Rong-Huei Yi

New exciplex-forming D:A blend is created to serve as the exciton-harvesting cohost of a tailor-made fluorescent emitter for realizing efficient and stable near-infrared OLEDs with emission wavelengths beyond 800 nm via Förster resonance energy transfer (FRET) process.

Abstract

Two new acceptors, C-BPhCN and N-BPhCN, with 2,3-dicyanopyrazinophenanthrene and pyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile core and ortho-linked biphenyl peripherals were synthesized and characterized. The exciplex formation of C-BPhCN and N-BPhCN as acceptor (A) and SBFC-G1 as donor (D) was examined. Through optimization, the device using a D:A (2:1) blend in emitting layer (EML) exhibited maximum external quantum efficiency (EQE_max) of 8.26% and 5.25% with electroluminescence peak (EL λ _max) centered at 581 and 595 nm, respectively. A new D–A–D configured near-infrared (NIR) fluorescent emitter DMACBBT was introduced as a dopant in the exciplex-forming cohost system. By tuning the thickness of the electron transporting layer (ETL), the EQE_max of the device employing SBFC-G1:C-BPhCN (2:1): 9 wt % DMACBBT as the EML reached 1.03% with EL λ _max at 808 nm. The counterpart device utilizing SBFC-G1:N-BPhCN (2:1): 9 wt % DMACBBT as the EML exhibited an EQE_max of 1.01% and EL λ _max at 817 nm. The stability of the NIR OLED device was measured, yielding lifetimes (T ₆₀) of 182 and 126 h for SBFC-G1:C-BPhCN and SBFC-G1:N-BPhCN cohost-based devices, respectively. This work highlights the high efficiency of NIR OLEDs that can be practically realized by using the exciplex cohost systems with a tailor-made NIR fluorescent emitter.

By copolymerizing with a host material, a multiple resonance thermally activated delayed fluorescent (MR-TADF) emitter quinolino[3,2,1-de]acridine-5,9-dione (QAO) is successfully assembled into water-dispersible polymer dots (Pdots). Photophysical measurements and multichannel fluorescence imaging studies show that the narrowband emission from the MR moiety is preserved, highlighting the potential of MR-TADF Pdots for multiplexed bioimaging applications.

Abstract

Materials exhibiting multiple resonance thermally activated delayed fluorescence (MR-TADF) have attracted attention in organic electronics because of their high photoluminescence quantum yield and narrowband emission. While these superior properties are also desirable for probes for biological imaging, MR-TADF emitters have yet to be broadly exploited in this field. Here, the MR-TADF emitter QAO is incorporated into polymer nanoparticles to construct biological imaging probes with narrowband emission and delayed fluorescence. A monomer QAO-NB is first designed by incorporating a norbornene polymerization handle onto the QAO core, which is then incorporated into a polymer backbone by ring-opening metathesis polymerization to afford water-dispersible fluorescent polymer dots, or Pdots. Photophysical characterization confirmed that narrowband emission is preserved in both the polymers and Pdots (λ_max = 467–478 nm, FWHM ≤ 50 nm), with a delayed fluorescence lifetime of 455 µs in the Pdots. Multichannel fluorescence imaging studies using comparisons against donor-acceptor TADF materials underscore the potential of MR-TADF for the development of narrowband emissive bioimaging probes.

A unique regioselective interior/exterior borylation strategy is presented for the one-shot synthesis of two distinct organoboron luminophores from a common macrocyclic azacalix[3]arene. This interior/exterior borylation imparts the material with remarkable TADF properties and narrowband emission capabilities, spanning the violet to blue spectral region.

Abstract

The development of high-performance organoboron luminophores with narrowband emission capabilities is desirable for advancing organic light-emitting diodes (OLEDs). Here, a unique regioselective interior/exterior borylation strategy is introduced for macrocyclic azacalix[3]arenes, enabling the synthesis of two distinct organoboron luminophores with narrowband thermally activated delayed fluorescence (TADF). By applying two different one-shot borylation conditions, we selectively obtain (i) internally borylated in-B-ACCz, which exhibits narrowband violet TADF, and (ii) externally borylated ex-B-ACCz, which achieves narrowband blue TADF with an exceptionally high photoluminescence quantum yield approaching 100%. Furthermore, OLEDs utilizing ex-B-ACCz demonstrate narrowband blue electroluminescence with a maximum external quantum efficiency of 23.1%. This study offers valuable insights into the design and synthesis of novel organoboron luminophores and photofunctional π-conjugated macrocycles.

