Highly Efficient Au(I) Alkynyl Emitters: Thermally Activated Delayed Fluorescence and Solution-Processed OLEDs

Two-coordinate donor-metal-acceptor type coinage metal complexes displaying efficient thermally activated delayed fluorescence (TADF) have been unveiled to be highly appealing candidates as emitters for organic light-emitting diodes (OLEDs). Herein a series of green to yellow TADF gold(I) complexes with alkynyl ligands has been developed for the first time. The complexes exhibit high photoluminescence quantum yields (PLQYs) of up to 0.76 in doped films (5 wt % in PMMA) at room temperature. The modifications of alkynyl ligands with electron-donating amino groups together with the use of electron-deficient carbene ligands induce ligand-to-ligand charge transfer excited states that give rise to TADF emission. Spectroscopic and density functional theory (DFT) calculations reveal the roles of electron-donating capability of the alkynyl ligand in tuning the excited-state properties. Solution-processed organic light-emitting diodes (OLEDs) using the present complexes as emitters achieve maximum external quantum efficiency (EQE) of up to 20 %.     

Highly Efficient Blue Thermally Activated Delayed Fluorescence Emitters Based on Multi-Donor Modified Oxygen-Bridged Boron Acceptor

The employment of thermally activated delayed fluorescence (TADF) emitters is one of the most promising ways to realize the external quantum efficiency (EQE) of over 25% for organic light-emitting diodes (OLEDs). In addition, the TADF emitter based on oxygen-bridged boron (BO) fragment can maintain blue emission with high color purity. Herein, we constructed two blue TADF emitters, 3TBO and 5TBO, for OLEDs application. Both emitters consist of three donors linked at the oxygen-bridged boron acceptor. OLED devices based on 3TBO and 5TBO exhibited both high excellent device efficiency and high color purity with a maximum EQE; full-width at half-maximum (FWHM); and CIE coordinates of 17.3%, 47 nm, (0.120, 0.294), and 26.2%, 57 nm, (0.125, 0.275), respectively.     

Highly Efficient Deep-Red Non-Doped Diodes Based on a T-Shape Thermally Activated Delayed Fluorescence Emitter

Device simplification is of practical significance for organic light emitting diodes (OLEDs), and remains the great challenge for deep-red emitters. Herein, a deep-red thermally activated delayed fluorescence molecule ( p TPA-DPPZ ) is reported which features a T shaped structure containing two triphenylamine (TPA) donors, one either side of a planar dipyridophenazine (DPPZ) acceptor. The rational spatial arrangement of the functional groups leads to limited but sufficient molecular packing for effective carrier transport. The neat p TPA-DPPZ film achieves an around 90-fold improved radiation rate constant of 10⁷ s⁻¹ and the nearly unitary reverse intersystem crossing (RISC) efficiency, as well as accelerated emission decays for quenching suppression. The high radiation and RISC result in a photoluminescence quantum yield of 87 %. The bilayer OLED based on the p TPA-DPPZ emissive layer achieved the record external quantum efficiencies of 12.3 % for maximum and 10.4 % at 1000 nits, accompanied by the deep-red electroluminescence with the excellent color purity.     

Molecular Engineering of Push-Pull Diphenylsulfone Derivatives towards Aggregation-Induced Narrowband Deep Blue Thermally Activated Delayed Fluorescence (TADF) Emitters

Narrowband deep blue thermally activated delayed fluorescent (TADF) materials have attracted significant attention. Herein, four asymmetrical structured TADF emitters based on diphenylsulfone (DPS) acceptor and 9,9-dimethyl-9,10-dihydroacridine (DMAC) donor with progressive performances were developed. The tert-butyloxy auxiliary electron-donor was adopted to restrict the intramolecular rotations and provide efficient steric hindrance. Regioisomerization by altering the substitution position of DMAC on DPS unit further enhanced the intra- and inter-molecular interactions. The accompanying effects yielded increased energy level, minimized reorganization energy, and inhibited non-radiative transitions in the crystals of t BuO-SOmAD , which achieved narrowband deep-blue emission peaking at 424 nm (FWHM=64 nm, Φ_F=33.6 %) through aggregation-induced, blue-shifted emission (AIBSE). In addition, deep-blue organic light emitting diodes (OLEDs) based on t BuO-SOmAD realized the electroluminescence (EL) spectrum peaking located at 435 nm and CIE coordination of (0.12, 0.09).     

