| Summary: | Heteroleptic [Cu(BIPHEP)(N^N)][PF<sub>6</sub>] 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<sub>2</sub>bpy), have been synthesized and characterized using multinuclear NMR spectroscopies and electrospray ionization mass spectrometry. The single crystal structures of [Cu(BIPHEP)(bpy)][PF<sub>6</sub>]∙CH<sub>2</sub>Cl<sub>2</sub>, [Cu(BIPHEP)(5,5′-Me<sub>2</sub>bpy)][PF<sub>6</sub>]∙CH<sub>2</sub>Cl<sub>2</sub>, [Cu(BIPHEP)(6-Mebpy)][PF<sub>6</sub>]∙Et<sub>2</sub>O∙0.5H<sub>2</sub>O and [Cu(BIPHEP)(6-Etbpy)][PF<sub>6</sub>] confirm distorted tetrahedral {Cu(P^P)(N^N)} coordination environments. Each compound shows a quasi-reversible Cu<sup>+</sup>/Cu<sup>2+</sup> process. In deaerated solution, the compounds are weak emitters. Powdered samples are yellow emitters (<i>λ</i><sub>em</sub><sup>max</sup> in the range 558−583 nm) and [Cu(BIPHEP)(5,5′-Me<sub>2</sub>bpy)][PF<sub>6</sub>] 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 <i>τ</i><sub>1/2</sub> < 8 μs, to <i>τ</i><sub>1/2</sub> values of up to 53 μs, consistent with the compounds with N^N = 6-Mebpy, 6-Etbpy and 5,5′-Me<sub>2</sub>bpy exhibiting thermally activated delayed fluorescence (TADF).
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