Tuning the optoelectronic properties of phenothiazine-based D?A-type emitters through changing acceptor pattern

Article abstract of DOI:10.1016/j.dyepig.2017.07.064

Two novel blue emitters, 10-ethyl-3-(1,4,5-triphenyl-1H-imidazol-2-yl)-phenothiazine (PTHBI) and 10-ethyl-3-(1-phenyl-1H-phenanthro[9,10-d]imidazole-2-yl)-phenothiazine (PTHPI), employing a donor (10-ethyl-phenothiazine) and an acceptor (1,4,5-triphenyl-1

Full text of DOI:10.1016/j.dyepig.2017.07.064

Dyes and Pigments 147 (2017) 6e15
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Tuning the optoelectronic properties of phenothiazine-based D‒A-
type emitters through changing acceptor pattern
Xu Qiu, Jinjin Shi, Xin Xu, Yuansheng Lu, Qikun Sun, Shanfeng Xue*, Wenjun Yang
Key Laboratory of Rubber-Plastics of Ministry of Education/Shandong Province (QUST), School of Polymer Science & Engineering, Qingdao University of
Science & Technology, 53-Zhengzhou Road, Qingdao 266042, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel blue emitters, 10-ethyl-3-(1,4,5-triphenyl-1H-imidazol-2-yl)-phenothiazine (PTHBI) and 10-
Received 12 June 2017
Received in revised form
19 July 2017
Accepted 26 July 2017
Available online 27 July 2017
ethyl-3-(1-phenyl-1H-phenanthro[9,10-d]imidazole-2-yl)-phenothiazine (PTHPI), employing
a donor
(10-ethyl-phenothiazine) and an acceptor (1,4,5-triphenyl-1H-imidazole and phenanthro[9,10-d]imid-
azole, respectively), were designed and synthesized. By breaking the C‒C bond of the phenanthro[9,10-d]
imidazole acceptor, we obtain the acceptor, 1,4,5-triphenyl-1H-imidazole, with a more twisted confor-
mation. This subtle molecular structure change leads to large differences in the optoelectronic properties
of PTHBI and PTHPI. The fluorescent organic light-emitting diodes (OLEDs) were fabricated and used as
the emitting layer, and all of the OLEDs showed strong blue emission. Thus, the PTHBI-based non-doped
device exhibited an emission peak at 460 nm, which is bluer than the emission peak (476 nm) for the
PTHPI-based device. However, the PTHPI-based OLED device exhibited a maximum current efficiency of
7.20 cd Aꢀ1, maximum power efficiency of 4.00 lm Wꢀ1, and external quantum efficiency of 3.41%, which
are better than the corresponding values for the PTHBI-based device (luminous efficiency ¼ 3.44 cd Aꢀ1
and external quantum efficiency ¼ 2.38%). This is caused by the twisting degree of the acceptor units.
This is the first observation of blue emission using a phenothiazine-based derivative in non-doped de-
vices. This study demonstrates a strategy toward blue emitters by tuning the acceptor pattern.
© 2017 Elsevier Ltd. All rights reserved.
Keywords:
Phenothiazine
Phenanthro[9,10-d]imidazole
Optoelectronic properties
Non-doped device
Acceptor pattern
1. Introduction
with electron-rich nitrogen and sulfur heteroatoms among plenty
of electron donor moieties [23‒29]. Compared to donors that only
Since first reported by Tang and VanSlyke in 1987, organic light-
emitting diodes (OLEDs) have made huge breakthroughs in flat
displays and solid-state lighting resources over the last several
decades [1‒4]. Three primary colours (blue, green, and red) are
needed to achieve full colour displays [5]. Moreover, the blue
emitter is of paramount importance because it emits one of the
three primary colours and can generate other emissions through
energy transfer [6‒10]. A widely used approach for engineering
high-performance blue fluorescent emitters is to develop a suitable
degree of intramolecular charge transfer by joining the electron
donor (D) and acceptor (A) moieties to form D‒A-type fluorophores
[11‒16].
contain nitrogen atoms, such as carbazole and diphenylamine, the
additional electron-rich sulfur atom in phenothiazine could provide
an enhanced electron-donating ability. Meanwhile, the non-planar
“butterfly” structure of phenothiazine could hinder intermolecular
p-stacking aggregation and excimer formation [30‒34]. Unfortu-
nately, most phenothiazine-based fluorescent molecules emit
green and red light; therefore, it is not easy to realize blue light
emission because of the strong donor-donating property [35,36].
