Functional derivatives of (bi)phenyl-substituted carbazoles as building blocks for electro-active polymers

Article abstract of DOI:10.1016/j.reactfunctpolym.2010.03.007

(Bi)phenyl-substituted carbazoles containing reactive functional groups were synthesized by the multi-step synthetic rout. The monomers were examined by various techniques including thermogravimetry, differential scanning calorimetry, UV and fluorescence spectrometry as well as electron photoemission technique. These derivatives were also tested as hole transporting materials in bilayer OLEDs with Alq₃ as the emitter. The devices exhibited promising overall performance with a turn-on voltage of ～3 V, a maximal photometric efficiency of 5.1 cd/A and maximum brightness of 12,200-15,600 cd/m².

Full text of DOI:10.1016/j.reactfunctpolym.2010.03.007

Reactive & Functional Polymers 70 (2010) 477–481
Contents lists available at ScienceDirect
Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Functional derivatives of (bi)phenyl-substituted carbazoles as building blocks
for electro-active polymers
b
b
S. Lengvinaite a, J.V. Grazulevicius a, S. Grigalevicius a, , B. Zhang , Z. Xie
*
a Department of Organic Technology, Kaunas University of Technology, Radvilenu Plentas 19, LT50254 Kaunas, Lithuania
b State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
a r t i c l e i n f o
a b s t r a c t
Article history:
(Bi)phenyl-substituted carbazoles containing reactive functional groups were synthesized by the multi-
step synthetic rout. The monomers were examined by various techniques including thermogravimetry,
differential scanning calorimetry, UV and fluorescence spectrometry as well as electron photoemission
technique. These derivatives were also tested as hole transporting materials in bilayer OLEDs with Alq3
as the emitter. The devices exhibited promising overall performance with a turn-on voltage of ꢀ3 V, a
maximal photometric efficiency of 5.1 cd/A and maximum brightness of 12,200–15,600 cd/m2.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 25 February 2010
Received in revised form 18 March 2010
Accepted 20 March 2010
Available online 27 March 2010
Keywords:
Electroactive material
Reactive functional group
Monomer
Ionization potential
Light emitting diode
1. Introduction
2. Experimental
Charge-transporting organic materials capable of forming
amorphous films are favoured for various applications such as of
electrophotographic devices [1], organic light emitting diodes
(OLEDs) [2,3], photovoltaic cells [4] and photorefractive materials
[5,6].
2.1. Instrumentation
1H NMR spectra were recorded using a Varian Unity Inova
(300 MHz) apparatus. Mass spectra were obtained on a Waters
ZQ 2000 spectrometer. FTIR spectra were recorded using a Perkin
Elmer FT-IR System. UV spectra were measured with a Spectronic
Genesys™ 8 spectrometer. Fluorescence (FL) spectra were recorded
with a MPF-4 spectrometer. Differential scanning calorimetry
(DSC) measurements were carried out using a Bruker Reflex II ther-
mosystem. Thermogravimetric analysis (TGA) was performed on a
Netzsch STA 409. The TGA and DSC curves were recorded in a
nitrogen atmosphere at a heating rate of 10 °C/min.
The ionization potentials of the layers of the compounds syn-
thesized were measured by the electron photoemission method
in air, which was described earlier [12]. The measurement method
was, in principle, similar to that described by Miyamoto et al. [13].
The samples for the ionization potential measurements were pre-
pared as follows [14]. The materials were dissolved in THF and
Carbazole-containing polymers and low-molecular-weight
derivatives are among the most studied materials for optoelec-
tronic and electronic applications due to their high hole mobility
and excellent photoconductive properties [7]. Some carbazole
based materials have been commercialized in a number of devices
and processes (photocopy machines, laser printers, etc.) [8]. We
have synthesized series of hole-transporting carbazole and
indolo[3,2-b]carbazole derivatives [9,10]. We have observed that
phenyl substituted indolo[3,2-b]carbazole-based derivatives dem-
onstrate better charge transporting properties than the derivatives
containing unsubstituted indolo[3,2-b]carbazole fragments [11].
In this work, we have designed and synthesized phenyl or
biphenyl-substituted carbazole derivatives, which were expected
to show enhanced hole injection and transport properties. On the
other hand, the compounds contain reactive oxetanyl moieties
and could be polymerized in solution to yield electro-active
polymers.
were coated on Al plates pre-coated with ꢀ0.5
lm thick methyl-
methacrylate and methacrylic acid copolymer (MKM) adhesive
layer. The function of this layer is not only to improve adhesion,
but also to eliminate the electron photoemission from Al layer. In
addition, the MKM layer is conductive enough to avoid charge
accumulation on it during the measurements. The thickness of
* Corresponding author.
the layers was 0.5–1 lm.
