Rong-Huei Yi

A series of extended pure hydrocarbon (PHC) materials with a high triplet energy, a wide HOMO/LUMO gap and improved thermal properties compared to reported PHCs have been constructed by the assembly of three spirobifluorene fragments. When used as the host in blue phosphorescent OLEDs (PhOLEDs), a high external quantum efficiency (EQE) of 24 % with a low-efficiency roll-off was reached.

Abstract

Pure aromatic hydrocarbon materials (PHCs) represent a new generation of host materials for phosphorescent OLEDs (PhOLEDs), free of heteroatoms. They reduce the molecular complexity, can be easily synthesized and are an important direction towards robust devices. As heteroatoms can be involved in bonds dissociations in operating OLEDs through exciton induced degradation processes, developing novel PHCs appear particularly relevant for the future of this technology. In the present work, we report a series of extended PHCs constructed by the assembly of three spirobifluorene fragments. The resulting positional isomers present a high triplet energy level, a wide HOMO/LUMO difference and improved thermal and morphological properties compared to previously reported PHCs. These characteristics are beneficial for the next generation of host materials for PhOLEDs and provide relevant design guidelines. When used as a host in blue-emitting PhOLEDs, which are still the weakest link of the field, a very high EQE of 24 % and low threshold voltage of 3.56 V were obtained with a low-efficiency roll-off. This high performance strengthens the position of PHC strategy as an efficient alternative for OLED technology and opens the way to a more simple electronic.

Thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP) can be controlled in an organic molecule by applying pressure that helps in transferring J-aggregation in the crystal, initially showing TADF into H-aggregation to generate RTP.

Abstract

Considering that regulation of thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP) is necessary for a fluorophore to survive and properly show its characteristic features, it is successfully shown herein that a carefully designed novel pyrene derivative can exhibit remarkable aggregation-induced emission (AIE) overcoming the characteristic fast excitonic decay of pyrene compounds. Moreover, it is established that J-aggregation in the red-emitting molecular crystals of the compound reduces the singlet-triplet energy gap (∆E _ST), allowing TADF with a very high photoluminescence quantum yield (PLQY) of 72%. Whereas, anisotropic grinding of the same crystals generates well-defined microcrystals that show RTP as the nature of aggregation changes. Impressively, under isotropic hydrostatic pressure, the crystals show near-infra-red (NIR) emission with a very high piezochromic luminescence sensitivity of 46.2 nm GPa⁻¹. The results have established that formation of stable H-aggregates in the microcrystals is responsible for the ultra-long RTP. A highly sophisticated full-spectrum Mueller-matrix analysis is used for the first time in such systems to demonstrate the details of the effect of perturbation (pressure) on molecular conformation.

In this paper, a novel donor-π-donor-σ-acceptor type deep-blue TSCT emitter is developed. The corresponding molecule exhibits deep-blue photoluminescence at 448 nm and features a small ΔE _ST (0.09 eV). The doped device achieves a maximum external quantum efficiency of 20.5%.

Abstract

In the realm of organic light-emitting diode (OLED), through-space charge transfer (TSCT) thermally activated delayed fluorescence (TADF) emitters have demonstrated remarkable achievements. Nevertheless, realizing both deep-blue emission and high efficiency by the TSCT strategy remains a persistent challenge. Here, the novel donor-π-donor-σ-acceptor (D-π-D-σ-A) type deep-blue TSCT emitter, namely D-tCz-BO, is formulated to investigate its performance in OLED. D-tCz-BO show a narrow deep-blue emission and efficient TADF character (λ _PL = 448, FWHM = 54 nm, Φ _PL = 82%, and ΔE _ST = 0.09 eV). More importantly, the OLED based on D-tCz-BO as the emitter showed a maximum external quantum efficiency (EQE_max) of 20.5%, with a Commission Internationale de l'Eclairage (CIE) coordinate of (0.14, 0.12). This represents one of the best deep-blue TSCT-TADF emitters to date.

Integrating chiral elements within a multi-resonance framework presents a promising avenue for engineering tailored emitters suited to cutting-edge circularly polarized organic light–emitting diodes. Consequently, a considerable g _PL of 3.3 × 10⁻³ along with exceptional device performance characterized by a peak external quantum efficiency of 36.6%, and desirable CIE coordinates (0.19, 0.71) can be achieved simultaneously.

