Rong-Huei Yi

Nature Photonics, Published online: 11 October 2024; doi:10.1038/s41566-024-01527-7

Here, a novel “short-range charge-transfer region regulation” design strategy is proposed toward the goal of ultra-narrowband blue emitters. The corresponding device exhibits leading performance for pure-blue OLEDs, with excellent efficiencies of 36.4%, 49.1 cd A⁻¹, and 51.4 lm W⁻¹, a record high luminescence of 9.0 × 10⁵ cd m⁻², an ultra-small FWHM of 15 nm and a CIEy coordinate of 0.20.

Abstract

Ultra-narrowband multiple resonance (MR) emitters are a key component in the fabrication of highly efficient and stable blue organic light-emitting diodes (OLEDs). To explore the theoretical boundaries of wavelength and full width at half maximum (FWHM) in blue emitters, the currently narrowest boron-based MR emitter is carefully designed by integrating the superior v-DABNA and BBCz-DB structures under the auspices of the ingenious short-range charge-transfer region regulation strategy. The target tetraboron compound TB-PB demonstrates a blue emission with an emission maximum of 473 nm, a small FWHM of 12 nm and a CIEy coordinate of 0.14. Benefiting from the emitter's high photoluminescence quantum yield (99%), low excited-state energy (2.74 eV) and short delayed fluorescence lifetime (0.53 µs), the corresponding OLED achieves exceptional efficiencies of 36.4%, 49.1 cd A⁻¹, and 51.4 lm W⁻¹ with a record-high luminescence of 9.0 × 10⁵ cd m⁻², an ultra-narrow FWHM of 15 nm and a CIEy coordinate of 0.20. These breakthroughs will accelerate the development of next-generation blue emitters and lead to the advancement of OLED technology.

The ToC illustrates the molecular structure of DtCzBN-CNBT2 along with a brief overview of the performance of the corresponding OLED. The green and blue-colored moieties represent the MR-TADF emitting core and the pendant D–A TADF shield, respectively.

Abstract

Here the utility and potential of an emitter design are demonstrated, consisting of a narrowband-emitting multiresonant thermally activated delayed fluorescent (MR-TADF) core that is decorated with a suitably higher energy donor-acceptor TADF moiety. Not only does this D–A TADF group offer additional channels for triplet exciton harvesting and confers faster reverse intersystem crossing (RISC) kinetics but it also acts as a steric shield, insulating the emissive MR-TADF core from aggregation-caused quenching. Two emitters, DtCzBN-CNBT1 and DtCzBN-CNBT2, demonstrate enhanced photophysical properties leading to outstanding performance of the organic light-emitting diodes (OLEDs). DtCzBN-CNBT2, containing a D–A TADF moiety, has a faster k _RISC (1.1 × 10⁵ s⁻¹) and higher photoluminescence quantum yield (Φ _PL: 97%) compared to DtCzBN-CNBT1 (0.2 × 10⁵ s⁻¹, Φ _PL: 90%), which contains a D–A moiety that itself is not TADF. The sensitizer-free OLEDs with DtCzBN-CNBT2 achieve a record-high maximum external quantum efficiency (EQE_max) of 40.2% and showed milder efficiency roll-off (EQE₁₀₀₀ of 20.7%) compared to the DtCzBN-CNBT1-based devices (EQE_max of 37.1% and EQE₁₀₀₀ of 11.9%).

Based on a novel B–N embedded framework, a peripheral substitution engineering has been proposed to afford ultrapure blue MR-TADF emitters, revealing a record-high EQE of 20.3%, a narrow FWHM of 20 nm and CIE coordinates of (0.152, 0.046) that align with the BT.2020 blue standard.

Abstract

Compared with the classical boron/nitrogen (B/N) doped ones, multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters embedded with B–N covalent bond behave a significantly blue-shifted narrowband TADF, and thus show a greater potential in ultrapure blue organic light-emitting diodes (OLEDs). As a proof of concept, herein a peripheral substitution engineering is demonstrated based on such a B‒N embedded parent core. The simple approach is found to ensure easy synthesis via a one-pot lithium-free borylation-annulation, manipulate the excited states through different electronic coupling between core and substituent, and introduce the steric hindrance to minimize the unwanted spectral broadening. Impressively, ultrapure blue OLEDs are realized to give a high external quantum efficiency of 20.3% together with Commission Internationale de l’Éclairage coordinates of (0.152, 0.046). The performance is well competent with those of B/N doped MR-TADF emitters, clearly highlighting that the B‒N embedded framework is a novel promising paradigm towards efficient BT.2020 blue standard.

