Heteroleptic [Cu(BIPHEP)(NˆN)][PF6 ] complexes (BIPHEP = 1,10 -biphenyl-2,20 - diylbis(diphenylphosphane)), in which NˆN is 2,20 -bipyridine (bpy), 6-methyl-2,20 -bipyridine (6-Mebpy), 6-ethyl-2,20 -bipyridine (6-Etbpy), or 5,50 -dimethyl-2,20 -bipyridine (5,50 -Me2 bpy), have been synthesized and characterized using multinuclear NMR spectroscopies and electrospray ionization mass spectrometry. The single crystal structures of [Cu(BIPHEP)(bpy)][PF6 ] CH2 Cl2 , [Cu(BIPHEP)(5,50 -Me2 bpy)][PF6 ] CH2 Cl2 , [Cu(BIPHEP)(6-Mebpy)][PF6 ] Et2 O 0.5H2 O and [Cu(BIPHEP) (6-Etbpy)][PF6 ] confirm distorted tetrahedral {Cu(PˆP)(NˆN)} coordination environments. Each compound shows a quasi-reversible Cu+/ Cu2+ process. In deaerated solution, the compounds are weak emitters. Powdered samples are yellow emitters (em max in the range 558-583 nm) and [Cu(BIPHEP)(5,50 -Me2 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 1/2 < 8 s, to 1/2 values of up to 53 s, consistent with the compounds with NˆN = 6-Mebpy, 6-Etbpy and 5,50 -Me2 bpy exhibiting thermally activated delayed fluorescence (TADF)
