Protonation-Dependent Luminescence of an Iridium(III) Bibenzimidazole Chromophore

We present the synthesis and a detailed structural and photophysical investigation of the bis(phenylpyridine)iridium(III) complex of 4,4’,5,5’-tetramethyl-2,2’-bibenzimidazole [Ir(tmBBI)-H2] and its homodinuclear complex [Ir(tmBBI)Ir]. Their structures were determined by single-crystal X-ray diffraction analysis and NMR spectroscopy, and detailed UV/ Vis and emission spectroscopy studies were performed. Ir(tmBBI)-Hx shows strong protonation-state-dependent luminescence and is luminescent in all three of its protonation states, which allows for protonation-state-dependent color tuning from the green to blue spectral region. The diastereomeric meso and rac forms of the dinuclear Ir(tmBBI)- Ir could be separated by fractional crystallization and both showed strong luminescence. A comparison with structurally related ruthenium complexes showed pronounced differences in their photophysical properties.