Manipulating the LUMO distribution of quinoxaline-containing architectures to design electron transport materials: Efficient blue phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1016/j.orgel.2016.07.020

A series of new quinoxaline-containing compounds, namely, 2,3,6,7-tetrakis(3-(pyridin-3-yl)phenyl)quinoxaline (Tm3PyQ), 2,3,6,7-tetrakis(3-(pyridin-4-yl)phenyl)quinoxaline (Tm4PyQ), 1,4-bis(2,3-dimethyl-7-(pyridin-3-yl)quinoxalin-6-yl)benzene (3PyDQB), and 1,4-bis(2,3-dimethyl-7-(pyridin-4-yl)quinoxalin-6-yl)benzene (4PyDQB) were designed and synthesized as electronic transporting materials. The lowest unoccupied molecular orbital (LUMO) distributions of these compounds vary with the locations of quinoxaline moieties, which result in adjustable intermolecular charge-transfer integrals. All the compounds exhibit favorable electron affinity (2.73–2.88 eV) and good thermostability (glass transition temperatures in the range of 112–148 °C). Using these compounds as electron transport layers, the bis(4,6-(difluorophenyl)pyridinato-N,C^2′)picolinate iridium(Ⅲ) (Firpic)-based blue phosphorescent organic light emitting diodes (PhOLEDs) achieve good performances with a maximum current efficiency (η_c,max) of 30.2 cd A^?1 and a maximum external quantum efficiency (η_ext,max) of 14.2%. Moreover, these efficiencies reveal small roll-offs at high luminance.

Full text of DOI:10.1016/j.orgel.2016.07.020

Organic Electronics 37 (2016) 439e447
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Manipulating the LUMO distribution of quinoxaline-containing
architectures to design electron transport materials: Efficient blue
phosphorescent organic light-emitting diodes
,
**
Xiaojun Yin a, Hengda Sun b, Weixuan Zeng a, Yepeng Xiang a, Tao Zhou a, Dongge Ma b
,
,
*
Chuluo Yang a
a Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials,
Department of Chemistry, Wuhan University, Wuhan 430072, People's Republic of China
b State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022,
People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new quinoxaline-containing compounds, namely, 2,3,6,7-tetrakis(3-(pyridin-3-yl)phenyl)
quinoxaline (Tm3PyQ), 2,3,6,7-tetrakis(3-(pyridin-4-yl)phenyl)quinoxaline (Tm4PyQ), 1,4-bis(2,3-
dimethyl-7-(pyridin-3-yl)quinoxalin-6-yl)benzene (3PyDQB), and 1,4-bis(2,3-dimethyl-7-(pyridin-4-yl)
quinoxalin-6-yl)benzene (4PyDQB) were designed and synthesized as electronic transporting materials.
The lowest unoccupied molecular orbital (LUMO) distributions of these compounds vary with the lo-
cations of quinoxaline moieties, which result in adjustable intermolecular charge-transfer integrals. All
the compounds exhibit favorable electron affinity (2.73e2.88 eV) and good thermostability (glass
Received 2 May 2016
Received in revised form
16 July 2016
Accepted 16 July 2016
Keywords:
transition temperatures in the range of 112e148 ꢀC). Using these compounds as electron transport layers,
Electron transport materials
Organic light emitting diodes
Phosphorescence
0
the bis(4,6-(difluorophenyl)pyridinato-N,C2 )picolinate iridium(Ⅲ) (Firpic)-based blue phosphorescent
organic light emitting diodes (PhOLEDs) achieve good performances with a maximum current efficiency
(
hc,max) of 30.2 cd Aꢁ1 and a maximum external quantum efficiency (hext,max) of 14.2%. Moreover, these
Quinoxaline
efficiencies reveal small roll-offs at high luminance.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
p-conjugation of the molecule [26e30] or utilizing the intra-/
intermolecular hydrogen bonds [31e34]. Beyond that, manipu-
lating the lowest unoccupied molecular orbital (LUMO) distribu-
tions of the molecule to afford effective LUMO overlaps between
neighboring molecules is also a feasible way [35e37], because the
effective LUMO overlaps could enlarge the intermolecular charge-
transfer integrals, and then facilitate the electron transfer from
one molecule to another [38,39].
Nitrogen-embedded monocyclic compounds such as pyridine
and pyrimidine are generally recognized as desired electron-
transporting units in constructing new ETMs with both high
triplet energy level (ET) and high me [40e44]. For example, the well-
known ETM of 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB)
has a ET of 2.75 eV and a me of 1.0 ꢂ 10ꢁ3 cm2 Vꢁ1 sꢁ1 simultaneously
[45]. Nevertheless, their thermal stabilities are often undesirable,
which may lead to a poor morphological stability in the OLED and
hence resulting in inferior device performance at high brightness
[46e49]. In order to enhance the glass transfer temperature (Tg) of
Charge transport is of significant importance in organic light-
emitting diodes (OLEDs) [1e6], since the generation of light in
OLED is the consequence of the recombination of holes and elec-
trons injected from the electrodes to the emitting layer [7e12], A
balanced charge carrier injection and transport in OLED is consid-
ered to be essential for achieving high performance [13e16].
However, the hole mobility is generally 2e3 orders of magnitude
higher than that of electron in the OLED due to the electron trap-
ping [17e20]. Developing new electron-transporting materials
(ETMs) with enhanced electron mobility (me) to alleviate such
contradiction is of significance [21e25]. Several strategies have
been adapted to promote the me of the ETMs, such as extending the
* Corresponding author.
** Corresponding author.
the compounds, extended p-conjugation of the aza-aromatics such
E-mail addresses: md1014@ciac.jl.cn (D. Ma), clyang@whu.edu.cn (C. Yang).
http://dx.doi.org/10.1016/j.orgel.2016.07.020
1566-1199/© 2016 Elsevier B.V. All rights reserved.

440
X. Yin et al. / Organic Electronics 37 (2016) 439e447
as quinoline, quinoxaline, naphthyridine, etc., have been incorpo-
rated into the -conjugated skeleton [50e54].
p
In this article, four new quinoxaline-based ETMs, namely,
Tm3PyQ, Tm4PyQ, 3PyDQB and 4PyDQB were designed and syn-
thesized (Scheme 1). The ortho-linkage is intended to decrease the
p-extension, and thus maintain a high ET of the molecule (Fig. 1).
Density functional theory (DFT) simulation reveals that their LUMO
distributions can be manipulated by adjusting the location of qui-
noxaline units. All these ETMs exhibit favorable electron affinities
(2.73e2.88 eV) and des0irable Tgs (>112 ꢀC). The bis(4,6-(difluor-
ophenyl)pyridinato-N,C2 )picolinate iridium(Ⅲ) (Firpic)-based blue
phosphorescent OLEDs employ them as electron transport layers
(ETLs) achieved good performance, with a maximum current effi-
ciency (hc,max) of 30.2 cd Aꢁ1 and a maximum external quantum
(hext,max) of 14.2%.