Exciplex Formation and Electromer Blocking for Highly Efficient Blue Thermally Activated Delayed Fluorescence OLEDs with All-Solution-Processed Organic Layers

Highly efficient solution-processable emitters are greatly desired to develop low-cost organic light-emitting diodes (OLEDs). The recently developed thermally activated delayed fluorescence (TADF) materials are promising candidates, but blue TADF materials compatible with the all-solution-process have still not been achieved. Here, a series of TADF materials, named X-4CzCN, are developed by introducing the bulky units through an unconjugated linker, which realizes high molecular weight to enhance the solvent resistance ability without disturbing the blue TADF feature. Meanwhile, the peripheral wrapping groups efficiently inhibit the triplet–triplet and triplet–polaron quenching by isolating the energy-transfer and charge-transporting channels. The photophysical measurements indicate that a small variation in peripheral unit will have a noticeable effect on the luminescence efficiency. The enlarged volume of peripheral units will make the electroluminescent spectra blueshift, while enhancing the energy transfer of exciplex and blocking the energy leakage of electromer can facilitate the exciton utilization. As a result, the fully solution-processed blue OLED achieves a CIE of (0.16, 0.27), a low turn on voltage of 2.9 eV, and a high external quantum efficiency of 20.6 %. As far as we known, this is the first report of all-solution-processed TADF OLEDs with blue emission, which exhibits a high efficiency even comparable to the vacuum-deposited devices.     

Molecular Design Tactics for Highly Efficient Thermally Activated Delayed Fluorescence Emitters for Organic Light Emitting Diodes

Recently, pure organic thermally activated delayed fluorescence (TADF) emitters have attracted considerable interest from the scientific community in the field of organic light emitting diodes (OLEDs) as they can theoretically realize 100 % of the internal quantum efficiency by exploiting both the singlet and triplet excitons via the reverse intersystem crossing enabled by small singlet‐triplet energy splitting. Currently, the external quantum efficiency of the TADF emitters is reaching the level of phosphorescent emitters. Therefore, the TADF approach is considered as a potential alternative to the low efficiency conventional fluorescent and expensive phosphorescent emitters. In this account, we summarized our recent development of blue and green TADF molecular designs to improve the device performances of the TADF devices.     

Molecular Design Strategy for Orange Red Thermally Activated Delayed Fluorescence Emitters in Organic Light-Emitting Diodes (OLEDs)

Pure organic molecules based thermally activated delayed fluorescence (TADF) emitters have been successfully developed in recent years for their propitious application in highly efficient organic light emitting diodes (OLEDs). In the case of orange red emitters, the non-radiative process is known to be a serious issue due to its lower lying singlet energy level. However, recent studies indicate that there are tremendous efforts put to develop efficient orange red TADF emitters. In addition, the external quantum efficiency (EQE) of heteroaromatic based orange red TADF OLEDs surpassed 30 %. Such heteroaromatic type emitters showed wide emission spectra; therefore, more attention is being paid to develop highly efficient orange red TADF emitters along with good color purity. Herein, the recent progress of orange red TADF emitters based on molecular structures, such as cyanobenzene, heteroaromatic, naphthalimide, and boron-based acceptors, are reviewed. Further, our insight on these acceptors has been provided by their photophysical studies and device performances. Future perspectives of orange red TADF emitters for real practical applications are discussed.     

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 %.     

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.     

Asymmetric Host Molecule Bearing Pyridine Core for Highly Efficient Blue Thermally Activated Delayed Fluorescence OLEDs

In this study, two host materials, pCzBzbCz and pCzPybCz , are synthesized to achieve a high efficiency and long lifetime of blue thermally activated delayed fluorescence organic light-emitting diodes (TADF-OLEDs). The molecular design strategy involves the introduction of a pyridine group into the core structure of pCzPybCz as an electron-withdrawing unit, and an electron-donating phenyl group into the structure of pCzBzbCz . These host materials demonstrate good thermal stability and high triplet energy (T₁=3.07 eV for pCzBzbCz and 3.06 eV for pCzPybCz ) for the fabrication of blue TADF-OLEDs. In particular, pCzPybCz -based OLED devices demonstrate an external quantum efficiency (EQE) of 22.7 % and an operational lifetime of 24 h (LT₉₀, time to attain 90 % of initial luminance) at an initial luminance of 1000 cd m⁻². This superior lifetime could be explained by the C−N bond dissociation energy (BDE) in the host molecular structure. Furthermore, a mixed-host system using the electron-deficient 2,4-bis(dibenzo[b,d]furan-2-yl)-6-phenyl-1,3,5-triazine (DDBFT) is proposed to inhibit the formation of the anion state of our host materials. In short, the device operational lifetime is further improved by applying DDBFT. The carbazole-based asymmetric host molecule containing a pyridine core realizes a high-efficiency blue TADF-OLED showing a positive effect on the operating lifetime, and can provide useful strategies for designing new host materials.     

