Small Singlet-triplet Energy Splitting Research Articles

High-performance fully solution-processed OLEDs based on carbazole-benzonitrile TADF emitter with long alkyl chains

The introduction of alkyl chains into the luminescent molecules will increase solubility and film-forming performance for fabricating solution-processed organic semiconductor devices. In this work, 5CzBN-9CSP and 5CzBN-12CSP with long alkyl chains were synthesized based on the typical thermally activated delayed fluorescence (TADF) molecule 5CzBN as core, and bipolar 9,9′-spirobi[fluorene] (SP) groups as peripheral dendrons. Not only the excellent solution processability is invested, but also the uniform photophysical property is sustained, including small singlet-triplet splitting energy (ΔEST) and high reverse intersystem crossing (RISC) rate. However, excessive length of alkyl chains degrading thermodynamic stability and carrier transport and balance, results in a maximum external quantum efficiency (EQEmax) of 17.6% for 5CzBN-12CSP-based fully solution-processed non-doped device, which is lower than that for 5CzBN-9CSP counterpart. Furthermore, TADF sensitized (TSF) solution-processed device obtains a high EQEmax of 25.9% within 5CzBN-12CSP host and multiple-resonance (MR) final emitter thanks to an efficient energy transfer.

Molecular Engineering Modulating the Singlet-Triplet Energy Splitting of Indolocarbazole-Based TADF Emitters Exhibiting AIE Properties for Nondoped Blue OLEDs with EQE of Nearly 20.

The development of efficient blue thermally activated delayed fluorescence (TADF) emitters with an aggregation-induced emission (AIE) nature, for the construction of organic light-emitting diodes (OLEDs), is still insufficient. This can be attributed to the challenges encountered in molecular design, including the inherent trade-off between radiative decay and reverse intersystem crossing (RISC), as well as small singlet-triplet energy splitting (ΔEST) and the requirement for high photoluminescence quantum yields (ΦPL). Herein, we present the design of three highly efficient blue TADF molecules with AIE characteristics by combining π-extended donors with different acceptors to modulate the differences in the electron-donating and electron-withdrawing abilities. This approach not only ensures high emission efficiency by suppressing close π-π stacking, weakening nonradiative relaxation, and enhancing radiative transition but also maintains the equilibrium ratio between the triplet and singlet excitons by facilitating the process of RISC. These emitters exhibit AIE and TADF properties, featuring quick radiative rates and low nonradiative rates. The ΦPL of these emitters reached an impressive 88%. Based on their excellent comprehensive performance, nondoped PICzPMO and PICzPMO OLEDs achieved excellent electroluminescence performance, exhibiting maximum external quantum efficiency (EQEmax) of up to 19.5%, while the doped device achieved a higher EQEmax of 20.8%. This work demonstrated that by fusing π-extended large rigid donors with different acceptors, it is possible to regulate the difference in electron-donating and electron-withdrawing abilities, resulting in a small ΔEST, high ΦPL, and fast RISC process, which is a highly feasible strategy for designing efficient TADF molecules.

Tuning photophysical properties of bicarbazole-based blue TADF emitters with AIE feature: Trade-off between small singlet-triplet energy splitting and high photoluminescence quantum yield

Three aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF) blue light-emitting materials were successfully synthesized using bicarbazole and phenylsulfonyl/benzophenone moieties as electron donors and acceptors, respectively, via Buchwald-Hartwig reaction. Density functional theory calculations showed that the three emitters have a moderate dihedral angle between the two carbazole planes and between the donor and acceptor, resulting in a good conjugated structure and a trade-off between the small ΔEST and high photoluminescence quantum yield of the BCzDPM molecule. The cyclic voltammetry, thermogravimetric analysis and differential scanning calorimetry tests demonstrated that the three materials had high electrochemical and thermal stability. The three emitters exhibited obvious intramolecular charge transfer characteristics in solution and film. According to the fluorescence and low-temperature phosphorescence spectra, the lowest singlet and triplet excited state energy levels of BCzDPM were 2.96 eV and 2.78 eV, respectively, with a ΔEST of 0.18 eV. Using these emitters as non-doped emissive layers, efficient blue electroluminescent devices were fabricated. The maximum current efficiency and external quantum efficiency reached 15.4 cd/A and 7.05% for the BCzDPM device, respectively, and the efficiency roll-off of BCzDPM was smaller than that of PSOBCz and PBCzPM, under high current density.