In this work, two HLCT blue materials is designed and synthesized with high PLQY, PPIBA, and PPIXO. The exciton utilization of the devices is enhanced by introducing a TTA up-conversion channel in the emitting layer of OLED devices. Ultimately, the PPIBA and PPIXO device performances are increased from 6.6% and 5.2% to 8.7% and 9.1% with no efficiency roll-off.

Abstract

Hybridized local and charge-transfer (HLCT) materials exhibit great potential for high-efficiency organic light-emitting diodes (OLEDs) due to their rapid high-energy reverse intersystem crossing (hRISC) and high photoluminescence quantum yield (PLQY). However, their exciton utilization efficiency (EUE) is constrained by energy loss from higher triplet states (T_n, n ≥ 2) to T₁ via internal conversion and direct generation of T₁ excitons during device operation. To address this limitation, two HLCT blue emitters, PPIBA and PPIXO, are designed and synthesized. The PLQY of PPIBA and PPIXO reach 89.78% and 90.75%, whereas the EUE of their devices is only 39.7% and 31.4%. To further boost the EUE, a triplet-triplet annihilation up-conversion (TTA-UC) material, 1-(2,5-dimethyl-4-(1-pyrenyl)phenyl)pyrene (DMPPP), is introduced to recycle the T₁ excitons of the emitter. This strategy significantly enhances the EUE of the PPIBA- and PPIXO-based devices, reaching 65.6% and 63.5%, respectively. Compared to non-doped devices, incorporating TTA-UC leads to substantial performance enhancements of 31.8% and 75.0% for PPIBA and PPIXO devices, respectively. Notably, the optimized devices exhibit negligible efficiency roll-off until damage. This work demonstrates that TTA-UC as an assistant host can overcome intrinsic limitations of HLCT materials in the emitting layer, providing a viable pathway for high-performance OLEDs.

Secondary donor positional engineering at the C2 and C3 positions of a NAI–DMAc core yields efficient orange‑red TADF emitters. The C3‑isomer ND37DBT exhibits enhanced spin orbit coupling and RISC processes, achieving 70% PLQY. and 22.1% EQE in solution‑processed organic light-emitting diodes (OLEDs).

Abstract

Achieving efficient orange-red emission in thermally activated delayed fluorescence (TADF) emitters remains challenging due to the hard to achieve great balance between a small singlet-triplet energy gap and high radiative decay rates. In this study, a modification site engineering strategy for secondary donors to optimize excited-state properties for orange-red emitters is proposed. Two isomeric emitters, ND28DBT and ND37DBT, were synthesized by introducing dibenzothiophene (DBT) units at different positions of a naphthalimide–dimethylacridine (NAI–DMAc) core. The theoretical analysis indicates that ND37DBT possesses a more favorable excited-state configuration, which facilitates an efficient reverse intersystem crossing (RISC) process and accelerates the radiative decay rate. As a result, ND37DBT showed a high photoluminescence quantum yield (PLQY) (70%), a fast RISC rate (6.25 × 10⁵ s⁻¹), and an excellent external quantum efficiency (EQE) (22.1%) with minimal roll-off in 100 cd m⁻². This work demonstrates that precise control of secondary donor modification sites offers a powerful molecular design strategy for developing high-efficiency orange-red TADF materials.

A π-extended, heavy-atom design enables multi-resonance thermally activated delayed fluorescence emitters with an ultrafast reverse intersystem crossing rate of 3.0 × 10⁶ s⁻¹ while preserving narrowband emission in the deep-blue region. The best-performing device achieves an emission maximum at 456 nm, a full-width at half-maximum of 18 nm, a record-high external quantum efficiency of 40.5%, and a Blue Index of 422, charting a promising route toward next-generation deep-blue display technologies.

