Extending rigid electron-deficient skeletons and appending electron-rich units to build high-efficiency red-emitting pyrene-derived TADF materials

Abstract

In order to develop novel pyrene-derived thermally activated delayed fluorescence (TADF) materials and deeply study their structure–property relationship, two emitters of PyPzCN-TPA₂ and PyQlCN-TPA₂ were designed and synthesized by alternating dicyano pyrenopyrazine (PyPzCN) and dicyano pyrenoquinoxaline (PyQlCN) as the rigid electron-deficient skeleton, as well as triphenylamine (TPA) as an electron-rich unit anchored at the K-region of pyrene, respectively. The effects of the rigid electron-deficient skeleton and electron-rich unit on photophysical and electroluminescence (EL) properties were primarily studied. The extending rigid electron-deficient skeleton made PyQlCN-TPA₂ exhibit red-shifted emission at 658 nm in toluene and 840 nm in its neat film. The appending electron-rich unit at the K-region made both emitters exhibit large steric hindrance with a torsion angle of more than 70°, which resulted in a small ΔE_ST value of about 0.032 eV and a high k_RISC of about 1.6–4.7 × 10⁵ s⁻¹. As a result, the solution-processed PyPzCN-TPA₂ and PyQlCN-TPA₂-doped OLEDs simultaneously exhibited high EQEs of 9.30% at 588 nm and 8.94% at 630 nm, respectively. The EQE roll-off was efficiently suppressed with an increase of emission wavelength. This is the first time that high-efficiency red-emitting pyrene-derived TADF materials were obtained by extending the rigid electron-deficient skeleton and appending the electron-rich unit at the K-region for solution-processed OLEDs.