Bis(carbazol-9-ylphenyl)aniline end-capped oligoarylenes as solution-processed nondoped emitters for full-emission color tuning organic light-emitting diodes

Article abstract of DOI:10.1021/jo4008332

A series of bis(3,6-di-tert-butylcarbazol-9-ylphenyl)aniline end-capped oligoarylenes, BCPA-Ars, are synthesized by double palladium-catalyzed cross-coupling reactions. By using this bis(carbazol-9-yl)triphenylamine moiety as an end-cap, we are able to reduce the crystallization and retain the high-emission ability of these planar fluorescent oligoarylene cores in the solid state, as well as improve the amorphous stability and solubility of the materials. The results of optical and electrochemical studies show that their HOMOs, LUMOs, and energy gaps can be easily modified or fine-tuned by either varying the degree of π-conjugation or using electron affinities of the aryl cores which include fluorene, oligothiophenes, 2,1,3-benzothiadiazole, 4,7-diphenyl-4-yl-2,1,3-benzothiadiazole, and 4,7-dithien-2-yl-2,1,3- benzothiadiazole. As a result, their emission spectra measured in solution and thin films can cover the full UV-vis spectrum (426-644 nm). Remarkably, solution-processed nondoped BCPA-Ars-based OLEDs could show moderate to excellent device performance with emission colors spanning the whole visible spectrum (deep blue to red). Particularly, the RGB (red, green, blue) OLEDs exhibit good color purity close to the pure RGB colors. This report offers a practical approach for both decorating the highly efficient but planar fluorophores and tuning their emission colors to be suitable for applications in nondoped and solution-processable full-color emission OLEDs.

Full text of DOI:10.1021/jo4008332

Article
pubs.acs.org/joc
Bis(carbazol-9-ylphenyl)aniline End-Capped Oligoarylenes as
Solution-Processed Nondoped Emitters for Full-Emission Color
Tuning Organic Light-Emitting Diodes
Tanika Khanasa,† Narid Prachumrak,† Rattanawaree Rattanawan,† Siriporn Jungsuttiwong,†
Tinnagon Keawin,† Taweesak Sudyoadsuk,† Thawatchai Tuntulani,‡ and Vinich Promarak*,§
†Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Ubon Ratchathani University,
Ubon Ratchathani, 34190 Thailand
‡Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok, 10330 Thailand
§School of Chemistry and Center of Excellence for Innovation in Chemistry, Institute of Science, Suranaree University of Technology,
Muang District, Nakhon Ratchasima, 30000 Thailand
S
* Supporting Information
ABSTRACT: A series of bis(3,6-di-tert-butylcarbazol-9-ylphenyl)aniline end-capped
oligoarylenes, BCPA-Ars, are synthesized by double palladium-catalyzed cross-coupling
reactions. By using this bis(carbazol-9-yl)triphenylamine moiety as an end-cap, we are
able to reduce the crystallization and retain the high-emission ability of these planar
fluorescent oligoarylene cores in the solid state, as well as improve the amorphous
stability and solubility of the materials. The results of optical and electrochemical studies
show that their HOMOs, LUMOs, and energy gaps can be easily modified or fine-tuned
by either varying the degree of π-conjugation or using electron affinities of the aryl cores
which include fluorene, oligothiophenes, 2,1,3-benzothiadiazole, 4,7-diphenyl-4-yl-2,1,3-
benzothiadiazole, and 4,7-dithien-2-yl-2,1,3-benzothiadiazole. As a result, their emission
spectra measured in solution and thin films can cover the full UV−vis spectrum (426−
644 nm). Remarkably, solution-processed nondoped BCPA-Ars-based OLEDs could
show moderate to excellent device performance with emission colors spanning the whole
visible spectrum (deep blue to red). Particularly, the RGB (red, green, blue) OLEDs
exhibit good color purity close to the pure RGB colors. This report offers a practical approach for both decorating the highly
efficient but planar fluorophores and tuning their emission colors to be suitable for applications in nondoped and solution-
processable full-color emission OLEDs.
and high stability; however, tuning emission color over the
entire visible range has also emerged as an important ongoing
research task.4 A simple and efficient way to achieve full-
emission color tuning OLEDs is to change the molecular
structure of the emitting materials. Color tuning has been
achieved in conjugated polymers and small molecules by
systematic modification of the chemical structure of, for
example, polythiophenes,5 poly(p-phenylene vinylene) deriva-
tives,6 1,6-bis(N-phenyl-p-(R)-phenylamino)pyrenes,7 bis-dipo-
lar diphenylamino-end-capped oligoarylfluorenes,8 tris(5-aryl-8-
quinolinolate)Al(III) complexes,9 and iridium complexes.10
However, the limitation of these materials is that their emission
colors and the EL spectra of the OLEDs do not span the entire
visible spectrum. In terms of material development, it is
worthwhile to explore a new group of organic materials that can
emit at wavelengths across the whole visible spectrum, while
possessing similar physical properties and processability. In this
paper, we report the design, synthesis, and light-emitting
INTRODUCTION
■
Since the pioneering work on the first organic light-emitting
diodes (OLEDs) by Tang in 1987,1 OLEDs and displays made
from these devices have received tremendous interest due to
their excellent technological aspects such as low-cost, ease of
fabrication using standard techniques (e.g., vacuum deposition
or solution processing), the possibility of realizing flexible or
large-area displays, and use in lighting applications.2 In today’s
developments of OLED technologies, the trends of OLEDs are
mainly focusing both on optimizations of device structures and
developments of new emitting materials. In terms of the device
fabrication, solution-processed OLEDs that are fabricated using
small molecules will have great advantages because the
materials that are used are easy to synthesize and purify,
while the fabricating method is convenient, low-cost, and has
large-scale manufacturability with less material usage.3 The
efficient tunability of the emission spectra of OLEDs to a
desired color is an important consideration when designing
materials. Obviously, the key point of materials development
for full-color flat display is to determine materials emitting pure
colors of red, green, and blue with excellent emission efficiency
Received: May 15, 2013
© XXXX American Chemical Society
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properties of a group of novel bis(3,6-di-tert-butylcarbazol-9-
ylphenyl)aniline end-capped oligoarylenes, BCPA-Ars (Scheme
1), in our aim to develop solution-processable small molecules
simple structure full-emission color tuning nondoped OLEDs
can be fabricated. Nondoped OLEDs can avoid reproducibility
problems associated with achieving the optimum doping
concentration during processing and are easily adapted to a
mass production line. Herein, we describe a detailed synthesis
of BCPA-Ars including physical and photophysical properties.
The investigation of a solution-processed OLED device
fabrication and performance is also reported.
