Evaporation- and Solution-Process-Feasible Highly Efficient Thianthrene-9,9′,10,10′-Tetraoxide-Based Thermally Activated Delayed Fluorescence Emitters with Reduced Efficiency Roll-Off

Article abstract of DOI:10.1002/adma.201503225

A study was conducted to exploit evaporation- and solution-process-feasible thermally Activated Delayed Fluorescence (TADF) materials with reduced efficiency roll-off. Two novel emitters ACRDSO2 and PXZDSO2, containing thianthrene-9,9',10,10'-tetraoxide (TEDSO2), as a brand-new electron-acceptor moiety were designed and synthesized for the investigations. 9,9-Dimethyl-9,10-dihydroacridine (ACR) and phenoxazine (PXZ) with good hole-transporting ability were introduced as the electron-donor units to give a shallow HOMO energy level and thus improved hole injection.

Full text of DOI:10.1002/adma.201503225

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Evaporation- and Solution-Process-Feasible Highly Efficient
Thianthrene-9,9′,10,10′-Tetraoxide-Based Thermally
Activated Delayed Fluorescence Emitters with Reduced
Efficiency Roll-Off
Gaozhan Xie, Xianglong Li, Dongjun Chen, Zhiheng Wang, Xinyi Cai, Dongcheng Chen,
Yunchuan Li, Kunkun Liu, Yong Cao, and Shi-Jian Su*
Organic light-emitting diodes (OLEDs) have been a research
focus for almost 30 years since they have great potential appli-
cations in flat-panel displays, solid-state lighting, and wearable
electronics.[1–3] It is well known that the recombination of holes
and electrons under electrical excitation typically generates
25% singlet excitons and 75% triplet excitons.[4] However, for
traditional fluorescent OLEDs, only 25% singlet excitons can
be utilized to emit light and the other 75% triplet excitons are
generally wasted through nonradiative transition. Phospho-
rescent OLEDs can reach 100% internal quantum efficiency
theoretically because they are able to harvest both singlet and
triplet excitons by incorporating heavy metals into the organic
aromatic frameworks to increase spin–orbit interactions. Nev-
ertheless, phosphorescent materials, based on transition-metal
complexes, have to use noble metals such as iridium and plat-
inum, which are expensive and nonrenewable.[5] In contrast,
metal-free thermally activated delayed fluorescence (TADF)
emitters that can realize 100% exciton utilization through effi-
cient reverse intersystem crossing (RISC) of triplet excitons are
regarded as promising materials for next-generation OLEDs.[6]
To date, numerous TADF emitters have been synthesized
and applied in vacuum-deposited devices with commendable
efficiencies.[6–20] Compared with vacuum deposition, the solu-
tion process is generally accepted as a more feasible means
to realize low-cost, large-area OLED displays or lighting prod-
ucts.[21,22] To the best of our knowledge, only several green
solution-processed TADF OLEDs have been achieved up to the
present,[23–27] and almost all the devices are based on unitary
carbazole derivatives. For solution-processed OLEDs, an emit-
ting layer (EML) is generally spin coated on a wet-processed
PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrenesu-
lfonate)) layer without any hole-transporting material because
spin coating of the emitting layer may dissolve the underlying
hole-transporting layer.[23–27] A relatively higher lying HOMO
(highest occupied molecular orbital) energy level is generally
required for emitting materials to match the HOMO energy
level of PEDOT:PSS and thus to ensure efficient hole injection.