Abstract

The integration of chiral elements within a multiple resonance (MR) motif affords a prospective avenue to construct satisfying emitters tailored for state-of-the-art circularly polarized organic light–emitting diodes (CP-OLEDs). However, the concurrently realizing of both high luminescence efficiency and favorable dissymmetry factors (g _PL) still remains a formidable challenge, particularly when aligning with the requirement of high color purity. Herein, a dual-pronged approach is proposed to reconcile such trade-offs by directly fusing a secondary chiral donor onto the MR scaffold, thereby facilitating a hybrid short/long-range charge-transfer with fine-tuned compositions. Theoretical calculations unveil the pronounced impact of the chiral donor on meticulously refining the characteristics of excited states, therefore yielding a considerable g _PL of 3.3 × 10⁻³, along with a high fluorescence quantum yield of 0.97, and a rapid reverse intersystem crossing rate of 3.06 × 10⁵ s⁻¹ in one embodiment. Leveraging these merits, electroluminescence devices incorporating them as chiral dopants exhibit exceptional performance, showcasing a peak external quantum efficiency of 36.6% and remarkable Commission Internationale de L'Eclairage coordinates of (0.19, 0.71), which represent one of the most notable achievements among pure-green CP-OLEDs.

A strategy of controlling electron density on amino of diaminoterephthalates to modulate their photophysical properties and functionalities is proposed. Based on this strategy, a series of single-benzene fluorophores is facilely synthesized and exhibits tunable bright dual-state emission in solution and solid-state, acidichromism, Cu²⁺ detection, and lipid droplets staining.

Abstract

Single-benzene fluorophores have attracted increasing interest owing to their simple molecular structure and unique photophysical properties. Developing novel single-benzene fluorophores with facile synthesis, bright and tunable dual-state emissions, and diversified functions is highly desirable. Herein, a strategy of controlling electron density on amino of 2,5-diaminoterephthalates is proposed to modulate their dual-state emission, acidichromism, Cu²⁺ detection, and lipid droplets (LDs) staining. A series of efficient dual-state single-benzene fluorophores based on 2,5-diaminoterephthalate skeleton with tunable emissions in blue to red region and large Stokes shifts is facilely developed. The obtained fluorophores with various substituents on amino groups exhibit different acidichromism property. Two of fluorophores (ABB and ABM) with high electron density on amino groups exhibit high sensitivity and selectivity toward Cu²⁺ detection with a ratiometric and “turn-off” fluorescence mechanism for ABM and ABB, respectively. The limit of detection for Cu²⁺ is determined to be 1.40 × 10⁻⁷ and 4.35 × 10⁻⁹ mol L⁻¹ for ABB and ABM, respectively. Moreover, three of fluorophores (ABT, ABB, and ABM) with green to red emission display superior LDs-targeting capability with high specificity, and brightness. This molecular design philosophy provides a new way of designing highly bright dual-state fluorophores for practical applications.

Bright deep-blue thermally activated delayed fluorescent emitters, carbene-gold-amides (CMA), possess near unity photoluminescent quantum yields and record high radiative rates up to 4.6 × 10⁶ s⁻¹. Molecular design rules are explained to achieve and predict deep-blue CMA materials. Organic light-emitting diodes (OLED) emit deep-blue electroluminescence with efficiency up to 25.7% with operating stability of 1 h at practical brightness.

Abstract

Linear gold complexes of the “carbene–metal–amide” (CMA) type are prepared with a rigid benzoguanidine amide donor and various carbene ligands. These complexes emit in the deep-blue range at 424 and 466 nm with 100% quantum yields in all media. The deep-blue thermally activates delayed fluorescence originates from a charge transfer state with an excited state lifetime as low as 213 ns, resulting in fast radiative rates of 4.7 × 10⁶ s⁻¹. The high thermal and photo-stability of these carbene–metal–amide (CMA) materials enabled the authors to fabricate highly energy-efficient organic light-emitting diodes (OLED) in host–guest architectures. Deep-blue OLED devices with electroluminescence at 416 and 457 nm with practical external quantum efficiencies of up to 23% at 100 cd m⁻² with excellent color coordinates CIE (x; y) = 0.16; 0.07 and 0.17; 0.18 are reported. The operating stability of these OLEDs is the longest reported to date (LT₅₀ = 1 h) for deep-blue CMA emitters, indicating a high promise for further development of blue OLED devices. These findings inform the molecular design strategy and correlation between delayed luminescence with high radiative rates and CMA OLED device operating stability.

