for wide-gamut displays. Although boron-doped polycyclic aromatic hydrocarbons in MR compounds share common structural features, their molecular design traditionally involves iterative approaches with repeated attempts until success. To address this, we implemented machine learning algorithms to establish quantitative structure-property relationship models, predicting key optoelectronic characteristics, such as full width at half maximum (FWHM) and main peak wavelength, for deep-blue MR candidates. Using these methodologies, we crafted ν-DABNA-O-xy and developed deep-blue organic light-emitting diodes featuring a Commission Internationale de l'Eclairage y of 0.07 and an FWHM of 19 nm. The maximum external quantum efficiency reached ca. 27.5% with a binary emission layer, which increased to 41.3% with the hyperfluorescent architecture, effectively mitigating efficiency roll-off. These findings are expected to guide the systematic design of MR-type TADF clusters, unlocking their full potential.

Manipulating photophysical properties through asymmetric modification of triptycene‐fused multi‐resonance TADF emitter

AbstractDeveloping multi‐resonance (MR) effect‐induced thermally activated delayed fluorescence (TADF) emitters that exhibit excellent photophysical properties and a sterically shielded structure is crucial for achieving high‐efficiency and stable organic light‐emitting diodes (OLEDs). Herein, we propose a new synthetic approach to asymmetric bulky MR‐TADF emitters based on a B,N core. Three asymmetric Tp‐fused MR emitters (1–3) are produced by attaching a bulky and rigid triptycene (Tp) moiety on one side of the MR core and introducing different N‐containing functional groups on the other side. All emitters exhibit high photoluminescence quantum yield, narrow full width at half maximum, and TADF properties with reasonable reverse intersystem crossing rates (~104 s−1) in a rigid matrix. Notably, the emission color of the emitters ranges from deep blue to sky blue, depending on the N‐containing functional groups. Electrochemical and theoretical studies further demonstrate that the frontier molecular orbitals are distributed over the MR core formed by the two moieties, and the resulting energy levels vary accordingly. The findings of this study will be useful for the design of various color‐tunable yet sterically shielded MR‐TADF emitters.

Sym- and Asym-Expanded Heterohelicene Isomers Featuring Extended Multi-Resonance Skeleton for Narrowband Deep-Blue Fluorescence.

Expanded heterohelicenes composed of alternating linearly and angularly fused multi-resonance (MR) skeletons have gained wide interest owing to their promising narrowband emissions. Herein, a pair of sym- and asym-expanded heterohelicene isomers was obtained by merging boron/oxygen (B/O)-embedded MR triangulene and indolo[3,2,1-jk]carbazole units via one-pot synthesis. Owing to their fully resonating extended helical skeleton, the target heterohelicenes exhibit a significantly narrowed spectra bandwidth while emission red-shifting, thus affording deep-blue narrowband emission with a peak at approximately 460 nm, full-width-at-half-maximum (FWHM) of only 18 nm, and near-unity photoluminescence quantum yields. In comparison to the symmetrical structure, the asym-expanded heterohelicene displays suppressed aggregation within doped films, thereby showing superior narrowband electroluminescence in devices with CIE coordinates of (0.12, 0.18) and a high external quantum efficiency of up to 25 %, which retains 18.1 % even at a high luminance of 10,000 cd m-1. Overall, this work provides a novel paradigm for the development of narrowband expanded heterohelicenes.

The degradation mechanism of multi-resonance thermally activated delayed fluorescence materials

1,4-Azaborine-based arenes are promising electroluminescent emitters with thermally activated delayed fluorescence (TADF), offering narrow emission spectra and high quantum yields due to a multi-resonance (MR) effect. However, their practical application is constrained by their limited operational stability. This study investigates the degradation mechanism of MR-TADF molecules. Electroluminescent devices incorporating these compounds display varied operational lifetimes, uncorrelated with excitonic stability or external quantum efficiency roll-off. Bulk electrolysis reveals significant instability in the radical cationic forms of MR-TADF compounds, with device lifetime linked to the Faradaic yield of oxidation. Comprehensive chemical analyses corroborate that the degradation byproducts originated from intramolecular cyclization of radical cation, followed by hydrogen atom transfer. The mechanism is further supported by enhanced stability observed in a deuterated MR-TADF emitter, attributed to a secondary kinetic isotope effect. These findings provide insights into the stabilizing effects of deuteration and mechanism-driven strategies for designing MR-TADF compounds with improved operational longevity.

