High-Performance Ultraviolet Organic Light-Emitting Diodes Enabled by Double Boron-Oxygen-Embedded Benzo[m]tetraphene Emitters

A High-Efficiency Ultraviolet Organic Light-Emitting Diode Employing a Double Boron-Oxygen-Nitrogen-Based Emitter

Designing high-efficiency ultraviolet organic light-emitting diodes (UV OLEDs) remains challenging due to the need for efficient utilization of triplet excitons while maintaining a wide bandgap. In this study, we designed double boron-oxygen-nitrogen-based polycyclic aromatic hydrocarbons (dBON-PAHs) with rigid planar structures and developed a novel UV emitter, BO-N, featuring hybridized local and charge-transfer (HLCT) properties. BO-N exhibited UV emission in toluene solution and 1,3-di(9H-carbazol-9-yl)benzene (mCP) film, with photoluminescence (PL) peaks of 391 and 400 nm and narrow full width at half-maximum (FWHM) values of 15 and 34 nm, respectively. The device doped with 5 wt% BO-N achieved a narrowband UV emission with an FWHM of 37 nm, an electroluminescence peak (λEL) of 399 nm, and CIE coordinates of (0.166, 0.030). Moreover, the device attained a record-high maximum external quantum efficiency (EQEmax) of 18.6% among reported HLCT-based UV OLEDs with CIEy < 0.05. These findings highlight the great potential of double BON-PAHs as robust emitters for high-performance UV OLEDs.

Boosting Exciton Utilization Rate in Organic Electroluminescence Devices with BN-Embedded Triangulenes and Triplet Fluorescence Emission

Organic diradicals with triplet ground states hold the potential to directly harness triplet excitons in electroluminescent devices, thus circumventing the slow reverse intersystem crossing process inherent in conventional organic fluorescent materials. Despite their significant promise, research into diradicals has remained relatively underexplored. [3]Triangulene, a polycyclic aromatic hydrocarbon, possesses a triplet ground state; however, it is nonfluorescent due to its alternating symmetry. This study introduces a novel strategy to break this symmetry by substituting carbon with boron or nitrogen, which may enable diradical fluorescence by facilitating triplet state transitions. Computational design of BN-integrated [3]triangulene derivatives reveals the presence of narrowband triplet fluorescence in the red and infrared regions, characterized by high quantum yields. By strategically positioning substituents at the periphery, we further achieve tunable luminescence colors and photoluminescence quantum yields that approach 100%. Our research highlights the promising potential of BN-embedded triangulenes in the advancement of high-performance organic light-emitting devices.

Preparation, Thermal, and Optical Properties of D-A-Type Molecules Based on 1,3,5-Triazine for Violet-Blue Fluorescent Materials

Organic violet-blue fluorescent materials have garnered significant interest for a broad spectrum of applications. A series of triazine-based molecules, that is, 2,4,6-tri(9H-carbazol-9-yl)-1,3,5-triazine (TCZT), 2,4,6-tri(1H-indol-1-yl)-1,3,5-triazine (TIDT), and 2,4,6-tris(3,6-di-tert-butyl-9H-carbazol-9-yl)-1,3,5-triazine (TDBCZT), exhibiting violet-blue emission were synthesized via a catalyst-free aromatic nucleophilic substitution reaction. These compounds possess a non-planar and twisted structure with favorable charge-transfer characteristics, demonstrating excellent thermal stability (decomposition temperatures of 370 °C, 384 °C, and 230 °C, respectively). Cyclic voltammetry analysis, combined with time-dependent density functional theory (TD-DFT) calculations at the B3LYP/6-31G(d) level, offered detailed insights into their electronic structures and electrochemical properties. Optical properties were systematically characterized using Ultraviolet-visible (UV-Vis) absorption and photoluminescence (PL) spectroscopy. The compounds exhibited violet-blue luminescence with emission peaks located at 397 nm, 383 nm, and 402 nm in toluene, respectively. In their respective films, the compounds exhibited varying degrees of spectral shifts, with emission peaks at 408 nm, 381 nm, and 369 nm. Moreover, the CIE (Commission Internationale de l'Éclairage) coordinates of TIDT in toluene were (0.155, 0.067), indicative of excellent violet purity. These compounds demonstrated significant two-photon absorption (TPA) properties, with cross-sections of 4.6 GM, 15.3 GM, and 7.4 GM, respectively. Notably, they exhibited large molar absorptivities and substantial photoluminescence quantum yields (PLQYs), suggesting their potential for practical applications as violet-blue fluorescent materials.