2. Experimental
Fig. 1. Molecular design strategy of the quinoxaline-based compounds.
2.1. Instrumentation
thermogravimetric analysis (TGA) and differential scanning calo-
rimetry (DSC) of the compounds were undertaken with a NETZSCH
STA 449C instrument and NETZSCH DSC 200 PC unit, respectively.
Cyclic voltammetry (CV) measured in nitrogen-purged dimethyl
formamide (DMF) solution by using a CHI voltammetric analyzer.
The Bu4NPF6 (0.1 M) was used as the supporting electrolyte with
ferrocenium-ferrocene (Fc/Fcþ) was identified as the internal
standard, and scan rate was 100 mV sꢁ1. The lowest unoccupied
molecular orbital (LUMO) energy level (eV) of the samples were
1H NMR and 13C NMR spectra of the compounds were recorded
on a Varian MERCURY-VX300 spectrometer. The molecular weights
were evaluated by VJ-ZAB-3F or Waters ZQ2000 LC/MS Mass
spectrometer. Elemental analyses of C, H and N were performed on
a Perkin-Elmer 204B microanalyzer. UVeVis absorption spectra
and photoluminescence (PL) spectra were obtained on Shimadzu
UV-2500 spectrophotometer and Hitachi F-4600 fluorescence
spectrophotometer, respectively. Thermodynamic investigation of
Scheme 1. Synthetic routes of the quinoxaline-containing compounds.

X. Yin et al. / Organic Electronics 37 (2016) 439e447
441
calculated through the following formula: ꢁ[4.8 þ (Eonset, red ꢁ E1/
148.9,148.8,143.2,141.0,140.1,139.8,138.1,137.9,136.4,136.3,136.2,
134.5, 130.6, 129.8, 129.7, 129.6, 129.5, 129.2, 128.0, 126.4, 123.9,
123.8, 123.7. MS (ESI): m/z 765 [MþNa]þ. Anal. calcd for C52H34N6: C
84.07, H 4.61, N 11.31; found: C 83.95, H 4.65, N 11.39.
2(Fc/Fcþ))] eV.
2.2. Materials preparation
4,5-bis(3-(pyridin-4-yl)phenyl)benzene-1,2-diamine
(3):
A
Commercially available reagents were used directly only if
indicated; Solvents were purified according to standard pro-
cedures. The starting material of 1,2-bis(3-bromophenyl)ethane-
1,2-dione [55], 4,5-dibromobenzene-1,2-diamine [56], 3-(3-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)pyridine, 4-
(3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)pyridine
[57], and 1,4-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)ben-
zene [58], were synthesized according to the literature procedures.
1,2-bis(3-(pyridin-3-yl)phenyl)ethane-1,2-dione (1): A mixture of
1,2-bis(3-bromophenyl)ethane-1,2-dione (4.4 g, 12.1 mmol), pyri-
dine-3-boronic acid (4.5 g, 36.2 mmol), Pd(PPh3)4 (300 mg) and
potassium carbonate (8.2 g) were added to a 250 mL flask. Then, the
degassed toluene (50 mL), ethanol (20 mL) and deionized water
(30 mL) was added under argon atmosphere. The mixture was
stirred at 110 ꢀC for 24 h. After cooling down, the mixture was
allowed to pour into water and then filtration. The remaining solid
was washed by water and ethanol successively, and then dissolved
in acetic acid. The acetic acid solution was then neutralized with
sodium hydroxide, and the precipitation was separated by filtration
to give the title compound as yellow solid (3.73 g, yield: 85%). 1H
mixture of 4,5-dibromobenzene-1,2-diamine (852 mg, 3.2 mmol),
4-(3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)pyridine
(2.25 g, 8.0 mmol), potassium carbonate (5.5 g), Pd(PPh3)4 (200 mg)
were added to a 150 mL of flask, then the degassed toluene (40 mL),
ethanol (10 mL) and deionized water (20 mL) were added. The
mixture was stirred at 110 ꢀC for 24 h. After cooling down, the
mixture was poured into water and extracted with ethyl acetate
(50 mL ꢂ 3). The combined organic layer was washed with water
and brine, dried over Na2SO4, and then, the solvent was removed by
vacuum distillation and the rest solid was purified with a silica gel
column. Using dichloromethane/methanol (v/v) 20:1 as eluent and
gave the target compound as white solid (1.01 g, yield: 76%). 1H
NMR (300 MHz, CDCl3)
d
[ppm]: 8.54 (d, J ¼ 4.2 Hz, 4H), 7.44 (d,
J ¼ 6.9 Hz, 2H), 7.37 (m, 4H), 7.27 (d, J ¼ 6.9 Hz, 2H), 7.21 (d,
J ¼ 4.2 Hz, 4H), 6.88 (s, 2H), 3.49 (s, 4H). 13C NMR (75 MHz, CDCl3)
d
[ppm]: 150.1, 148.5, 142.5, 137.7, 134.7, 132.0, 130.6, 129.3, 129.0,
124.7, 121.7, 118.7. MS (EI): m/z 414.2 [Mþ]. Anal. calcd for C28H22N4:
C 81.13, H 5.35, N 13.52; found: C 81.33, H 5.43, N 13.64.
2,3-bis(3-bromophenyl)-6,7-bis(3-(pyridin-4-yl)phenyl)quinoxa-
line (4): Follow the same procedure of compound 2, with the
participation of 1,2-bis(3-bromophenyl)ethane-1,2-dione (1.15 g,
3.2 mmol) and intermediate 3 (1.3 g, 3.1 mmol) to give the title
compound as light yellow solid (2.2 g, yield: 95%). 1H NMR
NMR (300 MHz, CDCl3)
d
[ppm]: 8.87 (s, 2H), 8.65 (d, J ¼ 4.2 Hz, 2H),
8.24 (s, 2H), 8.03 (d, J ¼ 7.8 Hz, 2H), 7.91 (d, J ¼ 7.8 Hz, 4H), 7.67 (t,
J ¼ 7.8 Hz, 2H), 7.42 (t, J ¼ 5.1 Hz, 2H). 13C NMR (75 MHz, CDCl3)
d
[ppm]: 193.8, 149.4, 148.3, 139.1, 134.7, 133.7, 130.1, 129.8, 128.4,
(300 MHz, CDCl3)
(s, 2H), 7.46e7.61 (m, 10H), 7.35 (d, J ¼ 7.8 Hz, 2H), 7.19e7.24 (m,
6H). 13C NMR (75 MHz, CDCl3)
[ppm]: 152.1, 149.6, 148.2, 142.9,
d
[ppm]: 8.60 (d, J ¼ 4.2 Hz, 4H), 8.36 (s, 2H), 7.90
123.9. MS (EI): m/z 364.3 [Mþ]. Anal. calcd for C24H16N2O2: C 79.11,
H 4.43, N 7.69; found: C 79.22, H 4.40, N 7.61.
