excited state (corresponding to the ICT band). In terms of emissive properties, both
isomers emit green fluorescence with a maximum emission wavelength of 521 nm
for g-9 and of 531 nm for m-10. Interestingly, the values of the quantum yields were
found to be highly dependent on the presence of oxygen in the toluene solution.
Indeed, g-9 and m-10 possess quantum yields of 0.46 and 0.34 in degassed toluene
solution, respectively, whereas these values drop to 0.068 and 0.049 in presence of
oxygen. This oxygen dependence on fluorescence efficiencies can be explained by
the oxygen triplet state quenching and preludes therefore the possibility of exciton
exchanges via efficient intersystem crossing. TADF properties were confirmed by
the measurement of fluorescent decays in toluene solution. Biexponential decays
were observed for both molecules with prompt fluorescence lifetimes of 17 ns for g-9
and 22 ns for m-10 and delayed fluorescence lifetimes of 0.38 μs for g-9 and 0.22 μs
for m-10. Those TADF active molecules show also good quantum yields at the solid
state with a value of 0.53 for m-10 and 0.33 for g-9. Because of its better
photophysical performances, g-9 was selected to study the chiroptical properties of
such chiral TADF paracyclophanes. The enantiomers of g-9 were synthesized
starting from enantiopure pseudo-gem-substituted bromo amines g-H (see Scheme
13.4), which were obtained through optical resolution of its racemic form with (R)(À)-10-camphorsulfonyl chloride. Both enantiomers display a small CD signal for
the ICT band centered at 374 nm and a larger signal around 313 nm. The dissymmetry factor (g abs ) of absorbance at the longest wavelength (374 nm) was determined
to be 1.44 Â 10
À3 . These compounds were also proved to be CPL active and possess
a g lum of 4.24 Â 10
À3 , the highest value measured in solution so far for CPTADF
molecules. Despite their promising photophysical and chiroptical properties, the
authors did not report so far about the use of such chiral paracyclophanes as emitters
to construct efficient CP-OLED.
13.4 Conclusion and Perspectives
After the first report about CPTADF molecules in mid-2015, only three other
molecular designs were described in the literature so far (see Fig. 13.9).
This small number of effective compounds can be explained by the inherent
difficulty related to the merging of CPL and TADF properties in a small organic
molecule. Indeed, such molecules should possess a low ΔE ST and a large magnetic
dipole transition moment while maintaining a strong oscillator strength of the
transition involved in the fluorescence emission. More systematic studies are needed
in order to establish general guidelines for the design of efficient CPTADF emitters,
as well as other approaches to combine CPL and TADF properties should be sought.
So far, only two of these four efficient molecular designs have furnished molecules
successfully used as emitters to construct CP-OLEDs. This highlights another
important requirement to integrate in the future design of such molecules that is
related their configurational stability. Indeed, in order to be incorporated in OLED
stacks fabricated through CVD (the fabrication process affording the highest level of
13 Design of Circularly Polarized Thermally Activated Delayed Fluorescence Emitters
305
isomers emit green fluorescence with a maximum emission wavelength of 521 nm
for g-9 and of 531 nm for m-10. Interestingly, the values of the quantum yields were
found to be highly dependent on the presence of oxygen in the toluene solution.
Indeed, g-9 and m-10 possess quantum yields of 0.46 and 0.34 in degassed toluene
solution, respectively, whereas these values drop to 0.068 and 0.049 in presence of
oxygen. This oxygen dependence on fluorescence efficiencies can be explained by
the oxygen triplet state quenching and preludes therefore the possibility of exciton
exchanges via efficient intersystem crossing. TADF properties were confirmed by
the measurement of fluorescent decays in toluene solution. Biexponential decays
were observed for both molecules with prompt fluorescence lifetimes of 17 ns for g-9
and 22 ns for m-10 and delayed fluorescence lifetimes of 0.38 μs for g-9 and 0.22 μs
for m-10. Those TADF active molecules show also good quantum yields at the solid
state with a value of 0.53 for m-10 and 0.33 for g-9. Because of its better
photophysical performances, g-9 was selected to study the chiroptical properties of
such chiral TADF paracyclophanes. The enantiomers of g-9 were synthesized
starting from enantiopure pseudo-gem-substituted bromo amines g-H (see Scheme
13.4), which were obtained through optical resolution of its racemic form with (R)(À)-10-camphorsulfonyl chloride. Both enantiomers display a small CD signal for
the ICT band centered at 374 nm and a larger signal around 313 nm. The dissymmetry factor (g abs ) of absorbance at the longest wavelength (374 nm) was determined
to be 1.44 Â 10
À3 . These compounds were also proved to be CPL active and possess
a g lum of 4.24 Â 10
À3 , the highest value measured in solution so far for CPTADF
molecules. Despite their promising photophysical and chiroptical properties, the
authors did not report so far about the use of such chiral paracyclophanes as emitters
to construct efficient CP-OLED.
13.4 Conclusion and Perspectives
After the first report about CPTADF molecules in mid-2015, only three other
molecular designs were described in the literature so far (see Fig. 13.9).
This small number of effective compounds can be explained by the inherent
difficulty related to the merging of CPL and TADF properties in a small organic
molecule. Indeed, such molecules should possess a low ΔE ST and a large magnetic
dipole transition moment while maintaining a strong oscillator strength of the
transition involved in the fluorescence emission. More systematic studies are needed
in order to establish general guidelines for the design of efficient CPTADF emitters,
as well as other approaches to combine CPL and TADF properties should be sought.
So far, only two of these four efficient molecular designs have furnished molecules
successfully used as emitters to construct CP-OLEDs. This highlights another
important requirement to integrate in the future design of such molecules that is
related their configurational stability. Indeed, in order to be incorporated in OLED
stacks fabricated through CVD (the fabrication process affording the highest level of
13 Design of Circularly Polarized Thermally Activated Delayed Fluorescence Emitters
305