Fluorination of Cyclometalated Iridium β-Ketoiminate and β-Diketiminate Complexes: Extreme Redox Tuning and Ligand-Centered Excited States

Abstract

In this work we describe a series of bis-cyclometalated iridium complexes with ancillary β-ketoiminate (acNac) and β-diketiminate (NacNac) ligands, prepared by a general synthetic route. Fluorination of these ligands by introducing CF3 substituents onto the ligand backbone and/or N-aryl substituent(s) leads to pronounced changes in the redox properties and photophysical properties. All of the complexes show a reversible IrIV/IrIII redox couple that is sensitive to the degree of fluorination on the ancillary ligand. Introduction of CF3 groups at the 3- and 5-positions of the N-aryl substituent shifts the potential positive by ca. 50–70 mV per CF3 group, whereas fluorination of the acNac or NacNac backbone induces larger shifts of ca. 200–300 mV per CF3 group. Fluorination of the NacNac backbone gives rise to substantially altered excited-state properties. Complexes with backbone CF3 groups luminesce in the red and near-infrared regions out of an excited state that is predominately a π → π* NacNac-centered triplet state. A preponderance of evidence supports the assignment of this low-energy feature, including minimal dependence of this emission feature on the identity of the cyclometalating ligand, pronounced vibronic structure, and microsecond lifetimes. These results demonstrate that acNac and NacNac ancillary ligands can engender cyclometalated iridium complexes with desirable and readily tailorable redox and optical properties, motivating continued application of this important ligand class to the design of phosphorescent organometallic molecules.