Phenothiazine dioxide-containing derivatives as efficient hosts for blue, green and yellow thermally activated delayed fluorescence OLEDs

Article abstract of DOI:10.1039/c9tc05917b

By conjugating 10-phenyl-10H-phenothiazine-5,5-dioxide (2PTO) with aromatic amine substituents (PhCz and DMACMN), three novel host materials namely 10-(9-phenyl-9H-carbazol-3-yl)-10H-phenothiazine 5,5-dioxide (3CzNPTO), (10-(4-(9H-carbazol-9-yl)phenyl)-10H-phenothiazine 5,5-dioxide (9CzNPTO) and 10-(4-(9,9-dimethylacridin-10(9H)-yl)-2,5-dimethylphenyl)-10H-phenothiazine 5,5-dioxide (DMACMNPTO) were designed and synthesized. DMACMNPTO with thermally activated delayed fluorescence (TADF) properties exhibited bipolar characteristics resulting from the completely separated HOMO/LUMO distribution. Using DMACMNPTO as a host for TADF emitters, devices showed a maximum external quantum efficiency (EQE_max), current efficiency (CE_max) and power efficiency (PE_max) of 18.3%, 33.3 cd A^-1 and 37.4 lm W^-1 in blue-emitting diodes, 18.6%, 49.1 cd A^-1 and 47.2 lm W^-1 in green-emitting diodes, and 19.1%, 59.2 cd A^-1 and 66.0 lm W^-1 in yellow-emitting diodes, respectively. These results corroborated the potential of phenothiazine dioxide-containing derivatives as host materials in a sequence of color TADF-OLEDs.

Full text of DOI:10.1039/c9tc05917b

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Nosang V. Myung, Yong-Ho Choa et al.
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DOI: 10.1039/C9TC05917B
Phenothiazine dioxide-containing derivatives as efficient
hosts for blue, green and yellow thermally activated delayed
fluorescence OLEDs
Runda Guo, Yaxiong Wang, Zhi Huang, Qing Zhang, Songpo Xiang, Shaofeng Ye, Wei Liu and
Lei Wang*
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and
Technology, Wuhan, 430074, P. R. China
Email: wanglei@mail.hust.edu.cn
Abstract
By conjugating 10-phenyl-10H-phenothiazine-5,5-dioxide (2PTO) with aromatic
amine substituents (PhCz and DMACMN), three novel host materials namely 10-(9-
phenyl-9H-carbazol-3-yl)-10H-phenothiazine 5,5-dioxide (3CzNPTO), (10-(4-(9H-
carbazol-9-yl)phenyl)-10H-phenothiazine 5,5-dioxide (9CzNPTO) and 10-(4-(9,9-
dimethylacridin-10(9H)-yl)-2,5-dimethylphenyl)-10H-phenothiazine
5,5-dioxide
(DMACMNPTO) were designed and synthesized. DMACMNPTO with thermally
activated delayed fluorescence (TADF) feature exhibited the bipolar characteristic
resulting from the completely separated HOMO/LUMO distribution. Using
DMACMNPTO as a host for TADF emitters, the devices showed a maximum external
quantum efficiencies (EQEmax), current efficiency (CEmax) and power efficiency (PEmax
of 18.3%, 33.3 cd/A and 37.4 lm/W in the blue-emitting diode, 18.6%, 49.1 cd/A and
7.2 lm/W in the green-emitting diode, and 19.1%, 59.2 cd/A and 66.0 lm/W in the
)
4
yellow-emitting diode, respectively. These results corroborated the potential of
phenothiazine dioxide-containing derivatives as host materials in a sequence of colors
TADF-OLEDs.
Keywords: Organic light-emitting devices, thermally activated delayed fluorescence,
host, phenothiazine dioxide

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Introduction
Thermally activated delayed fluorescence (TADF) organic molecules have emerged as
the most promising third-generation materials that can be utilized in developing high
performance organic light-emitting devices (OLEDs) since the pioneering work of C.
Adachi et al.[1-3] By efficiently harvesting both singlet and triplet excitons, the quantum
efficiencies of TADF emitters are comparable to that of the phosphorescent
molecules.[1-5] In addition, thanks to the whole organic entities assembled strategy
without noble-metals, it makes the TADF-OLEDs even more affordable and featuring
high molecular design flexibility.[6-8] Although TADF-OLEDs have attracted
considerable attention due to the superiorities compared with conventional fluorescent
and phosphorescent OLEDs, there remain several obstacles to be overcome before they
could actually be commercialized.[9-11] To design highly efficient devices with
improved efficiency roll-offs, the selection of a proper material system for emitting
layer (EML) is of great importance.[13-15] Besides the dopants, the choices of reasonable
host materials in EML can affect the energy transfer process and improve device
efficiency. In general, emitters are typically dispersed into an appropriate host matrix
to prevent back energy transfer and alleviate concentration quenching.[16-19] The ideal
host materials should meet certain requirements, e.g. higher triplet energy level (ET),
suitable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) matching with adjacent carrier transporting layers, favorable thermal
and morphological stabilities to lengthen the device lifetime.[20-24]
It’s desirable to find out an efficient host material suitable for different color dyes,
which can simplify the manufacture process in industrialization of OLEDs. However,
in many cases the same general host may not be suitable for all colors. Such as
oxybis(2,1-phenylene))bis-(diphenylphosphine oxide (DPEPO) which exhibits
excellent performance in blue OLEDs but underperforming in green or red devices due
to the wide-band-gap nature.[25, 26] The low E of host 4,4’-di(9H-carbazol-9-yl)-1,1’-
T
biphenyl (CBP) and poor thermal stability of 1,3-bis(9-carbazolyl)benzene (mCP)
obstructed their common application.[27-31] Meanwhile, the unipolar transporting
property leads to charge recombination localized in unilateral interface between the