High Color Purity Deep‐Blue Multi‐Resonance TADF Material with Narrowband Emission Toward BT.2020 Standard

AbstractDue to higher exciton energy, the deep‐blue organic light‐emitting diodes (OLEDs) are the most challenging among trichromatic emitters. In this work, three novel blue emitters with multi‐resonance (MR) effects are reported based on N,N,5,9‐tetraphenyl‐5,9‐dihydro‐5,9‐diaza‐13b‐boranaphtho[3,2,1‐de]anthracen‐7‐amine (PAB) skeleton, incorporating fluorine and methyl groups. Compound 2FPAB and MePAB exhibit ultra‐pure deep‐blue emission peaks at 436 and 448 nm respectively. The device based on MePAB achieves a maximum external quantum efficiency of 19.8% with an ultralow CIEy value of 0.046 and a full‐width‐at‐half‐maximum (FWHM) of 29 nm in the bottom emitting device. Additionally, the peripheral cladding of fluorine atoms extends the conjugation framework and slightly enhances the electron acceptance character of the boron atom at the ortho‐position. This leads to a redshift in the emission spectrum and enhanced photoluminescence quantum yield (PLQY) of up to 93% for MePABF. Furthermore, the potential application of MePAB as a deep‐blue sensitizer for MePABF is examined by fabricating a TADF‐sensitized‐TADF (TST) device. The results show a significantly reduced efficiency roll‐off and improved device performance, with a maximum external quantum efficiency (EQEmax) of 25.1% and a FWHM of 29 nm.

High‐Performance Solution‐Processable Organic Light‐Emitting Diode Based on a Narrowband Near‐Ultraviolet Emitter and a Hot Exciton Strategy

AbstractAchieving high efficiency narrowband near‐ultraviolet (NUV) emitters in organic light emitting diode (OLED) is still a formidable challenge. Herein, a proof‐of‐concept hybridized local and charge transfer (HLCT) molecule, named ICz‐BO, is prepared and characterized, in which both multiresonant (MR) skeletons are integrated via conjugation connection. A slightly distorted structure and weak intramolecular charge transfer (CT) interaction between two MR subunits lead to a high‐lying reverse intersystem crossing (h‐RISC) channel of T6→S1, also evidenced by both experimental and calculated results. Impressively, the ICz‐BO emitter exhibits outstanding narrowband NUV emission at 404 nm with a full‐width at half maximum of 28 nm in toluene solution. The solution processable OLED shows an excellent device performance with the recorded maximum external quantum efficiency of 12.01 %, concomitant with an extremely low y‐axis Commission Internationale de l’Éclairage (CIEy) value of 0.031. To the best of our knowledge, this is the highest efficiency reported for the HLCT‐based NUV‐OLEDs to date. This research proves that the MR skeleton plays a positive effect on the narrowband hot exciton emitter, which provides an alternative paradigm for developing high‐efficiency NUV emitters.

High-Performance Solution-Processable Organic Light-Emitting Diode Based on a Narrowband Near-Ultraviolet Emitter and a Hot Exciton Strategy.

Achieving high efficiency narrowband near-ultraviolet (NUV) emitters in organic light emitting diode (OLED) is still a formidable challenge. Herein, a proof-of-concept hybridized local and charge transfer (HLCT) molecule, named ICz-BO, is prepared and characterized, in which both multiresonant (MR) skeletons are integrated via conjugation connection. A slightly distorted structure and weak intramolecular charge transfer (CT) interaction between two MR subunits lead to a high-lying reverse intersystem crossing (h-RISC) channel of T6→S1, also evidenced by both experimental and calculated results. Impressively, the ICz-BO emitter exhibits outstanding narrowband NUV emission at 404 nm with a full-width at half maximum of 28 nm in toluene solution. The solution processable OLED shows an excellent device performance with the recorded maximum external quantum efficiency of 12.01 %, concomitant with an extremely low y-axis Commission Internationale de l'Éclairage (CIEy) value of 0.031. To the best of our knowledge, this is the highest efficiency reported for the HLCT-based NUV-OLEDs to date. This research proves that the MR skeleton plays a positive effect on the narrowband hot exciton emitter, which provides an alternative paradigm for developing high-efficiency NUV emitters.

Enabling High-Performance Multi-Resonant TADF Emitters via Intramolecular Hydrogen Bond.