Synthesis and Properties of Short-Lifetime Thermally Activated Delayed Fluorescence Materials.

Compounds MBZ-mPXZ, MBZ-2PXZ, MBZ-oPXZ, EBZ-PXZ, and TBZ-PXZ were conveniently synthesized, and they were found to exhibit TADF properties with lifetimes of 857, 575, 561, 768, and 600 ns, respectively. These short lifetimes of the compounds might be due to the combination of small singlet-triplet splitting energy (ΔEST) and benzoate group, which could be an efficient strategy for the further design of short-lifetime TADF materials.

Steric modulation of thermally activated delayed fluorescence-type host for high quantum efficiency in blue phosphorescent organic light-emitting diodes

In organic light-emitting diodes (OLEDs), host engineering is important in improving the device performance. however, it remains as a big challenge in blue phosphorescent OLEDs because of the high triplet energy of blue phosphors. In this work, we develop a sterically modulated thermally activated delayed fluorescence-type host based on an ortho-linked carbazole–triazine backbone structure. An additional π-spacer spatially twisting the two moieties delivers a short delayed decay time of 5.0 μs without any loss of the photoluminescence quantum yield while realizing high triplet and small singlet-triplet energy splitting. The photophysical modulation enables a more efficient utilization of the triplet excitons of the two blue phosphorescent emitters in OLEDs. The fabricated blue phosphorescent OLEDs exhibit a significantly improved device performance in all efficiency parameters compared to reference devices with a maximum external quantum efficiency of 23.6%. This work demonstrates a simple and powerful thermally activated delayed fluorescence host design strategy to realize high efficiency blue-phosphorescence OLEDs.

Enhanced Efficiency of Exciplex Emission from a 9-Phenylfluorene Derivative.

The exciplex-thermally activated delayed fluorescence (exciplex-TADF) system is an excellent candidate for the fabrication of high-efficiency organic light-emitting diodes (OLEDs) because of its more easily achieved small singlet-triplet energy splitting (ΔEST) and doping control. However, exciplex-TADF is still faced with the problems of low external quantum efficiency (ηext) and unclear effect of structure modification in electron acceptors. Herein, we provide a steric hindrance increase strategy to obtain high-efficiency exciplex emissions. Through introducing a 9-phenylfluorene group into N-ethylcarbazole of the dicyano-substituted 9-phenylfluorene, an electron acceptor material with increased steric hindrance is obtained, which helps the exciplex harvest a larger driving force and higher emission efficiencies. Encouragingly, the obtained OLED displays a maximum ηext of 25.8%, which is one of the best efficiency values among reported exciplex-OLEDs, simultaneously possessing excellent current efficiency of 83.6 cd A-1 and power efficiency of 93.7 lm W-1. It is expected that this work will offer a new avenue for designing electron acceptors for highly efficient exciplex emissions.

Recent Progress in Blue Thermally Activated Delayed Fluorescence Emitters and Their Applications in OLEDs: Beyond Pure Organic Molecules with Twist D-π-A Structures.

Thermally activated delayed fluorescence (TADF) materials, which can harvest all excitons and emit light without the use of noble metals, are an appealing class of functional materials emerging as next-generation organic electroluminescent materials. Triplet excitons can be upconverted to the singlet state with the aid of ambient thermal energy under the reverse inter-system crossing owing to the small singlet-triplet splitting energy (ΔEST). This results from a specific molecular design consisting of minimal overlap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, due to the spatial separation of the electron-donating and electron-releasing part. When a well-designed device structure is applied, high-performance blue-emitting TADF organic light-emitting diodes can be realized with an appropriate molecular design. Unlike the previous literature that has reviewed general blue-emitting TADF materials, in this paper, we focus on materials other than pure organic molecules with twist D-π-A structures, including multi-resonance TADF, through-space charge transfer TADF, and metal-TADF materials. Cutting-edge molecules with extremely small and even negative ΔEST values are also introduced as candidates for next-generation TADF materials. In addition, OLED structures used to exploit the merits of the abovementioned TADF emitters are also described in this review.