Therefore, the choice and control of the electron acceptor structure
is very important to construct a phenothiazine-based blue fluo-
rescent molecule. As for electron acceptors, phenanthro[9,10-d]
imidazole has some merits as a new constructing group for blue
fluorescent materials, such as a high luminous efficiency, excellent
Construction of DeA molecules can improve the charge injec-
tion and carrier transport of organic semiconductor materials.
Accordingly, it is crucial to choose appropriate D and A moieties
[17‒22]. Phenothiazine is a well-known heterocyclic compound
thermal stability, and
a relative balance of carrier injection
and transport properties [37‒44]. However, the aggregation of
materials containing phenanthro[9,10-d]imidazole may cause
bathochromic shifts of the photoluminescence (PL) and electrolu-
minescence (EL) spectra compared to that in the solution state
* Corresponding author.
E-mail address: sfxue@qust.edu.cn (S. Xue).
http://dx.doi.org/10.1016/j.dyepig.2017.07.064
0143-7208/© 2017 Elsevier Ltd. All rights reserved.

X. Qiu et al. / Dyes and Pigments 147 (2017) 6e15
7
because of the planarity and rigidity of this moiety. Changing the
2. Experimental section
structure of the acceptor could provide blue luminescent materials.
Several researchers demonstrated that blue-emitting materials can
be obtained by breaking a chemical bond in the phenanthrene part
2.1. Materials
of phenanthro[9,10-d]imidazole. Li et al. synthesized
a
fully
Starting materials and solvents were purchased from Aldrich
Chemical Co., or Energy Chemical Co., China. Tetrahydrofuran (THF)
was distilled over metallic sodium and dimethylformamide (DMF)
over calcium hydride before use. Dipyrazino[2,3-f:20,30-h]qui-
twisting DeA molecule, TPAePIM, which was composed of 1,4,5-
triphenyl-1H-imidazole (PIM) as an acceptor moiety and triphe-
nylamine (TPA) as a donor. Compared to TPA-PPI, whose acceptor
moiety is phenanthro[9,10-d]imidazole (PPI), TPA-PIM shows a
deep blue EL emission (lmax ¼ 420 nm) [45]. Analogously, He et al.
reported a new deep blue material, DPA-PIM, with asymmetrically
twisted conformations between the anthracene and imidazole
noxaline-2,3,6,7,10,11-hexacarbonitrile
(HATCN),
N,N0-di(1-
naphthyl)-N,N0-diphenylbenzidine (NPB), 4,40,400-tri(9-carbazoyl)
triphenylamine (TCTA), 1,3,5-tris(1-phenyl-benzoimidazol-2-yl)
benzene (TPBi), and lithium fluoride (LiF) were purchased from
Aldrich Chemical Co. They were used without further purification.
units, which could efficiently interrupt molecular
Small conjugation degree units, e.g., 1,4,5-triphenyl-1H-imidazole,
could further inhibit the tight intermolecular interactions. The
p-conjugation.
p-p
2.2. Measurements
OLED device using DPA-PIM as a blue emitter shows a purer blue
emission (Commission Internationale de I'Eclairage (CIE) co-
ordinates of (0.15, 0.08)) than that using DPA-PPI, which could be
attributed to the fully twisted molecular structure of DPA-PIM [46].
Therefore, breaking a single bond in the phenanthro[9,10-d]imid-
azole moiety, which is effectively validated by a multitude of
representative studies, is an exceedingly excellent approach to
construct blue-emitting materials [47,48]. However, no reports
have been published on exploiting phenothiazine-imidazole hy-
brids as efficient blue EL emitters.
In this work, we designed and synthesized two DeA-type
molecules (Scheme 1) with phenothiazine and imidazole as donor
and acceptor moieties, respectively. 10-Ethyl-3-(1,4,5-triphenyl-
1H-imidazol-2-yl)-phenothiazine (PTHBI) could be considered as a
derivative of 10-Ethyl-3-(1-phenyl-1H-phenanthro[9,10-d]imid-
azole-2-yl)-phenothiazine (PTHPI) by breaking a single chemical
bond of phenanthro[9,10-d]imidazole. Thus, we could manipulate
the molecular planarization and conjugation degree to tune the
optoelectronic properties. We fabricated non-doped OLED devices
with PTHBI or PTHPI as the emitting layer. The PTHPI-based device
exhibited a higher luminance, luminance efficiency, and external
quantum efficiency, and a lower turn-on voltage than the PTHBI-
based device. The PTHBI-based device produced bluer EL spectra
(460 nm) than the PTHPI-based device (476 nm).
