Sky-Blue Thermally Activated Delayed Fluorescence with Intramolecular Spatial Charge Transfer Based on a Dibenzothiophene Sulfone Emitter

Engineering the Macrocyclic Donor Structures towards Deep-Blue Thermally Activated Delayed Fluorescence Emitters

Deep-blue thermally activated delayed fluorescence (TADF) molecules present promising potential in organic light-emitting diodes (OLEDs), especially for display applications. Here, an efficient molecular engineering approach to modifying the donor or acceptor features of the D-π-A-configured TADF molecules for deep-blue emission is reported. By introducing oxygen and sulfone as a bridge unit onto the macrocyclic donor, two emitters, c-ON-MeTRZ and c-NS-MeTRZ, are synthesized and characterized, respectively. The reduced donor strength of c-ON-MeTRZ and c-NS-MeTRZ as compared to that of the model molecule c-NN-MeTRZ leads to blue-shifted emissions with high photoluminescence quantum yields (PLQYs) and retains TADF characters, while the new emitter c-NN-MePym with the most blue-shifted emission only exhibits a pure fluorescent nature because of the electron-accepting feature of pyrimidine that is insufficient for inducing the TADF property. In the presence of macrocyclic donors, these new emitters show high horizontal dipole ratios (Θ_// = 85-89%), which are beneficial for improving the light out-coupling efficiency. Deep-blue TADF OLEDs incorporating c-ON-MeTRZ as an emitter doped in the mCPCN host achieves a high maximum external quantum efficiency (EQE_max) of 30.2% together with 1931 Commission Internationale de I'Eclairage (CIE) coordinates of (0.14, 0.13), while the counter device employing c-NS-MeTRZ as a dopant gives EQE_max of 15.4% and CIE coordinates of (0.14, 0.09). The EQE_max of c-ON-MeTRZ- and c-NS-MeTRZ-based devices can be significantly improved to 34.4 and 29.3%, respectively, with a polar host DPEPO, which stabilizes the charge transfer (CT) S₁ state to give lower ΔE_ST for improving the reverse intersystem crossing process. The efficient TADF character, high PLQYs, and high anisotropic emission dipole ratios work together to render the superior electroluminescence (EL) efficiencies. Based on the detailed characterizations of physical properties, theoretical analyses, and comprehensive study on the corresponding devices, a clear structure-property-performance relationship has been successfully established to verify the effective molecular design strategy of modulating the macrocyclic donor characters for efficient deep-blue TADF emitters.

Synthesis, photoluminescence and electroluminescence properties of a new blue emitter with aggregation-induced emission and thermally activated delayed fluorescence characteristics

The emitters with aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF) characteristics are in high demand in organic light-emitting diodes (OLEDs) owing to their strong fluorescence and high exciton utilization under electrical excitation. Herein, a blue emitter, 10-(3-((3,5-di(9H-carbazol-9-yl)phenyl)sulfonyl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (m-CZ-DPS-DMAC), was synthesized by incorporating carbazole as skeleton, acridine as electron donor, and diphenyl sulfone as electron acceptor. m-CZ-DPS-DMAC emits weak fluorescence in good solvent, while it is obviously enhanced in the aggregate state, which is typical of AIE molecules. Meanwhile, the energy levels of the singlet and triplet states (ΔE_ST) of the molecule is relatively small, and it also exhibits obvious temperature dependence and oxygen sensitivity, which directly proves its TADF properties. In view of the above properties, a series of non-doped and doped OLEDs were prepared using m-CZ-DPS-DMAC as light-emitting layers. Among them, non-doped OLED (device A) displays blue emission (488 nm) with the turn-on voltage (V_on), the maximum luminance (L_max), the maximum current efficiency (CE_max), the maximum power efficiency (PE_max) and the maximum external quantum efficiency (EQE_max) of 2.6 V, 3460 cd m^-2, 26.09 cd A^-1, 29.26 lm W^-1 and 10.05%, respectively. Doped OLED (device C) constructed based on m-CZ-DPS-DMAC doped 30% in DPEPO shows the satisfactory performance with the maximum emission peak of 486 nm, the V_on of 2.8 V, the L_max of 4571 cd m^-2, the CE_max of 21.37 cd A^-1, the PE_max of 22.37 lm W^-1, and the EQE_max of 9.44%, respectively. The outstanding performance of m-CZ-DPS-DMAC proves that it is a potential material for designing blue OLEDs with AIE-TADF properties.