Scheme 1. Molecular Structures of a Series of Bis(3,6-di-tert-
butylcarbazol-9-ylphenyl)aniline End-Capped Oligoarylenes
(BCPA-Ars)
RESULTS AND DISCUSSION
■
Synthesis. The target BCPA-Ars were then synthesized by
means of double palladium-catalyzed cross-coupling reactions
of the iodo/bromo intermediates including 4-(5-bromo-
thiophen-2-yl)-N,N-bis(4-(3,6-di-tert-butylcarbazol-9-yl)-
phenyl)-4-iodoaniline (1), 4-(5-bromothiophen-2-yl)-N,N-bis-
(4-(3,6-di-tert-butylcarbazol-9-yl)phenyl)aniline (3), 4-(5′-
[2,2′-bithiophen]-5-yl)-N,N-bis(4-(3,6-di-tert-butyl carbazol-9-
yl)phenyl)aniline (5), and 4-(4-bromophenyl)-N,N-bis(4-(3,6-
di-tert-butylcarbazol-9-yl)phenyl)aniline (6), as outlined in
Scheme 2. The bromo intermediates 3 and 5 were synthesized
in good yields from 1 using a reaction sequence of Suzuki cross-
coupling reaction with 2-thiophene boronic acid catalyzed by
Pd(PPh3)4/Na2CO3 in THF and a bromination reaction with
NBS in THF in an iterative manner. The cross-coupling of 1
with 4-bromophenylboronic acid gave 6 in 76% yield. Cross-
coupling reactions of these iodo/bromo intermediates 1, 3, 5,
and 6 either with 2,1,3-benzothiadiazole-4,7-bis(boronic acid
pinacol ester), 9,9-dipropylfluorene-2,7-bis(boronic acid pina-
col ester), and 2,5-thiophenediboronic acid under Suzuki
coupling conditions or with hexabutylditin under Stille coupling
conditions were performed to afford the desired bis(carbazol-9-
yl)triphenylamine end-capped oligoarylenes (BCPA-Ars) in
moderate to good yields of 58−79%. Colors of the solid
products were spread from colorless to dark red. All the newly
which are capable of acting as nondoped emitters to achieve
efficient full-emission color tuning in OLEDs. The use of
bis(carbazol-9-yl)triphenylamine moieties as end-capping
groups aims to fulfill the following purposes: (1) to form
nonplanar molecules for suppression of aggregation-induced
fluorescence quenching of the planar conjugated core as well as
improve solubility, thermal stability, and prevention of
intermolecular interaction, (2) to increase the hole-transporting
capability so as to make the target molecules suitable as
nondoped emitters, and (3) to provide sufficient molecular
weight and viscosity to make the products suitable for solution
process. Many of the carbazole and triarylamine derivatives
have been employed as excellent hole-transporting materials
with superior amorphous and thermal stability.11 The structural
modifications of oligoarylene fluorescent cores by either varying
the degree of π-conjugation or using the electron-withdrawing
effect of the aryl cores such as fluorene, oligothiophenes, 2,1,3-
benzothiadiazole, 4,7-diphenyl-4-yl-2,1,3-benzothiadiazole, and
4,7-dithien-2-yl-2,1,3-benzothiadiazole will allow the fine-tuning
of the emission colors that cover the whole UV−vis spectrum.
Using fluorene as a core will give a deep blue-emitting
molecule,12 while using oligothiophenes will extend the π-
conjugation, and the molecules will emit in longer wavelengths
(sky blue to yellow).13 By employing a push−pull character of
the donor feature of triarylamine end-caps and the acceptor
nature of 2,1,3-benzothiadiazole core, combined with using
either phenyl or thiophene spacers to control the push−pull
extent, the molecules will emit in the long- wavelengths region
(yellow to red). With this design, simple solution-processed
hole-transporting nondoped emitters can be obtained, and
1
synthesized BCPA-Ars were fully characterized with H NMR,
13C NMR, FTIR, and high-resolution mass spectrometry and
were found to be in good agreement with the structures. These
materials show high solubility in most organic solvents allowing
their thin films to be fabricated by a spin-coating process. The
morphology of their spin-cast films, which is another key factor
for OLED fabrication, was investigated by atomic force
microscopy (AFM). All thin films spin-coated from CHCl3/
toluene solution have a fairly smooth and pinhole-free surface,
indicating that they have good film-forming properties and can
be fabricated by low-cost solution processes (Figure 1 and
Figure SI_1, Supporting Information).
Quantum Chemical Calculations. To understand the
geometries and electronic properties of the BCPA-Ar series,
quantum chemical calculations were performed using the
TDDFT/B3LYP/6-31 G(d,p) method.14 Their optimized
structures reveal that the bis(carbazol-9-yl)triphenylamine
moieties attached to the end of the molecules adopt a bulky
conformation creating nonplanar geometry around the planar
oligoarylene cores, which could help to prevent the close π−π
contact between molecules and enhance their thermal proper-
ties.15 In the HOMO of BCPA-Ars, π-electrons are delocalized
over the entire oligoarylene core and end-capping moieties. In
the LUMOs of BCPA-F and BCPA-T1−4, having fluorene and
oligothiophenes as cores, respectively, the excited electrons
delocalize over the quinoid-like fluorene and oligothiophene
planes, while in the LUMOs of BCPA-B, BCPA-PB, and BCPA-
TB, which have an electron-deficient 2,1,3-benzothiadiazole as a
central aryl core, these electrons delocalize mainly on the
B
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a
Scheme 2. Synthetic Route to Oligoarylenes BCPA-Ars
a
(i) Pd(PPh3)4, 2 M Na2CO3,THF, heat; (ii) NBS, THF; (iii) (Bu)3Sn−Sn(Bu)3, Pd(PPh3)4, toluene, heat.
requires a linear combination of a number of excited
configurations.
Photophysical Properties. Optical properties of the
BCPA-Ar series were investigated in CH2Cl2 solution and
thin films spin-coated on quartz substrates. The pertinent data
are summarized in Table 1. The solution UV−vis absorption
spectra of BCPA-Ars exhibit two common absorption bands at
298 nm that assigns to the π−π* electron transition of the
peripheral carbazole moieties, and 332−349 nm that is
consistent with the π−π* electron transition of the bis-
(carbazol-9-yl)triphenylamine end-caps (Figure 3a). The
spectra of BCPA-F and BCPA-T1−4 reveal that the extra
absorption bands at longer wavelengths (373−450 nm) are
assigned to the π−π* electron transition of their corresponding
fluorene and oligothiophenes cores. These bands are gradually
red-shifted from 373 to 450 nm with the increasing number of
thiophene rings indicating an extended π-conjugated molecule,
as expected. However, the absorption bands of BCPA-B,
BCPA-PB, and BCPA-TB are shifted to longer wavelengths
(422−528 nm) and belong to an intramolecular charge-transfer
(ICT) transition from the donor (end-capping groups) to the
acceptor (2,1,3-benzothiadiazole core). The solution photo-
luminescence (PL) spectra of BCPA-Ars exhibit featureless
emission bands covering the entire UV−vis spectrum (422−
640 nm), and upon illumination, the emission colors display a
range from deep blue to red (Figure 3b). This indicates that by
Figure 1. AFM image of the spin-coated film of BCPA-B.
quinoid-like 2,1,3-benzothiadiazole plane, creating a donor−
acceptor characteristic (Figure 2 and Figure SI_2, Supporting
Information). The HOMO−LUMO energy gaps (Eg cal) of
BCPA-Ars were calculated and are summarized in Table 1.
These Eg cal values (1.78−3.21 eV) are slightly deviated from
those estimated from the absorption onset (2.02−3.13 eV).
There are factors responsible for the errors in the calculated
results because the orbital energy difference between HOMO
and LUMO is still an approximate estimation of the transition
energy because the transition energy also contains significant
contributions from two-electron integrals. The real situation is
that an accurate description of the lowest singlet excited-state
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Figure 2. HOMO and LUMO orbitals of BCPA-T1 and BCPA-B calculated by the TDDFT/B3LYP/6-31G(d,p) method.
Table 1. Physical Data of Oligoarylenes BCPA-Ars
Stokes
a
a
b
c
d
e
f
g
h
ox
λabs (log ε)
λem
λem
shift
Tg/Tm/T5d
(°C)
Eonset
Eg
Eg cal
HOMO/LUMO
(eV)
a
e
comp
(nm, M−1 cm−1
)
(nm) (nm)
ΦF
(nm)
E1/2 vs Ag/Ag+ (V)
(V)
(eV)
(eV)
BCPA
349 (5.00)
373 (4.64)
396 (4.74)
422
443
465
426
444
488
0.55
0.71
0.65
73
70
69
223/ - /326
234/ - /358
242/ - /333
0.73, 0.85, 1.11, 1.45
0.80, 1.11, 1.40, 1.67
0.74, 0.82, 1.15, 1.40
0.69
0.73
0.68
3.13
2.99
2.77
3.21
3.04
2.79
−5.13/−2.00
−5.17/−2.18
−5.12/−2.35
BCPA-F
BCPA-
T1
BCPA-
T2
418 (4.85)
450 (4.93)
494
529
517
545
0.24
0.14
76
79
241/324/344 0.74, 1.13, 1.33, 1.55
0.66
0.63
2.58
2.39
2.55
2.25
−5.10/−2.52
−5.07/−2.68
BCPA-
T4
247/372/378 0.66, 0.77, 0.94, 1.12,
1.21, 1.26, 1.45, 1.59
BCPA-B
462 (4.40)
422 (4.32)
600
513
601
528
0.05
0.03
138
91
243/343/330 0.82, 1.11, 1.45
0.76
0.76
2.32
2.53
2.06
2.16
−5.20/−2.88
−5.20/−2.67
BCPA-
PB
243/363/368 0.80, 1.11, 1.43, 1.73
BCPA-
TB
528 (4.33)
640
644
0.08
112
255/368/339 0.74, 0.80, 1.08, 1.29, 1.48
0.72
2.02
1.78
−5.16/−3.14
a
b
c
d
Measured in CH2Cl2. Measured in thin film spin-coated on quartz substrate. Calculated from the difference of λmaxabs and λmaxem. Measured by
e
DSC/TGA at a heating rate of 10 °C min−1. Obtained from CV and DPV at a scan rate of 50 mV·s−1 in CH2Cl2 and n-Bu4NPF6 as the electrolyte.