E-mail address: saulius.grigalevicius@ktu.lt (S. Grigalevicius).
1381-5148/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.reactfunctpolym.2010.03.007

478
S. Lengvinaite et al. / Reactive & Functional Polymers 70 (2010) 477–481
The multilayer electroluminescent devices were fabricated on
(0.09 mmol) of PdCl2(PPh3)2 and 0.51 g (13.2 mmol) of powdered
potassium carbonate were stirred in 7 ml of THF containing water
(0.25 ml) at 80 °C under nitrogen for 24 h. After TLC control the
reaction mixture was cooled and quenched by the addition of ice
water. The product was extracted by ethyl acetate and the com-
bined extract was dried over anhydrous MgSO4. The crude product
was purified by silica gel column chromatography using the mix-
ture of ethyl acetate and hexane (vol. ratio 1:1) as an eluent. Yield:
0.4 g (76%) of white crystals. M.p.: 228–229.5 °C.
glass substrates and had the typical structure with the organic lay-
ers sandwiched between a bottom ITO anode and a top metal cath-
ode. Before use in device fabrication, the ITO-coated glass
substrates were carefully cleaned and treated with UV/ozone right
before deposition of the organic layers. PEDOT layers were depos-
ited by spin-coating and heated at 120 °C for 30 min. It was de-
scribed earlier that these conditions are suitable for preparation
of PEDOT layers with optimum electrical properties [15]. The hole
transporting layers (HTL) were prepared by spin-coating a 25 nm
layer of the derivatives 5 or 7. Tris(quinolin-8-olato)aluminium
(Alq3) was used as green light emitter. Evaporation of Alq3 as well
as of LiF/Al cathode was done at a pressure of 3 Â 10À6 mbar in
vacuum evaporation equipment.
1H NMR spectrum (300 MHz, CDCl3, d, ppm): 8.33–8.11 (m, 2H,
Ar), 7.74–7.20 (m, 14H, Ar), 4.76 (d, 2H, CH2 of oxetane ring,
J = 6 Hz), 4.44 (s, 2H, CH2N), 4.34 (d, 2H, CH2 of oxetane ring,
J = 6 Hz), 1.43 (s, 3H, CH3). MS (APCI+, 20 V), m/z (%): 404.5
([M+H]+, 100). IR (KBr,
m
, cmÀ1): 3051 (aromatic CÀH), 2955;
The current–voltage and luminance–voltage characteristics
were recorded under forward bias using a computer controlled
Keithley 2400 source meter and a PR650 Spectrometer. All mea-
surements were performed at ambient conditions in air [16].
2866 (aliphatic CÀH), 1477; 1461 (CÀN), 977 (CÀOÀC in oxetane
ring), 808, 766(CH@CH of phenyl group).
2.2.2. 3,6-Di(4-biphenyl)-9-(3-methyloxetan-3-ylmethyl)carbazole
(6).
2.2. Materials
0.5 g
(1 mmol)
of
3,6-diiodo-9-(3-methyloxetan-3-
ylmethyl)carbazole (4), 0.6 g (3 mmol) of biphenyl-4-boronic acid,
0.05 g (0.07 mmol) of PdCl2(PPh3)2 and 0.4 g (10 mmol) of pow-
dered potassium carbonate were stirred in 5 ml of THF containing
water (0.25 ml) at 80 °C under nitrogen for 24 h. After TLC control
the reaction mixture was cooled and quenched by addition of ice
water. The product was extracted by ethyl acetate. The combined
extract was dried over anhydrous MgSO4. The crude product was
purified by silica gel column chromatography using the mixture
of ethyl acetate and hexane (vol. ratio 1:1) as an eluent. Yield:
0.3 g (54%) of white crystals. M.p.: 248–249 °C.
9H-carbazole, 4-biphenylboronic acid, phenylboronic acid 1,3-
propanediol ester, bis(triphenylphosphine)palladium(II) dichloride
(Pd(PPh3)2Cl2), tetrabutylammonium hydrogen sulphate (TBAHS),
Alq3, K2CO3 and potassium hydroxide were purchased from Aldrich
and used as received.