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EML and the carrier transporting layer, which is detrimental to the device efficiency.
Therefore, it is crucial to explore new strategies to develop host materials improving
the exciton utilization efficiency.
Previously, we designed and synthesized phenothiazine dioxide moieties (2PTO: 10-
phenyl-10H-phenothiazine-5,5-dioxide) as the acceptor. Linking at 3-position of 2PTO
with strong donor of phenoxazine (PXZ), an excellent AIE-TADF molecule
(PXZ2PTO) with a highly stereoscopic structure endow the doped devices with
maximum external quantum efficiency (EQEmax) of 16.3%.[32] Compared with the
traditional sulfonyldibenzene (DPS) acceptor, 2PTO possesses one supernumerary
aromatic amine ring, which affords 2PTO with the weaker electron-withdrawing nature.
A stronger acceptor combined with donor moieties would result in a weak acceptor,
which is beneficial to constructing host materials with broadened energy gap. It is worth
noting that the conjugation effect of electron for 10-substitution position of 2PTO is
clearly smaller than its 3-substitution derivative, resulting in high E and efficient
T
separation of electron cloud for HOMO and LUMO, which can render host with
efficient exciton confinement and bipolar feature. As a result, matching 10-substitution
position of 2PTO with substituent such as 9-phenyl-9H-carbazole (PhCz) and 9,9-
dimethyl-9,10-dihydroacridine (DMAc) is a feasible strategy to design excellent host
and further explore the nature of 2PTO.
Herein, with the purpose of obtaining TADF hosts using 2PTO acceptor, we have
designed D-A type molecules successively. Firstly by introducing the carbazole
electron-donating substituents, 10-(9-phenyl-9H-carbazol-3-yl)-10H-phenothiazine
5
,5-dioxide (3CzNPTO) and (10-(4-(9H-carbazol-9-yl)phenyl)-10H-phenothiazine
,5-dioxide (9CzNPTO) were designed and synthesized. Although the linking
5
positions have been optimized, however, no TADF characteristics achieved by
CzNPTO and 9CzNPTO with the crooked molecule structure resulting from 2PTO.
3
We further changed bridging phenyl to xylen and introduced 9,9-dimethyl-9,10-
dihydroacridine donor. On one hand, the methyl of xylen prevented the 2PTO to be
crooked. Moreover, DMAc displayed stronger electron donating ability than 3Cz and
9
Cz. As a result, the completed intramolecular charge-transfer (ICT) and tiny EST

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10-(4-(9,9-dimethylacridin-10(9H)-yl)-2,5-dimethylphenyl)-10H-
rendered
phenothiazine 5,5-dioxide (DMACMNPTO) with TADF feature. Meanwhile,
DMACMNPTO presented more balanced bipolar transporting characteristics, which
was beneficial to improve the performance of devices.
Based on the highest E and the best bipolar property and TADF feature,
T
DMACMNPTO hosted blue, green and yellow TADF-OLEDs have achieved excellent
performance, respectively. In particular, DMACMNPTO hosted blue device with
SPF34pTr[33] as dopant achieved a maximum EQE, current efficiency (CE) and power
efficiency (PE) as high as 18.3%, 33.3 cd/A and 37.4 lm/W and Commission
Internationale de l’Eclairage (CIE) color coordinates of (0.17, 0.24). The same hosted
green and yellow OLEDs adopting PXZ2PTO[32] and PTZ-AD[40] as emitters also
achieved outstanding performances. The green device achieved a maximum
EQE/CE/PE of 18.6%/49.1cd/A/47.2 lm/W with CIE color coordinates of (0.22, 0.46)
and the yellow device achieved a maximum EQE/CE/PE of 19.1%/59.2 cd/A/66.0
lm/W with CIE color coordinates of (0.34, 0.56).
Results and discussion
Synthesis and structure characteristics
The synthetic routes and chemical structures of 3CzNPTO, 9CzNPTO and
DMACMNPTO were showed in Scheme 1. For target compounds 3CzNPTO and
9
CzNPTO, the first two steps (a) and (b) were CuI-catalytic C-N Ulmann coupling
reaction and bromination reaction, respectively. The final products were obtained after
palladium-catalyzed Buchwald-Hartwig reaction (c) and oxidized reaction (d) by
aqueous hydrogen peroxide solution. The synthetic condition for DMACMNPTO is a
bit harsh. In Ulmann coupling reaction (e), 1,1'-bis(diphenylphosphino)ferrocene
replaced Tri-tert-butylphosphine tetrafluoroborate as bridging ligand in synthetic of
intermediate compound 6. Step (f) is oxidation reaction, and 3-chloroperbenzoic acid,
a milder oxidizer compared with hydrogen peroxide, was adopted to prevent the methyl
group from being oxidized. Finally, desired molecule was synthesized by the
palladium-catalyzed Buchwald-Hartwig reaction with a high yield. In the Experimental
section, detailed synthesis steps of 3CzNPTO, 9CzNPTO and DMACMNPTO could

