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The quest for highly efficient electroluminescent materials that can be used in organic light emitting diodes (OLEDs) has been attracting continuously attention. OLEDs are becoming a considerable market for specialist manufacturer, as a consequence of the growing demand for efficient solution in lighting and displays. Therefore it is very important for material scientists to know how to influence the properties of these devices. This mini-review aims to give an overview of the possible metallic and metal-free emitters that have been used in OLEDs, focusing on the emission behaviour of such complexes. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) and TADF-organic light-emitting diodes (OLEDs) systems are being given increasing attention in research nowadays. Much more work has been done for organic-based materials in this field, but the use of TADF organometallic systems has also emerged in recent years. In particular, TADF-based gold compounds have not been particularly well-explored, with a higher number of examples of Au(I)-molecules and fewer for the higher oxidation state Au(III) derivatives. Nevertheless, the novelty and observed results deserve attention. A careful analysis has been performed in this review by classifying the reported compounds into two different groups regarding the oxidation state of the metal, and within each group, the ancillary ligands. Specific examples to illustrate their potential applications are included in the different sections.

Heteroleptic [Cu(BIPHEP)(N^N)][PF6] complexes (BIPHEP = 1,1′-biphenyl-2,2′-diylbis(diphenylphosphane)), in which N^N is 2,2′-bipyridine (bpy), 6-methyl-2,2′-bipyridine (6-Mebpy), 6-ethyl-2,2′-bipyridine (6-Etbpy), or 5,5′-dimethyl-2,2′-bipyridine (5,5′-Me2bpy), have been synthesized and characterized using multinuclear NMR spectroscopies and electrospray ionization mass spectrometry. The single crystal structures of [Cu(BIPHEP)(bpy)][PF6]∙CH2Cl2, [Cu(BIPHEP)(5,5′-Me2bpy)][PF6]∙CH2Cl2, [Cu(BIPHEP)(6-Mebpy)][PF6]∙Et2O∙0.5H2O and [Cu(BIPHEP)(6-Etbpy)][PF6] confirm distorted tetrahedral {Cu(P^P)(N^N)} coordination environments. Each compound shows a quasi-reversible Cu+/Cu2+ process. In deaerated solution, the compounds are weak emitters. Powdered samples are yellow emitters (λemmax in the range 558−583 nm) and [Cu(BIPHEP)(5,5′-Me2bpy)][PF6] exhibits the highest photoluminescence quantum yield (PLQY = 14%). On cooling to 77 K (frozen 2-methyloxolane), the emission maxima are red-shifted and the excited state lifetimes increase from τ1/2 < 8 μs, to τ1/2 values of up to 53 μs, consistent with the compounds with N^N = 6-Mebpy, 6-Etbpy and 5,5′-Me2bpy exhibiting thermally activated delayed fluorescence (TADF).

To achieve significant efficiency and low roll-off in thermally activated delayed fluorescence (TADF) material organic light-emitting diodes (OLEDs), it is essential to choose a host that has suitable high triplet energy (T1) and bipolar character to boost the TADF characteristics as a dopant and avoid exciton annihilation. Herein, we present the effect of different host materials on the efficiency of organic light-emitting diodes (OLEDs) based on bis[4-(3,6 dimethoxycarbazole)phenyl]sulfone (DMOC-DPS) deep-blue emitter. The devices with 10 wt.% of an emitter in different electron types of host bis[2-(diphenylphosphino) phenyl] ether oxide (DPEPO), and hole types of host 1,3-bis(N-carbazolyl)benzene (mCP), were fabricated to study the effect on device performance. The results show that an external quantum efficiency (EQE) of 4% and maximum current efficiency (ƞc) up to 5.77 cd/A with high luminescence (lmax) 8185 cd/m2 in DPEPO was achieved, compared to 2.63% EQE, ƞc 4.12 cd/A with lmax 5338 cd/m2 in mCP in a very simple device structure. As a remarkable result, the roll-off is suppressed at 1000 cd/m2, and for maximum brightness, the roll-off is less than 50%. Further general applications are discussed.