Abstract

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters hold great promise for high-resolution OLEDs, yet achieving both ultranarrow emission and efficient triplet utilization in the deep-blue region remains challenging. Here, a synergistic molecular design is reported that combines π-extension and heavy-atom incorporation to effectively reconcile the trade-off between color purity and fast reverse intersystem crossing (RISC). In this approach, π-extension narrows the emission bandwidth and reduces the singlet–triplet energy gap, while the strategic introduction of chalcogen atoms selectively enhances spin–orbit coupling with minimal impact on the emission spectrum. As a result, the new emitter exhibits a peak emission at 453 nm with an exceptionally narrow full width at half maximum (FWHM) of 17 nm and a high RISC rate constant of 3.0 × 10⁶ s⁻¹. When incorporated into a non-sensitized OLED, the emitter meets the European Broadcast Union (EBU) deep-blue standard with CIE coordinates as low as (0.140, 0.059), and sustains a brightness exceeding 30,000 cd m⁻². Notably, the device achieves a record-high external quantum efficiency (EQE_max) of 40.5% with minimal roll-off—retaining 38.4% and 28.2% at 100 and 1,000 cd m⁻², respectively—and attains a Blue Index (BI) of 422 cd A⁻¹ CIE_y ⁻¹. These findings highlight the effectiveness of our tactic in overcoming prior limitations where heavy-atom doping often compromises color purity, paving the way for next-generation emitters in advanced display and lighting applications.

Solution-processable deep-blue OLEDs are achieved by designing a linearly fully fused acceptor–donor–acceptor-type tetraborate emitter with intramoleuclar noncovalent interactions and highly distorted structure, since such emitter promots high color purity satisfying the BT.2020 standard and a fast reverse intersystem crossing rate. The nonsensitized solution-processed deep-blue device achieves the best performance among wet LEDs with CIE_y < 0.08.

Abstract

Solution-processable organic light-emitting diodes (OLEDs) have attracted much attention from academia and industry because of their advantages such as low production cost and suitability for large-scale production. However, solution-processable deep-blue OLEDs that simultaneously have high efficiencies and satisfy the BT.2020 standard remain still a great challenge. To address this issue, here a tetraboron multiresonance thermally activated delayed fluorescence (MR-TADF) emitter, tBO-4B, embedded with two soluble 2,12-di-tert-butyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene groups is designed and synthesized with a linearly fully fused acceptor–donor–acceptor-type molecular structure. tBO-4B not only achieves an ultranarrow full width at half maximum of 12 nm but also has a negligibly small singlet-triplet energy gap and large spin‒orbit coupling, eventually leading to very fast reverse intersystem crossing rate (4.23 × 10⁶ s⁻¹). The sensitizer-free solution-processed OLED exploiting tBO-4B as the emitter achieves an ultrahigh maximum external quantum efficiency (EQE_max) of 30.3%, with Commission Internationale de l’Éclairage (CIE) coordinates of (0.147, 0.042) meeting the BT.2020 blue standard. In addition, the corresponding sensitizer-free vacuum-processed deep-blue devices also exhibit an impressive EQE_max of 39.6% and mild efficiency roll-off with CIE coordinates of (0.147, 0.043). This work will facilitate the development of high-efficiency ultrapure deep-blue MR-TADF materials for solution- and vacuum-processed OLEDs.

Efficient organic D-π-A scintillators with temperature adaptability are presented, achieved through strategic fluorine atom modification of π-bridge. These materials simultaneously exhibit TADF, AIDF, and RTP that dynamically respond to environmental changes under ultraviolet excitation. Their PLQYs can reach up to 88% in PMMA films. Notably, 1FAT@PMMA exhibits excellent radioluminescence performance, achieving a remarkably low detection limit of 62.27 nGy s⁻¹ and an impressive spatial resolution of 20 lp mm⁻¹.

