CPL emitters, respectively), but these devices were strongly limited in efficiency
(EQE~0.05%, notably because of the low luminescence quantum yield of these
types of chiral emitter). Impressive results have also been reported by Fuchter et al.
[9], using phosphorescent organometallic helicenic complex as emitters which has
allowed to generate a degree of CP-electroluminescence (g El ¼ 0.4) sufficient to give
a 19% increased brightness compared to unpolarized OLED of similar performances. Nevertheless, the corresponding luminescence and power efficiencies
were still low (EQE < 1%), highlighting the need of molecular engineering to
design more efficient chiral emitters. In the field related to the development of
OLED technology, the design of Thermally Activated Delayed Fluorescence
(TADF) materials is an exploding area of research (For recent reviews see: [10–
12]). Indeed, in such materials both singlet and triplet excitons can be harvested for
light emission by a reverse intersystem crossing process thanks to a small energy gap
between their singlet and triplet states (ΔE ST ). This property has recently motivated
numerous research works because of the possibility to develop, in theory, OLEDs
with maximum efficiency. Therefore, the design of new molecular architectures
presenting both TADF and CPL emission properties appears as a cornerstone for
enhancement of the performances of OLED devices. In order to merge these two
properties, several molecular designs have been imagined during last 4 years. This
chapter covers the design principles allowing to combine TADF and CPL emissions
in simple organic molecules, the synthesis and photophysical performances of such
compounds and their ability to generate CP electroluminescence once incorporated
in OLED devices.
13.2 Principles of TADF and Design Rules
Fluorescence and electroluminescence can be produced in organic molecules upon
and photo- and electrical excitation, respectively. Generally, fluorescent small
organic molecules exhibit prompt fluorescence (PF), and in this case, emission
takes place following photon absorption (S 0 ! S n ) and excited-state relaxation to
the lowest excited singlet state (S 1 ). On the other hand, delayed fluorescence can be
generated via another mechanism involving the participation of the triplet-excited
state. Once S 1 is attained, if the energy difference between the two lowest excited
states S 1 and T 1 (ΔE ST ) is sufficiently small (ΔE ST < 100 meV), the molecule
undergoes intersystem crossing (ISC) to the lowest triplet-excited state. Then due
to thermal activation, the molecule may go back to S 1 via a process named RISC for
Reverse InterSystem Crossing and generates delayed fluorescence owning the same
emission spectrum than PF but with a longer lifetime, generally in the micro- to
millisecond timeframe (see Fig. 13.1). In the emissive layer of an OLED, electronhole recombination in the organic emitters creates 25% singlet excitons and 75%
triplet excitons. This means that in the case of an OLED using a classical fluorophore
as emitter the maximal internal efficiency is limited to 25%. Because in TADF
molecules both singlet and triplet excitons can be harvested for light emission, they
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G. Pieters and L. Frederic
(EQE~0.05%, notably because of the low luminescence quantum yield of these
types of chiral emitter). Impressive results have also been reported by Fuchter et al.
[9], using phosphorescent organometallic helicenic complex as emitters which has
allowed to generate a degree of CP-electroluminescence (g El ¼ 0.4) sufficient to give
a 19% increased brightness compared to unpolarized OLED of similar performances. Nevertheless, the corresponding luminescence and power efficiencies
were still low (EQE < 1%), highlighting the need of molecular engineering to
design more efficient chiral emitters. In the field related to the development of
OLED technology, the design of Thermally Activated Delayed Fluorescence
(TADF) materials is an exploding area of research (For recent reviews see: [10–
12]). Indeed, in such materials both singlet and triplet excitons can be harvested for
light emission by a reverse intersystem crossing process thanks to a small energy gap
between their singlet and triplet states (ΔE ST ). This property has recently motivated
numerous research works because of the possibility to develop, in theory, OLEDs
with maximum efficiency. Therefore, the design of new molecular architectures
presenting both TADF and CPL emission properties appears as a cornerstone for
enhancement of the performances of OLED devices. In order to merge these two
properties, several molecular designs have been imagined during last 4 years. This
chapter covers the design principles allowing to combine TADF and CPL emissions
in simple organic molecules, the synthesis and photophysical performances of such
compounds and their ability to generate CP electroluminescence once incorporated
in OLED devices.
13.2 Principles of TADF and Design Rules
Fluorescence and electroluminescence can be produced in organic molecules upon
and photo- and electrical excitation, respectively. Generally, fluorescent small
organic molecules exhibit prompt fluorescence (PF), and in this case, emission
takes place following photon absorption (S 0 ! S n ) and excited-state relaxation to
the lowest excited singlet state (S 1 ). On the other hand, delayed fluorescence can be
generated via another mechanism involving the participation of the triplet-excited
state. Once S 1 is attained, if the energy difference between the two lowest excited
states S 1 and T 1 (ΔE ST ) is sufficiently small (ΔE ST < 100 meV), the molecule
undergoes intersystem crossing (ISC) to the lowest triplet-excited state. Then due
to thermal activation, the molecule may go back to S 1 via a process named RISC for
Reverse InterSystem Crossing and generates delayed fluorescence owning the same
emission spectrum than PF but with a longer lifetime, generally in the micro- to
millisecond timeframe (see Fig. 13.1). In the emissive layer of an OLED, electronhole recombination in the organic emitters creates 25% singlet excitons and 75%
triplet excitons. This means that in the case of an OLED using a classical fluorophore
as emitter the maximal internal efficiency is limited to 25%. Because in TADF
molecules both singlet and triplet excitons can be harvested for light emission, they
294
G. Pieters and L. Frederic