  Organic light emitting diodes (OLEDs) based on thermally activated delayed
fluorescence (TADF) utilize molecular systems with a small energy splitting
between singlet and triplet states. This can either be realized in
intramolecular charge transfer states of molecules with near-orthogonal donor
and acceptor moieties or in intermolecular exciplex states formed between a
suitable combination of individual donor and acceptor materials. Here, we
investigate 4,4'-(9H,9'H-[3,3'-bicarbazole]-9,9'-diyl)bis(3-(trifluoromethyl)
benzonitrile) (pCNBCzoCF3), which shows intramolecular TADF but can also form
exciplex states in combination with
4,4',4''-tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA). Orange emitting
exciplex-based OLEDs additionally generate a sky-blue emission from the
intramolecular emitter with an intensity that can be voltage-controlled. We
apply electroluminescence detected magnetic resonance (ELDMR) to study the
thermally activated spin-dependent triplet to singlet up-conversion in
operating devices. Thereby, we can investigate intermediate excited states
involved in OLED operation and derive the corresponding activation energy for
both, intra- and intermolecular based TADF. Furthermore, we give a lower
estimate for the extent of the triplet wavefunction to be >1.2 nm.
Photoluminescence detected magnetic resonance (PLDMR) reveals the population of
molecular triplets in optically excited thin films. Overall, our findings allow
us to draw a comprehensive picture of the spin-dependent emission from intra-
and intermolecular TADF OLEDs.