By hybridized long-short axis strategy, TPA-CN-BN shows a rapid RISC rate of 1.4×10⁵ s⁻¹, narrow FWHM of 23 nm, and small ΔE _ST of 0.06 eV. The OLED without sensitizer exhibits a narrowband green emission with the FWHM of 31 nm, EQE_max of 37.9 %, and low efficiency roll-off as the EQEs maintain a high level of 34.8 % and 21.8 % at 100 and 1000 cd m⁻².

Abstract

Multi-resonance thermally activated delayed fluorescence (MR-TADF) molecules have experienced great success in organic light-emitting diodes (OLEDs) owing to their outstanding quantum efficiencies and narrow full width at half-maximums (FWHMs). However, the reverse intersystem crossing (RISC) rates of MR-TADF emitters are usually small, which will lead to relatively long triplet exciton lifetime and severe efficiency roll-off. Here, we report an effective molecular design strategy to introduce multichannel RISC pathways and thus increase RISC rates without compromising the color fidelity and emission efficiency by the “hybridized long-short axis (HLSA)” strategy. The TPA-CN-BN shows a near-unity photoluminescence quantum yield, rapid RISC rate of 1.4×10⁵ s⁻¹, narrow FWHM of 23 nm, and small singlet-triplet energy gap (ΔE _ST) of 0.06 eV in solution. The non-sensitized OLED based on TPA-CN-BN exhibits a narrowband emission with the FWHM of 31 nm, in company with external quantum efficiency (EQE) of 37.9 %. Notably, the device exhibits the low efficiency roll-off as the EQEs maintain 34.8 % and 21.8 % at 100 and 1000 cd m⁻², respectively, representing the best performance for single-host OLEDs based on the BCzBN skeleton. This study provides a fresh and promising approach to realize high-performance OLEDs with high color purity and remarkable device efficiency.

Engineering charge transfer(CT) and planar chirality by judicious choice of donor unit is achieved in enforced densely π-stacking D/A Cyclophanes. CT absorption band can be tuned from visible to near-infrared region. As for the two planar chiral CT cyclophanes, one shows chiral selectivity for incident circularly polarized(CP) light for photothermal conversion, while another can stack into helical structure to amplify molecular planar chirality.

Abstract

We report herein a series of macrocycles in which the densely π-stacked charge-transfer (CT) donor/acceptor with naphthalenediimides (NDIs) or perylene diimide (PDI) as acceptor moiety pairing various donor moieties are locked by covalent bond. The X-ray crystallography of C8BDT-NDI reveals a short intramolecular π-stacking distance around 3.4 Å and the existence of intermolecular donor/acceptor π-stacking (3.7 Å). The intramolecular CT is highly dependent on the electron-donating ability of donor moiety and replacing carbazole (C8KZ) with benzo[1,2-b:4,5-b′]dithiophene (C8BDT) or dihydroindolo[3,2-b]indole (C8DN) redshift CT absorption into NIR region. Notably, both C8BDT-NDI and C8DN-NDI demonstrate excellent photothermal performance, which is a result of the active non-radiative pathways. Interestingly, the different molecular symmetry between donor and acceptor moiety in cyclophanes endow C8BDT-NDI and C8DN-NDI with intrinsic planar chirality. The enantiomeric C8BDT-NDI shows chiral selectivity for incident light, i.e., when irradiated by left-circularly polarized light, (R)- C8BDT-NDI is more sensitive and a higher maximum stable temperature is achieved. While, enantiomeric C8DN-NDI pack with different orientations forming M- and P-handedness helix, respectively, demonstrating molecular planar chirality being transferred and amplified through molecular assembly. These results provide insight into the intramolecular charge transfer in enforced D/A π-stacks in which CT interactions and planar chirality would be engineered through structural control.