一种热活化延迟荧光黄光材料的合成、性能及高效率电致发光器件

利用吩噁嗪和嘧啶分别作为电子给体和电子受体，通过Buchwald-Hartwig和Suzuki偶联反应成功合成了一种热活化延迟荧光黄光材料pPBPXZ.密度泛函理论计算显示，pPBPXZ分子中吩噁嗪和嘧啶结构单元间的二面角接近90°，而两个嘧啶结构单元与连接二者的苯环间的二面角接近0°；pPBPXZ的最高电子占据轨道主要分布在吩噁嗪结构单元上，最低电子未占轨道主要分布在嘧啶环和苯环上，两种分子轨道只有很小部分重叠.循环伏安、热重和差热测试表明，pPBPXZ具有高的电化学稳定性和热稳定性.在甲苯溶液中，pPBPXZ在360~495 nm显示出了明显的分子内电荷转移跃迁吸收，室温发光峰出现在535 nm.根据低温（77 K）荧光和磷光光谱，计算得到pPBPXZ的最低激发单重态和最低激发三重态能级分别为2.57 eV和2.48 eV，能级差（△E_ST）仅为0.09 eV.利用pPBPXZ作为发光层客体掺杂材料，制备出了高效率的黄光电致发光器件.器件的发射峰出现在552~560 nm，最大电流效率、功率效率和外量子效率分别达到了49.9 cd/A、49.0 lm/W和15.7%，而且发光效率受pPBPXZ掺杂浓度影响较小.     

Tetradentate Gold(III) Complexes as Thermally Activated Delayed Fluorescence (TADF) Emitters: Microwave‐Assisted Synthesis and High‐Performance OLEDs with Long Operational Lifetime

Structurally robust tetradentate gold(III)‐emitters have potent material applications but are rare and unprecedented for those displaying thermally activated delayed fluorescence (TADF). Herein, a novel synthetic route leading to the preparation of highly emissive, charge‐neutral tetradentate [C^C^N^C] gold(III) complexes with 5‐5‐6‐membered chelate rings has been developed through microwave‐assisted C?H bond activation. These complexes show high thermal stability and with emission origin (³IL, ³ILCT, and TADF) tuned by varying the substituents of the C^C^N^C ligand. With phenoxazine/diphenylamine substituent, we prepared the first tetradentate gold(III) complexes that are TADF emitters with emission quantum yields of up to 94 % and emission lifetimes of down to 0.62 μs in deoxygenated toluene. These tetradentate Au^III TADF emitters showed good performance in vacuum‐deposited OLEDs with maximum EQEs of up to 25 % and LT₉₅ of up to 5280 h at 100 cd m^?2.     

White Light Luminescence from a Homo-conjugated Molecule with Thermally Activated Delayed Fluorescence

Luminophores with tunable emission properties are appealing due to various applications. Among those properties, thermally activated delayed fluorescence (TADF) has been attracting enormous research interests. Herein, we synthesized a 9,9’-spirobifluorene based homo-conjugated molecule 1 , which connects a diphenylamino moiety as electron donor and a naphthalimide group as electron acceptor via 2,2’-positions of spirofluorene. Compound 1 displays dual emission behaviour with both blue and orange fluorescence. The one orange fluorescence around 555 nmshows sensitivity to oxygen and a prolonged lifetime of 284 ns in degassed toluene. Such characteristics imply TADF nature for this emission from a charge-transfer excited state. The other emission at 440 nm with blue colour displayed resistance to oxygen quenching and a normal fluorescence lifetime of 1.5 ns. Compared with control molecule, this emission band is assigned as conventional fluorescence from a localized excited state. In addition, dual emission property allows molecule 1 to be modulated to emit white photoluminescence in thin film with a CIE color coordinate of (0.25, 0.33).     

