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

A design strategy is presented for the development of Ag(I)-based materials for thermally activated delayed fluorescence (TADF). Although Ag(I) complexes usually do not show TADF, the designed material, Ag(dbp)(P2-nCB) (with dbp = 2,9-di-n-butyl-1,10-phenanthroline and P2-nCB = nido-carborane-bis-(diphenylphosphine)), shows a TADF efficiency breakthrough exhibiting an emission decay time of τ(TADF) = 1.4 µs at a quantum yield of ΦPL = 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 ∆E(S1-T1) = 650 cm-1 (80 meV). (ii) The allowedness of the S1→S0 transition is more than one order of magnitude higher than found for other TADF metal complexes, as shown experimentally and by TD-DFT calculations. Both parameters favor 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 n-butyl substitutions at the 2,9-positions of phenanthroline which 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 ΦPL = 100 % reported so far suggests the use of this outstanding material for 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