f
g
h
Estimated from the optical absorption edge, Eg = 1240/λonset. Obtained from quantum calculation using TDDFT/B3LYP/6-31G(d,p). Calculated
by HOMO = −(4.44 + Eonsetox), and LUMO = HOMO − Eg, where Eonset is the onset potential of the oxidation.
ox
using different π-extended and electron-affinitive aryl cores, the
fine-tuning of the optical properties of molecular materials can
be achieved. Generally, with a longer π-conjugation length or
stronger electron-withdrawing strength of the central core,
there is a longer emission-wavelength shift. Moreover, the
fluorescence spectra are excitation-wavelength-independent,
and because of excitation at either bis(carbazol-9-yl)-
triphenylamine end-caps or oligoarylene cores, the emission
spectra are identical. This suggests that energy or excitation can
be efficiently transferred from the triarylamine moieties to the
emissive oligoarylene cores, as observed in the quantum
calculation. The fluorescence spectra of BCPA-B, BCPA-PB,
and BCPA-TB are solvent-dependent, in which the spectra
exhibit red-shifts with an increase in solvent polarity. For
instance, the emission maxima of BCPA-TB, BCPA-B, and
BCPA-PB exhibit a solvatochromatic shift of 50, 47, and 40 nm,
respectively, changing from cyclohexane to DMSO (Figure 3c).
This suggests that the emission excited states of these 2,1,3-
benzothiadiazole derivatives possess charge-transfer character.
BCPA, BCPA-F, and BCPA-T1−4 show small Stokes shifts
(69−79 nm), while BCPA-B, BCPA-PB, and BCPA-TB exhibit
large Stokes shifts (91−138 nm) in solution, which are typically
associated with compounds that have strong ICT character.
The fluorescence quantum yields (ΦF) of BCPA-Ar series that
were measured in CH2Cl2 using quinine sulfate or coumarin 6
as a standard are in the range of 0.03−0.71 (Table 1). In this
series, BCPA-F has the highest ΦF value of 0.71 due to the
better chromophore of the fluorene core. In contrast, the
donor−acceptor character of BCPA-B, BCPA-PB, and BCPA-
TB accounts for their low ΦF (0.03−0.08) due to fluorescence
quenching by electron exchange according to the Dexter
mechanism.16 The ΦF values of BCPA-T1−4 (0.14−0.65) are
found to decrease with the increasing number of thiophene
units, which is commonly observed in most cases of
oligothiophene derivatives.17 Due to the long oligothiophenes,
the molecules will become extended planar structures and will
be prone to fluorescence quenching brought about by ππ
stacking interactions of the oligomer segments. However, these
materials show good fluorescence in the solid state. The thin
film photoluminescence (PL) spectra of BCPA-Ar series also
show featureless emission bands and exhibit a small to
moderate red shift (≈1−23 nm) in emission maxima as
compared to those obtained from dichloromethane (Figure
3d). This suggests that the bulky molecular structure of end-
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Figure 3. (a) Absorption spectra of BCPA-Ars measured in CH2Cl2. (b) PL spectra of BCPA-Ars in CH2Cl2 and their emission colors upon
illumination. (c) PL spectra of BCPA-B in different solvents. (d) PL spectra of BCPA-Ars in thin films spin-coated on quartz substrates.
capping groups prevents the intermolecular π−π interactions of
the fluorescent core in the solid state. The most apparent
difference in emission spectra comes from those bearing
oligothiophene as a central core in which the red shift is
attributed to the improvement in the coplanarity of the thienyl
moieties in the solid state. Importantly, the emission spectra of
the thin film PL of BCPA-Ars also cover the full visible
spectrum.
Electrochemical and Thermal Properties. To analyze
the redox properties of these bis(carbazol-9-yl)triphenylamine
end-capped oligoarylenes, cyclic voltammetry (CV) and
differential pulse voltammetry (DPV) were performed in a
three-electrode cell setup with 0.1 M Bu4NPF6 as a supporting
electrolyte in CH2Cl2. The results are tabulated in Table 1.
DPV and CV curves of the BCPA-Ar series exhibit well-defined
multiple quasi-reversible oxidation waves (Figure 4 and Figure
SI_3, Supporting Information). Under this experimental
condition, no reduction is observed in all cases. The first
oxidation wave is assigned to the removal of electrons from the
peripheral carbazoles, resulting in radical cations. Their multiple
CV scans reveal identical CV curves with no additional peak at
a lower potential on the cathodic scan (Epc), suggesting that no
electrochemical coupling at the 3,6-positions of the carbazole
peripheries occurs as they are protected by tert-butyl groups,
indicating electrochemically stable molecules, as expected
(Figure SI_4, Supporting Information).18 It was determined
that, in cases of BCPA-F and BCPA-T1−4, the first oxidation
potentials gradually shift to lower potentials with increasing size
of the π-conjugation system, and this finding agrees with the
optical results. Similar behavior has also been observed in other
end-capped oligothiophenes.19 The HOMO and LUMO
Figure 4. Traces of differential pulse voltammetric measurements of
BCPA-Ars at a scan rate of 50 mV s−1.
energy levels of BCPA-Ars are calculated from the oxidation
onset potentials (Eonsetox) and energy gaps (Eg), and the results
are listed in Table 1. The HOMO energy levels of these
materials are in the range of −5.07 to −5.20 eV, matching well
with the work function of the indium tin oxide (ITO) electrode
and favoring the injection and transport of holes. Their LUMO
energy levels vary widely with a range of −2.00 to −3.14 eV.
Because of the electron-withdrawing nature of the aryl central
cores, the LUMOs of BCPA-Ar are stabilized in various degrees
as compared to that of BCPA (LUMO = −2.00 eV), which has
E
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no aryl core. The LUMOs of BCPA-B, BCPA-PB, and BCPA-
TB (−2.67 to −3.14 eV) having an electron-poor 2,1,3-
benzothiadiazole as a central aryl core are lower and closer to
the work function of the LiF/Al anode than those of BCPA-F
and BCPA-T1−4 (−2.00 to −2.68 eV). Consistently, the
stronger the electron-withdrawing strength, the more stabilized
the LUMO. These results further support that the LUMOs,
HOMOs, and energy gaps of these oligoarylenes can be easily
modified or tuned by the use of different π-extended and
electron-affinitive aryl cores.
