Phosphane tuning in heteroleptic [Cu(N^N)(P^P)]+ complexes for light-emitting electrochemical cells

Abstract

The synthesis and characterization of five [Cu(P^P)(N^N)][PF₆] complexes in which P^P = 2,7-bis(tert-butyl)-4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (^tBu₂xantphos) or the chiral 4,5-bis(mesitylphenylphosphino)-9,9-dimethylxanthene (xantphosMes₂) and N^N = 2,2′-bipyridine (bpy), 6-methyl-2,2′-bipyridine (6-Mebpy) or 6,6′-dimethyl-2,2′-bipyridine (6,6′-Me₂bpy) are reported. Single crystal structures of four of the compounds confirm that the copper(I) centre is in a distorted tetrahedral environment. In [Cu(xantphosMes₂)(6-Mebpy)][PF₆], the 6-Mebpy unit is disordered over two equally populated orientations and this disorder parallels a combination of two dynamic processes which we propose for [Cu(xantphosMes₂)(N^N)]⁺ cations in solution. Density functional theory (DFT) calculations reveal that the energy difference between the two conformers observed in the solid-state structure of [Cu(xantphosMes₂)(6-Mebpy)][PF₆] differ in energy by only 0.28 kcal mol⁻¹. Upon excitation into the MLCT region (λ_exc = 365 nm), the [Cu(P^P)(N^N)][PF₆] compounds are yellow to orange emitters. Increasing the number of Me groups in the bpy unit shifts the emission to higher energies, and moves the Cu⁺/Cu²⁺ oxidation to higher potentials. Photoluminescence quantum yields (PLQYs) of the compounds are low in solution, but in the solid state PLQYs of up to 59% (for [Cu(^tBu₂xantphos)(6,6′-Me₂bpy)]⁺) are observed. Increased excited-state lifetimes at low temperature are consistent with the complexes exhibiting thermally activated delayed fluorescence (TADF). This is supported by the small energy difference calculated between the lowest-energy singlet and triplet excited states (0.17–0.25 eV). The compounds were tested in simple bilayer light-emitting electrochemical cells (LECs). The optoelectronic performances of complexes containing xantphosMes₂ were generally lower with respect to those with ^tBu₂xantphos, which led to bright and efficient devices. The best performing LECs were obtained for the complex [Cu(^tBu₂xantphos)(6,6′-Me₂bpy)][PF₆] due to the increased steric hindrance at the N^N ligand, resulting in higher PLQY.