However, carbazole derivatives generally possess a lower-lying
HOMO energy level, which gives an improved hole-injection
barrier from the PEDOT:PSS layer. Furthermore, most solu-
tion-processed materials consist of bulky soluble groups and
are generally more complicated to synthesize and difficult to
purify by sublimation compared with evaporation-processed
materials. Apparently, it is quite valuable for a TADF emitter
that can take both advantages of the evaporation and solution
process. It is noteworthy that hitherto 4CzIPN ((4s,6s)-2,4,5,6-
tetra(9H-carbazol-9-yl)isophthalonitrile)[6] is the only reported
evaporation- and solution-process-feasible TADF emitter.[26]
As thus, more high-efficiency and diversified evaporation- and
solution−process-feasible emitters with various emission colors
are highly challenging. Besides, both vacuum-deposited and
solution-processed TADF OLEDs are facing an urgent problem
of efficiency roll-off. Generally, efficiency roll-off can be quan-
tified by the critical current density J0, which represents the
current density at the half maximum of the external quantum
efficiency (EQE). The smaller the J0 value is, the more severe
the efficiency roll-off. For most reported TADF OLEDs, the J0
values are less than 10 mA cm−2, which needs to be further
improved.[9,18,19,28–31]
In this work, we were devoted to exploiting evaporation- and
solution-process-feasible TADF materials with reduced effi-
ciency roll-off. Two novel emitters ACRDSO2 and PXZDSO2,
containing thianthrene-9,9′,10,10′-tetraoxide (TEDSO2), as a
brand-new electron-acceptor moiety were designed and synthe-
sized (Figure 1). 9,9-Dimethyl-9,10-dihydroacridine (ACR) and
phenoxazine (PXZ) with good hole-transporting ability were
introduced as the electron-donor units to give a shallow HOMO
energy level and thus improved hole injection. Although a
large torsional angle between the electron donor and electron
acceptor units has been reported for compounds consisting
of ACR and PXZ, which is critical to separate the HOMO and
LUMO (lowest unoccupied molecular orbital) to achieve a small
energy gap (ΔEST) between the lowest singlet excited state (S1)
and the lowest triplet excited state (T1),[14,17,32] an overly large
dihedral angle will prevent the S1–S0 transition for the seriously
reduced transition moment according to Fermi’s golden rule.[33]
Thus, a benzene ring was introduced as a π bridge between the
donor and the acceptor since the elongated distance between
G. Xie, X. Li, D. Chen, Z. Wang, X. Cai, Dr. D. Chen,
Y. Li, K. Liu, Prof. Y. Cao, Prof. S.-J. Su
State Key Laboratory of Luminescent Materials and
Devices and Institute of Polymer Optoelectronic
Materials and Devices
South China University of Technology
Guangzhou 510640, China
E-mail: mssjsu@scut.edu.cn
DOI: 10.1002/adma.201503225
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As shown in Figure S1 (Supporting
Information) and Table 1, ACRDSO2 and
PXZDSO2 exhibit adequate thermal sta-
bility with similar decomposition tempera-
tures (Td) of 355 and 351 °C at 5 wt% weight
loss. According to their thermogravimetric
analysis (TGA) curves, they are likely to be
sublimed before decomposition. In addition,
no phase transition, like the glass transition
and the melting temperature, was observed
in the tested temperature range, indicating
their amorphous characteristics (Figure S2,
Supporting Information). UV–vis absorp-
tion and photoluminescence (PL) spectra of
ACRDSO2 and PXZDSO2 in thin solid films
are shown in Figure S3 (Supporting Informa-
tion). Both the two compounds show a sharp
and strong absorption peak at 289 nm, which
can be ascribed to the π–π* transitions of
their conjugated backbone, while ACRDSO2
shows a broad and weak absorption peak at
405 nm, which is assigned to the CT absorp-
tion mainly derived from charge transfer
from the electron donor ACR to the elec-
tron acceptor TEDSO2. The CT absorption
band of PXZDSO2 is redshifted and stronger
in comparison with ACRDSO2 due to the
stronger electron-donating ability of PXZ
than that of ACR. A similar bathochromic
Figure 1. Molecular structures, optimized geometries and HOMO, LUMO distributions of
ACRDSO2 and PXZDSO2 calculated at the TD-DFT/B3LYP/6-31G** level by Gaussian 09W.