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be seen. Specific synthesis process of all the other intermediate compounds were given
in Electronic Supplementary Information (ESI) file. All the target and intermediate
1
13
compounds were characterized by H and C nuclear magnetic resonance (NMR)
spectroscopic analysis, mass spectroscopy and elemental analysis. Before devices
fabrication, 3CzNPTO, 9CzNPTO and DMACMNPTO were further purified by
vacuum temperature-gradient sublimation.
O
O
S
S
N
N
H
N
(a)
(b)
(c)
(d)
N
N
N
N
Br
1
2
3
3
9
CzNPTO
CzNPTO
O
O
S
S
Br
N
N
H
N
(a)
(c)
(d)
N
N
N
4
5
O
O
S
O
S
O
S
Br
Br
N
S
N
(e)
(f)
N
(c)
DMACMNPTO
N
H
N
Br
Br
6
7
Scheme 1. Synthetic route of 3CzNPTO, 9CzNPTO and DMACMNPTO: (a) CuI, K CO , 18-
2
3
o
Crown-6, N, N-Dimethylformamide (DMF), 140 C, 24 h; (b) N-Bromosuccinimide (NBS), DMF,
0 oC, 12 h; (c) Palladium acetate, Tri-tert-butylphosphine tetrafluoroborate, sodium tert-butoxide,
o
o
methylbenzene, 110 C, 12 h; (d) Acetic acid, hydrogen peroxide (H O ) (30%), 120 C, 2 h; (e)
2
2
Palladium acetate,1,1'-Bis(diphenylphosphino)ferrocene (dppf), Sodium tert-butoxide,
o
o
methylbenzene, 90 C, 8 h; (f) 3-Chloroperbenzoic acid (mCPBA), dichloromethane, 0 C, 12 h.
Theoretical calculations
The spatial configuration and frontier molecular orbital (FMO) distribution of
3
CzNPTO, 9CzNPTO and DMACMNPTO were analyzed using density functional
theory (DFT) calculations by Gaussian 09 (B3LYP/6-31(d) level) software. Fig. 1

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shows the distribution of the FMO and the calculated energy levels of 3CzNPTO,
9
CzNPTO and DMACMNPTO. For 9CzNPTO, the HOMOs and LUMOs were
mainly located on the carbazole moieties and the bridging phenyl, respectively. In
DMACMNPTO molecule, dimethylacridine acted as donor and the HOMO
distributions localize predominately on it. The LUMOs were located on the 2PTO.
Meanwhile, the two methyl moieties introduced on the bridging phenyl endow the
DMACMNPTO with orthogonal structure and the separated distributions between
HOMO and LUMO, which make DMACMNPTO possess the best bipolar
characteristic. However, abnormal HOMO/LUMO distributions of 3CzNPTO were
exhibited as the HOMOs were located on the 2PTO moieties, whereas their LUMOs
were mainly located on the carbazole moieties. We speculated that the anomalies
originating from unreasonable geometry optimization so that other calculation methods
were adopted. The ground state was performed using the B3LYP exchange correlation
functional, the def2-SVP basis set and the density functional dispersion correction with
Becke-Johnson damping [DFT-D3(BJ)], as shown in Fig. S5. Furthermore, M06-2X
method was also adopted to evaluate 3CzNPTO and calculations were listed in Fig. S6.
However, it found that the new results were the same as the previous calculation using
B3LYP/6-31(d). Actually, the roles of acceptors and donors are decided by their
relative electron-withdrawing and electron-donating abilities.
We further calculated the excited states using time-dependent density functional
theory (TD-DFT) at the B3LYP/6-31(d) level (Seen in Table 1). The theoretical singlet-
triplet energy splitting (ΔE ) of 3CzNPTO, 9CzNPTO and DMACMNPTO were
ST
0
.69 eV, 0.73 eVand 0.13 eV. Such small ΔE of 0.13 eV afford DMACMNPTO with
ST
an efficient reverse intersystem crossing (RISC) process for TADF and boost energy
transfer from host to dopant.

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Fig. 1 The distribution of the FMO levels and the calculated energy levels of 3CzNPTO, 9CzNPTO
and DMACMNPTO.
Thermal and electrochemical properties
The thermal properties of 3CzNPTO, 9CzNPTO and DMACMNPTO were examined
by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC)
measurements (shown in Fig. 2). Defined a decomposition temperature (Td)
o
corresponding to 5% weight loss, DMACMNPTO displayed a T of 357 C.
d
o
3
CzNPTO and 9CzNPTO have even high T s over 380 C. There were no obvious
d
glass transition temperatures (T s) for all three compounds. Good thermal stability of
g
the compounds were beneficial for good film-forming and enabled them to function
steadily as hosts in devices fabrication.
The electrochemical properties of these compounds are probed by cyclic
voltammetry (CV). As shown in Fig. 3, the HOMO energy levels were estimated with
onset potentials using the energy level of ferrocene (Fc) (4.8 eV) as a reference and
+
calibrated by comparing with Eonset(Fc/Fc ). The LUMO energy levels were calculated
from the HOMO energy levels and energy gaps (E , optical band-gap obtained from the
g
absorption spectra in neat films). Summarized in Table 1, the HOMO/LUMO levles of
3
CzNPTO, 9CzNPTO and DMACMNPTO were -5.72 eV/-2.37 eV, -5.78 eV/-2.32
eV and -5.40 eV/-1.91 eV, respectively. For DMACMNPTO, dimethylacridine is a

