A carbazole-oxadiazole diad molecule for single-emitting-component white organic light-emitting devices (WOLEDs)

Article abstract of DOI:10.1016/j.tet.2014.01.073

The synthesis is reported of a new molecule consisting of N-phenylcarbazole linked through a butadiyne bridge to 2,5-diphenyl-1,3,4-oxadiazole. The compound shows blue photoluminescence with positive solvatochromism: e.g., λ_max 401 nm in cyclohexane, 461 nm in chloroform solution (quantum yield Φ_f 0.55) and 428 nm in thin film, with a low intensity band extending to ca. 700 nm in films. OLEDs with a device configuration of [ITO/PEDOT:PSS (50 nm)/molecule:PVK:OXD-7 (80 nm)/Bphen (20 nm)/LiF (0.8 nm)/Al (120 nm)] were fabricated. Pure white-light emission with Commission Internationale De L'Eclairage (CIE) coordinates of (0.313, 0.330) at 13 V and good colour stability with a very small offset of coordinates (0.034, 0.033) under biases of 9-15 V has been achieved.

Full text of DOI:10.1016/j.tet.2014.01.073

Tetrahedron 70 (2014) 2015e2019
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
A carbazoleeoxadiazole diad molecule for single-emitting-
component white organic light-emitting devices (WOLEDs)
,
a
a
b
b
Xiaoming Wu a , Li Wang , Yulin Hua , Changsheng Wang , Andrei S. Batsanov ,
*
Martin R. Bryce b
,
*
a School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, PR China
b Department of Chemistry, Durham University, Durham DH1 3LE, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis is reported of a new molecule consisting of N-phenylcarbazole linked through a butadiyne
bridge to 2,5-diphenyl-1,3,4-oxadiazole. The compound shows blue photoluminescence with positive
solvatochromism: e.g., lmax 401 nm in cyclohexane, 461 nm in chloroform solution (quantum yield Ff
0.55) and 428 nm in thin film, with a low intensity band extending to ca. 700 nm in films. OLEDs with
a device configuration of [ITO/PEDOT:PSS (50 nm)/molecule:PVK:OXD-7 (80 nm)/Bphen (20 nm)/LiF
(0.8 nm)/Al (120 nm)] were fabricated. Pure white-light emission with Commission Internationale De
L’Eclairage (CIE) coordinates of (0.313, 0.330) at 13 V and good colour stability with a very small offset of
coordinates (0.034, 0.033) under biases of 9e15 V has been achieved.
Received 5 November 2013
Received in revised form 15 January 2014
Accepted 27 January 2014
Available online 1 February 2014
Keywords:
Carbazole
Oxadiazole
Fluorescence
Ó 2014 Elsevier Ltd. All rights reserved.
Donoreacceptor system
Organic light-emitting device
1. Introduction
Efficiencies of these devices are often low, although very recently
a peak external quantum efficiency (EQE) of >20%, Commission
The vast majority of organic materials that have been developed
for organic light-emitting diodes (OLEDs)1 emit monochromic light
with a relatively narrow electroluminescent (EL) spectral band.2
Therefore, the broad electroluminescence spectrum required for
white light emission (WOLEDs)3 is usually generated from the
combined emission of three primary colours or two complemen-
tary colours in a complicated device architecture.4e7 However, this
approach is problematic because energy readily transfers from the
higher energy to the lower energy dyes and careful adjustment of
the concentration of each dye is required to achieve a balanced
white emission colour. This energy transfer problem is partly
mitigated if the dyes are segregated into different layers.8 For ex-
ample, Reineke et al. achieved fluorescent tube efficiencies of
81 lm Wꢀ1 at 1000 cd/m2 using lens-based outcoupling (ꢁ2.7 en-
hancement) with three-component primary red, green and blue
(RGB) phosphorescent emitters.3a It is easier to adjust the compo-
sition of a two-chromophore system and a more convenient way to
obtain white emission is to use complementary blue and yellow/
orange emitters.9 WOLEDs have also been fabricated utilising
multiple emissions from monomers and excimers or exciplexes.10
Internationale De L’Eclairage (CIE) coordinates of (0.33, 0.33) and
a colour rendering index (CRI) value of 80 has been reported for an
excimer-based phosphorescent WOLED;10f these data are compa-
rable to state-of-the-art WOLEDs with multiple dopants.
