Bipolar 9-linked carbazole-π-dimesitylborane fluorophores for nondoped blue OLEDs and red phosphorescent OLEDs

Article abstract of DOI:10.1016/j.dyepig.2018.04.035

Three dimesitylborane-containing fluorophores with various π-conjugated systems attached at the 9th position of carbazole, namely, 9-(4′-bromobiphenyl-4-yl)-9H-carbazole (Cz⁹Ph₂B), 9-(4-(5-(dimesitylboryl)thiophen-2-yl)phenyl)-9H-carbazole (Cz⁹ThPhB), and 9-(4-(4-(dimesitylboryl)styryl)phenyl)-9H-carbazole (Cz⁹SB) were synthesized and their photophysical and electroluminescent properties were investigated for application in nondoped blue OLEDs as well as red phosphorescent OLEDs (PhOLEDs). The electron-accepting dimesitylboryl group and various π-conjugated segments appended to the electron-donating carbazole moiety impart the three fluorophores with bipolar transporting ability, and their energy levels are matched with those of the adjacent carrier-transporting layers. These bulky fluorophores are thermally stable with glass transition temperatures and degradation temperatures reaching up to 105 and 383 °C, respectively. In addition, efficient nondoped Cz⁹PhThB- and Cz⁹SB-based blue OLEDs with maximum currents of 1.51 and 4.03 cd A^?1 and external quantum efficiencies (EQE) of 2.30 and 4.72% were achieved, respectively. Notably, the Cz⁹Ph₂B-based red PhOLEDs exhibits a relatively low turn-on voltage (3.3 V) and high electroluminescence efficiencies (maximum current = 23.12 cd A^?1 and EQE = 14.10%). Their performance is superior to that of the corresponding device using conventional 4,4′-N,N′-dicarbazolbiphenyl as the host material. Moreover, a maximum brightness of 39700 cd m^?2 was also achieved.

Full text of DOI:10.1016/j.dyepig.2018.04.035

DyesandPigments157(2018)101–108
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Bipolar 9-linked carbazole-π-dimesitylborane fluorophores for nondoped
blue OLEDs and red phosphorescent OLEDs
Chi-Kan Liua, Ying-Hsiao Chenb, Yi-Jun Longa, Pai-Tao Saha, Wei-Che Changa, Li-Hsin Chana,∗
Jhao-Lin Wuc, Ru-Jong Jengd,e,∗∗, Shih-Chieh Yehc,d, Chin-Ti Chenc,∗∗∗
,
a
Department of Applied Materials and Optoelectronic Engineering, National Chi Nan University, Nantou 54561, Taiwan, ROC
Plastic Industry Development Center, Taichung 40768, Taiwan, ROC
Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan, ROC
Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan, ROC
b
c
d
e
Advanced Research Center of Green Materials Science and Technology, National Taiwan University, Taipei 10617, Taiwan, ROC
A R T I C L E I N F O
A B S T R A C T
Keywords:
Organic light-emitting diodes
Blue light
Red light
Bipolar host material
Carbazole derivatives
Three dimesitylborane-containing fluorophores with various π-conjugated systems attached at the 9th position
of carbazole, namely, 9-(4′-bromobiphenyl-4-yl)-9H-carbazole (Cz9Ph2B), 9-(4-(5-(dimesitylboryl)thiophen-2-yl)
phenyl)-9H-carbazole (Cz9ThPhB), and 9-(4-(4-(dimesitylboryl)styryl)phenyl)-9H-carbazole (Cz9SB) were syn-
thesized and their photophysical and electroluminescent properties were investigated for application in non-
doped blue OLEDs as well as red phosphorescent OLEDs (PhOLEDs). The electron-accepting dimesitylboryl
group and various π-conjugated segments appended to the electron-donating carbazole moiety impart the three
fluorophores with bipolar transporting ability, and their energy levels are matched with those of the adjacent
carrier-transporting layers. These bulky fluorophores are thermally stable with glass transition temperatures and
degradation temperatures reaching up to 105 and 383 °C, respectively. In addition, efficient nondoped
Cz9PhThB- and Cz9SB-based blue OLEDs with maximum currents of 1.51 and 4.03 cd A−1 and external quantum
efficiencies (EQE) of 2.30 and 4.72% were achieved, respectively. Notably, the Cz9Ph2B-based red PhOLEDs
exhibits
a relatively low turn-on voltage (3.3 V) and high electroluminescence efficiencies (maximum
current = 23.12 cd A−1 and EQE = 14.10%). Their performance is superior to that of the corresponding device
using conventional 4,4′-N,N′-dicarbazolbiphenyl as the host material. Moreover, a maximum brightness of
39700 cd m−2 was also achieved.
