Tetracoordinate Boron-Based Multifunctional Chiral Thermally Activated Delayed Fluorescence Emitters

Sterically Tuned Ortho-Phenylene-Linked Donor-Acceptor Benzothiazole-Based Boron Difluoride Complexes as Thermally-Activated Delayed Fluorescence Emitters for Organic Light-Emitting Diodes

Two donor-acceptor dyes with an ortho-phenylene-linked carbazole electron donor and a benzothiazole-fused boron heterocyclic acceptor were designed, synthesized, and spectroscopically investigated. Due to the steric effects of boron heterocyclic units, the dyes demonstrate different conformations in the crystalline state. The presence of numerous hydrogen-bonding intermolecular interactions and the very weak π-π stacking in the molecular packing results in intense solid-state emission with photoluminescence quantum yields of 40 and 18% for crystals and 50 and 42% for host-based light-emitting layers. The compounds show aggregation-induced emission and thermally activated delayed fluorescence (TADF). The received ionization potential and electron affinity values suggested good charge-injecting ability and bipolar charge-transporting properties of the developed dyes. Transport of holes and electrons was detected in layers of one dye by the time-of-flight measurements. The benzothiazole-based boron difluoride complexes showed high electron mobility of 1.5 × 10-4 and 0.7 × 10-4 cm2 V-1 s-1 at an electric field of 1.35 × 106 V cm-1. Therefore, these dyes were successfully applied as emitters in organic light-emitting diodes with external quantum efficiencies of 15 and 13%, respectively. Our study marks a critical advancement in the area of solid-state emissive boron difluoride dyes, which can be applied as TADF emitters into organic light-emitting diodes. The obtained results reveal that the orientation of the acceptor unit in the ortho-phenylene-linked donor-acceptor dyes makes a significant impact on the TADF activity.

Introducing Internal Host Component to Thermally Activated Delayed Fluorescence Emitter for Efficient NIR Nondoped Organic Light-Emitting Diodes

Developing thermally activated delayed fluorescence (TADF) near-infrared (NIR) organic light-emitting diodes (OLEDs) based on nondoped emitting layers is intriguing yet challenging, limited by low exciton utilization and notorious concentration quenching. Herein, a facile strategy is proposed to address this issue by incorporating an internal host component onto a traditional donor (D)-acceptor (A)-type red TADF molecule. A proof-of-concept emitter with an internal host is accordingly developed as well as a control one without an internal host. In the case of their monomer states, both emitters exhibit similar emission spectra due to their identical D-A pairs. However, under nondoped conditions, significant improvement in exciton utilization and quenching-resistant features are observed for the molecule with the internal host. The corresponding nondoped OLED yielded a maximum external quantum efficiency of 2.4%, with NIR emission peaking at 765 nm, which was a nearly 10-fold improvement relative to the efficiency based on the control molecule without an internal host. To the best of our knowledge, this result is on par with those of state-of-the art nondoped NIR TADF OLEDs in a similar emission region. These results offer a feasible pathway for the design and development of high-efficiency NIR nondoped OLEDs.

Synthetic progress of organic thermally activated delayed fluorescence emitters via C-H activation and functionalization

Thermally activated delayed fluorescence (TADF) emitters have become increasingly prominent due to their promising applications across various fields, prompting a continuous demand for developing reliable synthetic methods to access them. This review aims to highlight the progress made in the last decade in synthesizing organic TADF compounds through C-H bond activation and functionalization. The review begins with a brief introduction to the basic features and design principles of TADF emitters. It then provides an overview of the advantages and concise development of C-H bond transformations in constructing TADF emitters. Subsequently, it summarizes both transition-metal-catalyzed and non-transition-metal-promoted C-H bond transformations used for the synthesis of TADF emitters. Finally, the review gives an outlook on further challenges and potential directions in this field.

Integration of fine-tuned chiral donor with hybrid long/short-range charge-transfer for high-performance circularly polarized electroluminescence

The synergistic integration of a fine-tuned chiral donor with a hybrid long/short-range charge-transfer mechanism offers an accessible pathway to construct highly efficient circularly polarized emitters. Consequently, a notable dissymmetry factor of 1.6 × 10-3, concomitantly with a record-setting maximum external quantum efficiency of 37.4%, is synchronously realized within a single embodiment.

