Steric Switching for Thermally Activated Delayed Fluorescence by Controlling the Dihedral Angles between Donor and Acceptor in Organoboron Emitters

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.

Modulating Efficiency and Color of Thermally Activated Delayed Fluorescence by Rationalizing the Substitution Effect

Thermally activated delayed fluorescence (TADF) constitutes the process by which third-generation organic light-emitting diodes (OLEDs) are being designed and produced. Despite several years of trial-and-error attempts, mainly driven by chemical intuition about how to improve a certain aspect of the process, few studies focused on the in-depth description of its two key properties: efficiency of the T1 → S1 intersystem crossing and further S1 → S0 emission. Here, by means of a newly developed theoretical formalism, we propose a systematic rationalization of the substituent effect in a paradigmatic class of OLED compounds, based on phenothiazine-dibenzothiophene-S,S-dioxide, known as PTZ-DBTO2. Our methodology allows to discern among geometrical and electronic effects induced by the substituent, deeply understanding the relationships existing between charge transfer, spin density, geometrical deformations, and energy modulations between electronic states. By our results, we can finally elucidate, depending on the substituent, the fate of the overall TADF process, quantitatively assessing its efficiency and predicting the color emission. Moreover, the general terms by which this methodology was developed allow its application to any chromophore of interest.

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

Ultraviolet organic light-emitting diodes (UV OLEDs) have attracted increasing attention because of their promising applications in healthcare, industry, and agriculture; however, their development has been hindered by the shortage of robust UV emitters. Herein, we embedded double boron-oxygen units into nonlinear polycyclic aromatic hydrocarbons (BO-PAHs) to regulate their molecular configurations and excited-state properties, enabling novel bent BO-biphenyl (BO-bPh) and helical BO-naphthyl (BO-Nap) emitters with hybridized local and charge-transfer (HLCT) characteristics. They could be facilely synthesized in gram-scale amounts via a highly efficient two-step route. BO-bPh and BO-Nap showed strong UV and violet-blue photoluminescence in toluene with full width at half-maximum values of 25 and 37 nm, along with quantum efficiencies of 98 and 99%, respectively. A BO-bPh-based OLED showed high color purity UV electroluminescence peaking at 394 nm with Commission Internationale de l'Eclairage (CIE) coordinates of (0.166, 0.021). Moreover, the device demonstrated a record-high maximum external quantum efficiency (EQE) of 11.3%, achieved by successful hot exciton utilization. This work demonstrates the promising potential of double BO-PAHs as robust emitters for future UV OLEDs.

Modifications of Pyridine-3,5-dicarbonitrile Acceptor for Highly Efficient Green-to-Red Organic Light-Emitting Diodes

The strategy of acceptor modification is a powerful technique for tuning the emission color of thermally activated delayed fluorescence (TADF) emitters. In this study, we have successfully designed and synthesized three TADF emitters with donor-acceptor (D-A) structures using a 4-(diphenylamino)-2,6-dimethylphenyl (TPAm) donor and various pyridine-3,5-dicarbonitrile (PC) acceptor units. As a result, three compounds named TPAmbPPC, TPAm2NPC, and TPAmCPPC exhibited greenish-yellow to orange-red emissions with high photoluminescent quantum yields (76-100%) in thin films. Remarkably, a greenish-yellow device based on TPAmbPPC and TPAm2NPC showed a high maximum external quantum efficiency (EQE_max) of 39.1 and 39.0%, respectively. Furthermore, benefiting from the suitable steric hindrance between the acceptor and donor, the nondoped organic light-emitting diodes (OLEDs) based on TPAmbPPC demonstrated an exceptional EQE_max of 21.6%, indicating its promising potential as an efficient emitter for the application of OLED applications. Furthermore, orange-red OLED devices based on TPAmCPPC exhibited a high EQE_max of 26.2%, a CE of 50.1 cd A^-1, and a PE of 52.4 lm W^-1.