1. Introduction
[6–8]. Different classes of core structures have been studied and used as
the skeletal backbone for constructing emitters for OLEDs. Among these
Organic light-emitting diodes (OLEDs) have attracted great atten-
tion owing to their application in full-color flat panel displays and solid-
state lighting [1–3]. Since Tang and Van Slyke reported the first mul-
tilayered OLEDs, much research has been directed to improve the OLED
performance in terms of their efficiency, color stability, and lifetime
[4,5]. To meet the demand of commercial application, three primary
colors, blue, green, and red are basically required. During the past few
decades, green and red electroluminescence (EL) devices have been
improved significantly; however, for full-color application, there is an
ongoing search for efficient materials (including host or dopant mate-
rials) for developing EL devices based on all three primary colors with
appropriate Commission Internationale de ľEclairage (CIE) coordinates
materials, carbazole-based compounds are one of the promising can-
didates, owing to their large triplet energy, good hole-transporting
ability, and relatively weak π-donor ability [9–13]. The large triplet
energy is useful for their application as host materials in phosphor-
escent organic light-emitting diodes (PhOLEDs). A weak π-donating
property is particularly useful in generating short-wavelength emis-
sions, which is advantageous for designing blue light-emitting mate-
rials. Moreover, to achieve bipolarity, various electron-accepting moi-
eties such as pyridine, triazole, trazine, phenanthroline, oxadiazole,
benzimidazole, phosphine oxide, and phosphine sulfide, are included in
the molecular designs based on carbazole [14–26]. Another class of
electron-acceptors, boron-containing derivatives, has attracted
∗
Corresponding author. Department of Applied Materials and Optoelectronic Engineering, National Chi Nan University, Nantou 54561, Taiwan, ROC.
Corresponding author.
Corresponding author.
∗∗
∗∗∗
E-mail addresses: lhchan@ncnu.edu.tw (L.-H. Chan), rujong@ntu.edu.tw (R.-J. Jeng), chintchen@gate.sinica.edu.tw (C.-T. Chen).
https://doi.org/10.1016/j.dyepig.2018.04.035
Received 25 October 2017; Received in revised form 17 April 2018; Accepted 19 April 2018
Availableonline25April2018
0143-7208/©2018ElsevierLtd.Allrightsreserved.

C.-K. Liu et al.
DyesandPigments157(2018)101–108
significant attention owing to the vacant p-orbital of the boron atom.
These derivatives possess electron-transporting capability and have
been demonstrated to be useful for various applications such as non-
linear optics, anion sensing, and luminescent devices [11,27–31]. Using
a steric protection method in which the boron center is appended with
bulky substituents, compounds that are claimed to be stable under air
and moisture were prepared [27]. A representative example of the
steric protection strategy is the use of dimesitylboryl group, wherein the
boron atom is protected effectively by two ortho-methyl substituted
mesityl groups. As such, this moiety has been extensively employed as
an electron acceptor for constructing air-stable organoboron materials.
Considering that the electron-accepting ability of the boryl group plays
an important role in determining the overall performance of the ma-
terial, incorporation of this moiety is expected to lead to further im-
provements in OLEDs. Moreover, with the nonplanar structure of the
dimesitylboryl moiety in the molecular framework, the π-π stacking
between the molecules can be diminished and thus the fluorescence
quantum yield is increased. According to these considerations, three
dimesitylborane-containing fluorophores with various π-conjugated
systems attached at the 9th position of carbazole, viz., 9-(4′-bromobi-
phenyl-4-yl)-9H-carbazole (Cz9Ph2B), 9-(4-(5-(dimesitylboryl)thio-
phen-2-yl)phenyl)-9H-carbazole (Cz9ThPhB), and 9-(4-(4-(dimesi-
tylboryl)styryl)phenyl)-9H-carbazole (Cz9SB), were synthesized and
their photophysical and electroluminescent properties were in-
vestigated toward application in nondoped blue OLEDs. Further, be-
cause of their large triplet energies, these compounds are suitable host
materials for PhOLED application; therefore, red PhOLEDs were fabri-
cated and their EL properties were also examined.
Cz9PhBr using 9H-carbazole (5.02 g, 30 mmol), 4,4′-dibromobiphenyl
(18.63 g, 60 mmol), K2CO3 (16.54 g, 120 mmol), Cu powder (1.92 g,
30 mmol), 18-crown-6 (3.73 g, 15 mmol), and anhydrous o-DCB
(150 mL), affording Cz9Ph2Br as a white solid (5.36 g, 45%). 1H NMR
(300 MHz, CDCl3, δ/ppm): 8.16 (d, J = 7.5 Hz, 2H), 7.78 (d,
J = 8.4 Hz, 2H), 7.62–7.66 (m, 4H), 7.31 (t, J = 6.6 Hz, 2H). 13C NMR
(75 MHz, CDCl3, δ/ppm): 140.77, 139.17, 139.01, 137.26, 132.07,
128.69, 128.31, 127.43, 125.99, 123.47, 121.96, 120.34, 120.06,
109.76. HRMS (m/z): calcd for C24H16BrN: 397.0466. Found: 397.0464.
2.4. 4-(9H-carbazol-9-yl)benzaldehyde (Cz9PhCHO)
In a 250 mL two-necked flask, anhydrous DMF (100 mL) was added
to a mixture of 9H-carbazole (8.40 g, 50 mmol) and potassium tert-
butoxide (5.52 g, 50 mmol). The mixture was stirred at 110 °C for
30 min under nitrogen atmosphere. Then, 4-fluorobenzaldehyde
(1.24 g, 10 mmol) was added and stirred for 36 h. Then, the reaction
mixture was cooled to room temperature and the reaction solvent was
removed by distillation under reduced pressure. The residue was ex-
tracted with ethyl acetate and water. Further purification was carried
out by silica gel chromatography to afford Cz9PhCHO as a light yellow
solid (9.30 g, 68%). 1H NMR (300 MHz, CDCl3, δ/ppm): 10.11 (s, 1H),
8.12–8.16 (m, 4H), 7.79 (d, J = 8.4Hz, 2H), 7.51 (d, J = 7.5 Hz, 2H),
7.44 (t, J = 7.2 Hz, 2H), 7.33 (t, J = 7.8 Hz, 2H); 13C NMR (75 MHz,
CDCl3, δ/ppm): 191.03, 143.39, 140.07, 134.64, 131.42, 126.82,
126.34, 124.00, 120.88, 120.56, 109.81. HRMS (m/z): calcd for
C19H13NO: 271.0997. Found: 271.0996.
