Thermally activated delayed fluorescence
The first evidence of thermally activated delayed fluorescence in an organic molecule was discovered in 1961 investigating the compound eosin.
Application of the TADF mechanism for efficient light generation in OLEDs was proposed in 2008 by Yersin and coworkers [3] and subsequently intensively studied.
This mechanism was soon considered as possible high efficiency alternative to traditional fluorescent and also phosphorescent compounds used in lighting displays so far.
In the electroluminescent process, which is observed in OLEDs, an electrical excitation leads to population of singlet and triplet states of the TADF molecules.
Several key kinetic properties of TADF materials determine their ability to efficiently generate light through delayed fluorescence, while minimizing thermal loss pathways.
The most effective strategies employed so far to synthesize TADF molecules are based on donor and acceptor moieties spaced apart or twisted with respect to each other.
Moreover, the TADF decay time, representing another key parameter, should be as short as possible in order to reduce unwanted chemical reactions during excited state population.
In a recent design strategy, electron donating and accepting moieties are separated by two bridges, leading to the DSH molecule.
Thus, ultra-small energy gap ΔEST between the lowest excited singlet and triplet states of only about 1 meV is obtained.
In particular, Cu(I) compounds synthesized with different ligands display a wide range of ΔEST values, extending from around 33 to 160 meV.
[13] Systematic photophysical including theoretical studies of a large number of Cu(I) compounds result in a detailed understanding of TADF properties.
Even robust materials for OLEDs showing long operational device live time (LT90 > 1000 hours at 1000 cd m−2) were reported.
However, the vast majority of research on TADF-based materials is still focusing on improving efficiency, operational device lifetime, and color purity, though first OLED displays[17] that use TADF emitters are already on the market.
They represent donors for efficient radiationless energy transfer to fluorescent acceptors, which finally emit light.
[19] The fluorescein derivative DCF-MPYM has shown success in the field of bioimaging as its long lifetime allows time-resolved fluorescence imaging in living cells.
[20] TADF compounds can also be synthesized to exhibit a tunable color change based on the macroscopic particle size in powder form.
Specifically, asymmetric compounds with diphenyl sulfoxide and phenothiazine moieties have been synthesized displaying linearly tuneable mechanochromism due to a combination of fluorescence and TADF.
The compound named SCP shows dual emission peaks in its photoluminescence spectrum and changes from a green to blue color through mechanical grinding.
Likely the biggest hurdle is the difficulty in producing a blue light emitting TADF molecules with a reasonable operational lifetime.
Fabrication of long operational lifetime of blue light emitting OLEDs is a challenge not only for TADF, but also for phosphorescent materials.
Another difficulty in producing efficient TADF materials is the lack of sufficient knowledge concerning detailed structure-property relations for rational molecular design.
