Aromatic-Imide-Based Thermally Activated Delayed Fluorescence Materials for Highly Efficient Organic Light-Emitting Diodes

Aromatic-imide-based thermally activated delayed fluorescence (TADF) materials with a twisted donor–acceptor–donor skeleton were efficiently synthesized and exhibited excellent thermal stability and high photoluminescence quantum yields. The small ΔE_ST value (<0.1 eV) along with the clear temperature-dependent delayed component of their transient photoluminescence (PL) spectra demonstrated their excellent TADF properties. Moreover, the performance of organic light-emitting diodes in which TADF materials AI-Cz and AI-TBCz were used as dopants were outstanding, with external quantum efficiencies up to 23.2 and 21.1 %, respectively.     

Aggregation‐Induced Delayed Fluorescence Based on Donor/Acceptor‐Tethered Janus Carborane Triads: Unique Photophysical Properties of Nondoped OLEDs

Luminescent materials consisting of boron clusters, such as carboranes, have attracted immense interest in recent years. In this study, luminescent organic–inorganic conjugated systems based on o‐carboranes directly bonded to electron‐donating and electron‐accepting π‐conjugated units were elaborated as novel optoelectronic materials. These o‐carborane derivatives simultaneously possessed aggregation‐induced emission (AIE) and thermally activated delayed fluorescence (TADF) capabilities, and showed strong yellow‐to‐red emissions with high photoluminescence quantum efficiencies of up to 97 % in their aggregated states or in solid neat films. Organic light‐emitting diodes utilizing these o‐carborane derivatives as a nondoped emission layer exhibited maximum external electroluminescence quantum efficiencies as high as 11 %, originating from TADF.     

Molecular Design of Thermally Activated Delayed‐Fluorescent Emitters Using 2,2′‐Bipyrimidine as the Acceptor in Donor–Acceptor Structures

Donor–acceptor–donor (D–A–D)‐type thermally activated delayed fluorescence (TADF) emitters 5,5′‐bis{4‐[9,9‐dimethylacridin‐10(9H )‐yl]phenyl}‐2,2′‐bipyrimidine (Ac‐bpm) and 5,5′‐bis[4‐(10H ‐phenoxazin‐10‐yl)phenyl]‐2,2′‐bipyrimidine (Px‐bpm), based on the 2,2′‐bipyrimidine accepting unit, were developed and their TADF devices were fabricated. The orthogonal geometry between the donor unit and the 2,2′‐bipyrimidine accepting core facilitated a HOMO/LUMO spatial separation, thus realizing thermally activated delayed fluorescence. The exhibited electroluminescence ranged from green to yellow, depending on the donor unit, with maximum external quantum efficiencies of up to 17.1 %.     

Red/Near‐Infrared Thermally Activated Delayed Fluorescence OLEDs with Near 100 % Internal Quantum Efficiency

Developing red thermally activated delayed fluorescence (TADF) emitters, attainable for both high‐efficient red organic light‐emitting diodes (OLEDs) and non‐doped deep red/near‐infrared (NIR) OLEDs, is challenging. Now, two red emitters, BPPZ‐PXZ and mDPBPZ‐PXZ, with twisted donor–acceptor structures were designed and synthesized to study molecular design strategies of high‐efficiency red TADF emitters. BPPZ‐PXZ employs the strictest molecular restrictions to suppress energy loss and realizes red emission with a photoluminescence quantum yield (Φ_PL) of 100±0.8 % and external quantum efficiency (EQE) of 25.2 % in a doped OLED. Its non‐doped OLED has an EQE of 2.5 % owing to unavoidable intermolecular π–π interactions. mDPBPZ‐PXZ releases two pyridine substituents from its fused acceptor moiety. Although mDPBPZ‐PXZ realizes a lower EQE of 21.7 % in the doped OLED, its non‐doped device shows a superior EQE of 5.2 % with a deep red/NIR emission at peak of 680 nm.     

