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Bright Blue Phosphorescence from Cationic Bis-Cyclometalated Iridium(III) Isocyanide Complexes

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journal contribution
posted on 22.02.2016, 05:30 by Nail M. Shavaleev, Filippo Monti, Rubén D. Costa, Rosario Scopelliti, Henk J. Bolink, Enrique Ortí, Gianluca Accorsi, Nicola Armaroli, Etienne Baranoff, Michael Grätzel, Mohammad K. Nazeeruddin
We report new bis-cyclometalated cationic iridium­(III) complexes [(CN)2Ir­(CN-tert-Bu)2]­(CF3SO3) that have tert-butyl isocyanides as neutral auxiliary ligands and 2-phenylpyridine or 2-(4′-fluorophenyl)-R-pyridines (where R is 4-methoxy, 4-tert-butyl, or 5-trifluoromethyl) as CN ligands. The complexes are white or pale yellow solids that show irreversible reduction and oxidation processes and have a large electrochemical gap of 3.58–3.83 V. They emit blue or blue-green phosphorescence in liquid/solid solutions from a cyclometalating-ligand-centered excited state. Their emission spectra show vibronic structure with the highest-energy luminescence peak at 440–459 nm. The corresponding quantum yields and observed excited-state lifetimes are up to 76% and 46 μs, respectively, and the calculated radiative lifetimes are in the range of 46–82 μs. In solution, the photophysical properties of the complexes are solvent-independent, and their emission color is tuned by variation of the substituents in the cyclometalating ligand. For most of the complexes, an emission color red shift occurs in going from solution to neat solids. However, the shift is minimal for the complexes with bulky tert-butyl or trifluoromethyl groups on the cyclometalating ligands that prevent aggregation. We report the first example of an iridium­(III) isocyanide complex that emits blue phosphorescence not only in solution but also as a neat solid.