Single-component fluorescent emittersdsmall molecules and
short oligomers11 or polymers12dwith a broad emission spectrum
have only rarely been utilised in WOLEDs. A single white-emitting
molecule offers the advantages of simplified processing, as well as
avoiding differential aging of multiple emitters and morphological
changes upon device operation that adversely affect colour
stability.
We now report on the synthesis, photophysical characterisation
and electroluminescence of the new fluorescent compound 4.
‘Pure’ white light (CIE: 0.313, 0.330) with a high CRI of 84e88 has
been achieved using 4 as a single-component emitter in WOLEDs.
Compound 4 is a donoreacceptor system and is structurally very
distinct from compounds used previously for WOLEDs.
2. Results and discussion
2.1. Molecular design, synthesis and characterisation
Carbazole (Cz)13 and diaryl-1,3,4-oxadiazole (OXD)14 are well
known as thermally stable, hole- and electron-transporting
* Corresponding authors. E-mail addresses: wxm@mail.nankai.edu.cn (X. Wu), m.
r.bryce@durham.ac.uk (M.R. Bryce).
0040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2014.01.073

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X. Wu et al. / Tetrahedron 70 (2014) 2015e2019
materials, respectively, and their derivatives have been widely used
as emitter and/or charge-transport layers in OLEDs. Bipolar co-
valent Cz-OXD diad molecules have received attention as blue and
green emitters15 but they have not been used previously as white-
light emitters. The novel feature of compound 4 is the incorporation
of the rigid, conjugated butadiyne linker between the Cz and OXD
units. The synthesis of 4 via Sonogashira cross-coupling of 2 and 3 is
shown in Scheme 1. The key reagents 116 and 317 have been re-
ported previously.
in film emission, intermediate between non-polar and polar sol-
vents, has been reported previously for dipolar carbazole de-
rivatives.20 For films of 4 a low intensity tail in the emission band
extends to ca. 700 nm, which is not observed in the solution spectra.
This has precedent in previous work and is generally ascribed to p-
stacking of the molecules20 and/or excimers in the film.11c
Fig. 2. UVevis absorption and emission spectra for compound 4.
Scheme 1. Reagents, conditions and yields: i) N-bromosuccinimide, AgNO3, acetone,
22 ꢂC, 77% yield; ii) 3, Pd(PPh3)2Cl2, CuI, triethylamine, THF, 22 ꢂC to 50 ꢂC, 42% yield.
For OLED studies (see below) poly(N-vinylcarbazole) (PVK) was
used as the host material. Fig. 3 shows the transient photo-
luminescence decay curves for films of neat 4, and 4 (3% by weight)
blended in PVK. The decays are exponential with fluorescence
lifetimes of 38.0 ns (neat film) and 51.5 ns (4:PVK). These data in-
dicate that the quenching of excitons in the 4:PVK blend film is
significantly reduced, and partial energy transfer exists between
the host and dopant.21 Atomic force microscopy (AFM) images re-
veal that the 4:PVK blend film has a lower surface roughness (Ra:
0.303) compared to the neat 4 film (Ra: 0.603) as shown in Fig. 4.
The PVK should thus facilitate the transport of charge carriers at the
interfaces of the emitter:PVK/transport layers in OLEDs.
The structure of compound 4 was unambiguously established by
NMR spectroscopy, mass spectrometry, elemental analysis and
single-crystal X-ray crystallography of 4$1/4 PhMe (Fig. 1). Moder-
ately twisted diphenyloxadiazole moieties (iiieivev) form a slanted
ꢀ
pe
p
stack with mean interplanar separations of ca. 3.6 A, whereas
effective stacking of carbazole moieties i is prevented by almost
perpendicular orientation of the adjacent benzene ring ii (see SD
for an additional figure).