Optimal Dihedral Angle in Twisted Donor-Acceptor Organic Emitters for Maximized Thermally Activated Delayed Fluorescence

The twisted donor-acceptor (D-A) organic formwork with a large dihedral angle (θ_DA ) is usually adopted to narrow the singlet-triplet energy gap for obtaining excellent thermally activated delayed fluorescence (TADF) emitters. However, the dependence of overall TADF properties on θ_DA has not been systematically investigated to this day. Taking new designed CzBP, CzBP-1M and CzBP-2M via introducing methyl as investigated models, it is found that (i) with increasing θ_DA , the charge transfer component in S₁ is larger than that in T₁ in varying degrees, leading to non-monotonic spin-orbit couplings; (ii) the electron-vibration couplings between S₁ and T₁ states become the largest when θ_DA approaching 80°, facilitating phonon-driven up-conversion; (iii) the overall TADF rate reaches a peak at θ_DA ≈80°. By this, the TADF on/off switching is realized via methyl moiety for regulating θ_DA from theoretical prediction to experimental confirmation. Importantly, the θ_DA near 80° would be a good descriptor for screening excellent D-A type TADF emitters.

Highly emissive planarized B,N-diarylated benzonaphthoazaborine compounds for narrowband blue fluorescence

Highly fluorescent blue emitters with high color purity are of great significance for optical applications. Herein, a series of planarized B,N-diarylated benzonaphthoazaborine compounds, namely, BzNp (1), BuBzNp (2), Bu₂BzNp (3), Bu₂BzMeNp (4), and Bu₂BzBuNp (5), where electron-donating ^t Bu and Me groups are differently introduced into the B-Ph, N-Ph, or benzoazaborine rings, are prepared and characterized. All compounds exhibit low-energy absorptions (λ _abs = 462-467 nm) and emissions (λ _PL = 472-478 nm) remarkably red-shifted compared with those found for the pristine dibenzoazaborine compound (404 and 415 nm, respectively). Although the expansion of π-conjugation in the azaborine ring by replacing one phenyl ring with a naphthyl ring is mainly responsible for the redshifts, the emission is also fine-tuned by attached alkyl groups, which have a greater impact on the B-centered LUMO level at the azaborine ring than at the B-Ph ring. The bandgap control and emission tuning are further supported by electrochemical and theoretical studies. Notably, blue to sky-blue fluorescence of all compounds exhibits unitary photoluminescence quantum yields, narrow full width at half maximum values (∼20 nm), and small Stokes shifts (∼11 nm), indicating strong emissions with high color purity.

The Rise of 1,4-BN-Heteroarenes: Synthesis, Properties, and Applications

BN-heteroarenes, which employ both boron and nitrogen in aromatic hydrocarbons, have gained great attention in the fields of organic chemistry and materials science. Nevertheless, the extensive studies on BN-heteroarenes are largely limited to 1,2-azaborine-based compounds with B-N covalent bonds, whereas 1,3- and 1,4-BN-heteroarenes are relatively rare due to their greater challenge in the synthesis. Recently, significant progresses have been achieved in the synthesis and applications of BN-heteroarenes featuring 1,4-azaborines, especially driven by their significant potential as multiresonant thermally activated delayed fluorescence (MR-TADF) materials. Therefore, it is timely to review these advances from the chemistry perspective. This review summarizes the synthetic methods and recent achievements of 1,4-azaborine-based BN-heteroarenes and discusses their unique properties and potential applications of this emerging class of materials, highlighting the value of 1,4-BN-heteroarenes beyond MR-TADF materials. It is hoped that this review would stimulate the conversation and cooperation between chemists who are interested in azaborine chemistry and materials scientists working in the fields of organic optoelectronics, metal catalysis, and carbon-based nanoscience etc.

Catalytic aerobic photooxidation of triarylphosphines using dibenzo-fused 1,4-azaborines

Although dibenzo-fused 1,4-heteroaromatics are utilized as strongly reducing photocatalysts in organic synthesis and polymerization, they have rarely been employed in catalytic photooxidation. Moreover, to date, their boron-analogs, dibenzo-fused 1,4-azaborines (DBABs), have not been applied in photocatalysis despite their promising potential as photocatalysts. Accordingly, herein, aerobic photooxidation of triarylphosphines (Ar₃P) was performed using DBABs as photocatalysts. The reaction smoothly proceeded in an aprotic solvent, and phosphine oxides were obtained in appropriate yields. Density functional theory calculations suggested that DBAB captured and activated phosphadioxirane intermediates, which were generated by the interaction of Ar₃P with ¹O₂, at the Lewis acidic boron center.

