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

We report new bis-cyclometalated cationic iridium­(III)
complexes
[(C∧N)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 C∧N 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.