A comparative study of carbazole-based thermally activated delayed fluorescence emitters with different steric hindrance

Article abstract of DOI:10.1039/c7tc00681k

Thermally activated delayed fluorescence (TADF) emitters based on carbazole groups have been reported to realize different efficiencies in devices. Herein, we report two carbazole-based TADF emitters, namely 2-(9H-carbazol-9-yl)thianthrene 5,5,10,10-tetraoxide (CZ-TTR), which has one free-rotation carbazole, and 2,3-di(9H-carbazol-9-yl)thianthrene 5,5,10,10-tetraoxide (DCZ-TTR), which has two mutually restricted carbazole groups, and investigated the influence of steric hindrance on their properties. Both compounds employed the same donor and acceptor segments and connecting mode. However, due to steric hindrance between the two carbazole segments, DCZ-TTR exhibited a smaller singlet-triplet splitting of 0.03 eV compared with that of CZ-TTR (0.10 eV). The device containing DCZ-TTR showed significantly higher efficiencies (20.1% for external quantum efficiency (EQE), 58.5 lm W^-1 for power efficiency (PE), and 59.6 cd A^-1 for current efficiency (CE)) than those of the CZ-TTR-based device (EQE = 14.4%; PE = 32.9 lm W^-1; CE = 32.5 cd A^-1). These results clearly proved the necessity of introducing suitable steric hindrance when designing highly efficient TADF emitters based on carbazole groups.

Full text of DOI:10.1039/c7tc00681k

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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JPolueransael odfoMnaotteraiadljsuCsht emmairsgtriynsC
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DOI: 10.1039/C7TC00681K
Journal Name
ARTICLE
A comparative study on carbazole-based thermally activated
delayed fluorescence emitters with different steric hindrance
Kai Wang,a,d Wei Liu, a Cai-Jun Zheng,*a,b Yi-Zhong Shi,c Ke Liang,c Ming Zhang,b Xue-Mei Ouc and
Received 00th January 20xx,
Accepted 00th January 20xx
Xiao-Hong Zhang*a,c
DOI: 10.1039/x0xx00000x
Thermally activated delayed fluorescence (TADF) emitters based on carbazole group have been reported to realize different
efficiencies in the devices. Here we report a couple of carbazole based TADF emitters, 2-(9H-carbazol-9-yl)thianthrene
5,5,10,10-tetraoxide (CZ-TTR), with one free-rotation carbazole and 2,3-di(9H-carbazol-9-yl)thianthrene 5,5,10,10-
tetraoxide (DCZ-TTR), with two mutually restricted carbazole groups, studying the influence of steric hindrance in the
molecules. Both the compounds employ exactly same constituent segments and connecting mode. However, due to steric
hindrance between the two carbazole segements, DCZ-TTR exhibits a smaller singlet–triplet splitting of 0.03 eV comparing
that of 0.10 eV for CZ-TTR. The device containing DCZ-TTR shows significantly higher efficiencies of 20.1% for external
quantum efficicency (EQE), 58.5 lm/W for power efficiency (PE) and 59.6 cd/A for current efficiency (CE), which is
significantly higher than that of the device based on CZ-TTR (EQE = 14.4%; PE = 32.9 lm/W; CE = 32.5 cd/A). These results
clearly prove the necessity of introduction of suitable steric hindrance on designing highly efficient TADF emitters based on
carbazole.
www.rsc.org/
need of noble metals, the TADF-based OLEDs show great
advantage on the cost, thus are considered as the new
generation of OLEDs.19
Introduction
To be an efficient TADF emitters, the key point is the small
Organic light emitting diodes (OLEDs) have been
developed and considered as the future display and lightning
technology during the past three decades.1-12 How to use them
efficiently and economically is always the most important issue.
In devices, singlet and triplet excitons are generated with a ratio
of 1:3.3 Traditional fluorescent OLEDs can only harvest singlet
excitons, resulting in three quarters of excitons waste. For
better exciton utilization, phosphorescent OLEDs were once
taken as a hotspot.10-14 However, their necessity of noble metals
leads a great disadvantage on the cost. Recently, the
introduction of thermally activated delayed fluorescence
enough singlet-triplet energy splitting (ΔEST) to lead
a
remarkable RISC efficiency, which requires a harsh separation
between the highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) of the molecule.
Thus, the most common way to build TADF emitters is
combining electron-donor (D) and electron-acceptor (A)
segments in one molecule6, 20-22 and isolating the HOMOs on D
segments and LUMOs on A segments respectively. Till now,
carbazole (CZ) and its derivatives are widely used to construct
TADF emitters for their suitable electron-donating abilities and
excellent stabilities during electroluminescence.5, 23-30 In 2012,
Adachi et al. reported first highly efficient TADF OLEDs with a
high external quantum efficiency (EQE) of 19.3% by using a CZ
derivative 4CZIPN as the emitter.5 And then a series of CZ
derivatives were developed as TADF emitters and realized high
EQEs about 20% in the devices.31-33 However, beyond them,
there are also a lot of CZ-based TADF emitters realizing
unsatisfactory efficiencies in the devices.7, 34-36 In 2012, Zhang
et al. designed a series of CZ-based blue TADF emitters, but the
maximum EQE of the devices was only 9.9%.34 Recently, Park et
al. reported CZ-based TADF emitters with a low maximum EQE
of 8.7% in the devices.35 Such huge differences on the
efficiencies of these devices disorder the further design of CZ-
based TADF emitters.
