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

Abstract

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-din -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 4dorbitals 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 2,9positions 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.

Cyanide‐Assembled d10Coordination Polymers and Cycles: Excited State Metallophilic Modulation of Solid‐State Luminescence

“…This similarity indicates the same nature of the emissive excited state, which was originally assigned to the mixed ( 3 IL+ 3 MLCT) character at low temperature and is expectedly independent on the counterion. However, a dramatic variation in the emission decay time of 2 upon cooling from 298 K ( τ av =5.6 μs) to 77 K ( τ av =908.2 μs) found in the present study very likely evidences that different excited states operate at low and room temperatures, thus apparently indicative of TADF behaviour, which has been already found for some mononuclear silver complexes …”

“…The gold‐copper polymer 5 has the shortest emission decay time amongst the title complexes at 298 K ( τ av =2.4 μs), that is also one of smallest values reported for TADF materials . Taking into account zero oscillator strength calculated for the S 0 ↔ S 1 transition that should prevent high rate of prompt fluorescence k ( S 1 → S 0 ), short τ might testify to a remarkably small energy separation Δ E ( T 1 – S 1 ), which is also in line with TADF process observed even at 77 K …”

TADF Material Design: Photophysical Background and Case Studies Focusing on CuIand AgIComplexes

“…However, these shortcomings may be suppressed to a large extent by rigidifying the molecular structure either by sterically demanding ligands or by a rigid environment. This behavior has already been discussed frequently in the literature . We will address these properties in Sections 3 and 6.…”

“…Interestingly, design of a material, a silver complex, with Φ PL of 100 % becomes possible by following this strategy of rigidifying the molecular structure (see refs. and Section 6). Well designed TADF materials should exhibit relatively small energy separations Δ E (S 1 –T 1 ). For organo‐transition metal compounds this is related to the occurrence of metal‐to‐ligand charge transfer (MLCT) (and ligand‐to‐ligand charge transfer (LL′CT)) states having frontier orbitals, HOMO and LUMO, that are spatially largely separated.…”

“…Interestingly, design of a material, a silver complex, with Φ PL of 100 % becomes possible by following this strategy of rigidifying the molecular structure (see refs. and Section 6).…”