Introducing sterically demanding substituents and π-π-interactions into [Cu(P^P)(N^N)]⁺ complexes

A series of ten N^N chelating ligands based on a 2,2'-bipyridine (bpy) metal-binding domain and featuring sterically hindering substituents in the 6- and 6,6'-positions has been synthesized and characterized. The ligands have been incorporated into a family of 15 heteroleptic complexes of type [Cu(P^P)(N^N)][PF₆] where P^P is the wide bite-angle bisphosphane ligand bis(2(diphenylphosphanyl)phenyl)ether (POP) or (9,9-dimethyl-9H-xanthene-4,5-diyl)bis(diphenylphosphane) (xantphos). Substituents in several of the N^N ligands ligands possess phenyl rings remotely tethered to enable intra- and intermolecular π-π-interactions in the [Cu(P^P)(N^N)]⁺ cations. Single crystal X-ray structures of 12 complexes are reported. The effects of the functional groups in the bpy ligand on the photophysical properties of the complexes have been studied; solid-state emission maxima range from 518 to 567 nm. Values of the solid-state photoluminescence quantum yields (PLQYs) of the [Cu(P^P)(N^N)][PF₆] compounds respond to the nature of the N^N ligand. In general, we observed that the [Cu(P^P)(N^N)]⁺ complexes containing 6,6'-disubstituted complexes with phenyl moieties connected via a CH₂CH₂ or CH₂CH₂CH₂ spacer to the bpy domain have the highest values of PLQY. The most significant compounds are [Cu(POP)((2-PhEt)₂bpy)][PF₆] (PLQY = 67%) and [Cu(POP)((3-PhPr)₂bpy)][PF₆] (PLQY = 72%) where (2-PhEt)₂bpy = 6,6'-bis(2-phenylethyl)-2,2'-bipyridine and (3-PhPr)₂bpy = 6,6'-bis(3-phenylpropyl)-2,2'-bipyridine. These PLQY values are among the best performing previously reported families of [Cu(P^P)(N^N)][PF₆] compounds.