Synthesis and characterization of phosphorescent cyclometalated iridium complexes

Article abstract of DOI:10.1021/ic0008969

The preparation, photophysics, and solid state structures of octahedral organometallic Ir complexes with several different cyclometalated ligands are reported. IrC1₃·nH₂O cleanly cyclometalates a number of different compounds (i.e., 2-phenylpyridine, 2-(p-tolyl)pyridine, benzoquinoline, 2-phenylbenzothiazole, 2-(1-naphthyl)benzothiazole, and 2-phenylquinoline), forming the corresponding chloride-bridged dimers, C∧N₂Ir(μ-C1)₂IrC∧N₂ (C∧Nis a cyclometalated ligand) in good yield. These chloride-bridged dimers react with acetyl acetone (acacH) and other bidentate, monoanionic ligands such as picolinic acid (picH) and N-methylsalicylimine (salH), to give monomeric C∧N₂Ir(LX) complexes (LX = acac, pic, sal). The emission spectra of these complexes are largely governed by the nature of the cyclometalating ligand, leading to λ_max values from 510 to 606 nm for the complexes reported here. The strong spin-orbit coupling of iridium mixes the formally forbidden ³MLCT and ³π-π*transitions with the allowed ¹MLCT, leading to a strong phosphorescence with good quantum efficiencies (0.1-0.4) and room temperature lifetimes in the microsecond regime. The emission spectra of the C∧N₂Ir(LX) complexes are surprisingly similar to the fac-IrC∧N₃ complex of the same ligand, even though the structures of the two complexes are markedly different. The crystal structures of two of the C∧N₂Ir(acac) complexes (i.e., C∧N = ppy and tpy) have been determined. Both complexes show cis-C,C′, trans-N,N′ disposition of the two cyclometalated ligands, similar to the structures reported for other complexes with a C∧N₂Ir fragment. NMR data (¹H and ¹³C) support a similar structure for all of the C∧N₂Ir(LX) complexes. Close intermolecular contacts in both (ppy)₂Ir(acac) and (tpy)₂Ir(acac) lead to significantly red shifted emission spectra for crystalline samples of the ppy and tpy complexes relative to their solution spectra.