Donor–σ–Acceptor Motifs: Thermally Activated Delayed Fluorescence Emitters with Dual Upconversion

A family of organic emitters with a donor–σ–acceptor (D‐σ‐A) motif is presented. Owing to the weakly coupled D‐σ‐A intramolecular charge‐transfer state, a transition from the localized excited triplet state (³LE) and charge‐transfer triplet state (³CT) to the charge‐transfer singlet state (¹CT) occurred with a small activation energy and high photoluminescence quantum efficiency. Two thermally activated delayed fluorescence (TADF) components were identified, one of which has a very short lifetime of 200–400 ns and the other a longer TADF lifetime of the order of microseconds. In particular, the two D‐σ‐A materials presented strong blue emission with TADF properties in toluene. These results will shed light on the molecular design of new TADF emitters with short delayed lifetimes.     

High Power Efficiency Blue‐to‐Green Organic Light‐Emitting Diodes Using Isonicotinonitrile‐Based Fluorescent Emitters

Herein, 9,10‐dihydro‐9,9‐dimethylacridine (Ac) or phenoxazine (PXZ)‐substituted isonicotinonitrile (INN) derivatives, denoted as 2AcINN , 26AcINN , and 26PXZINN , were developed as a series of thermally activated delayed fluorescence (TADF) emitters. These emitters showed reasonably high photoluminescence quantum yields of 71–79 % in the host films and high power efficiency organic light‐emitting diodes (OLEDs). Sky‐blue emitter 26AcINN exhibited a low turn‐on voltage of 2.9 V, a high external quantum efficiency (η _ext) of 22 %, and a high power efficiency (η _p) of 66 lm W⁻¹ with Commission Internationale de l′Eclairage (CIE) chromaticity coordinates of (0.22, 0.45), whereas green emitter 26PXZINN exhibited a low turn‐on voltage of 2.2 V, a high η _ext of 22 %, and a high η _p of 99 lm W⁻¹ with CIE chromaticity coordinates of (0.37, 0.58). These performances are among the best for TADF OLEDs to date.     

Dibenzo[a,j]phenazine‐Cored Donor–Acceptor–Donor Compounds as Green‐to‐Red/NIR Thermally Activated Delayed Fluorescence Organic Light Emitters

A new family of thermally activated delayed fluorescence (TADF) emitters based on U‐shaped D‐A‐D architecture with a novel accepting unit has been developed. All investigated compounds have small singlet‐triplet energy splitting (ΔE_ST) ranging from 0.02 to 0.20 eV and showed efficient TADF properties. The lowest triplet state of the acceptor unit plays the key role in the TADF mechanism. OLEDs fabricated with these TADF emitters achieved excellent efficiencies up to 16 % external quantum efficiency (EQE).     

Carbazole Dendrimers as Solution‐Processable Thermally Activated Delayed‐Fluorescence Materials

Recently, thermally activated delayed fluorescence (TADF) materials have received increasing attention as effective emitters for organic light‐emitting diodes (OLEDs). However, most of them are usually employed as dopants in a host material. In this report, carbazole dendrimers with a triphenyl‐s‐triazine core are reported, which are the first solution‐processable, non‐doped, high‐molecular‐weight TADF materials. The dendrimers were obtained by a new and facile synthetic route using the tert‐butyldimethylsilyl moiety as a protecting group. All dendrimers showed TADF in toluene. Measurements of the temperature‐dependent luminescence lifetime revealed that spin‐coated neat films also showed TADF with moderate quantum yields. OLED devices incorporating these dendrimers as spin‐coated emitting layers gave external quantum efficiencies of up to a 3.4 %, which suggests that this device is harvesting triplet excitons. This result indicates that carbazole dendrimers with attached acceptors are potential TADF materials owing to their polarized electronic structure (with HOMO–LUMO separation).     

Highly Efficient Near‐Infrared Delayed Fluorescence Organic Light Emitting Diodes Using a Phenanthrene‐Based Charge‐Transfer Compound

Significant efforts have been made to develop high‐efficiency organic light‐emitting diodes (OLEDs) employing thermally activated delayed fluorescence (TADF) emitters with blue, green, yellow, and orange–red colors. However, efficient TADF materials with colors ranging from red, to deep‐red, to near‐infrared (NIR) have been rarely reported owing to the difficulty in molecular design. Herein, we report the first NIR TADF molecule TPA‐DCPP (TPA=triphenylamine; DCPP=2,3‐dicyanopyrazino phenanthrene) which has a small singlet–triplet splitting (ΔE_ST) of 0.13 eV. Its nondoped OLED device exhibits a maximum external quantum efficiency (EQE) of 2.1 % with a Commission International de L′Éclairage (CIE) coordinate of (0.70, 0.29). Moreover, an extremely high EQE of nearly 10 % with an emission band at λ=668 nm has been achieved in the doped device, which is comparable to the most‐efficient deep‐red/NIR phosphorescent OLEDs with similar electroluminescent spectra.     