Blue TADF Emitters Based on B-Heterotriangulene Acceptors for Highly Efficient OLEDs with Reduced Efficiency Roll-Off

The design of robust boron acceptors plays a key role in the development of boron-based thermally activated delayed fluorescence (TADF) emitters for the realization of efficient and stable blue organic light-emitting diodes (OLEDs). Herein, we report a set of donor (D)-acceptor (A)-type blue TADF compounds (1-3) comprising triply bridged triarylboryl acceptors, the so-called B-heterotriangulenes, which differ depending on the identity of one of the bridging groups: methylene (1), dimethylmethylene (2), or oxo (3). The X-ray crystal structures of 2 and 3 reveal a highly twisted D-A connectivity and a completely planar geometry for the B-heterotriangulene rings. All compounds exhibit blue emissions with the unitary photoluminescence quantum yields and small singlet-triplet energy splitting (<0.1 eV) in their doped host films. The compounds exhibit a fast reverse intersystem crossing rate (k_RISC ≈ 10⁶ s^-1) with short-lived delayed fluorescence (τ_d ≈ 2 μs), which is found to be promoted by the strong spin-orbit coupling between the local triplet excited state (³LE, T₂) and singlet (S₁) states. Using compounds 1-3 as the emitters, highly efficient blue TADF-OLEDs are realized. The devices based on the emitters with B-heterotriangulenes exhibit better performances than the device incorporating a singly bridged reference emitter over the whole luminance range. Notably, the device based on the fully dimethylmethylene-bridged emitter (2) achieves the highest maximum external quantum efficiency (EQE) of 28.2% and the lowest efficiency roll-off, maintaining a high EQE value of 21.2% at 1000 cd/m².

6,12-Dihydro-6,12-diboradibenzo[def,mno]chrysene: A Doubly Boron-Doped Polycyclic Aromatic Hydrocarbon for Organic Light Emitting Diodes by a One-Pot Synthesis

One-pot synthesis of a new doubly boron-doped polycyclic aromatic hydrocarbon of 6,12-dimesityl-6,12-dihydro-6,12-diboradibenzo[def,mno]chrysene (MDBDBC) was reported. MDBDBC features a rigid planar electron-deficient core structure and demonstrates good chemical and thermal stabilities. A low-lying LUMO of -3.53 eV, a low locally excited triplet energy of 1.92 eV, as well as green electroluminescence with maximum EQE of 4.9% were found for MDBDBC, suggesting its potential as an n-type unit for future organic light emitting diode applications.

Thermally Activated Delayed Fluorescence in 1,3,4-Oxadiazoles with π-Extended Donors

Here, we describe the synthesis of five 1,3,4-oxadiazole-based donor-acceptor materials, using dendritic carbazole-based donors 9'H-9,3':6'9″-tercarbazole (terCBz) and N3,N3,N6,N6-tetra-p-tolyl-9H-carbazole-3,6-diamine (TTAC). Due to the strongly donating and highly twisted nature of the TTAC donor as well as the spatially separated hole-particle wavefunctions, three of the five compounds exhibited thermally activated delayed fluorescence (TADF) in spite of a relatively large ΔE_ST measured through phosphorimetry (0.33-0.37 eV). These materials demonstrated photoluminescence quantum yields as high as 0.89 in toluene, with emission maxima ranging from 474 to 495 nm in the solid state. Additionally, two materials containing only terCBZ donor(s) exhibited deep blue fluorescence, with Commission Internationale de l'éclairage coordinates of (0.16, 0.05); the weaker nature of the terCBz donor results in a prohibitively large ΔE_ST (0.68-0.77 eV). A gap-tuned range-separated hybrid functional (ωB97XD*) was used to rigorously calculate triplet energies, while a systematic analysis of electronic structures and photophysical properties provided further insight into the properties of these materials. These findings ultimately contribute a synthetically facile approach toward highly emissive TADF emitters using a 1,3,4-oxadiazole motif.