3-Bromomethyl-3-methyloxetane was received from Chemada
Fine Chemicals and used without further purification.
3-Iodo-9H-carbazole (1) and 3,6-diiodo-9H-carbazole (2) were
obtained by a procedure of Tucker [17].
3-Iodo-9-(3-methyloxetan-3-ylmethyl)carbazole (3) and 3,6-
diiodo-9-(3-methyloxetan-3-ylmethyl)carbazole (4) were pre-
pared by the reaction of 3-iodo-9H-carbazole (1) or 3,6-diiodo-
9H-carbazole (2) with large excess of 3-bromomethyl-3-meth-
yloxetane under basic conditions in the presence of the phase
transfer catalyst – TBAHS. The synthesis was described earlier
[18,19].
1H NMR spectrum (300 MHz, CDCl3, d, ppm): 8.43 (s, 2H, Ar),
7.83–7.65 (m, 14H, Ar), 7.51–7.33 (m, 8H, Ar), 4.82 (d, 2H, CH2 of
oxetane ring, J = 7 Hz), 4.52 (s, 2H, CH2N), 4.41 (d, 2H, CH2 of oxe-
tane ring, J = 7 Hz), 1.50 (s, 3H, CH3). MS (APCI+, 20 V), m/z (%): 556
([M+H]+, 100). IR (KBr,
(aliphatic CÀH), 1476; 1461 (CÀN), 978 (CÀOÀC in oxetane ring),
m
, cmÀ1): 3052 (aromatic CÀH), 2955; 2867
809, 766 (CH@CH of phenyl group).
2.2.1. 3-(4-Biphenyl)-9-(3-methyloxetan-3-ylmethyl)carbazole (5)
0.5 g (1.32 mmol) of 3-iodo-9-(3-methyloxetan-3-ylmethyl)
carbazole (3), 0.39 g (2 mmol) of biphenyl-4-boronic acid, 0.064 g
2.2.3. 3,6-Diphenyl-9-(3-methyloxetan-3-ylmethyl)carbazole (7).
0.7 g
(1.4 mmol)
of
3,6-diiodo-9-(3-methyloxetan-3-
ylmethyl)carbazole (4), 0.68 g (4.2 mmol) of phenylboronic acid
O
O
O
OH
B
H
N
N
OH
Br
N
I
I
( b )
( a )
5
3
1
O
OH
N
B
OH
O
O
H
N
6
Br
N
( b )
I
I
I
I
( a )
4
O
2
( b )
N
O
B
O
7
Scheme 1. (a) KOH, TBAHS and (b) PdCl2(PPh3)2, K2CO3.

S. Lengvinaite et al. / Reactive & Functional Polymers 70 (2010) 477–481
479
1,3-propanediol ester, 0.06 g (0.1 mmol) of PdCl2(PPh3)2 and 0.54 g
(14 mmol) of powdered potassium carbonate were stirred in 10 ml
of THF containing water (0.5 ml) at 80 °C under nitrogen for 18 h.
After TLC control the reaction mixture was cooled and quenched
by the addition of ice water. The product was extracted by ethyl
acetate. The combined extract was dried over anhydrous MgSO4.
The crude product was purified by silica gel column chromatogra-
phy using the mixture of ethyl acetate and hexane (vol. ratio 1:1)
as an eluent. Yield: 0.35 g (62%) of white crystals. M.p.: 168–
169.5 °C.
1H NMR spectrum (300 MHz, CDCl3, d, ppm): 8.28 (s, 2H, Ar),
7.67–7.61 (m, 6H, Ar), 7.43–7.25 (m, 8H, Ar), 4.71 (d, 2H, CH2 of
oxetane ring, J = 7 Hz), 4.6 (s, 2H, CH2N), 4.30 (d, 2H, CH2 of oxetane
ring, J = 7 Hz), 1.38 (s, 3H, CH3). MS (APCI+, 20 V), m/z (%): 404
([M+H]+, 100). IR (KBr,
m
, cmÀ1): 3054 (aromatic CÀH), 2959;
2867 (aliphatic CÀH), 1476; 1462 (CÀN), 978 (CÀOÀC in oxetane
ring), 812, 762 (CH@CH of phenyl group).