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stronger electron donor and resulted in a shallower HOMO level, which is beneficial to
reducing carrier injection barrier. 9CzNPTO presented clear oxidation behaviors which
assigned to the oxidation of the carbazole moiety and revealed an irreversible oxidation
behavior due to the exposure of unstable radical cations at both the 3 and 6-positions of
carbazole. The experimental results coincide with those obtained from calculation.
However, the calculated results of 3CzNPTO indeed did not agree with cyclic
voltammetry. Here, we found the cyclic voltammetric data have the more priority than
theory calculation. Further study was carried out, the CV scans of 3CzNPTO, carbazole
and 2PTO monomer were given in Fig. S7. For HOMO test, curves 1 and 2 were both
of 3CzNPTO. Curve 1 was identical to previous Fig.3. Curve 2 with an extended
detection range exhibited two oxidation characteristic peaks. Compared to the oxidation
characteristic peak of Cz and 2PTO monomer, it demonstrated that the first
characteristic peak of 3CzNPTO could originate from Cz which trend to be donor. For
LUMO test, different from 2PTO, there were no characteristic peaks of 3CzNPTO and
Cz. It meant that a deeper LUMO of 3CzNPTO than 2PTO is logical and the 2PTO
served as acceptor.
Fig.2 (a) Thermogravimetric analysis (TGA) and (b) Differential scanning calorimetry (DSC)
o
curves of 3CzNPTO, 9CzNPTO and DMACMNPTO recorded at a heating rate of 10 C/min.

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Fig. 3 Cyclic voltammetric scans of 3CzNPTO, 9CzNPTO and DMACMNPTO.
Photophysical properties
The absorption (UV-abs) and photoluminescence (PL) spectra in toluene at room
temperature, fluorescence (Fl) and phosphorescence (Phos) spectra in 2-methyl-
tetrahydrofuran at 77 K of 3CzNPTO, 9CzNPTO and DMACMNPTO were depicted
in Fig. 4 (a), (b) and (c). The partial enlarged drawing of Fl and Phos spectra in 2-
methyl-tetrahydrofuran at 77 K of DMACMNPTO was shown in Fig. S8. The
corresponding data were summarized in Table 1. 3CzNPTO and 9CzNPTO comprised
identical function moieties so that the two compounds exhibited similar UV-abs
characteristics in toluene. While the stronger higher energy absorptions at 290 nm were
attributed to the π-π* transitions of the conjugated carbazole units, the absorption band
at around 330 nm could be attributed to the n-π* transitions of 2PTO. For
DMACMNPTO, the π-π* transitions of dimethylacridine and the n-π* transitions of
phenothiazine dioxide are redshift to 316 nm and 331 nm. As shown in Fig. 4 (d), the
UV-abs and PL spectra of the three compounds in neat films were measured. The E of
g
3
CzNPTO, 9CzNPTO and DMACMNPTO were 3.35, 3.46 and 3.49 eV, respectively,
which were obtained from the onset of the absorption spectra. Besides, the emission
spectra in film is narrower than that in solution, which due to the molecular vibrations
were suppressed in film compared with solution.[41]
The fluorescence emission spectra of the three novel compounds measured in a series
of solvents with different polarities at room temperature are shown in Fig. S1. There
was just one peak of DMACMNPTO in different polarity solvents and the peak

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performed red-shift along with the increase of solvent polarity that indicated ICT-state
emission. Different from DMACMNPTO, 3CzNPTO and 9CzNPTO exhibited two
emission peaks in all solvents. The dual emission peaks phenomenon is attributed to
the crooked six-membered heterocycles donors (e.g. acridine, phenothiazine). As
similar to the phenothiazine group, the crooked 2PTO in 3CzNPTO and 9CzNPTO
also presented crooked molecule structure. As a result, for 3CzNPTO and 9CzNPTO,
the ICT is incomplete, LE-state emission around 350 nm can be observed as well.[42-45]
The short wavelength LE-state emission was invariable but the long wavelength ICT-
state emission peak performed red-shift along with the increase of solvent polarity. The
emission spectra of 3CzNPTO and 9CzNPTO in neat films were only LE-state which
are quite different from that in toluene solvent. Single peak with trailing at long-wave
band was obtained from the emission spectra of DMACMNPTO in neat film attributed
to intermolecular interaction. For 3Cz and 9Cz hosts, incomplete intramolecular
charge-transfer (ICT) result in coexist of CT and LE states. The energy level mainly
depended on monomer. But for DMACMNPTO, the energy gap would be narrower
due to complete ICT and resulted in a shallower LUMO.[46] To estimate the first singlet
state energy (S ) and triplet state energy (T ) values, the Fl and Phos spectra in 2-
1
1
methyltetrahydrofuran (2-MeTHF) solutions at 77 K were measured. For 3CzNPTO
and 9CzNPTO, the low temperature Fl at 355 nm and 345 nm were similar to the LE-
state emission at room temperature, it was because that molecular rotation was
restricted at low temperature in concretionary state 2-MeTHF. According to the peaks
of the low temperature Phos spectra, the singlet (E )/triplet energy (E ) levels and E
S
T
ST
were estimated to be 3.49/3.02/0.47 eV for 3CzNPTO, 3.59/3.03/0.56 eV for
CzNPTO and 3.42/3.14/0.28 eV for DMACMNPTO, respectively. We further tested
9
the transient PL decay curves of DMACMNPTO in toluene, as shown in Fig. 5 (a), a
second order exponential decay characteristics were displayed after deoxygenize with
a delayed lifetime τ = 308.5 μs. Then the DMACMNPTO was doped into PMMA
d
spin-coated film, a delayed lifetime τ = 247.2 μs was obtained. The delay emission
d
ratio in film was higher than that in solvent. Drastic intermolecular collision in solvent
lead to severe non-radiative transition, more long lifetime triplet excitons quenching