the donor and the acceptor can properly weaken the nonradia-
tive internal conversion rate constant to obtain a large fluores-
cence rate constant, meanwhile maintaining a tiny ΔEST.[20]
Time-dependent density functional theory (TD-DFT) calcula-
tions were employed to gain insight into the structure−property
relationships of ACRDSO2 and PXZDSO2. Their ground-state
geometries were optimized on B3LYP/6-31G** level in the gas
phase. For vertical transition energy computation, M06-2X/6-
31G* was utilized to better describe the excited states based on
the optimized ground-state geometries.[6] As expected, the dihe-
dral angles between the donor units and the phenyl bridge (θ1)
are almost vertical in the optimized geometries shown in Figure 1
and Table S1 (Supporting Information). The severely distorted
angles can break the electronic transition of the charge-transfer
(CT) excited state effectively, owing to the spatial separation
of the donor and acceptor moieties. In contrast, the dihedral
angles between TEDSO2 and the phenyl bridge (θ2) are rather
smaller compared to θ1. Besides, for both the two compounds,
the bond lengths between TEDSO2 and the phenyl bridge (L2)
are much shorter than those between the donor and the phenyl
bridge (L1), indicating the phenyl bridge has stronger conjuga-
tion with the TEDSO2 acceptor unit and therefore increases its
electron-withdrawing ability. As a consequence, the HOMOs
are mainly located on the ACR and PXZ moieties due to their
strong electron-donating property, while the LUMOs are dis-
persed over the phenyl bridge and the TEDSO2 acceptor unit
because of the strong electron-withdrawing ability of TEDSO2
and the elongated conjugation between them. The almost-sep-
arated HOMOs and LUMOs result in extremely tiny calculated
ΔEST, 0.0050 eV for ACRDSO2 and 0.0059 eV for PXZDSO2.
shift is also observed for their PL spectra recorded in thin films,
and the neat films of ACRDSO2 and PXZDSO2 emit yellow and
orange light centered at 575 and 608 nm, respectively. In addi-
tion, phosphorescence spectra of the ACRDSO2 and PXZDSO2
neat films were measured at 77 K (Figure S4, Supporting Infor-
mation). According to the fluorescence and phosphorescence
spectra, ΔEST of ACRDSO2 and PXZDSO2 were estimated to
be 0.058 and 0.048 eV, respectively, which is favorable for effi-
cient reverse intersystem crossing.
The electrochemical properties of ACRDSO2 and PXZDSO2
were examined by using cyclic voltammetry. As shown in
Figure S5 (Supporting Information) and Table 1, the HOMO
and LUMO energy levels are estimated to be −5.26 and −2.65 eV
for ACRDSO2, and −5.06 and −2.67 eV for PXZDSO2, respec-
tively. Compared to ACRDSO2, the stronger electron-donating
Table 1. Thermal, photophysical, and electrochemical properties of
ACRDSO2 and PXZDSO2.
Compound
Tda)/Tgb)
[°C]
HOMOe)/LUMOf)
[eV]
c)
d)
λabs
λPL
[nm]
[nm]
575
608
ACRDSO2
PXZDSO2
355/N.A.
351/N.A.
289/405
289/420
−5.26/−2.65
−5.06/−2.67
a)Decomposition temperature (Td) at 5 wt% weight loss obtained from TGA meas-
urements; b)Glass transition temperature (Tg) obtained from differential scanning
calorimetry measurements; c)UV−vis absorption spectra measured in thin solid
films; d)PL spectra measured in thin solid films; e)HOMO energy level calculated
from the empirical formula: EHOMO = −(Eox + 4.4); f)LUMO energy level estimated
from the absorption edge of the thin films (λonset) and EHOMO, using empirical for-
mula: ELUMO = EHOMO + 1240/λonset
.
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group of PXZ makes the HOMO energy level of PXZDSO2
0.2 eV shallower.
efficiency of 30% (φp), and the delayed component has a decay
lifetime of 8.3 µs (τd) with an efficiency of 41% (τd). The life-
times of the prompt and delayed ingredients for the doped
film of PXZDSO2:CBP are 36 ns (τp) and 5.0 µs (τd), and
their contributions to the total φPL are 33% (φp) and 29% (φd),
respectively. Noticeably, the quite short lifetimes of the delayed
components, which are close to general phosphorescent mate-
rials, are important to achieve high efficiency and reduced effi-
ciency roll-off.[15,17,19,29,30,32,36] Then, the efficiencies of inter-
system crossing (ISC) (φISC) and RISC (φRISC) were estimated
from Equation (1) and (2), respectively:[37–39]
To further investigate the luminescent properties of
ACRDSO2 and PXZDSO2, their PL quantum yields (PLQYs)
in dilute toluene solutions (10−5 mol L−1) were measured.