3. Results and discussion
The synthesis of oxetanes containing phenyl- or biphenyl-
substituted carbazole rings (5–7) was carried out by a multi-step
synthetic route as shown in Scheme 1. Iodo-derivatives of carba-
zole (1 and 2) as key materials were synthesized from commer-
cially available 9H-carbazole by Tucker iodination [17] with KI/
KIO3 in acetic acid. The compounds 1 and 2 were converted to oxe-
tanyl-functionalized derivatives 3 and 4 by reaction with large ex-
cess of 3-bromomethyl-3-methyloxetane under basic conditions in
the presence of a phase transfer catalyst. The monomers 5–7 were
obtained from 3-iodo-9-(3-methyloxetan-3-ylmethyl)carbazole or
3,6-diiodo-9-(3-methyloxetan-3-ylmethyl)carbazole by Suzuki
reaction with derivatives of boronic acid, i.e. 4-biphenylboronic
acid or phenylboronic acid 1,3-propanediol ester.
(a)
T
m = 218 oC
1st heating
2nd heating
Cooling
Tg = 116 oC
150
All the newly synthesized monomers were identified by mass
spectrometry, IR-, and 1H NMR spectroscopy. The data were found
to be in good agreement with the proposed structures. The
materials 5–7 were soluble in common organic solvents, such as
chloroform and THF at room temperature. Transparent thin films
50
100
200
250
Temperature (ºC)
(b)
(a)
7
6
5
Tm = 179 oC
EtCr
1st heating
2nd heating
Tg = 72 oC
Cooling
200 240
0
40
80
120
160
240
280
320
360
400
Temperature (ºC)
Wavelength, nm
Tm = 258 oC
(c)
(b)
2nd heating
o
T
= 199 C
cr
6
5
7
Cooling
1st heating
330
360
390
420
450
480
30
60
90
120
150
180
210
240
270
Wavelength (nm)
Temperature (ºC)
Fig. 2. UV absorption (a) and FL (b) spectra of dilute THF solutions of the
compounds 5–7, kex = 290 nm.
Fig. 1. DSC curves of compounds 5 (a), 7 (b) and 6 (c). Heating rate: 10 °C/min.

480
S. Lengvinaite et al. / Reactive & Functional Polymers 70 (2010) 477–481
of these materials could be prepared by spin-coating from
solutions.
applied the bright green electroluminescence of Alq3 was observed
with an emission maximum at around 520 nm [24,25]. This implies
that the hole mobility in the HT layers of 5 and 7 is fully sufficient
for an effective charge carrier recombination occurring within the
Alq3 layer. No exciplex formation at the interface between HT and
Alq3 layer was observed.
Fig. 3 shows current–voltage and luminance–voltage character-
istics as well as the efficiencies for the OLEDs containing the HTL of
5 and 7. The characteristics of a device without HTL are presented
for the comparison. The devices containing 5 or 7 exhibit turn-on
The behaviour under heating of compounds 5–7 was studied by
DSC and TGA under a nitrogen atmosphere. The compounds dem-
onstrated high thermal stability. The onsets of mass loss were
360 °C for 5, 345 °C for 6 and 358 °C for 7, as confirmed by TGA
with a heating rate of 10 °C/min.
Compounds 5–7 were obtained as crystalline materials. When
the crystalline samples were heated, endothermic peaks due to
melting were observed in the region from 179 °C to 258 °C by
DSC. However, the melt samples of 5 and 7 readily formed glasses
when they were cooled on standing in air or with liquid nitrogen.
The DSC thermo-grams of 7 are shown in Fig. 1b. When the crystal-
line sample of 7 was heated, an endothermic peak due to solid–so-
lid phase transition was observed at 160 °C, where the crystals
were transformed into a different crystal form. On further heating,
the crystals melted at 179 °C to give an isotropic liquid. This is not
the first observation of polymorphism in the compounds which are
able to form glass. Earlier assumption of the different crystalline
forms of the similar glass-forming materials was described by Shi-
rota [20]. When the isotropic liquid was rapidly cooled, glass was
formed. When the amorphous sample was heated, the glass-transi-
tion was observed at 72 °C, and on further heating no peaks due to
crystallisation and melting appeared. The crystalline sample of
compound 5 melted on first heating at 218 °C and formed glass
(Tg = 116 °C) upon cooling (Fig. 1a).
(a) 1000
100
10
1
without HTL
5
7
0.1
0.01
1E-3
1E-4
0
2
4
6
8
10
12
14
Derivative 6 did not form glass. The DSC thermo-grams of 6 are
shown in Fig. 1c. When the crystalline sample was heated, an
endothermic melting peak was observed at 258°. When the melted
sample was cooled, the crystallisation was observed at 199 °C to
give the same crystals as obtained after synthesis, which melted
at 258 °C.