2.3. Electroluminescence (EL) properties of OLEDs of 4
For OLED fabrication PVK was used as the host for the emitter,
not only to enhance the EL performance but also to improve the
film uniformity of the emitting layer assembled by spin-coating.22
The device architecture used is: [ITO/PEDOT:PSS (50 nm)/
4:PVK:OXD-7 (80 nm)/Bphen (20 nm)/LiF (0.8 nm)/Al (120 nm)].
For these devices, PEDOT:PSS is a hole transport layer (HTL); the
emitting layer is PVK doped compound 4 and 1,3-bis[(4-tert-
butylphenyl)-1,3,4-oxadiazolyl]phenylene (OXD-7); 4,7-diphenyl-
1,10-phenanthroline (BPhen) serves as an additional electron
transport layer adjacent to the LiF/Al cathode.
Fig. 1. X-ray molecular structure of 4. Interplanar angles (ꢂ): i/ii 62.7, ii/iii 87.8, iii/iv 6.5,
iv/v 11.1, iii/v 17.6.
2.2. Photophysical studies
The EL spectra of the WOLED under biases of 9e15 V are illus-
trated in Fig. 5, with the two main peaks at lmax ca. 450 and 575 nm
corresponding to blue and orange-red emission. The peak width at
half height covers a wide wavelength range of ca. 430e640 nm,
which results in a high colour rendering index (CRI) of 84e88 for
the white light emission. As the driving voltage is increased from 9
to 15 V, the higher energy emission band rises in relative intensity
until ca. 12e14 V and then begins to reduce. On the contrary, the
lower energy emission displays the opposite trend. Fig. 6 depicts
the variation of CIE coordinates as the applied voltage varies from 9
to 15 V. The best coordinates of (0.313, 0.330) are achieved at 13 V
and they locate very close to the equal-energy white point of (0.333,
0.333) with a very small offset of (0.034, 0.033) under the various
biases. These data demonstrate that pure white emission and
Normalised absorption and photoluminescence spectra for
compound 4 are shown in Fig. 2 and the data are collated in Table 1.
The absorption spectra in cyclohexane, chloroform and thin film
have very similar profiles with a strong band at ca. 340 nm and
a lower intensity shoulder at ca. 375 nm. The main absorption fea-
tures are attributed to
pep* transitions, following literature pre-
cedents for these types of compound.18 Blue to green emissions are
observed in solutions (lmax: cyclohexane 401, chloroform 461,
dichloromethane 487, acetonitrile 549 nm). The significant positive
solvatochromism is consistent with dipolar interactions of the
donorep
eacceptor molecule 4 with the polar solvent.19 In the thin
film state the lmax of photoluminescence (428 nm) is located be-
tween that observed in non-polar and polar solvents. A similar shift

X. Wu et al. / Tetrahedron 70 (2014) 2015e2019
2017
Table 1
Photophysical data for 4
Abs, cyclohexane, lmax/nm
Abs, chloroform, lmax/nm
(ε Mꢀ1 cmꢀ1
340 (2.19ꢁ105)
Abs, thin film, lmax/nm
Em, cyclohexane, lmax/nm
401; Ff 0.92
Em, chloroform, lmax/nm
461; Ff 0.55
Em, thin film, lmax/nm
428
(ε Mꢀ1 cmꢀ1
)
)
338 (1.77ꢁ105)
340
Fig. 3. Transient photoluminescence decays of a film of neat 4 (black) and 4 (3% by
weight) blended in PVK film (red) at room temperature; lexc 340 nm.
