Oligomers containing pyridinyl-substituted carbazole rings as host materials for phosphorescent OLEDs

Article abstract of DOI:10.1080/15421406.2018.1542076

Two oligoethers containing electroactive pendent pyridinyl-carbazole moieties have been synthesized by the multi-step synthetic route. Full characterization of their structures is presented. The oligomers represent derivatives of very high thermal stability with initial thermal degradation temperatures exceeding 400 °C. Glass transition temperatures of the amorphous materials were also very high and reached values of 124 °C and 145 °C, respectively. Bipolar chemical structure having oligomer, i.e. poly{3-(2-methoxy-3-pyridinyl)-9-(3-methyloxetan-3-yl)methyl-carbazole} was tested as host material for green phosphorescent organic light emitting diode using tris(2-phenylpyridine)iridium(III) as a triplet emitter. A green device containing 10 wt% of the green guest demonstrated among all the devices the best performance with current efficiency of 8.8 cd/A and power efficiency of 5.1 lm/W at 100 cd/m². At higher brightness, such as 1000 cd/m², used for illumination applications, this PhOLED showed enhanced efficiency of 11.7 cd/A (5.4 lm/W) with brightness exceeding 4000 cd/m².

Full text of DOI:10.1080/15421406.2018.1542076

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Oligomers containing pyridinyl-substituted
carbazole rings as host materials for
phosphorescent OLEDs
G. Krucaite, D. Tavgeniene, D. Blazevicius, U. Baranauskyte & E. Zaleckas
To cite this article: G. Krucaite, D. Tavgeniene, D. Blazevicius, U. Baranauskyte & E.
Zaleckas (2018) Oligomers containing pyridinyl-substituted carbazole rings as host materials
for phosphorescent OLEDs, Molecular Crystals and Liquid Crystals, 670:1, 160-167, DOI:
10.1080/15421406.2018.1542076
To link to this article: https://doi.org/10.1080/15421406.2018.1542076
Published online: 07 Jun 2019.
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
018, VOL. 670, NO. 1, 160–167
2
https://doi.org/10.1080/15421406.2018.1542076
RESEARCH-ARTICLE
Oligomers containing pyridinyl-substituted carbazole rings
as host materials for phosphorescent OLEDs
a
a
a
a
b
G. Krucaite , D. Tavgeniene , D. Blazevicius , U. Baranauskyte , and E. Zaleckas
a
Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilenu
b
plentas 19, LT50254, Kaunas, Lithuania; Aleksandras Stulginskis University, Studentu 11, Akademija,
LT53361, Kaunas district, Lithuania
ABSTRACT
KEYWORDS
Carbazole derivative;
oligomer; thermal stability;
organic light emitting diode
Two oligoethers containing electroactive pendent pyridinyl-carbazole
moieties have been synthesized by the multi-step synthetic route.
Full characterization of their structures is presented. The oligomers
represent derivatives of very high thermal stability with initial ther-
ꢀ
mal degradation temperatures exceeding 400 C. Glass transition
temperatures of the amorphous materials were also very high and
ꢀ
ꢀ
reached values of 124 C and 145 C, respectively. Bipolar chemical
structure having oligomer, i.e. poly{3-(2-methoxy-3-pyridinyl)-9-(3-
methyloxetan-3-yl)methyl-carbazole} was tested as host material for
green phosphorescent organic light emitting diode using tris(2-
phenylpyridine)iridium(III) as a triplet emitter. A green device con-
taining 10 wt% of the green guest demonstrated among all the devi-
ces the best performance with current efficiency of 8.8 cd/A and
2
power efficiency of 5.1 lm/W at 100cd/m . At higher brightness, such
2
as 1000 cd/m , used for illumination applications, this PhOLED
showed enhanced efficiency of 11.7 cd/A (5.4 lm/W) with brightness
2
exceeding 4000 cd/m .
1. Introduction
Organic light emitting diode (OLED) is a potential technology to realize high quality
displays and solid state lightings [1, 2]. Nowadays, wide range of OLED based portable
display products and few large size displays have already been in the market [3, 4]. The
efficiencies of the devices have advanced rapidly in recent years because of the develop-