1H and 13C NMR spectra were recorded on a Bruker AC500
(500 MHz) spectrometer at 298 K by utilizing deuterated chloro-
form (CDCl3) as the solvent and tetramethylsilane (TMS) as the
internal standard. The MALDI-TOF MS mass spectra were recorded
using an AXIMA-CFRTM plus instrument. Elemental analysis was
performed on a PerkinElmer 2400. UV-vis absorption spectra were
recorded on a Hitachi U-4100 spectrophotometer. Fluorescence
measurements were carried out with a Hitachi F-4600 spectro-
photometer. Solution fluorescence quantum yield (FF) was deter-
mined at room temperature by the dilution method using 0.1 M
quinine sulfate as the reference. The quantum efficiencies of solid
films and single crystals were carried out with FLS980 Spectrom-
eter. Thermal gravimetric analysis (TGA) was undertaken on a
PerkinElmer thermal analysis system at a heating rate of 10 ꢁC
minꢀ1 and a nitrogen flow rate of 80 mL minꢀ1. Differential scan-
ning calorimetry (DSC) was performed on a NETZSCH (DSC-204)
unit at a heating rate of 10 ꢁC minꢀ1 under nitrogen. Cyclic vol-
tammetry (CV) was performed using a BAS 100 W (Bioanalytical
Systems), using a glass carbon disk (diameter ¼ 3 mm) as the
working electrode, a platinum wire with a porous ceramic wick as
the auxiliary electrode, and Ag/Ag þ as the reference electrode
standardized by the redox couple ferrocenium/ferrocene. Anhy-
drous N,N-dimethylformamide (DMF) and dichloromethane
Scheme 1. Synthetic routes and chemical structures of the target materials PTHBI and PTHPI.

8
X. Qiu et al. / Dyes and Pigments 147 (2017) 6e15
(CH2Cl2) containing 0.1 M tetrakis(n-butyl)-ammonium hexa-
fluorophosphate (NBu4PF6) as the supporting electrolyte were used
as solvents under a nitrogen atmosphere. All solutions were purged
with a nitrogen stream for 10 min before measurements. The
procedure was performed at room temperature, and a nitrogen
atmosphere was maintained over the solution during measure-
ments. A scan rate of 50 mV sꢀ1 was applied.
dried over anhydrous MgSO4 and evaporating the organic solvent
using a rotary evaporator. The crude product was purified by silica
gel column chromatography using ethyl acetate/petroleum ether
(1/10; v/v) as the eluent. The light yellow solid product yield was
(0.44 g, yield: 43.2%). 1H NMR (500 MHz, CDCl3, ppm):
d 9.80 (s,1H),
7.57e7.65 (m, 2H), 7.09e7.19 (m, 2H), 6.88e6.98 (m, 3H), 3.98 (m,
2H), 1.45 (t, 3H).
2.3. Theoretical calculation
2.5.3. 10-Ethyl-3-(1,4,5-triphenyl-1H-imidazol-2-yl)-phenothiazine
(PTHBI)
The ground-state geometries were optimized at the level of
B3LYP/6-31G(d,p), spatial distributions of the HOMO and LUMO of
the compounds were obtained from the optimized ground state
structures.