Hydrogen bonds (HBs), prevalent strong interactions in organic compounds, can effectively constrain single bond rotation, leading to rigid planar configurations. This rigidity enhances emission efficiency and narrows the emission spectrum of luminescent materials. Recent advances have leveraged HBs to advance high-performance donor-acceptor thermally activated delayed fluorescence (TADF) materials. However, their application in multi-resonance (MR) TADF emitters remains limited. We herein developed MR-TADF emitters incorporating intramolecular hydrogen bonds (IHBs) with pyrimidine as the HB acceptor. The rigid planar conformation induced by IHBs significantly improved photoluminescence quantum yield, extended emission wavelength, reduced full-width at half-maximum, and decreased non-radiative decay rates for BN-2Pm compared to BN-5Pm. Devices based on BN-2Pm achieved a maximum external quantum efficiency of 36.5 %, a current efficiency of 102.3 cd A-1, a power efficiency of 84.6 lm W-1, and Commission Internationale de l'Éclairage (CIE) coordinates of (0.18, 0.71). In contrast, BN-5Pm exhibited lower values: 14.6 %, 36.8 cd A-1, 27.6 lm W-1, and (0.12, 0.57), respectively.

Endo-Encapsulated Multi-Resonance Dendrimers with Through-Space Interactions for Efficient Narrowband Blue-Emitting Solution-Processed OLEDs.

Different from conventional luminescent dendrimers with fluorophore tethered outside to dendron, here we first developed endo-encapsulated luminescent dendrimers with multi-resonance (MR) fluorophore embedded inside of carbazole dendrons by growing dendrons through 1,8-positions of central carbazole moiety to create a cavity for accommodating the fluorophore. This endo-encapsulated structure not only shields the fluorophore to fully resist aggregation-caused spectral broadening, but also induce through-space interactions between dendron and fluorophore via intramolecular π-stacking, giving lowered singlet state energy and reduced singlet-triplet energy splitting to accelerate reverse intersystem crossing (RISC) from triplet to singlet states. The resultant dendrimer containing 1,8-linked second-generation carbazole dendrons and boron, sulfur-doped polycyclic MR fluorophore exhibits narrowband blue emission at 471 nm with FWHM kept at 34 nm even in neat film, together with ~4 times enhancement of RISC rate constant compared to its exo-tethered counterpart. Solution-processed OLEDs based on the endo-encapsulated dendrimer reveal efficient narrowband blue emissions with maximum external quantum efficiency of 22.6 %, representing the best device efficiency for blue-emitting multi-resonance dendrimers so far.

Endo‐Encapsulated Multi‐Resonance Dendrimers with Through‐Space Interactions for Efficient Narrowband Blue‐Emitting Solution‐Processed OLEDs

AbstractDifferent from conventional luminescent dendrimers with fluorophore tethered outside to dendron, here we first developed endo‐encapsulated luminescent dendrimers with multi‐resonance (MR) fluorophore embedded inside of carbazole dendrons by growing dendrons through 1,8‐positions of central carbazole moiety to create a cavity for accommodating the fluorophore. This endo‐encapsulated structure not only shields the fluorophore to fully resist aggregation‐caused spectral broadening, but also induce through‐space interactions between dendron and fluorophore via intramolecular π‐stacking, giving lowered singlet state energy and reduced singlet‐triplet energy splitting to accelerate reverse intersystem crossing (RISC) from triplet to singlet states. The resultant dendrimer containing 1,8‐linked second‐generation carbazole dendrons and boron, sulfur‐doped polycyclic MR fluorophore exhibits narrowband blue emission at 471 nm with FWHM kept at 34 nm even in neat film, together with ~4 times enhancement of RISC rate constant compared to its exo‐tethered counterpart. Solution‐processed OLEDs based on the endo‐encapsulated dendrimer reveal efficient narrowband blue emissions with maximum external quantum efficiency of 22.6 %, representing the best device efficiency for blue‐emitting multi‐resonance dendrimers so far.