d
6,7-dibromo-2,3-bis(3-(pyridin-3-yl)phenyl)quinoxaline (2): In-
termediate 1 (3.17 g, 8.7 mmol) and 4,5-dibromobenzene-1,2-
diamine (3.02 g, 11.35 mmol) were dissolved in 90 mL acetic acid,
and then heated to reflux for 16 h. After cooling down, the resulting
mixture was poured into water and neutralized by sodium hy-
droxide. The suspension was extracted with chloroform
(50 mL ꢂ 3), and the combined organic layer was washed with
water and dried over anhydrous sodium sulfate. After that, the
solvent was removed by vacuum distillation and the rest solid was
purified with a silica gel column. Using dichloromethane/methanol
(v/v) 20:1 as eluent and gave the target compound as light yellow
140.7, 140.6, 140.3, 137.9, 132.7, 132.3, 130.6, 130.4, 129.7, 129.4,
129.1, 128.6, 126.2, 122.8, 121.7. MS (ESI): m/z 747 [MþH]þ. Anal.
calcd for C42H26Br2N4: C 67.58, H 3.51, N 7.51; found: C 67.53, H 3.61,
N 7.46.
2,3,6,7-tetrakis(3-(pyridin-4-yl)phenyl)quinoxaline
(Tm4PyQ):
Follow the same procedure of compound Tm3PyQ, with the
participation of intermediate 4 (1.95 g, 2.61 mmol) and pyridine-4-
boronic acid (1.6 g, 13 mmol) to give the title compound as white
solid (2.2 g, yield: 95%). 1H NMR (300 MHz, CDCl3)
d [ppm]: 8.63 (d,
J ¼ 5.7 Hz, 4H), 8.59 (d, J ¼ 5.4 Hz, 4H), 8.41 (s, 2H), 7.89 (s, 2H), 7.72
(d, J ¼ 8.1 Hz, 4H), 7.49e7.61 (m, 10H), 7.35 (d, J ¼ 5.4 Hz, 4H), 7.25
solid (4.3 g, yield: 83%). 1H NMR (300 MHz, CDCl3)
d
[ppm]: 8.62 (s,
(d, J ¼ 6.6 Hz, 4H). 13C NMR (75 MHz, CDCl3)
d [ppm]: 153.4, 150.3,
147.9, 147.6, 143.0, 140.8, 139.7, 138.5, 138.2, 130.5, 129.5, 129.2,
2H), 8.57 (d, J ¼ 3.9 Hz, 2H), 8.53 (s, 2H), 7.65e7.73 (m, 8H) 7.55 (t,
J ¼ 7.5 Hz, 2H), 7.33 (t, J ¼ 5.1 Hz, 2H). 13C NMR (75 MHz, CDCl3)
128.9, 127.8, 126.2, 121.6. MS (ESI): m/z 765 [MþNa]þ. Anal. calcd for
d
[ppm]: 153.9, 148.7, 148.0, 140.6, 139.1, 138.0, 136.0, 134.6, 133.4,
C52H34N6: C 84.07, H 4.61, N 11.31; found: C 84.12, H 4.69, N 11.28.
129.7, 129.6, 129.0, 128.3, 127.1, 123.8. MS (EI): m/z 593.6 [Mþ]. Anal.
calcd for C30H18Br2N4: C 60.63, H 3.05, N 9.43; found: C 60.53, H
3.10, N 9.48.
6,7-dibromo-2,3-dimethylquinoxaline (5): Follow the same pro-
cedure of compound 2, with the participation of 4,5-
dibromobenzene-1,2-diamine (2.3 g, 8.6 mmol) and biacetyl
(0.75 g, 8.6 mmol), using petroleum ether/ethyl acetate (v/v) 5:1 as
eluent to give the title compound as white solid (2.1 g, yield: 76%).
2,3,6,7-tetrakis(3-(pyridin-3-yl)phenyl)quinoxaline (Tm3PyQ): To
a mixture of intermediate 2 (1.95 g, 3.2 mmol), 3-(3-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)pyridine
(2.3
g,
1H NMR (300 MHz, CDCl3)
d
[ppm]: 8.28 (s, 2H), 2.71 (s, 6H). 13C
8.2 mmol), potassium carbonate (5.6 g) and Pd(PPh3)4 (300 mg),
was added degassed toluene (40 mL), ethanol (10 mL) and deion-
ized water (20 mL). The suspension was stirred at 110 ꢀC for 24 h.
After cooling to room temperature, the resulting mixture was
poured into water and extracted with dichloromethane
(50 mL ꢂ 3). The combined organic layer was washed with water
and brine, and then dried over Na2SO4. After that, the solvent was
removed by vacuum distillation and the rest solid was purified with
a silica gel column. Using dichloromethane/methanol (v/v) 20:1 as
eluent and gave the final product as white solid (2.1 g, yield: 89%).
NMR (75 MHz, CDCl3) d [ppm]: 157.1, 138.8, 132.7, 105.4, 23.5. MS
(EI): m/z 315.8 [Mþ]. Anal. calcd for C10H8Br2N2: C 38.01, H 2.55, N
8.87; found: C 38.13, H 2.49, N 8.81.
6-bromo-2,3-dimethyl-7-(pyridin-3-yl)quinoxaline (6): Follow
the same procedure of compound 3, with the participation of in-
termediate 5 (1.87 g, 5.93 mmol) and pyridine-3-boronic acid
(875 mg, 7.11 mmol) to give the title compound as white solid
(1.41 g, yield: 76%). 1H NMR (300 MHz, CDCl3)
d [ppm]: 8.77 (s, 1H),
8.69 (d, J ¼ 3.6 Hz, 1H), 8.37 (s, 1H), 8.08 (s, 1H), 7.86 (d, J ¼ 7.5 Hz,
1H), 7.43 (t, J ¼ 5.1 Hz, 1H), 2.77 (s, 3H), 2.74 (s, 3H). 13C NMR
1H NMR (300 MHz, CDCl3)
8.40 (s, 2H), 7.79 (s, 2H), 7.71e7.75 (m, 4H), 7.51e7.68 (m, 12H), 7.43
(s, 2H), 7.29e7.35 (m, 4H). 13C NMR (75 MHz, CDCl3)
[ppm]: 153.6,
d
[ppm]: 8.64 (s, 2H), 8.54e8.58 (m, 6H),
(75 MHz, CDCl3) d [ppm]: 155.2, 154.9, 150.1, 149.3, 141.1, 140.0,
139.5, 137.1, 135.9, 132.6, 130.3, 123.1, 123.0, 23.4. MS (EI): m/z 314.2
d
[Mþ]. Anal. calcd for C15H12BrN3: C 57.34, H 3.85, N 13.37; found: C

442
X. Yin et al. / Organic Electronics 37 (2016) 439e447
57.53, H 3.81, N 13.22.
standard of the theory by using TDDFT method. The molecular
orbitals of these compounds were visualized using Gauss view 5.0.
