Cross-linkable fluorenyl-substituted aromatic amines for polymeric hole transporting networks

Article abstract of DOI:10.1016/j.reactfunctpolym.2011.02.007

Fluorenyl-substituted aromatic amines containing reactive oxetanyl groups have been synthesized. Full characterization of their structure by NMR-, IR- and mass spectrometry is presented. The synthesized materials were examined by various techniques includ

Full text of DOI:10.1016/j.reactfunctpolym.2011.02.007

Reactive & Functional Polymers 71 (2011) 574–578
Contents lists available at ScienceDirect
Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Cross-linkable fluorenyl-substituted aromatic amines for polymeric hole
transporting networks
S. Lengvinaite a, J.V. Grazulevicius a, B. Zhang b, Z. Xie b, S. Grigalevicius a,
⇑
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:
Received 20 July 2010
Received in revised form 31 January 2011
Accepted 27 February 2011
Available online 5 March 2011
Fluorenyl-substituted aromatic amines containing reactive oxetanyl groups have been synthesized. Full
characterization of their structure by NMR-, IR- and mass spectrometry is presented. The synthesized
materials were examined by various techniques including differential scanning calorimetry, thermo-
gravimetry, electron photoemission spectrometry and xerographic time of flight technique. The electron
photoemission spectra of the layers of the derivatives showed ionization potentials of 5.4–5.65 eV. These
derivatives have been tested as hole transporting materials in bilayer OLEDs with Alq3 as the emitter. The
device with derivative containing benzidine core exhibited the best overall performance with turn-on
voltage of ꢀ2.5 V, a maximum luminance efficiency of 4 cd/A and maximum brightness of about
6000 cd/m2. The derivative was also used for the preparation of cross-linked hole transporting layers
by photoinduced polymerization. The layers obtained were also tested in light emitting diodes. The
devices demonstrated maximum photometric efficiency of 1.65 cd/A and maximum brightness of about
3000 cd/m2.
Keywords:
Aromatic amine
Cross-linkable material
Ionization potential
Light emitting diode
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
additions after polymerization nor decrease the concentration of
charge transporting chromophores. The advantages as well as
Organic light- emitting diodes (OLEDs) based on organic small
molecules and polymers have attracted much attention because
of their potential use in flat panel displays and lighting [1]. Efficient
OLEDs can be obtained only by building multilayer structures [2–
4]. The main difficulty in the preparation of such devices by spin-
coating is the solubility of the material which forms the bottom
layer onto which the top layer has to be cast, because most organic
semiconductors are soluble in the same solvents. One approach
that has been employed to circumvent this problem is the appli-
ance of bi-functional electro-active derivatives, which could be
converted into insoluble networks by cross-linking reactions. Sev-
eral series of photo-cross-linkable polymers and monomers for the
fabrication of OLEDs have been reported [5–9]. Among the deriva-
tives earlier studied, triphenyldiamine (TPD) core having deriva-
tives were mostly used. For the purpose of developing of organic
layers with new optoelectronic properties, we have been studying
the synthesis and applications of new photo-cross-linkable deriva-
tives with fluorenyl fragments. Films of the compounds should
demonstrate suitable ionization potentials and could be used as
additional layers in multilayer devices. On the other hand, we have
used photo-cross-linkable oxetanyl moieties which neither form
cross-linking mechanism of oxetanyl functionalized derivatives
containing electro-active fragments is well demonstrated in the lit-
erature [5–7,10].
2. Experimental
2.1. Instrumentation
1H NMR spectra were recorded using Varian Unity Inova
(300 MHz) apparatus. Mass spectra were obtained on a Waters
ZQ 2000 spectrometer. FTIR spectra were recorded using 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 [11]. The measurement method
was, in principle, similar to that described by Miyamoto et al.
[12].The samples for the ionization potential measurement were
prepared as we described earlier [13].
⇑
Corresponding author. Tel./fax: +370 37 456 525.
E-mail address: saugrig@ktu.lt (S. Grigalevicius).