A number of commercial organic compounds (m-MTDATA, PBD, CBP, TAPC, NPB, TPBi, etc) as well as several donor-acceptor-donor (D-A-D) compounds were investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The compounds were chosen as they are promising components of small-molecule-based high efficiency Thermally Activated Delayed Fluorescence (TADF) emitters in Organic Light Emitting Diodes (OLEDs). Electrochemical impedance spectra of a Pt electrode in CH 2 Cl 2 solutions containing the investigated compound and Bu 4 NBF 4 as the electrolyte were obtained and analyzed by electrochemical electrical circuit methods. Charge transfer resistance, double layer capacitance, Warburg constant and other parameters were determined and represented as a function of the potential. Analysis of charge transfer resistance as a function of potential allowed an estimation of standard redox rate constants for the compounds' oxidation and reduction processes. Two main features concerning the redox reaction rates of OLED-active compounds were revealed: (i) the oxidation and reduction rates of ambipolar compounds, i.e. containing both donor and acceptor parts, were found to be much higher than those of unipolar donor-only and acceptor-only molecules; (ii) the relationship between the oxidation and reduction rate constants was shown to be related to the compounds' conductivity type in the solid state. [ABSTRACT FROM AUTHOR]

A self-host thermally activated delayed fluorescence (TADF) dendrimer, namely MPPA-MCBP, for solution-processed nondoped near-infrared organic light-emitting diodes (OLEDs) was designed and synthesized, in which the bipolar CBP derivatives is introduced to render balanced charge transportation. This structural modification that the dendrons are well-bounded by the flexible alkyl chains, which can efficiently reduce the intermolecular interactions between the emissive cores and ensure the stable operation of the solution-processed device. As a result of the spin-coated OLEDs employing MPPA-MCBP as the host-free near-infrared emitter exhibits a highest external quantum efficiency (EQE) of 0.62% and a peak wavelength at 698 nm, which indicates the design of self-host TADF dendrimers containing bipolar dendrons will be a promising strategy to improve the electroluminescent performance of solution-processed nondoped device. [ABSTRACT FROM AUTHOR]

In this work, exciplex-based OLEDs (ExOLEDs) are fabricated with 1,1-bis((di-4-tolylamino)phenyl)cyclohexane (TAPC) as the donor and a series triazine derivatives, e.g., 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T), 2,4,6-tris(3-(1H-pyrazol-1-yl)phenyl)-1,3,5-triazine (3P-T2T), and 2,4,6-tris(m-(diphenylphosphinoyl)phenyl)-1,3,5-triazine (PO-T2T), as the acceptors. With different acceptors and mole ratios of donor:acceptor, the effects of the photoluminescence quantum yield (PLQY), charge carrier balance, and exciton lifetime on the EQE and efficiency roll-off are systematically investigated. It is found that the external quantum efficiency (EQE) of the ExOLEDs is primary determined by the PLQY of the mixed donor:acceptor film and tuned in a certain extent by the charge carrier balance. Furthermore, the efficiency roll-offs of different ExOLEDs are all less than 20%, which are simultaneously determined by the photoluminescence (PL) lifetime and the charge carrier balance. [ABSTRACT FROM AUTHOR]

We fabricated highly efficient thermally activated delayed fluorescence (TADF)-based tandem green- and white-emitting organic light emitting devices (OLEDs). By using 4CzIPN as a green TADF emitter, mCP:TPBI as a co-host, and BPhen:Li/HAT-CN as a connecting unit, we demonstrated the first TADF-based green-emitting tandem OLED with a maximum current efficiency (CE) and external quantum efficiency (EQE) of 117.5 cd/A and 32.5%, respectively. This is by far the highest EQE achieved to date for TADF devices. We also explored the first hybrid tandem white-emitting OLED using DMAC-DPS and PO-01 as blue TADF and orange phosphorescent emitters, respectively. The device exhibited a maximum CE and EQE of 78.5 cd/A and 28.5%, respectively, with CIE ( x, y) of (0.33, 0.45) at a brightness of 1000 cd m−2. From these data, we suggest that blue TADF materials have a great potential to replace the conventional fluorescent blue emitters for highly efficient white OLEDs composed of hybrid tandem structures. [ABSTRACT FROM AUTHOR]