6-bromo-2,3-dimethyl-7-(pyridin-4-yl)quinoxaline (7): Follow
the same procedure of compound 3, with the participation of in-
termediate 5 (2.79 g, 8.8 mmol) and pyridine-4-boronic acid (1.19 g,
9.76 mmol) to give the title compound as white solid (1.79 g, yield:
3. Results and discussion
65%). 1H NMR (300 MHz, CDCl3)
d
[ppm]:
8.36 (s, 1H), 7.94 (s, 1H), 7.45 (d, J ¼ 4.5 Hz, 2H), 2.77 (s, 3H), 2.75 (s,
3H). 13C NMR (75 MHz, CDCl3)
[ppm]: 155.4, 155.0, 149.7, 147.8,
d
8.74 (d, J ¼ 4.5 Hz, 2H),
3.1. Synthesis and characterization
d
Scheme 1 depicted the synthetic routes of these quinoxaline-
containing compounds. The quinoxaline-based bromide in-
termediates of 2, 4 and 5 were synthesized from a,b-diketone and
141.2, 140.3, 139.9, 132.7, 130.0, 124.5, 121.9, 23.4. MS (EI): m/z 314.0
[Mþ]. Anal. calcd for C15H12BrN3: C 57.34, H 3.85, N 13.37; found: C
57.51, H 3.82, N 13.23.
1,2-diaminobenzene precursors. The final products of Tm3PyQ,
Tm4PyQ, 3PyDQB and 4PyDQB were synthesized via Suzuki
coupling reaction of the various bromide intermediates and the
corresponding boric acid esters. All these intermediates and final
products were successively purified by column chromatography
and recrystallization, and the accurate molecular structures were
well confirmed by 1H NMR and 13C NMR spectroscopies, elemental
analysis and mass spectrometry.
1,4-bis(2,3-dimethyl-7-(pyridin-3-yl)quinoxalin-6-yl)benzene
(3PyDQB): Follow the same procedure of compound Tm3PyQ, with
the participation of intermediate 6 (1.29 g, 4.1 mmol) and 1,4-
bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzene (540 mg,
1.64 mmol) to give the final compound as white solid (508 mg,
yield: 57%). 1H NMR (300 MHz, CDCl3)
d [ppm]: 8.55 (s, 2H), 8.53 (d,
J ¼ 6.3 Hz, 2H), 8.08 (s, 2H), 8.05 (s, 2H), 7.45 (d, J ¼ 8.1 Hz, 2H), 7.27
(t, J ¼ 5.7 Hz, 2H), 7.12 (s, 4H), 2.78 (s, 12H). 13C NMR (75 MHz,
CDCl3)
d [ppm]: 154.6, 154.4, 150.3, 148.3, 141.3, 140.6, 140.3, 138.8,
3.2. Thermal properties
138.2, 137.2, 136.1, 130.0, 129.9, 123.0, 23.4. MS (EI): m/z 544.1 [Mþ].
Anal. calcd for C36H28N6: C 79.39, H 5.18, N 15.43; found: C 79.42, H
5.21, N 15.37.
TGA and DSC measurements are carried out to investigate the
impact of embedded quinoxaline unit on their thermal properties.
The TGA curves (Fig. 2) reveal that the thermal-decomposition
temperatures (Td, 5% weight loss) of Tm3PyQ and Tm4PyQ are
463 ꢀC and 485 ꢀC, respectively, which are significantly higher than
that of 3PyDQB and 4PyDQB (397 ꢀC). The inferior Tds of the latter
two can be ascribed to the lower molecular weight and less
1,4-bis(2,3-dimethyl-7-(pyridin-4-yl)quinoxalin-6-yl)benzene
(4PyDQB): Follow the same procedure of compound Tm3PyQ, with
the participation of intermediate 7 and 1,4-bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)benzene to give the title compound as
white solid (yield: 73%). 1H NMR (300 MHz, CDCl3)
d
[ppm]: 8.54 (d,
J ¼ 5.7 Hz, 4H), 8.06 (d, J ¼ 4.8 Hz, 4H), 7.14e7.17 (m, 8H), 2.79 (s,
extended
p-conjugation. The DSC traces exhibit gradually
12H). 13C NMR (75 MHz, CDCl3)
d [ppm]: 155.0, 154.6, 149.6, 148.4,
enhanced glass-transition temperatures (Tg), from 112 ꢀC for
Tm3PyQ, 132 ꢀC for Tm4PyQ, 138 ꢀC for 3PyDQB to 148 ꢀC for
4PyDQB (inset of Fig. 2). Compared to the generally pyridine-
containing ETMs, such as TmPyPB (Tg ~ 79 ꢀC) [45], the signifi-
cantly enhanced Tgs of these compounds are presumably due to the
introduction of quinoxaline units.
140.9, 140.3, 138.9, 130.1, 129.9, 124.8, 23.4. MS (EI): m/z 544.5 [Mþ].
Anal. calcd for C36H28N6: C 79.39, H 5.18, N 15.43; found: C 79.46, H
5.19, N 15.35.
2.3. Device fabrication and measurement
The OLED were fabricated on the patterned 180 nm-thick in-
3.3. Photophysical properties
dium tin oxide (ITO)-coated glass with sheet resistance of 10
U per
square. The ITO surface was carefully cleaned in an ultrasonic sol-
vent bath, and dried at 120 ꢀC, then treated with UV ozone for
15 min. After that, the substrates were transferred to the deposition
system, and then different functional layers were sequentially
deposited onto the substrates by thermal evaporation without
breaking vacuum (~5 ꢂ 10ꢁ4 Pa). The host and the guest were ar-
ranged into different effusion sources before the deposition of the
emissive layer, and the deposition rates of the materials were
controlled with their correspondingly independent quartz crystal
oscillators. The thicknesses of these functional layers were cali-
brated by a Dektak 6 M profiler (Veeco). The electroluminescent
(EL) spectra were measured by PR650 spectrometer. The current-
voltage-brightness (IꢁVꢁB) features of the devices were
measured with a Keithey 2400 Source meter and a Keithey 2000
Source multimeter equipped with a calibrated silicon photodiode.
All measurements were carried out at room temperature under
ambient conditions, and the light emissive area of the devices was
0.16 cm2.