Fig. 6. The CIE chromaticity diagram at driving voltages of 9e15 V.
brightness of 32 cd/m2 at 20 V, and a maximum current efficiency
of 0.03 cd/A at 25.1 mA/cm2. We rationalise this as follows. Holes
transported from the PEDOT:PSS could be easily injected into the
EML as the host PVK has hole transport character. However, the
lowest occupied molecular orbital (LUMO) of PVK is 2.2 eV,23which
is much higher than that of the ETL (Bphen) (2.9 eV).24 Even with
this ETL layer, the efficiency of electron injection into the EML will
be low due to the high energy barrier of 0.7 eV between EML and
ETL, which results in unbalanced charge carrier distribution in the
EML. For this reason, OXD-7 with a lower LUMO of 3.0 eV,25 was
blended into the emitting layer to facilitate the electron injection
from the ETL to EML via the lower energy barrier of 0.1 eV. The
balance of the carriers is improved accordingly, which will result in
an enhancement of the recombination rate for the holeeelectron
pairs. Based on this approach, the optimum WOLED presented
a lower turn-on voltage of 8 V at 1.7 cd/m2, and a maximum
brightness of 116 cd/m2 was achieved at 14.5 V, as shown in the
current densityevoltageeluminance (JeVeL) curves (Fig. 7). The
device exhibited a maximum current efficiency of 0.12 cd/A at
12.5 mA/cm2 (10 V) as shown in Fig. 8. The inset of Fig. 8 is a pho-
tograph of the WOLED at 10 V; pure white light emission with
a uniform emitting area is seen.
Fig. 4. Atomic force microscopy images of (a) ITO/compound 4 and (b) ITO/compound
4 (3% by weight) blended in PVK.
Fig. 5. Normalised EL spectra of the white OLEDs [ITO/PEDOT:PSS/4:PVK:OXD-7/
Bphen/LiF/Al] under driving voltages of 9e15 V.
remarkably good colour stability have been achieved using com-
pound 4. These data also suggest that a balanced distribution of
holes and electrons has been achieved within the recombination
zone in the emitting layer. This could be facilitated by the
donoreacceptor (bipolar) structure of the emitter.
Devices were initially fabricated without the addition of OXD-7
into the emitting layer (EML) and although pure white light could
be achieved, the device exhibited only a relatively weak perfor-
mance with a high turn-on voltage of 12 V at 1.8 cd/m2, a maximum
Fig. 7. The current densityevoltageeluminance (JeVeL) curves of the white OLED.

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X. Wu et al. / Tetrahedron 70 (2014) 2015e2019
4.2. Synthesis of materials
4.2.1. 2-(4-tert-Butylphenyl)-5-[4-(2-bromo-ethynyl)phenyl]-1,3,4-
oxadiazole (2). To the solution of 2-(4-tert-butylphenyl)-5-(4-
ethynylphenyl)-1,3,4-oxadiazole (1)16 (1.24 g, 4.1 mmol) in ace-
tone (50 cm3) was added N-bromosuccinimide (NBS) (0.80 g,
4.49 mmol) and silver nitrate (0.10 g, 5.9 mmol). The mixture was
stirred at 22 ꢂC for 1 h to yield a pale-yellow suspension. The
precipitate was removed by suction filtration through a Celite pad
and the filtrate was vacuum concentrated to dryness. The solid
residue was recrystallised from ethanol to afford product 2 as off-
white crystals (1.21 g, 77%). Elemental analysis (%) calcd for
C
20H17BrN2O: C, 63.00; H, 4.49; N, 7.35. Found: C, 62.84; H, 4.46; N,
7.34. HR-MS (ASAPþ) m/z calcd for [M]þ 380.0524, found m/z:
380.0526. 1H NMR (CDCl3, 400 MHz)
¼1.36 (s, 9H), 7.54 (d, J 8.8 Hz,
2H), 7.58 (d, J 8.8 Hz, 2H), 8.04 (d, J 8.4 Hz, 2H), 8.07 (d, J 8.8 Hz, 2H).