2.5. 9-(4-(thiophen-2-yl)phenyl)-9H-carbazole (Cz9PhTh)
2. Material and method
A mixture of Cz9PhBr (3.22 g, 10 mmol), 2-(tri-butylstannyl)thio-
phene (3.74 g, 10 mmol), and PdCl2(PPh3)2 (0.35 g, 0.5 mmol) was
dissolved in anhydrous toluene (100 mL) and the mixture was refluxed
for 48 h. After cooling, the reaction mixture was poured into ice water
and then extracted with dichloromethane. The combined organic ex-
tracts were dried over magnesium sulfate and the organic solvent was
removed by distillation under reduced pressure. Finally, the residue
was purified by column chromatography on silica gel to afford Cz9PhTh
as a white solid (2.34 g, 72%). 1H NMR (300 MHz, CDCl3, δ/ppm): 8.14
(d, J = 7.8 Hz, 2H), 7.70 (d, J = 8.7 Hz, 2H), 7.56 (d, J = 8.4 Hz, 2H),
7.39–7.44 (m, 5H), 7.28–7.35 (m, 3H), 7.12 (t, J = 3.9 Hz, 1H); 13C
NMR (75 MHz, CDCl3) δ143.44, 140.78, 136.82, 133.56, 128.29,
127.47, 127.28, 126.03, 125.38, 123.63, 123.46, 120.38, 120.07,
109.83. HRMS (m/z): calcd for C22H15NS:325.0925. Found: 325.0928.
2.1. Materials
All chemicals were obtained from Aldrich, Alfa Aesar, and TCI
Chemical Co., and were used as received unless otherwise noted. All the
solvents, including o-dichlorobenzene (o-DCB), tetrahydrofuran (THF),
N,N-dimethylformamide (DMF), and toluene were freshly distilled and
dried over appropriate drying agents before use. The hole-transporting
material, di-[4-(N,N-ditolyl-amino)phenyl]cyclohexane (TAPC), and
electron-transporting material, 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene
(TmPyPB), were purchased from Lumtec Corp., and used without pur-
ification.
2.2. 9-(4-bromophenyl)-9H-carbazole (Cz9PhBr)
The Cz9PhBr was synthesized according to a previously published
procedure [32,33]. Briefly, under nitrogen atmosphere, 9H-carbazole
(5.02 g, 30 mmol), 1,4-dibromobenzene (14.14 g, 60 mmol), K2CO3
(16.54 g, 120 mmol), Cu powder (1.92 g, 30 mmol), 18-crown-6 (3.73 g,
15 mmol), and anhydrous o-DCB (150 mL) were taken in a 500 mL two-
neck flask. The mixture was stirred and then heated to reflux for 12 h.
After cooling, the resulting mixture was filtered and the collected or-
ganic solvent was removed by distillation under reduced pressure. The
residue was extracted with ethyl acetate and water. The organic layer
was collected and dried over magnesium sulfate. The product was
purified further by silica gel chromatography using hexanes as the
eluent, to obtain Cz9PhBr as a white solid (7.02 g, 73%). 1H NMR
(300 MHz, CDCl3, δ/ppm): 8.15 (d, J = 7.5 Hz, 2H), 7.73 (d,
J = 8.4 Hz, 2H), 7.36–7.47 (m, 6H), 7.30 (t, J = 7.2 Hz, 2H); 13C NMR
(75 MHz, CDCl3, δ/ppm): 140.66, 136.86, 133.16, 128.76, 126.15,
123.57, 120.93, 120.47, 120.30, 109.61. HRMS (m/z): calcd for
2.6. 9-(4-(4-bromostyryl)phenyl)-9H-carbazole (Cz9SBr)
Previously reported procedures were employed for the synthesis of
Cz9SBr [9]. Under nitrogen atmosphere and at −20 °C, a solution of 4-
bromobenzyltriphenylphosphonium bromide (3.07 g, 10 mmol) in an-
hydrous THF (20 mL) was added in portions over a period of 10 min to a
mixture of sodium hydride (0.36 g, 15 mmol) in anhydrous THF
(80 mL). After 10 min, a mixture of Cz9PhCHO (2.71 g, 10 mmol) and
dry THF (50 mL) was added to the reaction mixture and stirred for
another 8 h at room temperature under nitrogen atmosphere. The re-
sulting product was poured into water, neutralized with dilute hydro-
chloric acid, and extracted with dichloromethane. Then, the obtained
organic layer was dried over magnesium sulfate. After the organic
solvent was removed by distillation under reduced pressure, the residue
was purified by column chromatography on silica gel to yield Cz9SBr as
a green-yellow solid (2.92 g, 70%). 1H NMR (400 MHz, CDCl3, δ/ppm):
8.12 (d, J = 7.5 Hz, 2H), 7.69 (d, J = 9.0 Hz, 2H), 7.54 (d, J = 8.4 Hz,
2H), 7.49 (d, J = 8.4 Hz, 2H), 7.37–7.45 (m, 6H), 7.25 (t, J = 8.1 Hz,
2H), 7.16 (d, J = 16.5 Hz, 1H), 7.07 (d, J = 16.5 Hz, 1H); 13C NMR
(100 MHz, CDCl3) δ 140.74, 137.14, 136.10, 136.06, 131.90, 128.39,
128.27, 128.08, 127.88, 127.25, 125.99, 123.46, 121.65, 120.35,
C
18H12BrN:321.0153. Found: 321.0162.