Abstract

Achieving highly efficient organic scintillators for X-ray imaging remains a significant challenge, primarily owing to the inherent difficulty in facilitating rapid radiative decays of both singlet and triplet excitons. To address this limitation, a novel design strategy is introduced that incorporated fluorine atoms to modify the π-bridge of D-π-A molecules, thereby fine-tuning their electronic structures and photophysical properties. The emitters, namely 1FAT and 2FAT, show excellent thermally activated delayed fluorescence (TADF), aggregation-induced delayed fluorescence (AIDF), and room temperature phosphorescence (RTP) excited by ultraviolet light varying with surrounding environments. Notably, the doped PMMA film of 1FAT exhibites intense emission across a broad temperature range from 77 to 363 K, showcasing its adaptability to diverse thermal conditions. More importantly, 1FAT@PMMA film exhibits excellent radioluminescence performance under X-ray excitation with a low detection limit of 62.27 nGy s⁻¹ and a high spatial resolution of over 20 lp mm⁻¹. This study introduces a novel and fundamental design strategy for developing efficient, temperature-adaptive D-π-A organic scintillators, significantly expanding their potential applications in flexible, stretchable X-ray detectors and advanced imaging technologies.

Two pairs of chiral multiple resonance enantiomers with increased g factors are obtained by using different chiral sources to modulate the angle between transition electric and magnetic dipole moments. The enantiomers show excellent circularly polarized luminescence properties with dissymmetry factors (|g _PL|) of 5.8/2.5 × 10⁻³. Ultimately, the optimized circularly polarized organic light-emitting diodes exhibit high maximum external quantum efficiencies of 24.7%/33.3% with |g _EL| factors of 3.4/1.2 × 10⁻³.

Abstract

The most challenging task in the development of chiral luminescent materials is currently increasing the dissymmetry factors (g _PL, g _EL) of circularly polarized luminescence (CPL). Current research primarily focuses on enhancing the molecular transition magnetic dipole moment, while the angle between the transition electric and magnetic dipole moment (θ _e,m) has received comparatively limited attention. In this study, two pairs of intrinsically axial chiral fluorescence (CP-MR-F) and thermally activated delayed fluorescence (CP-MR-TADF) enantiomers (R/S-BBCz-BN and R/S-OBBCz-BN) with increased g factors by modulating θ _e,m are reported. Based on R/S-5H,5′H -6,6′-bibenzo[b]carbazole units, the theoretically calculated θ _e,m and g _cal for S-BBCz-BN are 17.7° and 1.1 × 10⁻², respectively, enabling excellent CPL properties with |g _PL| of 5.8 × 10⁻³ in toluene, representing one of the highest g values among MR materials. By employing the less conjugated R/S-7,7′,8,8′,9,9′,10,10′-octahydro-5H,5′H-6,6′-bibenzo[b]carbazole groups with better electron donating ability, R/S-OBBCz-BN demonstrate good MR-TADF and CPL properties with a peak at 467 nm, a full width at half-maximum of 27 nm, and |g _PL| of 2.5 × 10⁻³. Furthermore, the optimized circularly polarized organic light-emitting diodes based on these two pairs of enantiomers showcase high maximum external quantum efficiencies of 24.7%/33.3% and mirror symmetrical circularly polarized electroluminescence with |g _EL| factors of 3.4/1.2 × 10⁻³, respectively.

A spiro-acridine framework incorporating boron–nitrogen covalent bond fusion enables the construction of multi-resonance thermally activated delayed fluorescence emitters with narrowband emission (full width at half maximum: 20 and 19 nm). The corresponding organic light-emitting diode devices exhibit maximum external quantum efficiencies of 27.2% and 30.8%, demonstrating a balanced molecular design strategy for high spectral purity and excellent performance.