An organic solid-state laser under continuous-wave (CW) excitation is one of the most challenging areas in organic optoelectronics. Recent advances in long-pulsed organic lasers are comprehensively summarized with respect to molecular designs, optical-resonator architectures, triplet scavenging, and potential triplet-contribution strategies. Future directions and perspectives for CW operation are discussed.

Abstract

A continuous-wave (CW) organic solid-state laser is highly desirable for spectroscopy, sensing, and communications, but is a significant challenge in optoelectronics. The accumulation of long-lived triplet excitons and relevant excited-state absorptions, as well as singlet–triplet annihilation, are the main obstacles to CW lasing. Here, progress in singlet- and triplet-state utilizations in organic gain media is reviewed to reveal the issues in working with triplets. Then, exciton behaviors that inhibit light oscillations during long excitation pulses are discussed. Further, recent advances in increasing organic lasing pulse widths from microseconds toward the indication of CW operation are summarized with respect to molecular designs, advanced resonator architectures, triplet scavenging, and potential triplet contribution strategies. Finally, future directions and perspectives are proposed for achieving stable CW organic lasers with significant triplet contribution.

A promising light-pressing modulated nanowelding process is proposed for fabricating large-area AgNWs flexible transparent conductive electrodes (FTCEs) toward next generation flexible optoelectronics, and the resulting AgNWs FTCEs-based green phosphorescent organic light-emitting diode achieves an unprecedented external quantum efficiency of 23.7% and a current efficiency as high as 81.5 cd A⁻¹, demonstrating a uniform large-area emission of 25 × 25 mm².

Abstract

Despite considerable interest, uniform and robust flexible transparent conducting electrodes (FTCEs) that can be seamlessly integrated and used for highly efficient large-area flexible oganic light-emitting diodes (OLEDs) remain elusive. In this study, a large-area fabrication of uniform transparent electrodes for high-performance flexible OLEDs by exploiting the rapid nanowelding process of silver nanowires (AgNWs) onto polyethylene terephthalate substrate under Xe-lamp irradiation and mechanical pressing treatment is reported. The performance of AgNWs FTCEs is significantly enhanced by applying the Xe-lamp beam irradiation for 5 s and subsequent compression at 20 MPa for 15 s, achieving a low sheet resistance of 26.5 Ω sq⁻¹, a high transmittance of 95.2% (at 550 nm), and very smooth surfaces with root-mean-square of 5.4 nm. Meanwhile, the nanowelded AgNWs FTCEs maintain excellent electrical conductivity (only a 2.96% increase in ΔR/R ₀) after 1000 bending cycles. The resulting AgNWs FTCEs-based green phosphorescent OLED achieves an unprecedented external quantum efficiency (EQE) of 23.7% and a current efficiency as high as 81.5 cd A⁻¹. Benefiting from the uniform properties for resulting AgNWs FTCEs, the fabricated flexible OLED with a large area of 25 × 25 mm² still retains a high EQE of 22.2% and a current efficiency of 78.0 cd A⁻¹ _.

A linear D–A–D triad UV emitter, namely CDFDB, which possesses high-lying reverse intersystem crossing (hRISC) property. Thanks to its integrated narrowband UV emission, good hot exciton conversion capability and large horizontal dipole ratio, the CDFDB-based UV OLED can emit highly efficient near UV EL at 398 nm (CIE_x,y: 0.161, 0.040) with a maximum EQE of 12.0 %.

Abstract

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

A ternary charge-transfer (CT) cocrystal with weakened exciton coupling strength due to the changed geometric structure and enlarged donor-acceptor distance is successfully synthesized by rationally selecting monomers. This leads to a prolonged CT exciton lifetime which remarkably improves its photocurrent response and photocatalytic performance, highlighting the future application of CT cocrystals in light conversion.

Abstract

Charge-transfer (CT) cocrystals have attracted continuous interest for their promising optical and optoelectronic applications. To improve the performance of this class of material, a CT cocrystal with long-lived CT excitons is highly desired. Herein, the development of a pyrene-doped trans-1,2-diphenylethylene-1,2,4,5-tetracyanobenzene ternary cocrystal (named P-T_S-T_C) is reported. Compared to the undoped binary cocrystals (without pyrene), P-T_S-T_C exhibits a two times longer CT exciton lifetime (≈60.2 ns), and thus 8.8- and 16.6-times improvement in photocurrent response and photocatalytic H₂ evolution activity. By using transient photoluminescence spectroscopy, it is uncovered that the absorbed photon energy in P-T_S-T_C is localized to a lower energy CT state through an efficient energy transfer process (≈389.8 ps) between two co-existing CT states. The CT exciton lifetime is prolonged due to the weakened CT coupling strength, as a result of the enlarged donor-acceptor distance and the change of geometric structure. The result is expected to inspire the design of cocrystals with manipulatable CT exciton properties and to promote the potential application of multi-component CT cocrystals.