2.3. 9-(4′-bromobiphenyl-4-yl)-9H-carbazole (Cz9Ph2Br)
Cz9Ph2Br was synthesized using the same procedure described for
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C.-K. Liu et al.
DyesandPigments157(2018)101–108
120.05, 109.81. HRMS (m/z): calcd for C26H18BrN: 423.0623. Found:
423.0624.
under a nitrogen flow. Thermogravimetric analysis (TGA) was per-
formed on a Perkin-Elmer TGA-7 analyzer at a scanning rate of 10 °C
min−1 in the temperature range of 30–800 °C under nitrogen atmo-
sphere. UV–visible absorption spectra were recorded on Hewlett-
Packard 8453 spectrometer and fluorescence and EL spectra were re-
corded on a Hitachi F-7000 spectrophotometer. Energy levels were
determined by low energy photo-electron spectrometry using a Riken-
Keiki AC-2 Photoelectron Spectrometer. The sample was prepared by
vacuum deposition on clean indium tin oxide (ITO)-coated glass sub-
strates. The photoluminescence quantum yield (Φf) was determined
from the corrected fluorescence spectrum of the individual substances
in diluted deoxygenated 10−6 M ethyl acetate solution using the stan-
dard, coumarin 1 according a process reported previously [34]. In ad-
dition, Φf of the material in the solid film state was determined by the
integrating-sphere method described by Mello et al. [11,35] All the EL
devices were fabricated as follows: Glass substrates (with a sheet re-
sistance of 15 Ω sq−1) coated with ITO were patterned lithographically,
then sequentially cleansed with a detergent, ultrasonicated in acetone
and isopropyl alcohol, and dried on a hot plate at 120 °C for 5 min, and
finally treated with oxygen plasma for 5 min. For the EL devices, 4,4′-
bis[N-(1-naphthyl)eN-phenylamino] biphenyl (NPB) or di-[4-(N,N-di-
tolyl-amino)phenyl]cyclohexane (TAPC) was used as the hole trans-
porting material, Cz9Ph2B, Cz9ThPhB, or Cz9SB was used as the emitter
for nondoped blue OLEDs whereas Cz9Ph2B was used as the host ma-
terial for red PhOLEDs, 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene
(TmPyPB) or 1,3,5-tris(N-phenyl benzimidizol-2-yl)benzene (TPBI) was
the electron-transporting layer, dimethyl-4,7-diphenyl-1,10-phenan-
throline (BCP) was used as the hole-blocking material, and lithium
fluoride (LiF) and Al electrode were used as the cathode. All the ma-
terials were deposited sequentially on top of the ITO glass substrate
through a shadow mask, without breaking the vacuum. The thermal
evaporation of the organic materials was carried out using ULVAC
Cryogenics at a chamber pressure of 10−6 Torr. Current–voltage–lu-
minescence (IeV–L) characteristics and CIE color coordinates of the
devices were measured simultaneously using a Keithley 2400 source
meter and a Newport 1835C optical meter equipped with a Newport
818-ST silicon photodiode, respectively. All electrical measurements
were made in air.
2.7. 9-(4′-dimesitylboryl)biphenyl-4-yl)-9H-carbazole (Cz9Ph2B)
A solution of 2.5 M n-butyllithium (10 mmol, 4 mL) in n-hexane was
added to a solution of Cz9Ph2Br (3.98 g, 10 mmol) in anhydrous THF
(100 mL) at −78 °C. The mixture was stirred for 1 h and dimesitylboron
fluoride (2.68 g, 10 mmol) was slowly added. Stirring was continued for
1 h at −78 °C and for further 12 h at room temperature. The reaction
mixture was quenched with water and then extracted with di-
chloromethane. The organic layer was dried over magnesium sulfate,
filtered, and evaporated to dryness under reduced pressure. The crude
product was purified by column chromatography to obtain Cz9Ph2B as a
white solid (3.01 g, 53%). 1H NMR (300 MHz, CDCl3, δ/ppm) δ 8.16 (d,
J = 7.8 Hz, 2H), 7.89 (d, J = 8.1 Hz, 2H), 7.64–7.70 (m, 5H), 7.40–7.49
(m, 5H), 7.30 (t, J = 6.6 Hz, 2H), 6.86 (s, 4H), 2.33 (s, 6H), 2.06 (s,
12H); 13C NMR (75 MHz, CDCl3, δ/ppm) δ 145.05, 143.42, 141.86,
140.99, 139.85, 138.87, 137.45, 137.33, 128.78, 128.37, 127.46,
126.68, 126.13, 123.60, 120.49, 120.17, 109.96, 23.50, 21.23. HRMS
(m/z): calcd for C42H38BN: 567.3097. Found: 567.3101. Anal. Calcd. for
C42H38BN (%): C, 88.88; H, 6.75; N, 2.47; found: C, 88.98; H, 6.79; N,
2.48.
2.8. 9-(4-(5-(dimesitylboryl)thiophen-2-yl)phenyl)-9H-carbazole
(Cz9PhThB)
Cz9PhThB was synthesized according to a method reported pre-
viously [32], using Cz9PhTh (3.25 g, 10 mmol), 2.5 M n-butyllithium
(10 mmol, 4 mL) in n-hexane, dimesitylboron fluoride (2.68 g,
10 mmol), and anhydrous THF (100 mL), to yield Cz9PhThB as a white
solid (3.44 g, 60%). 1H NMR (300 MHz, CDCl3, δ/ppm): 8.15 (d,
J = 7.8 Hz, 2H), 7.90 (d, J = 8.4 Hz, 2H), 7.54–7.60 (m, 3H), 7.38–7.51
(m, 5H), 7.28–7.35 (m, 2H), 6.86 (s, 4H), 2.33 (s, 6H), 2.19 (s, 12H).