(TADF) emitters provided a new approach to realize highly
15-18
efficient OLEDs.5-7,
Via an effective reverse intersystem
crossing (RISC) process from the lowest triplet states (T1) to the
lowest singlet states (S1), TADF emitters can also utilize
nonradiative triplet excitons for emission, so that all excitons
can be totally utilized in the devices.6 Moreover, without the
a.Nano-organic Photoelectronic Laboratory, Technical Institute of Physics and
Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R. China. *Email:
xhzhang@mail.ipc.ac.cn
b.University of Electronic Science and Technology of China (UESTC), Chengdu,
Sichuan, 610054, P.R. China. *Email: zhengcaijun@uestc.edu.cn
c. Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory
for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou,
Jiangsu, 215123, P.R. China.
d.University of Chinese Academy of Sciences, Beijing 100049, P.R. China.
Electronic Supplementary Information (ESI) available: [Cyclic voltammetry, TGA and
DSC thermograms, transient PL decay curve, device optimization, EL performances
and kinetic parameters]. See DOI: 10.1039/x0xx00000x
In this work, we designed and synthesized a couple of CZ-
based
TADF
emitters,
2-(9H-carbazol-9-yl)thianthrene
5,5,10,10-tetraoxide (CZ-TTR), with one free-rotation CZ group
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and 2,3-di(9H-carbazol-9-yl)thianthrene 5,5,10,10-tetraoxide
(DCZ-TTR), with two mutually restricted CZ groups, to study the
influence of steric hindrance on molecular properties. In
consequence of the same constituent D and A segments and
connecting mode, both two emitters exhibit similar HOMO and
LUMO energy levels. However, without an extra limitation
between CZ as D segment and thianthrene 5,5,10,10-tetraoxide
(TTR) as A segment for CZ-TTR, a common dihedral angle of 47o
is realized between CZ and TTR segments. While in DCZ-TTR, the
steric hindrance between two CZ groups enlarges the
corresponding dihedral angles to 60o, resulting in a smaller
overlap between HOMO and LUMO. As expected, CZ-TTR
exhibits a ΔEST of 0.10 eV, whereas DCZ-TTR exhibits a
significantly smaller ΔEST of 0.03 eV. In the OLEDs, DCZ-TTR
shows significantly higher EQE of 20.1% than that of 14.4% for
CZ-TTR. These results not only prove the necessary role of
extremely small ΔEST to TADF emitters, but also clearly
demonstrate the great importance of introducing appropriate
steric hindrance on designing CZ-based TADF emitters.
View Article Online
DOI: 10.1039/C7TC00681K
Theoretical calculations and electrochemical properties
(a) (b)
Fig. 1. Calculated spatial distributions of the HOMO (up)
and LUMO (below) energy densities of (a) CZ-TTR and (b)
DCZ-TTR.
Results and Discussion
Synthesis
To gain the insight into the structure-property relationship,
we performed density functional theoretical (DFT) calculations
on two carbazole derivatives. As shown in Figure 1, due to the
same constituent D and A segments, in both two compounds,
the electron distributions of LUMO are mainly located at TTR
moiety, and the HOMO is primarily confined at the CZ region.
Obviously, there are only small overlaps between HOMO and
LUMO for both CZ-TTR and DCZ-TTR, which will insure both two
compounds realizing small ΔESTs and TADF characteristics.
However, due to the different steric hindrances in the molecular
structures, two molecules exhibit difference in the dihedral
angles between CZ and TTR segments. The dihedral angle in CZ-
TTR is 47o, which is consistent with common carbazole based D-
Scheme 1. Synthetic routes and molecular structures of CZ-
TTR and DCZ-TTR.
A structure molecules with free rotations between D and A
34
segments.30,
While in DCZ-TTR, with the steric hindrance
between two nearly parallel CZ groups, two dihedral angles are
both 60o, clearly larger than that in CZ-TTR. These larger
dihedral angles will lead more evident separation and weaker
electron interaction between HOMO and LUMO, theoretically
resulting in a smaller ΔEST of DCZ-TTR than that of CZ-TTR.
The synthetic routes of CZ-TTR and DCZ-TTR are depicted
in Scheme 1. First, 2-bromothianthrene was synthesized via
bromination reaction; whereas for DCZ-TTR, 2,3-
difluorothianthrene was synthesized via nucleophilic loop
reaction by benzene-1,2-dithiol and 1,2,4,5-tetrafluorobenzene.
Then both the resulting halides were oxidized by hydrogen
peroxide in acetic acid solution. Finally, CZ-TTR was synthesized
via the Pd(OAc)2 catalyzed cross-coupling reaction between 2-
bromothianthrene 5,5,10,10-tetraoxide and carbazole. While
DCZ-TTR was synthesized via nucleophilic cross-coupling
reaction between 2,3-difluorothianthrene 5,5,10,10-tetraoxide
and carbazole. Besides, both the target compounds were
purified via sublimation process before their further utilizations.
The chemical structures of the intermediates and target
compounds were fully characterized and confirmed by nuclear
magnetic resonance (NMR) spectroscopy and mass
spectrometry (MS).
