Organic Semiconductors – Jun.-Prof. Sebastian Reineke
Investigation of thermally activated delayed fluorescence (TADF)
via solvatochromism
Introduction: The most important quest driving the development of OLEDs is finding the best
emitter molecules that allow internal quantum efficiencies close to 100%. A very unique property
of soft organic, amorphous materials is that excited states are formed statistically upon charge
injection. The numbers are simple: three triplet excitons are formed for each singlet exciton
(simply because the triplet state has a multiplicity of 3).
Figure 1, general scheme of TADF.
Recently, pure organic materials where reported that realize 100% internal quantum efficiency,
but from the fluorescence (singlet) channel. The trick played is as simple as it is striking:
Molecules, whose energy levels of singlet and triplet states are so close in energy that thermal
energy at room temperature can assist up-conversion from triplet to singlet states, can radiatively
utilize all excitons. This group of molecules is called thermally activated delayed fluorescence
(TADF)-based molecules and is considered the latest (3rd) generation of emitter molecules. TADF
has globally triggered an unexpected turn in material development. New TADF emitters are
currently developed in our organic chemistry group, calling for effective methods to characterize
their TADF properties.
The topic: The TADF properties of new emitters should be investigated in this thesis by making
use of solvatochromism. As mentioned, TADF works effectively when the singlet-triplet splitting
is very small, which is realized in intramolecular charge transfer (CT) states. This charge transfer
state character introduces a strong dipole moment in the excited state. The variation of solvents
with different polarity will therefore alter the overall emission (as seen in Figure 2), i.e. either
stabilize or destabilize the CT state seen as spectral shifts. Thus, the so-called solvatochromatic
effect made use of mainly in steady-state photoluminescence spectroscopy should be the central
point of this thesis to understand the photo-physical properties.
Importance: It often takes a fair amount of time from the synthesis of new emitter materials to
the final assessment of their suitability as an efficient OLED emitter. Partly, this is due to the
missing evaluation techniques that allow rapid and trustworthy screening of materials. This thesis
aims to establish solvatochromism here to fulfill those urgent needs.
Organic Semiconductors – Jun.-Prof. Sebastian Reineke
Figure 2, taken from http://dx.doi.org/10.1063/1.4749285
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