Synthesis and Characterization of Blue Phosphorescent NHC-Ir(III) Complexes with Annulated Heterocyclic 1,2,4-Triazolophenanthridine Derivatives for Highly Efficient PhOLEDs

Efficient tris-bidentate Ir(III) phosphorescent dopants were prepared using a series of 1,2,4-triazolo[4,3-f]phenanthridine (tzp) moieties modified with aryl substituents (phenyl, tolyl, and xylenyl) as the main phenylimidazole-based N-heterocyclic carbene (NHC) chelates (C∧C:). According to the degree of the bulkiness of the aryl substituent and the ligation mode, the five prepared Ir(tzpC∧C:) complexes include four homoleptic NHC-Ir(III) complexes, fac-Ir(tzpPh)3, fac-/mer-Ir(tzpTol)3, and mer-Ir(tzpXyl)3, and one heteroleptic NHC-Ir(III) complex, cis-Ir(tzpPh)2(tzpPh)′, in which the phenyl moiety of one tzpPh ligand is abnormally ligated to the Ir metal center, unlike other tzp ligands. The Ir(III) complexes ligated by carbene ligands (tzpC∧C:) exhibited highly efficient emissions in the solid state (Φem = 23.2–54.0%). Electrochemical and theoretical studies revealed that the excited-state properties of these NHC-Ir(III) complexes are variable on the extent of planarity and π-conjugation of the tzpC∧C: chelating ligand. Due to its enhanced rigidity and low excited-state energy, a result of abnormal tzpPh ligand ligation, the heteroleptic cis-Ir(tzpPh)2(tzpPh)′ exhibited the most efficient emission properties in solution (Φem = 21.4%) and solid (Φem = 54.0%) media. Of the devices fabricated with Ir(tzpC∧C:)3 complexes as emitters, that doped with cis-Ir(tzpPh)2(tzpPh)′ exhibited superior electroluminescence efficiencies (external quantum efficiency (EQE) of 16.3%, current efficiency of 27.6 cd A–1, and power efficiency of 22.1 lm W–1) and CIE coordinates of [0.17,0.26], which are superior to those of other Ir(tzpC∧C:)3 complexes and Ir(dmp)3 (dmp = 3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridine). This study provides insight into the molecular-level engineering of Ir(III) dopant materials for improving the emission efficiencies of phosphorescent OLEDs.