Nature, Published online: 08 May 2024; doi:10.1038/s41586-024-07246-x

Nature, Published online: 08 May 2024; doi:10.1038/d41586-024-01170-w

Azepine-substituted ν-DABNA derivatives exhibit narrowband deep-blue emission with thermally activated delayed fluorescence properties owing to multiple-resonance (MR) effects. Organic light-emitting diodes (OLEDs) that use these emitters simultaneously satisfy high color purity, quantum efficiency, luminous efficacy, and device durability, surpassing those reported in previous studies. The azepine unit shows great potential for enhancing the performance of OLEDs based on MR materials.

Abstract

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

Two TADF emitters, APTT and APTI, which have the same D/A backbone but different attaching groups at the APDC core, are reported. The appended regulating groups can not only suppress the D/A rotation due to the space confinement effects but also modulate the locally excited triplet state (³LE). This work can provide an effective approach for researchers to develop red/NIR TADF emitters.

Abstract

Developing highly efficient red/near-infrared (NIR) thermally activated delayed fluorescence (TADF) materials is important for organic light-emitting diodes (OLEDs). Here, two TADF emitters, APTT and APTI, which have the same D/A backbone but different attaching groups at acenaphtho-[1,2-b]pyrazine-8,9-dicarbonitrile (APDC) core, are reported. The appended regulating groups can not only suppress the D/A rotation due to the space confinement effects but also modulate the locally excited triplet state (³LE). The improved molecular rigidity suppresses the non-radiative process, accounting for the improved photoluminescence quantum yields (PLQYs), while the modulated ³LE promotes the reverse intersystem crossing (RISC) process due to the high utilization efficiency of triplets. Consequently, both APTT and APTI demonstrate high PLQY and fast RISC process, thereby enhancing TADF efficiency. The doped devices based on APTT and APTI achieve maximum external quantum efficiency (EQE_max) values of 20.5% and 25.4% with emission peaks at 664 and 670 nm, respectively. The non-doped devices of APTT and APTI achieve the EQE_max of 2.8% and 2.9% with emission peaks at 788 and 794 nm, respectively. Encouragingly, the non-doped devices of APTI have set new records for near-infrared TADF OLEDs based on the APDC core. This study provides an efficient approach to modulating the optoelectronic properties of highly efficient NIR TADF OLEDs.

The strategy of peripheral Se modification is proposed to disclose the impact of heavy atom on the structure-performance relationship of blue MR-TADF emitters. As the proof of concept, the OLEDs based on Se-modified emitters achieved excellent performances, with a maximum EQE of 25.5% and the EQE still maintains to be 24.5% and 19.6% at the luminance of 100 and 1000 cd m^-2, respectively.

Abstract

Multi-resonance thermally activated delayed fluorescence (MR-TADF) molecules have attracted much attention in the academia owing to their unique photoelectrical properties. However, MR-TADF emitters usually show slow reverse intersystem crossing (RISC) rate, resulting in high efficiency roll-off of organic light-emitting diodes (OLEDs) and seriously limiting their further development. Here, a peripheral selenium (Se) modification is presented for MR-TADF molecules to promote the RISC process while keeping the narrowband emission for high-performance blue OLEDs. Compared to the parent molecules (NBN and tBuNBN), SeNBN and SetBuNBN exhibited narrower full-width at half maximum (FWHM) value of 23 nm and more obvious delayed fluorescence properties with a high efficiency of delayed fluorescence up to 86%, shorter delayed lifetime of 2.4 µs as well as a faster RISC rate of 3.34×10⁵ s⁻¹. Therefore, high-performance OLEDs based on these two Se modified MR-TADF emitters are achieved with a high maximum external quantum efficiency (EQE) up to 25.5% and extremely suppressed efficiency roll-offs of 3.9% at 100 cd m⁻² and 24.4% at 1000 cd m⁻². This work demonstrated that the introduction of peripheral Se atom can achieve high-performance organic semiconductors with both narrowband emission and fast RISC rate constant for high-performance organic optoelectronic devices.