Achieving over 36% EQE in blue OLEDs using rigid TADF emitters based on spiro-donor and spiro-B-heterotriangulene acceptors

The design of robust thermally activated delayed fluorescence (TADF) emitters is crucial in realizing highly efficient and stable blue TADF-organic light-emitting diodes (OLEDs). Here boron-based donor–acceptor-type blue TADF compounds incorporating a triply-bridged, spiro-type boron acceptor; namely, spiro-fluorenyl B-heterotriangulene, and 9,9-diphenyl-9,10-dihydroacridine (DPAC, 1) or 10H-spiro[acridine-9,9′-fluorene] (sAC, 2) donors are presented. Their doped host films show blue emissions with narrow full width at half maximum values of 52 nm (0.29 eV) and unitary photoluminescence quantum yields. The compounds exhibit short-lived delayed fluorescence (τd = 1.8–2.7 μs) and undergo fast reverse intersystem crossing with the rate constants of ∼106 s−1 in the film state. These are attributable to the small singlet–triplet energy splitting of ∼40 meV and the effective spin–orbit coupling between the singlet (1CT, S1) and local triplet (3LE, T2) excited states. From outstanding horizontal transition dipole orientation ratios of 0.89 and 0.90 for the host films of 1 and 2, respectively, high-efficiency blue TADF-OLEDs showing maximum external quantum efficiencies (EQEs) of over 30% are realized. Notably, the device with the fully spiro 2 achieves a state-of-the-art EQEmax of 36.4% and a suppressed efficiency roll-off, with a high EQE of 26.3% even at 1,000 cd/m2, highlighting the synergistic effect between the spiro-structure and the B-heterotriangulene acceptor skeleton.

Boron‐Based Narrowband Multiresonance Delayed Fluorescent Emitters for Organic Light‐Emitting Diodes

Recently, the exploration of boron (B)/heteroatom‐embedded polycyclic nanographites featuring multiresonance thermally activated delayed fluorescence (MR‐TADF) garners astonishing attention to promote the advancement of organic light‐emitting diodes (OLEDs). Contrary to the traditional donor–acceptor (D–A)‐type TADF emitters, the MR‐TADF emitters manifest narrowband emission with full width at half maximum (FWHM ≤ 40 nm) and superior photoluminescence quantum yield (PLQY) coupled with the small singlet–triplet energy splitting, which appeal their potential as promising candidates in fabricating efficient OLEDs. Growingly, MR‐TADF emitters deliver benchmark device performance comparable to the conventional TADF/phosphorescent emitters. However, they are suffering from the major drawbacks such as difficult to realize full‐color emitters, slow exciton upconversion dynamics, aggregation‐caused emission quenching, severe efficiency roll‐off, and poor operational lifetime, which jeopardizes their practical applicability. Herein, a comprehensive review on B‐based MR‐TADF emitters reported till date is presented, focusing on the different design strategies documented for circumventing the aforementioned shortcomings. This review is divided into several subgroups based on the emission color of the materials to draw the attention of organic electronics community toward constructing efficient full‐color MR‐OLEDs. Finally, challenges and opportunities in the MR‐TADF emitters are discussed.

Facile access to high-performance reverse intersystem crossing OLED materials through an unsymmetrical D-A-D’ molecular scaffold

