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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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Strongly luminescent neutral copper(I) complexes of the type Cu(pop)(NN), with pop = bis(2-(diphenylphosphanyl)phenyl)ether and NN = bis(pyrazol-1-yl)borohydrate (pz2BH2), tetrakis(pyrazol-1-yl)borate (pz4B), or bis(pyrazol-1-yl)-biphenyl-borate (pz2Bph2), are readily accessible in reactions of Cu(acetonitrile)4 + with equimolar amounts of the pop and NN ligands at ambient temperature. All products were characterized by means of single crystal X-ray diffractometry. The compounds exhibit very strong blue/white luminescence with emission quantum yields of up to 90%. Investigations of spectroscopic properties and the emission decay behavior in the temperature range between 1.6 K and ambient temperature allow us to assign the emitting electronic states. Below 100 K, the emission decay times are in the order of many hundreds of microseconds. Therefore, it is concluded that the emission stems from the lowest triplet state. This state is assigned to a metal-to-ligand charge-transfer state (3MLCT) involving Cu-3d and pop-π* orbitals. With temperature increase, the emission decay time is drastically reduced to e.g. to 13 μs (Cu(pop)(pz2Bph2)) at ambient temperature. At this temperature, the complexes exhibit high emission quantum yields, as neat material or doped into poly(methyl methacrylate) (PMMA). This behavior is assigned to an efficient thermal population of a singlet state (being classified as 1MLCT), which lies only 800 to 1300 cm–1 above the triplet state, depending on the individual complex. Thus, the resulting emission at ambient temperature largely represents a fluorescence. For applications in OLEDs and LEECs, for example, this type of thermally activated delayed fluorescence (TADF) creates a new mechanism that allows to harvest both singlet and triplet excitons (excitations) in the lowest singlet state. This effect of singlet harvesting leads to drastically higher radiative rates than obtainable for emissions from triplet states of Cu(I) complexes.

The emitting triplet state of fac-Ir(ppy)(3) (fac-tris(2-phenylpyridine)iridium) is studied for the first time on the basis of highly resolved optical spectra in the range of the electronic 0-0 transitions. For the compound dissolved in CH(2)Cl(2) and cooled to cryogenic temperatures, three 0-0 transitions corresponding to the triplet substates I, II, and III are identified. They lie at 19,693 cm(-1) (507.79 nm, I → 0), 19,712 cm(-1) (507.31 nm, II → 0), and 19,863 cm(-1) (503.45 nm, III → 0). From the large total zero-field splitting (ZFS) of 170 cm(-1), the assignment of the emitting triplet term as a (3)MLCT state (metal-to-ligand charge transfer state) is substantiated, and it is seen that spin-orbit couplings to higher lying (1,3)MLCT states are very effective. Moreover, the studies provide emission decay times for the three individual substates of τ(I) = 116 μs, τ(II) = 6.4 μs, and τ(III) = 200 ns. Further, group-theoretical considerations and investigations under application of high magnetic fields up to B = 12 T allow us to conclude that all three substates are nondegenerate and that the symmetry of the complex in the CH(2)Cl(2) matrix cage is lower than C(3). It follows that the triplet parent term is of (3)A character. Studies of the emission decay time and photoluminescence quantum yield, Φ(PL), of Ir(ppy)(3) in poly(methylmethacrylate) (PMMA) in the temperature range of 1.5 ≤ T ≤ 370 K reveal average and individual radiative and nonradiative decay rates and quantum yields of the substates. In the range 80 ≤ T ≤ 370 K, Φ(PL) is as high as almost 100%. The quantum yield Φ(PL) drops to ∼88% when cooled to T = 1.5 K. The investigations show further that the emission properties of Ir(ppy)(3) depend distinctly on the complex's environment or the matrix cage according to distinct changes of spin-orbit coupling effectiveness. These issues also have consequences for optimizations of the material's properties if applied as an organic light-emitting diode (OLED) emitter.