The effects of introducing terminal alkenyl substituents into the 2,2'-bipyridine domain in [Cu(N^N)(P^P)]+ coordination compounds

The N^N chelating ligands 6,6'-bis(but-3-en-1-yl)-2,2'-bipyridine ( 1 ), 6-(but-3-en-1-yl)-6'-methyl-2,2'-bipyridine ( 2 ), 6,6'-bis(pent-4-en-1-yl)-2,2'-bipyridine ( 3 ), 6-(pent-4-en-1-yl)-6'-methyl-2,2'-bipyridine ( 4 ) have been prepared, characterized, and incorporated into the heteroleptic [Cu(N^N)(P^P)][PF 6 ] complexes in which P^P is either POP (bis(2-(diphenylphosphanyl)phenyl)ether) or xantphos (9,9-dimethyl-9 H -xanthene-4,5-diyl)bis(diphenylphosphane). The eight coordination compounds have been fully characterized, including the single crystal structures of [Cu( 1 )(xantphos)][PF 6 ], [Cu( 1 )(POP)][PF 6 ]·CH 2 Cl 2 , [Cu( 2 )(xantphos)][PF 6 ], [Cu( 2 )(POP)][PF 6 ] and [Cu(3)(POP)][PF 6 ]·0.5Et 2 O. The [Cu(N^N)(P^P)] + cations exhibit a partially reversible or irreversible Cu + /Cu 2+ oxidation at more positive potentials than the benchmark [Cu(bpy)(P^P)]+ and [Cu(Me2bpy)(P^P)]+ complexes consistent with the increase in steric hindrance of the terminal alkenyl substituents. When excited in the region of the metal-to-ligand charge transfer (MLCT) absorption, solutions of the [Cu(N^N)(P^P)][PF 6 ] complexes are weak emitters with in the range 565-578 nm. However, powdered samples achieve photoluminescence quantum yields in the range to 28.5 to 62.3%, with the highest PLQY found for [Cu( 3 )(POP)][PF 6 ] with an excited-state lifetime, , of 16.1 μs. For [Cu( 3 )(POP)][PF 6 ], the excited state lifetime was measured in MeTHF at 293 and 77K, and the increase in from 1.77 to 59.4 μs upon cooling supports thermally activated delayed fluorescence (TADF) at ambient temperatures