ment of efficient phosphorescent guests. In phosphor-doped OLEDs (PhOLEDs), to
reduce quenching associated with relatively long excited-state lifetimes of triplet emitters
and triplet-triplet annihilation etc., the triplet emitters of heavy-metal complexes are
normally used as emitting guests in a host material, and thus suitable host materials are
of equal importance for the phosphorescent devices [5–7]. For electrophosphorescence
from triplet guests, it is essential that the triplet level of the host is 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 [8–10].
It was reported earlier that carbazole-based derivatives demonstrate rather large trip-
let energies and are potential host materials for blue electrophosphorescent devices
ꢁCONTACT Dr. Gintare Krucaite
gintare.krucaite@ktu.lt
ß 2018 Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
161
[
[
11–15]. The derivatives are also used as hole transporting materials and emitters
16–21]. Here, we report on bipolar solution process-able host materials containing elec-
troactive pendent pyridinyl-carbazole moieties.
2. Experimental
2.1. Instrumentation
1
H NMR spectra were recorded using a Varian Unity Inova (300 MHz) apparatus. Mass
spectra were obtained on a Waters ZQ 2000 spectrometer. Differential scanning calor-
imetry (DSC) measurements were carried out using a Bruker Reflex II thermosystem.
Thermogravimetric analysis (TGA) was performed on a Netzsch STA 409. TGA and
ꢀ
DSC curves were recorded in a nitrogen atmosphere at a heating rate of 10 C/min.
The PhOLEDs were fabricated on glass substrates and consisted of multiple organic
layers sandwiched between the bottom indium tin oxide (ITO) anode and the top metal
cathode (Al). The device structure used was ITO/PEDOT:PSS (ca. 30 nm)/new oligo-
meric host doped with Ir(ppy) )/LiF (0,5 nm)/Al (150 nm), where the conducting poly-
3
mer poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) was used as
the hole-injection layer [22], the oligomeric host 7 doped with the green phosphorescent
tris(2-phenylpyridine)iridium(III) (Ir(ppy) ) was used as the emitting layers (ꢂ40 nm),
3
and LiF was used as the electron-injecting layer for the devices [23]. All the emitting
layers were prepared by spin coating from solutions. Thickness of the layers was con-
trolled by changing speed (rpm) of the substrate. The layers of LiF and of Al cathode
ꢄ5
were then deposited at 1 ꢃ 10 torr.
The luminance and CIE chromatic coordinates of the resulting OLEDs were meas-
ured using a Minolta CS-100 luminance meter. A Keithley 2400 electrometer was used
to measure the current-voltage (I-V) characteristics. All the devices were characterized
without encapsulation and all the measurements were carried out under ambient condi-
2
tion. The emission area of all the devices was 25 mm and only the luminance in the
forward direction was measured.
2
.2. Materials
9
2
H-Carbazole (1), 3-bromomethyl-3-methyloxetane, 6-methoxy-3-pyridinylboronic acid,
-methoxy-3-pyridinylboronic acid, bis(triphenylphosphine)palladium(II) dichloride
[
Pd(PPh ) Cl ], boron trifluoride diethyl etherate [BF ꢅO(C H ) ], tetra-N-butylammo-
3
2
2
3
2
5 2
nium bromide, Na SO and KOH were purchased from Aldrich and used as received.
2
4
3
-Iodo-9H-carbazole (2) [24] and 3-iodo-9-(3-methyloxetan-3-yl)methylcarbazole (3)
25] were synthesized according to the procedures outlined in literature.
-(6-Methoxy-3-pyridinyl)-9-(3-methyloxetan-3-yl)methylcarbazole (4). 1.4g (3.7mmol)
[
3
of 3-iodo-9-(3-methyloxetan-3-yl)methylcarbazole (3), 1g (6.6mmol) of 6-methoxy-3-pyri-
dinylboronic acid, 0.14g (0.19mmol) of PdCl (PPh ) and 1.4g (24.9 mmol) of powdered
2
3 2
potassium hydroxide were stirred in 20ml of THF containing degassed water (1.5 ml) at
ꢀ
8
0 C under nitrogen for 24h. After TLC control the reaction mixture was cooled and
quenched by the addition of ice water. The product was extracted by ethylacetate. The
combined extract was dried over anhydrous Na SO . The crude product was purified by
2
4