A mixture of M1 (1.00 g, 3.92 mmol), benzil (0.82 g, 3.92 mmol),
aminobenzene (1.46 g, 15.67 mmol), and ammonium acetate
(1.51 g, 19.58 mmol) with 20 mL acetic acid were added into a clean
100 mL flask and refluxed under N2 in a 120 ꢁC oil bath for 2 h. After
cooling down, the solid product was filtrated and washed with
30 mL water/acetic acid (1/1; v/v) and 30 mL water successively,
dissolved in CH2Cl2 and dried over anhydrous MgSO4, purified by
silica gel column chromatography using dichloromethane/petro-
leum ether (1/1; v/v) as the eluent and the white solid product yield
2.4. Device fabrication
Indiumꢀtin oxide (ITO) coated glass with a sheet resistance of
15e20
U
cmꢀ2 was used as the substrate. Before device fabrication,
the ITO glass substrates were cleaned with acetone, detergent,
deionized water, and isopropanol, dried in an oven at 120 ꢁC, and
treated with UV-zone for 20 min. After that, the samples were
transferred into a deposition system. The devices were fabricated
by the multiple source organic molecular beam deposition method
in a vacuum at a pressure of 4 ꢂ 10ꢀ6 mbar. Evaporation rates of
0.4 Å sꢀ1 for the organic materials and 1e4 Å sꢀ1 for the metal
electrodes were applied. The thickness of each deposition layer was
monitored using a quartz crystal thickness/ratio monitor (STM-
100/MF, Sycon). The structure of the device was ITO/HATCN (5 nm)/
NPB (50 nm)/TCTA (5 nm)/PTHBI or PTHPI (20 nm)/TPBi (30 nm)/LiF
(0.5 nm)/Al (100 nm). Electroluminescence (EL) spectra were
measured using a PR650 fluorescence spectrophotometer. Lumi-
nanceevoltage and current densityevoltage characteristics were
recorded simultaneously by combining the spectrometer with a
Keithley model 2400 programmable voltageecurrent source.
External quantum efficiency (EQE) was calculated from the lumi-
nance, current density, and EL efficiency according to the literature
[28].
was (1.64 g, yield: 80.0%). 1H NMR (500 MHz, CDCl3, ppm):
d 7.58 (d,
J ¼ 8.0 Hz, 2H), 7.32e7.15 (m, 10H), 7.11 (t, J ¼ 6.9 Hz, 3H), 7.05 (t,
J ¼ 6.8 Hz, 4H), 6.88 (t, J ¼ 7.5 Hz, 1H), 6.81 (d, J ¼ 8.2 Hz, 1H), 6.64
(d, J ¼ 8.6 Hz, 1H), 3.85 (q, J ¼ 6.9 Hz, 2H), 1.36 (t, J ¼ 6.9 Hz, 3H). 13
C
NMR (125 MHz, CDCl3, ppm):
d 146.01, 144.93, 144.22, 137.80,
136.93, 133.99, 131.11, 130.65, 130.42, 129.16, 128.42, 128.32, 128.15,
127.97, 127.46, 127.21, 126.69, 123.97, 122.48, 115.00, 114.20, 41.82,
12.85. MALDI-TOF MS (mass m/z): 521.1 [Mþ]. Anal. Calcd. for
C35H27N3S: C, 80.58; H, 5.22; N, 8.05; S, 6.15. Found: C, 80.78; H,
5.12; N, 8.04; S, 6.06.
2.5.4. 10-Ethyl-3-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)-
phenothiazine (PTHPI)
A mixture of M1 (1.50 g, 5.87 mmol), phenanthrenequinone
(1.22 g, 5.87 mmol), aminobenzene (2.19 g, 23.49 mmol), and
ammonium acetate (2.26 g, 29.37 mmol) with 20 mL acetic acid
were added into a clean 100 mL flask and refluxed under N2 in a
120 ꢁC oil bath for 2 h. After cooling down, the solid product was
filtrated and washed with 30 mL water/acetic acid (1/1; v/v) and
30 mL water successively, dissolved in CH2Cl2 and dried over
anhydrous MgSO4, purified by silica gel column chromatography
using dichloromethane/petroleum ether (1/1; v/v) as the eluent
and the white solid product yield was (2.53 g, yield: 83.0%). 1H NMR
2.5. Synthesis
2.5.1. 10-Ethyl-phenothiazine
To a phenothiazine (9.96 g, 50.11 mmol) solution in 150 mL
DMSO, sodium hydroxide (12 g, 300.01 mmol) and bromoethane
(4.47 mL, 60.04 mmol) were added at room temperature and
reacted for 8 h. Then the reaction mixture was poured into water
and extracted with dichloromethane. The organic phase was
collected and dried over anhydrous MgSO4. After removing the
solvent, the residue was purified by silica gel column chromatog-
raphy using petroleum ether/dichloromethane (10/1; v/v) as the
eluent to give the product as white solid (9.66 g, yield: 85.0%). 1H
(500 MHz, CDCl3, ppm):
d
