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 (λexc = 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.