ACRDSO2 and PXZDSO2 exhibit low PLQY values of 17% and
18% in solution under air condition, respectively. However, after
excluding oxygen by bubbling argon for 10 min, their PLQY
values increase to 34% and 37%, almost two times larger than
before, indicating that triplet excitons quenched by oxygen are
inhibited and then reverse to S1 to generate delayed emission.
Furthermore, two doped films in CBP (4,4′-dicarbazolyl-1,1′-
biphenyl) were also vacuum co-deposited at a concentration of
6 wt% for PLQY measurement. CBP has a triplet energy of 2.6 eV,
which is high enough to suppress back energy transfer from
the guest to the host to confine the triplet excitons within the
guest molecules.[34] As is shown in Table S2 (Supporting Infor-
mation), the PLQYs of the co-deposited films of ACRDSO2:CBP
and PXZDSO2:CBP are 71% and 62%, respectively. Compared
to the PLQY values of the toluene solutions, the PLQY values of
the doped films increased significantly, which can be ascribed
to efficient restriction of collision-induced intramolecular
nonradiative transitions and bimolecular processes in the
rigid matrix.[35] Also, as appended in Figure S6 (Supporting
Information), the co-deposited films of ACRDSO2:CBP and
PXZDSO2:CBP emit green light with λmax = 520 nm and yellow
light with λmax = 540 nm, respectively, which are significantly
blueshifted compared with their neat films.
In order to investigate the emission mechanism of the co-
deposited films, time-resolved transient PL decay measure-
ments were carried out at room temperature. As shown in
Figure 2, both transient PL decay curves consist of a nano-
second-order prompt decay component and a microsecond-
order delayed decay component as fitted by double-exponential
decay. According to the integration of the prompt and delayed
component of the time-resolved transient PL decay curves, the
PLQY values of their respective components are calculated. For
the doped film of ACRDSO2:CBP, the prompt component has
a transient decay lifetime of 36 ns (τp) with a corresponding
(1)
(2)
φISC = 1−φp
φRISC = φd /φISC
where the nonradiative decay from S1 to S0 is assumed to be
negligible. The φISC and φRISC for ACRDSO2:CBP are estimated
to be 70% and 59%, respectively, which means 30% of the sin-
glet excitons radiatively decay directly and 70% of singlet exci-
tons give the T1 state through ISC; meanwhile, 59% of the tri-
plet excitons are upconverted into the S1 state and decay radia-
tively. As for PXZDSO2:CBP, φISC and φRISC are 67% and 43%,
respectively. In addition, their temperature-dependent transient
PL decays were also measured from 79 to 300 K. As shown in
Figure S7 (Supporting Information), both the two doped films
exhibit slightly enhanced delayed fluorescence intensity at high
temperature, which can be ascribed to predominant activation
by thermal energy, but in the further longer delayed times,
the slightly decreased delayed fluorescence may be due to the
simultaneously enhanced nonradiative deactivation processes
of the triplet excitons.
Vacuum-deposited devices were fabricated in a structure of
indium tin oxide (95 nm)/(dipyrazino(2,3-f:2′,3′-h)-quinoxaline-
2,3,6,7,10,11-hexacarbonitrile) (5 nm)/1,1-bis[4-[N,N-di(p-tolyl)-
amino]phenyl]cyclohexane (20 nm)/EML (35 nm)/1,3,5-tri(m-
pyrid-3-yl-phenyl)benzene (TmPyPB)[40] (55 nm)/LiF (1 nm)/Al
(100 nm), where ACRDSO2 and PXZDSO2 were co-deposited
with CBP in a concentration of 6 wt% to serve as the EML. As
shown in Table 2 and Figure 3, the devices based on ACRDSO2
and PXZDSO2 exhibit quite low turn-on voltages (Von, at the
luminance of 1 cd m−2) of 3.5 and 3.7 V, indicating efficient
carrier injection and transport into the EML in such a device
structure. The device based on ACRDSO2 emits green light
with maximal current efficiency (CE) of 61.8 cd A−1, power effi-
ciency (PE) of 54.0 lm W−1, and EQE of 19.2%, which is compa-
rable to the previously reported vacuum-deposited green TADF
OLED based on 4CzIPN as the emitter and CBP as the host
(EQEmax = 19.3%).[6] In addition, the device based on PXZDSO2
exhibits a yellow emission with maximal CE of 49.3 cd A−1, PE
of 38.5 lm W−1, EQE of 16.7%, and maximum luminance over
17 000 cd m−2. The peak wavelengths of the electrolumines-
cence (EL) spectra are 534 and 560 nm for the devices based
on ACRDSO2 and PXZDSO2, respectively, which are 14 and
20 nm bathochromic shifted compared with the corresponding
PL spectra of the co-deposited films.