UVeVis absorption and photoluminescence (PL) spectra of the
quinoxaline-based compounds in dilute dichloromethane (DCM)
solution are displayed in Fig. 3 and the key data are presented in
Table 1. As displayed in Fig. 3(a), Tm3PyQ, Tm4PyQ, 3PyDQB and
4PyDQB show absorption peaks at 372, 371, 339 and 336 nm,
respectively, assigned to their
p-
p* transitions. Their fluorescence
2.4. DFT calculations
The DFT simulations of the compounds were performed using a
suite of Gaussian 09 program packages [59]. The molecular struc-
tures was optimized by means of the B3LYP-GD3 with the 6-
311G(d, p) atomic basis set [60,61], The electronic energy levels of
the compounds were simulated at
[62], and the triplet energy level was calculated at the equal
t
HCTHhyb/6-311þþG(d, p) level
Fig. 2. TGA and DSC (inset) curves of the compounds.

X. Yin et al. / Organic Electronics 37 (2016) 439e447
443
Fig. 3. Normalized UVevis absorption in dichloromethane solution and phosphorescent emission in 2-methyltetrahydrofuran at 77 K (a), and fluorescence emission in
dichloromethane solution (b).
Table 1
Photophysical, electrochemical and thermal data of the compounds.
LUMOf/HOMOg [eV]
LUMO/HOMOh [eV]
a
b
c
e
Compounds
labs,maxa [nm]
lem,max [nm]
Eg [eV]
ET [eV]
Tgd/Tmd/Td [ꢀC]
Tm3PyQ
Tm4PyQ
3PyDQB
4PyDQB
372/382
371/381
339/348
336/342
415/425
413/422
396/410
404/416
3.13
3.14
3.39
3.38
2.52
2.52
2.74
2.69
112/219/463
132/278/485
138/338/397
148/338/397
ꢁ2.88/ꢁ6.01
ꢁ2.88/ꢁ6.02
ꢁ2.73/ꢁ6.12
ꢁ2.75/ꢁ6.13
ꢁ2.15/ꢁ6.03
ꢁ2.36/ꢁ6.37
ꢁ1.86/ꢁ5.91
ꢁ2.03/ꢁ6.09
a
Measured in DCM solution/films.
Estimated from the optical absorption edges.
Estimated from phosphorescent spectra in 2-methyltetrahydrofuran at 77 K.
Obtained from DSC curves.
Obtained from TGA curves.
b
c
d
e
f
Determined from the onset of reduction potentials.
Deduced from the LUMO and Eg.
g
h
Obtained from the DFT simulations.
emission spectra exhibit peaks at 415 (Tm3PyQ), 413 (Tm4PyQ),
396 (3PyDQB) and 404 nm (4PyDQB), respectively (Fig. 3(b)). The
absorption and emission spectra of the compounds in films
(Fig. S1) reveal only 6e14 nm red-shifts in comparison with those
in dilute DCM solution. This can be ascribed to the twisted
determined to be ꢁ2.88 eV (Tm3PyQ and Tm4PyQ), ꢁ2.73 eV
(3PyDQB) and ꢁ2.75 eV (4PyDQB), respectively. Unfortunately,
the oxidation process were not observed for these compounds,
and thus their HOMO energy levels are deduced to
be ꢁ6.01 eV (Tm3PyQ), ꢁ6.02 eV (Tm4PyQ), ꢁ6.12 eV (3PyDQB)
and ꢁ6.13 eV (4PyDQB), from the optical energy gaps (Eg) and
LUMO levels (Table 1). The suitable LUMO/HOMO energy levels of
these quinoxaline-based compounds enable them as favorable
electron transport and hole blocking materials for OLEDs.
configuration in restraining the intermolecular
p-p stacking. Both
Tm3PyQ and Tm4PyQ show similar photophysical properties,
with ca. 0.33 eV of Stokes shift. However, 3PyDQB and 4PyDQB
exhibit Stokes shifts of 0.53 and 0.63 eV, respectively, which may
origin from the diverse sites of pyridine substituents via direct
connection to the quinoxaline units. The small Stoke shift will
result in low energy loss upon reorganization in the excited state
[63e65]. Their triplet energy levels (ETs) are estimated to be
2.52 eV for Tm3PyQ and Tm4PyQ, 2.74 eV for 3PyDQB and
2.69 eV for 4PyDQB, respectively, from the onset of the phos-
phorescence spectra in frozen 2-methyltetrahydrofuran matrix at
77 K (Fig. 3(a)). Noticeably, Tm3PyQ and Tm4PyQ reveal distinctly
lower ETs than those of 3PyDQB and 4PyDQB, and the difference
will be discussed below.
3.5. Quantum chemical calculations
The DFT simulations are performed to investigate the geomet-
rical and electronic properties of the compounds at the molecular
level. The calculated LUMO/HOMO energy levels and orbitals of
these quinoxaline-based compounds are shown in Fig. 5. The re-
sults show that the LUMO orbitals of Tm3PyQ and Tm4PyQ
distribute mostly on the quinoxaline core, while the highest occu-
pied molecular orbitals (HOMOs) distribute from central quinoxa-
line core to the adjacent benzene rings. Similarly, the LUMO orbitals
of 3PyDQB and 4PyDQB mainly locate at the peripheral quinoxa-
line units, while the HOMO orbitals distribute across the central
benzene ring and quinoxaline units. Obviously, the LUMOs of the
four molecules can be effectively confined on the quinoxaline units.
Therefore, it can be speculated that the 3PyDQB/4PyDQB would
offer larger charge-transfer integrals for electron transport than
Tm3PyQ/Tm4PyQ, because the outside LUMO distribution and
double quinoxaline units will benefit for effective LUMO orbitals
overlaps between neighboring molecules.
3.4. Electrochemical properties
The electrochemical analysis was performed to evaluate the
electron affinity and hole blocking ability of the compounds. The
electrochemical behaviors were investigated by cyclic voltamme-
try (CV), and Fc/Fcþ couple were chosen as the internal reference.
As shown in Fig. 4, all the four compounds exhibit reversible
reduction waves in DMF solution. The onset reduction potentials
(Eonset,red) are ꢁ1.92 V for Tm3PyQ and Tm4PyQ, ꢁ2.07 V for
3PyDQB and ꢁ2.05 V for 4PyDQB, respectively. According to the
equation of ELUMO ¼ - [4.8 þ (Eonset,red eE1/2(Fc/Fcþ))] eV, the LUMO
energy levels of these quinoxaline-based compounds are
The calculated HOMO/LUMO values of these molecules
are ꢁ2.15/-6.03 eV for Tm3PyQ, ꢁ2.36/-6.27 eV for
Tm4PyQ, ꢁ1.86/-5.91 eV for 3PyDQB and ꢁ2.03/-6.09 eV for

444
X. Yin et al. / Organic Electronics 37 (2016) 439e447
Fig. 4. Cyclic voltammograms of the compounds in DMF solution for reduction.
Fig. 5. Calculated LUMO/HOMO energy levels and orbitals of these quinoxaline-based compounds.
