1,4-Bis(trifluoromethyl)benzene as a new acceptor for the design and synthesis of emitters exhibiting efficient thermally activated delayed fluorescence and electroluminescence: experimental and computational guidance

Abstract

1,4-Bis(trifluoromethyl)benzene as a new acceptor with hydrogen bonding sites together with phenoxazine, phenothiazine or 9,9-dimethyl-9-10-dihydroacridine as donor moieties was used for the design and synthesis of compounds with symmetrical donor–acceptor–donor architectures as emitters exhibiting thermally activated delayed fluorescence (TADF). The molecules exhibited large dihedral angles between the donor and acceptor moieties which are close to 80° as was shown by single crystal X-ray analysis and theoretical calculations. The compounds showed very broad charge-transfer-state (¹CT) absorption which can be accounted for by multiple ¹CTs as indicated by quantum molecular dynamics simulations. The magnitude of oscillatory strength increases with deviation away from the orthogonality of the dihedral angle between the donor and acceptor and the presence of in-plane bending of the two donors where the donors swing back and forth with respect to the acceptor at C–N bonds. The localised triplet excited states (³LEs) were experimentally obtained. Although a very small and similar singlet and triplet splitting of ca. 20 meV was observed for the compounds, its reverse intersystem crossing rates were different and ranged from 1.92 × 10⁴ to 5.45 × 10⁵ s⁻¹ due to the different energy gap between the ¹CT and ³LE. A 9,9-dimethyl-9-10-dihydroacridine based compound was shown to be a promising cyan TADF emitter. The selection of the right donor with the appropriate ³LE that matches the charge transfer states is important to obtain an efficient TADF emitter. The X-Ray study of the packing pattern in the crystals of the compounds revealed that the molecules are held together through many weak van der Waals intramolecular bonds, which are formed between the CF₃ fluorine atoms and hydrogen atoms of methyl groups or the carbon and hydrogen atoms of phenyl rings (C–H⋯F, C–F⋯N, C–H⋯H and C–H⋯C with distances smaller than 2.85 Å). Because of that, this compound emitted cyan electroluminescence with unusually stable colours at different emitter concentrations and different voltages in devices. The efficiency at a brightness of 1000 cd m⁻² was practically the same as the maximum one due to the extremely low efficiency roll-off.