Improvement of the stability of blue phosphorescent dopant material is one of the key factors for real application of organic light-emitting diodes (OLEDs). In this study, we found that the intramolecular hydrogen bonding in an ancillary ligand from a heteroleptic Ir(III) complex can play an important role in the stability of blue phosphorescence. To rationalize the role of intramolecular hydrogen bonding, a series of Ir(III) complexes is designed and prepared: Ir(dfppy)2(pic-OH) (1a), Ir(dfppy)2(pic-OMe) (1b), Ir(ppy)2(pic-OH) (2a), and Ir(ppy)2(pic-OMe) (2b). The emission lifetime of Ir(dfppy)2(pic-OH) (1a) (τem = 3.19 μs) in dichloromethane solution was found to be significantly longer than that of Ir(dfppy)2(pic-OMe) (1b) (τem = 0.94 μs), because of a substantial difference in the nonradiative decay rate (knr = 0.28 × 10(5) s(-1) for (1a) vs 2.99 × 10(5) s(-1) for (1b)). These results were attributed to the intramolecular OH···O═C hydrogen bond of the 3-hydroxy-picolinato ligand. Finally, device lifetime was significantly improved when 1a was used as the dopant compared to FIrpic, a well-known blue dopant. Device III (1a as dopant) achieved an operational lifetime of 34.3 h for an initial luminance of 400 nits compared to that of device IV (FIrpic as dopant), a value of 20.1 h, indicating that the intramolecular hydrogen bond in ancillary ligand is playing an important role in device stability.

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.

To explore the excited-state electronic structure of the blue-emitting Ir(dmp) 3 dopant material (dmp = 3-(2,6-dimethylphenyl)-7-methylimidazo[1,2- f ]phenanthridine), which is notable for durable blue phosphorescent organic light-emitting diode (PhOLED), a series of homoleptic dmp-based Ir(III) complexes ( DMP–R , tris[3-(2,6-dimethylphenyl)-7- R -imidazo[1,2- f ]phenanthridin-12-yl-κ C 12 ,κ N 1 ]iridium, R = H, CH 3 , F, and CF 3 ) were prepared by introducing an electron-donating group (EDG; −CH 3 ) or an electron-withdrawing group (EWG; −F and −CF 3 ) at the 7-position of the imidazo-phenanthridine ligand. The photophysical analysis demonstrated that the alteration from EDG to EWGs led to redshifted structureless emission profiles, which were correlated with variations in the 3 MLCT/ 3 ILCT ratio in the T 1 excited state. From electrochemical studies and density functional theory calculations, it turned out that the excited-state nature of the dmp-based Ir(III) complexes was significantly affected by the inductive effect of the 7-substituent of the cyclometalating dmp ligand. As a result of the lowest unoccupied molecular orbital energy stabilization by the EWGs that suppressed the non-radiative pathway from the emissive triplet excited state to the 3 d – d state, the F- and CF 3 -modified Ir(dmp) 3 complexes ( DMP–F and DMP–CF 3 ) showed quantum yields of 27–30% in the solution state, which were at least 4- or 5-fold higher than those shown by DMP–H and DMP–CH 3 . A PhOLED device based on DMP–CF 3 [CIE chromaticity (0.17, 0.39)], which demonstrated a distinct 3 MLCT characteristic, exhibited better electroluminescent efficiencies with an external quantum efficiency of 13.5% than that based on DMP–CH 3 .

To elucidate the key parameters governing the emission properties of phenylimidazole (pim)-based Ir(III) emitters, including their electronic structure and the bulky aryl substitution effect, a series of pim-based iridium(III) complexes (Ir( R pim-X) 3 , R pim-X = 1-R-2-(X-phenyl)-1H-imidazole) bearing secondary pendants of increasing bulkiness [R = methyl (Me), phenyl (Ph), terphenyl (TPh), or 4-isopropyl terphenyl (ITPh)] and three different primary pim ligands (X = F, F 2 , and CN) were designed and synthesized. Based on photophysical and electrochemical analyses, it was found that the excited state properties are highly dependent on the bulkiness of the secondary substituent and the inductive nature of the primary pim ligand. The incorporation of bulky TPh/ITPh substituents in the second coordination sphere significantly enhanced the emission efficiencies in the solid state (Φ PL = 72.1−84.9%) compared to those of the methyl-or phenyl-substituted Ir(III) complexes (Φ PL = 30.4% for Ir( Me pim) 3 and 63.7% for Ir( Ph pim) 3 ). Further modification of the secondary aryl substituent (Ir( TPh pim) 3 → Ir( ITPh pim) 3 ) through the incorporation of an isopropyl group and F substitution on the primary pim ligand (Ir( TPh/ITPh pim) 3 → Ir( TPh/ITPh pim-F/F 2 ) 3 ) resulted in a slight decrease in the LUMO and a significant decrease in the HOMO energy levels, respectively; these energy level adjustments consequently amplified emission blue shifts, thereby enabling efficient blue electroluminescence in phosphorescent organic light-emitting diodes. Theoretical calculations revealed that the excited-state properties of pim-based Ir(III) complexes can be modulated by the nature of the peripheral substituent and the presence of an EWG substituent. Among the fabricated blueemitting TPh/ITPh-substituted Ir(III) complexes, Ir( ITPh pim-F) 3 , Ir( TPh pim-F 2 ) 3 , and Ir( ITPh pim-F 2 ) 3 were tested as blue-emitting dopants for blue phosphorescent OLEDs owing to their high solid radiative quantum yields (Φ PL = 75.9−84.9%). The Ir( ITPh pim-F) 3 -doped multilayer device displayed the best performance with a maximum external quantum efficiency of 21.0%, a maximum current efficiency of 43.6 cd/A, and CIE coordinates of 0.18 and 0.31.

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