Molecular Design Strategy of Thermally Activated Delayed Fluorescent Emitters Using CN‐Substituted Imidazopyrazine as a New Electron‐Accepting Unit

Thermally activated delayed fluorescence (TADF)‐based organic light‐emitting diodes (OLEDs) have attracted enormous attention recently due to their capability to replace conventional phosphorescent organic light‐emitting diodes for practical applications. In this work, a newly designed CN‐substituted imidazopyrazine moiety was utilized as an electron‐accepting unit in a TADF emitter. Two TADF emitters, 8‐(3‐cyano‐4‐(9,9‐dimethylacridin‐10(9H)‐yl)phenyl)‐2‐phenylimidazo[1,2‐a]pyrazine‐3‐carbonitrile (Ac‐CNImPyr) and 8‐(3‐cyano‐4‐(10H‐phenoxazin‐10‐yl)phenyl)‐2‐phenylimidazo[1,2‐a]pyrazine‐3‐carbonitrile (PXZ‐CNImPyr), were developed based on the CN‐substituted imidazopyrazine acceptor combined with acridine and phenoxazine donor, respectively. A CN‐substituted phenyl spacer was introduced between the donor and acceptor for a sufficiently small singlet‐triplet energy gap (ΔE_ST) and molecular orbital management. Small ΔE_ST of 0.07 eV was achieved for the phenoxazine donor‐based PXZ‐CNImPyr emitter. As a result, an organic light‐emitting diode based on the PXZ‐CNImPyr emitter exhibited a high external quantum efficiency of up to 12.7 %, which surpassed the EQE limit of common fluorescent emitters. Hence, the CN‐modified imidazopyrazine unit can be introduced as a new acceptor for further modifications to develop efficient TADF‐based OLEDs.     

A Simple and Strong Electron‐Deficient 5,6‐Dicyano[2,1,3]benzothiadiazole‐Cored Donor‐Acceptor‐Donor Compound for Efficient Near Infrared Thermally Activated Delayed Fluorescence

Despite the success of thermally activated delayed fluorescent (TADF) materials in steering the next generation of organic light‐emitting diodes (OLEDs), effective near infrared (NIR) TADF emitters are still very rare. Here, we present a simple and extremely high electron‐deficient compound, 5,6‐dicyano[2,1,3]benzothiadiazole (CNBz), as a strong electron‐accepting unit to develop a sufficiently strong donor‐acceptor (D?A) interaction for NIR emission. End‐capping with the electron‐donating triphenylamine (TPA) unit created an effective D?A?D type system, giving rise to an efficient NIR TADF emissive molecule (λ_em=750 nm) with a very small ΔE_ST of 0.06 eV. The electroluminescent device using this NIR TADF emitter exhibited an excellent performance with a high maximum radiance of 10020 mW Sr^?1 m^?2, a maximum EQE of 6.57% and a peak wavelength of 712 nm.     

Meta Junction Promoting Efficient Thermally Activated Delayed Fluorescence in Donor‐Acceptor Conjugated Polymers

The meta junction is proposed to realize efficient thermally activated delayed fluorescence (TADF) in donor–acceptor (D‐A) conjugated polymers. Based on triphenylamine as D and dicyanobenzene as A, as a proof of concept, a series of D‐A conjugated polymers has been developed by changing their connection sites. When the junction between D and A is tuned from para to meta, the singlet–triplet energy splitting (ΔE_ST) is found to be significantly decreased from 0.44 to 0.10 eV because of the increasing hole–electron separation. Unlike the para‐linked analogue with no TADF, consequently, the meta‐linked polymer shows a strong delayed fluorescence. Its corresponding solution‐processed organic light‐emitting diodes (OLEDs) achieve a promising external quantum efficiency (EQE) of 15.4 % (51.9 cd A^?1, 50.9 lm W^?1) and CIE coordinates of (0.34, 0.57). The results highlight the bright future of D‐A conjugated polymers used for TADF OLEDs.     

