Cyano-Isocyanide Iridium(III) Complexes with Pure\nBlue Phosphorescence

Abstract

In this paper, we report a series of six neutral, blue-phosphorescent\ncyclometalated iridium complexes of the type Ir­(C^Y)₂(CNAr)­(CN).\nThe cyclometalating ligands in these compounds (C^Y) are either aryl-substituted\n1,2,4-triazole or NHC ligands, known to produce complexes with blue\nphosphorescence. These cyclometalating ligands are paired with π-acidic,\nstrongly σ-donating cyano and aryl isocyanide (CNAr) ancillary\nligands, the hypothesis being that these ancillary ligands would destabilize\nthe higher-lying ligand-field (<i>d</i>–<i>d</i>) excited states, allowing efficient blue photoluminescence. The\ncompounds are prepared by substituting the cyanide ancillary ligand\nonto a chloride precursor and are characterized by NMR, mass spectrometry,\ninfrared spectroscopy, and, for five of the compounds, by X-ray crystallography.\nCyclic voltammetry establishes that these compounds have large HOMO–LUMO\ngaps. The mixed cyano-isocyanide compounds are weakly luminescent\nin solution, but they phosphoresce with moderate to good efficiency\nwhen doped into poly­(methyl methacrylate) films, with Commission Internationale\nde L’Eclairage coordinates that indicate deep blue emission\nfor five of the six compounds. The photophysical studies show that\nthe photoluminescence quantum yields are greatly enhanced in the cyano\ncomplexes relative to the chloride precursors, affirming the benefit\nof strong-field ancillary ligands in the design of blue-phosphorescent\ncomplexes. Density functional theory calculations confirm that this\nenhancement arises from a significant destabilization of the higher-energy\nligand-field states in the cyanide complexes relative to the chloride\nprecursors.