Mono-versus Bicyclic Carbene Metal Amide Photoemitters: Which Design Leads to the Best Performance?

Abstract

New luminescent “carbene–metal–amide” (CMA) Cu, Ag, and Au complexes based on monocyclic (C6) or bicyclic six-ring (BIC6) cyclic (alkyl)(amino)carbene ligands illustrate the effects of LUMO energy stabilization, conformational flexibility, excited state energy, and geometry on the photoluminescent properties, leading to 100% luminescence quantum yields, short excited state lifetimes Cu > Au > Ag down to 0.5 μs, and high radiative rates of 106 s–1. Gold complexes with the BIC6 ligand exhibit exceptional photostability under hard and soft UV light compared with analogous complexes with C5 and C6 carbenes. Steady-state and time-resolved photoluminescence spectroscopy at 298 and 77 K enabled an estimate of the energy levels of the charge transfer (CT) and locally excited (LE) states with singlet and triplet character. A four-state model is applied to describe thermally activated delayed fluorescence (TADF) properties in CMA materials and correlates excited state lifetimes with the energy difference between LE and CT states.