Thermally Activated Delayed Fluorescence Sensitizer with Through‐Space Charge Transfer Manipulated by Atom Distribution for Solution‐Processed Deep‐Blue Multi‐Resonance OLEDs

AbstractOrganic light‐emitting diodes (OLEDs) employing multi‐resonance (MR) emitters have attracted much attention due to their high luminescent efficiency and color purity, however, the development of deep‐blue multi‐resonance OLEDs is hindered by the lack of suitable sensitizers. Here a novel design strategy is reported for thermally activated delayed fluorescence sensitizer with high reverse intersystem crossing (kRISC) and radiative transition (kR) rate by regulating atom distribution of a spatially‐aligned carbazole donor/diphenylpyrimidine acceptor structure to manipulate through‐space charge transfer (CT) process. It is found that diphenylpyrimidine acceptor with 1,3‐position nitrogen atoms is un‐conjugated to bridging phenyl moiety by the conjugation node of pyrimidine, while the acceptor with 1,5‐position nitrogen atoms is conjugated to bridging phenyl through conjugation site, opening through‐bond CT pathway via bridging phenyl besides through‐space CT from spatial donor‐acceptor interactions. As a result, the sensitizer with 1,5‐position nitrogen atoms exhibits enhanced oscillator strength by 3.5 folds and accelerated kR of 3.57 × 107 s−1, while keeping high kRISC of 4.03 × 106 s−1 and high singlet (S1) state (2.88 eV). Solution‐processed OLEDs using the sensitizer and deep‐blue multi‐resonance emitter exhibit efficient narrowband electroluminescence with CIE coordinates of (0.14, 0.13) and EQEmax of 15.6%, representing the state‐of‐the‐art device performance for deep‐blue multi‐resonance OLEDs by solution process.

Gold Coordination-Accelerated Multi-Resonance TADF Emission for Efficient Solution-Processible Ultrapure Deep-Blue OLEDs.

Multi-resonance (MR) type emitters have emerged as highly promising candidates for high-resolution organic light-emitting diodes (OLEDs). However, thermally activated delayed fluorescence (TADF) emissions with simultaneous short excited state lifetimes and ultrapure blue color (a CIEy close to 0.046 and an emission peak >440 nm) have rarely been obtained for MR emitters. Herein, we report a design of dual gold-coordinated MR molecules to achieve efficient and short-lived ultrapure blue TADF emission. The dinuclear Au(I) complex, namely iPrAuBN, shows a narrowband deep-blue emission with a peak maximum of 448 nm and a full width at half maximum (FWHM) of 29 nm in doped film. The coordination with two Au atoms significantly shortens the delayed fluorescence lifetime to 7.8 μs in comparison to 60.6 μs for the parental organic analogue. Solution-processed OLED doped with iPrAuBN demonstrates an ultrapure blue electroluminescence with a peak maximum of 442 nm, a FWHM of 19 nm, CIE coordinates of (0.154, 0.036), and a maximum external quantum efficiency of 14.8 %.

Enhancing Spin-Orbit Coupling in an Indolocarbazole Multiresonance Emitter by a Sulfur-Containing Peripheral Substituent for a Fast Reverse Intersystem Crossing.

A fast reverse intersystem crossing (RISC) remains an ongoing pursuit for multiresonance (MR) emitters but faces formidable challenges, particularly for indolocarbazole (ICz) derived ones. Here, heavy-atom effect is introduced first to construct ICz-MR emitter using a sulfur-containing substitute, simultaneously enhancing both spin-orbit and spin-vibronic coupling to afford a fast RISC with a rate of 1.2×105s-1, nearly one order of magnitude higher than previous maximum values. The emitter also exhibits an extremely narrow deep-blue emission peaking at 456nm with full-width at half-maxima of merely 12nm and a photoluminescence quantum yield of 92%. Benefiting from its efficient triplet upconversion capability, this emitter achieves not only a high maximum external quantum efficiency (EQE) of 31.1% in organic light-emitting diodes but also greatly alleviates efficiency roll-off, affording record-high EQEs of 29.9% at 1000cdm-2 and 18.7% at 5000cdm-2 among devices with ICz-MR emitters.

Design of a miniaturized multi resonance resonator based highly selective dual wideband bandpass filter

This paper presents the design of a highly selective dual-wideband bandpass filter. The filter is designed by using multi-resonance resonator consisting of two stub-loaded step impedance resonators separated by an inter-digital capacitor. The inter-digital capacitor generates capacitive coupling, resulting in multiple resonating modes. The proposed filter generates two passband frequencies centered at 5.75 GHz and 11.5 GHz. Experimental measurements show that the maximum insertion loss in the passbands is 0.83 dB and 0.87 dB, while the reflection losses are better than 10.8 dB and 12.6 dB, respectively. The resonant frequencies can be adjusted by changing the length of the loaded stubs. The proposed dual-wideband bandpass filter offers improved out-of-band rejection and selectivity by generating seven transmission poles and seven transmission zeros around both passbands. The experimental results confirm the effectiveness of the proposed filter. Furthermore, the filter has a compact size of (0.61×0.66)λg, making it suitable for integration into microwave sensing devices.