Highly Efficient Sky-Blue π-Stacked Thermally Activated Delayed Fluorescence Emitter with Multi-Stimulus Response Properties

Organic materials with multi-stimulus response (MSR) properties have demonstrated many potential and practical applications. Herein, a π-stacked thermally activated delayed fluorescence (TADF) material with multi-stimulus response (MSR) properties, named SDMAC, was designed and synthesized using distorted 9,9-dimethyl-10-phenyl-9,10-dihydroacridine as a donor. SDMAC possesses a rigid π-stacked configuration with intramolecular through-space interactions and exhibits aggregation-induced emission enhancement (AIEE), solvatochromic, piezochromic, and circularly polarized luminescence (CPL) under different external stimuli. The rigid molecular structure and efficient TADF properties of SDMAC can be used in displays and lighting. Using SDMAC as an emitter, the maximum external quantum efficiency (EQE) of the fabricated organic light-emitting diodes (OLEDs) is as high as 28.4 %, which make them the most efficient CP-TADF OLEDs based on the through-space charge transfer strategy. The CP organic light-emitting diodes (CP-OLEDs) exhibit circularly polarized electroluminescence (CPEL) signals.

Research Progress of Intramolecular π-Stacked Small Molecules for Device Applications

Organic semiconductors can be designed and constructed in π-stacked structures instead of the conventional π-conjugated structures. Through-space interaction (TSI) occurs in π-stacked optoelectronic materials. Thus, unlike electronic coupling along the conjugated chain, the functional groups can stack closely to facilitate spatial electron communication. Using π-stacked motifs, chemists and materials scientists can find new ways for constructing materials with aggregation-induced emission (AIE), thermally activated delayed fluorescence (TADF), circularly polarized luminescence (CPL), and room-temperature phosphorescence (RTP), as well as enhanced molecular conductance. Organic optoelectronic devices based on π-stacked molecules have exhibited very promising performance, with some of them exceeding π-conjugated analogues. Recently, reports on various organic π-stacked structures have grown rapidly, prompting this review. Representative molecular scaffolds and newly developed π-stacked systems could stimulate more attention on through-space charge transfer the well-known through-bond charge transfer. Finally, the opportunities and challenges for utilizing and improving particular materials are discussed. The previous achievements and upcoming prospects may provide new insights into the theory, materials, and devices in the field of organic semiconductors.

Thermally Activated Delayed Fluorescence Polysiloxanes with Short Delay Fluorescence Lifetimes

Blue-emitting thermally activated delayed fluorescence (TADF) polymers are still in demand for high-efficiency display materials. Through-space charge transfer (TSCT) strategy is promising for keeping color purity of blue-emitting polymers with non-conjugated main chains. It is, however, hard to synthesize copolymers with well-dispersed donors or acceptors utilizing traditional polyethylene backbones via radical polymerization. Herein, two series of blue-emiting polysiloxane with TADF properties, random and order-controlled copolysiloxanes, were succsuccfully designed and synthesized and their photophysical properties were investigated and compared in detail. All of them display short prompt and delay fluorescence lifetimes, very fast reverse intersystem crossing (RISC) rate of 10⁷ s^-1 . Compared with random copolysiloxanes, acceptors are well seperated by donors for order-controlled copolysiloxanes, whcih exhibit the faster RISC processes and the higher photoluminescence quantum yield. Therefore, the order-controlled architecture provides a guide for improving light-emitting efficiency of TSCT-type TADF polymers. This article is protected by copyright. All rights reserved.