Customizing circularly polarized afterglow by stepwise chiral amplification in BINAPs/BINAPOs

Overcoming spin-forbidden radiation in chiral phosphors has attracted enormous attention because of their capacity to exhibit circularly polarized organic ultra-long room temperature phosphorescence (CP-OURTP). However, their development has been hindered by the short lifetimes and low dissymmetry factors, which are attributed to the differing parity selection rules that govern the electric and magnetic dipole moments in chiral molecules and poor triplet populations via intersystem crossing (ISC). Considering stepwise chiral amplification at molecular and supramolecular aspects, herein, we first reported donor-decorated BINAPs/BINAPOs with tunable D-A character and triplet incubation, which could enable hybridized local and charge-transfer (HLCT) characteristics, heavy atoms, and p-π* effects. These emitters could serve as guests in the polymer matrix. The doped phosphorescent polymer exhibits unimolecular circularly polarized luminescence (C) with high quantum efficiency, impressive CP-OURTP lifetimes (up to 1.02 s), and decent dissymmetry factors (10-3 level). Comprehensive studies unveil that the impressive CP-OURTP from monomer emission is ascribed to the 1HLCT-controlled ISC, long-lived 3LE-governing triplet radiation, and superior electric-magnetic dipole moment environments. Moreover, given the high RTP activity of rigid polymerization, we demonstrate their potential application in CP-OURTP amplification. Using in situ chiral liquid crystal polymerization, RM257 liquid crystals doped with 0.1-1.0 wt% PO1 guests demonstrate a secondary helical assembly, showing an amplified g CP-RTP factor (±0.11) and a long lifetime (0.83 s) after photopolymerization. The current materials' excellent performance in CP-OURTP and structural dependence could lead to their use in afterglow patterns for multiple optical encryption.

Dual-Ring-Locking Strategy Enables Persistent Blue Room Temperature Phosphorescence in Benzo[b]phospholiums

Commercial phosphines and phosphoniums were commonly reported to have unstable triplet dissipation because of the flexible C-P pyramidal geometry, resulting in extremely weak or no phosphorescence. To boost triplet populations and stability by restricting the molecular motion and rebuilding the electronic structures, we reported that the dual-ring-locking strategy could enable elevated intersystem crossing (ISC) and triplet radiation for the rigid benzo[b]phospholium configuration, exhibiting intense persistent room temperature phosphorescence (RTP) in poly(vinyl alcohol) (PVA). Among them, dual-ring-locked [P1]+[Cl]- showed near-ultraviolet fluorescence maximized at 400 nm in dichloromethane and blue RTP emission at 453 nm (Φphos ≈ 12.4%, τphos > 1200 ms) in the PVA matrix. In contrast, [P2]+[Cl]- possessed a single ring-locked nucleus that had red-shifted emission and weak phosphorescence (Φphos < 1.8%, τphos = 74.2 ms). Time-dependent density functional theory (TD-DFT) disclosed that the improved spin-flipping of phosphoniums benefited from the integrated π-π*/n-π* transition, rational split energy, and rigid excited states. The impressive OU-RTP duration could function as an afterglow pattern for optical encryption or as an emitting layer for light-emitting diode (LED) applications.

Intramolecular-locking modification enables efficient asymmetric donor-acceptor-donor' type ultraviolet emitters for high-performance OLEDs with reduced efficiency roll-off and high color purity