Hybridization of Long‐Range and Short‐Range Charger Transfer Enables Efficient Thermally Activated Delayed Fluorescent and Hyperfluorescent OLEDs

AbstractExploring possible strategy to balance the radiative decay and reverse intersystem crossing (RISC) are of great significance in thermally activated delayed fluorescent (TADF) emitters for enabling both high efficiency and low efficiency roll‐off. Herein, by introducing a multi‐resonance (MR) type acceptor, two TADF emitters featuring hybridized short‐range charge transfer (SRCT) and long‐rang charge transfer (LRCT) are designed. LRCT‐dominate photoluminescent nature is maintained with a short delayed lifetime of 1.5–3.4 µs. While the incorporated SRCT accelerates the radiative decay. Consequently, both emitters demonstrate remarkable electroluminescent performance with maximum EQE (EQEmax) of 40.8% for IT‐BO and 39.0% for IA‐BO, respectively, along with ultra‐high luminance of up to 120 000 cd m−2. More encouragingly, both emitters can enable small efficiency roll‐off with a minimum value of only 4%, highlighting the significance of enhanced kr in suppressing the roll‐off. Moreover, when served as sensitizers, both compounds can also endow excellent performances with EQEmax ≈38%. This work presents an innovative approach by hybridizing SRCT and LRCT for developing practical TADF emitters.

Multi‐Resonance TADF Conjugated Polymers toward Highly Efficient Solution‐Processible Narrowband Green OLEDs

AbstractDue to the extended conjugation along the polymeric backbone and the scarcity of suitable multi‐resonance (MR) emitting units for polymerization, precisely adjusting the colors of MR‐thermally activated delayed fluorescence (MR‐TADF) conjugated polymers to green and red region remains to be a formidable challenge. Herein, this work develops a series of green MR‐TADF polymers for the first time by attaching MR emitting moiety as pendant to an acceptor triphenyltriazine while simultaneously embedding acceptor moiety into polycarbazole backbone. Benefitting from the enhanced intramolecular charge transfer (ICT) interaction by introducing acceptor on the para‐carbon position of boron (B) atom of the MR unit, as well as extended conjugation along backbone, all polymers exhibit red‐shifted emissions with emission peaks of 505–517 nm and full‐width at half maximums (FWHMs) of below 48 nm compared to the pendant bluish‐green MR unit peaking at 484 nm. The solution‐processed devices based on these polymers exhibit excellent performances with maximum quantum efficiency (EQE) of 22.2%, emission peak at 511 nm and FWHM of 44 nm, which represents the state‐of‐the‐art performance among the MR‐TADF polymer‐based narrowband OLEDs.

Nitrogen-Embedding Strategy for Short-Range Charge Transfer Excited States and Efficient Narrowband Deep-Blue Organic Light Emitting Diodes.

The development of deep-blue organic light-emitting diodes (OLEDs) featuring high efficiency and narrowband emission is of great importance for ultrahigh-definition displays with wide color gamut. Herein, based on the nitrogen-embedding strategy for modifying the short range charge transfer excited state energies of multi-resonance (MR) thermally activated delayed fluorescence (TADF) emitters, we introduce one or two nitrogen atoms into the central benzene ring of a versatile boron-embedded 1,3-bis(carbazol-9-yl)benzene skeleton. This approach resulted in the stabilization of the highest occupied molecular orbital energy levels and the formation of intramolecular hydrogen bonds, and thus systematic hypsochromic shifts and narrowing spectra. In toluene solution, two heterocyclic-based MR-TADF molecules, Py-BN and Pm-BN, exhibit deep-blue emissions with high photoluminescence quantum yields of 93 % and 94 %, and narrow full width at half maximum of 14 and 13 nm, respectively. A deep-blue hyperfluorescent OLED based on Py-BN exhibited a maximum external quantum efficiency of 27.7 % and desired color purity with Commission Internationale de L'Eclairage (CIE) coordinates of (0.150, 0.052). These results demonstrate the significant potential for the development of deep blue narrowband MR-TADF emitters.