By employing topologically chiral [2]catenane as the chiral platform and switching scaffold, the first stimuli-responsive topologically chiral multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitter with |g _PL| reaching up to 1.6×10⁻² was successfully constructed. It further serves as an excellent emitter layer for the fabrication of efficient circularly polarized organic light-emitting diodes (CP-OLEDs).

Abstract

Aiming at the fabrication of circularly polarized organic light-emitting diodes (CP-OLEDs) with high dissymmetry factors (g _EL) and color purity through the employment of novel chiral source, topologically chiral [2]catenanes were first utilized as the key chiral skeleton to construct novel multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters. Impressively, the efficient chirality induction and unique switchable feature of topologically chiral [2]catenane not only lead to a high |g _PL| value up to 1.6×10⁻² but also facilitate in situ dynamic switching of the full-width at half-maximum (FWHM) and circularly polarized luminescence (CPL). Furthermore, the solution-processed CP-OLEDs based on the resultant topologically chiral emitters exhibit a narrow FWHM of 36 nm, maximum external quantum efficiency of 17.6 %, and CPEL with |g _EL| of 2.1×10⁻³. This study demonstrates the successful construction of the first CP-MR-TADF emitters based on topological chirality with the highest |g _PL| among the reported CP-MR-TADF emitters and excellent device performance to the best of our knowledge. Moreover, it endowed the MR-TADF emitter with distinctive switchable CPL performances, thus providing a novel design strategy as well as a promising platform for developing intelligent CP-OLEDs.

Chiral co-assemblies responsive to acid and thermal stimuli were prepared based on R/S-5011 and polymer PFIQ containing iso-quinoline. The chiral co-assemblies showed tunable circularly polarized luminescence (CPL) with high |g _lum| values of up to 0.3. Furthermore, the chiral co-assemblies induced achiral emitters to generate full-color CPL signals through CPL energy transfer, with the corresponding |g _lum| values above 0.2.

Abstract

Stimuli-responsive circularly polarized luminescence (CPL) materials have been attaching wide attention in the field of optical information storage and encryption, while still facing the challenge of the realization of high luminescence dissymmetry factors (g _lum). This work presents a pair of stimuli-responsive chiral co-assemblies P7R3 and P7S3 by combining polymer PFIQ containing iso-quinoline units with chiral inducers. The obtained chiral co-assemblies can reversibly undergo significant modification in CPL behavior under trifluoroacetic acid (TFA) fumigation and annealing treatment, with the |g _lum | values exhibiting a reversible shift between 0.2 and 0.3. Moreover, the chiral co-assemblies before TFA fumigating can effectively induce achiral emitters to generate intense full-color CPL signals through CPL energy transfer (CPL-ET), with the corresponding |g _lum| values larger than 0.2. Moreover, information encryption and decryption as well as a multi-level logic gates application are achieved by leveraging the reversible stimuli-responsive CPL activity of the chiral co-assembly. This work provides a new perspective for the construction of stimuli-responsive chiral luminescent materials with large |g _lum| values and the activation of CPL behavior in achiral emitters.

A multimode radioluminescence process is developed by thermally activating the release of triplet excitons from organic phosphorescent scintillators. These scintillators achieve a maximum photoluminescence efficiency of 65.8% and a minimum X-ray radiation detection limit of 110 nGy s⁻¹, enabling efficient radiographic imaging with a spatial resolution of ≈10.0 lp mm⁻¹.

Abstract

The development of organic phosphorescent scintillators with high exciton utilization efficiency has attracted significant attention but remains a difficult challenge because of the inherent spin-forbidden feature of X-ray-induced triplet excitons. Herein, a design strategy is proposed to develop organic phosphorescent scintillators through thermally activated exciton release to convert stabilized spin-forbidden triplet excitons to spin-allowed singlet excitons, which enables singlet exciton-dominated multi-mode emission simultaneously from the lowest singlet, triplet, and stabilized triplet states. The resultant scintillators demonstrate a maximum photoluminescence efficiency of 65.8% and a minimum X-ray radiation detection limit of 110 nGy s⁻¹; this allows efficient radiography imaging with a spatial resolution of ≈10.0 lp mm⁻¹. This study advances the fundamental understanding of exciton dynamics under X-ray excitation, significantly broadening the practical use of phosphorescent materials for safety-critical industries and medical diagnostics.

