Well defined carbazol-3,9-diyl based oligomers as host materials for organic electro-phosphorescent devices

Article abstract of DOI:10.1080/15421400701229495

Well defined carbazol-3,9-diyl-based derivatives as host materials for electro-phosphorescent devices have been synthesized by the modified Ullmann reaction as a key step. The full characterization of their structure by mass spectrometry, infrared absorption and proton magnetic resonance spectroscopy is presented. The amorphous compounds with glass transition temperatures of 103-154C possess high thermal stability with onset decomposition temperatures exceeding 360C. Conjugation of the 3,9-linked systems is rather confined within one carbazole unit, retaining large triplet energies suitable for green and red electro-phosphorescence. Using the new host material, efficient green phosphorescent organic light emitting diodes was demonstrated.

Full text of DOI:10.1080/15421400701229495

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Well Defined Carbazol-3,9-
Diyl Based Oligomers as Host
Materials for Organic Electro-
Phosphorescent Devices
Saulius Grigalevicius a , Juozas V. Grazulevicius a
Ming-Han Tsai b & Chung-Chih Wu b
,
a Department of Organic Technology , Kaunas
University of Technology , Kaunas, Lithuania
b Department of Electrical Engineering , Graduate
Institute of Electro-optical Engineering and
Graduate Institute of Electronics Engineering,
National Taiwan University , Taipei, Taiwan
Published online: 22 Sep 2010.
To cite this article: Saulius Grigalevicius , Juozas V. Grazulevicius , Ming-Han Tsai
& Chung-Chih Wu (2007) Well Defined Carbazol-3,9-Diyl Based Oligomers as Host
Materials for Organic Electro-Phosphorescent Devices, Molecular Crystals and Liquid
Crystals, 468:1, 77/[429]-86/[438], DOI: 10.1080/15421400701229495
To link to this article: http://dx.doi.org/10.1080/15421400701229495
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Mol. Cryst. Liq. Cryst., Vol. 468, pp. 77=[429]–86=[438], 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701229495
Well Defined Carbazol-3,9-Diyl Based Oligomers as Host
Materials for Organic Electro-Phosphorescent Devices
Saulius Grigalevicius
Juozas V. Grazulevicius
Department of Organic Technology, Kaunas University of Technology,
Kaunas, Lithuania
Ming-Han Tsai
Chung-Chih Wu
Department of Electrical Engineering, Graduate Institute of Electro-
optical Engineering and Graduate Institute of Electronics Engineering,
National Taiwan University, Taipei, Taiwan
Well defined carbazol-3,9-diyl-based derivatives as host materials for electro-
phosphorescent devices have been synthesized by the modified Ullmann reaction
as a key step. The full characterization of their structure by mass spectrometry,
infrared absorption and proton magnetic resonance spectroscopy is presented.
The amorphous compounds with glass transition temperatures of 103–154ꢀC
possess high thermal stability with onset decomposition temperatures exceeding
360ꢀC. Conjugation of the 3,9-linked systems is rather confined within one
carbazole unit, retaining large triplet energies suitable for green and red electro-
phosphorescence. Using the new host material, efficient green phosphorescent
organic light emitting diodes was demonstrated.
Keywords: amorphous material; aromatic amine; electrophosphorescence; light emit-
ting diode
INTRODUCTION
Improvements in organic light emitting diodes (OLEDs) performance
over the past decade have resulted in commercially available products.
Financial support of this research from the National Science Council of Taiwan and
from the Lithuanian Science and Studies Foundation is gratefully acknowledged.
Address correspondence to Juozas V. Grazulevicius, Department of Organic Tech-
nology, Kaunas University of Technology, Radvilenu Plentas 19, Kaunas, LT 50254,
Lithuania. E-mail: juogra@ktu.lt
77=[429]