The thermal properties of these oligoarylenes were analyzed
by differential scanning calorimetry (DSC) and thermogravi-
metric analysis (TGA) (Table 1, Figure SI_5, Supporting
Information). With the incorporation of bis(3,6-di-tert-butyl-
carbazol-9-ylphenyl)aniline as the end-caps, all the newly
synthesized BCPA-Ar series exhibit an exceptionally high
glass transition temperature (Tg) in the range of 223−255
°C, indicating that they have the ability to form a
morphologically stable amorphous thin film. They also show
high thermal stabilities with a decomposition temperature at 5%
weight loss (T5d) at a temperature well over 326 °C (Figure
SI_5, Supporting Information). The Tg values of BCPA-T1−4,
for example, are higher than those of triarylamine end-capped
oligothiophenes (Tg = 90−98 °C)20 and triarylamine-
substituted carbazole-based dendritic oligothiophenes (Tg =
139−196 °C).21 These results prove that the use of
bis(carbazol-9-yl)triphenylamine as end-capping groups could
reduce the crystallization of a planar-conjugated core and
improve the amorphous stability of the material and the thin
film, which in turn could increase the service time in device
operation.22
Electroluminescence Properties. According to the
above-discussed excellent properties including low first
ionization potential, good luminescence properties, high
thermal stability, good amorphous morphological stability,
and ability to form good thin film from the solution spin-
coating process, BCPA-Ars show great potential for use as
solution-processed nondoped hole-transport emitters in
OLEDs. First, single-layer devices with the structure of ITO/
PEDOT:PSS/BCPA-Ars(spin-coating)/LiF(0.5 nm):Al(150
nm) were fabricated and investigated. The BCPA-Ar layer
was spin-coated from a CHCl3/toluene (5:1) solution with
controlled thickness. The electroluminescent characteristics of
the devices are shown in Figure 5 and summarized in Table 2.
It is found that only the devices using BCPA-T4 (device I),
BCPA-B (device II), BCPA-PB (device III), and BCPA-TB
(device IV) as an emitting layer (EML) show light emission.
Analysis of the energy-band diagram of all fabricated single-
layer devices reveals that the LUMO energy levels of BPPA,
BCPA-F, and BCPA-T1−2 are too high and mismatch with the
work function of the LiF/Al (4.20 eV) cathode with the
electron injection barriers at the EML/LiF/Al interface as high
as 2.17−2.20 eV (Figure 6a). The results reveal that the ICT
effect and the favorable contributions are so well-balanced in
2,1,3-benzothiadiazole-based molecules (BCPA-B, BCPA-PB,
and BCPA-TB) that they produce the best light-emission
performance in single-layer OLEDs among all these analogues.
The EL spectra of these devices (I−IV) exhibit a featureless
emission peak matching well with the PL spectra of thin films
indicating that the EL originates purely from the oligoarylenes
(Figure 5b). The devices (II−IV) using BCPA-B, BCPA-P, and
BCPA-TB as EML exhibit good to excellent device perform-
ance with maximum luminance (Lmax) of 4523−7851 cd m−2
Figure 5. Plots of (a) current density−luminance−voltage and (b) EL
spectra of the single-layer OLEDs (devices I−IV).
and luminance efficiencies (ηmax) up to 1.53−3.85 cd A−1
(Figures SI_6, Supporting Information). Particularly, the
BCPA-TB-based single-layer device (IV) emits a bright red
luminescence with excellent color purity, an emission peak at
652 nm, and a full width at half-maximum of ≈ 100 nm. The
coordinates of the emitted light specified by the Commission
Internationale De L′Eclairage (CIE) are x = 0.65 and y = 0.33,
which are very close to the pure red color (x = 0.67, y = 0.33)
standardized by the National Television System Committee
(NTSC).23 This device emits better red color purity and has
less complex device structure than those of the devices
featuring 1,10-dicyano-substituted bis-styrylnaphthalene deriv-
ative (BSN),24 (bis(4-(N-(1-naphthyl)phenylamino)phenyl)-
fumaronitrile) (NPAFN),25 or a star-shaped thieno-3,4-b-
pyrazine (TPNA)26 as the nondoped materials. This solution-
processed single-layer red OLED (device IV) shows an Lmax of
4523 cd m−2 at 14.8 V, ηmax of 1.53 cd A−1, and turn-on voltage
(Von) of 2.9 V. The BCPA-PB-based device (III) gives a yellow
emission with a peak maximum (λem) at 553 nm, a high
efficiency (Lmax = 7851 cd m−2 at 14.8 V, ηmax = 3.85 cd A−1),
and a low Von of 2.4 V, while the BCPA-B-based device (II)
emits a bright orange color (λem = 586 nm) with a slightly lower
efficiency (ηmax = 2.46 cd A−1).
To improve the efficiency of the single-layer OLED and
achieve light emission from BPPA, BCPA-F and BCPA-T1−2, a
multilayer device is necessary. This could be done by balancing
a charge injection in the device. In this case, BCPA-Ars are
more hole-injection EML as their HOMOs (−5.07 to −5.20
eV) match well with the work function of the ITO/
PEDOT:PSS anode (−5.00 eV), hence an electron-injection/
hole-blocking layer is required (Figure 6a). Dimethyl-4,7-
diphenyl-1,10-phenanthroline (BCP) is known to enhance
performance in multilayer devices fabricated with predom-
F
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Table 2. Electroluminescent Data of the Single-Layer OLEDs
a
b
c
d
e
device
EML
λmaxEL (nm)
Von/V100 (V)
Lmax/V (cd m−2, V)
Jmax (mA cm−2
)
ηmax (cd A−1
)
CIE (x, y)
I
BCPA-T4
BCPA-B
540
586
553
2.9/5.9
2.9/4.9
2.4/4.7
2.7/5.7
873/12.4
5212/14.6
7851/16.2
1307
456
607
732
0.16
2.46
3.85
0.39, 0.55
0.52, 0.46
0.43, 0.55
0.65, 0.33
II
III
BCPA-PB
BCPA-TB
IV
625
4523/14.8
1.53
a
b
c
d
ITO/PEDOT:PSS/EML/LiF:Al. Turn-on voltages at 1 and 100 cd m−2. Maximum luminance at applied voltage. Current density at maximum
e
luminance. Luminance efficiency.
Figure 6. Schematic energy diagrams of the fabricated OLEDs using (a) BCPA-Ars as EML and (b) BCPA and BCPA-F as HTL.
Figure 7. Plots of (a) EL spectra and emission colors, (b) luminance−voltage, (c) current density−voltage, and (d) efficiency−current density of the
double-layer OLEDs (device V−XIII).
inantly hole-transporting emitters.27 Simple double-layer
devices with the structure of ITO/PEDOT:PSS/BCPA-Ars-
(spin-coating)/BCP(40 nm)/LiF(0.5 nm)/Al(150 nm) were
fabricated and investigated. The detailed EL and J−V−L data
are shown in Figure 7 and summarized in Table 3. The spin-
coated double-layer OLEDs using BCPA-T4 (device V),
BCPA-B (device VI), BCPA-PB (device VII), and BCPA-TB
(device VIII) as EML exhibit a massive improvement in the
device performance compared to their single-layer devices with
an Lmax of 12325−47497 cd m−2 and ηmax up to 2.18−5.19 cd
G
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Table 3. Electroluminescent Data of the Double-Layer OLEDs
c
d
e
f
device
EML/HTL
λmaxEL (nm)
Von/V100 (V)
Lmax/V (cd m−2, V)
Jmax (mA cm−2
)
ηmax (cd A−1
)
CIE (x, y)
a
V
BCPA-T4
BCPA-B
BCPA-PB
BCPA-TB
BCPA-F
BCPA-T1
BCPA-T2
BCPA-F
BCPA
542
587
557
653
426
484
510
521
520
518
3.0/5.1
2.7/5.0
2.9/6.2
2.2/4.7
4.0/6.7
4.0/10.8
3.7/6.9
2.4/4.1
2.5/3.8
4.2/5.4
2.5/3.7
18699/13.0
47497/15.4
32817/15.8
12325/15.2
1572/10.0
640/17.8
858
2.81
5.19
4.17
3.97
0.49
0.57
0.49
4.55
4.37
0.91
0.43, 0.55
0.53, 0.46
0.44, 0.54
0.66, 0.33
0.16, 0.08
0.19, 0.33
0.22, 0.52
0.29, 0.53
0.29, 0.53
0.30, 0.54
0.29, 0.53
a
VI
1221
1440
1009
561
a
VII
a
VIII
a
IX
a
X
764
a
XI
8489/16.0
22238/10.6
11275/10.0
4961/10.0
1768
1046
888
b
XII
b
XIII
b
XIV
693
b
XV
TPD
519
22539/8.8
1296
4.05
a
b
c
d
ITO/PEDOT:PSS/EML/BCP/LiF:Al. ITO/PEDOT:PSS/HTL/Alq3/LiF:Al. Turn-on voltages at 1 and 100 cd m−2. Maximum luminance at
e
f
applied voltage. Current density at maximum luminance. Luminance efficiency.