1381-5148/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.reactfunctpolym.2011.02.007

S. Lengvinaite et al. / Reactive & Functional Polymers 71 (2011) 574–578
575
Thin films of the materials were prepared by spin-coating from
their chloroform solutions. Solutions for preparation of cross-
linked films contained 7 wt% of {4-[(2-hydroxytetradecyl)-oxyl]-
phenyl}-phenyliodonium hexafluorantimonate as initiator for the
cationic polymerization of oxetane units of 3 or 4. The cross-link-
able films were irradiated (standard handheld UV lamp) for
2 min at room temperature. Afterwards, to further advance the
cross-linking process in the growing network, the films were an-
nealed at 180 °C for 7 min. The resulting polymer networks were
found to be insoluble in organic solvents.
m
, cmꢂ1): 3048 (aromatic CAH), 2963, 2858 (aliphatic CAH), 976
(CAOAC in oxetane ring), 741; 694 (CH@CH).
2.2.3. N,N-Di[9,9-di(3-methyloxetan-3-ylmethyl)fluoren-2-yl]-N-
phenylamine (3).
A 23.5 mg (0.025 mmol) of Pd2(dba)3, 10.5 mg, (0.05 mmol) of t-
Bu3P and toluene (7 ml) were charged under nitrogen in a two neck
flask equipped with a reflux condenser. The mixture was stirred at
room temperature for 10 min. Then 1.06 g (2.6 mmol) of 2-bromo-
9,9-di(3-methyloxetan-3-ylmethyl)fluorene (2), 0.1 g (1.1 mmol)
of aniline and 1.47 g (15.6 mmol) of t-BuONa were added into
the reaction mixture, and it was stirred under nitrogen for 24 h
at 80 °C. The mixture was cooled and quenched by an addition of
ice water. The product was extracted with ethyl acetate. The com-
bined extract was dried over anhydrous MgSO4. The crude product
was purified by silica gel column chromatography using ethyl ace-
tate and hexane mixture (vol. ratio 1:3) as an eluent. Yield: 0.23 g
(31%) of yellowish crystals. M.p.: 253–254 °C.
The multilayer electroluminescent devices were fabricated on
glass substrates and had the typical structure with the organic layers
sandwiched between a bottom ITO anode and a top metal cathode.
Before use in device fabrication, the ITO-coated glass substrates
were carefullycleaned andtreated with UV/ozone rightbefore depo-
sition of the organic layers. PEDOT layers were deposited by spin-
coating and heated at 120 °C for 30 min. The hole transporting layers
(HTL)weremadebyspin-coatinga25–30 nmlayerofthederivatives
3 or 4. Cross-linkable HTL contained the mentioned initiator and
were converted to insoluble networks as described above. Tris(quin-
olin-8-olato)aluminum (Alq3) or poly(9,9-dioctylfluorene-2,7-diyl-
co-2,5-di(phenyl-4-yl)-2,1,3-benzothiadiazole) (PFBT)were used
as green light emitters. The polymer was spin-coated from its tolu-
ene solution onto the cross-linked hole transporting network. Evap-
oration of Alq3 as well as of Al cathode was done at a pressure of
3 ꢁ 10ꢂ6 mbar in vacuum evaporation equipment.
1H NMR spectrum (300 MHz, CDCl3, d, ppm): 7.66–7.05 (m, 19H,
Ar), 4.28 (d, 4H, CH2 of oxetane ring, J = 6 Hz), 4.23 (d, 4H, CH2 of oxe-
tane ring, J = 6 Hz), 3.7 (d, 4H, CH2 of oxetane ring, J = 6 Hz), 3.55 (d,
2H, CH2 of oxetane ring, J = 6 Hz), 2.48 (d, 4H, 2 ꢁ CCH2C, J = 14 Hz),
2.33 (d, 4H, 2 ꢁ CCH2C, J = 14 Hz), 0.57 (s, 12H, 4 ꢁ CH3). 13C NMR
spectrum (75.4 MHz, CDCl3, d, ppm): 24.41, 39.97, 49.75, 53.01,
77.31, 83.92, 84.57, 120.04, 121.40, 123.31, 124.22, 124.53, 125.11,
126.22, 128.09, 129.62, 134.99, 136.45, 147.99, 149.97.