Abstract

Developing the modified skeleton of multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters can feasibly regulate their optoelectronic properties. However, research on MR-TADF materials incorporating B─N covalent bonds remains relatively limited. Herein, a strategy is first proposed utilizing the twin-spiro conformation, a dispirofluorene acridine skeleton (DSAF), via the amine-directed double borylation for framework fusion to construct MR-TADF emitters, DSAF-TBDPA and DSAF-TBCz. B─N covalent bonds and twin-spiro fusion are employed to enhance the structural rigidity of the molecular framework. Furthermore, the introduction of a twin-spiro structure imparts steric hindrance, thereby weakening the π–π interactions and mitigating exciton quenching. As a result, DSAF-TBDPA and DSAF-TBCz exhibit narrowband emission with a full width at half maximum of 20 and 19 nm. The doped organic light-emitting diodes (OLEDs) based on DSAF-TBDPA and DSAF-TBCz exhibit maximum external quantum efficiencies (EQE_max) of 27.2% and 30.8%, respectively. These results underscore that molecular design tactics not only expand the diversity of MR frameworks but also deliver critical insights for the design of high-performance OLED emitters.

A premixed exciplex co-host composed of electron donor and acceptor molecules is proposed with similar relative molecular masses and evaporating temperatures, which is evaporated in one heating crucible. Continuous parallelly fabricated OLED devices employing the co-host exhibit indistinctive performance with high luminance over 250 000 cd m⁻², negligible efficiency roll-offs, and long-term operational lifetimes, predicting practical application prospects.

Abstract

Exciplex, characterized by intermolecular charge transfer and thermally activated delayed fluorescence (TADF) properties, plays a significant role in organic light-emitting diodes (OLEDs), particularly as co-hosts. The rapid rate of reverse intersystem crossing (RISC) and balanced carrier mobility contribute to improved efficiency and suppressive efficiency roll-off at high current density. Despite these advantages, the fabrication of devices using two-component exciplexes is challenging, especially when the emitting layers require the simultaneous evaporation of three or four materials from separate crucibles. To address this issue, a pair of premixed exciplex co-hosts is developed and utilized as the co-host for Ir(ppy)₃. The consistent performance of continuous parallelly fabricated devices with the same premixed co-host sample indicates the long-term stability of the premixed exciplex co-host and the stable evaporation ratio of the electron donor and electron acceptor molecules. The devices achieve maximum luminance over 250 000 cd m⁻², a maximum external quantum efficiency of 21.9%, a regardless efficiency roll-off of 4.6% at 10 000 cd m⁻², along with a prolonged operational LT₉₅(lifetime to 95% of the initial luminance) of 165 h at the current density of 10 mA cm⁻². Further enhancement in device performance is observed through co-doping a multiple resonance TADF (MR-TADF) material in the emitting layer, underscoring the significant potential for industrial application.

A highly luminescent propeller-shaped PDI hexamer exhibiting solvent-responsive circularly polarized luminescence (CPL) was synthesized via a one-pot reaction. The propeller chirality undergoes reversible inversion, enabling discrimination between mixtures of CH₂Cl₂ and CHCl₃.

Abstract

Circularly polarized luminescence (CPL) has the potential for next-generation optoelectronic applications. One of the major challenges in this field is the development of CPL emitters, whose emission properties can be modulated by external stimuli, such as solvents. However, CPL-active materials are often synthetically demanding and typically require chiral separation. To address these limitations, we have synthesized a propeller-shaped molecule, (R/S)-Bz-6PDI 1, via a one-pot nucleophilic aromatic substitution reaction, using a chiral pyrrole-fused perylene diimide (PDI) 4 as the blade. The introduction of a chiral auxiliary into the blade enabled the induction of propeller chirality without the need for chiral chromatographic separation. (R)-Bz-6PDI 1 exhibited high CPL brightness in solution (B _CPL = 103–369 M⁻¹ cm⁻¹). Furthermore, the propeller chirality proved highly sensitive to the solvent environment, leading to significant modulation of both the sign and intensity of the CD and CPL signals, including complete signal inversion in CH₂Cl₂ and CHCl₃. CD spectral analysis combined with DFT calculations revealed that the propeller chirality is governed by the orientation of the hydrogen atom in the chiral auxiliary.

The work presents a novel synthesis methodology for multiple-resonance (MR) materials. The selective carbon–nitrogen bond cleavage of MR frameworks enables the introduction of a wide range of aryl groups that are otherwise difficult to incorporate via conventional borylation methods. This late-stage functionalization approach boosts the optoelectronic properties of MR materials. As a representative example, DABNA-dimer exhibited improved EL performance.