8.86 (d, J ¼ 7.5 Hz, 1H), 8.75 (d, J ¼ 8.3 Hz,
1H), 8.69 (d, J ¼ 8.3 Hz, 1H), 7.73 (t, J ¼ 7.5 Hz, 1H), 7.67e7.58 (m,
4H), 7.53e7.45 (m, 3H), 7.25 (m, 3H), 7.18e7.03 (m, 3H), 6.89 (t,
J ¼ 7.5 Hz, 1H), 6.82 (d, J ¼ 8.1 Hz, 1H), 6.69 (d, J ¼ 8.6 Hz, 1H), 3.87
(q, J ¼ 7.0 Hz, 2H), 1.37 (t, J ¼ 7.0 Hz, 3H). 13C NMR (125 MHz, CDCl3,
ppm): d 150.02, 145.25, 144.13, 138.81, 137.22, 130.19, 129.82, 129.10,
128.20, 127.96, 127.22, 126.18, 125.52, 124.71, 124.36, 124.05, 123.78,
123.02, 122.78, 122.55, 120.72, 115.01, 114.22, 41.85, 12.81. MALDI-
TOF MS (mass m/z): 519.2 [Mþ]. Anal. Calcd. for C35H27N3S: C,
80.89; H, 4.85; N, 8.09; S, 6.17. Found: C, 81.01; H, 4.82; N, 8.07; S,
6.10.
NMR (500 MHz, CDCl3, ppm):
4H), 3.92 (m, 2H), 1.42 (t, 3H).
d 7.11e7.16 (m, 4H), 6.85e6.88 (m,
2.5.2. 10-Ethyl-phenothiazine-3-carbaldehyde (M1)
3. Results and discussion
A portion (1.09 mL, 12.01 mmol) of POCl3 was slowly added into
0.40 mL of N,N-dimethylformamide (DMF) at 0 ꢁC. The reaction
mixture was stirred for an hour at this temperature. This reaction
mixture was slowly added into a solution of (0.91 g, 4.02 mmol) of
10-Ethyl-phenothiazine in 20 mL of 1,2-dichloroethane at 0 ꢁC.
After complete addition of DMF and POCl3 mixture, the reaction
mixture was stirred at 100 ꢁC for 16 h. After cooling to room tem-
perature, the reaction mixture was neutralized with aqueous K2CO3
and extracted with 100 mL of dichloromethane three times. The
combined organic layer was washed with 300 mL of water and
3.1. X-ray crystal structure
We obtained single crystals of PTHBI and PTHPI by evaporating
tetrahydrofuran (THF) and methanol from the mixture solution.
The molecular structures determined by single crystal diffraction
are shown in Fig. 1, and the corresponding data and structural pa-
rameters are summarized in the supporting information (Table S1
and Table S2).
As shown in Fig. 1a, PTHBI has a twisted molecular structure.

X. Qiu et al. / Dyes and Pigments 147 (2017) 6e15
9
demonstrated by their high decomposition temperatures (Td, cor-
responding to 5% weight loss) of 348 ꢁC and 372 ꢁC, respectively.
Meanwhile, PTHBI and PTHPI show glass transition temperatures
(Tg) of 92 ꢁC and 103 ꢁC, and endothermic melting temperatures
(Tm) of 203 ꢁC and 205 ꢁC, respectively. The detailed data are
summarized in Table 1. Compared to PTHPI, the more flexible mo-
lecular structure of PTHBI shows a relatively poorer thermal sta-
bility, which confirms that the planar conjugation of phenanthro
[9,10-d]imidazole makes PTHPI more rigid, which leads to a better
thermal stability. Their good thermal stabilities indicate the high
morphological stability of their deposited films, which is a pre-
requisite for their applications in OLEDs.
3.3. Theoretical calculations
Density functional theory calculations (B3LYP with the 6-31G(d)
basis set) were performed for PTHBI and PTHPI using the Gaussian
09 program. The optimized molecular geometries and the calcu-
lated highest occupied molecular orbital (HOMO) and lowest un-
occupied molecular orbital (LUMO) density maps are shown in
Fig. 3. The PTHBI molecule could emit blue fluorescence because of
the interrupted
p-conjugation by breaking the single bond in the
phenanthrene part, which is in accordance with the optimized
molecular geometry. In addition, the outside phenyl unit at the N-1
position of the imidazole ring has a large twist angle of 70e80ꢁ
because of the large steric hindrance between the adjacent phenyl
rings and the strong repulsion between the neighbouring hydrogen
atoms. This conformation tends to inhibit intimate
pep intermo-
lecular interactions. The theoretical results are in accordance with
the X-ray crystal structures. PTHBI and PTHPI have almost the same
HOMO and LUMO orbitals shapes; i.e., the HOMO and LUMO are
located on the whole molecule, and the HOMO cannot be located on
the outside phenyl units at the N-1 position of the imidazole ring.
