Luminescent Iridium(III) Complexes Supported by N-Heterocyclic Carbene-based C^C^C-Pincer Ligands and Aromatic Diimines

Abstract

Iridium(III) hydrido complexes containing N-heterocyclic carbene (NHC)-based pincer ligand 1,3-bis(1-butylimidazolin-2-ylidene)phenyl anion (C1^C^C1) or 1,3-bis(3-butylbenzimidazolin-2-ylidene)phenyl anion (C2^C^C2) and aromatic diimine (2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), 4,4′-dimethyl-2,2′-bipyridine (Me2bpy), or dipyrido-[3,2-f:2′,3′-h]-quinoxaline (dpq)) in the form of [Ir(C^C^C)(N^N)(H)]+ have been prepared. Crystal structures for these complexes show that the Ir–CNHC distances are 2.043(5)–2.056(5) Å. The hydride chemical shifts for complexes bearing C1^C^C1 (−20.6 to −20.3 ppm) are more upfield than those with C2^C^C2 (−19.5 and −19.2 ppm), revealing that C1^C^C1 is a better electron donor than C2^C^C2. Spectroscopic comparisons and time-dependent density functional theory (TD-DFT) calculations suggest that the lowest-energy electronic transition associated with these complexes (λ = 340–530 nm (ε ≤ 103 dm3 mol−1 cm−1)) originate from a dπ(IrIII) → π*(N^N) metal-to-ligand charge transfer transition, where the dπ(IrIII) level contain significant contribution from the C^C^C ligands. All these complexes are emissive in the yellow-spectral region (553–604 nm in CH3CN and CH2Cl2) upon photo-excitation with quantum yields of 10−3–10−1.

Phosphorescent [3 + 2 + 1] coordinated Ir(iii) cyano complexes for achieving efficient phosphors and their application in OLED devices

“… 44 However, [3 + 2 + 1] iridium complexes containing tridentate ligands which bear bis(NHC) ligands are yet to be investigated. Wong et al reported a class of [3 + 2 + 1] iridium complexes 45 containing tridentate bis(NHC) ligands, wherein bipyridine served as the bidentate ligand and hydride as the monodentate ligand. As a result, these complexes are inherently salts, leading a hurdle in utilization through evaporated process into fabrication as PhOLEDs.…”

Iridium(III) Complexes Bearing Tridentate Chromophoric Chelate: Phosphorescence Fine-Tuned by Phosphine and Hydride Ancillary

“…This class of dianionic chelates is distinctive from tridentate chelates such as 1,3-bis­(benzimidazol-2-yl)­benzene, bis­(imidazol-2-ylidene) benzene, and tridentate polypyridyl chelate, employed by the research groups of Haga, De Cola, and Bernhard, from which higher emission efficiency was also recorded for the isolated Ir­(III) complexes. Moreover, the monoanionic ancillary chelate can be the bidentate or tridentate heteroaromatic cyclometalate with either terminal pyridyl-based donor(s) (i.e., chelates L (2) and L (3) ) or imidazolylidene appendage(s) (i.e., chelates L (4) , L (5) , and L (6) ). − Since the C-donor of imidazolylidene and analogues is known to exert relatively stronger metal–ligand bonding (or ligand field strength) than that of the N-based donor such as pyridine, the imidazolylidene-coordinated Ir­(III) emitters would exhibit better emission efficiency than those possessing only the N-donor. This strategy is especially crucial to the blue emitters with higher emission energy. − One recent report is the syntheses of bis-tridentate Ir­(III) phosphors bearing both 2-phenyl-6-(pyrazol-3-yl) pyridine class of chelate L (1) and di-imidazolylidene pincer of ancillary L (6) , and fabrication of the true-blue phosphorescent OLEDs using the same .…”

Fine Emission Tuning from Near-Ultraviolet to Saturated Blue with Rationally Designed Carbene-Based [3 + 2 + 1] Iridium(III) Complexes

“…340 nm (ε < 9.1 × 10 3 M −1 cm −1 ) are mainly assigned as the triplet metal-to-ligand and intraligand charge transfer ( 3 MLCT and 3 ILCT) transitions. 55 S49 in the Supporting Information). It has been well reported that emitters with a short lifetime will help alleviate the triplet−triplet annihilation), which is known as the triplet excitons' relaxation process and is the principal reason of the efficiency roll-off.…”

“…Apart from these, the [3 + 2 + 1]-coordinated iridium­(III) complexes with one tridentate, bidentate, and monodentate ligand have been increasingly reported recently. − Such versatile molecular design may offer an alternative pathway to fine-tune the photophysical and electrochemical properties of the emitters. Figure b illustrates the schematic structures of iridium­(III) complexes in [2 + 2 + 2], [3 + 3], or [3 + 2 + 1] coordination modes. ,,, Upon varying the bidentate ligand from substituted arylpyridine to NHC ligands, the emission energy of the [3 + 2 + 1]-coordinated iridium­(III) complexes could be effectively tuned to cover the whole visible region, ranging from 386 to 609 nm.…”