Cationic Ir(III) Complexes Featuring Phenylimidazole-type Cyclometalated Ligands: Fluorine-Free Blue Phosphorescent Emitters for Light-Emitting Devices

Abstract

The development of blue emissive cationic Ir­(III) complexes with no fluorine substitutions but with sufficient blue color purity and high phosphorescence efficiency has remained challenging. Here, fluorine-free cyan to deep blue emissive cationic Ir­(III) complexes with phenylimidazole-type cyclometalated ligands (C∧N) are reported, which are [Ir­(dphim)2(dmapzpy)]­PF6 (1), [Ir­(ipr-dphim)2(dmapzpy)]­PF6 (2), [Ir­(ipr-dphim)2(bipz)]­PF6 (3), and [Ir­(ipr-dphim)2(bicb)]­PF6 (4). 1,2-Diphenyl-1H-imidazole (dphim) and 1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole (ipr-dphim) are the phenylimidazole-type C∧N ligands, and 4-dimethylamino-2-(1H-pyrazol-1-yl)­pyridine (dmapzpy), di­(1H-pyrazol-1-yl)­methane (bipz), and 3,3′-methylenebis­(1-methyl-1H-imidazol-3-ium-2-ide) (bicb) are the neutral ancillary ligands (A∧A). In both solution and diluted films, complex 1 shows a cyan emission with the emission maxima at ∼472 and 495 nm, and complexes 2–4 provide a deep blue emission with the emission maxima at ∼460 and 480 nm. While the complexes exhibit low to moderate phosphorescence efficiencies (0.05–0.35) in a degassed CH3CN solution, they exhibit high phosphorescence efficiencies (up to 0.82) in diluted films. Theoretical calculations revealed that the mixed 3π–π* (C∧N-centered)/3MLCT (Ir → C∧N) states are responsible for the emission afforded by complexes 1–4, which undergo nonradiative deactivations induced by different types of metal-centered states. Organic light-emitting diodes with complexes 1–4 as phosphorescent dopants are fabricated by a solution process, which affords a blue-green to blue emission with the emission maxima at ∼460 and 490 nm for the blue devices and a high current efficiency at 28.1 cd A–1 for the blue-green device. Solid-state light-emitting electrochemical cells are also fabricated with complexes 1–2 as phosphorescent dopants, which provide green-blue to blue emission with a high luminance (up to 840 cd m–2) and current-efficiency (up to 16.8 cd A–1) under a constant-current driving. The work reveals that, by using phenylimidazole-type C∧N ligands and optimized A∧A ligands, blue emissive cationic Ir­(III) complexes with no fluorine substitutions but with sufficient blue-color purity and a high phosphorescence efficiency can be developed.

On the importance of equatorial metal-centered excited states in the photophysics of cyclometallated Ir(iii) complexes

“…Regarding their energy positions, it is important to stress that in all the cases reported in the literature, the 3 MC eq state appears to be lower in energy than the 3 MC ax one, consequently making its involvement in principle more relevant than that of the 3 MC ax state. [18][19][20][21][22][23][24] As a final remark, similar MC states were also found while studying the photorelease of N^N ligands in some complexes of the [Ru(N^N) 3 ] 2+ family. [25][26][27][28][29] In the present work, the photophysical properties of [Ir ( ppy) 2 (bpy)] + , [Ir( ppy) 2 ( pbpy)] + and [Ir( ppy) 2 (dpbpy)] + (hereafter complexes 1, 2 and 3, respectively, see Fig.…”

“…Regarding their energy positions, it is important to stress that in all the cases reported in the literature, the 3 MC eq state appears to be lower in energy than the 3 MC ax one, consequently making its involvement in principle more relevant than that of the 3 MC ax state. 18–24 As a final remark, similar MC states were also found while studying the photorelease of N^N ligands in some complexes of the [Ru(N^N) 3 ] 2+ family. 25–29…”

Tailoring the π-system of benzimidazole ligands towards stable light-harvesting cyclometalated iridium(iii) complexes

“…Unique photophysical properties of octahedral d 6 cyclometalated iridium complexes in combination with their high stability determine current and potential application of these complexes [1][2][3][4][5] in organic light emitting diodes (OLEDs), [6][7][8] photodynamic therapy and bioimaging, [9][10][11][12][13] as homogeneous photocatalysts, 14,15 biosensors, [16][17][18] building blocks for supramolecular architectures [19][20][21] and photosensitizers in dyesensitized solar cells (DSSC). [22][23][24][25][26][27] The latter technology is experiencing a renaissance, 28 being highly attractive for several niche applications (mostly indoors) for which diffuse light performance, transparency and flexibility are more important factors than the high efficiency.…”