Fig. 1. The single molecule structure of PTHBI (a) and PTHPI (b) determined by single
crystal.
The benzene moieties that are connected to the imidazole five-
membered ring show twist angles of 69.67ꢁ
(
b
), 86.57ꢁ
(
g
), and
), respectively. The “butterfly” angle (DHA-1) of the
phenothiazine ring in PTHBI is 137.71ꢁ with a twist angle (
) of
42.20ꢁ between the phenothiazine and imidazole five-membered
ring. CeH … interactions ranging from approximately 3.0 to
3.4. Optical properties
18.66ꢁ
(d
We collected UV-Vis absorption and PL spectra of PTHBI and
PTHPI in dilute THF solutions (10ꢀ5 mol Lꢀ1) and in the thin film
state to evaluate their optical properties, as shown in Fig. 4. The key
photophysical data are summarized in Table 1. A higher intensity
absorption peak at around 265 nm can be seen, which originates
a
p
3.2 Å (Fig. S3) are observed, which are responsible for providing
molecular rigidity and solid-state emission. The twisted structure
of PTHBI effectively diminishes the ðeconjugation and suppresses
the intermolecular charge transfer. In addition, the fluorescence
quenching induced by aggregation in the solid state is also reduced.
For PTHPI (Fig. 1b), the benzene moieties connected to imidazole
from the
molecule, the absorption peak at around 318 nm originates from
the * transition of the phenanthro[9,10-d]imidazole moiety.
The lower intensity absorption peak near 370 nm originates from
the * transition, which is generated from the phenothiazine
pep* transition of the benzene ring. For the PTHPI
p‒p
ꢁ
p‒p
^
show a twisted angle of 71.76 (a). The “butterfly” angle (DHA-2) oꢁf
ꢁ
and phenanthro[9,10-d]imidazole moieties.
ꢀ
the phenothiazine ring is 129.97 with a twisted angle (a) of 42.81
Concerning the PL spectra, the PTHBI emission is bluer than that
from the PTHPI molecule. The maximum emission wavelengths of
PTHBI and PTHPI are 454 nm and 466 nm, respectively, in THF
solutions. The emission peaks in the film state are red-shifted by
5 nm for PTHBI and 10 nm for PTHPI compared to those in the THF
solution. The more twisted molecular structure causes the PL
spectra of PTHBI to exhibit a hypochromic shift compared to that of
PTHPI. Breaking a bond in the phenanthro[9,10-d]imidazole moiety
between the phenothiazine and phenanthro[9,10-d]imidazole unit.
In molecular packing of PTHPI, no tight
observed between the aromatic units; rather, CeH …
p
e
p
interactions are
interaction
p
distances ranging from approximately 2.6 to 4.2 Å are observed
(Fig. S4). The phenothiazine groups in both PTHBI and PTHPI have
non-coplanar structures, and the butterfly shape could suppress
the intermolecular close packing. Further, the non-coplanar struc-
ture of PTHBI and PTHPI can weaken the pep close packing in the
film state and keep the molecule in an amorphous state.
can efficiently limit
p-conjugation, which induces blue emission.
The PL quantum yields (FF) of PTHBI and PTHPI are approximately
18.58% and 28.40% in THF using quinine sulfate as the standard.
We also measured the PL spectra in eight solvents with different
polarities. As shown in Fig. 5, from low-polarity n-hexane to high-
polarity acetonitrile, the fluorescence of PTHPI exhibits obvious
solvatochromic effects, and the total redshift of 33 nm is much
larger than that of PTHBI (16 nm), indicating that PTHPI has a more
obvious charge-transfer emitting character.
