Large Increase in External Quantum Efficiency by Dihedral Angle Tuning in a Sky‐Blue Thermally Activated Delayed Fluorescence Emitter

Abstract Efficient and stable blue emitters for organic light‐emitting diodes are urgently needed for next‐generation display and lighting applications. The discovery of thermally activated delayed fluorescence (TADF) has revealed a new class of promising candidates. After pairing the iminodibenzyl donor with the triazine acceptor via a phenylene linker, dihedral angle tuning is employed to regulate the difference between the energy levels of singlet and triplet excited states. Enhanced reverse intersystem crossing rates are observed in response to increased methylation at the phenylene linker. This behavior agrees with the density functional theory calculations. Photoluminescence quantum yields of up to 98% are achieved upon doping into a solid‐state matrix. When incorporated in devices, the maximum external quantum efficiency is 28.3% for the emitter with the most favorable trade‐off between singlet–triplet splitting and fluorescent oscillator strength. This result highlights the general applicability of dihedral angle tuning, a molecular design strategy that can be used to improve the performance of donor–acceptor type TADF emitters without significantly changing their emission spectra.