13C NMR (75 MHz, CDCl3, δ/ppm): 155.89, 150.14, 141.88, 141.28,
141.02, 140.78, 138.78, 137.82, 133.25, 128.37, 127.70, 127.49,
126.18, 125.92, 123.65, 120.52, 120.23, 109.90, 23.65, 21.41. HRMS
(m/z): calcd for C40H36BNS: 573.2662. Found: 573.2668. Anal. Calcd.
for C40H36BNS (%): C, 83.76; H, 6.33; N, 2.44; found: C, 83.52; H, 6.40;
N, 2.44.
3. Results and discussion
2.9. 9-(4-(4-(dimesitylboryl)styryl)phenyl)-9H-carbazole (Cz9SB)
3.1. Synthesis and characterization
Cz9SB was prepared using the same procedure described for
Cz9Ph2B using Cz9SBr (4.24 g, 10 mmol), 2.5 M n-butyllithium
(10 mmol, 4 mL) in n-hexane, dimesitylboron fluoride (2.68 g,
10 mmol), and anhydrous THF (100 mL), to obtain a white solid (3.50 g,
59%). 1H NMR (300 MHz, CDCl3, δ/ppm): 8.16 (d, J = 7.8 Hz, 2H),
7.76 (d, J = 8.7 Hz, 2H), 7.50–7.62 (m, 6H), 7.38–7.49 (m, 4H),
7.19–7.35 (m, 4H), 6.85 (s, 4H), 2.33 (s, 6H), 2.05 (s, 12H); 13C NMR
(75 MHz, CDCl3, δ/ppm): 145.64, 141.87, 140.98, 140.86, 140.57,
138.79, 137.23, 136.40, 129.55, 129.40, 128.33, 128.15, 127.33,
126.30, 126.12, 123.59, 120.48, 120.18, 109.98, 23.63, 21.40. HRMS
(m/z): calcd for C44H40BN: 593.3254. Found: 593.3260. Anal. Calcd. for
The synthetic routes of the three designed fluorophores, viz.,
Cz9Ph2B, Cz9PhThB, and Cz9SB, are illustrated in Scheme 1. The
synthesis of the intermediate, Cz9PhTh was carried out as described
elsewhere, whereas the intermediate, Cz9Ph2Br was prepared using 9H-
carbazole and 4,4′-dibromobiphenyl through Ullmann coupling reac-
tion. Another intermediate, Cz9SBr, was obtained by reacting
Cz9PhCHO with 4-bromobenzyltriphenylphosphonium bromide via the
Wittig reaction. The three target fluorophores were synthesized by
treating Cz9PhTh, Cz9Ph2Br, and Cz9SBr with n-butyllithium and di-
mesitylboron fluoride, following a same synthetic procedure. The che-
mical structures of these fluorophores (Cz9Ph2B, Cz9PhThB, and Cz9SB)
were confirmed by 1H NMR, 13C NMR, and molecular mass data (Figure
S1‒S9). These fluorophores showed good solubility in common organic
solvents such as ethyl acetate, THF, chloroform, and toluene. This high
solubility is possibly owing to the slightly twisted conformation due to
the introduction of nonplanar dimesitylboryl group, which increases
the steric hindrance of the molecule. Fig. 1 shows the thermal proper-
ties of these fluorophores, determined by DSC and TGA measurements.
The glass transition temperatures (Tg) together with the thermal de-
composition temperatures (Td) are summarized in Table 1. As shown in
Fig. 1(a), the DSC curves displayed no indication of crystallizing or
undergoing melting phase transition up to 250 °C, only a weak en-
dothermic transition was observed, corresponding to the glass
C
44H40BN (%): C, 89.03; H, 6.79; N, 2.36; found: C, 88.99; H, 6.76; N,
2.41.
2.10. Instrumentation
General. 1H NMR spectra were recorded on a Varian Unity Inova
300WB NMR spectrometer at room temperature. Elemental analyses
were performed on an Elementar Vario EL III elemental analyzer. High-
resolution mass spectrometry (HRMS) was performed with a Finnigan/
Thermo Quest MAT 95XL mass spectrometer. Differential scanning
calorimetry (DSC) was carried out using a SEIKO SII Model SSC5200 at
a scanning rate of 10 °C min−1 in the temperature range of 30–250 °C
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C.-K. Liu et al.
DyesandPigments157(2018)101–108
c
S
S
e
N
Br
BMes2
N
N
a
Cz9PhTh
Cz9PhThB
Cz9PhBr
H
N
e
a
B
N
Br
N
BMes2
BMes2 =
Cz9Ph2Br
Cz9Ph2B
b
d
BMes2
Br
e
N
CHO
N
N
Cz9PhCHO
Cz9SBr
Cz9SB
Scheme 1. Synthetic scheme of Cz9Ph2B, Cz9PhThB, and Cz9SB. Reaction condition: (a) 1,4-Dibromo-aryl, Cu, K2CO3, 18-crown-6-ether, o-dichlorobenzene, reflux,
12 h. (b) 4-Fluorobenzaldehyde, K2CO3, DMF, 110 °C, 36 h. (c) 2-(Tributylstannyl)thiophene, PdCl2(PPh3)2, toluene, reflux, 48 h. (d) 4-
Bromobenzyltriphenylphosphonium bromide, NaH, THF, 0 °C, 8 h. (e) n-BuLi, dimesitylboron fluoride, THF, −78 °C.