Impact of Boron Acceptors on the TADF Properties of Ortho-Donor-Appended Triarylboron Emitters

We report the impact of boron acceptors on the thermally activated delayed fluorescence (TADF) properties of ortho-donor-appended triarylboron compounds. Different boryl acceptor moieties, such as 9-boraanthryl (1), 10H-phenoxaboryl (2), and dimesitylboryl (BMes₂, 3) groups have been introduced into an ortho donor (D)–acceptor (A) backbone structure containing a 9,9-diphenylacridine (DPAC) donor. X-ray crystal diffraction and NMR spectroscopy evidence the presence of steric congestion around the boron atom along with a highly twisted D–A structure. A short contact of 2.906 Å between the N and B atoms, which is indicative of an N → B nonbonding electronic interaction, is observed in the crystal structure of 2. All compounds are highly emissive (PLQYs = 90–99%) and display strong TADF properties in both solution and solid state. The fluorescence bands of cyclic boryl-containing 1 and 2 are substantially blue-shifted compared to that of BMes₂-containing 3. In particular, the PL emission bandwidths of 1 and 2 are narrower than that of 3. High-efficiency TADF-OLEDs are realized using 1–3 as emitters. Among them, the devices based on the cyclic boryl emitters exhibit pure blue electroluminescence (EL) and narrower EL bands than the device with 3. Furthermore, the device fabricated with emitter 1 achieves a high external quantum efficiency of 25.8%.

Diboron-Based Delayed Fluorescent Emitters with Orange-to-Red Emission and Superior Organic Light-Emitting Diode Efficiency

For the application of organic light-emitting diodes (OLEDs) in lighting and panels, the basic requirement is to include a full spectrum color range. Compared with the development of blue and green luminophores in thermally activated delayed fluorescence (TADF) technology, the progress of orange-to-red materials is slow and needs further investigation. In this study, three diboron compound-based materials, dPhADBA, dmAcDBA, and SpAcDBA, were designed and synthesized by nucleophilic arylation of three amine donors on 9,10-diboraanthracene (DBA) in a two-step reaction. With increasing electron-donating ability, they show orange-to-red emission with TADF characteristics. The electroluminescence of these diboron compounds exhibits emissions λ_max at 613, 583, and 567 nm for dPhADBA, dmAcDBA, and SpAcDBA, respectively. It is noteworthy that the rod-like D-A-D structures can achieve high horizontal ratios (84-86%) and outstanding device performance for orange-to-red TADF OLEDs: the highest external quantum efficiencies for dPhADBA, dmAcDBA, and SpAcDBA are 11.1 ± 0.5, 24.9 ± 0.5, and 30.0 ± 0.8%, respectively. Therefore, these diboron-based molecules offer a promising avenue for the design of orange-to-red TADF emitters and the development of highly efficient orange-to-red OLEDs.

Achieving Submicrosecond Thermally Activated Delayed Fluorescence Lifetime and Highly Efficient Electroluminescence by Fine-Tuning of the Phenoxazine-Pyrimidine Structure

Thermally activated delayed fluorescence (TADF) materials, combining high fluorescence quantum efficiency and short delayed emission lifetime, are highly desirable for application in organic light-emitting diodes (OLEDs) with negligible external quantum efficiency (EQE) roll-off. Here, we present the pathway for shortening the TADF lifetime of highly emissive 4,6-bis[4-(10-phenoxazinyl)phenyl]pyrimidine derivatives. Tiny manipulation of the molecular structure with methyl groups was applied to tune the singlet-triplet energy-level scheme and the corresponding coupling strengths, enabling the boost of the reverse intersystem crossing (rISC) rate (from 0.7 to 6.5) × 10⁶ s^-1 and shorten the TADF lifetime down to only 800 ns in toluene solutions. An almost identical TADF lifetime of roughly 860 ns was attained also in solid films for the compound with the most rapid TADF decay in toluene despite the presence of inevitable conformational disorder. Concomitantly, the boost of fluorescence quantum efficiency to near unity was achieved in solid films due to the weakened nonradiative decay. Exceptional EQE peak values of 26.3-29.1% together with adjustable emission wavelength in the range of 502-536 nm were achieved in TADF OLEDs. Reduction of EQE roll-off was demonstrated by lowering the TADF lifetime.

