Blue Phosphorescence of Trifluoromethyl- and Trifluoromethoxy-Substituted Cationic Iridium(III) Isocyanide Complexes

We report the first comprehensive comparative synthetic,
structural,
electrochemical, and spectroscopic study of an extended series of
fluorocarbon-modified iridium­(III) complexes. We prepared seven new
cationic Ir­(III) complexes with tert-butyl isocyanide
and trifluoromethyl- or trifluoromethoxy-substituted cyclometalating
2-phenylpyridines, [(C∧N)2Ir­(CNtBu)2]­(CF3SO3), and characterized
five of them by crystal structure analysis. The redox potentials and
photophysical properties of Ir­(III) complexes are determined by the
type, position, and number of fluorocarbon groups in the cyclometalating
ligand. The complexes exhibit pale blue to yellow-green phosphorescence
at room temperature with quantum yields and excited-state lifetimes
up to 73% and 84 μs in solution (under argon) and 7.5% and 4.3
μs in neat solid (under air). The structured and solvent-independent
phosphorescence spectra, with 0–0 emission transition at 445–467
nm, and the long calculated radiative lifetimes, 43–160 μs,
indicate that the complexes emit from a cyclometalating-ligand-centered
triplet excited state. Bulky fluorocarbon groups prevent intermolecular
interaction (aggregation) of the complexes, thereby minimizing red-shift
of phosphorescence color in going from solution to neat solid.