1
62
G. KRUCAITE ET AL.
silica gel column chromatography using the mixture of ethylacetate and hexane (vol. ratio
ꢀ
1
:3) as an eluent. Yield: 0.8 g of yellowish crystals. M.p.: 134 C (DSC).
þ
1
MS (APCI , 20 V): 359.1 ([M þ H], 100%). H NMR (300 MHz, CDCl , d, ppm):
3
8
(
.48 (d, 1H, J ¼ 1,8Hz Ar), 8.22(d, 1H, J ¼ 2.1Hz Ar), 8.14 (d, 1H, J ¼ 7.2Hz, Ar), 7.90
dd, 1H, J =2.4Hz, J =8.7Hz Ar), 7.61 (dd, 1H, J =1.8Hz, J =8.7Hz Ar), 7.52-7.38(m,
1 2 1 2
4
H, Ar), 7.35-7.23 (m, 1H, Ar), 4.80 (d, 2H, J ¼ 6.0Hz, CH of oxetanyl fragment), 4,47
2
(s, 2H, NCH ), 4.38 (d, 2H, J ¼ 6.0Hz, CH of oxetanyl fragment), 4.01 (s, 3H, OCH ),
2
2
3
1.47 (s, 3H, CH ).
3
3-(2-Methoxy-3-pyridinyl)-9-(3-methyloxetan-3-yl)methylcarbazole (5). 1g (2.6mmol)
of 3-iodo-9-(3-methyloxetan-3-yl)methylcarbazole (3), 0.8g (5.3mmol) of 2-methoxy-3-
pyridinylboronic acid, 0.07 g (0.101 mmol) of PdCl (PPh ) and 0.7g (11 mmol) of
2
3 2
powdered potassium hydroxide were stirred in 20 ml of THF containing degassed
ꢀ
water (1.5 ml) at 80 C under nitrogen for 24 h. After TLC control the reaction mix-
ture was cooled and quenched by the addition of ice water. The product was extracted
by ethylacetate. The combined extract was dried over anhydrous Na SO . The crude
2
4
product was purified by silica gel column chromatography using the mixture of ethyl-
acetate and hexane (vol. ratio 1:2) as an eluent. Yield: 0.72 g of rosy crystals. M.p.:
ꢀ
1
17 C (DSC).
þ
1
MS (APCI , 20 V): 359.1 ([M þ H], 100%). H NMR (300 MHz, CDCl , d, ppm): 8.26
3
(
7
d, 1H, J ¼ 1.8Hz Ar), 8.17(dd, 1H, J =1.2Hz, J =4.8Hz Ar), 8.12 (d, 1H, J ¼ 8.1Hz Ar),
1
2
.65-7.63 (m, 2H, Ar), 7.50-7.36(m, 3H, Ar), 7.29-7.22 (m, 1H, Ar), 7.00(dd, 1H,
J =5.1Hz, J =7.2Hz Ar), 4.80(d, 2H, J ¼ 6.0Hz, CH of oxetanyl fragment), 4.47 (s, 2H,
1
2
2
NCH ), 4.38 (d, 2H, J ¼ 6.0Hz, CH of oxetanyl fragment), 4.01 (s, 3H, OCH ), 1,47 (s,
2
2
3
3
H, CH ).
3
Poly{3-(6-methoxy-3-pyridinyl)-9-(3-methyloxetan-3-yl)methylcarbazole}(6). Monomer 4
(
0.7g, 1.95mmol) was polymerized in 3.9 ml of 1,2-dichlorethane by using 8.3mg
ꢀ
(
0.058 mmol) of BF ꢅO(C H ) as an initiator. Polymerization mixture was stirred at 60 C
3
2 5 2
under nitrogen for 24 h. After precipitation into methanol, the low molecular weight frac-
tions of the polymer were removed by Soxhlet extraction of the raw material with metha-
1
nol. Yield: 0.3 g (42%) of yellowish amorphous powder. GPC: M = 1100, M = 860. H
w
n
NMR (400 MHz, CDCl , d, m.d.): 8.52-6.78 (m, 10H, Ar); 4.83-3.40 (m, 9H, NCH ir
3
2
CH OCH ir OCH ), 1.68-0.81 (m, 3H, CH ).
2
2
3
3
Poly{3-(2-methoxy-3-pyridinyl)-9-(3-methyloxetan-3-yl)methyl-carbazole}(7).
Monomer 5 (0.5g 1.38mmol) was polymerized in 2.7 ml of 1,2-dichlorethane by using
5
.9mg (0.042 mmol) of BF ꢅO(C H ) as an initiator. Polymerization mixture was stirred
3
2
5 2
ꢀ
at 60 C under nitrogen for 24 h. After precipitation into methanol, the low molecular
weight fractions of the polymer were removed by Soxhlet extraction of the raw material
with methanol. Yield: 0.3 g (42%) of white amorphous powder. GPC: M = 6350, M =
w
n
1
1920. H NMR (400 MHz, CDCl , d, m.d.): 8.23-6.71 (m, 10H, Ar); 4.40-3.15 (m, 9H,
3
NCH ir CH OCH ir OCH ), 1.24-0.73 (m, 3H, CH ) .
2
2
2
3
3
3. Results and discussion
Synthesis of the oligoethers (4-5) was carried out by a multi-step synthetic route as
shown in Figure 1. 3-Iodo-9H-carbazole (2) as a key material was synthesized from