78=[430]
S. Grigalevicius et al.
Organometallic small molecule phosphor-doped devices have demon-
strated by far the highest efficiency with >90% internal quantum
efficiency [1,2]. In phosphorescent OLEDs, to reduce quenching asso-
ciated with relatively long excited-state lifetimes of triplet emitters
and triplet-triplet annihilation etc., triplet emitters of heavy-metal
complexes are normally used as emitting guests in a host material,
and thus effective host materials are of equal importance for efficient
phosphorescent OLEDs. For electrophosphorescence from triplet
guests, it is essential that the triplet level of the host be larger than
that of the triplet emitter to prevent reverse energy transfer from
the guest back to the host and to effectively confine triplet excitons
on guest molecules [3,4].
Here, we report a molecular design for large-gap materials adopting
the non-conjugated linking of carbazolyl moieties. The 3,90-linked car-
bazoles demonstrate large triplet energies and serve as effective host
materials for green electro-phosphorescent devices.
2. EXPERIMENTAL
2.1. Instrumentation
Fourier transform infrared (FTIR) spectra were measured using a
Bio-Rad Digilab FTS-40 spectrometer. 1H NMR spectra were recorded
using a Bruker DPX 250 apparatus. Mass spectra were obtained on a
Varian MAT-312-Spectrometer Waters ZQ 200 spectrochromatograph.
Ultraviolet (UV) spectra were measured with a Shimadzu UV-
1601PC spectrophotometer. Photoluminescence (PL) spectra were
measured using a charge-coupled-device (CCD) spectragraph and
the 325-nm line of the He-Cd laser as the excitation source. Phosphor-
escence spectra of compounds were measured at 77 K with an IBH
picosecond photon detection module and a sub-microsecond Xenon
flash lamp as an excitation source using a 5 ms delay time between
the excitation with a microsecond flash lamp and the measurement.
Differential scanning calorimetry (DSC) measurements were car-
ried out using a TA Instrument DSC-2920 calorimeter. Thermogravi-
metric analysis (TGA) was performed on a TA Instrument 2950 TGA
thermal analyzer. The measurements were recorded in a nitrogen
atmosphere at a heating rate of 10ꢀ C=min.
OLEDs were fabricated on the ITO-coated glass substrates with
multiple organic layers sandwiched between the transparent bottom
indium-tin-oxide (ITO) anode and the top metal cathode. The
PEDT:PSS layer was prepared by spin coating, and other material
layers were deposited by vacuum evaporation in a vacuum chamber

Carbazole Oligomers for Electro-Phosphorescent Devices
79=[431]
with a base pressure of <10ꢁ6 torr. The deposition rate of organic
layers was kept at ꢂ0.2 nm=s. The active area of the device is
1 mm2, as defined by the shadow mask for cathode deposition. The cur-
rent-voltage-brightness characterization of the light-emitting devices
was performed with a source-measurement unit and a Si photodiode
calibrated with Photo Research PR650. Electroluminescence (EL)
spectra of the devices were collected by a calibrated CCD spectra-
graph.
2.2. Materials
Polyethylene dioxythiophene=polystyrene sulphonate (PEDOT=PSS)
was received from Bayer Corp. 1,4-Bis(1-naphthylphenylamino)-
biphenyl (NPD), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP),
tris(2-phenylpyridine)-iridium (Ir(ppy)3) and tris-(8-hydroxyquinoline)
aluminium (Alq3) were received from SynTec Gmbh. The materials
were used in this study for preparation of multilayer electrolumines-
cent devices.
9H-Carbazole (1), t-butylchloride, diphenylamine, copper powder, 4-
iodoanizole and 18-crown-6 were purchased from Aldrich and used as
received. 3,6-Di(t-butyl)-9H-carbazole (2) was synthesized from
9H-carbazole (1) and t-butylchloride using Friedel- Crafts alkylation
[5]. 3-Iodo-9H-carbazole (3) was obtained by a procedure of Tucker
[6]. 3-Iodo-9-ethylcarbazole (4) was prepared by alkylation of 3-iodo-
9H-carbazole (3) with ethyliodide in the presence of a phase transfer
catalyst [7]. Diphenylamino end-capped di(carbazol-3,9-diyl) with sec-
ondary amine functions (5) was prepared by similar procedure as we
described earlier [8].
3-[3,6-Di(t-butyl)carbazol-9-yl]-9-ethylcarbazole (6) 2 g (6.2 mmol) of
3-iodo-9-ethylcarbazole (4), 1 g (3.6 mmol) of 3,6-di(t-butyl)carbazole
(2), powdered potassium carbonate (1.9 g, 14.4 mmol), copper powder
(0.46 g, 7.2 mmol) and 18-crown-6 (0.17 g, 0.64 mmol) were refluxed
in o-dichlorobenzene (10 ml) under nitrogen for 24 h. Copper powder
and inorganic salts were then removed by filtration of the hot reaction
mixture. The solvent was distilled under reduced pressure and the
crude product was purified by silica gel column chromatography using
hexane=ethylacetate (vol. ratio 20:1) as an eluent. Yield of the com-
pound 6 was 71%. IR (KBr), n=cmꢁ1: 3054, 2973 (CꢁH, Ar), 1588,
1492 (CH CH, Ar), 1274 (CꢁN), 1230, 1067, 746 (CH CH, Ar). 1H
NMR (CDCl3), d (ppm): 1.41–1.63 (m, 21H, ꢁC(CH3)3 and ꢁCH3),
=
=
=
4.42 (q, 4H, Jꢁ7.2 Hz, NCH2ꢁ), 7.16–8.45 (m, 13H, Ar). MS, m=z
(%): 473 (100), 459 (47), 346 (22).

