Extreme Tuning of Redox and Optical Properties of Cationic Cyclometalated Iridium(III) Isocyanide Complexes

We report seven heteroleptic cationic iridium­(III) complexes
with
cyclometalating N-arylazoles and alkyl/aryl isocyanides, [(C∧N)2Ir­(CNR)2]­(CF3SO3),
and characterize two of them by crystal structure analysis. The complexes
are air- and moisture-stable white solids that have electronic transitions
at very high energy with absorption onset at 320–380 nm. The
complexes are difficult to reduce and oxidize; they exhibit irreversible
electrochemical processes with peak potentials (against ferrocene)
at −2.74 to −2.37 V (reduction) and 0.99–1.56
V (oxidation) and have a large redox gap of 3.49–4.26 V. The
reduction potential of the complex is determined by the azole heterocycle
(pyrazole or indazole) and by the isocyanide (tert-butyl or 2,6-dimethylphenyl) and the oxidation potential by the
Ir–aryl fragment [aryl = 2′,4′-R2-phenyl
(R = H/F), 9′,9′-dihexyl-2′-fluorenyl]. Three
of the complexes exhibit phosphorescence in argon-saturated dichloromethane
and acetonitrile solutions at room temperature with 0–0 transitions
at 473–478 nm (green color; the emission spectra are solvent-independent),
quantum yields of 3–25%, and long excited-state lifetimes of
62–350 μs. All of the complexes are phosphorescent at
77 K with 0–0 transitions at 387–474 nm (blue to green
color). The extremely long calculated radiative lifetimes, 0.5–3.5
ms, confirm that the complexes emit from a cyclometalating-ligand-centered
excited state.