Synthesis and electroluminescent performance of thermally activated delayed fluorescence‐conjugated polymers with simple formylphenyl as pendant acceptor

Formylphenyl has been demonstrated to act as an acceptor to construct thermally activated delayed fluorescence (TADF) emitter, and therefore a series of the TADF‐conjugated polymers with formylphenyl as pendant acceptor and carbazole/acridine as backbone donor are designed and synthesized. All polymers involve the twisted donor/acceptor structural moieties with the sufficiently spatial separation between the highest occupied molecular orbital and the lowest unoccupied molecular orbital as well as a small singlet/triplet splitting, and exhibit the legible TADF features confirmed by theoretical calculation and their transient decay spectra. The solution‐processed organic light‐emitting diodes using neat film of the polymers as emissive layer achieve excellent performance with the maximum external quantum efficiency (EQE) of up to 10.6%, the maximum current efficiency of up to 35.3 cd A⁻¹ and the low turn‐on voltage of 2.7 V. Moreover, the EQE still remains 10.3% at the luminance of 1000 cd m⁻² with the low driving voltage of 4.4 V and extremely small efficiency roll‐off. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1989–1996     

Molecular Orbital Controlling Donor Moiety for High‐Efficiency Thermally Activated Delayed Fluorescent Emitters

A new donor moiety, 7,7,13,13‐tetramethyl‐7,13‐dihydro‐5H‐indeno[1,2‐b]acridine (IAc), was developed to control the highest occupied molecular orbital (HOMO) dispersion of thermally activated delayed fluorescent (TADF) emitters. The IAc unit expanded the HOMO dispersion of the emitters and increased the quantum efficiency of the TADF devices up to 20.9 %.     

Twisted penta‐Carbazole/Benzophenone Hybrid Compound as Multifunctional Organic Host,Dopant or Non‐doped Emitter for Highly Efficient Solution‐Processed Delayed Fluorescence OLEDs

A new multi‐functional penta‐carbazole/benzophenone hybrid compound 5CzBP was designed and synthesized through a simple one‐step catalyst‐free C—N coupling reaction by using 2,3,4,5,6‐pentafluorobenzophenone and carbazole as starting materials. 5CzBP is very soluble in tetrahydrofuran (THF), which brings an environmentally friendly device fabrication for solution‐processed OLEDs instead of most widely used chlorinated solvents when 5CzBP is employed as the bulk‐phase of organic host or non‐doped emitter in the emissive layer. 5CzBP exhibits thermally activated delayed fluorescence (TADF) characteristic with relatively high triplet energy of 2.60 eV and a low ΔE_ST of 0.01 eV. By using the new TADF material as organic host for another green TADF emitter, maximum external quantum efficiency (EQE) of 12.5% has been achieved in simple solution‐processed OLED device. Besides, a maximum EQE of 8.9% and 5.7% was further obtained in TADF devices based on 5CzBP as dopant and non‐doped emitter, respectively. The simultaneously acting as efficient TADF host and non‐doped TADF emitter provides the potential guidance of the future simple single‐layer two‐color white OLEDs based on low‐cost pure organic TADF materials.     

Acridan‐Grafted Poly(biphenyl germanium) with High Triplet Energy,Low Polarizability,and an External Heavy‐Atom Effect for Highly Efficient Sky‐Blue TADF Electroluminescence

We propose the novel σ–π conjugated polymer poly(biphenyl germanium) grafted with two electron‐donating acridan moieties on the Ge atom for use as the host material in a polymer light‐emitting diode (PLED) with the sky‐blue‐emitting thermally activated delayed fluorescence (TADF) material DMAC‐TRZ as the guest. Its high triplet energy (E_T) of 2.86 eV is significantly higher than those of conventional π–π conjugated polymers (E_T=2.65 eV as the limit) and this guest emitter (E_T=2.77 eV). The TADF emitter emits bluer emission than in other host materials owing to the low orientation polarizability of the germanium‐based polymer host. The Ge atom also provides an external heavy‐atom effect, which increases the rate of reverse intersystem crossing in this TADF guest, so that more triplet excitons are harvested for light emission. The sky‐blue TADF electroluminescence with this host/guest pair gave a record‐high external quantum efficiency of 24.1 % at maximum and 22.8 % at 500 cd m^?2.     