3.2. Thermal properties
The thermal properties of PTHBI and PTHPI were characterized
by thermogravimetric analysis and differential scanning calorim-
etry under a N2 atmosphere at a scanning rate of 10 ꢁC minꢀ1. Fig. 2
shows the excellent thermal stability of PTHBI and PTHPI, as

10
X. Qiu et al. / Dyes and Pigments 147 (2017) 6e15
Fig. 2. (a) TGA data of PTHBI and PTHPI recorded at a heating rate of 10 ꢁC minꢀ1. (b) DSC traces recorded at a heating rate of 10 ꢁC minꢀ1
.
Table 1
Physical data of compounds.
donor for each. From the onset of the oxidation curves, the HOMO
levels are estimated as ꢀ5.05 eV and ꢀ5.06 eV for PTHBI and PTHPI,
respectively. From the onset of the reduction curves, the LUMO
levels are estimated as ꢀ2.11 eV and ꢀ2.23 eV for PTHBI and PTHPI,
respectively. Thus, the HOMO-LUMO band gaps of PTHBI and PTHPI
are 2.94 eV and 2.83 eV, respectively. The detailed data are sum-
marized in Table 1. Breaking the C‒C bond in the phenanthro[9,10-
d]imidazole moiety to form the 1,4,5-triphenyl-1H-imidazole
moiety can enlarge the band gap of PTHBI.
PTHBI
PTHPI
Td/Tg/Tm (oC)
UV-visa (nm)
UV-visb (nm)
PLsa/PLfb (nm)
FF (%)
348/92/203
267
277
372/103/205
262/316/370
267/322/376
466/476
454/459
18.58c/22.20d/15.75e
ꢀ5.05/ꢀ2.11
2.94
28.40c/66.13d/48.86e
ꢀ5.06/ꢀ2.23
2.83
HOMO/LUMOf (eV)
Egf (eV)
a
In tetrahydrofuran solution (room temperature).
Neat film.
In tetrahydrofuran using quinine sulfate as standard.
In neat film using an integrating sphere photometer.
Single crystal using an integrating sphere photometer.
Calculated by comparing with ferrocene (Fc).
b
c
d
e
f
3.6. Electroluminescence properties
To understand the structure and EL performance relationship,
we constructed non-doped devices using PTHBI or PTHPI, respec-
tively, as the emissive materials. The device structures were opti-
mized as: ITO/HATCN (5 nm)/NPB (50 nm)/TCTA (5 nm)/PTHBI or
PTHPI (20 nm)/TPBi (30 nm)/LiF (0.5 nm)/Al (100 nm). In this de-
vice, ITO (indium tin oxide) and LiF/Al are used as the anode and
cathode, respectively, HATCN (dipyrazino[2,3-f:20,30-h]quinoxa-
line-2,3,6,7,10,11- hexacarbonitrile) as the hole injection layer, NPB
(N,N0-bis-(1-naphthalenyl)-N,N0-bis-phenyl-(1,10-biphenyl)-4,40-
diamine) as the hole transporting layer, TCTA (4,40,400-tris(N-car-
bazolyl)triphenylamine) as the electron blocking layer, and TPBi
3.5. Electrochemical properties
The HOMO and LUMO energy levels were obtained for PTHBI
and PTHPI from the electrochemical cyclic voltammetry measure-
ments (Fig. 6). The oxidation potentials of PTHBI and PTHPI are
almost the same because the phenothiazine moiety is the electron
Fig. 3. The optimized molecular geometry and the calculated HOMO and LUMO density maps of PTHBI (top) and PTHPI (down) at the B3LYP/6-31G(d) level.

X. Qiu et al. / Dyes and Pigments 147 (2017) 6e15
11
Fig. 4. The UV-vis absorption spectra and PL spectra in THF solution (10ꢀ5 M) and thin film of PTHBI (a) and PTHPI (b).
Fig. 5. Solvatochromism PL spectra of PTHBI (a) and PTHPI (b).