Table 1
Thermal, photophysical and electrochemical properties of fluorophores.
Egopt
(V)
PL
abs
Fluorophore Tg
Td
HOMO/LUMO Φf
λ
λ
max
max
(°C) (°C)
(eV)
(%)
(nm)
a/b
(nm)
a/b
d
b/e
c
Cz9Ph2B
Cz9PhThB
Cz9SB
105 364 340/348 438/414 3.03 −6.01/–2.98
103 383 371/387 451/439 2.83 −5.92/–3.09
38/91
45/100
33/64
98
341 367/378 465/453 2.79 −5.91/–3.12
a. Measured in THF solution.
b. In thin film state.
c. Optical band gap was estimated from the wavelength of the optical edge of
the thin film.
d. The HOMO energy levels determined by low-energy photoelectron spectro-
metry. The LUMO energy levels were calculated according to: LUMO = Egopt
HOMO (eV).
+
e. Relative to 10−6 M ethyl acetate solution of Coumarin 1.
transition phase. This implies that these fluorophores are amorphous in
nature. The amorphous characteristic of the materials is ascribed to the
twisted conformation of the molecules due to the nonplanar dimesi-
tylboryl group. The Tg values corresponding to Cz9Ph2B, Cz9PhThB, and
Cz9SB are 105, 103, and 98 °C, respectively. Cz9Ph2B showed the
highest Tg among the materials, which is attributed to the rigid di-
phenyl ring in the bridge. The thermal stability of these fluorophores is
indicated by their high Td, as depicted in Fig. 1(b); the Td values
(corresponding to 5% weight loss) of these fluorophores are approxi-
mately 364, 383, and 341 °C corresponding to Cz9Ph2B, Cz9PhThB, and
Cz9SB, respectively. All the fluorophores clearly showed favorable
thermal stability up to 340 °C, which is suffice for their application in
OLEDs.
3.2. Photophysical properties
Fig. 2 displays the absorption spectra of these fluorophores in THF
solutions and in thin films. The corresponding absorption data of these
fluorophores are summarized in Table 1. In solution, all the fluor-
ophores exhibited two absorption peaks; the absorption band at the
shorter wavelength originated from the π-π∗ transition of the fluor-
ophore backbone whereas the absorption band at the longer wave-
length is assigned to the intramolecular charge transfer from the car-
bazole moiety acting as the donor to the dimesitylboryl group acceptor
Fig. 1. (a) DSC thermograms and (b) TGA curves of Cz9-based fluorophores.
104

C.-K. Liu et al.
DyesandPigments157(2018)101–108
the fluorophores exhibited high Φf in the thin film state, which is fa-
vorable for OLED application. The HOMO energy levels of these
fluorophores were determined by low-energy photoelectron spectro-
metry. The measured HOMO levels for Cz9Ph2B, Cz9PhThB, and Cz9SB
are −6.01, −5.92, and −5.91, respectively. Similar to the absorption
properties, the HOMO levels are affected by the electron-donating
ability and conjugation length of the fluorophore; therefore, Cz9Ph2B
showed the lowest HOMO level amongst the three fluorophores. The
LUMO energy levels of these fluorophores were estimated from their
HOMO levels and the energy bandgaps that were determined from the
UV–vis absorption threshold. The LUMO levels of Cz9Ph2B, Cz9PhThB,
and Cz9SB are −2.98, −3.09, and −3.12, respectively. Apparently,
these fluorophores exhibit wide bandgaps and this is suitable for their
use as blue emitters or as host materials in PhOLED applications. To
provide a better insight into the geometries and electron-state-density
distributions of these fluorophores, DFT calculation at B3LYP/6-31G(d)
level was performed, as provided in Figure S10‒S12. The HOMO state
density of the three fluorophores mainly distributes on the carbazole
and π‒bridge moieties, spreading slightly on the boron atom. On the
other hand, the LUMO state electron density mainly locates on dime-
sitylboryl and π‒bridge groups with a minor contribution from the
carbazole unit. Obviously, the HOMO and LUMO of the three fluor-
ophores are well overlapping, leading to the ambipolar property and
efficient fluorescence of these fluorophores [37,38]. The calculated
HOMO levels were −5.35, −5.33, −5.28 eV, while the LUMO levels
were −1.85, −2.02, −2.08 eV for Cz9Ph2B, Cz9PhThB, and Cz9SB,
respectively. The predicted HOMO and LUMO energies show good
agreement with the experimental results although there are some dis-
crepancies existing between the calculations and experiments. The di-
hedral angles between the carbazole plane and π‒bridge moiety were
predicted to be 56°, 54° and 53° for Cz9Ph2B, Cz9PhThB, and Cz9SB,
respectively, while the dihedral angles between π‒bridge moiety and
dimesitylboryl group were 25°, 16°, and 22° for Cz9Ph2B, Cz9PhThB,
and Cz9SB, respectively. The twisted conformation of the molecules
implied the amorphous characteristics and wide bandgaps of these
fluorophores. In addition, the relative low dihedral angle between
π‒bridge moiety and dimesitylboryl group of the Cz9PhThB indicated
Cz9PhThB showed stronger π‒π interactions of the molecules than the
others, and thus the Cz9PhThB exhibited the longest absorption max-
imum among the fluorophores.
Fig. 2. UV–Vis absorption spectra of Cz9-based fluorophores in THF solutions
and thin films.
