Hinged and Wide: A New P^P Ligand for Emissive [Cu(P^P)(N^N)][PF₆] Complexes

Heteroleptic [Cu(BIPHEP)(N^N)][PF₆] complexes (BIPHEP = 1,1'-biphenyl-2,2'-diylbis(diphenylphosphane)), in which N^N is 2,2'-bipyridine (bpy), 6-methyl-2,2'-bipyridine (6-Mebpy), 6-ethyl-2,2'-bipyridine (6-Etbpy), or 5,5'-dimethyl-2,2'-bipyridine (5,5'-Me₂bpy), have been synthesized and characterized using multinuclear NMR spectroscopies and electrospray ionization mass spectrometry. The single crystal structures of [Cu(BIPHEP)(bpy)][PF₆]∙CH₂Cl₂, [Cu(BIPHEP)(5,5'-Me₂bpy)][PF₆]∙CH₂Cl₂, [Cu(BIPHEP)(6-Mebpy)][PF₆]∙Et₂O∙0.5H₂O and [Cu(BIPHEP)(6-Etbpy)][PF₆] confirm distorted tetrahedral {Cu(P^P)(N^N)} coordination environments. Each compound shows a quasi-reversible Cu⁺/Cu²⁺ process. In deaerated solution, the compounds are weak emitters. Powdered samples are yellow emitters (λ_em^max in the range 558-583 nm) and [Cu(BIPHEP)(5,5'-Me₂bpy)][PF₆] exhibits the highest photoluminescence quantum yield (PLQY = 14%). On cooling to 77 K (frozen 2-methyloxolane), the emission maxima are red-shifted and the excited state lifetimes increase from τ_1/2 < 8 μs, to τ_1/2 values of up to 53 μs, consistent with the compounds with N^N = 6-Mebpy, 6-Etbpy and 5,5'-Me₂bpy exhibiting thermally activated delayed fluorescence (TADF).