To compare the differences on HOMOs and LUMOs, the
electrochemical properties of CZ-TTR and DCZ-TTR were then
investigated by cyclic voltammetry. As shown in Figure S1,
determined from the onset of oxidation curves with respect to
that of ferrocene, the HOMO energy levels are calculated to be
˗6.02 and ˗6.06 eV for CZ-TTR and DCZ-TTR, respectively.37
Likewise, their LUMO energy levels are calculated to be -3.30
and -3.44 eV, respectively, from the onset of the reduction
curves. Apparently, with the same constituent D and A
segments, the HOMO and LUMO values are very close for both
compounds. Meanwhile, both the HOMO and LUMO energy
levels of DCZ-TTR are lower than that of CZ-TTR. This difference
should be ascribed to the weaker electron interaction between
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CZ and TTR segment in DCZ-TTR molecule, which is consistent CZ-based TADF emitters with free rotations between D and A
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segments.36, 38, 39
DOI: 10.1039/C7TC00681K
with the larger dihedral angles caused by the steric hindrance.
Photophysical Properties
(a)
1.2
Cyclohexane
Toluene
1.0
0.8
0.6
0.4
0.2
0.0
1.0
Dichloromethane
CZ-TTR
0.5
0.0
1.0
DCZ-TTR
400
500
600
700
0.5
Wavelength (nm)
(b)
1.2
0.0
Cyclohexane
Toluene
Dichloromethane
300
400
500
600
Wavelength(nm)
1.0
0.8
0.6
0.4
0.2
0.0
Fig. 2. Absorption spectra (black line) of CZ-TTR and DCZ-
TTR in ethyl acetate at room temperature; and their
fluorescence and phosphorescence spectra (red and green
line) in mCP films at 77 K.
400
500
600
700
The photophyisical properties of the two compounds were
investigated as shown in Figure 2. Two compounds exhibit nearly the
same absorption spectral shape. The absorption below 350 nm
should be mainly attributed to the locally excited transition of
carbazole and TTR segments. And the absorption in range of 350 to
430 nm will primarily be attributed to intramolecular charge-transfer
(ICT) absorption from CZ group to TTR group. From the absorption
edges, the optical bandgaps of CZ-TTR and DCZ-TTR were estimated
to be 3.06 and 2.95 eV. This tendency is consistent with the HOMO
and LUMO results. Owing to the same constituent D and A segments
of two compounds, the shapes and positions of their
photoluminescence (PL) spectra are very close. By measuring 10 wt%
CZ-TTR and 6.5 wt% DCZ-TTR doped m-bis(N-carbazolyl)benzene
(mCP) films at 77 K, fluorescence and phosphorescence spectra were
respectively obtained with peaks at 487 and 497 nm for CZ-TTR, and
502 and 508 nm for DCZ-TTR. Accordingly, the S1 and T1 levels were
estimated to be 2.59 and 2.49 eV for CZ-TTR and 2.47 and 2.44 eV for
DCZ-TTR, respectively. With the influence of additional CZ group as
steric hindrance restricting the conjugations between CZ and TTR
segments, the more evident separation between CZ and TTR
segments in DCZ-TTR molecule are theoretically predicted and thus
results in our further theoretical prediction on the lower ΔEST of DCZ-
TTR than that of CZ-TTR. As listed in Table 1, the ΔEST of DCZ-TTR (0.03
eV) is indeed significantly smaller than that of CZ-TTR (0.10 eV). The
smaller ΔEST of DCZ-TTR will more enhance the RISC process from T1
to S1, leading a higher rate constant for RISC process (kRISC) and
benefiting the utilization of triplet excitons in the devices. Besides,
the value of 0.10 eV is also consistent with the ΔESTs of the reported
Wavelength (nm)
Fig. 3. PL spectra of (a) CZ-TTR and (b) DCZ-TTR in
cyclohexane, toluene and dichloromethane at room
temperature.
To investigate the ICT transition characteristic of both the
compounds, the PL spectra of CZ-TTR and DCZ-TTR are measured in
different solvents. As shown in Figure 3, significant solvatochromic
effect can be observed with the increase of environmental polarity.
the PL spectra of CZ-TTR shift from the peak of 424 nm in nonpolar
cyclohexane to the peak of 465 nm in higher polar toluene and the
peak of 518 nm in dichloromethane with highest polarity.
Correspondingly, the bathochromic shift is observed for PL spectra of
DCZ-TTR in the three kinds of solvents in the same order (peak at 477
nm in cyclohexane; at 501 nm in toluene and 545 nm in
dichloromethane). The Stokes shift of DCZ-TTR is evidently decreased
comparing CZ-TTR, which should be attributed to the less efficient
ICT transition with the larger dihedral angles.
PL quantum yields (PLQYs) of 10 wt% CZ-TTR and 6.5 wt% DCZ-
TTR doped mCP films were measured via integrating sphere
measurements in atmosphere. By exciting at 300 nm, high PLQY
values of 56.63% and 47.14% were obtained for CZ-TTR and DCZ-TTR
doped films, respectively. Considering the triplet excitons generated
from ISC process would be unavoidably quenched by oxygen during
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the measurement. The contribution of delayed component will be
suppressed, and these PLQY values should be significantly lower than
that in oxygen-free conditions for those TADF emitters, especially the
ones with higher delay component.40 With the theoretical prediction
of more effective RISC process, the suppression will be more
significant for DCZ-TTR. And the stronger electron interaction
between CZ and TTR segments in CZ-TTR will also lead higher prompt
quantum yield. These two factors cause the measured PLQY value of
DCZ-TTR is lower than that of CZ-TTR. Temperature depended
transient decay characteristics of 10 wt% CZ-TTR and 6.5 wt% DCZ-
TTR doped mCP films were then measured in nitrogen atmosphere
to prove their TADF characteristics. CZ-TTR-doped film showed a
prompt lifetime of 69.63 ns and a delayed lifetime of 28.1 μs;
whereas DCZ-TTR based film exhibited a prompt lifetime of 31.68 ns
and a delayed lifetime of 25.8 μs at room temperature. Moreover,
with the increase of test temperature from 200 to 300 K, both the
delayed lifetimes of CZ-TTR and DCZ-TTR show regular decline, which
further confirm their TADF characteristics, as presented in Figure S3.
