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The development of orange‐red/red thermally activated delayed fluorescence (TADF) materials with both high emission efficiencies and short lifetimes is highly desirable for electroluminescence (EL) applications, but remains a formidable challenge owing to the strict molecular design principles. Herein, two new orange‐red/red TADF emitters, namely AC‐PCNCF3 and TAC‐PCNCF3, composed of pyridine‐3,5‐dicarbonitrile‐derived electron‐acceptor (PCNCF3) and acridine electron‐donors (AC/TAC) are developed. These emitters in doped films exhibit excellent photophysical properties, including high photoluminescence quantum yields of up to 0.91, tiny singlet‐triplet energy gaps of 0.01 eV, and ultrashort TADF lifetimes of less than 1 µs. The TADF‐organic light‐emitting diodes employing the AC‐PCNCF3 as emitter achieve orange‐red and red EL with high external quantum efficiencies of up to 25.0% and nearly 20% at doping concentrations of 5 and 40 wt%, respectively, both accompanied by well‐suppressed efficiency roll‐offs. This work provides an efficient molecular design strategy for developing high‐performance red TADF materials. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) materials based on multiple resonance (MR) molecular structures can achieve advantages, such as high fluorescence efficiency, reduced structural relaxation and narrow emission. It still remains a challenge to design highly efficient MR structure-based TADF materials with fast reverse intersystem crossing processes and easy emission-color tuning. Herein, we theoretically investigated the recently reported TADF molecule, which has the MR structure of quinolino[3,2,1-de] acridine-5,9-dione (QAD) as the acceptor and the moiety of carbazole (Cz) as the donor, namely QAD-Cz. The calculated fluorescence quantum efficiency of QAD-Cz is as high as 96.5%, which agrees with the experimental results (99.6%) very well. Moreover, we designed two new TADF materials based on the MR-QAD structure with different electron-donating moieties, namely QAD-TPA and QAD-DPTZ. The two newly designed molecules can theoretically achieve high fluorescence quantum efficiencies (over 90%) and fast reverse intersystem crossing rates (e.g. as high as 107 s−1 for QAD-DPTZ). The calculated emission wavelength of the QAD-DPTZ molecule is 638 nm, which obviously widens the emission color range of the reported MR-QAD-based TADF materials. Furthermore, we have analyzed the aggregation effects of the QAD-DPTZ molecule and demonstrated that this novel compound can likewise exhibit excellent luminescence performance in amorphous states. [ABSTRACT FROM AUTHOR]

Organic donor–acceptor (D–A) type thermally activated delayed fluorescence (TADF) materials exhibit high luminous quantum efficiencies and therefore attract considerable research interest as organic light-emitting diodes (OLEDs). Accurately calculating the photoluminescence efficiency is crucial for the design of new TADF materials. In this work, we comprehensively quantify the luminous quantum efficiencies and related photophysical process rates of dimethylacridan-based blue-color TADF emitters (AC-OSO and ACRXTN), and demonstrate that the higher reverse intersystem crossing rates (e.g. KRISC = 4.98 × 108 s−1) from T1 to S1 and the suppressed non-radiative decay rates (e.g. K Snr = 1.68 × 106 s−1) together lead to a theoretical luminous quantum efficiency of around 100%. Our calculations predict well fluorescence efficiencies observed in experiments (Φcal = 95%). In both molecules, the nonradiative rates of S1–S0 and reverse intersystem crossing rates are affected by the reorganization energies due to the twisting vibration of the donor DMAC and acceptor moiety. Our theoretical study on the TADF mechanism will be beneficial for designing highly efficient TADF based OLED materials. [ABSTRACT FROM AUTHOR]