Abstract

Recent advances in materials development for organic light-emitting diodes (OLEDs) have been driven by an emerging design concept known as multiple-resonance (MR), which provides the benefits of high efficiency and color purity required for commercial OLED displays. Borylation reaction constructing MR frameworks is a key step for developing new π-conjugated systems, while this reaction includes the limitation of selectivity, making it challenging to control reactions in elaborate molecular structures. Although a wide variety of MR compounds have been prepared through carefully planned synthetic schemes based on retrosynthetic analysis, these tasks typically require significant effort for each individual target. Herein, we demonstrate a successful late-stage functionalization strategy that enables structural modifications after the borylation reaction, especially in hardly accessible regions in MR structures via carbon–nitrogen bond cleavage. Our new synthetic method also allows access to new products which were never feasible before. The newly developed material, DABNA-dimer, proves the impact of late-stage functionalization on OLED characteristics. The present study serves as a proof-of-concept for a diversity-orientated synthesis in the development of MR materials.

A salicylaldehyde azine-based CCOF exhibiting strong circular dichroism and solid-state circularly polarized fluorescence was constructed via chiral induction, featuring water-induced stimuli-responsive color change and chiroptical activity enhancement.

Abstract

Chiral covalent organic frameworks (CCOFs) are promising candidates for chiral optoelectronics and sensing, but their weak solid-state fluorescence and chiroptical responses often limit practical utility. Here, we introduce a novel CCOF synthesized from achiral monomers, 2-hydroxy-1,3,5-benzenetricarbaldehyde and hydrazine, via imine condensation with a chiral induction strategy, yielding salicylaldehyde azine units with aggregation-induced emission. Optimized catalyst and chiral inducer stoichiometry endow the framework with exceptional chiroptical properties (|g_abs| = 2.2 × 10⁻², ellipticity ≈ 1000 mdeg). In the solid state, the CCOF exhibits intense red fluorescence (λ_em ≈ 645 nm) with a large stoke shift and favorable circularly polarized luminescence (CPL, |g_lum| = 5.2 × 10⁻²), marking the first CCOF derived solely from achiral building blocks with robust solid-state CPL. When integrated into polydimethylsiloxane, it forms flexible and semitransparent composite films suitable for CPL-based applications. The CCOF also functions as a highly enantioselective fluorescent sensor for chiral analytes, including 2-aminocyclohenanol and dimethyl-1,2-cyclohexanediamine. Furthermore, it demonstrates reversible hydrochromism, transitioning from yellow to orange (ΔE ≈ 42.7), and water-induced chiroptical enhancement (ellipticity up to 2100 medg, |g_abs| = 5.5 × 10⁻²), achieving the highest ground-state chirality reported for CCOFs through enol-to-keto tautomerism upon water adsorption. This stimuli-responsive CCOF overcomes persistent limitations in solid-state CPL and paves the way for chiral sensing, optical displays, and responsive materials.

Luminescent lanthanide emitters typically need sensitization from an attached chromophore – an “antenna” – to have useful emission intensities. Here we show that the mechanical bond can be used to connect the antenna to the emitter, providing unique stimuli-responsiveness to the resulting assemblies. The resulting lanthanide-capped [2]rotaxanes are easy to synthesize, give good emission intensities and act as efficient metal ion sensors.

Abstract

Luminescent emitters based on lanthanide ions are of ubiquitous importance in the biological sciences, but typically need sensitization from a covalently attached adjacent chromophore – an “antenna” – to have suitable emission intensities. Here we show that the mechanical bond can be used to connect the antenna to the emitter, providing dynamic features and stimuli-responsiveness to the resulting assemblies. We outline a strategy to synthesize [2]rotaxanes capped with strong chelating groups, and establish that post-functionalization of the interlocked scaffold by lanthanide ion insertion is modular, high-yielding and straightforward. Photophysical studies revealed effective antenna-emitter energy transfer within the [2]rotaxane, and the sensitization mechanism as well as ring-thread dynamics were studied with spectroscopic and computational methods. The rotaxane was shown to have high selectivity toward Cu(II) ions, acting as an efficient turn-off sensor. This study validates the mechanical bond as a conjugation method between antennas and emitters, yielding otherwise hard-to-access and beneficial features to the resulting molecular systems.

