Cationic Cyclometalated Iridium Luminophores: Photophysical, Redox, and Structural Characterization

Abstract

The photophysical and electrochemical properties of a series of cationic cyclometalated Ir(III) complexes is reported. The complexes are of general formula [Ir(ppy)2(R,R‘-bpy)]+(1−5; PF6- as counterion) where ppy = 2-phenylpyridinato anion. Complexes 1−3 contain asymmetric bpy ligands with R and R‘ substituents in the 6‘ and 4‘ positions, while complexes 4 and 5 bear bpy ligands symmetrically substituted in the 4 and 4‘ positions. Complex 5 was structurally characterized by single-crystal X-ray crystallography, revealing a cis arrangement of the metalated C atoms of the ppy ligands. All the species exhibit strong absorption in the UV region, due to spin-allowed ligand-centered (LC) transitions, and moderately intense bands in the visible region, due to charge transfer (CT) transitions. Several redox processes have been evidenced in each complex and assigned to specific components. The complexes also exhibit relatively strong and long-lived (from 10-8 to 10-5 s, depending on temperature and matrix) luminescence, in all the experimental conditions used (acetonitrile solution and spin-coated films at 298 K; butyronitrile rigid matrix at 77 K). The substituents of the polypyridine ligands affect in a substantial way the redox and photophysical properties of the compounds. In particular, a phenyl substituent on the polypyridine chelating ligand in the 6‘ position (complexes 1−3) stabilizes oxidation of an orbital which receives significant contributions from the ppy ligands and leads to emission from triplet ligand-to-ligand charge transfer (LLCT) excited states. When such a phenyl is absent in the ligand structure (complexes 4 and 5), the usual triplet metal-to-ligand charge transfer (MLCT) emission predominates.