Role of the Bridging Group in Bis-Pyridyl Ligands: Enhancing Both the Photo- and Electroluminescent Features of Cationic (IPr)Cu^I Complexes

We report on the benefits of changing the bridging group X of bis-pyridyl ligands, that is, Py-X-Py where X is NH, CH₂, C(CH₃)₂, or PPh, on the photo- and electroluminescent properties of a new family of luminescent cationic H-heterocyclic carbene (NHC) copper(I) complexes. A joint experimental and theoretical study demonstrates that the bridging group affects the molecular conformation from a planar-like structure (X is NH and CH₂) to a boat-like structure (X is C(CH₃)₂ and PPh), leading to i) four-fold enhancement of the photoluminescence quantum yield (ϕ_em) without affecting the thermally activated delayed fluorescence mechanism, and ii) one order of magnitude reduction of the ionic conductivity (σ) of thin films. This leads to an overall enhancement of the device efficacy and luminance owing to the increased ϕ_em and the use of low applied driving currents.