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-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
ΔE(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 n-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
ΦPL 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.