Dinuclear pincer-type Ni(II) complexes bridged by formamidinate/α-carbolinato ligands and the dinuclear Pt(II) and Pd(II) analogues were studied. These Ni(II) complexes show short Ni−Ni distances, resulting in a singlet metal-metal-to-ligand charge transfer transition. At 77 K, they show phosphorescence (2.6–8.6 μs) in the NIR (800–1400 nm) spectral region in 2-MeTHF and in the solid state, which originates from the ³dd excited state as supported by DFT calculations.

Abstract

Facile non-radiative decay of low-lying metal-centered (MC) dd excited states has been well documented to pose a significant obstacle to the development of phosphorescent Ni^II complexes due to substantial structural distortions between the dd excited state and the ground state. Herein, we prepared a series of dinuclear Ni₂ ^II,II complexes by using strong σ-donating carbene-phenyl-carbene (C_NHC C_phenyl C_NHC) pincer ligands, and prepared their dinuclear Pt₂ ^II,II and Pd₂ ^II,II analogues. Dinuclear Ni₂ ^II,II complexes bridged by formamidinate/α-carbolinato ligand exhibit short Ni−Ni distances of 2.947–3.054 Å and singlet metal-metal-to-ligand charge transfer (¹MMLCT) transitions at 500–550 nm. Their ¹MMLCT absorption energies are red-shifted relative to the Pt₂ ^II,II and Pd₂ ^II,II analogues at ~450 nm and ≤420 nm respectively. One-electron oxidation of these Ni₂ ^II,II complexes produces valence-trapped dinuclear Ni₂ ^II,III species, which are characterized by EPR spectroscopy. Upon photoexcitation, these Ni₂ ^II,II complexes display phosphorescence (τ=2.6–8.6 μs) in the NIR (800–1400 nm) spectral region in 2-MeTHF and in the solid state at 77 K, which is insensitive to π-conjugation of the coordinated [C_NHC C_phenyl C_NHC] ligand. Combined with DFT calculations, the NIR emission is assigned to originate from the ³dd excited state. Studies have found that the dinuclear Ni₂ ^II,II complex can sensitize the formation of singlet oxygen and catalyze the oxidation of cyclo-dienes under light irradiation.

Exploring new strategies for construction of chiral covalent organic frameworks (COFs) is of paramount importance yet remains a challenge. Herein, we report the rational design and construction of chiral COFs through a linker decomposition chiral induction (LDCI) strategy. Three pairs of azine-linked chiral COFs are successfully synthesized by the condensation reactions of C3-symmetric 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde (Tz) with flexible chiral dihydrazide linkers derived from malic acid, aspartic acid and tartaric acid, respectively. Remarkably, upon complete or partial decomposition from flexible chiral dihydrazides to hydrazine during COF synthesis, the homochirality of these COFs, originating from the single-handedness conformation of propeller-like Tz cores, is well preserved. Such a stereoselective chiral memory realized via the LDCI strategy is confirmed by time-dependent powder X-ray diffraction (PXRD), Fourier transform infrared (FT-IR) and diffuse reflectance circular dichroism (DRCD). Moreover, the resultant azine-linked chiral COFs are used as the active materials to fabricate photodetectors to directly distinguish circularly polarized light (CPL), showing impressive recognition performances on the identification of left-handed circularly (LHC) and right-handed circularly (RHC) polarized lights. Notably, the residual undecomposed flexible chiral linkers within the COFs are found to be conducive to improving the polarization discrimination ratio.

The combination of boron-induced coordination-enhanced charge transfer (CE-CT) and a helically twisted conjugated framework has led to a family of helically twisted D-π-A conjugated systems showing red to near-infrared (NIR) emission, with the cis/trans configurational isomers exhibiting similar photophysical properties. The observed narrow, red-shifted emission could be explained by theoretical calculations.

