Hinged and Wide: A New PP Ligand for Emissive [Cu(PP)(NN)][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 '-Me(2)bpy), 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 '-Me(2)bpy)][PF6].CH2Cl2, [Cu(BIPHEP)(6-Mebpy)][PF6].Et2O.0.5H(2)O 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 (lambda(max)(em) in the range 558-583 nm) and [Cu(BIPHEP)(5,5 '-Me(2)bpy)][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 tau(1/2) < 8 mu s, to tau(1/2) values of up to 53 mu s, consistent with the compounds with N<^>N = 6-Mebpy, 6-Etbpy and 5,5 '-Me(2)bpy exhibiting thermally activated delayed fluorescence (TADF).