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

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posted on 16.09.2013, 00:00 authored by Filippo Monti, Florian Kessler, Manuel Delgado, Julien Frey, Federico Bazzanini, Gianluca Accorsi, Nicola Armaroli, Henk J. Bolink, Enrique Ortí, Rosario Scopelliti, Md. Khaja Nazeeruddin, Etienne Baranoff
Charged cyclometalated (CN) 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 CN ligands, in contrast to the classical design of charged complexes where NN 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 CN ligand is 2-phenylpyridine, whereas in the second one it is 2-(2,4-difluorophenyl)-pyridine. One bis-carbene (:CC:) and four different pyridine–carbene (NC:) 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 :CC: complexes show much larger photoluminescence quantum yields (ΦPL) of ca. 30%, compared to the NC: 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 NC:-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 :CC: analogues. This finding is supported by temperature-dependent excited-state lifetime measurements made on representative NC: and :CC: complexes.

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