Charged Bis-Cyclometalated Iridium(III) Complexes with Carbene-Based Ancillary Ligands

Abstract

Charged cyclometalated (C^∧N) iridium­(III) complexes with carbene-based ancillary ligands are a promising family of deep-blue phosphorescent compounds. Their emission properties are controlled primarily by the main C^∧N ligands, in contrast to the classical design of charged complexes where N^∧N ancillary ligands with low-energy π* orbitals, such as 2,2'-bipyridine, are generally used for this purpose. Herein we report two series of charged iridium complexes with various carbene-based ancillary ligands. In the first series the C^∧N ligand is 2-phenylpyridine, whereas in the second one it is 2-(2,4-difluorophenyl)-pyridine. One bis-carbene (:C^∧C:) and four different pyridine–carbene (N^∧C:) chelators are used as bidentate ancillary ligands in each series. Synthesis, X-ray crystal structures, and photophysical and electrochemical properties of the two series of complexes are described. At room temperature, the :C^∧C: complexes show much larger photoluminescence quantum yields (Φ_PL) of ca. 30%, compared to the N^∧C: analogues (around 1%). On the contrary, all of the investigated complexes are bright emitters in the solid state both at room temperature (1% poly­(methyl methacrylate) matrix, Φ_PL 30–60%) and at 77 K. Density functional theory calculations are used to rationalize the differences in the photophysical behavior observed upon change of the ancillary ligands. The N^∧C:-type complexes possess a low-lying triplet metal-centered (³MC) state mainly deactivating the excited state through nonradiative processes; in contrast, no such state is present for the :C^∧C: analogues. This finding is supported by temperature-dependent excited-state lifetime measurements made on representative N^∧C: and :C^∧C: complexes