4PyDQB, respectively (Table 1). The calculated ETs are 2.53 eV for
Tm3PyQ and Tm4PyQ, 2.71 eV for 3PyDQB and 2.68 eV for
4PyDQB, respectively, which match well with the measured values
(Table 1). It is worth noting that, though the highly twisted mo-
3.6. Transient electroluminescence measurements
Simple electroluminescent devices with the structures of ITO/
N,N0-di-1-naphthyl-N,N0-diphenylbenzidine (NPB) (40 nm)/ETMs:-
Rubrene (1%) (10 nm)/ETMs (50 nm)/LiF (0.7 nm)/Al (100 nm) were
fabricated to evaluate the electron transporting abilities of these
quinoxaline-containing compounds via transient electrolumines-
cence method. The electron mobilities of the compounds in
lecular configurations of these compounds decrease the
p-exten-
sion, the overlap of the HOMOs and LUMOs, especially for Tm3PyQ
and Tm4PyQ, result in a narrow energy bandgap and subsequently
lower their triplet energy levels.

X. Yin et al. / Organic Electronics 37 (2016) 439e447
445
time, and E was the electric field [66,67]. Fig. 6 displayed the
electron mobilities of the compounds under different electric field
intensities. As shown, the me of 3PyDQB lie in the range of
1.4 ꢂ 10ꢁ5 cm2 Vꢁ1 sꢁ1 to 2.9 ꢂ 10ꢁ5 cm2 Vꢁ1 sꢁ1 at an electric field
between 4.2 ꢂ 105 V cmꢁ1 and 1.1 ꢂ 106 V cmꢁ1, which is signifi-
cantly larger than those of Tm3PyQ, Tm4PyQ and 4PyDQB (Fig. 6).
The significantly improved intermolecular charge-transfer integrals
of 3PyDQB may lead to its superior electron transport ability. In
contrast, the inferior electron mobility of 4PyDQB than 3PyDQB is
presumably due to the larger energy loss upon reorganization in
the excited state.
3.0x10-5
2.5x10-5
2.0x10-5
1.5x10-5
1.0x10-5
5.0x10-6
Tm3PyQ
Tm4PyQ
3PyDQB
4PyDQB
3.7. Electroluminescence properties of the devices
600
700
800
900
1000
1100
1200
To evaluate the electron transport and exciton blocking prop-
erties of the quinoxaline-based compounds in electroluminescent
devices, the FIrpic-based blue phosphorescent OLEDs were fabri-
cated. The schematic diagram of the ETs of the used ETMs is shown
in Fig. 7(b). The ETs of Tm3PyQ and Tm4PyQ are lower than that of
FIrpic, and thus additional exciton blocking layer is required to
ensure the excitons confinement in emitting layer. The devices
Electric field E1/2 (V/cm)1/2
Fig. 6. Electric field dependence of electron mobilities of these compounds.
amorphous thin films can be calculated by me ¼ L/(tdE), approxi-
mately, where L was the thickness of ETMs layer, td was the delay
Fig. 7. Schematic diagram of the two types of device configuration (a), and the triplet energy levels of the used ETMs (b).
Fig. 8. The molecular structures and energy levels of the used materials.

446
X. Yin et al. / Organic Electronics 37 (2016) 439e447
cyclohexane (TAPC) was employed as the hole-transporting layer;
Device A1
Device A2
Device B1
Device B2
Device B3
500
400
300
200
100
0
1,3-di(9H-carbazol-9-yl)benzene (mCP) was used as the host ma-
terial, and FIrpic was used as the blue phosphor. The TmPyPB in
type A was used as exciton blocking layer. Quinoxaline-based ETMs
of Tm3PyQ, Tm4PyQ, 3PyDQB and 4PyDQB were used as the ETLs
for device A1, A2, B1, and B2, respectively. For reference, device B3
with the commonly used ETLs of TmPyPB was fabricated. The
molecular structures and energy levels of the used materials are
shown in Fig. 8 [68].
10k
1k
100
10
1
The current density-voltage-brightness (I-V-B) characteristics of
these devices are shown in Fig. 9. The power efficiency, current
efficiency and external quantum efficiency (EQE) as functions of the
brightness for all these devices are shown in Fig. 10, and the key
data of these devices are summarized in Table 2. As displayed in
Fig. 9, Devices A1 and A2 based on Tm3PyQ and Tm4PyQ exhibited
the turn-on voltages of 3.8 V and 3.6 V, respectively, which are
lower than those of devices B1 (4.5 V) and B2 (5.3 V) based on
3PyDQB and 4PyDQB. This should be attributed to the smaller
electron injection barriers of Tm3PyQ/Tm4PyQ than those of
3PyDQB/4PyDQB. However, devices A1 and A2 exhibited inferior
performance than B1 and B2 at high brightness, presumably due to
the weaker electron transport ability of Tm3PyQ/Tm4PyQ
compared to 3PyDQB/4PyDQB, which could result in imbalanced
charge-recombination. Among these devices, 3PyDQB-based de-
vice B1 achieved the best results with a hc,max of 30.2 cd Aꢁ1, a
maximum power efficiency (hp,max) of 18.3 lm Wꢁ1 and a hext,max of
14.2%, which are comparable to the TmPyPB-based device B3,
indicating the favorable electron-transporting and exciton-
blocking ability of 3PyDQB.
4
6
8
10
12
14
16
Voltage (V)
Fig. 9. (a) I-V-B characteristics of these devices.
It is worth mentioning that the efficiencies of the quinoxaline-
based devices show smaller roll-offs than the control device B3
(Fig. 10). For example, at the high brightness of 1000 cd mꢁ2, the
roll-off values of device B3 are 25% for hext, while the decays of B1
are barely 12%. Considering that most of the studies in the litera-
tures focused on the 0-150/200 mA/cm2 regions, which mean
insufficient device stability, the devices in this work are remarkably
stable since current densities as high as 500 mA/cm2 can be applied
for all PhOLEDs. Therefore, we believe that the pyridine-based
molecules incorporated with quinoxaline moieties would be a
promising way for designing new efficient ETMs.
Fig. 10. Current efficiency (a), power efficiency (b), and external quantum efficiency (c)
versus luminance of the devices.