View Article Online
DOI: 10.(160)39/C7TC00681K
퐤퐓퐧퐫=kd-kRISCΦF
Where ΦF and ΦTADF are the prompt and delayed fluorescence
quantum efficiencies; kp and kd are the prompt and delayed
fluorescence decay rate constants; kISC and 퐤퐓퐧퐫 are the rate constant
for intersystem crossing (ISC) process from S1 to T1 states and the
non-radiative decay rate constant at T-state, respectively. Here, we
estimated the ΦF and ΦTADF of both compounds via the PLQY values
in air condition as an alternative of those in oxygen-free environment.
ΦF and ΦTADF were determined using intensity ratio and total
emission quantum yield as literature reported. 40-42 Though there are
deviations in both the estimations, the trend still can easily be
noticed. As shown in Table S2, DCZ-TTR has a kRISC about 6 times
larger than CZ-TTR which is well matched our further estimation
based on the ΔEST difference between them. Also a small increase in
kd for DCZ-TTR can be seen. These increased rates naturally decrease
the non-radiative decay rate constant at T-state (퐤퐓퐧퐫) of DCZ-TTR
comparing CZ-TTR and would further result in a much higher
efficiency in devices.
To further comparing the TADF characteristic of both
compounds, the main kinetic parameters of both compounds were
estimated according to the literature40-42 (summarized in Table S2).
Ignoring the non-radiative decay of singlet excitons at room
temperature, we estimated the kinetic parameters of both
compounds via the following equations:41
Thermal Properties
Thermal properties of CZ-TTR and DCZ-TTR were measured
by thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) under nitrogen (as shown in Figure S2). The
decomposition temperatures (Td, corresponding to 5% weight
loss) of CZ-TTR and DCZ-TTR are respectively as high as 395 and
415 °C, respectively. The glass transition temperature (Tg) is as
high as 124°C for CZ-TTR, and DCZ-TTR even exhibits no obvious
glass transition in the range from 25 to 350 °C. These good
thermal stabilities ensure the utilization of both emitters in
OLEDs.
kp = 1/τp
kd = 1/τd
(1)
(2)
퐤퐒퐫=kpΦF
(3)
kISC=kp(1-ΦF)
(4)
(5)
퐤퐩퐤
퐝 횽퐓퐀퐃퐅
=
퐤퐈퐒퐂 횽퐅
퐤퐑퐈퐒퐂
Table 1. Physical properties of molecules in this work.
HOMO
/eV[a]
-6.02
-6.06
LUMO
/eV[b]
-3.30
-3.44
λfluo.
/nm[c]
487
S1
λphos.
/nm[e]
497
T1
ΔEST
/eV
Compounds
PLQY[g]
Td
Tg
/eV[d]
2.59
2.47
/nm[f]
2.49
2.44
CZ-TTR
0.10
0.03
56.63%
47.14%
395
415
124
-
DCZ-TTR
502
508
[a] Determined from the onset of oxidation potential with respect to that of ferrocence in acetonitrile; [b] Determined
from the onset of reduction potential with respect to that of ferrocence in dichloromethane; [c] Determined from the
energy peak of the fluorescence spectra of dopants mixed in mCP excited at 300 nm at 77 K; [d] Calculated from
S1=1241/λfluo.; [e] Determined from the energy peak of the phosphorescence spectra of dopants mixed in mCP excited
at 300 nm at 77 K; [f] Calculated from T1=1241/λphos.; [g] Measured dopants mixed in mCP in atmosphere.
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devices, ITO (indium tin oxide) and LiF/Al wereViuewseAdrticalesOntlhinee
anode and the cathode, respectively; TAPDCO(1I:,110-.1B0i3s[94/C-[7NTC,N0-0d6i8(1pK-
tolyl)amino]phenyl]cyclohexane) was used as the hole-
transporting
layer;
TCTA
(4,4’,4’’-tris(carbazol-9-
yi)triphenylamine) was used as the exciton-blocking layer, and
TmPyPb (1,3,5-tri[(32pyridyl)-phen-3-yl]benzene) was used as
the electron-transporting, hole-blocking and exciton-blocking
layer as well. The doping ratios are optimized as 10 wt% for CZ-
TTR and 6.5 wt% DCZ-TTR, respectively (seen in Figure S4).
The EL spectra of two devices are shown in Figure 4a. CZ-
TTR-based device exhibits a greenish blue emission with a CIE
coordinate of (0.21, 0.35) and a peak at 492 nm; whereas DCZ-
TTR-based device shows a green emission with a CIE coordinate
(0.25, 0.50) and a peak at 512 nm. This slight red shift is
consistent with the results of two emitters’ PL spectra.