Figure 2. Time-resolved transient PL decay profiles of the co-deposited
films of ACRDSO2:CBP and PXZDSO2:CBP in a concentration of 6 wt%
at room temperature.
The EQE values can also be divided into the prompt fluo-
rescence component and the delayed fluorescence component
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Table 2. Summary of the device performance of the vacuum-deposited and solution-processed devices based on ACRDSO2 and PXZDSO2.
At 100 cd m−2
At 1000 cd m−2
Process
Emitter
ACRDSO2
PXZDSO2
ACRDSO2
PXZDSO2
Vona) [V]
3.5
EQEmax [%]
19.2
CIEb) (x, y)
(0.34, 0.57)
(0.44, 0.54)
(0.32, 0.58)
(0.42, 0.55)
V [V]
EQE [%]
17.6
V [V]
EQE [%]
13.1
CEmax [cd A−1
61.8
]
Evaporation
4.9
5.2
4.8
5.7
6.4
6.8
5.6
7.0
3.7
49.3
16.7
16.3
13.7
Solution
3.7
53.3
17.5
17.5
14.0
4.1
45.1
15.2
15.2
13.3
a)At a luminance of 1 cd m−2 b)CIE, Commission Internationale Ed I’eclairage.
;
from the following equations, according to the PL characteris-
tics obtained from the doped films:[39,41]
based on ACRDSO2 consists of ηp (2.4%) and ηd (16.8%). The
ηp value is only 2.4% due to the large φISC value of 70%, and it
just resembles most conventional fluorescent devices with only
25% singlet exciton utilization. Thanks to the triplet exciton
utilization channel of TADF, ηd makes a main contribution
to the maximal EQE with the general nonradiative triplet exci-
tons utilized through RISC from T1 to S1. For the device based
on PXZDSO2, the ηp and ηd values are estimated to be 2.9%
and 13.8%, respectively, and the main contribution of ηd also
benefits from the TADF channel. On the assumption of γ = 1.0
and ηout = 0.3, the theoretical maximum EQEs for the devices
based on ACRDSO2 and PXZDSO2 are 18.6% and 14.4%,
respectively, which are a bit smaller than the corresponding
experimental values, and it might be due to the underesti-
mated PLQYs of the doped films measured in air. The con-
sistent theoretical and experimental EQE values further prove
(3)
(4)
(5)
ηEQE = ηp +ηd
ηp = 0.25×γ ×φp ×ηout
ηd = (0.25 × φISC + 0.75) × γ × φRISC ×ηout
where the external EL quantum efficiency ηp is the contribu-
tion of the prompt fluorescence component, the external EL
quantum efficiency ηd is the contribution of the delayed fluo-
rescence component, ηout is the optical out-coupling factor, and
γ is the carrier balance factor. As shown in Table S3 (Supporting
Information), the experimental maximal EQE of the device
Figure 3. Energy level diagrams of a) the vacuum-deposited and b) the solution-processed OLEDs based on ACRDSO2 and PXZDSO2. c) Current-
density–voltage–luminance and d) EQE–current-density characteristics of the devices. Inset: EL spectra of the devices measured at a current density of
10 mA cm−2
.
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a balanced carrier recombination should be achieved in the
multilayer vacuum-deposited devices.