Two fluorescent emitters, namely, phenylbis(4-(3,3″,6,6″-tetra- tert -butyl-9′H-[9,3′:6′,9″-tercarbazol]-9′-yl)phenyl)phosphineoxide (TPPOCz) and 9′,9‴′-(sulfonylbis(4,1-phenylene))bis(3,3″,6,6″-tetra- tert -butyl-9′H-9,3′:6′,9″- tercarbazole) (DBSOCz) are constructed. These compounds emit deep-blue and bluish-green signals and feature triphenylphosphine oxide/sulfonyldibenzene as electron acceptor core and carbazole dendrite as electron donor encapsulation groups. A simple construction strategy functionalizes these emitters to exhibit high thermal stability, suitable highest occupied molecular orbital/lowest unoccupied molecular orbital levels, relatively high photoluminescent quantum yields, and good film-forming capability. Emission spectrum and quantum computation were used to confirm that TPPOCz and DBSOCz possess small singlet–triplet energy splitting (0.22 and 0.10 eV). As a result, these fluorescent emitters exhibit significant thermally activated delayed fluorescence features with a short delayed fluorescence lifetime. Consequently, solution-processed OLEDs featuring DBSOCz as the non-doped emitter achieved a maximum current efficiency of 15.5 cd/A and external quantum efficiency of 7.65%. Surprisingly, the DBSOCz-based device exhibits an ultraslow current efficiency roll-off of 4.5% at a high luminance of 3000 cd/m 2 . The value represents almost the lowest attenuation of current efficiency in thermally activated delayed fluorescence-based OLEDs. [ABSTRACT FROM AUTHOR]

Two near-infrared (NIR) dendrimers with TADF characteristics are reported to develop the non-doped solution-processed OLED for the first time. The rigid ring end-capped aliphatic chain dendrons are introduced to improve the dissolvability and film-forming ability. The dendrimers possess excellent thermal and morphological stabilities. Simultaneously, the dendrimers exhibit self-host feature that the peripheral carbazole/tricarbazole dendrons can encapsulate the core to prevent concentration quenching. Employing MPPA-3Cz as the emitter, the non-doped solution-processed device exhibits a maximum external quantum efficiency (EQE) of 0.254% with a peak wavelength at 715 nm, which is comparable to the most-efficient solution-processed NIR FOLEDs with similar electroluminescent spectra. Moreover, the device shows negligible efficiency roll-off at high current density. Our results indicate that the design of long-wavelength TADF dendrimers will be a promising strategy for the efficient non-doped solution-processed NIR OLEDs. [ABSTRACT FROM AUTHOR]

We achieved high-efficiency and low-roll-off green fluorescent and phosphorescent organic light-emitting diodes (OLEDs) simultaneously by adopting the thermally activated delayed fluorescence material of bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone as the universal host. At a luminance of 1000 cd/m2, fluorescent OLEDs based on C545T get a current efficiency, power efficiency, and external quantum efficiency (EQE) of 31.8 cd/A, 25.0 lm/W, and 9.26%, respectively. This is almost the highest efficiency based on C545T at the luminance of 1000 cd/m2 to date. On the other hand, phosphorescent OLEDs with Ir(ppy)3 as the emitter realize a maximum current efficiency, power efficiency, and EQE of 64.3 cd/A, 62.4 lm/W, and 18.5%, respectively. More important, the EQE remains 17.8% at the representative luminance of 1000 cd/m2 and the roll-off ratio is just 3.78%. The transient photoluminescence decay measurement demonstrates that the up-conversion of host triplet excitons plays a key role in the high efficiency and low roll-off. More detailed discussions are also given. [ABSTRACT FROM AUTHOR]

To further develop a simple method for emission fine-tuning, two novel thermally activated delayed fluorescence (TADF) emitters, 5′-(10 H -phenoxazin-10-yl)-[1,1′:3′,1″-terphenyl]- 2′,4,4″- tricarbonitrile (mPTBC) and 2′-(10 H -phenoxazin-10-yl)-[1,1′:3′,1″- terphenyl]-4,4″,5′-tricarbonitrile (oPTBC) have been designed and synthesized by employing exactly the same functional units, a ter-benzonitrile and a phenoxazine (PXZ). By adjusting the linking position of the two electron-deficient benzonitrile arms from a meta -position to an ortho -position with the cyano core, the LUMO energy levels of mPTBC and oPTBC are varied to −2.98 and −3.13 eV, respectively; while the HOMO energy levels remain similar. In devices, the new TADF emitters successfully exhibit similar maximum efficiencies, but different-color emissions with a spectral peak at 516 nm for mPTBC and a spectral peak at 540 nm for oPTBC. These results prove the simple modification of adjusting the linking position of electron-deficient substituent groups in the molecules can effectively fine-tune the emissions of TADF emitters but without significantly changing other key performance. [ABSTRACT FROM AUTHOR]