MS (APCI+, 20 V), m/z (%): 759.2 ([M + H]+, 100). IR (KBr, , cmꢂ1):
3035 (aromatic CAH), 2959; 2864 (aliphatic CAH), 1487; 1450
(CAN), 976 (CAOAC in oxetane ring), 741; 694 (CH@CH of mono
substituted benzene). Elemental analysis for C52H55N4O4,% Calc.:
C 82.40, H 7.31, N 1.85, O 8.44;% Found: C 82.31, H 7.43, N 1.85.
The current–voltage and luminance-voltage characteristics
were recorded under forward bias using a computer controlled
Keithley 2400 source meter and a PR650 Spectrometer [14]. All
measurements were performed at ambient conditions in air.
2.2. Materials
2.2.4. N,N0-Di[9,9-di(3-methyloxetan-3-ylmethyl)fluoren-9-yl]-N,N0-
diphenylbenzidine (4)
2-Bromofluorene (1), benzyl triethyl ammonium chloride
(BEAC), NaOH, aniline, N,N-diphenylbenzidine, sodium tert-butox-
ide (t-BuONa), tri(tert-butyl)phosphine {(t-Bu)3P)} solution (1.0 M
in toluene), tris(dibenzylideneacetone)dipalladium(0) {Pd2(dba)3}
and Alq3 were received from Sigma–Aldrich.
A 10 mg (0.01 mmol) of Pd2(dba)3, 4.4 mg (0.02 mmol) of t-Bu3P
and toluene (4 ml) were charged under nitrogen in a two neck flask
equipped with a reflux condenser. The mixture was stirred at room
temperature for 10 min. Then 0.49 g (1.1 mmol) of 2-bromo-9,9-
di(3-methyloxetan-3-ylmethyl)fluorene (2), 0.1 g (0.3 mmol) of
N,N0-diphenylbenzidine and 0.69 g (6.6 mmol) of t-BuONa were
added into the reaction mixture, and it was stirred under nitrogen
for 18 h at 80 °C. The mixture was cooled and quenched by an addi-
tion of ice water. The product was extracted into ethyl acetate. The
combined extract was dried over anhydrous MgSO4. The crude
product was purified by silica gel column chromatography using
ethyl acetate and hexane (vol. ratio 1:3) as an eluent. Yield:
0.25 g (21%) of yellow amorphous powder.
2.2.1. 3-Bromomethyl-3-methyloxetane was obtained from Chemada
Fine Chemicals
Poly(9,9-dioctylfluorene-2,7-diyl-co-2,5-di(phenyl-4_-yl)-2,1,3-
benzothiadiazole) (PFBT) was prepared by the procedure as
described earlier [15]. The polymer was used as an green light
emitter in OLEDs.
2.2.2. 2-Bromo-9,9-di(3-methyloxetan-3-ylmethyl)fluorene (2)
A 5 g (20 mmol) of 2-bromofluorene (1) and 0.2 g (0.92 mmol)
of BEAC were dissolved in 20 ml of DMSO and 20 ml of aqueous so-
dium hydroxide solution (50%) were added. After 5 min 8.42 g
(50 mmol) of 3-bromomethyl-3-methyloxetane were added drop
wise into the reaction mixture. The two-phase system was stirred
at 80 °C for 6 h under argon atmosphere. After TLC control the reac-
tion mixture was treated with 2 N HCl and extracted with chloro-
form. After extraction the organic layer was dried over MgSO4.
After filtration and evaporation of the solvent, the crude product
was purified by silica gel column chromatography using ethyl ace-
tate and hexane (vol. ratio 1:3) as eluent. Yield: 5.6 g (68%) of
orange crystals. M.p.: 128–129 °C.