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and less light emission. But in the film, non-radiative transition was suppressed due to
molecular motion inhibition. For 3CzNPTO and 9CzNPTO, however, no TADF
character have been observed as expected due to the large E . As a result,
ST
DMACMNPTO would serve as a better host role for TADF dopants arising from its
higher T and TADF nature.
1
Fig. 4 Absorption (UV-abs) and photoluminescence (PL) spectra in toluene at room temperature,
fluorescence (Fl) and phosphorescence (Phos) spectra in 2-methyl-tetrahydrofuran at 77 K of (a)
3
9
CzNPTO (b) 9CzNPTO (c) DMACMNPTO; (d) UV-abs and PL spectra of 3CzNPTO,
CzNPTO and DMACMNPTO in neat films.
(b)
TADF-HOST
TADF-DOPANT
S1
RISC
ISC
T1
S1
RISC
T1
DF
PF
S0
Fig. 5 (a) Transient PL decay curves of DMACMNPTO in toluene and doped in PMMA spin-
coated film and (b) The energy transfer mechanism. PF: prompt fluorescence, DF: delay
fluorescence.

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Table 1 Thermal, electrochemical, and photophysical properties of 3CzNPTO, 9CzNPTO and
DMACMNPTO
HOMO/
S1/T1d
(eV)
S1/T1e
(eV)
Tg/Tdf
(oC)
Derivatives
λ
absa(nm)
λ
PLa(nm)
E
b(eV)
g
c
LUMO (eV)
3
9
CzNPTO
CzNPTO
292,331 355,371,486
293,323 344,359,443
3.35
3.46
3.49
-5.72/-2.37 3.86/3.17 3.49/3.02 -/383
-5.78/-2.32 3.90/3.17 3.59/3.03 -/385
DMACMNPTO 316,331
382
-5.40/-1.91 3.34/3.21 3.42/3.14 -/357
a
b
Measured in dilute toluene solution at room temperature. Obtained from the onset of the absorption
c
spectra in neat films. HOMO energy levels estimated from the redox potential in cyclicvoltammetry.
d
LUMO energy level estimated by LUMO = HOMO + Eg. Calculated from time-dependent density
e
functional theory (TD-DFT) at the B3LYP/6-31(d) level. Obtained from the peaks of fluorescence
f
spectra and phosphorescence spectra in 2-MeTHF at 77 K.； T obtained from TGA measurements and
d
Tg obtained from DSC measurements.
Electroluminescent properties
The well thermal and morphological stabilities, suitable energy levels inspired us to
further evaluate these three compounds as host materials, TADF-OLEDs were
fabricated with SPF34pTr , PXZ2PTO and PTZ-AD[40] as blue, green and yellow
dopants, respectively. Fig. 6 and Fig. 7 provided the energy-level diagram and chemical
structures of the materials used in the devices. The blue device configuration was:
[33]
[32]
ITO/MoO (10 nm)/TAPC (60 nm)/mCP (5 nm)/host: 10% SPF34pTr (25 nm)/DPEPO
3
(5 nm)/TmPyPb (30 nm)/LiF (1 nm)/Al; the green device configuration was ITO/MoO3
(10 nm)/TAPC (80 nm)/mCP (15 nm)/host: 60% PXZ2PTO (20 nm)/TmPyPb (30
nm)/LiF (1 nm)/Al; the yellow device configuration was ITO/MoO (10 nm)/TAPC (70
3
nm)/TCTA (10 nm)/host: 20% PTZ-AD (30 nm)/TmPyPb (40 nm)/LiF (1 nm)/Al.
Herein, SPF34pTr, PXZ2PTO and PTZ-AD were introduced as blue, green and yellow
emitters, respectively. MoO and TAPC (1,1-bis[4-[N,N-di(p-tolyl)-amino]phenyl]-
3
cyclohexane) served as the hole injecting and transporting layers, LiF and TmPyPb
(1,3,5-tri(m-pyrid-3-yl-phenyl) benzene) served as the electron injecting and
transporting layers. In blue and green devices, mCP was used as exciton-blocking layer
EBL-1 with the LUMO/HOMO of -2.4 eV/-6.1 eV. In yellow devices, the TCTA
(tris(4-(9H-carbazol-9-yl)phenyl)amine) was chosen as EBL-1 with the
LUMO/HOMO of -2.3 eV/-5.7 eV. An external DPEPO EBL-2 with the