80=[432]
S. Grigalevicius et al.
Ethyl-substituted ter(carbazol-3,9-diyl) with diphenylamino end-
cap (7) was prepared by the following procedure. 2.5 g (ꢂ5 mmol) of
the derivative having secondary amine function (5), 3.33 g
(10.35 mmol) of 3-iodo-9-ethylcarbazole (4), copper powder (1.13 g,
17.83 mmol), 18-crown-6 (0.23 g, 0.89 mmol) and potassium carbonate
(4.24 g, 30.76 mmol) were dissolved in o-dichlorobenzene. The solution
was stirred at 180ꢀC for 24 h. Copper powder and inorganic compo-
nents were then removed by filtration of the hot reaction mixture.
The crude product was precipitated into 250 ml of methyl alcohol
and purified by silica gel column chromatography using hexane=
ethylacetate (vol. ratio 45:1) as an eluent. Yield of the compound 7
was 30%. IR (KBr), n=cmꢁ1: 3053, 2974 (CꢁH, Ar), 1588, 1493
1
=
=
(CH CH, Ar), 1274 (CꢁN), 1229, 1067, 747 (CH CH, Ar). H NMR
=
=
(CDCl3), d (ppm): 1.49 (t, 6H, J 7.25 Hz, ꢁCH3), 4.39 (q, 4H, J 7.2
Hz, ꢁNCH2ꢁ), 6.86–8.55 (m, 31H, Ar). MS, m=z (%): 694 (26), 693
(47), 502 (100), 361 (22).
Methoxyphenyl-substituted di(carbazol-3,9-diyl) with diphenyla-
mino end-cap (8) was prepared by the following procedure. 2.5 g
(ꢂ5 mmol) of the derivative having secondary amine function (5),
2.25 g (9.9 mmol) of 4-iodoanizole, copper powder (1.13 g, 17.83 mmol),
18-crown-6 (0.23 g, 0.89 mmol) and potassium carbonate (4.24 g,
30.76 mmol) were dissolved in o-dichlorobenzene. The solution was
stirred at 180ꢀC for 24 h. Copper powder and inorganic components
were then removed by filtration of the hot reaction mixture. The crude
product was precipitated into 200 ml of methyl alcohol and purified
by silica gel column chromatography using hexane=ethylacetate
(vol. ratio 50:1) as an eluent. Yield of the compound 8 was 34%. IR
(KBr), n=cmꢁ1: 3073, 2969 (CꢁH, Ar), 2834 (OCH3), 1588, 1493
=
=
(CH CH, Ar), 1274 (CꢁN), 1229, 1067, 998, 789 747 (CH CH, Ar).
1H NMR (CDCl3), d (ppm): 3.87 (s, 3H, ꢁOCH3), 6.72–8.95 (m, 28H,
Ar). MS, m=z (%): 606 (100), 504 (74), 421 (82).
3. RESULTS AND DISCUSSION
The synthetic route to well defined carbazol-3,9-diyl-based oligomers
(6–8) is shown in Scheme 1. 3-[3,6-Di(t-butyl)carbazol-90-yl]-9-
ethylcarbazole (6) was prepared via the modified Ullmann coupling
[9] reaction of an excess of 3-iodo-9-ethylcarbazole (4) with 3,6-di
(t-butyl)-9H-carbazole (2). The iodo-derivative 4 was prepared by
alkylation of 3-iodo-9H-carbazole (3), which was synthesized by
Tucker iodination [6] of 9H-carbazole (1). 3,6-Di(t-butyl)-9H-carbazole
(2) was synthesized from 9H-carbazole(1) and t-butylchloride using
Friedel-Crafts alkylation [5].

