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) and 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)][PF6] complexes in which P<^>P is either POP (bis(2-(diphenylphosphanyl)phenyl)ether) or xantphos (9,9dimethyl-9H-xanthene-4,5-diyl)bis(diphenylphosphane). The eight coordination compounds have been fully characterized, including the single crystal structures of [Cu(1)(xantphos)][PF6], [Cu(1) (POP)][PF6].CH2Cl2, [Cu(2)(xantphos)][PF6], [Cu(2)(POP)][PF6] and [Cu(3)(POP)][PF6].0.5Et(2)O. The [Cu(N<^>N) (P<^>P)](+) cations exhibit a partially reversible or irreversible Cu+/Cu2+ 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)][PF6] complexes are weak emitters with.max em in the range 565-578 nm. However, powdered samples achieve photoluminescence quantum yields in the range of 28.5 to 62.3%, with the highest PLQY found for [Cu(3)(POP)][PF6] with an excitedstate lifetime, t, of 16.1 mu s. For [Cu(3)(POP)][PF6], the excited state lifetime was measured in MeTHF at 293 and 77 K, and the increase in t from 1.77 to 59.4 mu s upon cooling supports thermally activated delayed fluorescence (TADF) at ambient temperatures.