1H NMR spectrum (300 MHz, CDCl3, d, ppm): 7.73–7.05 (m, 32H,
Ar), 4.3–4.22 (m, 8H, CH2 of oxetane ring), 3.71 (d, 2H, CH2 of oxe-
tane ring, J = 5.5 Hz), 3.56 (d, 6H, CH2 of oxetane ring, J = 5.6 Hz),
2.55–2.36 (m, 8H, 4 ꢁ CCH2C), 0.56 (s, 12H, 4 ꢁ CH3). 13C NMR
spectrum (75.4 MHz, CDCl3, d, ppm): 24.41, 39.97, 49.75, 53.01,
77.31, 83.92, 84.57, 120.04, 121.40, 123.31, 124.22, 124.53,
125.11, 126.22, 127.62, 128.09, 129.62, 134.99, 136,45, 140.95,
147.12, 147.99, 148.01, 149.97.
MS (APCI+, 20 V), m/z (%): 1002.5 ([M + H]+, 100). IR (KBr, , cmꢂ1):
3035 (aromatic CAH), 2956; 2864 (aliphatic CAH), 1489; 1451
(CAN), 969 (CAOAC in oxetane ring), 744; 696 (CH@CH of mono
substituted benzene). Elemental analysis for C70H68N2O4% Calc.: C
83.97, H 6.85, N 2.80, O 6.39;% Found: C 83.91, H 6.80, N 2.84.
1H NMR spectrum (300 MHz, CDCl3, d, ppm): 7.62–7.22 (m, 7H,
Ar), 4.47 (d, 2H, CH2 of oxetane ring, J = 5.7 Hz), 4.32 (d, 2H, CH2 of
oxetane ring, J = 5.7 Hz), 3.59 (d, 2H, CH2 of oxetane ring,
J = 5.6 Hz), 3.50 (d, 2H, CH2 of oxetane ring, J = 5.6 Hz), 2.54 (d,
2H, CCH2C, J = 14 Hz), 2.45 (d, 2H, CCH2C, J = 14 Hz), 0.40 (s, 6H,
2 ꢁ CH3). MS (APCI+, 20 V), m/z (%): 414.4 ([M + H]+, 100). IR (KBr,
3. Results and discussion
The synthetic route of cross-linkable aromatic amines (3–4)
containing fluorenyl fragments is shown in Scheme 1. 2-Bromo-

576
S. Lengvinaite et al. / Reactive & Functional Polymers 71 (2011) 574–578
9,9-di(3-methyloxetan-3-ylmethyl)fluorene (2) as a key material
was synthesized from commercially available 2-bromofluorene
and 3-bromomethyl-3-methyloxetane in a two phase system by
similar procedure as it is described in the literature [16,17]. An
excess of the bromo compound 2 was reacted with aniline or
N,N0-diphenylbenzidine by the method of Buchwald-Hartwig [18]
to afford N,N0-di[9,9-di(3-methyloxetan-3-ylmethyl)fluorenyl]-N-
phenylamine (3) or N,N0-di[9,9-di(3-methyloxetan-3-ylmethyl)flu-
oren-9-yl]-N,N0-diphenylbenzidine (4), respectively.
The newly synthesized compounds 3 and 4 were identified by
mass spectrometry, IR and by 1H NMR spectroscopy. The data were
found to be in good agreement with the proposed structures. The
materials were soluble in common organic solvents, such as chlo-
roform and THF at room temperature. Transparent thin films of
these materials could be prepared by spin coating from their
solutions.
T = 122o
C
g
2nd heating
1st heating
Tm = 253oC
200 250
50
100
150
The behavior under heating of the compounds 3 and 4 was
studied by DSC and TGA under a nitrogen atmosphere. It was
established that both the materials demonstrate high thermal sta-
bility. The temperatures at which 5% loss of mass was observed are
384 °C for 3 and 400 °C for 4 as confirmed by TGA with a heating
rate of 10 °C/min.