A series of phenanthrene and phenanthroline derivatives were considered computationally by the application of Density Functional Theory at the B3LYP/6-31++G(d,p) level to investigate their potential usage as organic solar cell components, thermally activated delayed fluorescence and nonlinear optic compounds. The structures were constructed as a D-π-A motif in order to increase the ability to achieve intramolecular charge transfer enabling them to act as organic semiconductors. The inter-frontier energy gap of all compounds was found to be in the range of semiconductors. The thermally activated delayed fluorescence (TADF) properties of the compounds were also discussed in relation to the results obtained by TD-DFT calculations. Some of them possessed very narrow triplet-singlet transition energy leading to future TADF applications. Moreover, the nonlinear optic characteristics of all compounds were investigated through calculations of the total molecular dipole moment (μtot), linear polarizability (αtot) and hyperpolarizability (βtot). The results indicate the potential nonlinear optic property of all of the systems. [ABSTRACT FROM AUTHOR]

Two TADF dendrimers composed of diphenylsulfone core and oligo-carbazole dendrons were developed. Compared with the reported small molecular TADF emitter bis[4-(3,6-di- tert -butylcarbazole)phenyl]sulfone ( G1 ), dendrimers G2 and G3 inherited the blue light emission but exhibited a reduced singlet-triplet energy gap (Δ E ST ) with increasing the dendron generation. The efficient triplet-to-singlet (T 1 -to-S 1 ) transition processes, excellent thermal and amorphous stabilities, good solution processability, and appropriate HOMO/LUMO energy levels enabled these dendrimers to serve as non-doped emitting layers for solution-processed OLEDs. The G2 device achieved deep-blue light with emission peak at 428 nm, a CIE coordinate at (0.15, 0.12) and relatively low efficiency roll-off at high current density. [ABSTRACT FROM AUTHOR]

To enhance the efficiency of organic light-emitting diodes (OLEDs), the application of triplet excitons must be maximized. Two donor–acceptor-type molecules bearing bis-carbazolyl and tris-carbazolyl groups at the ortho position of a phenyl-attached triazine ring were synthesized. These molecules showed very small singlet–triplet energy gaps (ΔE ST ) of 0.06 and 0.03 eV, respectively, and transient photoluminescence (PL) characteristics, which indicate that they are thermally activated delayed fluorescence (TADF) materials. A high external quantum efficiency (EQE) of 16.5% with deep blue color coordinates of (0.15, 0.22) was observed using a TrisCz–Trz TADF emitter. This interaction between the electron donor and electron acceptor substituents plays a key role in achieving a very small ΔE ST and good intramolecular charge transfer (ICT) activities with high photoluminescence (PL). [ABSTRACT FROM AUTHOR]

To increase the efficiency of blue organic light-emitting diodes (OLEDs), new blue dopants, 2-(bis(4-(diphenylamino)phenyl)methylene)malononitrile (4-DPM) and 2-(bis(3-(diphenylamino)phenyl) methylene)malononitrile (3-DPM), with thermally activated delayed fluorescence were synthesized. 4-DPM with a p -phenyl linker between the electron-donating diphenylamine unit and the electron-withdrawing malonitrile unit showed a large overlap in the phenyl linker as well as the malonitrile unit, while 3-DPM with an m -phenyl linker showed a weak overlap in the phenyl linker. The OLED with the 3-DPM dopant showed higher efficiencies of 12.62 cd/A and 8.81 lm/W compared to that with the 4-DPM dopant (1.38 cd/A and 0.70 lm/W). This result was attributed to the small overlap between the electron-donating and electron-withdrawing units, the small charge-transfer singlet-triplet state splitting below 0.1 eV, and the well-aligned structure in the horizontal direction, as determined by grazing incidence X-ray diffraction. [ABSTRACT FROM AUTHOR]