Over the past decade, red-emissive organic molecular crystals have advanced through molecular design (donor-acceptor structures, π-conjugation) and crystal engineering to overcome aggregation-caused quenching and energy gap limitations. Featuring high quantum yields, tunable deep-red/NIR emission, and flexibility, they enable applications in flexible waveguides, lasers, and bio-integrated optoelectronics.

Abstract

Red-emissive organic materials play a pivotal role in optoelectronics, including displays, optical communications, organic lasers, and biomedicine, owing to their high penetration and low scattering properties. However, conventional polymer and thin-film emitters often face efficiency losses at long wavelengths and limited operational stability. Organic molecular crystals have emerged as promising alternatives by offering high purity, low defect density, and unique optical anisotropy. Among them, red-emissive organic molecular crystals (ROMCs) remain relatively underdeveloped compared to their blue and green counterparts. Moreover, the inherent brittleness of most crystals poses significant challenges for their integration into flexible and wearable devices. This review highlights recent advances in the design and development of ROMCs, emphasizing molecular and crystal engineering strategies to overcome photophysical limitations and impart mechanical flexibility. Emerging applications in organic lasers, optical waveguides, and bioimaging are discussed, alongside key challenges and future research directions. By bridging fundamental understanding and practical deployment, this perspective offers a comprehensive roadmap for the rational design of flexible, red-light-emitting crystalline materials for next-generation optoelectronic platforms.

Multi-π-stacked through-space charge-transfer emitters featuring thermally-activated delayed fluorescence are developed with a 3D [fixed acceptor]/donor/[fixed acceptor] structure formed on two individual fluorene bridges, which show markedly enhanced radiative decays rates (k _r,s). Organic light-emitting diodes using them as emitters/sensitizers show high external quantum efficiencies (EQEs) and largely suppressed efficiency roll-offs.

Abstract

Through-space charge-transfer (TSCT) thermally-activated delayed fluorescence (TADF) emitters have been extensively explored for organic light-emitting diodes (OLEDs), but their low radiative decay rates (k _r,s) have remained a formidable obstacle to enhancement of luminescent efficiency (Ф_PL) and suppression of efficiency roll-off of the OLED. Here, multi-π-stacked TSCT-TADF emitters with accelerated radiative decay are developed with a 3D [fixed acceptor]/donor/[fixed acceptor] structure formed on two individual fluorene bridges. Single-crystal structures and theoretical calculations reveal cis or trans-configurations of the two acceptors around the central donor and double TSCT channels in the emitter. In doped films, the emitters show green-blue TADF with high k _r,s up to 1.1 × 10⁷ s⁻¹ and near-unity Ф_PL, which are notably improved compared to the single-π-stacked analog (k _r,s = 2.1 × 10⁶ s⁻¹, Ф_PL = 0.89). OLEDs based on the emitters show high external quantum efficiencies (EQEs) up to 26.2% and the EQEs at 1000 cd m⁻² (EQE₁₀₀₀) remain up to 23.4%, which are superior over the device based on the single-π-stacked analog (EQE_max/EQE₁₀₀₀ = 20%/15.6%). A narrowband blue-green OLED with one emitter as a sensitizer shows high EQE_max/EQE₁₀₀₀ at 37.6%/27.7%. The work demonstrates the great potential of multi-π-stacked structure in enhancing the k _r,s of TSCT-TADF emitters for high-efficiency OLEDs with low efficiency roll-offs.

Triplet-triplet annihilation-based photon up-conversion (TTA-UC) system using an anthracene-based UC emission by assembling multiple chromophores linked to a boron to enhance TTA rate constant. Structural fluctuations driven by vibronic motion induce picosecond intramolecular triplet exciton hopping, expanding the triplet exciton distribution and enhancing intermolecular TTA reactivity.

