The development of organic light emitting diodes (OLEDs) and the use of emitting molecules have strongly stimulated scientific research of emitting compounds. In particular, for OLEDs it is required to harvest all singlet and triplet excitons that are generated in the emission layer. This can be achieved using the so-called triplet harvesting mechanism. However, the materials to be applied are based on high-cost rare metals and therefore, it has been proposed already more than one decade ago by our group to use the effect of thermally activated delayed fluorescence (TADF) to harvest all generated excitons in the lowest excited singlet state S . In this situation, the resulting emission is an S →S fluorescence, though a delayed one. Hence, this mechanism represents the singlet harvesting mechanism. Using this effect, high-cost and strong SOC-carrying rare metals are not required. This mechanism can very effectively be realized by use of Cu or Ag complexes and even by purely organic molecules. In this investigation, we focus on photoluminescence properties and on crucial requirements for designing Cu and Ag materials that exhibit short TADF decay times at high emission quantum yields. The decay times should be as short as possible to minimize non-radiative quenching and, in particular, chemical reactions that frequently occur in the excited state. Thus, a short TADF decay time can strongly increase the material's long-term stability. Here, we study crucial parameters and analyze their impact on the TADF decay time. For example, the energy separation ΔE(S -T ) between the lowest excited singlet state S and the triplet state T should be small. Accordingly, we present detailed photophysical properties of two case-study materials designed to exhibit a large ΔE(S -T ) value of 1000 cm (120 meV) and, for comparison, a small one of 370 cm (46 meV). From these studies-extended by investigations of many other Cu TADF compounds-we can conclude that just small ΔE(S -T ) is not a sufficient requirement for short TADF decay times. High allowedness of the transition from the emitting S state to the electronic ground state S , expressed by the radiative rate k (S →S ) or the oscillator strength f(S →S ), is also very important. However, mostly small ΔE(S -T ) is related to small k (S →S ). This relation results from an experimental investigation of a large number of Cu complexes and basic quantum mechanical considerations. As a consequence, a reduction of τ(TADF) to below a few μs might be problematic. However, new materials can be designed for which this disadvantage is not prevailing. A new TADF compound, Ag(dbp)(P -nCB) (with dbp=2,9-di-n-butyl-1,10-phenanthroline and P -nCB=bis-(diphenylphosphine)-nido-carborane) seems to represent such an example. Accordingly, this material shows TADF record properties, such as short TADF decay time at high emission quantum yield. These properties are based (i) on geometry optimizations of the Ag complex for a fast radiative S →S rate and (ii) on restricting the extent of geometry reorganiza...

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.

The four new Ag(I) complexes Ag(phen)(P-nCB) (1), Ag(idmp)(P-nCB) (2), Ag(dmp)(P-nCB) (3), and Ag(dbp)(P-nCB) (4) with P-nCB = bis(diphenylphosphine)-nido-carborane, phen = 1,10-phenanthroline, idmp = 4,7-dimethyl-1,10-phenanthroline, dmp = 2,9-dimethyl-1,10-phenanthroline, and dbp = 2,9-di-n-butyl-1,10-phenanthroline were designed to demonstrate how to develop Ag(I) complexes that exhibit highly efficient thermally activated delayed fluorescence (TADF). The substituents on the 1,10-phenanthroline ligand affect the photophysical properties strongly (i) electronically via influencing the radiative rate of the S → S transition and (ii) structurally by rigidifying the molecular geometry with respect to geometry changes occurring in the lowest excited S and T states. The oscillator strength of the S ↔ S transition f(S ↔ S)-an important parameter for the TADF efficiency being proportional to the radiative rate-can be increased from f(S ↔ S) = 0.0258 for Ag(phen)(P-nCB) (1) to f(S ↔ S) = 0.0536 for Ag(dbp)(P-nCB) (4), as calculated for the T state optimized geometries. This parameter governs the radiative TADF decay time (τ) at ambient temperature, found to be τ = 5.6 μs for Ag(phen)(P-nCB) (1) but only τ = 1.4 μs for Ag(dbp)(P-nCB) (4)-a record TADF value. In parallel, the photoluminescence quantum yield (Φ) measured for powder samples at ambient temperature is boosted up from Φ = 36% for Ag(phen)(P-nCB) (1) to Φ = 100% for Ag(dbp)(P-nCB) (4). This is a consequence of a cooperative effect of both decreasing the nonradiative decay rate and increasing the radiative decay rate in the series from Ag(phen)(P-nCB) (1), Ag(idmp)(P-nCB) (2), and Ag(dmp)(P-nCB) (3) to Ag(dbp)(P-nCB) (4). Another parameter important for the TADF behavior is the activation energy of the S state from the state T, ΔE(S-T). Experimentally it is determined for the complexes Ag(dmp)(P-nCB) (3) and Ag(dbp)(P-nCB) (4) to be of moderate size of ΔE(S-T) = 650 cm.