The EL spectra of the PTHBI-based and PTHPI-based devices at 6 V
are shown in Fig. 9. The inset exhibits the CIE coordinates of (0.16,
0.19) and (0.18, 0.34) for PTHBI and PTHPI, respectively, at 6 V. The
PTHBI-based device shows a maximum L of 6184 cd mꢀ2 at 12.4 V, a
maximum LE of 3.44 cd Aꢀ1, and a maximum power efficiency (PE)
of 1.57 lm Wꢀ1 at a current density of 5.73 mA cmꢀ2. The PTHPI-
based device exhibits a significantly improved performance, with
a maximum L of 16816 cd mꢀ2 at 12.8 V, maximum LE of 7.20 cd Aꢀ1
,
and a maximum PE of 4.0 lm Wꢀ1 at a current density of
5.65 mA cmꢀ2. Remarkably, the device performances show that the
maximum luminous efficiency of the PTHPI-based device is about
109% higher than that of the PTHBI-based device. In addition, the
PTHPI-based device also harvests a maximum EQE of 3.41%, which
is significantly higher than that of the PTHBI-based device (2.38%)
(Fig. 8b). Both devices exhibit excellent operational stabilities
following a little roll-off with increasing luminance. This is the first
observation of blue emission using a phenothiazine-based deriva-
tive in non-doped devices.
Fig. 6. Cyclic voltammograms of PTHBI (red) and PTHPI (blue). (For interpretation of
the references to colour in this figure legend, the reader is referred to the web version
of this article.)
4. Conclusions
In summary, two new electron DeA compounds (PTHBI and
PTHPI) were designed and synthesized using 10-ethyl-phenothia-
zine as the donor and imidazole as the acceptor. They were char-
acterized by NMR, MALDI-TOF MS, and X-ray single crystal
diffraction. Breaking the C‒C bond in the phenanthro[9,10-d]
imidazole moiety of PTHPI to form the 1,4,5-triphenyl-1H-imid-
azole moiety of PTHBI increased the twisting of the molecule and
enlarged the energy band gap, generating a bluer emission
(1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene) as the elec-
tron transporting layer and the hole blocking layer. The energy level
diagram and molecular structures of each layer are illustrated in
Fig. 7.
The luminance‒current density‒voltage (L‒J‒V) and the lumi-
nous efficiencyeexternal quantum efficiencyecurrent density
(LEeEQEeJ) characteristics of the non-doped devices are shown in
Fig. 8. Detailed data from the EL performance are listed in Table 2.
(
lEL ¼ 460 nm) than that for PTHPI (lEL ¼ 476 nm). The PTHPI-based
device had good device properties because of the planar molecular

12
X. Qiu et al. / Dyes and Pigments 147 (2017) 6e15
Fig. 7. Energy level diagram (a) and chemical structures (b) of the materials used in devices.

X. Qiu et al. / Dyes and Pigments 147 (2017) 6e15
13
Fig. 8. (a) The luminance‒current density‒voltage (L‒J‒V) and (b) the luminous efficiencyeexternal quantum efficiencyecurrent density (LEeEQEeJ) characteristics of the non-
doped devices.

14
X. Qiu et al. / Dyes and Pigments 147 (2017) 6e15
Table 2
Electroluminescence characteristics of the devices.
Device
The maximums of quantum efficiency
Lmax
Vturn-on
[V]e
lEL
[nm]f
d
[cd mꢀ2
]
Voltage
[V]
J
L
LEmax
PEmax
EQEmax
[%]c
[mA cmꢀ2
]
[cd mꢀ2
]
[cd Aꢀ1
]
[lm Wꢀ1
]
a
b
PTHBI
PTHPI
6.9
6.8
5.73
5.65
197
407
3.44
7.20
1.57
4.00
2.38
3.41
6184
16816
4.7
4.0
460
476
a
Maximum luminous efficiency.
Maximum power efficiency.
External quantum efficiency.
Maximum luminance.
b
c
d
e
f
Defined as the bias voltage at the luminance of 1 cd mꢀ2
.
Maximum peak of EL spectra at 6 V. Device structure: ITO/HATCN (5 nm)/NPB (50 nm)/TCTA (5 nm)/PTHBI or PTHPI (20 nm)/TPBi (30 nm)/LiF (0.5 nm)/Al (100 nm).
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Acknowledgment
We are grateful for financial support from the National Science
Foundation of China (No. 51573081, 51303091 and 51673105), the
Natural Science Foundation of Shandong Province of China (No.
ZR2016JL016), and the Natural Science Foundation of Qingdao City
of China (16-5-1-89-jch). We thank Open Project of the State Key
Laboratory of Supramolecular Structure and Materials of Jilin Uni-
versity (SKLSSM-201728).
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2017.07.064.
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infrared organic light-emitting diode based on
a
butterfly-shaped