Theoretical Study on Thermally Activated Delayed Fluorescence Emitters in White Organic Light-Emitting Diodes: Emission Mechanism and Molecular Design.

Employing dual-emission thermally activated delayed fluorescence (TADF) molecules to fabricate single emitters in highly efficient white organic light-emitting diodes (WOLEDs) is still a challenge. In this work, a systematic study on the luminescence mechanism of three TADF molecules with blue-orange dual emission is performed. It is found that the quasi-axial (ax) conformer is responsible for blue normal fluorescence (NF), while the quasi-equatorial (eq) conformer is responsible for orange TADF. In addition, the upconversion for the TADF molecules happens from the first triplet state (T) and the second triplet state (T) to the first singlet state (S) in the quasi-equatorial conformers. In addition, the transition from T to S and T to S also happens in the process of conformation transformation. Besides, our study also indicates that the donor with an asymmetric bond length in the six-membered ring could favor the generation of dual conformations and dual emission. Some molecules with PTZ or PXZ as donors are predicted as potential emitters with white light emission. Our research would favor the design of TADF emitters with dual emission in WOLEDs.