4. Conclusion
were fabricated with the following two types of configuration: Type
A, ITO/MoO3 (10 nm)/TAPC (70 nm)/FIrpic:mCP (8 wt %, 20 nm)/
TmPyPB (5 nm)/Tm3PyQ or Tm4PyQ (45 nm)/LiF (1 nm)/Al; Type
B, ITO/MoO3 (10 nm)/TAPC (70 nm)/Firpic:mCP (8 wt %, 20 nm)/
3PyDQB, 4PyDQB or TmPyPB (45 nm)/LiF (1 nm)/Al (Fig. 7(a)),
where LiF and MoO3 were served as the electron- and hole-
injecting layers, respectively; 1,1-bis[(di-4-tolylamino)phenyl]
In conclusion, we developed four new pyridine-containing
quinoxaline derivatives as n-type charge transport materials for
blue PhOLEDs. The introduction of quinoxaline units and the ortho-
linkage effectively restricted their LUMOs' distribution on the qui-
noxaline moieties. In this way, the LUMOs of the molecules can be
manipulated, and desirable n-type charge transport materials can
be obtained. These quinoxaline derivatives show favorable electron
Table 2
Key data of the device performances.
a
e
Device
ETL
Von [V]
Lmaxb,V [cd mꢁ2], V
hc,maxc [cd Aꢁ1
]
hp,maxd [lm Wꢁ1
]
hext,max [%]
At 1000 cd mꢁ2
hc hp hext
,
,
A1
A2
B1
B2
B3
Tm3PyQ
Tm4PyQ
3PyDQB
4PyDQB
TmPyPB
3.8
3.6
4.5
5.3
3.6
26243, 13.5
25302, 12.9
29774, 14.5
28773, 16.9
27962, 14.2
25.9
22.2
30.2
25.8
33.8
20.9
18.0
18.3
8.5
9.3
8.6
14.2
11.7
15.8
21.8, 9.1, 8.6
20.8, 9.3, 8.0
27.6, 9.7, 12.6
25.3, 7.1, 11.5
26.3, 10.2, 11.9
26.3
a
Turn-on voltage at luminance of 1 cd mꢁ2
Lmax: maximum brightness.
hc,max: maximum current efficiency.
hp,max: maximum power efficiency.
.
b
c
d
e
hext,max: maximum external quantum efficiency.

X. Yin et al. / Organic Electronics 37 (2016) 439e447
447
21 (2011) 1881.
affinities and good thermal stabilities. The FIrpic-based PhOLED
employed them as ETLs achieved good performances, with a hc,max
over 30 cd Aꢁ1 and a hext,max up to 15%, which are comparable to the
TmPyPB-based control device. More importantly, the devices
embedded with quinoxaline-based ETLs revealed smaller efficiency
roll-offs than that of TmPyPB-based control devices.
[27] Y. Sun, L. Duan, P. Wei, J. Qiao, G. Dong, L. Wang, Y. Qiu, Org. Lett. 11 (2009)
2069.
[28] J. Lim, T.A. Albright, B.R. Martin, O.S. Miljanic, J. Org. Chem. 76 (2011) 10207.
[29] B.C. Tlach, A.L. Tomlinson, A.G. Ryno, D.D. Knoble, D.L. Drochner, K.J. Krager,
M. Jeffries-El, J. Org. Chem. 78 (2013) 6570.
[30] T. Earmme, E. Ahmed, S.A. Jenekhe, J. Phys. Chem. C 113 (2009) 18448.
[31] X. Yin, D. Chen, Q. Peng, Y. Xiang, G. Xie, Z. Zhu, C. Zhong, F. Li, S. Su, C. Yang,
J. Mater. Chem. C 4 (2016) 1482.
ꢁ
ꢀ
[32] T. Kamata, H. Sasabe, Y. Watanabe, D. Yokoyama, H. Katagiri, J. Kido, J. Mater.
Chem. C 4 (2016) 1104.
Acknowledgement
[33] Y. Watanabe, H. Sasabe, D. Yokoyama, T. Beppu, H. Katagiri, J. Kido, J. Mater.
Chem. C 4 (2016) 3699.
[34] Y. Watanabe, H. Sasabe, D. Yokoyama, T. Beppu, H. Katagiri, Y.-J. Pu, J. Kido,
Adv. Opt. Mater. 3 (2015) 769.
We thank the National Natural Science Foundation of China
(Nos. 91433201), the National Basic Research Programs of China
(973 Programs: 2013CB834805 and 2015CB655002), the Innovative
Research Group of Hubei Province (No. 2015CFA014) for financial
support. We acknowledge Professor Feng Li in Jilin University for
the measurement of electron mobility.
[35] C. Wang, H. Dong, W. Hu, Y. Liu, D. Zhu, Chem. Rev. 112 (2011) 2208.
ꢀ
[36] J.L. Bredas, J.P. Calbert, D.A. da Silva Filho, J. Cornil, Proc. Natl. Acad. Sci. 99
(2002) 5804.
[37] B.C. Lin, C.P. Cheng, Z.-Q. You, C.-P. Hsu, J. Am. Chem. Soc. 127 (2005) 66.
[38] D. Yokoyama, J. Mater. Chem. 21 (2011) 19187.
[39] H. Li, L. Duan, Y. Sun, D. Zhang, L. Wang, Y. Qiu, J. Phys. Chem. C 117 (2013)
16336.
[40] M. Liu, S.-J. Su, M.-C. Jung, Y. Qi, W.-M. Zhao, J. Kido, Chem. Mater. 24 (2012)
3817.
[41] S.-J. Su, Y. Takahashi, T. Chiba, T. Takeda, J. Kido, Adv. Funct. Mater. 19 (2009)
1260.
[42] H. Ye, D. Chen, M. Liu, S.-J. Su, Y.-F. Wang, C.-C. Lo, A. Lien, J. Kido, Adv. Funct.
Mater. 24 (2014) 3268.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.orgel.2016.07.020.
References
[43] M. Ichikawa, T. Yamamoto, H.-G. Jeon, K. Kase, S. Hayashi, M. Nagaoka,
N. Yokoyama, J. Mater. Chem. 22 (2012) 6765.
[44] M. Ichikawa, K. Wakabayashi, S. Hayashi, N. Yokoyama, T. Koyama,
Y. Taniguchi, Org. Electron. 11 (2010) 1966.
ꢀ
[1] V. Coropceanu, J. Cornil, D.A. da Silva Filho, Y. Olivier, R. Silbey, J.-L. Bredas,
Chem. Rev. 107 (2007) 926.
[2] K. Walzer, B. Maennig, M. Pfeiffer, K. Leo, Chem. Rev. 107 (2007) 1233.
[3] X. Zhao, X. Zhan, Chem. Soc. Rev. 40 (2011) 3728.
[4] A.P. Kulkarni, C.J. Tonzola, A. Babel, S.A. Jenekhe, Chem. Mater. 16 (2004) 4556.
[5] H. Sasabe, J. Kido, Chem. Mater. 23 (2011) 621.
[6] G. Hughes, M.R. Bryce, J. Mater. Chem. 15 (2005) 94.
[7] J. Jia, L. Zhu, Y. Wei, Z. Wu, H. Xu, D. Ding, R. Chen, D. Ma, W. Huang, J. Mater.
Chem. C 3 (2015) 4890.
[45] S.-J. Su, T. Chiba, T. Takeda, J. Kido, Adv. Mater. 20 (2008) 2125.