[Display omitted] • New ionic TADF emitters based on O/S-fused arylphosphonium electron acceptors. • Simultaneously attaining high PLQYs, short exciton lifetimes, and high RISC rates. • Realizing solution-processed OLEDs with high EQEs and small efficiency roll-offs. Ionic thermally activated delayed fluorescence (iTADF) materials are newly emerging as promising candidates for constructing solution-processed electroluminescence devices. Developing iTADF materials with high emission efficiency, short exciton lifetime, and fast reverse intersystem crossing (RISC) is crucial for achieving stable and efficient organic light-emitting diodes (OLEDs), but it remains challenging. Here we report two high-performance iTADF emitters, namely DMAC-TPPO[BF 4 ] and DMAC-TPPS[BF 4 ], which consist of a conventional acridine as the electron-donor and newly developed oxygen/sulfur-fused arylphosphonium derivatives as the electron-acceptor. DMAC-TPPO[BF 4 ] and DMAC-TPPS[BF 4 ] exhibit high photoluminescence quantum yields of 77 % and 83 %, coupled with short exciton lifetimes of 1.2 and 1.1 μs, and RISC rate constants reaching up to 2.60 × 106 and 2.73 × 106 s−1, respectively. Theoretical and experimental investigations demonstrate that the imbedding of bridging heteroatoms into the phosphonium cation is pivotal for enhancing emission efficiency through molecular skeleton rigidification, as well as for accelerating the RISC process by capitalizing on the non-metal heavy-atom effect. Eventually, solution-processed OLEDs utilizing DMAC-TPPS[BF 4 ] realize a peak external quantum efficiency (EQE) of 17.8 % and show a minor efficiency roll-off of 7.3 % (EQE = 16.5 %) even at a practical high luminance of 1000 cd/m2. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) emitters in the long wavelength region usually suffer from severe nonradiative decay according to the energy-gap law, and show relatively large energy split (ΔE ST) between the lowest singlet and triplet excited states which compromise efficient up-conversion. Here, we report three orange and red TADF emitters comprising 5, 10-disubstituted phenazine electron-donor and quinoxaline-derived electron-acceptors, which show excellent TADF properties and yield efficient organic light-emitting diodes (OLEDs). These symmetrical acceptor-donor-acceptor (A-D-A) type and rodlike molecules concurrently exhibit high photoluminescence quantum yields up to 85% and fast reverse intersystem crossing. Experimental and theoretical studies reveal that these excellent photophysical properties are attributed the synergistic effect of structural rigidity, steric hindrance and strength of the donor and acceptor units. Doped OLEDs based on these emitters achieved high maximum external electroluminescence quantum efficiencies up to 19.9%, and maximum luminance surpassing 31240 cd/m2. [Display omitted] • Highly efficient orange to red TADF emitters with high PLQYs (up to 85%) and short TADF lifetimes. • Tiny S 1 -T 1 energy splits and fast spin-flip due to rational molecular design and excited-state regulation. • High-performance OLED with EQE nearly 20%. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) materials with through-space charge transfer (TSCT) have recently attracted extensive attention. However, how to rationally design high-performance TSCT-TADF emitters with small ΔE ST , high photoluminescence quantum yields (PLQYs) and high stability remains challenging. Herein, two triptycene-derived TADF molecules bearing ortho-donor (D)-acceptor (A) conformations are designed and characterized. These emitters exhibit unique V-shaped face-to-face D-A alignments with significant intramolecular D-A interactions, which mediate TSCT between the D and A moieties in addition to the through-bond charge transfer (TBCT) and facilitate realizing small ΔE ST and high efficiencies. Meanwhile, the rigid 3D triptycene scaffold endows the emitters with excellent thermal stability and high film-formation quality. These TADF emitters exhibit small ΔE ST of 6.7 and 10.8 meV and high photoluminescence quantum yields of 0.80 and 0.40 in doped films, respectively. The doped OLEDs employing these emitters achieve high external quantum efficiencies up to 20.5%. [Display omitted] • 3D triptycene scaffold based TADF emitters with photoluminescent efficiency up to 80%. • V-shaped face-to-face D-A conformation for constructing through-space charge transfer (TSCT) pathway. • High performance OLED with EQE beyond 20%. [ABSTRACT FROM AUTHOR]

[Display omitted] • Lock/unlock strategy proposed to manipulate excited states and photophysical properties of TADF materials. • Efficient TADF emitters with high PLQYs (up to 0.91), short TADF lifetimes and excellent stability and film-quality. • High-performance OLED with EQE beyond 25%. Thermally activated delayed fluorescent (TADF) materials have drawn widespread attentions due to their great potential for practical applications in organic light-emitting diodes (OLEDs). Deep understanding of the structure-excited state-photophysical property relationships is a key to rationally design TADF emitters. Herein, two new TADF molecules, TBP-DPXZ and TBQ-DPXZ, which consist of locked (triptycene-fused dibenzophenazine) and unlocked (triptycene-fused 2, 3-diphenylquinoxaline) electron-acceptors respectively and phenoxazine electron donors are reported. The comparative study reveals that the lock/unlock strategy could substantially manipulate the lowest locally excited (LE) and charge transfer (CT) states of these emitters via changing the acceptor strength, structural rigidity, and conjugation length, and in turn lead to significantly different photoluminescence (PL) and electroluminescence (EL) performance. It is notable that the unlocked molecule TBQ-DPXZ emits efficient green fluorescence with photoluminescence quantum yield of 91% and TADF lifetime of 3.6 μs in doped film. The doped OLED based on TBQ-DPXZ achieves an external quantum efficiency (EQE) of 25.1%, a current efficiency (CE) of 79.7 cd/ A, a power efficiency (PE) of 80.0 lm/W, and a maximum luminance of 18400 cd/m2, which are better than those of the TBP-DPXZ-based device and are among the best results reported so far for green TADF-OLEDs. [ABSTRACT FROM AUTHOR]