Simultaneous enhancement of out-coupling efficiency, internal quantum efficiency, and color purity in thermally activated delayed fluorescence (TADF) emitters is highly desired for the practical application of these materials. We designed and synthesized two isomeric TADF emitters, 2DPyM-mDTC and 3DPyM-pDTC, based on di(pyridinyl)methanone (DPyM) cores as the new electron-accepting units and di(tert-butyl)carbazole (DTC) as the electron-donating units. 3DPyM-pDTC, which is structurally nearly planar with a very small ΔE, shows higher color purity, horizontal ratio, and quantum yield than 2DPyM-mDTC, which has a more flexible structure. An electroluminescence device based on 3DPyM-pDTC as the dopant emitter can reach an extremely high external quantum efficiency of 31.9% with a pure blue emission. This work also demonstrates a way to design materials with a high portion of horizontal molecular orientation to realize a highly efficient pure-blue device based on TADF emitters.

Two benzoylpyridine-carbazole based fluorescence materials DCBPy and DTCBPy, bearing two carbazolyl and 4-(t-butyl)carbazolyl groups, respectively, at the meta and ortho carbons of the benzoyl ring, were synthesized. These molecules show very small ΔEST of 0.03 and 0.04 eV and transient PL characteristics indicating that they are thermally activated delayed fluorescence (TADF) materials. In addition, they show extremely different photoluminescent quantum yields in solution and in the solid state: in cyclohexane the value are 14 and 36%, but in the thin films, the value increase to 88.0 and 91.4%, respectively. The OLEDs using DCBPy and DTCBPy as dopants emit blue and green light with EQEs of 24.0 and 27.2%, respectively, and with low efficiency roll-off at practical brightness level. The crystal structure of DTCBPy reveals a substantial interaction between the ortho donor (carbazolyl) and acceptor (4-pyridylcarbonyl) unit. This interaction between donor and acceptor substituents likely play a key role to achieve very small ΔEST with high photoluminescence quantum yield.

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We have successfully synthesized a series of blue thermally activated delayed fluorescence emitters, BPy-pC, BPy-pTC, BPy-p2C, and BPy-p3C, bearing a 4-benzoylpyridine core as the electron-accepting unit and carbazolyl, tert-butylcarbazolyl, dicarbazolyl, and tercarbazolyl groups as the electron-donating units, respectively. The density functional theory calculation shows that all of the compounds have their lowest unoccupied molecular orbitals on the benzoylpyridine moiety. However, the highest occupied molecular orbital (HOMO) of BPy-p3C is widely dispersed to the whole tercarbazolyl group, while the HOMOs of BPy-pC and BPy-pTC are mainly on the carbazolyl and extended to the phenyl ring. As a result, ΔE is reduced from 0.29 eV for BPy-pC to 0.05 eV for BPy-p3C, and the organic light-emitting diodes using these materials as dopants emit blue light and their maximum external quantum efficiencies (EQEs) increase from 4.2% to 23.9% for BPy-pC and BPy-p3C, respectively. The EQE of the BPy-p3C-based device increases 2 times more than that of the BPy-pTC-based device without a significant change in the color coordinates.