[46] S.-J. Yoo, H.-J. Yun, I. Kang, K. Thangaraju, S.-K. Kwon, Y.-H. Kim, J. Mater.
Chem. C 1 (2013) 2217.
[47] A.L. Von Ruden, L. Cosimbescu, E. Polikarpov, P.K. Koech, J.S. Swensen,
L. Wang, J.T. Darsell, A.B. Padmaperuma, Chem. Mater. 22 (2010) 5678.
[48] H. Tsuji, Y. Yokoi, Y. Sato, H. Tanaka, E. Nakamura, Chem. Asian J. 6 (2011)
2005.
[8] L. Xiao, B. Qi, X. Xing, L. Zheng, S. Kong, Z. Chen, B. Qu, L. Zhang, Z. Ji, Q. Gong,
J. Mater. Chem. 21 (2011) 19058.
[49] T.H. Huang, W.T. Whang, J.Y. Shen, Y.S. Wen, J.T. Lin, T.H. Ke, L.Y. Chen,
C.C. Wu, Adv. Funct. Mater. 16 (2006) 1449.
[9] K. Togashi, S. Nomura, N. Yokoyama, T. Yasuda, C. Adachi, J. Mater. Chem. 22
(2012) 20689.
[10] W. Jiang, H. Xu, X. Ban, G. Yuan, Y. Sun, B. Huang, L. Duan, Y. Qiu, Org. Lett. 16
(2014) 1140.
[11] K.S. Yook, J.Y. Lee, Adv. Mater. 24 (2012) 3169.
[12] C. Duan, C. Fan, Y. Wei, F. Han, W. Huang, H. Xu, Sci. Rep. 6 (2016) 19904.
[13] N. Chopra, J. Lee, Y. Zheng, S.-H. Eom, J. Xue, F. So, ACS Appl. Mater. Inter. 1
(2009) 1169.
[14] H. Tsuji, C. Mitsui, Y. Sato, E. Nakamura, Adv. Mater. 21 (2009) 3776.
[15] C. Mitsui, H. Tsuji, Y. Sato, E. Nakamura, Chem. Asian J. 7 (2012) 1443.
[16] W. Kan, L. Zhu, Y. Wei, D. Ma, M. Sun, Z. Wu, W. Huang, H. Xu, J. Mater. Chem.
C 3 (2015) 5430.
[50] T. Earmme, E. Ahmed, S.A. Jenekhe, Adv. Mater. 22 (2010) 4744.
[51] E. Ahmed, T. Earmme, S.A. Jenekhe, Adv. Funct. Mater. 21 (2011) 3889.
[52] C. Schmitz, P. Posch, M. Thelakkat, H.W. Schmidt, A. Montali, K. Feldman,
P. Smith, C. Weder, Adv. Funct. Mater. 11 (2001) 41.
[53] J.-F. Wang, Y. Kawabe, S.E. Shaheen, M.M. Morrell, G.E. Jabbour, P.A. Lee,
J. Anderson, N.R. Armstrong, B. Kippelen, E.A. Mash, N. Peyghambarian, Adv.
Mater. 10 (1998) 230.
[54] M. Jandke, P. Strohriegl, S. Berleb, E. Werner, W. Brütting, Macromolecules 31
(1998) 6434.
[55] F. Romanov-Michailidis, C. Besnard, A. Alexakis, Org. Lett. 14 (2012) 4906.
[56] J. Shao, J. Chang, C. Chi, Org. Biomol. Chem. 10 (2012) 7045.
[57] S.-J. Su, D. Tanaka, Y.-J. Li, H. Sasabe, T. Takeda, J. Kido, Org. Lett. 10 (2008) 941.
[58] M.R. Talipov, A. Boddeda, Q.K. Timerghazin, R. Rathore, J. Phys. Chem. C 118
(2014) 21400.
€
[17] X. Yin, T. Zhang, Q. Peng, T. Zhou, W. Zeng, Z. Zhu, G. Xie, F. Li, D. Ma, C. Yang,
J. Mater. Chem. C 3 (2015) 7589.
[18] C. Li, L. Duan, Y. Sun, H. Li, Y. Qiu, J. Phys. Chem. C 116 (2012) 19748.
[19] R.J. Holmes, B.W. D'Andrade, S.R. Forrest, X. Ren, J. Li, M.E. Thompson, Appl.
Phys. Lett. 83 (2003) 3818.
[59] G.W.T.M.J. Frisch, H.B. Schlegel, G.E. Scuseria, et al., Gaussian 09, Revision A.02,
Gaussian, W. Inc, CT, 2009.
[60] A.D. Becke, Phys. Rev. A 38 (1988) 3098.
[20] H.J. Park, O.Y. Kim, J.Y. Lee, Synth. Met. 176 (2013) 47.
[21] X. Yin, J. Miao, Y. Xiang, H. Wu, Y. Cao, C. Yang, Macromol. Rapid Commun. 36
(2015) 1658.
[22] S.-J. Yoo, H.-J. Yun, I. Kang, K. Thangaraju, S.-K. Kwon, Y.-H. Kim, J. Mater.
Chem. C 1 (2013) 2217.
[61] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.
[62] A.D. Boese, N.C. Handy, J. Chem. Phys. 116 (2002) 9559.
[63] G.E. Morse, T.P. Bender, ACS Appl. Mater Interfaces 4 (2012) 5055.
[64] J. Zhang, D. Ding, Y. Wei, F. Han, H. Xu, W. Huang, Adv. Mater. 28 (2016) 479.
ꢀ
[65] J.-L. Bredas, D. Beljonne, V. Coropceanu, J. Cornil, Chem. Rev. 104 (2004) 4971.
[23] S. Kwon, K.-R. Wee, A.-L. Kim, S.O. Kang, J. Phys. Chem. Lett. 1 (2010) 295.
[24] J. Liu, H. Zhang, H. Dong, L. Meng, L. Jiang, L. Jiang, Y. Wang, J. Yu, Y. Sun,
W. Hu, A.J. Heeger, Nat. Commun. 6 (2015) 10032.
[66] L. Xin, C. Li, F. Li, S. Liu, B. Hu, Appl. Phys. Lett. 95 (2009) 123306.
[67] S. Barth, P. Müller, H. Riel, P.F. Seidler, W. Rieß, H. Vestweber, H. Bassler,
€
J. Appl. Phys. 89 (2001) 3711.
[25] C. Fan, C. Duan, Y. Wei, D. Ding, H. Xu, W. Huang, Chem. Mater. 27 (2015)
5131.
[68] L. Xiao, Z. Chen, B. Qu, J. Luo, S. Kong, Q. Gong, J. Kido, Adv. Mater. 23 (2011)
926.
[26] Y. Sun, L. Duan, D. Zhang, J. Qiao, G. Dong, L. Wang, Y. Qiu, Adv. Funct. Mater.