The application of intramolecular non-covalent π–π interactions is broadened to the realm of single-molecule optoelectronic emitters, by featuring a face-to-face donor/acceptor stacking configuration with efficient through-space charge transfer. The resulting supramolecular foldamer showcases that the high-performance single-molecule exciplex thermally activates delayed fluorescence and demonstrates a record lifetime of 236 milliseconds for single-molecule deep-blue room temperature phosphorescence with a charge transfer feature.

Abstract

Although the π–π stacking has been widely applied for constructing aggregated emitters in optoelectronics fields, the role of intramolecular non-covalent π–π interactions has not been well studied. Here, a supramolecular foldermer M-σ-C, with the electron donor (D) and acceptor (A) units spatially separated with a non-covalent bond at a close distance by methylene linker is designed and synthesized. This gives a face-to-face D/A stacking configuration with supramolecular π–π interactions. Temperature-dependent nuclear magnetic resonance measurements and single crystal analyses confirm its folding configuration. In solutions, M-σ-C exhibits a single-molecule exciplex thermally activated delayed fluorescence (TADF) property ascribing to the efficient intramolecular through-space charge transfer (CT) process. While single-molecule deep-blue room temperature phosphorescence (RTP) with a long afterglow lifetime of 236 ms is observed in a nonpolar matrix, which represents the record lifetime among current ³CT-character featured RTP. This work indicates that intramolecular non-covalent interactions play an important role in manipulating high-performance single-molecule exciplex TADF and RTP, and provide a feasible molecular design strategy for supramolecular chemistry involving the development of optoelectronic materials.

The Table of Contents (TOC) image illustrates that novel organic supramolcular macrocycle scintillators with guest-induced TADF emission via host-guest through-space charge transfers, enabling efficient and color-tunable X-ray luminescence, as well as high-resolution imaging of 20 lp mm⁻¹ in devices.

Abstract

Organic scintillators, pivotal in security and medical applications, face challenges due to limited X-ray absorption and exciton utilization. Herein, a novel class of organic scintillators is introduced, named guest-induced thermally activated delayed fluorescence (TADF) within supramolecular macrocycles via host-guest through-space charge transfer (TSCT). Four co-crystals are obtained through orthogonal crystallizations involving calix[3]acridan (C[3]A) and calix[3]phenothiazine (C[3]P) macrocycles as hosts, along with 1,2-dicyanobenzene (DCB) and 4-bromo-1,2-benzenedicarbonitrile (BrDCB) as guests. Interestingly, DCB@C[3]A and BrDCB@C[3]A co-crystals exhibit strong host-guest TSCT with reduced single-triplet energy gap for efficient TADF emission, which leads to enhanced exciton utilization and X-ray absorption, yielding radioluminescence intensities over 29 and 25 times higher than C[3]A. Similarly, substituting C[3]A with C[3]P, the obtained TADF co-crystals also outperform C[3]P in scintillation performance. Additionally, the scintillation color of co-crystals can be adjusted by varying the electron-donating abilities of macrocycles and the electron-accepting abilities of guests, offering a simpler color-tuning mechanism than covalent-bonded scintillators. Furthermore, the flexible film based on DCB@C[3]A exhibits promising application in X-ray radiography, showcasing a high spatial resolution of 20 lp mm⁻¹ @MTF = 0.77. The innovative strategy of fabricating organic scintillators via reversible non-covalent interactions presents a novel solution for designing color-tunable and high-performance scintillators.

This study provides a better understanding of the degradation mechanism of blue organic light-emitting diodes (OLEDs) related to the molecular structures of electron transport materials (ETMs). The photodegradation experiment combined with electric currents indicates the importance of stability against hole currents for triazine-based ETMs. The optimized device shows better durability compared to the reference hyperfluorescence OLED.

Abstract

Recent advances in organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF)-assisted fluorescence (TAF) attest to the great promise of this technology in practical use. However, the simultaneous realization of high efficiency and device durability in blue OLEDs remains a significant challenge. Clarification of the degradation mechanisms correlated to molecular structure and device configuration is the key to extending the device lifetime. In this study, electron transport materials incorporating two triazine units in close proximity are adopted to use in hole-blocking and electron-transporting layers, resulting in superior device performances. In addition, a modified photodegradation experiment reveals that the degradation origins closely relate to charge carriers. The optimization of the device according to the obtained findings leads to 4.5 times extension in the lifetime of the TAF-OLED using a multiple resonance emitter. These results also provide guidelines for designing robust electron transport materials for blue OLEDs.