Peripheral Amplification of Multi‐Resonance Induced Thermally Activated Delayed Fluorescence for Highly Efficient OLEDs

Multi‐resonance induced by boron and nitrogen atoms in opposite resonance positions endows a thermally activated delayed fluorescence (MR‐TADF) emitter with a strikingly small full width at half maximum of only 26 nm and excellent photoluminescence quantum yield of up to 97.48 %. The introduction of a carbazole unit in the para position of the B‐substituted phenyl‐ring can significantly boost up the resonance effect without compromising the color fidelity, subsequently enhancing the performances of the corresponding pure blue TADF‐OLED, with an outstanding external quantum efficiency (EQE) up to 32.1 % and low efficiency roll‐off, making it one of the best TADF‐OLEDs in the blue region to date. Furthermore, utilizing this material as host for a yellow phosphorescent emitter, the device also shows a significantly reduced turn‐on voltage of 3.2 V and an EQE_max of 22.2 %.     

Acridan‐Grafted Poly(biphenyl germanium) with High Triplet Energy,Low Polarizability,and an External Heavy‐Atom Effect for Highly Efficient Sky‐Blue TADF Electroluminescence

We propose the novel σ–π conjugated polymer poly(biphenyl germanium) grafted with two electron‐donating acridan moieties on the Ge atom for use as the host material in a polymer light‐emitting diode (PLED) with the sky‐blue‐emitting thermally activated delayed fluorescence (TADF) material DMAC‐TRZ as the guest. Its high triplet energy (E_T) of 2.86 eV is significantly higher than those of conventional π–π conjugated polymers (E_T=2.65 eV as the limit) and this guest emitter (E_T=2.77 eV). The TADF emitter emits bluer emission than in other host materials owing to the low orientation polarizability of the germanium‐based polymer host. The Ge atom also provides an external heavy‐atom effect, which increases the rate of reverse intersystem crossing in this TADF guest, so that more triplet excitons are harvested for light emission. The sky‐blue TADF electroluminescence with this host/guest pair gave a record‐high external quantum efficiency of 24.1 % at maximum and 22.8 % at 500 cd m^?2.     

Nido‐Carboranes: Donors for Thermally Activated Delayed Fluorescence

An approach to the design of nido‐carborane‐based luminescent compounds that can exhibit thermally activated delayed fluorescence (TADF) is proposed. 7,8‐Dicarba‐nido‐undecaboranes (nido‐carboranes) having various 8‐R groups (R=H, Me, i‐Pr, Ph) are appended to the meta or para position of the phenyl ring of the dimesitylphenylborane (PhBMes₂) acceptor, forming donor–acceptor compounds (nido‐ m1 – m4 and nido‐ p1 – p4 ). The bulky 8‐R group and meta substitution of the nido‐carborane are essential to attain a highly twisted arrangement between the donor and acceptor moieties, leading to a very small energy splitting between the singlet and triplet excited states (ΔE_ST <0.05 eV for nido‐ m2 , ‐ m3 , and ‐ p3 ). These compounds exhibit efficient TADF with microsecond‐range lifetimes. In particular, nido‐ m2 and ‐ m3 display aggregation‐induced emission (AIE) with TADF properties.     

Multi‐Resonance Induced Thermally Activated Delayed Fluorophores for Narrowband Green OLEDs

High‐color‐purity emissions with small a full‐width at half‐maximum (FWHM) are an ongoing pursuit for high‐resolution displays. Though the flourishment of narrow‐band emissive materials with multi‐resonance induced thermally activated delayed fluorescence (MR‐TADF) in the blue region, such materials have not validated their potential in other color regions. By amplifying the influence of skeleton and peripheral units, a series of highly efficient green‐emitting MR‐TADF materials are firstly reported. Peripheral units with electron‐deficit properties can significantly narrow the energy gap for bathochromic emission without compromising the color fidelity. MR‐TADF emitters with photo‐luminance quantum yields of above 90 % with FWHMs of ≤25 nm are developed. The corresponding organic light‐emitting diodes show maximum external quantum efficiency/ power efficiency of 22.02 %/ 69.82 lm W^?1 with excellent long‐term stability.