Abstract

A novel design strategy to construct bright and narrow near-infrared (NIR) emission materials with suppressed shoulder peaks can significantly enhance their performance in various applications. Herein, we have successfully synthesized a series of helically twisted D-π-A conjugated systems bridged by boron atoms, achieving bright red to near-infrared (NIR) emissions with notably narrow full-width at half-maximum (FWHM) values of 35 nm (0.08 eV) and photoluminescence quantum yields (PLQY) up to 80 %. These compounds display red-shifted emissions up to 753 nm in higher concentrations. Cis/trans configurational isomers of multi-boron-bridged molecule BN3 exhibit similar photophysical properties. The unique combination of boron-induced coordination-enhanced charge transfer (CE-CT) and the helically twisted conjugated framework is pivotal in achieving the red-shifted, narrowband emission. X-ray crystallographic analysis of BN2 and BN3-a reveals that the extension of boron-bridged D-π-A skeletons significantly increases the distortion of the skeleton. Systematic theoretical calculations show how the boron CE-CT mechanism, in conjunction with the helical twist, leads to the narrowing of emission bands while simultaneously red-shifting them into the NIR region. This work could open new avenues for the development of advanced materials with tailored optical properties, particularly in the challenging and highly sought-after NIR spectrum.

Endo-encapsulated luminescent dendrimers consisting of multi-resonance fluorophore embedded inside of carbazole dendrons are first reported, which not only resist aggregation-caused spectral broadening but also induce through-space interactions between dendron and fluorophore to accelerate reverse intersystem crossing, giving state-of-the-art device efficiency with maximum external quantum efficiency of 22.6 % for narrowband luminescent dendrimers so far.

Abstract

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.

The intrinsic molecular chiroptical properties (electronic circular dichroism and circular polarized luminescence) of helicences are perfectly transferred from solution to the crystal state and to water-soluble nanoparticles by encapsulating the chiral emitter in supramolecular small-molecule, ionic isolation lattices (SMILES). Furthermore, CPL brightness is enhanced by combination with a rhodamine antenna system.

Abstract

Circularly polarized luminescence (CPL) from chiral molecules is attracting much attention due to its potential use in optical materials. However, formulation of CPL emitters as molecular solids typically deteriorates photophysical properties in the aggregated state leading to quenching and unpredictable changes in CPL behavior impeding materials development. To circumvent these shortcomings, a supramolecular approach can be used to isolate cationic dyes in a lattice of cyanostar-anion complexes that suppress aggregation-caused quenching and which we hypothesize can preserve the synthetically-crafted chiroptical properties. Herein, we verify that supramolecular assembly of small-molecule ionic isolation lattices (SMILES) allows translation of molecular ECD and CPL properties to solids. A series of cationic helicenes that display increasing chiroptical response is investigated. Crystal structures of three different packing motifs all show spatial isolation of dyes by the anion complexes. We observe the photophysical and chiroptical properties of all helicenes are seamlessly translated to water soluble nanoparticles by the SMILES method. Also, a DMQA helicene is used as FRET acceptor in SMILES nanoparticles of intensely absorbing rhodamine antennae to generate an 18-fold boost in CPL brightness. These features offer promise for reliably accessing bright materials with programmable CPL properties.

Hybrid local and charge transfer (HLCT) excited state materials, which possess weak donor-acceptor (D-A) pure organic structures, deserve one of the most promising efficient and stable blue emitters. Through high-lying reverse intersystem crossing (hRISC) process, 75% triplet excitons generated by electrical excitation could be harvested and utilized in organic light-emitting diodes (OLEDs). However, there are still significant challenges to achieve high-efficiency ultra-deep-blue HLCT emitters with low Commission Internationale de l’Eclairage (CIE) 1931 chromaticity coordinate y values. Here, a series of novel blue HLCT emitters based on spiro[1,8-diazafluorene-9,2'-imidazole] structure were designed and synthesized by fine-tuning the spiro[fluorene-9,2'-imidazole] core structure in our previous work through heteroatom substitution and hyperconjugation effect. The target emitters were endowed with excellent photophysical and electrochemical merits, thermal stability and solution processibility. The solution-processed OLED device based on 4',5'-bis(4-(9H-carbazol-9-yl)phenyl)spiro[1,8-diazafluorene-9,2'-imidazole] (NFIP-CZ) achieved efficient ultra-deep-blue emission (CIEx,y = 0.1581, 0.0422) with the maximum external quantum efficiency (EQEmax), maximum current efficiency (CEmax) and maximum power efficiency (PEmax) of 11.94%, 4.07 cd·A-1 and 2.56 lm·W-1. The record EQE is a breakthrough in both solution-processed and vacuum vapor deposition ultra-deep-blue HLCT-OLEDs currently.

