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

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 illustrates 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 K 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.