  Donor-Acceptor (D-A) type thermally activated delayed fluorescence (TADF)
emitters which constitute the key functional units in proposed Gen3 Organic
Light Emitting Diodes (OLEDs), are sensitive to the rigidity and polarity of
their local environment. In particular, the torsional freedom of the D-A
dihedral angle and the excited state dipole moments of the occurring charge
transfer states, condition a distribution of TADF dynamics over the emitter
ensemble, concealed in standard optical spectroscopy. Here we apply
spectroscopy on the single molecule level to directly access individual emitter
properties, and, thus, bypass the downside of ensemble averaging. By photon
correlation data and locally resolved spectral information on single D-A type
TADF molecules embedded in technological relevant host materials of different
polarity and rigidity, we derive host-dependent characteristics and
distributions in the TADF dynamics. Those can be related to the conformational
freedom and the dielectric environment imposed by the specific local host
rigidity and polarity, thereby pointing out new selection criteria for
host-emitter combinations in OLEDs.
