Planar and Rigid Pyrazine-Based TADF Emitter for Deep Blue Bright Organic Light-Emitting Diodes

Two blue thermally activated delayed fluorescence (TADF) emitters bearing di-tert-butyl carbazoles as the electron donor groups and pyrazine (DTCz-Pz) or dipyrazine (DTCz-Pz) as the electron acceptor are presented. The DFT calculations predict DTCz-Pz and DTCz-DPz to possess high S₁ energies (3.19 eV and 3.08 eV, respectively), and relatively large ΔE_ST values (0.52 eV and 0.56 eV, respectively). The closely layered intermediate triplet states between S₁ and T₁, predicted by DFT calculations, are expected to facilitate the reverse intersystem crossing (RISC) and improve spin-vibronic coupling efficiency between the excited states, even the relatively larger ΔE_STs. The ΔE_STs for DTCz-Pz and DTCz-DPz are 0.27 eV and 0.38 eV, and both molecules show high photoluminescence quantum yields (65 %, and 70 %, respectively). The decay lifetimes show temperature dependence in a PPT host, which is consistent with the fact that both molecules are TADF emitters in PPT. The OLEDs based on DTCz-Pz exhibit deep blue emission with λ_EL of 460 nm and CIE of (0.15, 0.16). The maximum external quantum efficiency (EQE_max) reaches 11.6 %, with a maximum luminance (L_max) of up to 6892 cd m⁻², while the device based on DTCz-DPz exhibits sky blue emission with λ_EL of 484 nm and CIE of (0.15, 0.30), an EQE_max of 7.2 %, and L_max of 8802 cd m⁻².