An exciplex is an excited state charge transfer complex formed between a donor molecule and an acceptor molecule upon excitation of the donor or acceptor molecule. Because of the charge transfer characteristics, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the exciplex are considered to be located on the donor and acceptor molecules, respectively. Since the overlap between the HOMO and the LUMO of the exciplex is small, the energy difference between the singlet and the triplet excited states can be very small. Therefore, exciplex forming materials have been used to harvest triplet as well as the singlet energy in OLEDs. Exciplex forming systems can also be used as co-hosts of phosphorescent dyes for efficient OLEDs. An exciplex forming co-host can be considered as a quasi-host material possessing its singlet and triplet excited state energy close to the energy difference between the LUMO of the electron transporting material (acceptor) and the HOMO of the hole transporting material (donor) of the co-host materials, which can be utilized to achieve high efficiency and low driving voltage in OLEDs. In this talk, we will present highly efficient phosphorescent and TADF OLEDs using exciplex host doped with phosphorescent dyes with preferred emitting dipole orientation along horizontal direction to realize the external quantum efficiency over 30%, low driving voltage, and low efficiency roll-off by doping. [ABSTRACT FROM AUTHOR]

A novel compound was designed and synthesized by connecting a dicyanobenzene acceptor and two 9,9-dimethyl-9,10-dihydroacridine donors to the 1,3,5-position of a phenyl ring by meta -position connection. This compound, which is a novel emitter for OLED devices, exhibits preferable heat stability. Moreover, the energy gap between its singlet and triplet states is as small as 0.04 eV, resulting in this molecule possesses thermally activated delayed fluorescence. Therefore, the corresponding device showed efficient electroluminescent performances. The maximum external quantum efficiency, maximum current efficiency, maximum power efficiency and maximum luminance were 16.5%, 40.8 cd A −1 , 45.8 lm W −1 and 5120 cd m −2 , respectively. In addition, the CIE x,y only changed from (0.22, 0.38) to (0.22, 0.39) over the entire operating voltage range, which confirms that the device possesses highly stable chromaticity with respect to the current density. Based on these experimental results, meta -connected type structures may provide a new approach for developing high-performance TADF emitters for OLED applications. [ABSTRACT FROM AUTHOR]

Here, some luminescent compounds, which comprise a central thianthrene-9,9′,10,10′-tetraoxide acceptor core coupled with various donor units, have been synthesized and reported as thermally activated delayed fluorescent emitters. This set of compounds allows systematic fine-tuning of the band gap that cover the entire visible range from sky-blue to orange-red. The results of optical and electrochemical studies show that their highest occupied molecular orbitals and energy gaps can be easily modified by carefully choosing appropriate donor units. Solution-processed organic light emitting diodes with high maximum external electroluminescence quantum efficiencies of up to 9.1% have been demonstrated by using these emitters. Our results demonstrate the method that systematic tuning the band gap as well as keeping the suitable the lowest unoccupied molecular orbital energy levels of emitters is a promising strategy to achieve highly efficient full-color organic light emitting diodes. [ABSTRACT FROM AUTHOR]

We designed and synthesized two dendrimers TA-Cz and TA-3Cz with TADF characteristics by using non-conjugated aliphatic chains carbazole/tricarbazole as peripheral dendrons. Both dendrimers possess excellent thermal and morphological stabilities. Introduced the phenyl bridge to increase the distance of the emission core TA between donor (D) and acceptor (A) is a promising route to simultaneously achieve small singlet–triplet energy splitting (ΔE ST ) and enhanced PL quantum yields (PLQYs). Furthermore, non-conjugated aliphatic chains carbazole/tricarbazole dendrons were conveniently introduced to the TADF core, which can effective encapsulate the emission core to restrain the concentration quenching effect and make the fluorescence of the core independent. By utilizing TA-3Cz emitter as the non-doped solution-processed emissive layers, the resulting yellow OLED achieved low driving voltage of 2.4 V and superior external quantum efficiency of 11.8%. Thus, our results here provide a facile strategy to obtain highly efficient non-doped solution-processed OLEDs by employing the reasonable molecular design of the TADF core and the utilization of flexible alkyl chain. [ABSTRACT FROM AUTHOR]

ABSTRACT The development of organic light emitting diodes (OLEDs) based on fluorescent materials has made a great progress in improving light emitting efficiency and full range colors. But it still encounters the low singlet excitons generation ratio of 25% in device. As a solution to this problem, thermally activated delayed fluorescent (TADF) materials can convert the triplet excitons to the singlet ones, thus achieve theoretically 100% exciton utilization efficiency. Up to now, the small TADF molecules have achieved great breakthrough in realizing high external quantum efficiency and full color range including blue, green, and red. While the OLED devices based on macromolecules possess the inherent advantages of simplicity and lower cost in the rapid deposition of large areas at room temperature, especially on large flexible substrates, it is still relatively difficult to realize TADF effect in macromolecules, although several reports have partially confirmed them promising candidates for practical applications. This review summarizes the recent progress in the field of TADF polymers and their device performances in OLEDs, and also gives some outlooks for the further exploration in this field at the end of this paper. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 575-584 [ABSTRACT FROM AUTHOR]

