TADF Material Design: Photophysical Background and Case Studies Focusing on Cu^Iand Ag^IComplexes

Abstract

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...

Remote Modification of Bidentate Phosphane Ligands Controlling the Photonic Properties in Their Complexes: Enhanced Performance of [Cu(RN‐xantphos)(N^N)][PF6] in Light‐Emitting Electrochemical Cells

“…The occurrence of TADF processes has been previously postulated for Cu(I) complexes with chemical structures similar to those of the complexes studied in this work. [ 6–8 ] A requirement for TADF is that the energy difference between the lowest‐lying T 1 triplet and the emitting singlet excited state S 1 has to be small to favor the reverse intersystem crossing (RISC) from T 1 to S 1 . For the [Cu(HN‐xantphos)(N ^ N)] + and [Cu(BnN‐xantphos)(N ^ N)] + complexes, the energy difference between S 1 and T 1 is predicted in the range 0.14–0.20 eV at the optimized geometry of S 0 (Table S1, Supporting Information), which is lower than that computed for the [Cu(xantphos)(bpy)] + complex (0.25 eV) and small enough to favor the RISC process (Figure 5).…”

“…The fact that many compounds in this family exhibit thermally activated delayed fluorescence (TADF), which results in an indirect harvesting of singlet‐state fluorescence and a concomitant enhancement in light emission, makes their investigation especially relevant. [ 6–8 ]…”

[Cu(POP)(N^S)][PF6] and [Cu(xantphos)(N^S)][PF6] compounds with 2-(thiophen-2-yl)pyridines

“…3,4 The discovery that [Cu(P^P)(N^N)] + complexes exhibit thermally activated delayed uorescence (TADF) leading to indirect harvesting of triplet-state uorescence has injected additional research vigour into the area. [5][6][7] The combination of hard and so donors at a metal centre to control electronic, photophysical, redox and reaction chemistry is an old and recurring theme in coordination chemistry. 8,9 The combination of both hard and so donors in the same ligand is also an established theme, illustrated in the motivation for the preparation of the rst synthetic macrocyclic complex by a template route.…”

“…As copper(I) is a classical so metal centre, we considered it likely that the thiophene sulfur might coordinate to the metal. We recently reported a series of [Cu(P^P)(N^S)][PF 6 ] complexes with P^P ¼ bis(2-(diphenylphosphino)phenyl)ether (POP) or 4,5bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos) (Scheme 1) and N^S being a 2-(alkylthio)pyridine. 23 These investigations were prompted by the fact that several previously reported copper(I) complexes combining heterocyclic thioamide and phosphane ligands are emissive 24,25 as is [Cu 2 (dppdtbpf) 2 (m-NCS) 2 ] (dppdtbpf ¼ 1-diphenylphosphino-1 0 -di-tert-butylphosphinoferrocene).…”

“…We now report an investigation of [Cu(POP)(N^S)][PF 6 ] and [Cu(xantphos)(N^S)][PF 6 ] complexes in which the N^S ligand is one of the series of 2-(thiophen-2-yl)pyridines shown in Scheme 1.…”