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)(P-2-nCB) [dbp = 2,9-di-n-butyl-1,10-phenanthroline, and P-2-nCB = nido-carborane-bis(diphenylphosphine)], shows a TADF efficiency breakthrough exhibiting an emission decay time of tau(TADF) = 1.4 mu s at a quantum yield of = Phi(PL) = 100%. This is a consequence of three optimized parameters. (i) The strongly electron donating negatively charged P-2-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 S-1 and triplet T-1 state of Delta E(S-1-T-1) = 650 cm (-1) (80 meV). (ii) The allowedness of the S-1 -> S-0 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)(P-2-nCB), resulting from the design strategy of introducing n-butyl substitutions at positions 2 and 9 of phenanthroline that sterically interact with the phenyl groups of the P-2-nCB ligand. In particular, the shortest TADF decay time of tau(TADF) = 1.4 mu s at a Phi(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)(P-2-nCB) is not subject to concentration quenching. Therefore, it may be applied even as a 100% emission layer