Organic light-emitting diode (OLED) materials based on a reverse intersystem crossing (RISC) triplet harvesting mechanism have attracted tremendous attention. Currently, many research efforts have been devoted to the construction of D-A or D-A-D RISC-OLED materials whose charge-transfer-featured singlet excited states (1CTD-A) are very close to the local triplet excited states of their D or/and A moieties (3LED or/and 3LEA), so that a small singlet–triplet energy splitting and a large spin–orbit coupling matrix element can be acquired concurrently. However, the chief technical difficulty lies in the accurate prediction and delicate tuning of the 1CTD–A energy levels of these compounds. Herein, by using PCz-TXO2 as an example, we demonstrated that high-performance RISC-OLED materials can be readily achieved by integrating an appropriate D’ subunit into a traditional D-A fluorophore that lacks RISC property, i.e., constructing a D–A–D’ triad whose 1CTD-A is close to its 3LED’. In comparison with its D-A or D-A-D counterpart of P-TXO2 or DP-TXO2 showing satisfactory photoluminescence efficiency, pure blue gamut but poor RISC, the presence of a carbazole-based D’ subunit has negligible influence on the transition feature of its lowest singlet excited state (S1), but triggers significantly enhanced RISC property. Consequently, PCz-TXO2 shows pure blue electroluminescence (EL) inherited from DP-TXO2 [CIE1931: (0.160, 0.089) vs. (0.154, 0.102)], but significantly enhanced external quantum efficiency (EQEmax: 9.5% vs. 6.1%) and exciton utilization efficiency (EUEmax: 93% vs. 42%). Our results present a new method to facilely access RISC materials and can greatly extend the design rationales for high-performance OLED materials.

Exploiting new feasibility of a phenylquinoline unit for establishing efficient green thermally activated delayed fluorescent emitter with short delayed fluorescent lifetime

Realizing the confluence of high photoluminescence quantum yields (PLQYs) and short delayed fluorescent lifetimes remains a challenge for thermally activated delayed fluorescent (TADF) emitters. A green TADF emitter, 10-(2,4-diphenylquinolin-6-yl)-10 H -phenoxazine (PhQLPXZ), is exploited by directly linking an 2,4-diphenylquinoline unit with an electron-donating (D) 10 H -phenoxazine moiety. For the first time, the phenylquinoline (PhQL) unit is revealed as an electron-accepting unit for fabricating efficient TADF emitters. The dihedral angle between the donor and the acceptor is nearly 90°, which induces a totally separation of frontier orbitals. As a result, the emitter reveals a small singlet-triplet splitting energy of 0.09 eV and a short delayed fluorescence lifetime of 1.3 μs. The green organic light-emitting diodes (OLEDs) employing PhQLPXZ as the emitter exhibit a maximum external quantum efficiency of 15.7%. This study demonstrates for the first time that phenylquinoline derivatives can be employed for constructing TADF emitters for high-efficiency fluorescent OLEDs. • Phenylquinoline derivatives were employed to construct a green TADF emitter with a short delayed lifetime of 1.3 μs. • The molecule endows a vertical dihedral angle (97°) between the acceptor and the donor and high photoluminescence quantum yield over 70% in film. • The device with a doping concentration of 6% exhibited a high EQE of 15.7%, which is one of the best among the devices based on quinolone cored emitters.

Spin-Enhanced Reverse Intersystem Crossing and Electroluminescence in Copper Acetate-Doped Thermally Activated Delayed Fluorescence Material

Thermally activated delayed fluorescence (TADF) materials are attractive for next-generation organic light-emitting diodes (OLEDs) because of their utilization of nonradiative triplets via reverse intersystem crossing (RISC), which requires not only a small singlet-triplet energy splitting but also the conservation of spin angular momentum. Here we use copper acetate as a spin sensitizer to facilitate RISC and thus enhance electroluminescence in TADF-exciplex OLEDs. Copper acetate is involved in the radiative decay process due to its coordination interaction with exciplex molecules having intermolecular charge-transfer characteristics, which causes significant changes in the photoluminescence intensity and lifetime. Meanwhile, magneto-photoluminescence reveals that the addition of copper acetate promotes spin conversion in the RISC process. It allows the enhancement of the electroluminescence (∼80%) from spin-sensitized OLEDs, accompanied by the suppression of magneto-electroluminescence upon the doping of copper acetate. These results illustrate that using a spin sensitizer may overcome the limitation of harvesting nonradiative triplets in organic luminescent materials and devices.

Significant role of spin-triplet state for exciton dissociation in organic solids.