A−1. The colorimetric purities of these double-layer devices are
also equivalent to their single-layer devices. The double-layer
OLEDs fabricated with BCPA-F (device IX) and BCPA-T1−2
(devices X and XI) as EML show moderate device perform-
ance. These devices exhibit an Lmax of 640−8489 cd m−2 and
ηmax up to 0.49−0.57 cd A−1. The EL spectra of all devices (V−
XI) exhibit a featureless emission peak identical to their PL
spectra of thin films and also their single-layer OLED devices,
indicating that they have similar relaxation processes (Figure
7a). Furthermore, no emission shoulder is detected at a longer
wavelength, which often occurs in devices fabricated from
EMLs with planar molecular structure due to excimer and
exciplex species formation at the interface of the EML and BCP
layers.28 Moreover, stable emission is obtained from all devices,
and the EL spectra did not change over the entire driven
voltages (Figure SI_7, Supporting Information). Unfortunately,
there is still not sufficient light emission from the BCPA-based
double-layer OLED. This may be the result of a higher LUMO
level of BCPA than is effective when used as EML. However,
the electroluminescence (EL) spectra of these BCPA-Ars-based
devices (V−XI) do cover the entire visible spectrum with
emission peaks ranging from 426 nm for the BCPA-F-based
device to 653 nm for the BCPA-TB-based device. The role of
the incorporated BPC layer into the BCPA-B- and BCPA-TB-
based double-layer devices (VI, VIII) is preferably a hole-
blocking layer as the LUMO of BCP (−3.00 eV) is similar to
that of both BCPA-B and BCPA-TB, while in other devices (V,
VII, IX−XI) BCP would act as both hole-blocking and electron-
transporting layers (Figure 6a). In these BCPA-Ar series, the
devices (VI, VII) fabricated with BCPA-B (orange emission)
and BCPA-PB (yellow emission) as EML have the best
performance in terms of Lmax (32817−47497 cd m−2) and ηmax
(4.17−5.19 cd A−1), while the BCPA-TB-based spin-coating
double-layer device (VIII) exhibits slightly lower performance
(Lmax = 12325 cd m−2, ηmax = 3.97 cd A−1) with excellent red
color purity (CIE = 0.66, 0.33) and low device turn-on voltage
(Von) of 2.2 V, which is considered to be one of the lowest turn-
on voltages for solution-processed red OLEDs reported so
far.29 As far as we know, the ability of BCPA-TB as a nondoped
red EML in red OLEDs, in terms of device performance,
solution processability, thermal property, red color purity, and
simple device structure, is outstanding compared with those
solution-processable red fluorescent emitters30 and is also
comparable with vacuum-deposited red fluorescent emitters
reported in recent years.31 Noticeably, the BCPA-F-based
double-layer device (IX) also emits an excellent deep blue color
(λem = 426 nm) with CIE coordinates of (0.16, 0.08), which is
close to the NTSC standard deep blue (0.14, 0.08),32 while the
green OLED of the BCPA-T2-based device (XI) shows the λem
around 510 nm with the CIE coordinates of (0.22, 0.52), which
is slightly deviated from the NTSC standard green (0.26,
0.65).33 The high EL efficiencies, high color purity, and
facilitating process for the light-emitting films indicate these
materials to be promising candidates for full-color-display
applications, especially BCPA-TB for red and BCPA-F for deep
blue.
As mentioned above, the simple structure OLEDs using
BCPA as EML did not give light emission; however, its HOMO
level (−5.13 eV) matched well with the work function of ITO
(4.8 eV) with high LUMO level, and BCPA may potentially
serve as an amorphous hole-transporting material (AHTM)
with superior thermal property. To test this hypothesis, double-
layer Alq3-based green OLEDs with the structure of ITO/
PEDOT:PSS/BCPA and BCPA-F (spin-coating)/Alq3(50
nm)/LiF(0.5 nm):Al(150 nm) were fabricated and inves-
tigated, where BCPA and BCPA-F were used as hole-
transporting layers (HTL) and tris(8-hydroxyquinoline)-
aluminum (Alq3) as the green light-emitting (EML) and
electron-transporting layers (ETL). The reference devices
(devices XIV and XV) fabricated with and without a
commercial HTM, N,N′-bis(3-methylphenyl)-N,N′-bis-
(phenyl)benzidine (TPD), as HTL were made for comparison.
The detailed EL and J−V−L data are shown in Figure 7 and
summarized in Table 3. The BCPA-F-based device (XII)
exhibits excellent performance with a high Lmax of 22238 cd
m
−2 for green OLED at 10.6 V, a low Von of 2.4 V, and ηmax of
4.55 cd A−1. The operating voltage at 100 cd m−2 is as low as
4.1 V. The BCPA-based device (XIII) displays slightly lower
performance in terms of a maximum brightness, turn-on
voltage, and maximum luminance efficiency. Both devices XII
and XIII emit a bright green luminescence (λem = 520 nm, CIE
coordinates of (0.29, 0.53)) under applied voltage. The EL
spectra match well with the PL spectrum of Alq3, the EL of the
reference devices (XIV and XV), and other reported
devices.11,34 No emission at longer wavelengths indicating the
formation of exciplex species at the interface is detected. From
these results, and in view of the fact that a barrier for electron
migration at the Alq3/HTL interface (≈0.9 eV) is higher than
that for hole migration at the HTL/Alq3 interface (≈0.6 eV)
(Figure 6b), both BCPA and BCPA-F will act only as HTL, and
Alq3 will act preferably as an electron blocker more than as a
hole blocker and charge recombination thus is confined to the
H
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desorption ionization (ESI) and measured in the MALDI-TOF/TOF
mode.
Alq3 layer. More importantly, a stable emission is obtained from
these diodes in which the EL spectra and CIE coordinates did
not change over the entire applied voltages (Figure SI_7,
Supporting Information). When devices are compared for the
performance of both devices (XII and XIII) with the reference
device (XIV), it is found that the incorporation of either BCPA
or BCPA-F in the device as an HTL not only increases the ηmax
from 0.91 to 4.37−4.55 cd A−1 but also significantly decreases
the Von from 4.2 to 2.4−2.5 V, indicating an excellent AHTM
ability of both materials. Moreover, the abilities as solution-
processed AHTM of BCPA and BCPA-F in terms of thermal
stability of amorphous film and their device luminance
efficiencies are also greater than commonly used TPD (Tg =
63 °C) (device XV).
N,N-Bis(4-(3,6-di-tert-butylcarbazol-9-yl)phenyl)-4-iodoani-
line (1). A mixture of tris(4-iodophenyl)amine (3.0 g, 4.8 mmol), 3,6-
di-tert-butylcarbazole (2.7 g, 9.6 mmol), CuI (0.9 g, 4.8 mmol), K3PO4
(3.3 g, 24.1 mmol), and -trans-1,2-diaminocyclohexane (0.6 mL, 4.8
mmol) in toluene (60 mL) was degassed with N2 for 5 min and then
heated at reflux under N2 atmosphere for 24 h. After being cooled, the
solid residue was filtered and washed with CH2Cl2 (50 mL). The
organic filtrate was washed with water (100 mL × 2) and brine
solution (100 mL), dried over anhydrous Na2SO4, and evaporated to
dryness. Purification by silica gel column chromatography using
CH2Cl2/hexane (1:9) as eluent gave a light gray solid (1.90 g, 44%):
mp > 250 °C; 1H NMR (300 MHz, CDCl3, ppm) δ 8.37 (4H, s), 7.80
(2H, d, J = 7.2 Hz), 7.50−7.68 (16H, m), 7.20 (2H, d, J = 7.2 Hz),
1.67 (36H, s); 13C NMR (75 MHz, CDCl3, ppm) δ 147.2, 145.9,
145.6, 142.6, 139.4, 133.3, 130.9, 128.7, 127.8, 126.8, 125.3, 124.5,
123.6, 123.3, 122.7, 116.3, 110.8, 109.2, 34.8, 32.0; IR (KBr, cm−1) ν
3042, 2955, 1507, 1483, 1312, 1295, 1262, 810; HRMS (ESI-TOF) m/
z [M + H]+ calcd for C58H61IN3 926.383, found 926.3902.
