Design Strategy for Ag(I)-Based Thermally Activated Delayed Fluorescence Reaching an Efficiency Breakthrough

A design strategy for the development of Ag­(I)-based materials for thermally activated delayed fluorescence (TADF) is presented. Although Ag­(I) complexes usually do not show TADF, the designed material, Ag­(dbp)­(P₂-nCB) [dbp = 2,9-di-<i>n</i>-butyl-1,10-phenanthroline, and P₂-nCB = <i>nido</i>-carborane-bis­(diphenylphosphine)], shows a TADF efficiency breakthrough exhibiting an emission decay time of τ­(TADF) = 1.4 μs at a quantum yield of Φ_PL = 100%. This is a consequence of three optimized parameters. (i) The strongly electron-donating negatively charged P₂-nCB ligand destabilizes the 4d orbitals and leads to low-lying charge (CT) states of MLL′CT character, with L and L′ being the two different ligands, thus giving a small energy separation between the lowest singlet S₁ and triplet T₁ state of Δ<i>E</i>(S₁–T₁) = 650 cm^–1 (80 meV). (ii) The allowedness of the S₁ → S₀ transition is more than 1 order of magnitude higher than those found for other TADF metal complexes, as shown experimentally and by time-dependent density functional theory calculations. Both parameters favor a short TADF decay time. (iii) The high quantum efficiency is dominantly related to the rigid molecular structure of Ag­(dbp)­(P₂-nCB), resulting from the design strategy of introducing <i>n</i>-butyl substitutions at positions 2 and 9 of phenanthroline that sterically interact with the phenyl groups of the P₂-nCB ligand. In particular, the shortest TADF decay time of τ­(TADF) = 1.4 μs at a Φ_PL value of 100%, reported so far, suggests the use of this outstanding material for organic light-emitting diodes (OLEDs). Importantly, the emission of Ag­(dbp)­(P₂-nCB) is not subject to concentration quenching. Therefore, it may be applied even as a 100% emission layer