Cu(I) complexes – Thermally activated delayed fluorescence. Photophysical approach and material design

Cu(I) complexes often show transitions of distinct metal-to-ligand charge transfer (MLCT) character. This can lead to small energy separations between the lowest singlet S1 and triplet T1 state. Hence, thermally activated delayed fluorescence (TADF) and, if applied to electroluminescent devices, singlet harvesting can become highly effective. In this contribution, we introduce the TADF mechanism and identify crucial parameters that are necessary to optimize materials’ properties, in particular, with respect to short emission decay times and high quantum yields at ambient temperature. In different case studies, we present a photophysical background for a deeper understanding of the materials’ properties. Accordingly, we elucidate strategies for obtaining high quantum yields. These are mainly based on enhancing the intrinsic rigidity of the complexes and of their environment. Efficient TADF essentially requires small energy separations ΔE(S1–T1) with preference below about 1000 cm−1 (≈120 meV). This is achievable with complexes that exhibit small spatial HOMO–LUMO overlap. Thus, energy separations below 300 cm−1 (≈37 meV) are obtained, giving short radiative TADF decay times of less than 5 μs. In a case study, it is shown that the TADF properties may be tuned or the TADF effect can even be turned off. However, very small ΔE(S1–T1) energy separations are related to small radiative rates or small oscillator strengths of the S1 → S0 transitions due to the (required) small HOMO–LUMO overlap, as discussed in a further case study. Moreover, large spin–orbit coupling (SOC) of the triplet state to higher lying singlet states can induce an additional phosphorescence decay path that leads to a luminescence consisting of TADF and phosphorescence, thus leading to a combined singlet harvesting and triplet harvesting mechanism. This gives an overall reduction of the decay time. Finally, in a strongly simplified model, the SOC efficiency is traced back to easily obtainable results from DFT calculations.