Ir(III)-Based Phosphors with Bipyrazolate Ancillaries; Rational Design, Photophysics, and Applications in Organic Light-Emitting Diodes

Liao, J.-L.; Chi, Y.; Sie, Z.-T.; Ku, C.-H.; Chang, C.-H.; Fox, M.A.; Low, P.J.; Tseng, M.-R.; Lee, G.-H.

Abstract

A series of three charge-neutral Ir(III) complexes bearing both neutral chelating ligands 4,4′-di-t-butyl-2,2′-bipyridine (dtbbpy) and monoanionic cyclometalated ligands derived from 2-phenylpyridine (ppyH), together with either two monoanionic ligands (i.e., chloride and monodentate pyrazolate) or a single dianionic chelate derived from 5,5′-di(trifluoromethyl)-3,3′-bipyrazole (bipzH2) or 5,5′-(1-methylethylidene)-bis-(3-trifluoromethyl-1H-pyrazole) (mepzH2), was successfully synthesized. These complexes are derived from a common, structurally characterized, Ir(III) intermediate complex [Ir(dtbbpy) (ppy)Cl2] (1), from treatment of IrCl3·3H2O with equal amount of the diimine (N^N) and precursor of the cyclometalated (C^N) ligands in a form of one-pot reaction. Treatment of 1 with various functional pyrazoles afforded [Ir(dtbbpy) (ppy) (pz)Cl] (2), [Ir(dtbbpy) (ppy) (bipz)] (3), and [Ir(dtbbpy) (ppy) (mepz)] (4), which display intense room-temperature emission with λmax spanning the region between 532 and 593 nm in both fluid and solid states. The Ir(III) complexes, 3 and 4, showcase rare examples of three distinctive chelates (i.e., neutral, anionic, and dianionic) assembled around the central Ir(III) cation. Hybrid density functional theory (DFT; B3LYP) electronic structure calculations on 1–4 reveal the lowest unoccupied molecular orbital to be π*(bpy) in character for all complexes and highest occupied molecular orbital (HOMO) offering d(Ir)−π(phenyl) character for 1, 2, and 4 and π(bipz) character for 3. The different HOMO composition of 3 and 4 is also predicted by calculations using pure DFT (BLYP) and wave function (MP2) methods. On the basis of time-dependent DFT calculations, the emissive processes are dominated by the phenyl group-to-bipyridine, ligand(ppy)-to-ligand(bpy) charge transfer admixed with metal-to-ligand transition for all Ir(III) complexes. Organic light emitting diodes were successfully fabricated. A double emitting layer design was adopted in the device architecture using Ir(III) metal complexes 3 and 4, attaining peak external quantum efficiencies, luminance efficiencies, and power efficiencies of 18.1% (59.0 cd/A and 38.6 lm/W) and 16.6% (53.3 cd/A and 33.5 lm/W), respectively.

Citation

Liao, J., Chi, Y., Sie, Z., Ku, C., Chang, C., Fox, M., …Lee, G. (2015). Ir(III)-Based Phosphors with Bipyrazolate Ancillaries; Rational Design, Photophysics, and Applications in Organic Light-Emitting Diodes. Inorganic Chemistry, 54(22), 10811-10821. https://doi.org/10.1021/acs.inorgchem.5b01835

• Journal Article Type: Article
• Acceptance Date: Oct 23, 2015
• Publication Date: Nov 16, 2015
• Deposit Date: Nov 26, 2015
• Publicly Available Date: Nov 3, 2016
• Journal: Inorganic Chemistry
• Print ISSN: 0020-1669
• Electronic ISSN: 1520-510X
• Publisher: American Chemical Society
• Peer Reviewed: Peer Reviewed
• Volume: 54
• Issue: 22
• Pages: 10811-10821
• DOI: https://doi.org/10.1021/acs.inorgchem.5b01835
• Public URL: https://durham-repository.worktribe.com/output/1426043