Page 13 of 22
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DOI: 10.1039/C9TC05917B
LUMO/HOMO of -2.5 eV/-6.5 eV was introduced into blue devices. There were no
EBL-2 for the green and yellow optimal devices. The thickness of the related functional
layers and the doping concentration for each TADF-OLED were optimized.
-
1.91
-
2.32
-
2.37
-
2.24
-2.54
-2.7
LUMO
LiF/Al
-
2.42
-
2.88
ITO/MoO3
HOMO
-5.2
-5.2
5.4
-5.15
-5.68
-
-5.72 -5.78
Three New Hosts
-6.7
Fig. 6 Energy-level diagram of blue, green and yellow TADF-OLEDs. The LUMO/HOMO levels
of 3CzNPTO, 9CzNPTO and DMACMNPTO are -2.37 eV/-5.72 eV, -2.32 eV/-5.78 eV and -1.91
eV/-5.40 eV in turn.
O
S
O
O
N
N
N
N
N
N
N
N
N
N
O
S
SPF34pTr
PXZ2PTO
PTZ-AD
mCP
N
N
N
N
N
N
N
P
O
O P
N
O
N
TAPC
TmPyPb
TCTA
DPEPO
Fig. 7 Chemical structures of the materials used in the devices.
Fig. 8 exhibited the current density-voltage-luminance (J-V-L) and EQE-luminance
characteristics of all the devices. The turn-on voltages of these devices were around 3
V and low operating voltages were obtained without any p or n doping technology.
According to the parallel contrast among these new hosts based devices,
DMACMNPTO provided all color devices with lowest driving voltages without

Journal of Materials Chemistry C
Page 14 of 22
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DOI: 10.1039/C9TC05917B
2
2
outcoupling or sensitization technology (<4.0 V at 100 cd/m and <6.2 V at 1000 cd/m
2
2
2
for blue, <4.6 V at 100 cd/m and <6.6 V at 1000 cd/m for green, <4.2 V at 100 cd/m
2
and <6.2 V at 1000 cd/m for yellow OLED, respectively). Considering the structure-
property correlations in terms of optoelectrical properties, its almost orthogonal
structure of DMACMNPTO endowed the carrier with respective transporting channel,
which result in better bipolar transporting characteristics and higher charge carrier
density. By contrast, neither 3CzNPTO nor 9CzNPTO with the same groups could
achieve a good property, although the linking positions would impact the relative
mobility of electrons and holes to some extent, it failed to bring about an essential
improvement.
Fig. 8 Current density-voltage-luminance (J-V-L) and EQE-luminance characteristics of blue ((a)
and (b)), green ((c) and (d)) and yellow ((e) and (f)) TADF-OLEDs.

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DOI: 10.1039/C9TC05917B
2
Fig. S2 displayed the normalized EL spectra of the devices at 10 mA/cm . It’s
obvious that their EL spectra originated exclusively from the three TADF emitters
without emission from the neighboring layers indicated sufficient exothermic energy
transfer from the hosts to emitters occurred in EMLs. It ultimately realized efficient
light-emitting. By comparing 3CzNPTO and 9CzNPTO hosted devices, as shown in
Fig. 8, the efficiencies of the blue and green devices based on 3CzNPTO were slightly
higher than that of 9CzNPTO based devices. But for the yellow device, just the
opposite result happened. However, in any case, the DMACMNPTO based devices
achieved the highest performance. The efficiency was affected by frontier molecules
orbits matching, balance of holes and electrons. DMACMNPTO based devices
achieved rather higher EQE than all the others with a maximum EQE of 18.3%, 18.6%
and 19.1% for SPF34pTr-based blue device, PXZ2PTO-based green device and PTZ-
AD-based yellow device, respectively. Meanwhile, relatively high CEs and PEs (33.3
cd/A and 37.4 lm/W for blue device, 49.1 cd/A and 47.2 lm/W for green device, 59.2
cd/A and 66.0 lm/W for yellow device) were also achieved. The CE- and PE-luminance
characteristics were shown in Fig. S3. The EL properties of these devices were
summarized in Table 2. From the perspective of device engineering, the well-matched
energy levels between the DMACMNPTO and bilateral transport layers gradually
decreased the injection barriers and improved the balance in EML. Besides, as
mentioned above, DMACMNPTO had an inherent higher T of 3.14 eV and smaller
1
ΔE of 0.28 eV than that of the other two hosts, it helped confine excitons in EML
ST
better and facilitate the RISC process. On the other hand, benefit from the efficient
energy transfer achieved through either Förster energy transfer or Dexter energy
transfer from TADF host to TADF dopant, higher exciton utilization efficiency could
also be achieved, along with relieved concentration quenching and triplet-triplet
annihilation. It was reasonable that DMACMNPTO based devices performed best and
the energy transfer mechanism was illustrated in Fig. 5 (b). The transient PL decay
curves of delayed emission for SPF34pTr, PXZ2PTO and PTZ-AD doped
DMACMNPTO film were shown in Fig. S9, and Fig. S10 given the PL decay curves
of DMACMNPTO: SPF34pTr, DMACMNPTO: PXZ2PTO and DMACMNPTO:

Journal of Materials Chemistry C
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DOI: 10.1039/C9TC05917B
PTZ-AD with increasing dopant concentration, measured at the 420 nm. According to
these experiments, the reverse intersystem crossing rate (kRISC) and other related
parameters were calculated, summarized in table S2 and table S3.[6, 47, 48] It verified
efficient host-dopant energy transfer in DMACMNPTO based devices. Obviously in
Fig. S11, enhancement of emission was achieved by DMACMNPTO doped host-
dopant system compared with PMMA doped ones, it also indirectly demonstrated the
energy transfer.
Furthermore, Single-carrier devices were fabricated to reveal the affection of carrier
transporting characteristics on device performance and the current density-voltage
characteristics were presented in Fig. S4.[49] The mobilities of the three materials were
given by SCLC fitting, as shown in Fig. S12. The results substantiated the inference
that an even more balance condition of holes and electrons could be achieved by
DMACMNPTO based devices. Other single-carrier devices that simulating the
operating device condition were fabricated to further investigate their carrier
transporting abilities, with the structure of ITO/MoO /TAPC (35 nm)/TCTA (5
3
nm)/EML (30 nm)/TCTA (5 nm)/TAPC (35 nm)/MoO /Al, and ITO/LiF/TmPyPb (40
3
nm)/EML (30 nm)/TmPyPb (40 nm)/LiF/Al. Herein, just a representative dopant was
selected to elucidate the mechanism (PTZ-AD with 20 wt% doped into different hosts).
The J-V curves were shown in fig. S13. It further implied that excellent charge carrier
transporting characteristics and more balanced carrier distribution in the emission
layers were achieved in DMACMNPTO hosted devices.
Through the whole research, it demonstrated that low driving voltages and high
efficiency could be simultaneously realized in different color OLEDs even if using a
common host without any more complicated structures.
It was fascinating to mention that by using DMACMNPTO with TADF feature as
the host, the performance of these basic-color devices all appears to be among of the
best ones that have been reported,[16, 34-39] and the electroluminescent performances for
some representative highly efficient OLEDs based on new hosts were listed in table S1.

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Table 2 EL performance of these TADF-OLEDs fabricated in this work
DOI: 10.1039/C9TC05917B
Maximum efficiency Efficiency
Host-Dopant
Vona
V)
Lmaxb
(cd/m2)
at 100 cd m2
CIE (x, y)
(
EQE/ CE/ PE
EQE/ CE/ PE
(%)/ (cd/A)/ (lm/W) (%)/ (cd/A)/ (lm/W)
SPF34pTr
3.4
3.2
3.2
2110
4940
12.2/ 19.7/ 18.1
15.2/ 41.4/ 32.4
9.2/ 13.8/ 7.5
15.1/ 41.1/ 24.8
7.6/ 24.3/ 13.2
(0.16, 0.20)
(0.23, 0.48)
(0.36, 0.57)
3
9
CzNPTO PXZ2PTO
PTZ-AD
16760 10.0/ 31.3/ 30.7
SPF34pTr
CzNPTO PXZ2PTO
PTZ-AD
3.4
3.2
3.2
1446
5263
7.4/ 11.2/ 10.7
12.8/ 36.1/ 29.7
6.0/ 8.3/ 4.7
(0.15, 0.18)
(0.24, 0.49)
(0.35, 0.57)
12.7/ 35.8/ 23.4
8.5/ 27.0/ 14.9
17040 11.9/ 36.0/ 35.3
SPF34pTr
2.8
3.0
2.8
4059
3933
18.3/ 33.3/ 37.4
18.6/ 49.1/ 47.2
13.6/ 24.2/ 20.0
18.0/ 47.8/ 34.0
17.8/ 56.0/ 44.0
(0.17, 0.24)
(0.22, 0.46)
(0.34, 0.56)
DMACM
NPTO
PXZ2PTO
PTZ-AD
23280 19.1/ 59.2/ 66.0
a
2
b
Turn-on voltage at the luminance of 1 cd/m . The maximum luminance achieved at high voltage. EQE: external
quantum efficiencies, CE: current efficiency, PE: power efficiency.
Conclusions
In summary, we have confirmed that it was an effective and workable strategy linking
1
0-substitution position of 2PTO moiety with aromatic amine substituent to construct
a series of host materials. Detailed molecule design and synthesis routes of the three
novel materials namely 3CzNPTO, 9CzNPTO and DMACMNPTO have been
described explicitly. Through the analysis of the systematic structure-property
correlation, DMACMNPTO exhibited excellent bipolar characteristic and TADF
feature, which avail to confine exciton in EML and enhance exciton utilization. The
devices based on DMACMNPTO exhibited high efficiency for doped TADF-OLEDs.
A blue, green and yellow OLED realized high maximum EQE, CE and PE of 18.3%,
3
3.3 cd/A and 37.4 lm/W with CIE of (0.17, 0.24), 18.6%, 49.1 cd/A and 47.2 lm/W
with CIE of (0.22, 0.46), and 19.1%, 59.2 cd/A and 66.0 lm/W with CIE of (0.34, 0.56),
respectively. These results indicated that a certain type of TADF material could be
applied as a host to blue, green and yellow devices using simple device architectures
but giving extremely high device efficiencies.