The compound 4 was obtained in nature as amorphous mate-
rial. When the sample of 4 was heated Tg was observed at 115 °C
and no peaks due to crystallization and melting appeared. Cooling
down and repeated heating revealed only the glass transition
again. The derivative 3 was obtained as crystalline material by
re-crystallization from solution, however it readily formed glass
when the melt sample was cooled on standing in air or with liquid
nitrogen. The DSC thermograms of 3 are shown in Fig. 1. When the
crystalline sample was heated, the endothermic peak due to melt-
ing was observed at 253 °C. When the melt sample was cooled
down and heated again, the glass-transition phenomenon was
observed at 122 °C and on further heating no peaks due to crystal-
lization and melting appeared.
Temperature [oC]
Fig. 1. DSC curves of the material 3. Heating rate 10 °C/min.
UV absorption and FL spectra of dilute solutions of the com-
pounds synthesized were recorded. The pertinent data are pre-
sented in Fig. 2. The electronic absorption energy of the
derivatives is similar, and the kmax values are in the range of
220–380 m. Their FL maxima were registered at 398 nm for 3
and at 409 nm for 4. Comparison of the FL spectra of these deriva-
tives demonstrates that benzidine fragment has an evident effect
on electronic structure of compound 4. It could be expected that
ionization potentials (Ip) of the materials 3 and 4 will be different.
On the other hand comparison of the UV absorption and FL spectra
of material 4 with those of similar triphenyldiamine (TPD)-based
derivatives [19] revealed that conjugation along the backbones of
the compound 4 is similar to that of TPD, and layers of 4 should
demonstrate Ip similar as of TPD.
Br
1
O
O
O
O
O
NaOH/ BEAC
DMSO/ H2O
N
Br
H N H
3
O
O
*
Br
2
O
O
O
O
*
N
N
N
H
H N
4
Scheme 1. ⁄Pd2(dba)3, t-Bu3P, and t-BuONa in toluene.

S. Lengvinaite et al. / Reactive & Functional Polymers 71 (2011) 574–578
577
3
4
3
4
240
280
320
360
400
440
360
390
420
450
480
510
Wavelength [nm]
Wavelength [nm]
Fig. 2. UV absorption and FL spectra of dilute THF solutions of 3 and 4.
The electron photoemission spectra of layers of the compounds
synthesized and values of their Ip are presented in Fig. 3. Ip of the
layer of material 4 was lower than that of 3. This observation indi-
cates that conjugation in molecules of derivative 4 is slightly larger
than that of 3. As it was expected from UV absorption and FL spec-
tra, Ip of layer of 4 was rather close to that of TPD derivatives [19].
It is evident that the benzidine fragment has considerable influ-
ence on Ip of the materials.
photometric efficiencies of 2–4 cd/A, and maximum brightness of
4450–5980 cd/m2. As it was expected from Ip values of the layers,
the device using derivative 4 as the HT material exhibited better
overall performance due to more effective hole injection and trans-
port. Efficiency of the device with 4 showed only a moderate drop
in the observed current density window up to 50 mA/cm2. For the
technically important brightness of 100 cd/m2, an efficiency above
3.5 cd/A was detected. It is also evident that HTL of the synthesized
The electron affinity of the materials was determined from their
optical band gaps (Eg) and ionization potentials (Eg = Ip ꢂ ELUMO).
The energy band gaps of 3 and 4, which were taken as the absorp-
tion onsets of the UV–vis spectra of the materials, were estimated
to be 3.15 and 3.05 eV, respectively. These values correspond to
LUMO of ꢂ2.35 eV for 3 and ꢂ2.5 eV for derivative 4.
