Synthesis, Diastereomer Separation, and Optoelectronic and Structural Properties of Dinuclear Cyclometalated Iridium(III) Complexes with Bridging Diarylhydrazide Ligands

A series of diiridium complexes 13–16 bridged by diarylhydrazine ligands and cyclometalated by phenylpyridine or phenylpyrazole ligands was synthesized. In all cases the ΛΔ meso and ΛΛ/ΔΔ rac diastereomers were separated and characterized by single-crystal X-ray diffraction, revealing intramolecular π–π stacking between arenes of the bridging and cyclometalating ligands. Density functional theory (DFT) calculations show that in general the HOMOs are mainly localized on the iridium centers, the cyclometalating phenyl moieties, and the central hydrazide components of the bridging ligands, while the LUMOs are primarily localized on the N-heterocycles (pyridine or pyrazole) of the cyclometalating ligands. This series of complexes, especially with the separated diastereomers, provides an ideal opportunity to study the effects of subtle structural changes on the optoelectronic properties of diiridium systems: significant differences are observed between the rac and meso isomers in some cases. A cyclic voltammetric study of the electrochemical properties of the eight complexes reveals strong intramolecular interactions between the iridium centers. The photophysical properties are reported in solution and in rigid poly­(methyl methacrylate) (PMMA) and 2-methyltetrahydrofuran (2-MeTHF) (at 77 K) matrices, where some of the complexes are strongly emissive in the turquoise and green regions (Φ_PL = 42–68 ± 10%) due to matrix-induced restricted intramolecular motion (RIM).