Moreover, DCZ-TTR-based device shows
a significantly
narrower full-width at half-maximum (FWHM) of 92nm than
that of 101 nm from CZ-TTR-based device, which can be
attributed to higher molecular restriction due to the extra
hindrance.37
Although both compounds consist of the same D and A
segments, devices using CZ-TTR and DCZ-TTR as the emitters
exhibit evidently different efficiency. As shown in Figure 4c, the
device based on CZ-TTR realizes a maximum EQE of 14.4%, a
maximum power efficiency (PE) of 32.9 lm/W and a maximum
current efficiency (CE) of 32.5 cd/A. These results are much
higher than the ceiling of traditional fluorescence based devices
(5%), but clearly lower than the reported highly efficient TADF
emitters (~20%). While for the device based on DCZ-TTR, an
extremely high maximum EQE of 20.1% is realized as well as a
maximum PE of 58.5 lm/W and a maximum CE of 59.6 cd/A,
which are among the best results of TADF based devices. To
better understand the mechanism of the exciton utilization in
the devices, we estimated the kinetic parameters of both
compounds not only using the PLQY values in air condition but
also based on the device efficiency by assuming the out-
coupling efficiency of the devices as 20%. Though both DCZ-TTR
and CZ-TTR have sufficiently low ΔEST for efficient TADF, a
further decrease of the ΔEST can still further significantly
increase the kRISC according to Boltzmann distribution.5
According to the literature, a small reduction in ΔEST of 0.05 eV
can increase kRISC by approximately 1 order of magnitude.43
Here, as summarized in Table S2, the kRISC of DCZ-TTR is about
5-6 time larger than that of CZ-TTR. In this case, the RISC process
of DCZ-TTR is much effective than that of CZ-TTR and results in
the significant higher efficiencies in the devices. Moreover, as
mentioned above, as the PLQY values were obtained in
atmosphere, the contribution of TADF will be suppressed due to
the unavoidable triplet exciton quenching by oxygen. With
more effective RISC process, the suppression will be more
significant for DCZ-TTR, which cause the measured PLQY value
of DCZ-TTR is lower than that of CZ-TTR. These results indicate
introducing suitable steric hindrance and enlarging the dihedral
angles between D and A segments in the molecule could
effectively improve the performance of CZ-based TADF emitters
in the OLEDs.
Fig. 4. a) The EL spectra, b) Current density-voltage-
luminance characteristics and c) The PE-EQE-luminance
characteristics of the devices based on CZ-TTR (red line) and
DCZ-TTR (black line).
Electroluminescence Properties
To compare the electroluminescence properties of two
emitters, OLEDs were fabricated with the optimized structure
of ITO/ TAPC (35 nm) /TCTA (10 nm)/ mCP: x wt% CZ-TTR or
DCZ-TTR (20 nm)/TmPyPb (40nm)/LiF (1 nm)/Al. In both
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dichloromethane. Concentration of the organic phase was followed
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DOI: 10.1039/C7TC00681K
by crude chromatography in petroleum ether. The residue of
removal of solvent from the main fraction was white solid (1.20 g,
yield: 40.0%). MS (EI) m/z: [M]+: calcd for C12H7BrS2 295.9152;
found,295.9155. Then it was oxidized by dissolution in 12 mL of
acetic acid, addition of 6 mL of 30% H2O2, and warming for 24 h at
85 °C. The cooled reaction was diluted with 800 mL of water, and the
resulting solid was removed by filtration, washed with water and
petroleum ether, and then dried, followed by crude chromatography
in dichloromethane, yielding 1.22 g of white solid (84.0%). 1H NMR
(400 MHz, Acetone) δ 8.39 (s, 1H), 8.32 (dd, J = 5.6, 3.4 Hz, 2H), 8.24
(s, 2H), 8.08 (dd, J = 5.8, 3.3 Hz, 2H). MS (EI) m/z: [M]+: calcd for
C12H7BrO4S2 359.8949; found,359.8948.
Conclusions
In conclusion, we developed two CZ based derivatives, CZ-
TTR and DCZ-TTR with the same constituent D and A segments
and connecting mode. With free rotation between D and A
segments, CZ-TTR exhibits a common dihedral angle of 47°
between CZ and TTR segment and a ΔEST of 0.10 eV. Whereas
for DCZ-TTR, with extra steric hindrance between two CZ groups,
larger dihedral angles of 60° between CZ and TTR segments are
realized as well as an extremely small ΔEST of 0.03 eV. In the
devices, DCZ-TTR shows much higher efficiency of 20.1% for
EQE, 58.5 lm/W for PE and 59.6 cd/A for CE, which is
significantly higher than that of CZ-TTR (EQE=14.4%; PE=32.9
lm/W; CE=32.5 cd/A), which should be ascribed to the much
lower ΔEST of DCZ-TTR. These significant improvements clearly
prove the necessity of introduction of suitable steric hindrance
between D and A segments on designing highly efficient TADF
emitters based on CZ group.
2,3-difluorothianthrene 5,5,10,10-tetraoxide. A solution in 50 mL of
sieve-dried N,N-Dimethylformamide (DMF) of 1.420 g (10 mmol) of
1,2-benzenedithiol was stirred for 30 min with strict exclusion of air
with 6,670 g (20.5 mmol) of Cs2CO3. Addition of 3.00 g (20 mmol) of
1,2,4,5-tetrafluorobenzene to the stirred reaction under N2 was
followed by 20 min of stirring, and the reaction was then heated at
60 °C for 12 h. After removal of most of the DMF by vacuum
distillation, the residue was partitioned between water and
dichloromethane. Concentration of the nonaqueous phase was
followed by crude chromatography in petroleum ether. The residue
of removal of solvent from the main fraction was white solid (2.40 g,
yield: 95.5 %). MS (EI) m/z: [M]+: calcd for C12H6F2S2 251.99; found,
252.02. Then it was oxidized by dissolution in 12 mL of acetic acid,
addition of 6 mL of 30% H2O2, and warming slowly first for 1.5 h at
45 °C and then for 1 h at 75 °C. The cooled reaction was diluted with
800 mL of water, and the resulting solid was removed by filtration,
washed with water and petroleum ether, and dried, followed by
crude chromatography in dichloromethane.yielding 2.72 g of white
solid (90.1%).1H NMR (400 MHz, DMSO) δ 8.59 (t, J = 8.0 Hz, 2H), 8.42
– 8.35 (m, 2H), 8.13 – 8.06 (m, 2H). MS (EI) m/z: [M]+: calcd for
Experimental
General information
Nuclear magnetic resonance (NMR) spectral data were
obtained using a Bruker Advance-400 spectrometer with
chemical shifts reported in ppm. Mass spectral data were
obtained using a Finnigan 4021C gas chromatography (GC)-
mass spectrometry (MS) instrument. Absorption and
photoluminescence (PL) spectra were obtained using a Hitachi
ultraviolet-visible (UV-Vis) spectrophotometer U-3010 and a
Hitachi fluorescence spectrometer F-4600, respectively. The C12H6Br2O4S2 315.97; found, 315.97.
