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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 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.