Rational design of orange-red iridium(iii) complexes by an isomer engineering strategy for improved performance of white organic light-emitting diodes

Abstract

White organic light-emitting diodes (WOLEDs) achieving high electroluminescence (EL) performance and tiny efficiency roll-off concurrently have been highly desired for practical application. Orange-red luminescence is well-known as an important component of WOLEDs, however, the development of such materials with high efficiency is still challenging due to the relatively poor emission caused by the energy-gap law and the concentration quenching phenomenon. Herein, two orange-red emitting iridium(III) complexes Ir(pq)₂pbi and Ir(piq)₂pbi with isomeric cyclometalated ligands, i.e. 2-phenylquinoline and 1-phenylisoquinoline, are designed and synthesized. The structural modulation adjusts not only the conjugation degree but also the molecular packing of both complexes, resulting in distinctly different photophysical and EL performance. The utility of Ir(pq)₂pbi in a monochromic device shows a current efficiency (η_c) of 36.7 cd A⁻¹, a power efficiency (η_p) of 22.6 lm W⁻¹, and an external quantum efficiency (η_ext) as high as 16.0% along with negligible efficiency roll-off, which are much better than those of Ir(piq)₂pbi. Two-color complementary WOLEDs based on Ir(pq)₂pbi as a dopant realize an η_c of 39.8 cd A⁻¹, an η_p of 31.3 lm W⁻¹, and an η_ext of 17.3% with a low η_ext roll-off ratio of 2.3%, respectively. Such excellent electroluminescence performance for monochromic and WOLEDs suggests the potential of Ir(pq)₂pbi for further display application and will guide further molecular design in the future.