Clarification of the role of the spin state that initiates exciton dissociation is critical to attaining a fundamental understanding of the mechanism of organic photovoltaics. Although an excited spin-triplet state with an energy lower than that of excited spin-singlet state is disadvantageous in exciton dissociation, a small electron exchange integral results in small singlet-triplet energy splitting in some material systems. This energy splitting leads to a nearly isoenergetic alignment of both excited states, raising a question about the role of excited spin states in exciton dissociation. Herein, we show that the spin-triplet rather than the spin-singlet plays a critical role in the exciton dissociation that leads to the formation of free carriers. This result indicates that the spin-triplet inherently acts as an intermediate, leading to exciton dissociation. Thus, our demonstration provides a fundamental understanding of the role of excited spin states of organic molecular systems in photoinduced charge-carrier generation.

Aggregation-induced delayed fluorescence molecules with mechanochromic behaviors for efficient blue organic light-emitting diodes

Aggregation-induced delayed fluorescence (AIDF) materials combining the advantages of aggregation-induced emission and thermally activated delayed fluorescence have attracted considerable attention. However, blue AIDF materials with satisfactory electroluminescence efficiencies are still few. In this work, two blue luminescent molecules (XT-DPDBA and XT-BDPDBA) with evident AIDF characteristics and high photoluminescence quantum yields are developed by using rigid electron acceptor xanthenone and weak electron donor 10-dihydrodibenzo[b,e][1,4]azasiline. Both molecules exhibit prominent mechanochromic luminescence (MCL) behaviors originating from conformational interconversion between quasi-axial and quasi-equatorial conformations. The quasi-equatorial conformation has small singlet–triplet energy splitting and thus delayed fluorescence characteristic. Efficient blue organic light-emitting diodes are fabricated based on both molecules, providing high external quantum efficiencies of up to 27% and small efficiency roll-offs. These blue AIDF molecules with MCL property have good application potentials in various optical and electronic devices.

Overcoming energy loss of thermally activated delayed fluorescence sensitized-OLEDs by developing a fluorescent dopant with a small singlet–triplet energy splitting

An approach was presented to suppress T1-state energy loss in OLEDs. That is developing comparable T1-energy FD with TADF co-host, and thus, T1 excitons on the FDs would escape and go back to the co-host and then transfer to the singlet state of the FDs, finally contributing to light-emission.

Thermally Activated Delayed Fluorescence Amorphous Molecular Materials for High-Performance Organic Light-Emitting Diodes.

Small-molecule thermally activated delayed fluorescence (TADF) materials have been extensively developed to actualize efficient organic LEDs (OLEDs). However, organic small molecules generally compromise thin film quality and stability due to the tendency of crystallization, aggregation, and phase separation, which hence degrade the efficiency and long-term stability of the OLEDs. Here, for the first time, we exploit the unique molecular configuration of the bimesitylene scaffold to design two highly efficient TADF amorphous molecular materials with excellent thermal and morphological stabilities. The twisted and rigid bimesitylene scaffold thwarts regular molecular packing and crystallization, thereby guaranteeing homogeneous and stable amorphous thin films. Meanwhile, the highly twisted geometry of the bimesitylene scaffold efficiently breaks the molecular conjugation and thus conserves the high energies of the lowest locally excited triplet states (3LE) above the lowest charge transfer states (1CT and 3CT), leading to small singlet-triplet energy splitting and fast reverse intersystem crossing. These TADF emitters exhibit high photoluminescence quantum yields of 0.90 and 0.69 and short TADF lifetimes of 4.94 and 1.44 μs in doped films, based on which the greenish-blue and greenish-yellow OLEDs achieve external quantum efficiencies of 23.2 and 16.2%, respectively, with small efficiency roll-off rates and perfect color stability.

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 (kRISC ≈ 106 s-1) with short-lived delayed fluorescence (τd ≈ 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.

Promoting Room Temperature Phosphorescence through Electron Transfer from Carbon Dots to Promethazine.

