A thermally activated delayed fluorescent platinum(ii) complex for red organic light emitting diodes with high efficiencies and small roll-off

Abstract

Although phosphorescent Pt(II) complexes have shown great potential for organic light-emitting diodes (OLEDs), red phosphorescence with a high photoluminescence quantum yield (PLQY) and a short emission lifetime is still scarce because of the limitation on the radiative decay rate. Transition metal complexes have proved advantageous for emitting thermally activated delayed fluorescence (TADF), but TADF Pt(II) complexes have rarely been reported. Herein, we describe the design and synthesis of two red TADF Pt(II) complexes, namely Pt-tCzDBPZ (Pt1) and Pt2-tCzDBPZ (Pt2). By using a donor–acceptor type monodentate ligand, the complexes have metal-perturbed intraligand charge-transfer (MPICT) excited states featuring small singlet–triplet energy gaps (0.086–0.089 eV) and a moderately large spin–orbit coupling matrix element (∼10 cm⁻¹). The complexes exhibit red TADF emissions (ca. 630 nm) with high PLQYs of 82–92% and delayed fluorescence lifetimes of 1.51–1.63 μs. The vacuum-deposited red OLEDs based on Pt-tCzDBPZ show high external quantum efficiencies of up to 35.6% and small roll-offs. In spite of the significant heavy atom effect of Pt for facilitating phosphorescence, this study demonstrates a feasible design of Pt(II)–TADF emitters which can enable them to outperform phosphorescent Pt(II) complexes.