[Cu(P P)(N N)][PF6] compounds with bis(phosphane) and 6-alkoxy, 6-alkylthio, 6-phenyloxy and 6-phenylthio- substituted 2,2-bipyridine ligands for light-emitting electrochemical cells

We report a series of [Cu(P<^>P)(N<^>N)][PF6] complexes with P<^>P = bis(2-(diphenylphosphino) phenyl) ether (POP) or 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos) and N<^>N = 6-methoxy-2,2'-bipyridine (MeObpy), 6-ethoxy-2,2'-bipyridine (EtObpy), 6-phenyloxy-2,2'-bipyridine (PhObpy), 6-methylthio-2,2'-bipyridine (MeSbpy), 6-ethylthio-2,2'-bipyridine (EtSbpy) and 6-phenylthio-2,2'-bipyridine (PhSbpy). The single crystal structures of all twelve compounds have been determined and confirm chelating modes for each N<^>N and P<^>P ligand, and a distorted tetrahedral geometry for copper(I). For the xantphos-containing complexes, the asymmetrical bpy ligand is arranged with the 6-substituent lying over the xanthene 'bowl'. The compounds have been characterized in solution by H-1, C-13 and P-31 NMR spectroscopies, and their photophysical and electrochemical properties are described. They are yellow emitters and solid samples show photoluminescence quantum yields in the range up to 38%, with emission lifetimes <10.2 mu s. On going from powder to frozen Me-THF, the excited state lifetimes increase which might suggest the presence of thermally activated delayed fluorescence (TADF). All the compounds have been tested in light-emitting electrochemical cells (LECs). Bright and stable LECs are obtained with complexes containing alkoxy- or phenyloxy-substituted ligands, making this family of compounds very relevant for the future development of copper-based electroluminescent devices.