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

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posted on 28.01.2013, 00:00 by Nail M. Shavaleev, Filippo Monti, Rosario Scopelliti, Andrea Baschieri, Letizia Sambri, Nicola Armaroli, Michael Grätzel, Mohammad K. Nazeeruddin
We report seven heteroleptic cationic iridium­(III) complexes with cyclometalating N-arylazoles and alkyl/aryl isocyanides, [(CN)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.