Steric and Electronic Influence of Aryl Isocyanides on the Properties of Iridium(III) Cyclometalates

Cyclometalated
iridium complexes with efficient phosphorescence and good electrochemical
stability are important candidates for optoelectronic devices. Isocyanide
ligands are strong-field ligands: when attached to transition metals,
they impart larger HOMO–LUMO energy gaps, engender higher oxidative
stability at the metal center, and support rugged organometallic complexes.
Aryl isocyanides offer more versatile steric and electronic control
by selective substitution at the aryl ring periphery. Despite a few
reports of alkyl isocyanide of cyclometalated iridium­(III), detailed
studies on analogous aryl isocyanide complexes are scant. We report
the synthesis, photophysical properties, and electrochemical properties
of 11 new luminescent cationic biscyclometalated bis­(aryl isocyanide)­iridium­(III)
complexes. Three different aryl isocyanides2,6-dimethylphenyl
isocyanide (CNArdmp), 2,6-diisopropylphenyl isocyanide
(CNArdipp), and 2-naphthyl isocyanide (CNArnap)were combined with four cyclometalating ligands with differential
π–π* energies2-phenylpyridine (ppy), 2,4-difluorophenylpyridine
(F2ppy), 2-benzothienylpyridine (btp), and 2-phenylbenzothiazole
(bt). Five of them were crystallographically characterized. All new
complexes show wide redox windows, with reduction potentials falling
in a narrow range of −2.02 to −2.37 V and oxidation
potentials spanning a wider range of 0.97–1.48 V. Efficient
structured emission spans from the blue region for [(F2ppy)2Ir­(CNAr)2]­PF6 to the orange
region for [(btp)2Ir­(CNAr)2]­PF6,
demonstrating that isocyanide ligands can support redox-stable luminescent
complexes with a range of emission colors. Emission quantum yields
were generally high, reaching a maximum of 0.37 for two complexes,
whereas btp-ligated complexes had quantum yields below 1%. The structure
of the CNAr ligand has a minimal effect on the photophysical properties,
which are shown to arise from ligand-centered excited states with
very little contribution from metal-to-ligand charge transfer in most
cases.