Different from the vacuum-deposited devices,
a wet-
processed 40 nm thin PEDOT:PSS layer was used as the
anode buffer layer, and the EMLs of ACRDSO2:CBP and
PXZDSO2:CBP were spin coated onto the PEDOT:PSS layer
without inserting any hole-transport layer. As shown in
Table 2 and Figure 3c, the current density of the solution-pro-
cessed devices is even higher than the corresponding vacuum-
deposited devices, indicating charge carriers could also be effec-
tively injected into the EML even without any hole-transport
layer. In addition, the driving voltages of the solution-processed
devices are even lower than the corresponding evaporation-
processed devices at high luminance. It is of interest, similar
to the evaporation-processed devices, that comparable impres-
sive EQE values of 17.5% and 15.2% were also achieved for the
solution-processed devices based on ACRDSO2 and PXZDSO2,
respectively, which are also very close to the theoretical max-
imum EQE values, indicating balanced carrier recombination
could also be achieved without inserting any hole-transport
layer. For comparison, the only reported evaporation- and solu-
tion-process-feasible TADF emitter 4CzIPN was also used to
fabricate a solution-processed device in the same architecture.
Its maximal CE and EQE values are 42.9 cd A−1 and 13.9%,
respectively, which are inferior to those of the device based on
the current green emitter ACRDSO2. Moreover, its driving volt-
ages at the luminances of 1, 100, and 1000 cd cm−2 are 0.9, 1.2,
and 1.2 V higher than those of the device based on ACRDSO2,
respectively, and the improved driving voltages for the device
based on 4CzIPN may be attributed to its lower-lying HOMO
energy level of −5.80 eV, which gives an improved hole-injec-
tion barrier from the anode.[42] In contrast, considering the
relatively shallow HOMO energy level of ACRDSO2 (−5.26 eV),
holes could be more easily injected into the EML from the
anode to give lower driving voltages. As for the device based
on PXZDSO2, it also exhibits lower driving voltages, mean-
while, higher efficiency due to its higher-lying HOMO energy
level compared with the device based on 4CzIPN. The EL spec-
trum of the device based on PXZDSO2 peaks at 552 nm, and
it is the first reported solution-processed TADF-based OLED
that exhibits competitive performance compared with the cor-
responding vacuum-deposited devices other than green TADF
OLEDs. Further improved efficiency could be expected by the
optimization of the host material, electron-transport material,
and device structure.
Figure 4. External quantum efficiency versus current density character-
istics of the a,b) vacuum-deposited and c,d) solution-processed devices
based on ACRDSO2 and PXZDSO2. The solid lines represent the simu-
lated EQE by employing the TTA model.
where η, η0, and J0 represent the EQE in the presence of TTA,
the EQE in the absence of TTA, and the current density at the
half maximum of the EQE, respectively. As shown in Figure 4
and Table S4 (Supporting Information), the fitted curves are in
good agreement with the experimental data for all the current
devices with correlation coefficients (R values) over 0.99, and it
confirms that the EQE declining mechanism can be primarily
ascribed to the TTA. In addition, the J0 values are 11.35 and
26.38 mA cm−2 for the solution-processed devices based on
ACRDSO2 and PXZDSO2, which are almost consistent with
those of the evaporation-processed devices based on ACRDSO2
(11.48 mA cm−2) and PXZDSO2 (20.06 mA cm−2). The rela-
tively higher J0 values for the PXZDSO2-based devices indi-
cate smaller efficiency roll-off, and higher efficiency could be
retained at a broad current density, and it could be attributed
to the relatively short lifetime of the delayed ingredient for the
doped film of PXZDSO2:CBP.