We present a molecular design where the dynamic chirality of donor-acceptor fluorophores is controlled using a configurationally stable chiral unit. Beyond illustrating the potential of dynamic chirality control to facilitate the access to chiral functional materials, this work highlight the positive influence of chiral perturbation from the configurationally stable chiral unit on the dissymmetry factors of the dynamically chiral fluorophores.

Abstract

Recently, the control of dynamic chirality has emerged as a powerful strategy to design chiral functional materials. In this context, we describe herein a molecular design in which a tethered configurationally stable binaphthyl chiral unit efficiently controls the dynamic chirality of donor-acceptor fluorophores, involving diverse indolocarbazoles as electron donors and terephthalonitrile as an electron acceptor. The high conformational discrimination in such a molecular system suggested by density functional theory calculations is experimentally probed using electronic and vibrational circular dichroism and confirmed by the crystallization of these chiral molecules in gel and their single crystal X-ray diffraction analysis. This work also highlights the positive effect of the configurationally stable chiral unit on the magnitude of the dissymmetry factors of the active dynamically chiral fluorophores, both in ground and excited states, through chiral perturbation.

We proposed a molecular pre-stacking strategy to minimize the defect density of states during the film formation process. Proof-of-concept emitter DspiroO-F-TRZ, featuring a fluorophenyl group, can form uniform nano-“cluster” structures in doped films, enabling fine-tuning of the film morphology during the film formation process, thereby reducing the density of defect states and minimizing the water/oxygen traps. Consequently, an optimized carrier dynamics process ensues, culminating in a lowered barrier for carrier injection, enhanced carrier transport rate, and increased recombination probability. Remarkably, the TADF-OLED based on DspiroO-F-TRZ has exhibited substantial enhancement compared to the control device, highlighting the contribution of nano-“cluster” structure in improving electroluminescence performance.

Abstract

The quality of organic thin films critically influences carrier dynamics in organic semiconductors. In neat/doped films, even tiny voids can be penetrated by water or oxygen molecules to create charge-trap states called water/oxygen-induced traps that significantly hinder carrier mobility. While the water/oxygen-induced traps in non-doped films and crystalline states have been investigated comprehensively, there is a lack of thorough examination regarding their properties in the doped state. Therefore, there is a high demand for a molecular design strategy that effectively modulates the molecular stacking behavior in doped films and practical devices and enhances the quality of these films. Herein, we proposed a versatile molecular design principle that enables the formation of “nano-cluster” structures on both the surface and interior of doped films in target molecule 10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1′-(4-fluorophenyl)-10H-spiro[acridine-9,9′-xanthene] (DspiroO-F-TRZ), which is modified with a fluorophenyl group. These “nano-cluster” structures exhibit more uniform shapes within doped films and effectively reduce defective state densities while enhancing carrier injection and transport properties, ultimately improving device performance. Notably, TADF-OLED based on DspiroO-F-TRZ demonstrates nearly twice as much efficiency as its control counterpart due to contributions from ′nano-cluster′ structure enhancements toward improved electroluminescence performance.

A novel boron/nitrogen-doped polycyclic aromatic hydrocarbon, L–DABNA-1, was synthesized through our proposed synthetic protocol of stepwise one-shot borylation reactions. It demonstrates high-efficiency narrowband thermally activated delayed fluorescence emission. In an organic light-emitting diode, L–DABNA-1 achieves yellow-green electroluminescence with maximum external quantum efficiency of over 40 % and recorded-high current efficiency of ~170 cd A⁻¹.