Liquid-crystalline circularly polarised TADF emitters for high-efficiency, solution-processable organic light-emitting diodes

Achieving a high emission efficiency and a large luminescence asymmetry factor (glum) in a single molecule exhibiting circularly polarised thermally activated delayed fluorescence (CP-TADF) remains a formidable challenge. In this work, a proof-of-concept, liquid-crystalline CP-TADF molecule is proposed to realise high glum by taking advantage of the order inherent in liquid crystals. Employing a chiral dinaphthol-based CP-TADF molecule as the emissive unit, a pair of liquid-crystalline CP-TADF molecules (R/S-4) is synthesised via the introduction of six mesogenic moieties. The enantiomers show intense emission at about 520 nm which has clear TADF and liquid-crystalline characteristics. Both enantiomers display symmetrical electronic circular dichroism (CD) and circular polarisation luminescence (CPL) signals as thin films. Impressively, relatively large glum values of 0.11 are realised for the films. Solution-processed devices were fabricated using R/S-4 as the dopants, with the TADF molecule CzAcSF as the sensitiser. The OLEDs so prepared show a very high maximum external quantum efficiency of 21.2%, revealing a novel strategy for realising large glum values in CP-TADF.

Functionalization of the Octahydro-Binaphthol Skeleton: A Universal Strategy for Directly Constructing D-A Type Axially Chiral Biphenyl Luminescent Molecules

D-A type axially chiral biphenyl luminescent molecules are directly constructed through ingenious functionalization of the octahydro-binaphthol skeleton without optical resolution. The circularly polarized organic light-emitting diodes based on them display remarkable circularly polarized electroluminescence emission, a high luminance of >10 000 cd m-2, a maximum external quantum efficiency of 6.6%, and an extremely low-efficiency roll-off. This work provides a universal strategy for developing efficient and diverse axially chiral biphenyl emitters.

Thermally activated delayed fluorescence emitters showing wide-range near-infrared piezochromism and their use in deep-red OLEDs

Organic small molecules exhibiting both thermally activated delayed fluorescence (TADF) and wide-ranging piezochromism (Δλ > 150 nm) in the near-infrared region have rarely been reported in the literature. We present three emitters MeTPA-BQ, tBuTPA-BQ and TPPA-BQ based on a hybrid acceptor, benzo[g]quinoxaline-5,10-dione, that emit via TADF, having photoluminescence quantum yields, ΦPL, of 39-42% at photoluminescence (PL) maxima, λPL, of 625-670 nm in 2 wt% doped films in 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP). Despite their similar chemical structures, the PL properties in the crystalline states of MeTPA-BQ (λem = 735 nm, ΦPL = 2%) and tBuTPA-BQ (λem = 657 nm, ΦPL = 11%) are significantly different. Further, compounds tBuTPA-BQ and TPPA-BQ showed a significant PL shift of ∼98 and ∼165 nm upon grinding of the crystalline samples, respectively. Deep-red organic light-emitting diodes with MeTPA-BQ and tBuTPA-BQ were also fabricated, which showed maximum external quantum efficiencies, EQEmax, of 10.1% (λEL = 650 nm) and 8.5% (λEL = 670 nm), respectively.

Recent advances in the design of afterglow materials: mechanisms, structural regulation strategies and applications

Afterglow materials are attracting widespread attention owing to their distinctive and long-lived optical emission properties which create exciting opportunities in various fields. Recent research has led to the discovery of many new afterglow materials featuring high photoluminescence quantum yields (PLQY) and lifetimes of up to several hours under ambient conditions. Afterglow materials are typically categorized according to their luminescence mechanism, such as long-persistent luminescence (LPL), room temperature phosphorescence (RTP), or thermally activated delayed fluorescence (TADF). Through rational design and novel synthetic strategies to modulate spin-orbit coupling (SOC) and populate triplet exciton states (T1), luminophores with long lifetimes and bright afterglow characteristics can be realized. Initial research towards afterglow materials focused mainly on pure inorganic materials, many of which possessed inherent disadvantages such as metal toxicity or low energy emissions. In recent years, organic-inorganic hybrid afterglow materials (OIHAMs) have been developed with high PLQY and long lifetimes. These hybrid materials exploit the tunable structure and easy processing of organic molecules, as well as enhanced SOC and intersystem crossing (ISC) processes involving heavy atom dopants, to achieve excellent afterglow performance. In this review, we begin by briefly discussing the structure and composition of inorganic and organic-inorganic hybrid afterglow materials, including strategies for regulating their lifetime, PLQY and luminescence wavelength. The specific advantages of organic-inorganic hybrid afterglow materials, including low manufacturing costs, diverse molecular/electronic structures, tunable structures and optical properties, and compatibility with a variety of substrates, are emphasized. Subsequently, we discuss in detail the fundamental mechanisms used by afterglow materials, their classification, design principles, and end applications (including sensing, anticounterfeiting, and photoelectric devices, among others). Finally, existing challenges and promising future directions are discussed, laying a platform for the design of afterglow materials for specific applications.