Spectrally resolved luminescence lifetime detection for measuring the energy splitting of the long-lived excited states

Molecular motion plays an important role in the reverse intersystem crossing of thermally activated delayed fluorescence (TADF) materials, since the conformation varies as the molecule vibrates, leading to potential changes in the energies of excited states. Although many theoretical simulations have researched the relationship between the excited states and the molecular conformations, there are still few experimental results showing the energy level difference between different long-lived excited states. Herein, a novel method for measuring spectrally resolved luminescence lifetimes is proposed to detect the energy splitting of the long-lived excited states of a classical TADF molecule, BTZ-DMAC. A set of the time-gated luminescence spectra with different delay times were captured by a spectrograph equipped on an auto-phase-locked system, and then used for lifetime analysis at each wavelength. Unlike traditional measurement techniques, the proposed novel method does not require ultrafast laser, high-speed detector and any phase matching circuitry, thus significantly reducing the cost. This method revealed a definite energy gap between the two excited states of BTZ-DMAC with different lifetimes, indicating different conformations caused by molecular vibration. This low-cost method could be also used to detect many other luminescence materials for investigating the detail mechanisms of multiple excited states.

Difluoroboron-Enabled Thermally Activated Delayed Fluorescence

A new series of tetracoordinated boron-enabled thermally activated delayed fluorescence (TADF) materials with a donor-acceptor BF₂-type framework were designed and conveniently synthesized. Difluoroboron plays a critical role and acts as a key to coordinate with the latent acceptor of the 2-(4-phenylpyridin-2-yl)phenol (PPyPOH) moiety to realize TADF. TADF materials are air-stable and have a high photoluminescence quantum yield of up to 99%. NOBF2-Cz- and NOBF2-DPCz-doped blue OLEDs demonstrated EQEs of 11.0% with CIE coordinates of (0.14, 0.16) and 15.8% with (0.14, 0.28) and high brightness of 6761 and 19383 cd/m² could be achieved, respectively. Moreover, the blue OLED doped with NOBF2-DPCz and the green OLED doped with NOBF2-DMAC achieved operational lifetimes at 50% of initial luminance (L₀ = 500 cd/m²), LT₅₀, of 54 and 920 h, respectively. This work indicates that these tetracoordinated difluoroboron molecules can act as efficient and stable TADF materials for OLED applications.

Achieving Efficient Blue Delayed Electrofluorescence by Shielding Acceptors with Carbazole Units

The design and synthesis of blue thermally activated delayed fluorescence (TADF) emitters that have high electroluminescence efficiency and low efficiency roll-off features remain a great challenge. Herein, we developed a facile and efficient strategy by shielding acceptors with carbazole units for constructing high-performance blue TADF emitters. Benzonitrile (BN), 9,9-diphenylacridan (DPAc), and carbazole (Cz) were adopted as the acceptor, donor, and protector, respectively, to build two TADF emitters named DPAc-DCzBN and DPAc-DtCzBN. The nondoped organic light-emitting diodes (OLEDs) of DPAc-DCzBN as the emitter exhibited a standard sky-blue emission with Commission Internationale de L'Eclairage (CIE) coordinates of (0.16, 0.26), high external quantum efficiency (EQE) of 20.0%, and low efficiency roll-off (EQEs of 19.5, 16.1, and 12.6% at 100, 500, and 1000 cd m^-2, respectively), which is an outstanding nondoped blue TADF OLED. The doped device of DPAc-DtCzBN displayed a pure blue emission and the corresponding CIE coordinates are (0.16, 0.15). Meanwhile, it also demonstrated high and stabilized EQE values of 23.1, 18.3, and 11.5% at maxima, 100 and 500 cd m^-2, respectively, which is a quite high level among the pure blue TADF OLEDs. This study testifies the feasibility of our strategy in constructing high-performance TADF electroluminescent materials.