transient photoluminance decay characteristics and
2-(9H-carbazol-9-yl)thianthrene 5,5,10,10-tetraoxide (CZ-TTR).
photoluminescence quantum yield of neat solid film were
measured using an Edinburgh Instruments F980 spectrometer.
thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) were measured by using a NETZSCH DSC204
instrument and a TAQ 500 thermogravimeter, respectively in
nitrogen atmosphere. Cyclic voltammetry (CV) was performed
Toluene (20 ml) and tri-tert-butyl phosphine solution 10% in pentane
(0.3 mL, 0.13 mmol) were added to a mixture of 2-bromothianthrene
5,5,10,10-tetraoxide (467 mg, 1.3 mmol), carbazole (270 mg, 1.625
mmol), palladium acetate (15 mg, 0.065 mmol), and sodium tert-
butoxide (156 mg, 1.625 mmol). With stirring, the suspension was
using a CHI660E electrochemical analyzer. The theoretical heated at 90°C for 24 h under nitrogen atmosphere. When cooled to
calculations were performed using the Gaussian-09 program.
Density functional theory (DFT) B3LPY/6-31G (d) was used to
determine and optimize the structures.
room temperature, the mixture was extracted with dichloromethane
and dried over Na2SO4. After the solvent had been removed, the
residue was purified by column chromatography on silica gel using
dichloromethane as the eluent to give a bright yellow solid, with an
85.1% yield (540 mg). 1H NMR (400 MHz, Acetone) δ 8.61 (d, J = 8.4
Materials and Synthesis
All commercially available reagents and chemicals were used without Hz, 1H), 8.56 (d, J = 2.1 Hz, 1H), 8.43 – 8.35 (m, 3H), 8.27 (d, J = 7.8
further purification.
Hz, 2H), 8.14 – 8.10 (m, 2H), 7.68 (s, 1H), 7.66 (s, 1H), 7.51 (t, J = 7.7
Hz, 2H), 7.39 (t, J = 7.5 Hz, 2H). 13C NMR (151 MHz, CDCl3) δ 143.27,
141.58, 139.60, 139.38, 139.34, 136.40, 133.94, 133.77, 130.37,
128.23, 126.80, 126.24, 126.04, 124.42, 123.24, 121.85, 120.75,
109.31. MS (EI) m/z: [M]+: calcd for C24H15NO4S2 445.04; found,
445.04. Anal. calcd for C, 64.70; H, 3.39; N, 3.14; O, 14.37; S, 14.39;
found: C, 64.66; H, 3.42; N, 3.17; S, 14.35.
2-bromothianthrene 5,5,10,10-tetraoxide. 2.16 g (10 mmol) of
thianthrene was added to a solution of 50 mL acetic acid and stirred
for 30 min with strict exclusion of air with 1.46 g (11 mmol) of
aluminum trichloride. Then 0.151 mL (3 mmol) bromine was added
to the stirred reaction under N2 was followed by 20 min of stirring,
and heated at 90 °C for 12 h. After reducing off the excessive bromine
by Na2S2O3 and removal of most of the acetic acid by vacuum 2,3-di(9H-carbazol-9-yl)thianthrene 5,5,10,10-tetraoxide (DCZ-
distillation, the residue was partitioned between water and TTR). Dimethyl sulphoxide (DMSO) of 10 ml was added to a mixture
6 | J. Name., 2012, 00, 1-3
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Page 7 of 9
Journal of Materials Chemistry C
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Journal Name
ARTICLE
4.
5.
6.
7.
A. Chaskar, H.-F. Chen and K.-T. Wong, VAiedwvA.rtiMcleaOtnelirn.e,
of 2,3-difluorothianthrene 5,5,10,10-tetraoxide (467 mg, 1.3 mmol),
carbazole (270 mg, 1.625 mmol) and 1.625 g (5 mmol) of Cs2CO3.
With stirring, the suspension was heated at 60 °C for 2 h under
nitrogen atmosphere. When cooled to room temperature, the
mixture was extracted with dichloromethane and dried over Na2SO4.