Four compounds ( TM-1∼4 ) based on N -(4-trifluoromethylphenyl)phenothiazine- S , S -dioxide and N -[3,5-bis(trifluoromethyl)phenyl]phenothiazine- S , S -dioxide are synthesized by Buchwald-Hartwig cross coupling reaction. The effect of different substituents in acceptors on the electronic, photophysical and electrochemical properties are studied by UV–Vis, fluorescence spectroscopy, cyclic voltammetry curves and theoretical calculations. The energy gaps (Δ E ST ) between singlet excited states ( S 1 ) and triplet excited states ( T 1 ) of these compounds decrease by addition of one para-substituted CF 3 or two meta-substituted CF 3 on N -phenyl in order to enhance the electron-withdrawing capability of the acceptors. Small Δ E ST of these compounds range from 0.09 eV to 0.18 eV. TM-2∼4 exhibit thermally activated delayed fluorescence (TADF) characteristics with short delayed fluorescence lifetimes ( τ d ) within 0.69 - 1.16 μs. TM-3 emits blue light with an emission wavelength of 475 nm. [ABSTRACT FROM AUTHOR]

In this study we present a guide to understanding of exciplex state: from physicochemical background to experimental procedures. Literature review is supported by original experimental results that exemplify the possible approaches. We show the experimental relationship between emission energy of solid-state exciplexes and electrochemically-derived ionization potential and electron affinity of donors and acceptors. [ABSTRACT FROM AUTHOR]

In this work we present a brief characterization of thermally activated delayed fluorescence (TADF) emitters and describe the spectroscopic methods used to investigate them. Both steady state and time-resolved methods are presented. The effect of oxygen on delayed fluorescence is described. Three different TADF emitters are compared to evaluate the effect of different electron- donating groups on their spectroscopic properties. [ABSTRACT FROM AUTHOR]

A series of chiral mechanochromic copper(I) cAAC (cAAC=cyclic (alkyl)(amino)carbene) complexes with a variety of amide ligands have been studied with regard to their photophysical and chiroptical properties to elucidate structure-property relationships for the design of efficient triplet exciton emitters exhibiting circularly polarized luminescence. Depending on the environment, which determines the excited state energies, either thermally activated delayed fluorescence (TADF) from 1/3 LLCT states or phosphorescence from 3 LLCT/LC states occurs. However, neither chiral moieties at the carbene nor at the carbazolate ligands provide detectable luminescence dissymmetries g lum . An exception is [Cu(phenoxazinyl)(cAAC)], showing orange to deep red TADF with λ max =601-715 nm in solution, powders and in PMMA. In this case, the amide ligand can undergo distortions in the excited state. This design motif leads to the first linear, non-aggregated CPL-active copper(I) complex with g lum of -3.4 ⋅ 10 -3 combined with a high radiative rate constant of 6.7 ⋅ 10 5  s -1 ., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Organic thermally activated delayed fluorescence (TADF) materials have been widely investigated due to their impressive electronic properties and applied potential for the third generation of organic light-emitting diodes (OLED). We present organic TADF material ( 4BGIPN ) based on the strained benzoguanidine donor and compare it with the benchmark carbazole-based material ( 4CzIPN ). Extended π-conjugation in 4BGIPN material results in yellow-green luminescence at 512 nm with a fast radiative rate of 5.5 × 10 -5 s -1 and a photoluminescence quantum yield of 46% in methylcyclohexane solution. Such a nitrogen-rich 4BGIPN material has a significantly stabilized highest occupied molecular orbital (HOMO) at -6.4 eV while the lowest unoccupied molecular orbital (LUMO) at -4.0 eV, indicating potential suitability for application as the electron transport layer or TADF class III emitter in OLEDs., (Copyright © 2023, Brannan et al.)