81=[433]

82=[434]
S. Grigalevicius et al.
Diphenylamino end-capped derivative with secondary amine func-
tion (5) was prepared by the modified Ullmann coupling reaction of
3-iodo-9H-carbazole (3) with a large excess of diphenylamine. The
large excess of the amine had to be used in order to prevent polycon-
densation of 3-iodo-9H-carbazole. After the reaction an excess of
diphenylamine was removed and the crude product 5 was treated
with an excess of iodocompound, i.e. 3-iodo-9-ethylcarbazole or p-
iodoanisole, respectively, by the Ullmann method, to provide ethyl-
or methoxypheny- substituted oligo(carbazol-3,9-diyl)s with dipheny-
lamino end-cap (7–8). All the compounds 6–8 were identified by mass
spectrometry, IR and electronic absorption, as well as 1H NMR
spectroscopy. Transparent amorphous films of these materials can
be prepared by vacuum deposition or spin coating from solutions.
Thermal properties of the materials 6–8 were examined by DSC and
TGA. The TGA measurements revealed that the oligomers synthesized
exhibited high thermal stability. The onset of decomposition for the
materials occured above 360ꢀC. Especially high thermal stability was
observed for oligomer 7, which has the highest molecular weight.
The temperature of initial decomposition for 7 was 372ꢀC, and 5%
weight loss was observed at 421ꢀC. All the materials were obtained
as amorphous materials as confirmed by DSC (Figure 1). When sam-
ples of 6–8 were heated the glass-transitions were observed at 103ꢀC
for 6, 154ꢀC for 7 and at 123ꢀC for 8, and no peaks due to crystalliza-
tion and melting appeared. Cooling down and the following repeated
FIGURE 1 DSC curves of the oligomers 6–8 at a heating rate of 10ꢀC/min.