After the solvent had been removed, the residue was purified by
column chromatography on silica gel using dichloromethane as the
eluent to give a yellow solid, with an 93% yield (737 mg). 1H NMR
(600 MHz, DMSO) δ 8.67 (s, 2H), 8.44 (dt, J = 7.4, 3.7 Hz, 2H), 8.17 –
8.09 (m, 2H), 7.95 (dd, J = 6.5, 1.9 Hz, 4H), 7.40 (dd, J = 6.9, 1.6 Hz,
4H), 7.20 – 7.00 (m, 8H). 13C NMR was not possible to measured due
to its low solubility. MALDI-TOF-MS: calcd for C36H22N2O4S2 610.10;
found, 610.17. Anal. calcd for C, 70.80; H, 3.63; N, 4.59; O, 10.48; S,
10.50; found: C, 70.85; H, 3.58; N, 4.62; S, 10.46.
2011, 23, 3876-3895.
DOI: 10.1039/C7TC00681K
H. Uoyama, K. Goushi, K. Shizu, H. Nomura and C.
Adachi, Nature, 2012, 492, 234-238.
Q. Zhang, B. Li, S. Huang, H. Nomura, H. Tanaka and C.
Adachi, Nat. Photonics, 2014, 8, 326-332.
S. Hirata, Y. Sakai, K. Masui, H. Tanaka, S. Y. Lee, H.
Nomura, N. Nakamura, M. Yasumatsu, H. Nakanotani,
Q. Zhang, K. Shizu, H. Miyazaki and C. Adachi, Nature
Mater., 2015, 14, 330-336.
R. Gomez-Bombarelli, J. Aguilera-Iparraguirre, T. D.
Hirzel, D. Duvenaud, D. Maclaurin, M. A. Blood-
Forsythe, H. S. Chae, M. Einzinger, D. G. Ha, T. Wu, G.
Markopoulos, S. Jeon, H. Kang, H. Miyazaki, M.
Numata, S. Kim, W. Huang, S. I. Hong, M. Baldo, R. P.
8.
Adams and A. Aspuru-Guzik, Nature Mater., 2016, 15
,
Device Fabrication and Measurement.
1120-1127.
9.
S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K.
Walzer, B. Lussem and K. Leo, Nature, 2009, 459, 234-
238.
X.-K. Liu, C.-J. Zheng, M.-F. Lo, J. Xiao, Z. Chen, C.-L. Liu,
C.-S. Lee, M.-K. Fung and X.-H. Zhang, Chem. Mater.,
2013, 25, 4454-4459.
C. J. Zheng, J. Wang, J. Ye, M. F. Lo, X. K. Liu, M. K. Fung,
X. H. Zhang and C. S. Lee, Adv. Mater., 2013, 25, 2205-
2211.
M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S.
Sibley, M. E. Thompson and S. R. Forrest, Nature, 1998,
395, 151-154.
S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq,
H.-E. Lee, C. Adachi, P. E. Burrows, S. R. Forrest and M.
E. Thompson, J. Am. Chem. Soc., 2001, 123, 4304-4312.
B. Zhang, G. Tan, C. S. Lam, B. Yao, C. L. Ho, L. Liu, Z.
Xie, W. Y. Wong, J. Ding and L. Wang, Adv. Mater.,
2012, 24, 1873-1877.
ITO-coated glasses with a sheet resistance of 30 Ω per square were
used as substrates. Before devices were fabricated, the substrates
were cleaned with acetone, ethanol, and ultrapure water. Then, they
were dried at 100 °C for 1.5 h and treated with UV-ozone under
ambient conditions for 15 min. Finally, the cleaned glasses were
transferred to a vacuum deposition system at approximately 1×10-6
Torr. Thermally evaporated organic layers were sequentially grown
onto the ITO substrates at rate of 1-2 Å s-1. The cathode was
completed via the thermal deposition of LiF at a rate of 0.1 Å s-1 and
then covered with Al metal deposited at rate of 10-20 Å s-1. EL
luminance, CIE color coordinates, and spectra were obtained using a
Spectrascan PR655 photometer, and the current-voltage
10.
11.
12.
13.
14.
characteristics were determined with
a computer-controlled
Keithley 2400 Source Meter in ambient atmosphere. EQE was
estimated from the current density, luminance, and EL spectrum,
assuming a Lambertian distribution.
15.
16.
17.
18.
W. Kan, C. Duan, M. Sun, C. Han and H. Xu, Chem.
Mater., 2016, 28, 7145-7157.
C. Fan, C. Duan, C. Han, B. Han and H. Xu, ACS Appl.
Mater. Interfaces, 2016, 8, 27383-27393.
C. Fan, C. Duan, Y. Wei, D. Ding, H. Xu and W. Huang,
Chem. Mater., 2015, 27, 5131-5140.
Z. Yang, Z. Mao, Z. Xie, Y. Zhang, S. Liu, J. Zhao, J. Xu, Z.
Chi and M. P. Aldred, Chem. Soc. Rev., 2017, 46, 915-
1016.
Acknowledgements
This work was supported by the National Key Research &
Development Program of China (Grant No. 2016YFB0401002),
the National Natural Science Foundation of China (Grant No.
51533005, 51373190), Collaborative Innovation Center of
Suzhou Nano Science & Technology, the Priority Academic
Program Development of Jiangsu Higher Education Institutions
(PAPD) and Qing Lan Project, P.R. China.
19.
20.
C. Adachi, Jpn. J. Appl. Phys., 2014, 53.
S. Y. Lee, T. Yasuda, Y. S. Yang, Q. Zhang and C. Adachi,
Angew. Chem. Int. Ed., 2014, 53, 6402–6406.
H. Hirai, K. Nakajima, S. Nakatsuka, K. Shiren, J. Ni, S.
Nomura, T. Ikuta and T. Hatakeyama, Angew. Chem.
Int. Ed. , 2015, 54, 13581-13585.
21.
22.
23.