Fig. 3. PL spectra of Cz9-based fluorophores in THF solutions and thin films.
3.3. EL properties
[36]. Apparently, the absorption maximum is red-shifted with an in-
crease in the conjugation length between the carbazole and dimesi-
tylboryl moieties. Cz9PhThB showed the longest absorption peak wa-
velength, which is ascribed to the electron-rich thiophene ring together
with the effect of the conjugation length in comparison to the other
fluorophores. A similar trend was observed for these fluorophores in the
thin film state. The structure-property relationship followed the same
trend, and thus, the Cz9PhThB showed the longest absorption maximum
among the fluorophores. The photoluminescence of these fluorophores
in THF solutions and in thin films are plotted in Fig. 3 and the corre-
sponding data including the fluorescence quantum yields (Φf) are also
listed in Table 1. These compounds in the solution state provide blue
luminescence with the wavelength maximum in the range of
438–465 nm, whereas the wavelength maxima in thin film state are in
the range of 414–453 nm Cz9SB showed a longer wavelength maximum
than that of Cz9PhThB, implying that Cz9SB is more coplanar than
Cz9PhThB, and this is further revealed by the fluorescence quantum
yields. As shown in Table 1, the Φf values in the solution state are 91,
100, and 64% for Cz9Ph2B, Cz9PhThB, and Cz9SB, respectively, whereas
the Φf values in the thin film state are 38, 45, and 33%, following the
same sequence as in the solution state. The Cz9SB exhibited lower Φf
than that of Cz9PhThB in the thin film state, and this suggests that
Cz9SB has stronger intermolecular ππ interactions of the molecules than
those of Cz9PhThB, which is consistent with the PL results. Notably, all
To assess the utility of these fluorophores for nondoped blue OLEDs
and red PhOLEDs, multilayer devices with various device configuration
were fabricated and investigated. Fig. 4 shows the nondoped blue
(devices A‒C) and phosphorescent red (device D) device configurations
along with the energy alignments of Cz9Ph2B, Cz9PhThB, and Cz9SB.
The electroluminescent characteristics of these devices are depicted in
Fig. 5 and the corresponding performance data are summarized in
Table 2. First, the Cz9Ph2B-based device (device A) was fabricated in
multilayer configuration, with the device structure of ITO/PEDOT:PSS
(40 nm)/TAPC (40 nm)/Cz9Ph2B (20 nm)/TPBI (40 nm)/LiF (1 nm)/Al
(100 nm). Under the applied voltage, device A emits near the ultraviolet
region with the peak wavelength centered at 416 nm. Device A exhibits
a maximum luminance, current, and external quantum efficiency of
770 cd m−2, 0.30 cd A−1, and 1.65%, respectively. The very similar
λmax observed in the EL and PL (λmax = 414 nm) spectra of Cz9Ph2B in
the thin film state indicates that the observed EL arises solely from the
emitter layer. However, an undesired shoulder emission at ∼380 nm is
observed in device A, which is attributed to the emission from TPBI.
The emission contribution from TPBI indicates leakage of holes from
Cz9Ph2B owing to the relatively low HOMO energy level of Cz9Ph2B. In
order to prevent the same issue in other devices, TPBI was replaced by
TmPyPB in devices B and C, owing to the low HOMO energy level of
TmPyPB. The thickness of each layer in devices B and C were the same
105

C.-K. Liu et al.
DyesandPigments157(2018)101–108
Fig. 4. Energy level diagrams for device A ‒ device D.
Fig. 5. EL characteristics of device A ‒ device C.
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C.-K. Liu et al.
DyesandPigments157(2018)101–108
Table 2
EL performances of devices A ‒ D.
a
b
b
b
b
Device
host
Von (V)
Lmax (cd m−2
)
ηc (max) (cd A−1
)
ηext (max) (%)
ηp(max)
CIE (x, y)
at 7 V
EL
λ
(nm)
max
(lm W−1
)
A
B
C
D
–
–
–
415
443
452
622
621
4.8
3.4
5.9
4.0
3.3
770
0.30
1.51
4.03
17.40
23.12
1.65
2.30
4.72
13.01
14.10
0.17
1.13
1.73
8.28
10.67
0.16, 0.03
0.15, 0.07
0.15, 0.10
0.66, 0.33
0.67, 0.33
7650
7110
25800
39700
CBP
Cz9Ph2B
a. Voltages at 1 cd m−2
b. The maximum values.
.
as in device A, except that the electron transporting material was re-
placed by TmPyPB. The EL properties of the optimized Cz9PhThB-based
device (device B) are also shown in Fig. 5. Device B emits pure blue
light with the peak wavelength centered at 442 nm and the CIEx,y co-
ordinates of (0.15, 0.07). Comparing this with the PL spectrum of the
Cz9PhThB thin film (λmax = 439 nm) clarifies that the emission origi-
nates from Cz9PhThB. Further, the EL spectrum exhibited a slight red-
shift compared to the PL spectrum. This can be explained by the space-
charge-induced spectral shift originating from the space charge redis-
tribution in response to the dipole moment change under electronic
excitation. Notably, device
B reaches a maximum brightness of
7650 cd m−2 with a full-width at half-maximum (FWHM) of 60 nm, a
maximum current efficiency of 1.51 cd A−1, and a maximum external
quantum efficiency (EQE) of 2.30%. The Cz9SB-based device (device C)
emitted blue light with a peak wavelength at 452 nm, with the CIEx,y
coordinates of (0.15, 0.10) and a FWHM of 70 nm, providing a max-
imum brightness of 7110 cd m−2, a maximum current efficiency of
4.03 cd A−1, and a maximum EQE of 4.72%. Similarly, the similarity
between the EL and PL spectra in thin film state of Cz9SB reveals that
the observed EL comes mainly from the emitter. The FWHM of EL in
device C is broader than that in device B, implying that the inter-
molecular π‒π interactions in Cz9SB are stronger than that in
Cz9PhThB, which agrees with the ΦF results. Interestingly, the Cz9SB-
based device exhibited better device performance than that of the
Cz9PhThB-based device. We attribute this to the better π‒π interactions
of Cz9SB that enhances the charge mobility between the molecules,
thereby improving the device performance. This also indicates that the
bulky structure with high Φf and strong π‒π interactions which facil-
itate good charge mobility are in a trade-off relationship in OLEDs.
