Bimetallic Coinage Metal Complexes of Tetradentate ortho-Ter-arylene Ligands

We report a series of photoluminescent bimetallic Cu(I), Ag(I), and Au(I) complexes supported by tetradentate donor-acceptor ligands containing two monoanionic amide donors (D) and two aza-arene acceptors (A). The metal complexes of the ligand containing carbazolide and pyridyl ligating groups possess high photoluminescence quantum yields (up to 65% in solution) and emission properties consistent with thermally-activated delayed fluorescence (TADF). Bimetallic Cu(I) complexes of similar ligands containing benzannulated donor (benzo[a]carbazolide) or acceptor (isoquinoline) moieties demonstrate that absorption/emission properties are tunable by modification of D/A groups. Single-crystal x-ray diffraction of the bimetallic complexes reveals metal-metal distances shorter than the sum of their van der Waals radii, suggesting metallophilic interactions between the d¹⁰ metal centers. Ultrafast excited-state kinetic analysis reveals nearly equal exergonic intersystem crossing rates between Cu(I) and Au(I) complexes, contrary to what is expected from each metal's spin-orbit coupling contributions. These findings are corroborated with theoretical modeling, which suggests that the optical transitions of these bimetallic complexes are characterized as metal-ligand-to-ligand charge transfer (MLLCT) states.