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

Abstract

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 (9,9-dimethyl-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]·0.5Et2O. 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 λmaxem 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 excited-state lifetime, τ, of 16.1 μs. For [Cu(3)(POP)][PF6], the excited state lifetime was measured in MeTHF at 293 and 77 K, and the increase in τ from 1.77 to 59.4 μs upon cooling supports thermally activated delayed fluorescence (TADF) at ambient temperatures.