Based on the high fluorescence Φf and wide bandgap of Cz9Ph2B,
red PhOLEDs were fabricated to access the utility of Cz9Ph2B for po-
tential application as a host material for red PhOLED. For comparison, a
device based on the commonly used host material, 4,4′-N,N′-dicarba-
zolbiphenyl (CBP), was also fabricated in the same device configura-
tion. The devices were fabricated in the configuration of ITO/
PEDOT:PSS (40 nm)/NPB (30 nm)/EML (40 nm)/BCP (6 nm)/Alq3
(30 nm)/LiF (1 nm)/Al (100 nm), as depicted in Fig. 4. Here, “EML”
refers to a layer of Cz9Ph2B or CBP doped with 10 wt% Ir(piq)3. Ir(piq)3
was chosen as the red phosphor, because of its commercial availability
and reliability, as reported elsewhere [39–41]. The EL spectra of these
red-emitting devices and the current density-voltage-luminance char-
acteristics together with the efficiency curves are illustrated in Fig. 6
and Fig. 7, respectively, and the EL data are listed in Table 2. As shown
in Fig. 6, both devices show red electrophosphorescence exclusively
from Ir(piq)3, with the maximum signal intensity at ∼621 nm, in-
dicating the excellent confinement of carriers within the emitting layer.
The Cz9Ph2B-based device exhibited a relatively lower operating vol-
tage compared to that of the CBP-based device, which is presumably
due to the lower energy barrier between BCP and Cz9Ph2B than that
between BCP and CBP. Notably, the Cz9Ph2B-based phosphorescent
device exhibited improved performance with enhanced brightness and
emission efficiency compared to those of the CBP-based phosphorescent
device (Fig. 7). A maximum current efficiency of 23.12 cd A−1 and a
Fig. 6. EL spectra of device D using CBP and Cz9Ph2B as host.
maximum EQE of 14.10% with a brightness of 39700 cd m−2 were
achieved from the Cz9Ph2B-based phosphorescent device, whereas the
CBP-based phosphorescent device showed a maximum current effi-
ciency of 17.40 cd A−1 and a maximum EQE of 13.01% with a bright-
ness of 25800 cd m−2. Compared to the CBP-based device, an increase
of almost 33, 11, and 55% were obtained in current efficiency, EQE,
and brightness, respectively, for the Cz9Ph2B-based device. The ex-
cellent performance of the Cz9Ph2B-based red phosphorescent device
can be ascribed to its bipolar nature that enhanced the charge transport
property and the wide bandgap of Cz9Ph2B, as revealed by the theo-
retical calculations. In addition, the relatively small electron injection
barrier at the Alq3/BCP/Cz9Ph2B interface (Fig. 4) owing to the low-
lying LUMO level of Cz9Ph2B is responsible for a higher current density
in the Cz9Ph2B-based device compared to that in the CBP-based device
and finally results in the enhancement of the device performance. No-
tably, the device using Cz9Ph2B as host showed less efficiency roll-off
with increased current density than that of device that used the CBP
host, which implies that the Cz9Ph2B-based phosphorescent device ex-
hibits more charge balance at a high current density than that of CBP-
based device.
4. Conclusion
In summary, three dimesitylborane-containing fluorophores with
various π-conjugated systems attached at the 9th position of carbazole,
referred as Cz9Ph2B, Cz9ThPhB, and Cz9STB were designed and syn-
thesized for application in nondoped blue OLEDs and red PhOLEDs. All
these fluorophores were thermally stable and provided strong emissions
both in the solution and thin film states. Importantly, Cz9ThPhB and
Cz9STB are promising as nondoped blue emitters with maximum cur-
rent efficiencies of 1.51 and 4.03 cd A−1 and external quantum effi-
ciencies of 2.30 and 4.72%, respectively. Notably, the red PhOLEDs
using Cz9Ph2B as the host material displayed a low efficiency roll-off,
107

C.-K. Liu et al.
DyesandPigments157(2018)101–108
doi.org/10.1016/j.dyepig.2018.04.035.
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with a relatively low turn-on voltage (3.3 V), a maximum brightness of
39700 cd m−2
, and high electroluminescence efficiencies (current,
23.12 cd A−1 and EQE, 14.10%). Their performance is superior to that
of the corresponding device using conventional CBP as the host mate-
rial. The finding here provide a molecular platform for achieving high
EL efficiency in blue OLEDs and red PhOLEDs applications.
Acknowledgments
The financial support of the Ministry of Science and Technology,
Taiwan (ROC), under contract no. MOST 104-2113-M-260-001, MOST
105-2113-M-260-003 and MOST 107-3017-F-002-001 are gratefully
acknowledged.
Appendix A. Supplementary data
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Supplementary data related to this article can be found at http://dx.
108