CONCLUSION
■
In summary, we demonstrated the design strategy and synthesis
of BCPA-Ars as nondoped solution-processed light emitters for
full-emission color tuning OLEDs. By varying the degree of π-
conjugation or using the electron affinities of the aryl cores,
their HOMOs, LUMOs, and energy gaps can be easily modified
or tuned. Their emission spectra in solution and thin film span
the entire visible light spectrum (422−640 nm), and the
emission colors range from deep blue to red upon illumination.
By using bis(3,6-di-tert-butylcarbazol-9-ylphenyl)aniline as end-
capping groups, we are able to reduce the crystallization and
retain the high-emissive ability of a planar fluorescent core in
the solid state, as well as improve the amorphous stability and
solubility of the materials. All of these materials can form
morphologically stable amorphous thin films with Tg as high as
223−255 °C. The ICT effect and the favorable contributions
are so well-balanced in 2,1,3-benzothiadiazole-based molecules
that they produce a high light-emission performance in
solution-processed nondoped single-layer OLEDs among all
these analogues. The single-layer OLED with a luminance
efficiency as high as 3.85 cd A−1 and a low turn-on voltage of
2.4 V is achieved. The solution-processed nondoped BCPA-
Ars-based double-layer OLEDs exhibit moderate to excellent
device performance with emission colors spanning the whole
visible spectrum (deep blue to red). The RGB (red, green,
blue) OLEDs based on these materials by a simple solution
process exhibit moderate to high luminescence efficiency and
good color purity close to the pure RGB colors. Especially,
double-layer red and blue OLEDs show an excellent red color
(λem = 653 nm) with CIE coordinates of (0.66, 0.33) and an
excellent deep blue color (λem = 426 nm) with CIE coordinates
of (0.16, 0.08), which are close to the NTSC standard red
(0.67, 0.33) and standard deep blue (0.14, 0.08), respectively.
The ability of BCPA-TB as a nondoped red light emitter, in
terms of device performance, is excellent compared with those
solution-processable red fluorescent materials and also
equivalent to the vacuum-deposited red fluorescent materials
reported in recent years. This report offers a useful strategy to
adorn the highly efficient but planar fluorophore to be suitable
for applications in solution-processable and nondoped OLEDs.
N,N-Bis(4-(3,6-di-tert-butylcarbazol-9-yl)phenyl)-4-(thio-
phen-2-yl)aniline (2). A mixture of 1 (2.00 g, 1.9 mmol), 2-
thiopheneboronic acid (0.27 g, 1.9 mmol), Pd(PPh3)4 (0.05 g, 0.04
mmol), and 2 M Na2CO3 aqueous solution (16 mL) in THF (40 mL)
was degassed with N2 for 5 min and then heated at reflux under N2
atmosphere for 24 h. After being cooled, CH2Cl2 (100 mL) was added,
and the organic phase was separated, washed with water (100 mL × 2)
and brine solution (100 mL), dried over anhydrous Na2SO4, filtered,
and evaporated to dryness. Purification by silica gel column
chromatography using CH2Cl2/hexane (1:9) as eluent gave a light
yellow solid (1.31 g, 72%): mp > 250 °C; 1H NMR (300 MHz,
CDCl3, ppm) δ 8.11 (4H, d, J = 5.4 Hz), 7.59 (2H, s), 7.24−7.48
(20H, m), 7.05 (1H, s), 1.40 (36H, s); 13C NMR (75 MHz, CDCl3,
ppm) δ 146.6, 145.9, 143.6, 142.7, 139.1, 132.7, 129.5, 128.5, 127.7,
121.1, 125.2, 124.9, 123.8, 123.1, 116.5, 109.5, 34.8, 32.2; IR (KBr,
cm−1) ν 3041, 2958, 1507, 1473, 1316, 1294, 1262, 809; HRMS (ESI-
TOF) m/z [M + H]+ calcd for C62H64N3S 882.4743, found 882.4819.
4-(5-Bromothiophen-2-yl)-N,N-bis(4-(3,6-di-tert-butyl-
carbazol-9-yl)phenyl)aniline (3). N-Bromosuccinimide (0.35 g, 2.0
mmol) was added in small portions to a solution of 2 (1.71 g, 1.9
mmol) in THF (30 mL). The mixture was stirred at room temperature
under N2 for 1 h. Water (30 mL) and CH2Cl2 (100 mL) were added.
The organic phase was separated, washed with water (100 mL × 2)
and brine solution (100 mL), dried over anhydrous Na2SO4, filtered,
and the solvents were removed to dryness. Purification by silica gel
column chromatography eluting with CH2Cl2/hexane (1:9) gave
brominated product as a light yellow solid (1.5 g, 91%): mp > 250 °C;
1H NMR (300 MHz, CDCl3, ppm) δ 8.15 (4H, d, J = 1.2 Hz), 7.26−
7.52 (21H, m), 7.03 (1H, s), 1.48 (36H, s); 13C NMR (75 MHz,
CDCl3, ppm) δ 147.3, 145.7, 142.9, 139.3, 138.5, 133.4, 127.7, 126.2,
125.2, 123.6, 123.3, 116.3, 109.2, 86.3, 34.7, 32.0; IR (KBr, cm−1) ν
3041, 2959, 1507, 1316, 1294, 1263, 809; HRMS (ESI-TOF) m/z [M
+ H]+ calcd for C62H63BrN3S 960.3848, found 960.3908.
4-(2,2′-Bithiophen-5-yl)-N,N-bis(4-(3,6-di-tert-butylcarbazol-
9-yl)phenyl)aniline (4). Compound 4 was synthesized as for 2 and
obtained as a light yellow-green solid (63%): mp > 250 °C; 1H NMR
(300 MHz, CDCl3, ppm) δ 8.16 (4H, s), 7.04−7.62 (25H, m), 1.48
(36H, s); 13C NMR (75 MHz, CDCl3, ppm) δ 146.7, 145.9, 142.8,
139.4, 133.2, 129.2, 127.8, 127.7, 126.7, 125.1, 124.6, 124.6, 124.6,
124.2, 123.9, 123.3, 123.2, 116.2, 109.2, 34.8, 32.0; IR (KBr, cm−1) ν
3041, 2953, 1507, 1316, 1293, 1262, 808; HRMS (ESI-TOF) m/z [M
+ H]+ calcd for C66H66N3S2 964.4620, found 964.4637.
4-(5′-Bromo-2,2′-bithiophen-5-yl)-N,N-bis(4-(3,6-di-tert-
butylcarbazol-9-yl)phenyl)aniline (5). Compound 5 was synthe-
sized as for 3 and obtained as a yellow-green solid (87%): mp > 250
EXPERIMENTAL SECTION
■
1
Synthesis and Characterization. The reagents (chemicals) were
purchased and used without further purification. All 1H and 13C NMR
spectra were recorded using CDCl3 as the solvent. Infrared (IR)
spectra were measured as KBr disc. High-resolution mass spectra
(micrOTOF) were obtained with electrospray and matrix-assisted laser
°C; H NMR (300 MHz, CDCl3, ppm) δ 8.16 (4H, s), 7.26−7.60
(21H, m), 7.18 (1H, d, J = 3.6 Hz), 7.09 (1H, d, J = 3.9 Hz), 6.98 (1H,
d, J = 3.6 Hz), 6.94 (1H, d, J = 3.9 Hz), 1.49 (36H, s); 13C NMR (75
MHz, CDCl3, ppm) δ 147.0, 145.8, 143.3, 142.8, 139.3, 139.0, 135.0,
133.2, 130.6, 127.7, 126.8, 125.2, 124.9, 124.4, 123.5, 123.3, 123.1,
I
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116.2, 109.2, 34.7, 32.0; IR (KBr, cm−1) ν 3041, 2959, 1507, 1318,
1294, 1263, 808; HRMS (ESI-TOF) m/z [M + H]+ calcd for
C66H65BrN3S2 1042.3725, found 1042.3773.
