compounds (Scheme 13.2). The first sequence consists in a highly selective
desymmetrization of the starting tetrafluoroterephthalonitrile C, involving
enantiopure BINOL D and K 2 CO 3 as a base, which led to the formation of a chiral
difluorinated intermediate. Then, carbazole E (2.1 equiv) and additional amount of
base were added to the reaction mixture, leading to the targeted molecule 2 with high
yield (83%) and enantiomeric excess (>99%, based on chiral HPLC) after purification over silica gel (see Scheme 13.2).
DFT calculations performed at the B3LYP/6-31G level have revealed some
interesting information about such compounds. First, the spatial separation of
HOMO and LUMO is significant with a HOMO mainly located on carbazolyl
units and a LUMO centered onto the terephthalonitrile substructure. Second, at the
ground state at least, no orbital coefficient has been found of the BINOL moiety.
This means that the BINOL unit plays, at least at the ground state, a role of chiral
perturbator for the potentially TADF active chromophore. Such compounds own
quite interesting photophysical properties with a strong positive solvatochromism
with Φ PF values between 0.74 and 0.06 depending on the solvent polarizability. By
contrast with the first example of CPTADF molecules described earlier, TADF
properties were clearly observed in toluene solution with an important increase in
the quantum yield in degassed toluene compared to the one in aerated solution (from
0.28 to 0.53). A bi-exponential fluorescence decay was observed with lifetime of
20 ns for prompt fluorescence and of 2.9 μs for the delayed component. The chiral
perturbation from the BINOL unit to the TADF active emitter was found to be
effective, both at ground and exited state, with CD signal located at the wavelengths
corresponding to the Internal Charge Transfer band (ICT band) of the TADF emitter
(centered at 420 nm) and a |g lum | value of 1.3 Â 10
À3 recorded in degassed toluene
solution (C ¼ 1 mM). These compounds were successfully applied as emissive
dopant in OLED devices built via Chemical Vapor Deposition (CVD). Using a
nonoptimized OLED architecture, a maximal external quantum efficiency of 9.1%
has been measured. Although the authors have confirmed the enantiopurity of the
compounds after the CVD process, no data were communicated about the potential
polarization light emanating from the device.
The potential of this molecular design to generate efficient emitters for
construction of CP-OLED was later confirmed by Tang and coworkers [17]. Indeed,
N
O
O
N
N
N
F
F
F
F
N
N
(R)-BINOL D, DMF, K 2 CO 3
then Carbazole E 2.1 éq., K 2 CO 3
Room temperature
(R)-2
83%
C
Scheme 13.2 Synthesis of the CPTADF molecule reported by Pieters et al
298
G. Pieters and L. Frederic
desymmetrization of the starting tetrafluoroterephthalonitrile C, involving
enantiopure BINOL D and K 2 CO 3 as a base, which led to the formation of a chiral
difluorinated intermediate. Then, carbazole E (2.1 equiv) and additional amount of
base were added to the reaction mixture, leading to the targeted molecule 2 with high
yield (83%) and enantiomeric excess (>99%, based on chiral HPLC) after purification over silica gel (see Scheme 13.2).
DFT calculations performed at the B3LYP/6-31G level have revealed some
interesting information about such compounds. First, the spatial separation of
HOMO and LUMO is significant with a HOMO mainly located on carbazolyl
units and a LUMO centered onto the terephthalonitrile substructure. Second, at the
ground state at least, no orbital coefficient has been found of the BINOL moiety.
This means that the BINOL unit plays, at least at the ground state, a role of chiral
perturbator for the potentially TADF active chromophore. Such compounds own
quite interesting photophysical properties with a strong positive solvatochromism
with Φ PF values between 0.74 and 0.06 depending on the solvent polarizability. By
contrast with the first example of CPTADF molecules described earlier, TADF
properties were clearly observed in toluene solution with an important increase in
the quantum yield in degassed toluene compared to the one in aerated solution (from
0.28 to 0.53). A bi-exponential fluorescence decay was observed with lifetime of
20 ns for prompt fluorescence and of 2.9 μs for the delayed component. The chiral
perturbation from the BINOL unit to the TADF active emitter was found to be
effective, both at ground and exited state, with CD signal located at the wavelengths
corresponding to the Internal Charge Transfer band (ICT band) of the TADF emitter
(centered at 420 nm) and a |g lum | value of 1.3 Â 10
À3 recorded in degassed toluene
solution (C ¼ 1 mM). These compounds were successfully applied as emissive
dopant in OLED devices built via Chemical Vapor Deposition (CVD). Using a
nonoptimized OLED architecture, a maximal external quantum efficiency of 9.1%
has been measured. Although the authors have confirmed the enantiopurity of the
compounds after the CVD process, no data were communicated about the potential
polarization light emanating from the device.
The potential of this molecular design to generate efficient emitters for
construction of CP-OLED was later confirmed by Tang and coworkers [17]. Indeed,
N
O
O
N
N
N
F
F
F
F
N
N
(R)-BINOL D, DMF, K 2 CO 3
then Carbazole E 2.1 éq., K 2 CO 3
Room temperature
(R)-2
83%
C
Scheme 13.2 Synthesis of the CPTADF molecule reported by Pieters et al
298
G. Pieters and L. Frederic