Purely Organic Crystals Exhibit Bright Thermally Activated Delayed Fluorescence

• Published in: Angewandte Chemie International Edition Vol. 58; no. 38; pp. 13522 - 13531
• Main Authors: Cai, Xinyi, Qiao, Zhenyang, Li, Mengke, Wu, Xiao, He, Yanmei, Jiang, Xiaofang, Cao, Yong, Su, Shi‐Jian
• Format: Journal Article
• Language: English
• Published: WEINHEIM Wiley 16.09.2019; Wiley Subscription Services, Inc
• Edition: International ed. in English
• Subjects: Aromatic compounds; Chemical compounds; Chemistry; Chemistry, Multidisciplinary; Crystal structure; Crystallinity; Crystals; Doped films; Emissions; Emitters; Energy transfer; Excitons; Fluorescence; intermolecular π–π stacking; luminescence; Optoelectronics; Organic chemistry; Organic crystals; Phosphorescence; Photoluminescence; Physical Sciences; Quantum chemistry; Quantum efficiency; Science & Technology; Sulfur; thermally activated delayed fluorescence; Triazine; ELECTROLUMINESCENCE EFFICIENCY; EXCITED SINGLET; energy transfer; ANISOTROPIC OPTICAL-PROPERTIES; INTRAMOLECULAR CHARGE-TRANSFER; LIGHT-EMITTING-DIODES; MOLECULAR DESIGN; thermally activated delayed fluorescence; luminescence; ROOM-TEMPERATURE PHOSPHORESCENCE; intermolecular pi-pi stacking; SINGLET OXYGEN GENERATION; TRIPLET ENERGY-LEVEL; QUANTUM EFFICIENCY; organic crystals; intermolecular π-π stacking
• ISSN: 1433-7851; 1521-3773; 1521-3773
• DOI: 10.1002/anie.201906371

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. A purely organic crystal exhibiting a bright thermally activated delayed fluorescence (TADF; photoluminescent quantum yield up to 87 %) and a strong intermolecular π–π stacking is presented. Built‐in spatially conjugated intra‐ and intermolecular transitions give rise to dual TADF emissions assisted by a hidden room temperature phosphorescent state.