4,7-Bis{N,N-bis4-(3,6-di-tert-butylcarbazol-9-yl)phenyl-
aminophen-4-yl}-2,1,3-benzothiadiazole (BCPA-B). Compound
BCPA-B was synthesized as for 2 and obtained as a red solid (79%):
mp > 250 °C; 1H NMR (300 MHz, CDCl3, ppm) δ 8.16 (8H, s), 8.04
(4H, d, J = 8.4 Hz), 7.86, (2H, s), 7.43−7.55 (36H, m), 1.48 (72H, s);
13C NMR (75 MHz, CDCl3, ppm) δ 154.2, 147.6, 146.0, 142.8, 139.4,
4′-Bromo-N,N-bis(4-(3,6-di-tert-butylcarbazol-9-yl)phenyl)-
1,1′-biphenyl-4-amine (6). Compound 6 was synthesized as for 2
1
and obtained a colorless solid (76%): mp > 250 °C; H NMR (300
MHz, CDCl3, ppm) δ 8.16 (4H, s), 7.43−7.59 (21H, m), 1.48 (36H,
s); 13C NMR (75 MHz, CDCl3, ppm) δ 142.85, 139.30, 131.90,
128.36, 127.70, 125.24, 123.34, 116.22, 109.31, 34.72, 32.06; IR (KBr,
cm−1) ν 2955, 1505, 1474, 1261, 808; HRMS (ESI-TOF) m/z [M +
H]+ calcd for C64H65BrN3 954.4362, found 954.4329.
132.2, 132.1, 130.3, 127.8, 127.7, 125.4, 123.9, 123.6, 123.3, 116.2,
109.2, 34.7, 32.0; IR (KBr, cm−1) ν 2955, 1505, 1474, 1261, 808;
MALDI-TOF m/z [M + H]+ calcd for C122H123N8S 1731.959, found
1731.957.
4,7-Bis{4,4′-(N,N-bis4-(3,6-di-tert-butylcarbazol-9-yl)phenyl-
amino)-1,1′-biphenyl}-2,1,3-benzothiadiazole (BCPA-PB). Com-
pound BCPA-PB was synthesized as for BCPA-B and obtained as a
reddish-orange solid (68%): mp > 250 °C; 1H NMR (300 MHz,
CDCl3, ppm) δ 8.16 (8H, d, J = 0.9 Hz), 8.10 (4H, d, J = 8.1 Hz), 7.90
(2H, s), 7.82 (4H, d, J = 8.1 Hz), 7.71 (4H, d, J = 8.4 Hz), 7.40−7.54
(36H, m), 1.49 (72H, s); 13C NMR (75 MHz, CDCl3, ppm) δ 154.2,
142.8, 139.3, 136.0, 133.2, 132.8, 129.7, 128.2, 127.7, 127.0, 125.1,
124.6, 123.5, 123.3, 123.3, 116.2, 109.2, 34.7, 32.0; IR (KBr, cm−1) ν
2958, 1505, 1474, 1261, 814, 793; MALDI-TOF m/z [M + H]+ calcd
for C134H131N8S 1884.022, found 1884.018.
4,4′-(N,N-Bis(4-(3,6-di-tert-butylcarbazol-9-yl)phenyl)-
amino)-1,1′-biphenyl (BCPA). A mixture of 1 (0.71 g, 0.76 mmol),
hexabutylditin (0.21 g, 0.37 mmol), and Pd(PPh3)4 (0.018 g, 0.015
mmol) in toluene (25 mL) was degassed with N2 for 5 min and then
heated at 80 °C under N2 atmosphere for 24 h. After the reaction
mixture was cooled to room temperature, water (10 mL) was added.
The mixture was extracted with CH2Cl2 (50 mL × 2). The combined
organic phase was washed with water (50 mL × 2) and brine solution
(50 mL) and dried over anhydrous Na2SO4. It was then filtered and
evaporated to dryness. Purification by silica gel column chromatog-
raphy using CH2Cl2/hexane (1:6) as eluent followed by recrystalliza-
tion with CH2Cl2/MeOH afforded a colorless solid (0.29 g, 51%): mp
4,7-Bis{5-(N,N-bis4-(3,6-di-tert-butylcarbazol-9-yl)phenyl-
aminophenyl)-thien-2-yl}-2,1,3-benzothiadiazole (BCPA-TB).
Compound BCPA-TB was synthesized as for 2 and obtained as a
1
> 250 °C; H NMR (300 MHz, CDCl3, ppm) δ 8.16 (8H, s), 7.64
1
(4H, d, J = 8.1 Hz), 7.42−7.52 (36H, m), 1.54 (72H, s); 13C NMR
(75 MHz, CDCl3, ppm) δ 145.2, 141.0, 140.6, 139.8, 139.3, 134.9,
134.9, 130.5, 129.9, 125.9, 125.5, 125.4, 124.3, 123.2, 123.1, 122.8,
117.6, 109.6, 34.4, 32.0; IR (KBr, cm−1) ν 2958, 1505, 1474, 1261,
793; MALDI-TOF m/z [M + H]+ calcd for C116H121N6 1597.965,
found 1597.961.
dark red solid (58%): mp > 250 °C; H NMR (300 MHz, CDCl3,
ppm) δ 8.17 (8H, s), 7.90 (2H, s), 7.71 (4H, d, J = 8.2 Hz), 7.40−7.55
(36H, m), 7.32 (4H, d, J = 8.2 Hz), 1.49 (72H, s); 13C NMR (75
MHz, CDCl3, ppm) δ 152.6, 147.1, 145.9, 145.2, 142.8, 139.4, 133.2,
129.1, 128.7, 127.8, 127.0, 125.4, 125.2, 124.4, 123.9, 123.3, 123.3,
116.2, 109.2, 34.7, 32.0; IR (KBr, cm−1) ν 2958, 1505, 1474, 1261,
814, 793; MALDI-TOF m/z [M + H]+ calcd for C130H127N8S3
1895.935, found 1895.933.
2,5-Bis{N,N-bis-4-(3,6-di-tert-butylcarbazol-9-yl)phenyl4-
aminophenyl}-9,9-dipropylfluorene (BCPA-F). Compound
BCPA-F was synthesized as for 2 and obtained as a yellow solid
(61%): mp > 250 °C; 1H NMR (300 MHz, CDCl3, ppm) δ 7.48−7.84
(46H, m), 2.13 (4H, m), 1.54 (74H, s), 0.78 (6H, m); 13C NMR (75
MHz, CDCl3, ppm) δ 146.7, 142.8, 139.4, 139.3, 136.8, 133.0, 128.3,
127.7, 125.7, 125.0, 123.3, 121.1, 120.1, 116.3, 109.3, 34.8, 32.0, 28.8,
24.4, 17.6; IR (KBr, cm−1) ν 2952, 1507, 1493, 1262, 807; MALDI-
TOF m/z [M]+ calcd for C135H140N6 1845.114, found 1845.118.
2,5-Bis{N,N-bis4-(3,6-di-tert-butylcarbazol-9-yl)phenyl4-
aminophenyl}thiophene (BCPA-T1). Compound BCPA-T1 was
synthesized as for 2 and obtained as a yellow solid (65%): mp > 250
Optical, Thermal, Electrochemical, and Physical Character-
izations. UV−vis absorption and photoluminescence spectra were
recorded as a dilute solution in spectroscopic grade CH2Cl2 and thin
film obtained by spin-casting. The fluorescence quantum yields (ΦF)
were determined by comparison with a fluorescence standard of
quinine sulfate in 0.01 M % H2SO4 (ΦF = 0.54) and coumarin 6 in
ethanol (ΦF = 0.78).35 Differential scanning calorimetry (DSC)
analysis and thermogravimetric analysis (TGA) were analyzed with a
heating rate of 10 °C min−1 under a N2 atmosphere. Cyclic
voltammetry (CV) and differential pulse voltammetry (DPV)
measurements were carried out with a three-electrode system
(platinum counter electrode, glassy carbon working electrode, and
Ag/Ag+ reference electrode) at a scan rate of 50 mV s−1 in CH2Cl2
under an Ar atmosphere. The concentration of analytical materials and
tetrabutyl ammonium hexafluorophosphate (n-Bu4NPF6) were 10−3
and 0.1 M, respectively. The atomic force microscopy (AFM) analysis
was performed using standard noncontact mode with a resonance of
316.17 kHz.
