Structurally robust phosphorescent [Pt(O^N^C^N)] emitters for high performance organic light-emitting devices with power efficiency up to 126 lm W−1 and external quantum efficiency over 20%

Abstract

A series of robust, bulky and strongly emissive platinum(II) complexes supported by tetradentate O^N^C^N ligands with tert-butyl groups (1–4), a bridging tertiary amine (5) or a biphenyl group with a spiro linkage (6) at the periphery of the [O^N^C^N] ligand scaffold have been prepared. Their photophysical properties were examined by absorption and emission spectroscopy, density functional theory calculations, and ultra-fast time-resolved emission measurements. These complexes display emission quantum yields of up to 95%, with emission maxima λ_max in the range of 522 to 570 nm, and have a good thermal stability of up to T_d > 423 °C. Notably, the k_q values of 4–6 are in the range of 8.5 × 10⁶ to 2.0 × 10⁷ mol⁻¹ dm³ s⁻¹, smaller than those (∼10⁸ to 10⁹ mol⁻¹ dm³ s⁻¹) of other reported Pt(II) complexes. The bulky groups at the periphery of the [O^N^C^N] ligand disfavour intermolecular interactions and hence excimer formation in solutions. These complexes are good light-emitting materials (dopants) for OLEDs, since the triplet–triplet annihilation (TTA) and concentration quenching effect arising from intermolecular interactions can be minimized even at a high dopant concentration. The efficiency of the devices fabricated with 4–6 increased with dopant concentration up to a high level of 10% with no extra emitting component or significant shift in the CIE observed. The maximum power efficiency (PE) values achieved for the 5 (yellow-emitting) and 6 (green-emitting) based devices were 118 and 126 lm W⁻¹, respectively. These PE values are the highest among the reported Pt(II)-OLEDs and comparable to those of the best reported Ir(III)-OLEDs without the out-coupling technique. Complex 7 is structurally analogous to, but less bulky than 3–6 and is prone to giving excimer emission in the solid state. A high PE of up to 55.5 lm W⁻¹ and external quantum efficiency of up to 25.1% have been realized in the white OLEDs fabricated with 7 as a single emitting material. These values are comparable with those of the best reported WOLEDs based on a single emitting material.