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>

Simultaneous enhancement of thermally activated delayed fluorescence and photoluminescence quantum yield via homoconjugation

“…As a direct consequence of the homoconjugation in 2, simultaneous improvements to PLQY, k S r , t TADF , k rISC , and DE ST were achieved, in-line with TD-DFT calculations which predicted a synergistic enhancement of oscillator strength (f) and spin-orbit coupling (SOC). Owing to the facile synthesis of this new system, and the ubiquity of the pyrazine moiety in state-of-the-art TADF materials across the electromagnetic spectrum from the deep blue to the nearinfrared (NIR), [44][45][46][47][48][49] this concept has great promise for generality. NIR TADF emitters in particular are poised to benefit from the enhancement in radiative transitions offered by homoconjugation and will be explored in future work.…”

“…Three factors led to the selection of this system: (1) the opportunity for comparison with the homologous fin 1, a known TBCT-TADF emitter, 43 (2) the facile synthesis (Scheme S2.1, ESI †) of quinoxaline-based iptycenes, and (3) the ubiquity of the pyrazine motif in TADF materials. [44][45][46][47][48][49][50] The NMR characterisation of the compounds can be found in the ESI † (Fig. S3.1-S3.5).…”

Synthesis of Stilbenes Using Various Catalysts and Investigation of their Optical Properties

“…Additionally, to verify the experimental results, absorption spectra of E and Z isomers were computed. For this purpose, meta-hybrid M06-2X functional 36 and 6-31G(d,p) 37 basis set were used for ground-state optimizations using the Gaussian09 software package. 38 Single point energy refinements were performed using B2PLYP, 39 PBE1PBE 40 and ωB97XD 41 levels of theory to obtain relative energies of E and Z isomers.…”