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

Abstract

<p>Two blue thermally activated delayed fluorescence (TADF) emitters bearing di-<i>tert</i>-butyl carbazoles as the electron donor groups and pyrazine (<b>DTCz-Pz</b>) or dipyrazine (<b>DTCz-Pz</b>) as the electron acceptor are presented. The DFT calculations predict <b>DTCz-Pz</b> and <b>DTCz-DPz</b> to possess high S<sub>1</sub> energies (3.19 eV and 3.08 eV, respectively), and relatively large E<sub>ST</sub> values (0.52 eV and 0.56 eV, respectively). The closely layered intermediate triplet states between S<sub>1</sub> and T<sub>1</sub>, 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<sub>ST</sub>s. The ΔE<sub>ST</sub>s for <b>DTCz-Pz</b> and <b>DTCz-DPz</b> are 0.27 eV and 0.38 eV, and both molecules show high photoluminescence quantum yields (65%, and 70%, respectively) and the decay lifetimes show temperature dependence in a PPT host, which is consistent that both molecules are TADF emitters in PPT. The OLEDs based on <b>DTCz-Pz</b> exhibit deep blue emission with λ<sub>EL</sub> of 460 nm and CIE of (0.15, 0.16). The maximum external quantum efficiency (EQE<sub>max</sub>) reaches 11.6%, with a maximum luminance (L<sub>max</sub>) of up to 6892 cd m<sup>-2</sup>, while the device based on <b>DTCz-DPz</b> exhibits sky blue emission with λ<sub>EL</sub> of 484 nm and CIE of (0.15, 0.30), an EQE<sub>max</sub> of 7.2%, and L<sub>max</sub> of 8802 cd m<sup>-2</sup>.</p>