10000
(a)
1000
100
The layers of compound 4 should demonstrate better hole injec-
tion and transport properties. This was studied by application of
the synthesized materials as HTL in OLEDs. The devices were of
the form: ITO/PEDOT/3 or 4/Alq3/LiF/Al with Alq3 as an electrolu-
minescent/electron transporting layer. When a positive voltage
was applied to the OLEDs the bright green electroluminescence
of Alq3 was observed with an emission maximum at around
520 nm. This implies that hole drift mobility in layers of the
compounds 3 and 4 is sufficient for an effective charge carrier
transport to Alq3 layer.
without HTL
10
with (25nm)
4
with 3 (25nm)
1
Fig. 4 shows efficiency and luminance – voltage characteristics
of the OLEDs containing HTL of the compounds 3 or 4. The charac-
teristics of a device without HTL are presented for comparison. The
devices containing HTL exhibit turn-on voltages of 2.5–4 V,
2
4
6
8
10
12
14
Voltage [V]
5
4
3
2
1
0
without HTL
(b)
with (25nm)
4
3 Ip = 5.65 eV
4 Ip = 5.4 eV
with 3 (25nm)
0,1
1
10
100
1000
Current density [mA/cm2]
6.0
5.6
5.8
6.4
6.2
5.0
5.4
5.2
hv [eV]
Fig. 4. Characteristics of the devices ITO/PEDOT/3 or 4/Alq3/LiF/Al; (a) brightness –
Fig. 3. Electron photoemission spectra of layers prepared using 3 and 4.
voltage, (b) current efficiency – current density.

578
S. Lengvinaite et al. / Reactive & Functional Polymers 71 (2011) 574–578
of 4, the turn-on voltage decreased to 10 V, showing the effect of
10000
1000
100
the improved hole injection and transport. EL efficiency of the
device reached 1.65 cd/A and the maximum brightness exceeded
3000 cd/m2.
It should be pointed out that these characteristics were
obtained in non-optimized test devices under ordinary labora-
tory conditions. The device performance may be further
improved by an optimization of the layer thicknesses and
processing conditions.
with crosslinked 4
without HTL
In conclusion, cross-linkable monomers with fluorenyl-substi-
tuted arylamine core as hole transport agent, and oxetanyl groups
as photocros-linking moieties have been synthesized and charac-
terized. The materials are thermally stable and form amorphous
films with glass transition temperatures in the range of 115–
122 °C. These derivatives have been studied as hole transporting
materials in bilayer OLEDs with Alq3 as an emitter. The devices
with derivative containing benzidine core exhibited the best over-
all performance (turn-on voltage: ꢀ2.5 V; maximum luminance
efficiency: 4 cd/A; maximum brightness: approx. 6000 cd/m2).
Cross-linked networks of the derivative have also been tested as
hole transporting structures in multilayer OLEDs with green light
emitting polymer. The device demonstrated maximum photomet-
ric efficiency of 1.65 cd/A and maximum brightness of about
3000 cd/m2.
10
5
10
15
20
25
Voltage (V)
10
without HTL
with crosslinked 4
Acknowledgements
1
This research was partly funded by a Grant No. TAP-02/2010
from the Research Council of Lithuania and by National Science
Council of Taiwan,ROC. Habil. dr. V. Gaidelis is thanked for the help
in ionization potential measurements.
References
[1] K. Mullen, U. Scherf (Eds.), Organic Light Emitting Devices
Properties and Applications, Wiley-VCH, Weinheim, 2005.
[2] U. Mitschke, P. Bauerle, J. Mater. Chem. 10 (2000) 913.
[3] Z. Li, Z. Jiang, G. Qiu, W. Wu, G. Yu, Y. Liu, J. Qin, Z. Li, Adv. Macromol. Chem.
Phys. 211 (2010) 1820–1825.
[4] Z. Li, Y. Liu, G. Yu, Y. Wen, Y. Guo, L. Ji, J. Qin, Z. Li, Adv. Funct. Mater. 19 (2009)
2677–2683.
[5] C.D. Muller, A. Falcou, N. Reckefuss, M. Rojahn, V. Wiederhirn, P. Rudati, H.
Frohne, O. Nuyken, H. Becker, K. Meerholz, Nature 421 (2003) 829. and
references therein.
– Synthesis,
0,1
0
100
200
300
400
Current Density (mA/cm2)
Fig. 5. LED characteristics of the devices ITO/HTL of cross-linked 4/PFBT/(Ca/Al)
and ITO/PFBT/(Ca/Al).