Through-Space Charge-Transfer Emitters Developed by Fixing the Acceptor for High-Efficiency Thermally Activated Delayed Fluorescence

Through-space charge-transfer (TSCT) emitters have been extensively explored for thermally activated delayed fluorescence (TADF), but arranging various donors and acceptors into rigid cofacial conformations for various efficient TSCT TADF emitters has remained challenging. Here, we report a "fixing acceptor" design to reach various efficient TSCT TADF emitters. By chemically fixing the acceptor (benzophenone) with a rigid spiro-structure and cofacially aligning various donors with the fixed acceptor, a series of efficient TSCT TADF emitters have been developed. Single-crystal structures and theoretical calculations have verified closely packed cofacial donor/acceptor conformations and favorable TSCT in the emitters. In doped films, the emitters afford sky blue to yellow TADF emission, with high photoluminescence efficiencies up to 0.92 and reverse intersystem crossing rates up to 1.0 × 10⁶ s^-1. Organic light-emitting diodes using the emitters afford sky blue to yellow electroluminescence with high external quantum efficiencies up to 20.9%. The work opens a new avenue toward a wide variety of efficient TSCT TADF emitters.

Organosulfur Materials with High Photo- and Photo-Oxidation Stability: 10-Anthryl Sulfoxides and Sulfones and Their Photophysical Properties Dependent on the Sulfur Oxidation State

While few studies show only symmetrical and poorly mono-SOn (n = 0-2) substituted acenes, in this study, we present a synthesis of a new group of unsymmetrical, significantly substituted derivatives, which revealed unique photophysical properties. Both sulfides (S), sulfoxides (SO) and sulfones (SO2) showed very high photochemical stabilities, unusual for these groups, during UV-irradiation at 254/365 nm (air O2 and Ar), which was higher than any found in the literature. For the (S)/(SO) series (254 nm), the stabilities of 80-519 min. (air O2 and Ar) were found. At 365 nm, stabilities of 124-812 min./(air O2) for (S)/(SO) and higher for (SO2) were observed. Photoluminescence lifetimes of (SOn) of the lower anthryl symmetry remained in the following order: (SO2) < (S) < (SO); those with full symmetry were in the following order: (S) < (SO) < (SO2). The enhanced photostability was explained with DFT/MS/Hammett's constants, which showed the leading role of the SOn groups in stabilization of HOMO/LUMO frontier orbitals. The SOn (n = 0-2) substituted acenes turned out to be tunable violet/blue/green emitters by oxidation of S atoms and the introduction of rich substitution.

Nonconjugated Triptycene-Spaced Donor-Acceptor-Type Emitters Showing Thermally Activated Delayed Fluorescence via Both Intra- and Intermolecular Charge-Transfer Transitions

Thermally activated delayed fluorescence (TADF) emitters have aroused considerable attention, particularly for their great potential in organic light-emitting diodes (OLEDs). In typical TADF molecules, intramolecular charge transfer (CT) between electron-donor (D) and electron-acceptor (A) moieties is the dominant transition. Actually, CT transitions can possibly occur between different molecules as well. Herein, we used a nonconjugated triptycene (TPE) moiety to space D and A moieties and developed two novel emitters tBuDMAC-TPE-TRZ and tBuDMAC-TPE-TTR to explore the roles of intra- and intermolecular CT transitions. Along with weak intramolecular CT transitions, intermolecular CT transitions are dominant for tBuDMAC-TPE-TRZ and tBuDMAC-TPE-TTR neat films. Particularly, tBuDMAC-TPE-TRZ showed a high maximum external quantum efficiency of 10.0% in a nondoped solution-processed OLED, which was evidently higher than that of a corresponding 10 wt % tBuDMAC-TPE-TRZ-doped OLED with 4,4',4″-tris(carbazol-9-yl)triphenylamine (TCTA) as the host matrix. The results prove that intermolecular CT transitions indeed participate in the CT transition process in these systems and they are helpful to enhance the electroluminescence performance of emitting systems with weak intramolecular CT transitions.