Carbazole Oligomers for Electro-Phosphorescent Devices
83=[435]
TABLE 1 Summary of Optical and Photophysical Characteristics of Cz and
the Compounds 6–8
Compound
konset, Abs. (nm)ꢃ
kmax, Fl. (nm)ꢃ
kmax, Phos. (nm)ꢃꢃ
Etriplet (eV)
Cz
6
7
379
381
417
412
351
404
427
420
409
420
455
456
3.02
2.95
2.71
2.71
8
ꢃIn thin film; ꢃꢃin ethanol at 77 K.
heating revealed only the glass transitions again. All the attempts to
crystallize the compounds failed.
It should be mentioned that the materials synthesized exhibit fairly
high glass transition temperatures, which are required to ensure the
formation of morphologically stable, uniform, amorphous thin films.
Such characteristics would render the devices using these materials
more resistant against joule heating and gives rises to a higher cur-
rent endurance. The more stable morphology would also reduce device
degradation.
The optical and photophysical characteristics of the carbazole com-
pounds are summarized in Table 1, in comparison with those of the
unsubstituted carbazole (Cz). Carbazole has relatively the largest
triplet energy Etriplet of 3.02 eV (i.e., the transition energy of T1!S0
estimated from the highest-energy phosphorescence (Phos) peak at
409 nm). Carbazole derivatives 7–8 with larger conjugation compared
to carbazole show a significant red shift in both fluorescence (Fl)
(ꢂ70 nm) and phosphorescence (ꢂ45 nm), leading to a substantially
lower triplet energy of 2.71 eV. In contrast, due to stronger localization
of triplet wave functions, dimer 6 shows a much less lowering of the
triplet energies, giving a triplet energy of 2.95 eV (420 nm). Such trip-
let energies would be large enough for applications in green or red
phosphorescent OLEDs. The results shown in Table 1 also show that
the 9th position of carbazole end-capped with phenyl substitution
(i.e., 7 and 8) hardly has any effect on the photophysical characteris-
tics, consistent with previous observations [10].
The compound 6 with the highest Etriplet was used as host in phos-
phorescent OLEDs studies. The conducting polymer PEDT=PSS was
used as the hole injection layer, a 40 nm NPD layer was used as the
hole-transport layer and Alq3 as the electron-transport layer. The
thickness of Alq3 layer was varied among devices from 30 nm to
50 nm. The emissive layer consisted of 20 nm of compound 6 doped
with 6 wt.% of the green phosphor Ir(ppy)3. A thin hole blocking layer

84=[436]
S. Grigalevicius et al.
of 10 nm of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) was
inserted between the emissive layer and the electron-transport layer
to provide exciton and carrier confinement. The cathode consists of a
thin LiF layer (0.5 nm) as the electron injection layer and a thick Al
layer (150 nm) as the metal electrode.
Figure 2 compares the fluorescence and phosphorescence spectra of
6, and the electroluminescence (EL) spectrum of the device. EL spec-
trum of the devices consisted entirely of the green emission of Ir(ppy)3,
and no emission from compound 6 was observed, indicative of complete
energy transfer from compound 6 to Ir(ppy)3. Such results are reason-
able considering the high triplet energy of the host compound. Charac-
teristics of the devices and external EL quantum efficiencies are
shown in Figure 3. These OLEDs exhibit turn-on voltages of ꢂ3.5 V
(defined as the voltage where EL becomes detectable), external EL
quantum efficiencies reaching 5% and maximum brightness of
11700 cd=m2 (at 13 V). The maximal power efficiency was 7.2 lm=W.
However, the devices show a gradual decrease of efficiency with
increasing current, which is often observed in phosphorescent OLEDs
and is usually attributed to triplet–triplet annihilation [11].
The device performances are promising, but not perfect yet, con-
sidering the host triplet energy to be as high as 2.95 eV. Such conse-
quences are most probably due to imperfect carrier injection into the
FIGURE 2 Fluorescence and phosphorescence spectra of compound 6 and EL
spectrum of device: ITO=PEDOT-PSS=NPD=Ir(ppy)3 in material 6 host=BCP=
Alq3=LiF=Al.

Carbazole Oligomers for Electro-Phosphorescent Devices
85=[437]
FIGURE 3 EL characteristics of devices: ITO=PEDOT-PSS=NPD=Ir(ppy)3 in
material 6 host=BCP=Alq3=LiF=Al (with Alq3 thickness of 30, 40 and 50 nm):
(a) current density-voltage, (b) brightness-voltage, and (c) external EL
quantum efficiency-current density.

86=[438]
S. Grigalevicius et al.
emission layer, as can be seen from the turn-on voltage, and probably
also to the exciton quenching in adjacent layers of lower triplet ener-
gies due to exciton diffusion. It is believed that further optimization
of the device structures (e.g., carrier-transport layers, electrodes etc.)
will further increase the device EL quantum efficiency.
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