Abstract

Boron/nitrogen (B/N)-doped polycyclic aromatic hydrocarbons (PAHs) with the multiple resonance (MR) effect are promising for organic light-emitting diodes (OLEDs) because of their narrowband emission and thermally activated delayed fluorescence (TADF) characteristics. Nevertheless, exploring the variety of such emitters is challenging because of the tricky and limited synthetic protocols. Herein, we designed a novel B/N-doped PAH, L–DABNA-1, whose backbone (L–DABNA) could not be achieved via conventional routes (e.g., one-pot borylation or one-shot borylation). We successfully synthesized it through stepwise one-shot borylations with precisely introducing decorations. The unique MR backbone with intersecting DABNA substructures sharing an aniline group, avoiding any para-N-π-B motif, allows L–DABNA-1 to maintain narrowband TADF emission while significantly redshifting to the yellow-green region with a reverse intersystem crossing rate (k _RISC) of 1.28×10⁵ s⁻¹. An L–DABNA-1-based OLED device achieved a maximum external quantum efficiency (EQE) of over 40 % and maintained a high EQE of 36.3 % at 1000 cd m⁻², with a current efficiency reaching ~170 cd A⁻¹. This work not only demonstrated the great potential of stepwise borylations in synthesizing B/N-doped PAH backbones, expanding their chemical space, but also provided a promising pathway for exploring MR-TADF emitters at longer wavelengths.

By introducing a prototypical π-conjugated polycyclic aromatic hydrocarbon unit, specifically the naphthalene unit, into the structure of a classical blue-light-emitting molecular framework, we can enhance the bonding properties of the target green emitters to meet the BT.2020 standard without sacrificing their optoelectronic performance.

Abstract

Developing fluorophores that conform to the Broadcast Service Television 2020 (BT.2020) standard presents a formidable challenge. Here, we propose an innovative approach that integrates two and three-boron/nitrogen (BN2)-embedded [4]helicene subunits with naphthalene, resulting in the synthesis of two novel narrowband bright green quasi-fluorescent emitters, NT-2B and NT-3B for ultra-high-definition displays. These emitters exhibit minimal reorganization energy and Huang–Rhys factor, emitting at 510 and 511 nm in dilute toluene solution with exceptionally narrow full width at half maximum values of 15 and 14 nm, respectively. Notably, NT-2B demonstrates an impressive photoluminescence quantum yield of 92.5 %, rapid radiative decay rate, and slow non-radiative decay rate. Owing to their narrowband emission characteristics and outstanding optoelectronic properties, corresponding OLEDs based on NT-2B demonstrate a high external quantum efficiency of 30.6 %, with an FWHM value of 21.5 nm and a CIEy of 0.74, positioning it as one of the leading narrow-band green emitters.

Novel NIR phosphors are constructed using a n-π-n molecular design strategy. Their phosphorescence can be readily modulated by altering the conjugated core structures. When incorporated into a 4-bromobenzophenone host, these materials exhibit efficient NIR phosphorescence ranging from 655–710 nm. Analysis of the exciton transition process reveals that the enhanced phosphorescence of the guest originates from the triplet energy transfer of abundant triplet excitons generated independently by the host material.

Abstract

Limited by the energy gap law, purely organic materials with efficient near-infrared room temperature phosphorescence are rare and difficult to achieve. Additionally, the exciton transition process among different emitting species in host–guest phosphorescent materials remains elusive, presenting a significant academic challenge. Herein, using a modular nonbonding orbital-π bridge-nonbonding orbital (n-π-n) molecular design strategy, we develop a series of heavy atom-free phosphors. Systematic modification of the π-conjugated cores enables the construction of a library with tunable near-infrared phosphorescence from 655 to 710 nm. These phosphors exhibit excellent performance under ambient conditions when dispersed into a 4-bromobenzophenone host matrix, achieving an extended lifetime of 11.25 ms and a maximum phosphorescence efficiency of 4.2 %. Notably, by eliminating the interference from host phosphorescence, the exciton transition process in hybrid materials can be visualized under various excitation conditions. Spectroscopic analysis reveals that the improved phosphorescent performance of the guest originates from the triplet-triplet energy transfer of abundant triplet excitons generated independently by the host, rather than from enhanced intersystem crossing efficiency between the guest singlet state and the host triplet state. The findings provide in-depth insights into constructing novel near-infrared phosphors and exploring emission mechanisms of host–guest materials.