[6] O. Nuyken, S. Jungermann, V. Wiederhirn, E. Bacher, K. Meerholz, Monatsch.
Chem. 137 (2006) 811. and references therein.
[7] F. Huang, Y.-J. Cheng, Y. Zhang, M.S. Liu, A.K.-Y. Jen, J. Mater. Chem. 18 (2008)
4495.
[8] G.-A. Wen, X.-R. Zhu, L.-H. Wang, J.-C. Feng, R. Zhu, W. Wei, B. Peng, Q.-B. Pei,
W. Huang, J. Polym. Sci. A: Polym. Chem. 45 (2007) 388.
[9] S. Lengvinaite, J.V. Grazulevicius, S. Grigalevicius, B. Zhang, J. Yang, Z. Xie, L.
Wang, Synth. Met. 158 (2008) 213.
[10] J. Schelter, G.F. Mielke, A. Kohnen, J. Wies, S. Kober, O. Nuyken, K. Meerholz,
Macromol. Rapid Commun. 31 (2010) 1560–1567.
[11] J. Simokaitiene, S. Grigalevicius, J.V. Grazulevicius, R. Rutkaite, K. Kazlauskas, S.
Jursenas, V. Jankauskas, J. Sidaravicius, J. Optoelectron. Adv. Mater. 8 (2006)
876–882.
[12] E. Miyamoto, Y. Yamaguchi, M. Yokoyama, Electrophotography 28 (1989) 364.
[13] V. Vaitkeviciene, S. Grigalevicius, J.V. Grazulevicius, V. Jankauskas, V.G.
Syromyatnikov, Eur. Polym. J. 42 (2006) 2254.
[14] S. Grigalevicius, L. Ma, G. Qian, Z. Xie, M. Forster, U. Scherf, Macromol. Chem.
Phys. 208 (2007) 349.
[15] US Patent PCT/US99/07876 (WO 00/46321).
materials improve considerably efficiencies of the devices as com-
pared with that of device without HTL.
Cross-linked layers of the material 4, which demonstrated
better characteristics in Alq3 based devices, were tested as hole
transporting networks in two layer LEDs of the configuration
ITO/cross-linked material 4/green light emitting polymer PFBT/
(Ca/Al). The hole transporting layers were made by spin-coating
from solution a 30–35 nm layer of the derivative 4 containing ini-
tiator onto the substrates with ITO and by following photoinitiated
cross-linking as described in experimental part. A similar device
without HTL was prepared for comparison.
When a positive voltage was applied to the device bright green
electroluminescence of PFBT was observed with an emission max-
imum at around 528 nm. This implies that the hole mobility in the
hole transporting network of 4 was sufficient for charge carrier
recombination occurring within the PFBT layer. Fig. 5 shows lumi-
nance- voltage characteristics as well as efficiencies of the OLEDs
containing hole transporting networks of 4. The characteristics of
a devise without the HTL are also presented in the figure.
Turn-on voltage of the single layer device was ca 12 V, maximum
brightness was 1270 cd/m2 and the maximum EL efficiency of
the device reached 0.96 cd/A. For the device having HT network
[16] G. Jiang, B. Yao, Y. Geng, Y. Cheng, Z. Xie, L. Wang, X. Jin, F. Wang,
Macromolecules 39 (2006) 1403.
[17] P.L. Burn, A. Kraft, D.R. Baigent, D.D.C. Bradley, A.R. Brown, R.H. Friend, R.W.
Gymer, A.B. Holmes, R.W. Jackson, J. Am. Chem. Soc. 115 (1993) 10117.
[18] J.F. Hartwig, M. Kawatsura, S.I. Hauck, K.H. Shaughnessy, L.M. Alcazar-Roman,
J. Org. Chem. 64 (1999) 5575.
[19] C. Takahashi, S. Moriya, H. Sato, N. Fugono, H.-C. Lee, Synth. Met. 129 (2002)
123.