Planar and Rigid Pyrazine‐Based TADF Emitter for Deep Blue Bright Organic Light‐Emitting Diodes
Pachaiyappan Rajamalli, Dongyang Chen, Subeesh Madayanad Suresh, Youichi Tsuchiya, Chihaya Adachi, Eli Zysman‐Colman
Abstract
Abstract 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 S 1 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 S 1 and T 1 , 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 ST s. The ΔE ST s 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 .