Abstract

Photon upconversion via triplet-triplet annihilation (TTA-UC) is a well-known process that converts low-energy light into higher-energy light. This process has attracted attention for its potential in various fields, including light-emitting devices, power generation and medical applications. It is desirable to develop TTA-UC materials of TTA emitters with large TTA rate constants (k _TTA). However, molecular design to accelerate the bimolecular rate constant of k _TTA has not been considered. We present a strategy to manipulate k _TTA by assembling multiple chromophores linked to a boron in a rotationally symmetric manner, causing asymmetric motions by a localized triplet exciton. We have studied tri(9-anthryl)borane, which consists of three anthracenes linked via boron, as a TTA emitter. Time-resolved luminescence measurements confirmed that k _TTA is improved compared to the conventional TTA-UC system using DPA, an anthracene-based monomer. Time-resolved electron paramagnetic resonance measurements showed that the improvement in k _TTA is due to fast intramolecular triplet exciton hopping coupled with vibrational motions in the trimer molecule, which extends the reactivity at the collision distance between the excitons through the pseudo-rotational motions. This molecular design that enhances TTA reactivity is expected to contribute to the future development of TTA-UC materials for sensing fluid environment.

Multiple resonance (MR) emitters bridged by multi-nitrogen indolocarbazoles to extend molecule skeletons could modify emission maximums without scarifying narrow full width at half maximum (FWHM), which, however, face synthesis challenges of uncontrollable borylation regioselectivity and spectral broadening issue from intermolecular aggregation. Here, a steric pre-substitution strategy is devised using tert-butylphenyl-functionalized indolo[3,2-b]carbazole as a bridge to extend MR skeletons, not only steering regioselective Bora-Friedel-Crafts borylation but also suppressing intermolecular interactions in films. The targeted greenish emitter therefore achieves a small electroluminescence FWHM of only 22 nm in device, matching the intrinsic photoluminescence one of 21 nm in dilute toluene. The extended skeleton also enhances the horizontal orientation of emitting dipole moment with a ratio of 90%, leading to a remarkable high maximum external quantum efficiency (EQE) of 41.0% and power efficiency of 106.5 lm W-1. The efficiency roll-off remains remarkably low with EQEs sustaining at 35.0% and 30.6% at luminance of 1,000 cd m-2 and 5,000 cd m-2, respectively. This work establishes a pre-substitution paradigm to concurrently optimize synthesis control and solid-state emission for high-performance MR emitters.

The synergistic effects of electronic and geometric engineering have been utilized to synthesize a diaza-octagon fused polycyclic aromatic hydrocarbons (PAHs) skeleton (8NN) with ultra-narrowband deep-blue emission. Its phenyl and boron derivatives 8NNP and 8NNB showed excellent performances in OLEDs.

Abstract

Developing new polycyclic aromatic hydrocarbons (PAHs) skeletons with narrowband emission is crucial for advancing ultra-high-definition displays. Inspired by the aesthetic configuration of negatively curved octagonal rings and the multiple resonance (MR) effect, we designed a novel diaza-octagon heterocycle fused with PAHs, abbreviated as 8NN, and its phenyl-substituted derivative 8NNP. Crystal data and theoretical calculations show that their peripheral PAHs exhibit significant rigidity, while the internal diaza-octagon ring confers skeleton flexibility and flipping properties. Notably, owing to the strategic embedding and unique bonding form of nitrogen atoms, their MR effect is significantly enhanced. Finally, 8NN and 8NNP achieve deep-blue emission peaking at 408 and 411 nm, with remarkably narrow full-width at half-maximum (FWHM) of 19 and 17 nm, respectively, and aggregation-enhanced emission characteristics. Additionally, to further tune emission range of this new skeleton, boron-functionalized 8NNB was synthesized, achieving blue emission peaking at 461 nm in toluene, with a FWHM of 31 nm. Finally, the OLEDs based on 8NNP and 8NNB exhibit maximum external quantum efficiencies of 3.47% and 24.24%, with CIE coordinates of (0.172, 0.043) and (0.132, 0.256), respectively. These results validate the rational design of the novel skeleton and facilitate the development of narrowband emission materials.