Developing high-performance ultraviolet organic light-emitting diodes with low efficiency roll-off and high color purity remains challenging due to their inherent wide-bandgap characteristics. In this work, we present an intramolecular noncovalent bond locking strategy to modulate donor-acceptor-donor' (D-A-D') type ultraviolet fluorophores (mPImCZ2F, mPIoCZ2F and mPImCP2F) with a hot-exciton mechanism. Notably, these asymmetric emitters exhibit significantly enhanced bipolar transport capacity and fluorescence efficiency compared to their counterparts. Among them, mPIoCZ2F exhibits a more remarkable intramolecular locking effect due to multiple C-H⋯F interactions and ortho-substitution-induced steric hindrance, which endows it with a higher radiation rate, narrower emission spectrum, and more balanced charge transport. Consequently, the mPIoCZ2F-based non-doped device achieves an electroluminescence (EL) peak at 393 nm with a maximum external quantum efficiency (EQE) of 6.62%. Moreover, in the doped device, mPIoCZ2F emits stable ultraviolet light with an EL peak at 391 nm and a full width at half maximum (FWHM) of 40 nm, corresponding to color coordinates of (0.167, 0.025). It also exhibits an exceptionally high EQE of 8.71% and minimal efficiency roll-off (7.95% at 1000 cd m-2), ranking among the best EL efficiencies reported for UV-OLEDs at high brightness levels.

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

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

Hyperconjugation Engineering of π-Extended Azaphosphinines for Designing Tunable Thermally Activated Delayed Fluorescence Emitters

Implanting heteroatoms into organic π-conjugated molecules (OCMS) offered a great opportunity to fine-tune the chemical structures and optoelectronic properties. This work describes a new family of 1,4-azaphosphinines with extended σ-π hyperconjugations. The photophysical studies revealed that azaphosphinines exhibited narrow-band thermally activated delayed fluorescence (TADF) ( full width at half-maximum: 26-40 nm). According to the orbital localization analysis and natural bond orbital analysis, the effective σ*-π* hyperconjugation is believed to induce the multiple-resonance (MR) TADF, which is distinct from the p-π conjugation-induced MR-TADF in BN systems. Although having the large ΔES1-T1s (>3.0 ev), the study suggested that σ*-π hyperconjugation endowed the system with the structural vibration favorable for the spin-vibronic-assisted RISC. Having the tunable p-centers (lp, O, S, Se, and Me+), azaphosphinines showed a fine-tuned TADF. Generally, azaphosphinines with strong σ*-π* hyperconjugations showed small ΔES1-T1s, efficient RISCs, and high PLQYs. Leveraging on the efficient hyperconjugations, TADF emission of the system spanned from UV-blue to green. Particularly, extended azaphosphinines exhibited the high photoluminescence quantum yields (74% in toluene and 92% in the 10% doped PMMA). As a proof of concept, two azaphosphinines with a PO center were applied as the light-emitting materials in organic lighting-emitting diodes. The devices showed the narrow-band UV- and deep-blue emission with EQE as high as 10.3%. The current study offered us a new strategy, namely, σ-π hyperconjugation-induced MR-TADF, for designing OCMs with tunable light-emitting properties.

Peripheral Substitution Engineering of MR-TADF Emitters Embedded With B‒N Covalent Bond Towards Efficient BT.2020 Blue Electroluminescence

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.

Four-Membered Ring-Embedded Cycloarene Enabling Anti-Aromaticity and Ultra-Narrowband Emission

The synthesis of fully fused π-conjugated cycloarenes embedded with nonbenzenoid aromatics is challenging. In this work, the first example of four-membered ring-embedded cycloarene (MF2) was designed and synthesized in single-crystal form by macrocyclization and ring fusion strategies. For comparison, single bond-linked chiral macrocycle MS2 without two fused four-membered rings and its linear-shaped polycyclic benzenoid monomer L1 were also synthesized. The pronounced anti-aromaticity of four-membered rings significantly adjusts the electronic structures and photophysical properties of cycloarene, resulting in an enhancement of the photoluminescence quantum yield (PLQY) from 10.66 % and 10.74 % for L1 and MS2, respectively, to 54.05 % for MF2, which is the highest PLQY among the reported cycloarenes. Notably, owing to the embedded anti-aromatic four-membered rings that reduce structural displacements, MF2 exhibits an ultra-narrowband emission with a single-digit full-width at half-maximum (FWHM) of only 7 nm (0.038 eV), which sets a new record among all reported organic narrowband luminescent molecules, and represents the first example of ultra-narrowband emission in conventional polycyclic aromatic hydrocarbons (PAHs) devoid of heteroatoms.