Despite promising optoelectronic features of N-doped polycyclic aromatic hydrocarbons (PAHs), their use as functional materials remains underdeveloped due to their limited post-functionalization. Facing this challenge, a novel design of N-doped PAHs with D-A-D electronic structure for thermally activated delayed fluorescence (TADF) emitters was performed. Implementing a set of auxiliary donors at the meta position of the protruding phenyl ring of quinoxaline triggers an increase in the charge-transfer property simultaneously decreasing the delayed fluorescence lifetime. This, in turn, contributes to a narrow (0.04-0.28 eV) singlet-triplet exchange energy split (Δ E ) and promotes a reverse intersystem crossing transition that is pivotal for an efficient TADF process. Boosting the electron-donating ability of our N-PAH scaffold leads to excellent photoluminescence quantum yield that was found in a solid-state matrix up to 96% (for phenoxazine-substituted derivatives, under air) with yellow or orange-red emission, depending on the specific compound. Organic light-emitting diodes (OLEDs) utilizing six, (D-A)-D, N-PAH emitters demonstrate a significant throughput with a maximum external quantum efficiency of 21.9% which is accompanied by remarkable luminance values which were found for all investigated devices in the range of 20,000-30,100 cd/m ST ) and promotes a reverse intersystem crossing transition that is pivotal for an efficient TADF process. Boosting the electron-donating ability of our N-PAH scaffold leads to excellent photoluminescence quantum yield that was found in a solid-state matrix up to 96% (for phenoxazine-substituted derivatives, under air) with yellow or orange-red emission, depending on the specific compound. Organic light-emitting diodes (OLEDs) utilizing six, (D-A)-D, N-PAH emitters demonstrate a significant throughput with a maximum external quantum efficiency of 21.9% which is accompanied by remarkable luminance values which were found for all investigated devices in the range of 20,000-30,100 cd/m 2 which is the highest reported to date for N-doped PAHs investigated in the OLED domain.

Designing multi-resonance (MR) emitters that can simultaneously achieve narrowband emission and suppressed intermolecular interactions is challenging for realizing high color purity and stable blue organic light-emitting diodes (OLEDs). Herein, a sterically shielded yet extremely rigid emitter based on a triptycene-fused B,N core (Tp-DABNA) is proposed to address the issue. Tp-DABNA exhibits intense deep blue emissions with a narrow full width at half maximum (FWHM) and a high horizontal transition dipole ratio, superior to the well-known bulky emitter, t-DABNA. The rigid MR skeleton of Tp-DABNA suppresses structural relaxation in the excited state, with reduced contributions from the medium- and high-frequency vibrational modes to spectral broadening. The hyperfluorescence (HF) film composed of a sensitizer and Tp-DABNA shows reduced Dexter energy transfer compared to those of t-DABNA and DABNA-1. Notably, deep blue TADF-OLEDs with the Tp-DABNA emitter display higher external quantum efficiencies (EQE max =24.8 %) and narrower FWHMs (≤26 nm) than t-DABNA-based OLEDs (EQE max =19.8 %). The HF-OLEDs based on the Tp-DABNA emitter further demonstrate improved performance with an EQE max of 28.7 % and mitigated efficiency roll-offs., (© 2023 Wiley-VCH GmbH.)

Developing double boron-based emitters with extremely narrow band spectrum and high efficiency in organic light-emitting diodes (OLEDs) is crucial and challenging. Herein, we report two materials, NO-DBMR and Cz-DBMR, hinge on polycyclic heteraborin skeletons based on role-play of the highest occupied molecular orbital (HOMO) energy levels. The NO-DBMR contains an oxygen atom, whereas the Cz-DBMR has a carbazole core in the double boron-embedded ν-DABNA structure. The synthesized materials resulted in an unsymmetrical pattern for NO-DBMR and surprisingly a symmetrical pattern for Cz-DBMR. Consequently, both materials showed extremely narrow full width at half maximum (FWHM) of 14 nm in hypsochromic (pure blue) and bathochromic (Bluish green) shifted emission without losing their high color fidelity. Furthermore, both materials show high photoluminescence quantum yield (PLQY) of over 82 %, and an extremely small singlet-triplet energy gap (ΔE ST ) of 0.04 eV, resulting in high reverse intersystem crossing process (k RISC ) of 10 5  s -1 . Due to the efficient thermally activated delayed fluorescence (TADF) characteristics, the fabricated OLEDs based on these heteraborins manifested maximum external quantum efficiency (EQE max ) of 33.7 and 29.8 % for NO-DBMR and Cz-DBMR, respectively. This is the first work reported with this type of strategy for achieving an extremely narrow emission spectrum in hypsochromic and bathochromic shifted emissions with a similar molecular skeleton., (© 2023 Wiley-VCH GmbH.)