13C NMR (CDCl3, 100 MHz)
¼31.3, 35.3, 79.6, 121.1, 124.1, 126.2,
126.3, 126.9, 127.0, 132.8, 155.7, 164.0, 165.1.
d
Fig. 8. The current efficiencyecurrent density curve of the white OLED. Inset: a pho-
tograph of a WOLED operating at 10 V.
d
The strong orange-red emission band in the EL (Fig. 5), which is
not observed in PL, is consistent with the formation of electric
field induced electromers during device operation.3d,11a,c,e This
lower energy band has not been observed in OLEDs of previous
covalent Cz-OXD diad molecules, all of which have more twisted
backbone structures than 4. The molecular rigidity imparted by
the butadiyne linkage in 4, may, therefore, be a key molecular
design feature for achieving white emission in this family of
compounds.
4.2.2. Compound
4. [4-(3,6-Di-tert-butyl-N-carbazolyl)]phenyl-
acetylene (3)17 (199 mg, 0.524 mmol), compound 2 (191 mg,
0.5 mmol), Pd(PPh3)2Cl2 (15 mg, 0.02 mmol) and CuI (5 mg,
0.026 mmol) were mixed with dry THF (10 cm3) and triethylamine
(10 cm3). The mixture was degassed by bubbling argon for 20 min
then stirred at 22 ꢂC for 12 h. The temperature was raised to 50 ꢂC
and the reaction was continued for an additional 2 h. The resultant
yellow suspension was vacuum evaporated to dryness and the
residue was column chromatographed on silica eluted with
DCMe2% diethyl ether to yield a yellow solid. The solid was
recrystallised from ethanol followed by a second recrystallisation
from tolueneeacetonitrile mixture to yield compound 4 as yellow
crystals (142 mg, 42%). Elemental analysis (%) calcd for C48H45N3O:
C, 84.80; H, 6.67; N, 6.18. Found: C, 84.97; H, 6.71; N, 5.78. HR-MS
(ASAPþ) m/z calcd for [M]þ 679.3563, found m/z: 379.3566. 1H
3. Conclusions
The synthesis, characterisation and properties of 4 as a single-
component WOLED fluorescent emitter have been described. The
OLED presents a promising combination of ‘pure’ white-light
emission with CIE coordinates of (0.313, 0.330) at 13 V and very
good colour stability under the various applied biases. The corre-
sponding high CRI value is in the range 84e88. To place this work in
a broader context, devices based on compound 4 demonstrate
a good trade-off between simple device architecture, efficiency, CRI
and colour stability, with WOLED data that are comparable with
recent values for other small-molecule single-component emit-
ters.11 Compound 4 is structurally very distinct from compounds
used previously for white-light emission. Our future research will
include systematic modifications to the chemical structure and
detailed photophysical studies to provide further understanding of
this interesting class of emitters.
NMR (CDCl3, 500 MHz)
d¼1.40 (s, 9H), 1.49 (s, 18H), 7.42 (d, J 8.5 Hz,
2H), 7.50 (dd, J1 8.5 Hz, J2 2.0 Hz, 2H), 7.58 (d, J 8.0 Hz, 2H), 7.60 (d, J
8.5 Hz, 2H), 7.72 (d, J 8.0 Hz, 2H), 7.77 (d, J 8.5 Hz, 2H), 8.09 (d, J
8.0 Hz, 2H), 8.16 (m 4H). 13C NMR (CDCl3, 125 MHz)
d¼31.1, 32.0,
34.7, 35.1, 74.3, 76.7, 80.9, 82.5,109.2,116.3,119.5,120.8,123.7,123.8,
124.2, 125.0, 126.1, 126.2, 126.78, 126.82, 133.1, 134.0, 138.6, 139.3,
143.4, 155.6, 163.7, 164.9. IR (solid)
n 2957, 2182, 2160, 1600, 1515,
1488, 1456, 1294, 1265, 847, 809 cmꢀ1. X-ray crystal structure: CCDC
number 956292.