A hot exciton organic glassy scintillator for high-resolution X-ray imaging

Hot exciton organic scintillators offer promising prospects due to their efficient generation of bright triplet excitons and ultrafast response time, having potential applications in security detection and medical diagnostics. However, fabricating large-area, highly transparent scintillator screens still remains challenging, impeding the realization of high-resolution X-ray imaging. Herein, we firstly demonstrate a novel highly-transparent hot exciton organic glassy scintillator (>87% transmittance @ 450-800 nm), produced using a low-temperature melt-quenching method with 2',5'-difluoro-N4,N4,N4'',N4''-tetraphenyl-[1,1':4',1''-terphenyl]-4,4''-diamine (DTPA2F) powder. Remarkably, compared to crystalline DTPA2F, which has a photoluminescence quantum yield of 67.8% and a relative light yield of 46 400 ± 406 photons MeV-1, the DTPA2F glass retains 49.8% and 28 341 ± 246 photons MeV-1, respectively. This results in a low detection limit of about 53.7 nGy s-1 and an ultrafast decay time of 1.66 ns for DTPA2F glass. Besides, it exhibits excellent environmental stability with no recrystallization or degradation after over 100 days of exposure to ambient conditions. Furthermore, the scintillator screen demonstrates exceptional spatial resolution of 38.5 lp mm-1 for X-ray imaging. It provides a simple molecular design strategy and a screen fabrication method for developing large-area, highly-transparent, efficient and ultrafast organic glassy scintillators.

Asymmetric Structural Engineering of Hot-Exciton Emitters Achieving a Breakthrough in Non-Doped BT.2020 Blue OLEDs with a Record 9.5% External Quantum Efficiency

High-efficiency non-doped deep-blue organic light-emitting diodes (OLEDs) meeting the standard of BT.2020 color gamut is desired but rarely reported. Herein, an asymmetric structural engineering based on crossed long-short axis (CLSA) strategy is developed to obtain three new deep-blue emitters (BICZ, PHDPYCZ, and PHPYCZ) with a hot-exciton characteristic. Compared to 2BuCz-CNCz featuring a symmetric single hole-transport framework, these asymmetric emitters with the introduction of different electron-transport units show the enhancement of photoluminescence efficiency and improvement of bipolar charge transport capacity. Further combined with high radiative exciton utilization efficiency and light outcoupling efficiency, the non-doped OLED based on PHPYCZ exhibits the best performance with an excellent current efficiency of 3.49%, a record-high maximum external quantum efficiency of 9.5%, and a CIE y coordinate of 0.049 approaching the BT.2020 blue point. The breakthrough obtained in this work can inspire the molecular design of deep-blue emitters for high-performance non-doped BT.2020 blue OLEDs.

A record-high EQE of 7.65%@3300 cd m-2 achieved in non-doped near-ultraviolet OLEDs based on novel D'-D-A type bipolar fluorophores upon molecular configuration engineering

Developing a high-performance near-ultraviolet (NUV) material and its simple non-doped device with a small efficiency roll-off and good color purity is a promising but challenging task. Here, we proposed a novel donor'-donor-acceptor (D'-D-A) type molecular strategy to largely solve the intrinsic contradictions among wide-bandgap NUV emission, fluorescence efficiency, carrier injection and transport. An efficient NUV fluorophore, 3,6-mPPICNC3, exhibiting a hybridized local and charge-transfer state, is achieved through precise molecular configuration engineering, realizing similar hole and electron mobilities at both low and high electric fields. Moreover, the planarized intramolecular charge transfer excited state and steric hindrance effect endow 3,6-mPPICNC3 with a considerable luminous efficiency and good color purity in the aggregation state. Consequently, the non-doped device emitting stable NUV light with Commission Internationale de l'Eclairage (CIE) coordinates of (0.160, 0.032) and a narrow full width at half maximum of 44 nm exhibits a state-of-the-art external quantum efficiency (EQE) of 7.67% and negligible efficiency roll-off over a luminance range from 0 to 3300 cd m-2. This is a record-high efficiency among all the reported non-doped NUV devices. Amazingly, an EQE of 7.85% and CIE coordinates of (0.161, 0.025) are achieved in the doped device. This demonstrates that the D'-D-A-type molecular structure has great potential for developing high-performance organic light-emitting materials and their optoelectronic applications.