24.
M. Numata, T. Yasuda and C. Adachi, Chem. Commun.,
2015, 51, 9443-9446.
J. Y. Li, D. Liu, Y. Q. Li, C. S. Lee, H. L. Kwong and S. T.
Lee, Chem. Mater., 2005, 17, 1208-1212.
Y. Tao, Q. Wang, C. Yang, Q. Wang, Z. Zhang, T. Zou, J.
Qin and D. Ma, Angew. Chem. Int. Ed., 2008, 47, 8104-
8107.
S.-L. Lin, L.-H. Chan, R.-H. Lee, M.-Y. Yen, W.-J. Kuo, C.-
T. Chen and R.-J. Jeng, Adv. Mater., 2008, 20, 3947-
3952.
Notes and references
1.
2.
C. Tang and S. VanSlyke, Appl. Phys. Lett., 1987, 51,
913-915.
J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N.
Marks, K. Mackay, R. H. Friend, P. L. Burn and A. B.
Holmes, Nature, 1990, 347, 539-541.
25.
26.
3.
M. Segal, M. A. Baldo, R. J. Holmes, S. R. Forrest and Z.
G. Soos, Phys. Rev. B, 2003, 68, 14.
J. Li, T. Zhang, Y. Liang and R. Yang, Adv. Funct. Mater.,
2013, 23, 619-628.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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Please do not adjust margins
Journal of Materials Chemistry C
Page 8 of 9
ARTICLE
27.
Journal Name
X. Wang, S. Wang, Z. Ma, J. Ding, L. Wang, X. Jing and
View Article Online
F. Wang, Adv. Funct. Mater., 2014, 24, 3413-3421.
C.-H. Chen, L.-C. Hsu, P. Rajamalli, Y.-W. Chang, F.-I.
Wu, C.-Y. Liao, M.-J. Chiu, P.-Y. Chou, M.-J. Huang and
DOI: 10.1039/C7TC00681K
28.
L.-K. Chu, J. Mater. Chem. C, 2014,
X.-D. Yuan, J. Liang, Y.-C. He, Q. Li, C. Zhong, Z.-Q. Jiang
and L.-S. Liao, J. Mater. Chem. C, 2014, , 6387-6394.
K. Liang, C. J. Zheng, K. Wang, W. Liu, Z. Y. Guo, Y. Y. Li
2, 6183-6191.
29.
30.
2
and X. H. Zhang, Phys. Chem. Chem. Phys., 2016, 18
,
26623-26629.
31.
Y. J. Cho, S. K. Jeon, B. D. Chin, E. Yu and J. Y. Lee,
Angew. Chem. Int. Ed., 2015, 54, 5201-5204.
32.
33.
34.
J. Li, D. Ding, Y. Tao, Y. Wei, R. Chen, L. Xie, W. Huang
and H. Xu, Adv. Mater., 2016, 28, 3122–3130
Y. J. Cho, K. S. Yook and J. Y. Lee, Adv. Mater., 2014, 26
,
6642-6646.
Q. Zhang, J. Li, K. Shizu, S. Huang, S. Hirata, H. Miyazaki
and C. Adachi, J. Am. Chem. Soc., 2012, 134, 14706-
14709.
35.
36.
I. S. Park, S. Y. Lee, C. Adachi and T. Yasuda, Adv. Funct.
Mater., 2016, 26, 1813-1821.
Q. Zhang, H. Kuwabara, W. J. Potscavage, Jr., S. Huang,
Y. Hatae, T. Shibata and C. Adachi, J. Am. Chem. Soc.,
2014, 136, 18070-18081.
37.
38.
39.
D.-Y. Chen, W. Liu, C.-J. Zheng, K. Wang, F. Li, S. L. Tao,
X.-M. Ou and X.-H. Zhang, ACS Appl. Mater. Interfaces,
2016, 8, 16791-16798.
Z. Wang, Y. Li, X. Cai, D. Chen, G. Xie, K. Liu, Y.-C. Wu,
C.-C. Lo, A. Lien, Y. Cao and S.-J. Su, ACS Appl. Mater.
Interfaces, 2016, 8, 8627-8636.
P. Rajamalli, N. Senthilkumar, P. Gandeepan, C.-C. Ren-
Wu, H.-W. Lin and C.-H. Cheng, ACS Appl. Mater.
Interfaces, 2016, 8, 27026-27034.
40.
41.
G. Mehes, H. Nomura, Q. Zhang, T. Nakagawa and C.
Adachi, Angew. Chem. Int. Ed., 2012, 51, 11311-11315.
J. S. Kang, T. R. Hong, H. J. Kim, Y. H. Son, R. Lampande,
B. Y. Kang, C. Lee, J.-K. Bin, B. S. Lee, J. H. Yang, J. Kim,
S. Park, M. J. Cho, J. H. Kwon and D. H. Choi, J. Mater.
Chem. C, 2016, 4, 4512-4520.
42.
43.
T. Nakagawa, S. Y. Ku, K. T. Wong and C. Adachi, Chem.
Commun., 2012, 48, 9580-9582.
P. K. Samanta, D. Kim, V. Coropceanu and J. L. Bredas,
J. Am. Chem. Soc., 2017, 139, 4042–4051.
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DOI: 10.1039/C7TC00681K
Table of Contents Entry
10
1000
100
10
1
0.1
0.01
1
0.1
1
10
100
1000
Luminance (cd/m2)
A
couple of carbazole based thermally activated delayed
fluorescence D-A structure emitters with different steric hindrance
were designed and compared.