Effect of Shielding Substituents on Performance of Blue Thermally Activated Delayed Fluorescent Molecules

In designing efficient and stable blue thermally activated delayed fluorescence (TADF) molecules aimed for the application of organic light-emitting diodes (OLEDs), accelerating the reverse intersystem crossing rate constant (k_RISC) is the primary factor in reducing the triplet population. However, despite achieving a relatively high k_RISC of over 10⁶ s⁻¹, the operational stability of blue TADF-OLEDs is still unsatisfactory. On the other hand, another critical issue originates from intermolecular interactions during electrical excitation, leading to unwanted degradation events such as exciton-polaron annihilation. Thus, “shielding” substituents are incorporated to protect the emitter molecules from unwanted interactions. To address this issue, six TADF emitters having different bulky shielding substituents: 1Ph, 2Ph, 3Ph, 4Ph, CzPh, and m2PhCz are designed, and confirmed that the OLED stability is extended by introducing shielding substituents.