Quantum Computer Calculation. The ground state geometries
of BCPA-Ars were fully optimized using density functional theory
(DFT) at the B3LYP/6-31G(d,p) level, as implemented in Gaussian
09.14 The TDDFT/B3LYP calculation of the lowest excitation
energies was performed at the optimized geometries of the ground
states.
Device Fabrication and Testing. OLED devices using BCPA-Ars
as a nondoped missive layer (EML) with the device configurations of
ITO/PEDOT:PSS/BCPA-Ar(spin-coating)/LiF(0.5 nm):Al(150 nm)
and ITO/PEDOT:PSS/BCPA-Ar(spin-coating)/BCP(40 nm)/LiF-
(0.5 nm):Al(150 nm) were fabricated and characterized as follows.
The patterned indium tin oxide (ITO) glass substrate with a sheet
resistance a 14 ohm sq−1 was thoroughly cleaned by successive
ultrasonic treatments in detergent, deionized water, isopropanol, and
acetone and then dried at 60 °C in a vacuum oven. A 50 nm thick
PEDOT/PSS hole-injection layer was spin-coated on top of ITO from
a 0.75 wt % dispersion in water at a spin speed of 3000 rpm for 20 s
and dried at 200 °C for 15 min under vacuum. Thin films of BCPA-
Ars were deposited on top of PEDOT/PSS layer by spin-coating in a
CHCl3/toluene solution (5:1) of BCPA-Ars (1.5% w/v) at a spin
1
°C; H NMR (300 MHz, CDCl3, ppm) δ 8.16 (8H, s), 7.17−7.65
(42H, m), 1.48 (72H, s); 13C NMR (75 MHz, CDCl3, ppm) δ 146.6,
146.2, 142.8, 139.4, 139.4, 139.3, 136.8, 133.0, 128.3, 127.7, 125.7,
125.0, 123.3, 121.1, 120.1, 116.3, 109.3, 34.8, 32.1; IR (KBr, cm−1) ν
2952, 1507, 1491, 1260, 786; MALDI-TOF m/z [M + H]+ calcd for
C120H123N6S 1679.953, found 1679.950.
5,5′-Bis{N,N-bis4-(3,6-di-tert-butylcarbazol-9-yl)phenyl4-
aminophenyl}-2,2′-bithiophene (BCPA-T2). Compound BCPA-
T2 was synthesized as for BCPA and obtained as a light green solid
(69%): mp > 250 °C; 1H NMR (300 MHz, CDCl3, ppm) δ (8H, d, J =
0.9 Hz), 7.61 (4H, d, J = 8.7 Hz), 7.40−7.53 (36H, m), 7.30 (4H, d, J
= 8.4 Hz), 7.22 (2H, d, J = 3.6 Hz), 7.19 (2H, d, J = 3.6 Hz), 1.49
(72H, s); 13C NMR (75 MHz, CDCl3, ppm) δ 146.8, 145.9, 142.8,
139.3, 136.3, 133.2, 129.1, 127.7, 126.7, 125.1, 124.4, 123.5, 116.2,
109.2, 34.5, 32.0; IR (KBr, cm−1) ν 2952, 1507, 1493, 1262, 807;
MALDI-TOF m/z [M + H]+ calcd for C124H125N6S2 1761.941, found
1761.944.
5,5‴-Bis{N,N-bis4-(3,6-di-tert-butylcarbazol-9-yl)phenyl4-
aminophenyl}-2,2′:5′,2″:5″,2‴-quaterthiophene (BCPA-T4).
Compound BCPA-T4 was synthesized as for BCPA and obtained as
1
an orange solid (73%): mp > 250 °C; H NMR (300 MHz, CDCl3,
ppm) δ 8.16 (8H, d, J = 1.2 Hz), 7.60 (4H, d, J = 8.4 Hz), 7.40−7.53
(32H, m), 7.29 (4H, d, J = 8.4 Hz), 7.22 (2H, d, J = 3.7 Hz), 7.19 (2H,
d, J = 3.7 Hz), 7.11 (4H, m), 1.49 (72H, s); 13C NMR (75 MHz,
CDCl3, ppm) δ 146.9, 145.9, 143.0, 142.8, 139.3, 136.4, 133.2, 129.1,
127.7, 126.7, 125.2, 124.4, 123.5, 116.2, 109.2, 34.7, 32.0; IR (KBr,
cm−1) ν 2952, 1507, 1491, 1260, 808; MALDI-TOF m/z [M + H]+
calcd for C132H129N6S4 1925.916, found 1925.911.
J
dx.doi.org/10.1021/jo4008332 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
speed of 3000 rpm for 30 s to create a film thickness of 30−40 nm.
The film thickness was measured using a Tencor π-Step 500 surface
profiler. For the double-layer device, BCP was deposited onto the
surface of the BCPA-Ars film as an electron-transporting layer (ETL)
with a thickness of 40 nm by evaporating from resistively heated
alumina crucibles at an evaporation rate of 0.5−1.0 nm s−1 in vacuum
evaporator deposition (ES280, ANS Technology) under a base
pressure of ≈10−5 mbar. The film thickness was monitored and
recorded by a quartz oscillator thickness meter (TM-350, MAXTEK).
The chamber was vented with dry air to load the cathode materials and
pumped back; a 0.5 nm thick LiF and a 150 nm thick aluminum (Al)
layer were subsequently deposited through a shadow mask on the top
of EML or BCP films without breaking vacuum to form active diode
areas of 4 mm2. The measurement of device efficiency was performed
according to M.E. Thompson’s protocol, and the device external
quantum efficiencies were calculated using procedure reported
previously.36 Current density−voltage−luminescence (J−V−L) char-
acteristics were measured simultaneously using a Keithley 2400 source
meter and a Newport 1835C power meter equipped with a Newport
818-UV/CM calibrated silicon photodiode. The EL spectra were
acquired by an Ocean Optics USB4000 multichannel spectrometer. All
the measurements were performed under ambient atmosphere at room
temperature immediately after breaking the chamber.
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Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
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N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
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Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.;
ASSOCIATED CONTENT
■
S
* Supporting Information
AFM images, DFT calculations data, CV curves, DSC/TGA
thermograms, OLED devices data, and NMR spectra. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: pvinich@sut.ac.th.
Notes
The authors declare no competing financial interest.
̈
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
ACKNOWLEDGMENTS
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.Gaussian 09,
revision A.1; Gaussian, Inc.: Wallingford, CT, 2009.
(15) Ning, Z. J.; Zhou, Y. C.; Zhang, Q.; Ma, D. G.; Zhang, J. J.; Tian,
■
This work was financially supported by the Thailand Research
Fund (RTA538000). We acknowledge the scholarship support
from the Center of Excellence for Innovation in Chemistry, the
Royal Golden Jubilee Ph.D. Program, and the Science
Achievement Scholarship of Thailand (SAST).
H. J. Photochem. Photobiol. A: Chem. 2007, 192, 8.
́ ́
(16) Collado, D.; Casado, J.; Rodríguez Gonzalez, S.; Lopez
Navarrete, J. T.; Suau, R.; Perez-Inestrosa, E.; Pappenfus, T. M.;
Raposo, M. M. Chem.Eur. J. 2011, 17, 498.
(17) Promarak, V.; Punkvuang, A.; Sudyoadsuk, T.; Jungsuttiwong,
S.; Saengsuwan, S.; Keawin, T.; Sirithip, K. Tetrahedron 2007, 83,
8881.
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