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
163
Table 1. Molecular weights and PDI of oligomers 6 and 7
Polymer
M
w
M
n
PDI
T
g
6
7
860
1920
1100
635
1.28
3.31
124
145
commercially available 9H-carbazole (1) by Tucker iodination procedure. The iodo-
derivative 2 was converted to oxetanyl-functionalized compound 3 by reaction with
large excess of 3-bromomethyl-3-methyloxetane under basic conditions in the presence
of a phase transfer catalyst. The monomers 4 and 5 were then obtained from 3-iodo-9-
(3-methyloxetan-3-ylmethyl)carbazole by its Suzuki reaction with 6-methoxy-3-pyridi-
nylboronic acid or 2-methoxy-3-pyridinylboronic acid, correspondingly. Oligomers 6
and 7 were prepared by cationic polymerization of the corresponding monomers 4 or 5
in 1,2-dichloretane solutions using BF ꢅO(C H ) as an initiator. Low-molecular-weight
3
2
5 2
fractions of the products of polymerizations were removed by Soxhlet extraction of the
raw polymers with methanol.
The newly synthesized derivatives were identified by mass spectrometry (low molar
mass derivatives) and NMR spectroscopy (oligomers). The data were found to be in
good agreement with the proposed structures. Oligomers 6 and 7 were soluble in
common organic solvents, such as chloroform and THF at room temperature.
Transparent thin films of these materials could be prepared by spin coating from
their solutions.
The number-average molecular weights (M ), weight-average molecular weights (M )
n
w
and polydispersity indexes (PDI) of these oligomers were estimated by GPC. The data
are presented in the Table 1. It was observed that low-molecular-weight oligomers, i.e.
oligomers were obtained after the cationic polymerization. It should be mentioned that
molecular weights of the oligomers 6 and 7 are very close to those of the earlier
described polyethers containing unsubstituted carbazole rings [26].
The behaviour under heating of the materials 6 and 7 was studied by DSC and TGA
under a nitrogen atmosphere. DSC measurements confirmed that the synthesized
oligomers are amorphous materials with high glass transition temperatures (T ), which
g
are desirable for application of the electroactive materials in OLEDs. When sample of
ꢀ
oligomer 6 was heated, glass transition was observed at 124 C and no peaks due to
ꢀ
ꢀ
crystallisation and melting appeared in the range from -30 C to 220 C. When the sam-
ꢀ
ple was cooled, it formed again the amorphous material with T of 124 C. The sample
g
of oligomer 7 demonstrated analogous behaviour during the DSC experiment. Its glass
ꢀ
transition was observed at 145 C and no peaks due to crystallisation and melting
ꢀ
ꢀ
appeared in the range from -30 C to 220 C.
The oligomers demonstrated rather high thermal stability during the TGA experiment
as it could be seen from Figure 2 as an example. The temperature at 5% weight loss
ꢀ
(
T ) of oligomer 7 was 412 C indicating good thermal stability of the material. The
ID
sample of oligomer 6 demonstrated similar behaviour during the DSC experiment. Its
ꢀ
TID was exceeded 400 C.
To evaluate the performance of the new electroactive oligomers as host materials,
phosphorescent green OLEDs were fabricated using green triplet emitter Ir(ppy) as the
3
guest. The structure of the multilayer devices was ITO/PEDOT:PSS/oligomeric host 7