Chiral Deep Eutectic Solvents Enable Full-Color and White Circularly Polarized Luminescence from Achiral Luminophores

Herein, chiral deep eutectic solvents (DES) are prepared by lauric acid as hydrogen bond donors (HBD) and chiral menthol as hydrogen bond acceptors (HBA). When achiral fluorescent molecules are dopedin the menthol-based chiral DES, they emit circularly polarized luminescence (CPL) with handedness controlled by the molecular chirality (l or d) of menthol. Remarkably, the strategy is universal and a series of achiral fluorescent molecules can be endowed with CPL activity, showing a full-color and white CPL upon appropriate mixing, which paves the way to prepare white CPL materials. Interestingly, CPL appears only in a certain temperature range in the DES. Variable-temperature spectra and other characterization methods reveal that the H-bond network in the chiral DES plays an important role in inducing CPL. This work unveils how the interior structure as well as the hydrogen-bond network of a chiral DES can transfer its chirality to achiral luminophores for the first time and realizes a full-color and white CPL in a DES.

Chiral Luminescent Aza[7]helicenes Functionalized with a Triarylborane Acceptor and Near-Infrared-Emissive Doublet-State Radicals

This paper presents new chiral luminescent molecules (N7-BMes2 and N7-TTM) using configurationally stable aza[7]helicene (1) as a universal heteroatom-doped chiral scaffold. The respective reactions of electron-donating 1 with a triarylborane acceptor via palladium-catalyzed Buchwald-Hartwig C-N coupling and with the open-shell doublet-state TTM radical via nucleophilic aromatic substitution (SN2Ar) resulted not only in tunable emissions from blue to the NIR domain but also in significantly enhanced emission quantum efficiency up to Φ = 50%.

Efficient Circularly Polarized Electroluminescence from Achiral Luminescent Materials

Circularly polarized electroluminescence (CP-EL) is generally produced in organic light-emitting diodes (OLEDs) based on special CP luminescent (CPL) materials, while common achiral luminescent materials are rarely considered to be capable of direct producing CP-EL. Herein, near ultraviolet CPL materials with high photoluminescence quantum yields and good CPL dissymmetry factors are developed, which can induce blue to red CPL for various achiral luminescent materials. Strong near ultraviolet CP-EL with the best external quantum efficiencies (η_ext s) of 9.0 % and small efficiency roll-offs are achieved by using them as emitters for CP-OLEDs. By adopting them as hosts or sensitizers, commercially available yellow-orange achiral phosphorescence, thermally activated delayed fluorescence (TADF) and multi-resonance (MR) TADF materials can generate intense CP-EL, with high dissymmetry factors and outstanding η_ext s (30.8 %), demonstrating a simple and universal avenue towards efficient CP-EL.

Bright Organic Mechanoluminescence and Remarkable Mechanofluorochromism from Circularly Polarized TADF Enantiomers with Aggregation-Induced Emission Properties

The development of circularly polarized thermally activated delayed fluorescence (CP-TADF) luminogens with stimuli-response characteristics remains challenging. Herein, a pair of organic enantiomers, S-CzTA and R-CzTA, with aggregation-induced emission properties, have been successfully developed by introducing chiral 1,2,3,4-tetrahydronaphthalene and carbazole to phthalimide. They present CP-TADF properties in toluene solutions, giving dissymmetric factors of 0.84×10^-3 and -1.03×10^-3 , respectively. In the crystalline state, both S-CzTA and R-CzTA can emit intense blue TADF and produce very bright sky-blue mechanoluminescence (ML) and remarkable mechanofluorochromism (MFC) under the stimuli of mechanical force. Single-crystal analysis and theoretical calculation results suggest that their ML activities are probably associated with their chiral and polar molecular structures and unique non-centrosymmetric molecular packing modes. Furthermore, the MFC properties of the enantiomers likely originate from the destruction of crystal structure, leading to the planarization of molecular conformation. This work may provide helpful guidance for developing new CP-TADF materials with force-stimuli-responsive properties.

Highly Efficient Sky-Blue π-Stacked Thermally Activated Delayed Fluorescence Emitter with Multi-Stimulus Response Properties

Organic materials with multi-stimulus response (MSR) properties have demonstrated many potential and practical applications. Herein, a π-stacked thermally activated delayed fluorescence (TADF) material with multi-stimulus response (MSR) properties, named SDMAC, was designed and synthesized using distorted 9,9-dimethyl-10-phenyl-9,10-dihydroacridine as a donor. SDMAC possesses a rigid π-stacked configuration with intramolecular through-space interactions and exhibits aggregation-induced emission enhancement (AIEE), solvatochromic, piezochromic, and circularly polarized luminescence (CPL) under different external stimuli. The rigid molecular structure and efficient TADF properties of SDMAC can be used in displays and lighting. Using SDMAC as an emitter, the maximum external quantum efficiency (EQE) of the fabricated organic light-emitting diodes (OLEDs) is as high as 28.4 %, which make them the most efficient CP-TADF OLEDs based on the through-space charge transfer strategy. The CP organic light-emitting diodes (CP-OLEDs) exhibit circularly polarized electroluminescence (CPEL) signals.