UV absorption and FL spectra of dilute solutions of the com-
pounds synthesized are shown in Fig. 2. For the comparison, the
UV spectra of 9-ethylcarbazole (EtCr) are given in the fiure. Com-
pounds 5–7 exhibit a broad absorption with kmax in the range of
250–330 nm and demonstrate a red shift of the UV absorption with
respect of that of EtCr. These red shifts show that phenyl or biphe-
nyl-substituted carbazole core has more extended conjugation and
Voltage(V)
(b)10000
1000
100
10
without HTL
5
7
1
that
p electrons are de-localized over the fragments.
The FL emission spectra of the synthesized derivatives also
show a red shift with respect of the EtCr spectrum [14]. The fluo-
rescence spectra of 5–7 have similar shape, however the spectra of
biphenyl substituted derivatives are slightly red shifted by ca. 3–
6 nm. The analysis of the UV and FL spectra suggests that layers
of (bi)phenyl-substituted carbazoles could have lower ionization
potentials and better hole injection properties than that of deriva-
tives containing unsubstituted carbazole rings [21].
0.1
0
2
4
6
8
10
12
14
Voltage (V)
6
(c)
The ionization potentials (Ip) of layers of the compounds syn-
thesized were measured by the electron photoemission method.
It was established that the Ip values of the layers range from 5.75
to 5.8 eV. It is evident that Ip of the derivatives are only slightly
lower than that of compounds containing unsubstituted carbazole
rings [22]. The Ip values demonstrate that these layers would be
suitable for application in optoelectronic devices. Positive charges
could be injected into the layers of 5–7 from compounds widely
used in electrophotography, such as titanyl phthalocyanines, pery-
lene derivatives, and bisazo pigments, which have Ip reaching
5.6 eV [23]. Compounds 5–7 can also be applicable as hole trans-
porting (HT) layers in multilayer electroluminescent devices.
The derivatives 5 and 7, which form thin amorphous films, were
tested in OLEDs as HT materials. The two layer OLED devices were
prepared using these compounds for the HT layers and Alq3 for the
electroluminescent/electron transporting layers. PEDOT was used
as a hole injecting layer. The cathode used was aluminium with a
thin LiF electron injection layer. When a positive voltage was
5
4
3
2
1
without HTL
5
7
0
0.1
1
10
100
1000
Current density(mA/cm2)
Fig. 3. OLED characteristics of the devices with the configuration: ITO/PEDOT/
compound 5 or 7/Alq3/LiF/Al; (a) current density–voltage; (b) brightness–voltage;
and (c) efficiency–current density plots.

S. Lengvinaite et al. / Reactive & Functional Polymers 70 (2010) 477–481
[4] K.M. Coakley, M.D. McGehee, Chem. Mater. 16 (2004) 4533.
481
voltages of ca. 3 V (defined as the voltage where electrolumines-
cence becomes detectable), a maximum brightness of 12,200–
15,600 cd/m2 (at 11–12 V) and a photometric efficiency of about
5.1 cd/A. It should be stated that the efficiency of the devices shows
an only a moderate drop in the observed current density window
up to 200 mA/cm2, for the technically important brightness of
100 cd/m2, an efficiency above 4.5 cd/A is detected. These findings
are rather promising also in comparison to similar Alq3-based bi-
layer devices [26,27] as well as poly(N-vinylcarbazole)-based de-
vices [28]. It should be pointed out that these characteristics
were obtained in a non-optimized test device under ordinary lab-
oratory conditions. The device performance may be further im-
proved by an optimization of the layer thicknesses and
processing conditions [29].
In conclusion, functional derivatives of (bi)phenyl-substituted
carbazole have been synthesized by a Suzuki-type aryl–aryl cou-
pling. The monomers show a promising thermal stability. They have
been tested as hole transporting layers in bilayer OLEDs with Alq3 as
the emitter. The devices exhibit good overall performance (turn-on
voltage: ꢀ3 V; maximum photometric efficiency: 5.1 cd/A; maxi-
mum brightness: 12,200–15,600 cd/m2). These OLEDs properties
are rather promising among Alq3-based two-layer devices.
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Acknowledgements
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Financial support of this research by the Research Council of
Lithuania is gratefully acknowledged. Habil Dr. V. Gaidelis is
thanked for the help in ionization potential measurements.
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