very small with a value of 0.07 eV indicating the potential of such molecular design
to generate TADF. Unfortunately, this molecule possesses a relatively small quantum yield (Φ PF ¼ 4%) in toluene solution and the TADF emission in solution state
was not demonstrated. The photophysical properties of compound 1 were then
studied in film state by spin casting a chloroform solution containing 9% wt of
compound 1 in N,N
0 -4,4
0 -dicarbazole-3,5-benzene (mCP) on quartz substrate. Interestingly at the solid state, higher quantum yields were obtained for prompt fluorescence (Φ PF ¼ 11%) and more importantly, TADF emission was observed
(Φ DF ¼ 15%) with a lifetime in the order of milliseconds. This moderate
photophysical performances can be explained by quite large ΔE ST values (experimentally determined to be 0.19 eV) and the small fluorescent rate constant (k F )
resulting in a deactivation of excitons from T 1 . Concerning chiroptical properties,
such molecules possess interesting performances with a dissymmetry factor, |g lum | of
1.1 Â 10
À3 measured in toluene solution, which is in the range of the value found for
purely organic chromophores (10
À5 to 10
À2 ). However, the possibility to use such
molecules as dopants in CP-OLED was not demonstrated.
13.3.2 TADF Molecules Possessing Axial Chirality
The second example of CPTADF-SOM has been reported by Pieters et al. few
months after the pioneer work from Hirata [14]. Conceptually, this other molecular
design involves the tethering of a chiral unit (derived from BINOL) to an active
TADF chromophore (see Fig. 13.3). Here, the proximity of the chiral unit is
anticipated to induce chiroptical properties (induced circular dichroism and circularly polarized luminescence) to the TADF emitter, like in the chiral O-BODIPY
dyes described by De la Moya et al. in 2015 [15]. The TADF unit used in this design
is a donor–acceptor system composed of carbazoles as electron donor and a
terephthalonitrile unit as acceptor. Such type of D-A systems has been previously
used to construct very efficient TADF molecules by Adachi and coworkers [16].
The target enantiomers, (R)-2 and (S)-2, were synthesized through a one-pot
sequential procedure at room temperature involving commercially available
Fig. 13.3 Design of CPTADF molecules reported by Pieters et al
13 Design of Circularly Polarized Thermally Activated Delayed Fluorescence Emitters
297
to generate TADF. Unfortunately, this molecule possesses a relatively small quantum yield (Φ PF ¼ 4%) in toluene solution and the TADF emission in solution state
was not demonstrated. The photophysical properties of compound 1 were then
studied in film state by spin casting a chloroform solution containing 9% wt of
compound 1 in N,N
0 -4,4
0 -dicarbazole-3,5-benzene (mCP) on quartz substrate. Interestingly at the solid state, higher quantum yields were obtained for prompt fluorescence (Φ PF ¼ 11%) and more importantly, TADF emission was observed
(Φ DF ¼ 15%) with a lifetime in the order of milliseconds. This moderate
photophysical performances can be explained by quite large ΔE ST values (experimentally determined to be 0.19 eV) and the small fluorescent rate constant (k F )
resulting in a deactivation of excitons from T 1 . Concerning chiroptical properties,
such molecules possess interesting performances with a dissymmetry factor, |g lum | of
1.1 Â 10
À3 measured in toluene solution, which is in the range of the value found for
purely organic chromophores (10
À5 to 10
À2 ). However, the possibility to use such
molecules as dopants in CP-OLED was not demonstrated.
13.3.2 TADF Molecules Possessing Axial Chirality
The second example of CPTADF-SOM has been reported by Pieters et al. few
months after the pioneer work from Hirata [14]. Conceptually, this other molecular
design involves the tethering of a chiral unit (derived from BINOL) to an active
TADF chromophore (see Fig. 13.3). Here, the proximity of the chiral unit is
anticipated to induce chiroptical properties (induced circular dichroism and circularly polarized luminescence) to the TADF emitter, like in the chiral O-BODIPY
dyes described by De la Moya et al. in 2015 [15]. The TADF unit used in this design
is a donor–acceptor system composed of carbazoles as electron donor and a
terephthalonitrile unit as acceptor. Such type of D-A systems has been previously
used to construct very efficient TADF molecules by Adachi and coworkers [16].
The target enantiomers, (R)-2 and (S)-2, were synthesized through a one-pot
sequential procedure at room temperature involving commercially available
Fig. 13.3 Design of CPTADF molecules reported by Pieters et al
13 Design of Circularly Polarized Thermally Activated Delayed Fluorescence Emitters
297