Room temperature phosphorescence (RTP) as a fascinating phenomenon shows great potential toward multiple applications. Howbeit, it is challengeable to improve the phosphorescence efficiency of carbon dots (CDs) owing to their short lifetime. Herein, we proposed a facile, rapid, and gram-scale strategy to synthesize the cross-linked carbon dots (named N-CDs) with both bright blue fluorescence and green RTP emissions. To be specific, the polymer of polyethylenimine (PEI) served as the cross-linking agent and carbon source, during which process phosphoric acid accelerated the formation of the compact carbon core within 30 s. Subsequently, the cross-linked carbon dots with the rigid network formed a small singlet-triplet energy splitting (ΔEST) of 0.490 eV, thus exhibiting a long RTP lifetime of 429.880 ms while coated on the filter paper through the hydrogen bonds. Taking advantage of the double luminescence, we successfully achieved the dual-channel detection of promethazine by N-CDs. The fluorescence of N-CDs was obviously quenched by promethazine through the electron-transfer process, displaying the linear range from 0.4 to 8 mM. Significantly, the electron transfer (ET) from carbon dots to promethazine boosted their phosphorescence efficiency and prolonged the lifetime to 565.190 ms, and the enhanced phosphorescence facilitated the sensitive recognition of promethazine with the concentration range of 1-3000 μM. Meanwhile, the possible autofluorescence interference from biological samples could be avoided through this RTP assaying mode, providing the more accurate results. Also, their RTP and fluorescence endowed the current N-CDs with the ability of dual-signal painting and imaging. This strategy may broaden the new approaches to produce the long-lifetime and high-efficiency RTP material toward the sensing purpose.

Design Approach of Lifetime Extending Thermally Activated Delayed Fluorescence Sensitizers for Highly Efficient Fluorescence Devices.

In this work, a design approach of three thermally activated delayed fluorescence (TADF) emitters to extend the device lifetime of the TADF sensitized fluorescent devices was studied. Three TADF materials, 5-{4,6-bis[4-(tert-butyl)phenyl]-1,3,5-triazin-2-yl}-2-(10,15-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazol-5-yl)benzonitrile (tTCNTruX), 4-[3-cyano-4-(10,15-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazol-5-yl)phenyl]-2,6-diphenylpyrimidine-5-carbonitrile (PCNTruX) and 4-(4-{10,15-bis[4-(tert-butyl)phenyl]-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazol-5-yl}-3-cyanophenyl)-2,6-diphenylpyrimidine-5-carbonitrile (PCNtTruX), were synthesized as sensitizers for TADF-sensitized fluorescent organic light-emitting diodes. The two tTCNTruX and PCNtTruX TADF emitters were designed to have Dexter energy transfer with blocking groups either in the donor or acceptor unit of the donor-acceptor-type TADF sensitizer. The TADF materials showed small singlet-triplet energy splitting and a high reverse intersystem crossing (RISC) rate for effective sensitization of the fluorescent emission of the fluorescent emitter. tTCNTruX- and PCNtTruX-sensitized fluorescent devices showed maximum external quantum efficiencies (EQEs) of 17.7 % and 11.5 % in the yellow and red devices, respectively, which were higher than those of TADF-sensitized devices with the corresponding TADF sensitizer without a blocking group. Moreover, the device lifetime was also extended by employing the tTCNTruX and PCNtTruX sensitizers. This work demonstrated that the tTCNTruX and PCNtTruX sensitizers are effective to improve the maximum EQE and device lifetime of TADF-sensitized fluorescent devices.

Multi‐Layer π‐Stacked Molecules as Efficient Thermally Activated Delayed Fluorescence Emitters

AbstractMulti‐layer π‐stacked emitters based on spatially confined donor/acceptor/donor (D/A/D) patterns have been developed to achieve high‐efficiency thermally activated delayed fluorescence (TADF). In this case, dual donor moieties and a single acceptor moiety are introduced to form two three‐dimensional (3D) emitters, DM‐BD1 and DM‐BD2, which rely on spatial charge transfer (CT). Owing to the enforced face‐to‐face D/A/D pattern, effective CT interactions are realized, which lead to high photoluminescence quantum yields (PLQYs) of 94.2 % and 92.8 % for the two molecules, respectively. The resulting emitters exhibit small singlet–triplet energy splitting (ΔEST) and fast reverse intersystem crossing (RISC) processes. Maximum external quantum efficiencies (EQEs) of 28.0 % and 26.6 % were realized for devices based on DM‐BD1 and DM‐BD2, respectively, which are higher than those of their D/A‐type analogues.