4.3. Photophysical characterisation
4. Experimental section
4.1. General information
Photoluminescence (PL) and absorption (Abs.) spectra of com-
pound 4 dissolved in solvents were measured with a Hitachi F-4500
fluorescence spectrophotometer and a Hitachi U-4100 UVeviseNIR
spectrophotometer. The PL transient decay characteristics in films
[film 1: compound 4 2 wt % in CHCl3 (60 nm), film 2: compound 4:
PVK (1:2, 5 wt % in CHCl3) (60 nm)] were measured using a Jobin
Yvon FL3-212-TCSPC fluorospectrophotometer. With an excitation
laser source of 340 nm, all the PL transient decay curves of films
were monitored at 430 nm. The photoluminescence quantum yield
data were obtained using rhodamine B as standard.26
All reactions were conducted under a blanket of argon, which
was dried by passage through a column of phosphorus pentoxide.
All commercial chemicals were used without further purification.
Anhydrous solvents were prepared using an Innovative Technology
Inc. solvent purification system. Column chromatography was
carried out using 40e60 mm mesh silica (Fluorochem). NMR spectra
were recorded on: Bruker Avance-400, Varian VNMRS 700
and Varian Inova 500 spectrometers. Chemical shifts are reported
in parts per million relative to tetramethylsilane (0.00 ppm).
Melting points were determined in open-ended capillaries using
a Stuart Scientific SMP40 melting point apparatus at a ramping
rate of 2 ꢂC/min. Mass spectra were measured on a Waters
Xevo OTofMS with an ASAP probe. Elemental analyses were
obtained on an Exeter Analytical Inc. CE-440 elemental analyser. IR
spectra were recorded on a Perkin Elmer FT-IR Paragon 1000
spectrometer.
4.4. Device fabrication and characterisation
All devices were prepared on patterned indium tin oxide (ITO)
coated glass substrate with a sheet resistance of 20
U/square. The
substrates were treated in an ultraviolet-ozone chamber after the
following pre-clean procedure: ultrasonically cleaned with de-
tergent, acetone, ethanol and deionised water. The hole transport
polymer PEDOT:PSS was spin-coated with a thickness of 50 nm.
After heat treatment, the emitting polymer consisting of PVK doped

X. Wu et al. / Tetrahedron 70 (2014) 2015e2019
6. D’Andrade, B. W.; Holmes, R. J.; Forrest, S. R. Adv. Mater. 2004, 16, 624.
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with compound 4 and OXD-7 at the optimal weight ratio of [12:7:3
w/w] in a concentration of 10 mg/ml CHCl3 solution was spin-
coated at 1200 rpm (low speed) for 5 s and at 4000 rpm (high
speed) for 30 s, respectively, which resulted in a film thickness of
80 nm. The ITO substrate coated with PEDOT:PSS and the emitting
layer was cured on a hot plate in a glove box to remove solvent at
100 ꢂC for 30 min. Then Bphen, LiF and Al was deposited onto the
above substrate sequentially without breaking the vacuum envi-
ronment at a base pressure <2ꢁ10ꢀ4 Pa. The evaporation rates
were 0.2 nm/s, 0.07 nm/s and 1 nm/s for the organic layer, LiF layer
and Al cathode, respectively. An active emitting area of 3ꢁ3 mm2
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was
determined
by
a
shadow
mask.
The
cur-
rentevoltageeluminance (JeVeL) characteristics were recorded
with a PR650 spectra scan spectrometer and a Keithley 2400 pro-
grammable voltageecurrent source. The electroluminescent spec-
tra, CIE coordinates and CRI of the devices were measured using
a PR650 spectra scan spectrometer. All the measurements were
carried out in ambient atmosphere at room temperature.
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This work was supported by the National Natural Science
Foundation of China (Grant No. 60906022), Scientific Developing
Foundation of Tianjin Education Commission, China (Grant No.
2011ZD02), and Tianjin Natural Science Council (Grant No.
10SYSYJC28100). The UK authors acknowledge the support of
EPSRC.
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