Phenoxazine-Dibenzothiophene Sulfoximine Emitters Featuring Both Thermally Activated Delayed Fluorescence and Aggregation Induced Emission

In this work, we demonstrate dibenzothiophene sulfoximine derivatives as building blocks for constructing emitters featuring both thermally activated delayed fluorescent (TADF) and aggregation-induced emission (AIE) properties, with multiple advantages including high chemical and thermal stability, facile functionalization, as well as tunable electron-accepting ability. A series of phenoxazine-dibenzothiophene sulfoximine structured TADF emitters were successfully synthesized and their photophysical and electroluminescent properties were evaluated. The electroluminescence devices based on these emitters displayed diverse emissions from yellow to orange and reached external quantum efficiencies (EQEs) of 5.8% with 16.7% efficiency roll-off at a high brightness of 1000 cd·m⁻².     

Highly Effective Thermally Activated Delayed Fluorescence Emitters Based on Symmetry and Asymmetry Nicotinonitrile Derivatives

In this study, we developed two thermally activated delayed fluorescence (TADF) emitters, ICzCN and ICzCYP, to apply to organic light-emitting diodes (OLEDs). These emitters involve indolocarbazole (ICz) donor units and nicotinonitrile acceptor units with a twisted donor-acceptor-donor (D-A-D) structure for small singlet (S₁) and triplet (T₁) state energy gap (ΔE_ST) to enable efficient exciton transfer from the T₁ to the S₁ state. Depending on the position of the cyano-substituent, ICzCN has a symmetric structure by introducing donor units at the 3,5-position of isonicotinonitrile, and ICzCYP has an asymmetric structure by introducing donor units at the 2,6-position of nicotinonitrile. These emitters have different properties, such as the maximum luminance (L_max) value. The L_max of ICzCN reached over 10000 cd m⁻². The external quantum efficiency (η_ext) was 14.8% for ICzCN and 14.9% for ICzCYP, and both achieved a low turn-on voltage (V_on) of less than 3.4 eV.     

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.     

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.     

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.     

Thermally Activated Delayed Fluorescence of a Dinuclear Platinum(II) Compound: Mechanism and Roles of an Upper Triplet State

A dinuclear Pt(II) compound was reported to exhibit thermally activated delayed fluorescence (TADF); however, the luminescence mechanism remains elusive. To reveal relevant excited-state properties and luminescence mechanism of this Pt(II) compound, both density function theory (DFT) and time-dependent DFT (TD-DFT) calculations were carried out in this work. In terms of the results, the S₁ and T₂ states show mixed intraligand charge transfer (ILCT)/metal-to-ligand CT (MLCT) characters while the T₁ state exhibits mixed ILCT/ligand-to-metal CT (LMCT) characters. Mechanistically, a four-state (S₀, S₁, T₁, and T₂) model is proposed to rationalize the TADF behavior. The reverse intersystem crossing (rISC) process from the initial T₁ to final S₁ states involves two up-conversion channels (direct T₁→S₁ and T₂-mediated T₁→T₂→S₁ pathways) and both play crucial roles in TADF. At 300 K, these two channels are much faster than the T₁ phosphorescence emission enabling TADF. However, at 80 K, these rISC rates are reduced by several orders of magnitude and become very small, which blocks the TADF emission; instead, only the phosphorescence is observed. These findings rationalize the experimental observation and could provide useful guidance to rational design of organometallic materials with superior TADF performances.     

Red/NIR Thermally Activated Delayed Fluorescence from Aza-BODIPYs

The search for long-lived red and NIR fluorescent dyes is challenging and hitherto scarcely reported. Herein, the viability of aza-BODIPY skeleton as a promising system for achieving thermal activated delayed fluorescent (TADF) probes emitting in this target region is demonstrated for the first time. The synthetic versatility of this scaffold allows the design of energy and charge transfer cassettes modulating the stereoelectronic properties of the energy donors, the spacer moieties and the linkage positions. Delayed emission from these architectures is recorded in the red spectral region (695–735 nm) with lifetimes longer than 100 μs in aerated solutions at room temperature. The computational-aided photophysical study under mild and hard irradiation regimes disclose the interplay between molecular structure and photonic performance to develop long-lived fluorescence red emitters through thermally activated reverse intersystem crossing. The efficient and long-lasting NIR emission of the newly synthesized aza-BODIPY systems provides a basis to develop advanced optical materials with exciting and appealing photonic response.