Chiral Spiro-Axis Induced Blue Thermally Activated Delayed Fluorescence Material for Efficient Circularly Polarized OLEDs with Low Efficiency Roll-Off

A spiro-axis skeleton not only introduces circularly polarized luminescence (CPL) into thermally activated delayed fluorescence (TADF) molecules but also enhances the intramolecular through space charge transfer (TSCT) process. Spiral distributed phenoxazine and 2-(trifluoromethyl)-9H-thioxanthen-9-one-10,10-dioxide act as donor and acceptor units, respectively. The resulting TADF enantiomers, (rac)-OSFSO, display emission maxima at 470 nm, small singlet-triplet energy gap (ΔE_ST ) of 0.022 eV and high photoluminescence quantum yield (PLQY) of 81.2 % in co-doped film. The circularly polarized OLEDs (CP-OLEDs) based on (R)-OSFSO and (S)-OSFSO display obvious circularly polarized electroluminescence (CPEL) signals with dissymmetry factor up to 3.0×10^-3 and maximum external quantum efficiency (EQE_max ) of 20.0 %. Moreover, the devices show remarkably low efficiency roll-off with an EQE of 19.3 % at 1000 cd m^-2 (roll-off ca. 3.5 %), which are among the top results of CP-OLEDs.

Harvesting triplet excitons for near-infrared electroluminescence via thermally activated delayed fluorescence channel

Near-infrared (NIR) emission is useful for numerous practical applications, such as communication, biomedical sensors, night vision, etc., which encourages researchers to develop materials and devices for the realization of efficient NIR organic light-emitting devices. Recently, the emerging organic thermally activated delayed fluorescence (TADF) emitters have attracted wide attention because of the full utilization of electron-generated excitons, which is crucial for achieving high device efficiency. Up to now, the TADF emitters have shown their potential in the deep red/NIR region. Considering the color purity and efficiency, however, the development of NIR TADF emitters still lags behind RGB TADF emitters, indicating that there is still much room to improve their performance. In this regard, this perspective mainly summarizes the past progress of molecular design on constructing TADF NIR emitters. We hope this perspective could provide a new vista in developing NIR materials and enlighten breakthroughs in both fundamental research and applications.

Face-to-Face Orientation of Quasiplanar Donor and Acceptor Enables Highly Efficient Intramolecular Exciplex Fluorescence

Intramolecular through-space charge-transfer (TSCT) excited states have been exploited for developing thermally activated delayed fluorescence (TADF) emitters, but the tuning of excited state dynamics by conformational engineering remains sparse. Designed here is a series of TSCT emitters with precisely controlled alignment of the donor and acceptor segments. With increasing intramolecular π-π interactions, the radiative decay rate of the lowest singlet excited state (S₁ ) progressively increased together with a suppression of nonradiative decay, leading to significantly enhanced photoluminescence quantum yields of up to 0.99 in doped thin films. A high-efficiency electroluminescence device, with a maximum external quantum efficiency (EQE) of 23.96 %, was achieved and maintains >20 % at a brightness of 1000 cd m^-2 . This work sheds light on the importance of conformation control for achieving high-efficiency intramolecular exciplex emitters.

The inspiration and challenge for through-space charge transfer architecture: from thermally activated delayed fluorescence to non-linear optical properties

Organic molecules consisting of electron donor (D) and electron acceptor (A) subunits linked by π-conjugated bridges are promising building blocks for thermally activated delayed fluorescence (TADF) and non-linear optics (NLO) materials due to their intramolecular charge transfer (CT) processes in response to external stimuli. According to the electron interaction pattern, the CT process in D-π-A architectures can be divided into two categories, through-bond/-space charge transfer (TB/TSCT). To date, research into the TADF properties of TSCT characteristic molecules has since seen significant growth. In fact, TSCT characteristic materials show great advantages in such NLO responses. In this perspective, we first briefly introduced the basic principles of NLO and TADF effects. Successively, we discuss the influence of TBCT and TSCT patterns on NLO and TADF properties, especially for TSCT characteristic. In the final part, we address the diversity and potential advantages of TSCT characteristic molecules as high-performance NLO materials. With these, it is expected that the greater structural flexibility of spatial conjugation can bring more functionality to NLO materials in the future.