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 E(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 s. 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 E(S1-T1) energy separations are related to small radiative rates or small

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