Journal of Materials Chemistry C
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DOI: 10.1039/C9TC05917B
Experimental section
Synthesis of target molecules
0-(9-phenyl-9H-carbazol-3-yl)-10H-phenothiazine 5,5-dioxide (3CzNPTO).
1
Compound 3 (2.0 g, 4.5 mmol) dissolved in 50 mL acetic acid, then 20 mL aqueous
o
hydrogen peroxide solution (30%) was introduced. The mixture refluxed under 120 C
for 12 h. After the mixture cooled to ambient temperature, turn the solvent into 200 mL
water and the white solid precipitate out. Saturated sodium bicarbonate in aqueous
solution was used for washing the cake, and then, the residue was dissolved in CH Cl .
2
2
The CH Cl organic phase was dried by anhydrous MgSO . The solvent was evaporated
2
2
4
by rotary evaporator, and the residue was purified by column silica gel chromatography
1
to afford a white solid (1.9 g, 89.6%). H NMR (400 MHz, CDCl ) δ [ppm]:8.21-8.18
3
(m, 2H), 8.15 (d, J = 1.6 Hz, 1H), 8.10 (d, J = 7.6 Hz, 2H), 7.70-7.63 (m, 5H), 7.56-
1
3
7
.46 (m, 3H), 7.38-7.32 (m, 4H), 7.25-7.22 (m, 1H), 6.74 (d, J = 8.8 Hz, 2H). C NMR
(
100 MHz, CDCl ) δ [ppm]: 141.74, 141.53,140.74, 137.06, 132.68, 130.80, 130.19,
3
1
28.20, 127.40, 127.19, 125.34, 123.31, 122.62, 122.51, 122.24, 121.87, 120.79,
20.69, 117.63, 112.19, 110.39. MS (APCI): calculated for C H N O S, 472.12;
1
3
0
20
2
2
found, 473.1, [M+H]+.
1
0-(4-(9H-carbazol-9-yl)phenyl)-10H-phenothiazine 5,5-dioxide (9CzNPTO).
CzNPTO was synthesized similarly to 3CzNPTO. Replace compound 3 with
9
compound 5 (1.9 g, 4.3 mmol). The residue was purified by column silica gel
1
chromatography to afford a white solid (1.7 g, 83.0%). H NMR (400 MHz, CDCl ) δ
3
[ppm]: 8.24-8.19 (m, 4H), 7.95 (d, J = 8.4 Hz, 2H), 7.66-7.59 (m, 4H), 7.50 (t, J = 7.6
1
3
Hz, 4H), 7.39-7.29 (m, 4H), 6.81 (d, J = 8.8 Hz, 2H). C NMR (100 MHz, CDCl ) δ
3
[ppm]: 140.77, 140.34, 139.18, 137.34, 132.98, 132.21, 129.40, 126.28, 123.86, 123.65,
1
22.96, 122.37, 120.73, 120.63, 117.11, 109.61. MS (APCI): calculated for
+
C H N O S, 472.12; found, 473.1, [M+H] .
3
0
20
2
2
1
0-(4-(9,9-dimethylacridin-10(9H)-yl)-2,5-dimethylphenyl)-10H-phenothiazine-5,5-
dioxide (DMACMNPTO).
Compound 7 (1.24 g, 3 mmol), 9,9-dimethylacridan (0.63 g, 3 mmol), palladium acetate
(0.03 g, 0.15 mmol), tri-tert-butylphosphine tetrafluoroborate (0.04 g, 0.15 mmol),

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sodium tert-butoxide (0.48 g, 5 mmol), 50 mL methylbenzene were successively added
o
into a 100 mL flask. The mixture was heated to 110 C and refluxed under nitrogen
atmosphere for 12 h. After cool-down, the product was extracted with CH Cl and dried
2
2
by anhydrous MgSO . The solvent was evaporated and the residue was purified by
4
1
column silica gel chromatography to afford a white solid (1.3 g, 80.0 %). H NMR (400
MHz, CDCl ) δ [ppm]: 8.25 (d, J = 7.6 Hz, 2H), 7.57-7.52 (m, 4H), 7.45 (s, 1H), 7.42
3
(s, 1H), 7.33 (t, J = 7.6 Hz, 2H), 7.11-7.07 (m, 2H), 7.01-6.97 (m, 2H), 6.74 (d, J = 8.4
Hz, 2H), 6.25 (d, J = 8.0 Hz, 2H), 2.14 (s, 3H), 2.03 (s, 3H), 1.76 (s, 3H), 1.73 (s, 3H).
1
3
C NMR (100 MHz, CDCl ) δ [ppm]: 140.90, 140.07, 139.61, 139.17, 138.51, 137.07,
3
1
1
35.50, 133.95, 133.28, 130.01, 126.79, 125.98, 123.77, 122.87, 122.32, 120.95,
16.43, 112.95, 35.99, 32.30, 32.09, 17.46, 16.99. MS (MALDI-TOF): calculated for
+
C H N O S, 542.20; found, 542.37, [M] .
3
5
30
2
2
Conflicts of interest
There are no conflicts to declare
Acknowledgements
This work was financially supported by the NSFC/China (51573065,51727809), China
Postdoctoral Science Foundation (2017M620321, 2018M632841), and the Science and
Technology Support Program of Hubei Province (2015BAA075). We are thankful to
SCTS/CGCL HPCC of Huazhong University of Science and Technology (HUST) for
providing computing resources and technical support. The Analytical and Testing
Center at HUST is acknowledged for characterization of new compounds. The Center
of Micro-Fabrication and Characterization (CMFC) of Wuhan National laboratory for
Optoelectronics (WNLO) of HUST is acknowledged for fabrication of the OLEDs.

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DOI: 10.1039/C9TC05917B
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DOI: 10.1039/C9TC05917B
TOC
New host for efficient heterochromatic TADF-OLEDs.