In summary, two novel evaporation- and solution-process-
feasible TADF emitters of ACRDSO2 and PXZDSO2, con-
sisting of thianthrene-9,9′,10,10′-tetraoxide as an electron
acceptor, a phenyl ring as a π bridge, and 9,9-dimethyl-9,10-
dihydroacridine or phenoxazine as an electron donor, were
designed and synthesized. Maximum EQE values of 19.2% and
16.7% were achieved for the vacuum-deposited OLEDs based
on green-light-emission ACRDSO2 and yellow-light-emission
PXZDSO2, respectively. Notably, the solution-processed device
Aside from the highly efficient efficiency roll-off for the
devices based on ACRDSO2 and PXZDSO2 are also obvi-
ously reduced compared with most reported TADF devices,
giving EQE values higher than 13% for all the current evapo-
ration- and solution-processed devices at the luminance of
1000 cd m−2. According to a previous study, triplet–triplet anni-
hilation (TTA) might be the main cause of efficiency roll-off
in the TADF OLEDs, owing to the long lifetime of the excited
triplet states (in the microsecond range).[43] The TTA model was
further employed to analyze the efficiency roll-off of the current
devices by using Equation (6) for TTA exciton deactivation:[24,34]
⎡
⎤
η/η0 = J0/4 J 1+ 8 J /J0 − 1
(6)
⎣
⎦
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Scheme 1. Synthetic routes of ACRDSO2 and PXZDSO2. i) Bromine, acetic acid, 80 °C; ii) H2O2 (30%), acetic acid, dichloromethane, 90 °C;
iii) 4-bromoiodobenzene, K2CO3, Cu, N,N-dimethylformamide, 160 °C; iv) bis(pinacolato)diboron, CH3COOK, Pd(dppf)Cl2, dioxane, 80 °C; v) Pd(PPh3)4,
K2CO3, toluene/EtOH = 3/1, 90 °C; vi) 1,4-dibromobenzene, K2CO3, toluene/EtOH = 3/1, 9 °C; and vii) phenoxazine, tri-tert-butylphosphine, sodium
tert-butoxide, palladium acetate, toluene, 110 °C.
126.65, 126.05, 125.40, 124.55, 120.94, 114.00. EI-MS (m/z): calcd for
C
based on ACRDSO2 without any hole-transport layer exhibits
30H19NO5S2 537.61; found, 538.0 [M+]. Anal. calcd for C30H19NO5S2: C
obviously lower driving voltage and higher EQE (17.5%) in
comparison with the well-known green TADF emitter 4CzIPN
in the same device structure. In addition, the PXZDSO2-based
solution-processed device exhibits a maximal EQE of 15.2%,
which is the first reported solution-processed TADF-based
OLED that exhibits competitive performance compared with
the corresponding vacuum-deposited devices other than green
TADF OLEDs. Moreover, all the devices exhibit slight efficiency
roll-off with EQE values exceeding 13% at the luminance of
1000 cd m−2, which can be mainly attributed to triplet–triplet
annihilation. The current findings validate a practical strategy
to develop evaporation- and solution-process-feasible metal-free
highly efficient fluorescent emitters for OLED applications.
67.02, H 3.56, N 2.61, S 11.93; Found: C 67.28, H 3.22, N 2.68, S 12.41.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
The authors greatly appreciate the financial support from the Ministry
of Science and Technology (2015CB655003 and 2014DFA52030), the
National Natural Science Foundation of China (91233116 and 51073057),
the Ministry of Education (NCET-11-0159), and the Guangdong Natural
Science Foundation (S2012030006232).
Experimental Section
Received: July 4, 2015
Revised: September 13, 2015
Published online: November 9, 2015
Materials: All solvents and reagents were used as received from
commercial suppliers without any further purification. 9,9-Dimethyl-
9,10-dihydroacridine (S3) was synthesized according to literature
procedures.[17] Synthetic routes of the compounds ACRDSO2 and
PXZDSO2 are shown in Scheme 1, and they were further purified by
repeated temperature gradient vacuum sublimation. Details of syntheses
are shown in the Supporting Information.
2-[4-(9,9-Dimethyl-9,10-dihydroacridine)phenyl]thianthrene-9,9′,10,10′-
tetraoxide (ACRDSO2): 1H NMR (500 MHz, CDCl3, δ) 8.54–8.55 (d,
J = 1.7 Hz, 1H), 8.37–8.38 (d, J = 8.1 Hz, 1H), 8.31–8.32 (m, 2H), 8.07–
8.12 (m, 1H), 7.86–7.91 (m, 4H), 7.47–7.53 (m, 4H), 6.96–7.00 (m, 4H),
6.29–6.31 (d, J = 8.0 Hz, 2H), 1.71(s, 6H). 13C NMR (125 MHz, CDCl3,
δ): 146.24, 142.97, 140.02, 137.97, 137.08, 133.76, 132.47, 131.68,
130.11, 127.35, 123.74, 120.94, 114.00, 31.24. EI-MS (electron ionization
mass spectrometry, m/z): calcd for C33H25NO4S2 563.69; found, 564.2
[M+]; Anal. calcd for C 70.31, H 4.47, N 2.48, S 11.38; Found: C 70.36, H
4.23, N 2.54, S 11.75.
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