LAUSR

(1 of 5) 1600926 TADF occurs in inter-[8,9] and intramolecular material systems.[4,6,10,11] In both cases, the wavefunction overlap of the two particles forming the emissive state defines the energetic difference between the singlet and triplet states ΔEST, to which the most dominant parameter is the distance between the participating charge carriers. If this is large enough both spin configurations will be energetically nearly degenerate and can be interconverted by thermal excitation and/or by spin–orbit or hyperfine interaction. The reference donor-material for TADF is 4,4′,4′′-Tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA), which can be combined with different acceptor molecules in order to obtain the pursued exciplex emission.[4,11] The donor material tri(9-hexylcarbazol-3-yl)amine (THCA) combined with different acceptors has recently also showed very promising properties in the development of white OLEDs.[12,13] Both material systems possess large external quantum efficiency values, rendering them highly attractive for lighting applications.[4,11–13] However, a clear proof that spin states are involved in recombination is still missing and the identification of recombination pathways is generally ambiguous, as thermal excitation (heating) is not spin-specific. Generally, the dependence of recombination processes on the spin states follows from the magnetic field effect (MFE).[14] In TADF materials and devices, MFE on electroluminescence (MEL), photoluminescence (MPL), and photocurrent (MPC) were also reported.[15–17] Strong temperature dependence of MEL, MPL, and MPC indicates a crucial role of the thermal activation in the generation of EL.[15,17] As pointed out in ref. [15], however, it is difficult to distinguish between recombination pathways involving exciplex states formed at donor–acceptor interfaces and weakly bound polaron pairs formed otherwise, as both are magnetic field dependent. Moreover, the formation of (weakly) bound excited pair (or exciplex) states from injected charge carriers may depend on charge transport properties of device active layers, which are intrinsically temperature dependent, e.g., due to imbalanced charge carrier mobility. Hence, measuring the influence of a magnetic field on the EL at various temperatures is important, but alone does not allow to decipher the underlying spin-dependent recombination channel.[18] Here, we apply an electron paramagnetic resonance (EPR) based technique, where spin populations in the triplet spin-sublevels are altered directly by microwaves with a fixed frequency. When these are applied to the sample in addition to a static magnetic field the condition Direct Observation of Spin States Involved in Organic Electroluminescence Based on Thermally Activated Delayed Fluorescence