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Thermally activated delayed fluorescent (TADF) materials generally suffer from severe concentration quenching. Efficient non‐doped TADF emitters are generally highly twisted aromatic amine‐based compounds with isolated chemical moieties. Herein we demonstrate that co‐facial packing and strong π–π intermolecular interactions give rise to bright TADF emissions in non‐doped film and crystalline states within the compound 2,4‐diphenyl‐6‐(thianthren‐1‐yl)‐1,3,5‐triazine (oTE‐DRZ). Quantum chemistry simulations indicate that a disperse outer orbital of sulfur atoms, a folded thianthrene plane (for a reduced donor–acceptor distance), and a triazine acceptor with n–π* character, generate a spatially conjugated transition with a small singlet–triplet splitting energy. In company with a highly emissive non‐doped film, the corresponding organic light‐emitting diode achieved a 20.6 % external quantum efficiency, verifying its potential for high‐performance optoelectronic applications. In a crystalline state, it was verified that intra‐ and intermolecular dual TADF assisted by a hidden room‐temperature phosphorescent state. This state could preserve the long‐lived excitons while suppressing non‐radiation, and it could serve as a "spring‐board" for cascade up‐conversion processes. The oTE‐DRZ crystal showed greenish‐blue emission with a very high photoluminescent quantum yield of approximately 87 %, which is the highest among all TADF crystals reported to date. [ABSTRACT FROM AUTHOR]

With the advantages of facile one-pot synthesis and THF-irrigating purification, we first developed novel cyclic compounds with deep blue thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP) dual emission even in the as-spun films. Careful crystal analysis, combined with theoretical calculations, reveal that abundant inter- and intra-molecular interactions, whatsoever the molecular packing, are obligated to unique RTP properties. These results open up a new study of compounds with multiple acceptors and donors, and represent an important step in the further development of deep blue TADF and easy RTP emission. [ABSTRACT FROM AUTHOR]

A novel nonaromatic amine containing hole‐transporting material, 3,3′‐(2‐methyl‐1,3‐phenylene)diphenoxathiine (mPXTMP), is developed and coevaporated with electron‐transporting materials to form comixed films. Pure‐blue, sky‐blue, and green thermally activated delayed fluorescent (TADF) exciplex emissions are observed. Though the mPXTMP:2,4,6‐tri([1,1′‐biphenyl]‐3‐yl)‐1,3,5‐triazine (T2T) bimolecular system exhibits a large singlet–triplet splitting energy (ΔEST) of 0.44 eV, its locally triplet excited states play vital roles in the cascade up‐conversion process, leading to an abnormal synergistic enhancement of TADF and phosphorescence from high‐lying triplet states at a low temperature, which violates Kasha's rule. Another bimolecular system, mPXTMP:PO‐T2T, achieves a fast up‐conversion rate of 106 s−1 benefiting from a small ΔEST of 0.10 eV and the enhanced spin–orbital coupling induced by the sulfur element rich environment. The corresponding organic light‐emitting diodes realize an exciton utilization ratio of 61.7–92.6%, verifying that these nonaromatic amine containing exciplex systems can be promising candidates toward purely organic electroluminescence with 100% internal quantum efficiency. [ABSTRACT FROM AUTHOR]