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

Practical strongly phosphorescent platinum( II) green to white OLED emitters with bulky tetradentate O^N^C^N ligands.

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 × 10 ⁶ to 2.0 × 10 ⁷ mol ⁻¹ dm ³ s ⁻¹, smaller than those (∼10 ⁸ to 10 ⁹ mol ⁻¹ dm ³ 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.