Angewandte
Communications
Chemie
7
7 K. Interestingly, the Phos spectra of Cz-TRZ1, Cz-TRZ3
and Cz-TRZ4 are well resolved and show characteristic
3
vibrational structures, indicating that their T states are LE
1
states. Conversely, Cz-TRZ2 exhibits a rather broad and less
structured Phos spectra, indicating that its T state can be
1
3
identified as a CT state. This difference in T state characters
1
can be explained well in terms of the molecular structures.
The introduction of two methyl substitutions in the 1, 8-
position of the 3, 6-dimethylcarbazole unit efficiently reduce
its IP despite the large twisted angle of 86.78, consequently
3
3
causing a CT energy that is lower than the LE energy in Cz-
TRZ2 and resulting in a T of Cz-TRZ2 that is dominated by
1
a charge transfer triplet state. However, the IP of 3, 6-
3
dimethylcarbazole is not enough to lower the CT energies of
Cz-TRZ1, Cz-TRZ3 and Cz-TRZ4, which implies that the
3
LEs are their lowest triplet states. Here we note that the T of
1
Cz-TRZ1 is 2.67 eV, which is appreciably lower than 2.85, 2.93
and 2.95 eV for Cz-TRZ2, Cz-TRZ3 and Cz-TRZ4, respec-
tively. To better understand these experimental results, the
natural transition orbitals (NTOs) were calculated to inves-
tigate their S !T transition characters of these four mole-
Figure 3. Transient PL decay spectra of Cz-TRZ1-4 doped into DPEPO
films (6 wt%) at room temperature.
PLQYs and the lifetime of the prompt and delayed compo-
nents. All the physical property data is listed in Table S2.
Unexpectedly, the kRISC of Cz-TRZ2 is the highest among the
four materials. Spin-orbit coupling is virtually not operative
0
1
cules (see Figure S1). Analysis of the NTOs obviously
demonstrates that the T state of Cz-TRZ2 is dominated by
1
CT character. On the other hand, Cz-TRZ1, Cz-TRZ3 and
1
3
Cz-TRZ4 show a main LE feature in their T states. There-
between the CT and CT states in Cz-TRZ2 because the
orbitals involved in both states are the same and thus the
1
fore, the hole-particle NTOs of Cz-TRZ1 expand across the
whole molecule due to its strong electronic coupling between
carbazole and cyaphenine units, while for Cz-TRZ3 and Cz-
TRZ4 they become localized on the cyaphenine segment,
which is thanks to their weak D-A electronic coupling
originating from their perpendicularly oriented structures.
1
3
[9a,10]
matrix element vanishes, h CT jH
j
CTi ¼ 0.
SOC
3
According to the Laporte rule, RISC processes from CT to
CT state is forbidden. Instead, another mechanism exists for
1
explaining the high kRISC of Cz-TRZ2. Hyperfine coupling has
3
1
been proposed to facilitate RISC processes from CT to CT
[
11]
Thus, the T energy of Cz-TRZ1 is significantly lower than
state
and this process can be a possible mechanism.
1
3
1
those of the other three ones and the DEST of Cz-TRZ1-4 are
calculated to be 0.43, 0.08, 0.17 and 0.15 eV, respectively. In
other words, the electronic coupling between the D and A
units and the oxidation potential of the D unit significantly
Conversely, for upconversion from LE to CT (Cz-TRZ3
and 4), RISC processes are strongly dominated by spin-orbit
coupling. While we cannot provide the conclusive mechanism,
the quite small DEST of Cz-TRZ2 should accelerate kRISC. At
any rate, the introduction of methyl groups into the 1, 8-
position of 3, 6-dimethylcarbazole (Cz-TRZ2) is rather
harmful for deep-blue emission due to the significant
decrease of the oxidation potential regardless of the accel-
erated kRISC. In short, substitution on the central phenylene
moieties (Cz-TRZ3 and 4) induced a large twisting, similar to
the 1, 8-substitution, that provided the localization of the T1
state while keeping the large optical band gap.
affect the singlet–triplet energy splitting and T state features
1
of TADF molecules.
The PL quantum yields (PLQYs) of Cz-TRZ1-4 in
oxygen-free toluene are 72%, 86%, 60% and 35%, respec-
tively. In contrast, the films of Cz-TRZ1-4 doped into DPEPO
(
bis[2-(di-(phenyl)phosphino)-phenyl]ether oxide) exhibited
higher PLQYs of 87%, 98%, 92% and 85%, respectively,
because intramolecular rotation and vibrational motions are
suppressed in their solid state. To better understand the
fluorescent behavior, the transient PL characteristics were
analyzed using a streak camera at room temperature.
Transient PL decay curves of 6% Cz-TRZ1-4: DPEPO co-
deposited films are shown in Figure 3. Notably, the Cz-TRZ1
doped film exhibits strong prompt and negligible delayed
fluorescence components, which can be ascribed to its large
The thermal properties of the four compounds were
investigated by thermogravimetric analysis (TGA). Their
decomposition temperatures (T ) with 5% loss are higher
d
than 4058C (see Figure S2), suggesting that they could form
morphologically stable and uniform amorphous films by
vacuum deposition for OLED fabrication.
To investigate the electroluminescence properties of all
four D-A molecules, we constructed the devices with
commonly used multi-layered structure: ITO/HAT-CN
(5 nm)/a-NPD (20 nm)/TCTA (20 nm)/mCP (10 nm)/
DPEPO: 6 wt% Cz-TRZ1-4 (20 nm)/ DPEPO (10 nm)/
TPBi (30 nm)/LiF (0.8 nm)/Al (120 nm) (dopant = Cz-TRZ1
for device 1, Cz-TRZ2 for device 2, Cz-TRZ3 for device 3 and
Cz-TRZ4 for device 4), in which 1,4,5,8,9,11-hexaazatriphe-
nylene-hexacarbonitrile (HAT-CN) and LiF served as hole-
DE . However, prompt and delayed fluorescence compo-
ST
nents are clearly identified in Cz-TRZ2-4 doped films. The
delayed fluorescence components are thus unambiguously
assigned to the TADF mechanism. The lifetimes of the
delayed components of Cz-TRZ2-4 doped films were 3.5, 13.0
and 10.3 ms, respectively. In addition, the radiative decay rate
constants of fluorescence (k ), internal crossing (kISC), and
r
reverse internal crossing (kRISC) are estimated from the
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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