How to tune luminescent Cu(i) complexes with strong donor carbenes towards TADF?

Abstract

Cu(I) carbene complexes are common emitters of current emerging interest for next-generation OLED technology based on their encountered thermally activated delayed fluorescence (TADF) properties. However, general molecular design principles for successfully observable TADF properties of such complexes usually rely on π-acidic N-heterocyclic carbene (NHC) ligands, while Cu(I) carbene complexes with purely σ-donating NHC ligands are still scarce. Based on a simple synthetic approach reported by us earlier that offers large-scale access to this class of Cu(I) carbene complexes, we aim to elucidate design principles to enhance their TADF properties. In this work, we combined temperature-dependent time-resolved luminescence spectroscopy on different time scales (ns, μs, ms) with high-level DFT/MRCI calculations on a number of representative Cu(I) NHC complexes with the anionic carbene An6DAC and additional pyridine-derived ligands. Good agreement between experiment and theory related to the electronic nature of the optical transitions, the energy levels of the electronic states and the kinetics of interconversion controlled by the respective energy gaps was found. We conclude that both the electron-rich or electron-poor nature of the pyridine ligands and the coordination geometry (linear, trigonal planar) play a significant role in the outcome of the optical properties of these complexes. While electron-rich pyridine moieties induce decoupled prompt fluorescence and phosphorescence with connected low photoluminescence quantum yields, electron-poor pyridine moieties enhance a “push–pull” effect mediated by Cu(I) that favours TADF properties so that the luminescence intensity is increased by 1–2 orders of magnitude if the complex is trigonal. In the trigonal formyl pyridine complex, theory predicts high MLCT contributions to the electronic wavefunctions, so that the T₁ states should exhibit large spin–orbit coupling matrix elements (SOCMEs) with several excited singlet states. Indeed, in temperature dependent luminescence measurements, the electronic states behave as an intimately coupled ensemble. At 270 K, 56% of its luminescence can be attributed to TADF. However, while the luminescence of the trigonal complexes is brighter, their chemical Cu–pyridine bond is more labile. Overall, this study provides a detailed overview of the possibilities of targeted molecular design to selectively address desirable optical properties in organometallic compounds for potential applications in lighting.