A series of 1,8-naphthalimide (NI)-phenothiazine (PTZ) electron donor-acceptor dyads were prepared to study the thermally activated delayed fluorescence (TADF) properties of the dyads, from a point of view of detection of the various transient species. The photophysical properties of the dyads were tuned by changing the electron-donating and the electron-withdrawing capability of the PTZ and NI moieties, respectively, by oxidation of the PTZ unit, or by using different aryl substituents attached to the NI unit. This tuning effect was manifested in the UV-vis absorption and fluorescence emission spectra, e.g., in the change of the charge transfer absorption bands. TADF was observed for the dyads containing the native PTZ unit, and the prompt and delayed fluorescence lifetimes changed with different aryl substituents on the imide part. In polar solvents, no TADF was observed. For the dyads with the PTZ unit oxidized, no TADF was observed as well. Femtosecond transient absorption spectra showed that the charge separation takes ca. 0.6 ps, and admixtures of locally excited ( 3 LE) state and charge separated ( 1 CS/ 3 CS) states formed (in n -hexane). The subsequent charge recombination from the 1 CS state takes ca. 7.92 ns. Upon oxidation of the PTZ unit, the beginning of charge separation is at 178 fs and formation of 3 LE state takes 4.53 ns. Nanosecond transient absorption (ns-TA) spectra showed that both 3 CS and 3 LE states were observed for the dyads showing TADF, whereas only 3 LE or 3 CS states were observed for the systems lacking TADF. This is a rare but unambiguous experimental evidence that the spin-vibronic coupling of 3 CS/ 3 LE states is crucial for TADF. Without the mediating effect of the 3 LE state, no TADF is resulted, even if the long-lived 3 CS state is populated (lifetime τ CS ≈ 140 ns). This experimental result confirms the 3 CS → 1 CS reverse intersystem crossing (rISC) is slow, without coupling with an approximate 3 LE state. These studies are useful for an in-depth understanding of the photophysical mechanisms of the TADF emitters, as well as for molecular structure design of new electron donor-acceptor TADF emitters., (Copyright © 2023, Cao et al.)

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 .

The synthetic feasibility and excellent luminescence features of organic molecules attracted much attention and were eventually found useful in lighting applications. In this context, a solvent-free organic liquid having attractive thermally activated delayed fluorescence features in bulk along with high processability has prime importance. Herein, we report a series of naphthalene monoimide-based solvent-free organic liquids exhibiting cyan to red thermally activated delayed fluorescence with luminescence quantum yields up to 80% and lifetimes between 10 to 45 μs. An effective approach explored energy transfer between liquid donors with various emitters exhibiting tunable emission colors, including white. The high processability of liquid emitters improved the compatibility with polylactic acid and was used for developing multicolor emissive objects using 3D printing. Our demonstration of the thermally activated delayed fluorescence liquid will be much appreciated as a processable alternate emissive material suitable for large-area lighting, display, and related applications., (© 2023 Wiley-VCH GmbH.)

The low efficiency of blue devices has been a bottleneck problem for OLED panel manufacturers for a long time. In this paper, we present the development of TASF materials technology for deep blue OLEDs with high Blue Index (current efficiency/CIE‐y) around 300 at CIE‐y=0.078 obtained at 1000 cd/m2. [ABSTRACT FROM AUTHOR]

Blue fluorescent OLEDs are still the bottleneck for the power consumption in today's OLED panels. In recent years, the TADF technology has shown fast progress towards high‐efficiency deep‐blue emitter systems. The TADF technology can be used in a self‐emitting or in a co‐emitting (or hyper) approach. To make the co‐emitting system work efficiently, the combination of TADF and fluorescent emitter has to be chosen carefully. In this paper, we will discuss how to match TADF and fluorescent emitter combinations for the co‐emitting approach. We will also show results for highly efficient deep‐blue hyper‐fluorescent devices. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) emitting materials have drawn a huge attention across the community since the first realization of pure organic and highly efficient organic light‐emitting diodes (OLEDs). Latest progress of Red, Green and Blue TADF and HyperfluorescenceTM development will be reported. [ABSTRACT FROM AUTHOR]