1
64
G. KRUCAITE ET AL.
Figure 1. Synthetic pathway of the electroactive oligomers.
100
90
80
70
60
50
40
30
20
10
0
100
200
300
400
500
600
700
Temperature, °C
ꢀ
Figure 2. TGA curve of the oligomer 7. Heating rate: 10 C/min.
doped with Ir(ppy) )/LiF/Al. Details of fabrication of the devices are described in
3
Experimental part.
The oligomer 7 demonstrated better film forming properties as well as compatibility
with Ir(ppy) guest. It was used in concentration-dependent PhOLED experiments with
3
the green guest amount ranging from 7 to 15wt%. The electro-phosphorescence was
found to originate only from Ir(ppy) at different bias voltages in all these devices as it is
3
demonstrated in Figure 3. No host and doped transport molecular emission was visible
from the OLEDs, indicating an energy transfer or charge transfer from the host to the
guest as well as the sufficient injection of both holes and electrons into the emitting layer.
Figure 4 shows characteristics of the devices. These PhOLEDs exhibited rather low
turn-on voltages of 4.0 ꢄ 5.1 V, maximal current efficiencies of 12.2 ꢄ 14.9 cd/A, max-
2
imal power efficiencies of 6.1 – 7.3 lm/W and maximal brightness of 3500-4073 cd/m .

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
165
1,0
0,8
0,6
0,4
0,2
0,0
dopant wt% in host 7
7
1
1
1
0
2.5
5
400
500
600
700
Wavelength (nm)
Figure 3. Electro-phosphorescent spectra of PhOLEDs with the green guest amount ranging from 7
to 15 wt%.
Efficiency roll-off at higher currents, which is typical for phosphorescent OLEDs, was
2
also observed here; however at high brightness of 1000 cd/m current efficiencies of
1
1.0 ꢄ 11.9 cd/A and maximal power efficiencies of 4.9 – 5.6 lm/W were still obtained. It
could be observed that rather high efficiencies of 6.8 – 7.3 cd/A (2.3 – 2.8 lm/W) were
established even at very high brightness of 3000 cd/m .
2
A green device containing 10 wt% of Ir(ppy) guest exhibited among all the devices
3
the best performance with current efficiency of 8.8 cd/A and power efficiency of 5.1 lm/
2
2
W at 100 cd/m . At higher brightness, such as 1000 cd/m , used for illumination appli-
cations, this PhOLED showed even enhanced efficiency of 11.7 cd/A (5.4 lm/W).
2
Maximal brightness of the device exceeded 4000 cd/m at 10 V.
In conclusion, new oligoethers with pyridinyl-carbazole electroactive units have been
synthesized by cationic polymerization of the corresponding oxetane-based monomers.
The amorphous oligomers have high thermal stability as well as very high glass transi-
ꢀ
ꢀ
tion temperatures (124 C and 145 C) and are expected to show also high morpho-
logical stability of thin electroactive films. Better film forming properties and
compatibility with triplet emitter of tris(2-phenylpyridine)iridium(III) (Ir(ppy) ) having
3
bipolar polymer was tested as host material for green phosphorescent organic light
emitting diode. The host was used in concentration-dependent PhOLED experiments
with the green guest amount ranging from 7 to 15 wt%. A green device containing
1
0 wt% of Ir(ppy) guest demonstrated among all the devices the best performance with
3
2
current efficiency of 8.8 cd/A and power efficiency of 5.1 lm/W at 100 cd/m . At higher
brightness, such as 1000 cd/m , used for illumination applications, this PhOLED showed
2
enhanced efficiency of 11.7 cd/A (5.4 lm/W). Maximal brightness of the device exceeded
2
4
000 cd/m at 10 V.
Acknowledgements
The OLED materials were developed in the frame of project funded by theKaunas University of
Technology (grant No. PDN2/18).

1
66
G. KRUCAITE ET AL.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
167
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