dissymmetry factor (g PL) (defined as 2(I L − I R)/(I L + I R) where I is the emission intensity), which takes a value between −2 and +2. [3] In addition, good emission efficiency is clearly important, especially in the condensed state. Over the past decades, several strategies to realize efficient CPL-active materials have been developed [4] and much work has concentrated on determining the most efficient method of introducing a chiral center into the emissive material. [5] To date, various kinds of chiral materials, including polymers and small molecules, [6] have been developed for CPL. However, most of the reported CPLactive emitters are fluorescent materials which therefore have an intrinsic internal quantum efficiency (IQE) of 25%, [7] leading to a limitation in the device. To enhance the IQE of the CPL emitter, various complexes with different transition metals were explored recently as they offer the prospect of quantitative IQE. [8] For these circularly polarized organic light-emitting diodes (CP-OLEDs), device performance, and the luminescence dissymmetry factor (g EL) of circularly polarized electroluminescence (CPEL) were two main aspects to evaluate the level of device. [9] To date, few examples have been reported with both high emission efficiency in solution-processable CP-OLED and a large luminescence dissymmetry factor. For example, Di Bari et al. reported a CP-OLED based on chiral Eu-complexes and its Circularly polarized luminescence (CPL) is of interest due to its wide potential application in semiconductors. To balance the emission efficiency and luminescence dissymmetry factor (g PL) of a CPL emitter, in this context, two chiral, phosphorescent and liquid-crystalline cyclometalated platinum complexes, abbreviated R-Pt and S-Pt, are prepared. The complexes, which show an intense green emission at 504 nm both in solution and in the solid state, contain a simple, ortho-metalated 2-phenylpyridine unit functionalized with a chiral 2-octanol chain, with liquid crystallinity being induced by modifying the β-diketonato ligand with mesogenic groups. Interestingly, both the chiral smectic (SmA*) and nematic (N*) phases are found by a combination of polarized optical microscopy, differential scanning calorimetry, and small-angle X-ray scattering. By annealing, distinct CPL emission is achieved in the solid state with a g PL around 0.02. Employing the chiral platinum complexes as the dopant, solution-processable organic light-emitting diodes present an external quantum efficiency of 11.3% and strong, circularly polarized electroluminescence with an extremely high luminescence dissymmetry value (g EL) of 0.06 after annealing at 100 °C. This work opens an avenue for designing CPL-active emitters with high emission efficiency and high dissymmetry factor.

Difluoroboron (BF2)-containing dyes have attracted great interest owing to their exceptionally high luminescence efficiency and good electron-withdrawing properties. However, only a few reports on difluoroboron-based thermally activated delayed fluorescence (TADF) have been addressed. In this contribution, a novel BF2-containing TADF molecule of BFOXD, which contains two acceptor fragments of oxadiazole (OXD) and BF2 and one donor unit of 9,9-dimethylacridine, was synthesized and characterized. For comparison, the precursor of OHOXD bearing one acceptor unit was also investigated. Both molecules clearly show TADF characteristics with sky-blue emission in solution and film state. Additionally, OHOXD undergoes excited-state intramolecular proton transfer-coupled intramolecular charge transfer processes. Using 9-(4-tert-butylphenyl)-3,6-bis­(triphenylsilyl)-9H-carbazole (CzSi) as the host, the organic light-emitting diodes fabricated via a solution process show maximum external quantum efficiency (EQE) of 2.98 and 13.8% for OHOXD- and BFOXD-based devices, respectively. While the bipolar TADF host of 10-(4-((4-(9H-carbazol-9-yl)­phenyl)­sulfonyl)­phenyl)-9,9-dimethyl-9,10-dihydroacridine (CzAcSF) is utilized instead of CzSi, the OHOXD- and BFOXD-based devices exhibit better performances with the maximum EQEs of 12.1 and 20.1%, respectively, which render the most efficient and the bluest emission ever reported for the BF2-based TADF molecules. This research demonstrates that introduction of one more acceptor unit into the TADF molecule could have a positive effect on emission efficiency, which opens a new way to design high-efficiency TADF molecules.

structures. [1] The promising mechanism is that TADF molecules convert the triplet states to singlet states via thermally activated reverse intersystem crossing (rISC). [2] Since the first report on the application of TADF molecule in organic light-emitting diodes (OLEDs) by Adachi group, [3] huge progress for TADF materials has been made over the past decades. The previous reports demonstrated that an effective strategy for TADF molecules is mainly composed by a twist donoracceptor (D-A) framework which could achieve a small energy gap (ΔE ST ) by the spatial separation of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). [4] To date, high external quantum efficiencies (EQEs) beyond 20% have been gained with the emission wavelength from deep blue to red. [5] However, most these high-efficiency devices were achieved by vacuum deposition procedure, which is unsuitable for the large area and inexpensive 3D printing owing to its complicated process and high cost. Therefore, it is necessary to develop the high-efficiency solution processable OLEDs based on TADF emitters.According to the design strategy of TADF molecules, proper donor and acceptor units should be carefully selected because Four crucifix-shaped molecules, named TPA-BPSB, DMAc-BPSB, MTPA-BPSB and MDMAc-BPSB, bearing the same acceptor fragment of bis(phenylsulfonyl)benzene (BPSB) and different donor segments (TPA and MTPA are the diphenylamine derivatives while DMAc and MDMAc are the 9,9-dimethylacridine derivatives) are synthesized and characterized by NMR, mass spectra, and single crystal X-ray crystallography. The molecular structure-property relationship of these crucifix-shaped molecules is systematically explored. All compounds display thermally activated delayed fluorescence (TADF) in the region of 500-550 nm. In addition, charming mechanochromic luminescence properties are observed for all these TADF molecules under external stimuli, such as grinding and exposure to CH 2 Cl 2 vapor. Four TADF molecules are used as the emitters in organic light-emitting diodes (OLEDs) fabricated via solution process. MTPA-BPSB-based device presents a best performance with a highest external quantum efficiency of ≈21%, which is among the highest efficiencies for reported BPSB-based solution-processable OLEDs so far. This research has an important significance in designing high-efficiency multifunctional TADF molecules.

Polymorphism, the intrinsic character of one chemical compound with at least two distinct phase arrangements, plays a very key role in the photophysical properties. In this contribution, four ′T′‐shaped molecules bearing the 2,1,3‐benzothiadiazole (BTD) skeleton, named 5 a–5 d, were prepared and characterized. All compounds exhibited excellent thermal stability and polymorphism in the solid state, evident from thermogravimetric analysis, differential scanning calorimetry, and polarized optical microscopy results. Intense emissions with high photoluminescent quantum yields were achieved both in solution (56–97 %) and neat films (33–98 %). All compounds possessed clearly pH‐dependent luminescence properties in solution. Additionally, compound 5 d showed useful mechanochromic luminescence owing to the transformation between the crystal and amorphous state. Employing compounds 5 a–5 d as the dopant, solution‐processable organic light‐emitting diodes (OLEDs) were fabricated and presented a highest external quantum efficiency of 6.15 %, which is higher than the theoretical value of fluorescence‐based OLEDs (∼5 %). This research provided a novel strategy for designing high‐efficiency BTD‐based polymorphic luminescent materials.