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American Chemical Society, Inorganic Chemistry, 18(52), p. 10292-10305, 2013

DOI: 10.1021/ic400600d

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

This paper is available in a repository.
This paper is available in a repository.

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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 (ΦPL) 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, ΦPL 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 ((3)MC) 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.