Unsymmetric 2-phenylpyridine (ppy)-type cyclometalated Ir(iii) complexes bearing both 5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene and phenylsulfonyl groups for tuning optoelectronic properties and electroluminescence abilities

Abstract

A series of 2-phenylpyridine (ppy)-type cyclometalating ligands containing rigid 5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene (OBA) units have been developed. Together with different ppy-type cyclometalating ligands with a phenylsulfonyl group (–SO₂Ph) substituted at different positions, new unsymmetric heteroleptic cyclometalated Ir(III) complexes have been successfully synthesized. Their photophysical, electrochemical and electroluminescence (EL) behaviors have been fully characterized. In addition, both time-dependent functional theory (TD-DFT) and natural transition orbital (NTO) theoretical results have been obtained to gain insight into the optoelectronic behaviors of these unsymmetric Ir(III) complexes with OBA units. The NTO results indicate that the features of the lowest triplet excited states (T₁) can be delicately tuned by combination between the OBA-based cyclometalating ligand and another ppy-type ligand with the –SO₂Ph group. Importantly, these unsymmetric Ir(III) complexes can exhibit very high phosphorescence quantum yield (Φ_p) higher than 0.90 due to the highly rigid OBA unit which can effectively restrain non-radiative processes. In addition, an electron injection/transporting (EI/ET) ability can be conferred to these unsymmetric heteroleptic cyclometalated Ir(III) complexes by both OBA and –SO₂Ph moieties. Taking advantage of these merits, organic light-emitting diodes (OLEDs) based on these unsymmetric heteroleptic cyclometalated Ir(III) complexes can show a high maximum external quantum efficiency (η_ext) of 12.8%, a current efficiency (η_L) of 51.1 cd A⁻¹, and a power efficiency (η_P) of 40.5 lm W⁻¹ for a yellowish-green device, while EL efficiencies for red devices are 6.8%, 13.9 cd A⁻¹, and 12.8 lm W⁻¹, respectively. All these results indicate the good potential of the rigid OBA unit in developing functional phosphorescent emitters.