Microwave-assisted Efficient H/D Exchange Method of 9H-Carbazole and 2-Phenylpyridine as Organic Light-emitting Materials

Full text of DOI:10.1002/bkcs.11642

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DOI: 10.1002/bkcs.11642
BULLETIN OF THE
S. R. Kim and C. W. Lim
KOREAN CHEMICAL SOCIETY
Microwave-assisted Efficient H/D Exchange Method of 9H-Carbazole
and 2-Phenylpyridine as Organic Light-emitting Materials
*
Seo Ra Kim and Choon Woo Lim
Department of Chemistry, College of Life Science and Nano technology, Hannam University, Daejeon
305-811, Republic of Korea. *E-mail: cwlim@hnu.kr
Received November 7, 2018, Accepted November 21, 2018
Keywords: 9H-Carbazole, H/D exchange, Microwave, Organic light-emitting material, 2-Phenylpyridine
Organic light-emitting materials have much interest in
material science and many researchers have developed vari-
ous materials with new conceptual synthetic strategy. Espe-
cially, research interest on phosphorescent and thermally
activated delayed fluorescence (TADF) materials have been
devoted to get high power efficiency of device, because of
their unique photophysical properties (e.g., theoretically
100% internal quantum efficiency) and the potential for a
new field of applications in organic light-emitting diodes
(OLEDs).1 Device life time and stability are important
issues and interestingly studied in terms of intrinsic material
stability as well as device architectures to improve life time
and power efficiency. Recently, isotope effect has been
reported to be advantageous for genuine material stability.
The deuterium exchange has been applied to many func-
tional layers such as host2 and dopant3 of emitting layer,
electron-transport layer (ETL), and hole-transport layer
(HTL),4 reporting longer device life time and higher quan-
tum efficiency.
Deuterated compounds were prepared via several deute-
rium exchange methods1a,5 and, for aromatic compounds
such as organic light-emitting materials, one of useful
approaches is the use of transition metal catalyst (Pd/C or
Pt/C) under hydrogen atmosphere and deuterated water. In
our study, we prepared deuterated 9H-carbazole and
2-phenylpyridine using microwave reactor. Carbazole is
useful precursor for hole-transport materials, electron-
transport materials and emitting materials with high triplet
energy (T1).4 Phenylpyridine is widely used as a ligand of
phosphorescent dopant, such as cyclometallated iridium
(III) complexes.1a Microwave reaction is known to be use-
ful in metal catalytic reaction.6 As shown in Scheme 1, we
run H/D exchange reactions (Pd/C or Pt/C, D2O) under
microwave irradiation and several conditions were investi-
gated to obtain highly deuterated compound.
Scheme 1. H/D exchange route of 9H-carbazole(1) and
2-phenylpyridine(2) in microwave.
hydrothermal conditions (240 ꢀC), which was gave 80%
yield and 94% deuterium of the aromatic proton sites.4
Microwave condition provides advantages of heterogeneous
metal catalyst reaction, which enable shorter reaction period
and lower reaction temperature than hydrothermal substitu-
tion methods.
In Table 2, H/D exchange condition showed
2-phenylpyridine was found to have efficient deuterium
ꢀ
substitution rate of 90% with 160 C, Pd/C catalyst 25 wt
% and 1 h running time. It was previously reported deuter-
ated 2-phenylpyridine was obtained from two different
methods. One was synthetic reaction from deuterated bro-
mobenzene and deuterated pyridine3 and the other was
hydrothermal reaction of 2-phenylpyridine using Pt/C cata-
lyst, Al powder in D2O.1a Compared with those two
methods, microwave-assisted H/D exchange scheme shows
direct and efficient deuteration of aromatic protons. From
the results of H/D exchange condition (Tables 1 and 2),
both Pd/C and Pt/C catalysts do not show any significant
difference of deuteration efficiency for aromatic protons
under microwave irradiation condition.
H/D exchange conditions were optimized, based on tem-
perature, amount of catalyst and running time using micro-
wave reactor (Tables 1 and 2). According to optimized
results, 9H-carbazole was found to have a high deuterium
substitution rate of 93% (Table 1. Entry 8) from the
repeated run of entry 7 product. Yamamoto et al.7 has
reported that carbazole-D8 was obtained for 48 h under
In 1H NMR spectrum (Figure 1), each proton of 9H-
carbazole (1) and 2-phenylpyridine (2) were individually
assigned and D content of each proton position was quanti-
tatively calculated from an internal standard (Table 3).
9H-carbazole (1) and 2-phenylpyridine (2) in Figure 1 cor-
respond to entry 8 in Tables 1 and 2, respectively. It has
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KOREAN CHEMICAL SOCIETY
Table 1. H/D exchange conditions of 9H-carbazole.a
Experimental
Temperature
(ꢀC)
10% Pt/C
(wt %)
Time
(h)
General. All the chemicals were obtained commercially
and used without further purification. 10% Palladium on
carbon and 10% platinum on carbon were purchased from
Aldrich Chemical Co. (St. Louis, MO, USA) and deuterium
oxide (D, 99.9%) was purchased from Cambridge Isotope
Laboratories, Inc. (Tewksbury, MA, USA).
The H/D exchange reactions were performed using a
CEM Discover® SP microwave reactor (CEM Corporation,
Matthews, NC, USA) (10 mL vessel capacity, 250 psi max-
imum pressure, 300 ꢀC maximum temperature). The 1H
NMR spectra were recorded on 600 MHz spectrometer
(Varian, Santa Clara, CA, USA). The high resolution FAB
mass spectra were recorded on JMS-700 (JEOL, Tokyo,
Japan) and 6890 series (Agilent, Santa Clara, CA, USA).
Entry
D content (%)b
1
2
3
4
5
6
7
8
a
80
10
10
5
10
10
10
25
25
1.5
1.5
1.5
0.5
1.5
3
25
57
36
55
79
84
81
93c
120
160
160
160
160
160
160
1.5
1.5
All data performed by microwave reactor.
The deuterium contents (D content) were determined by 1H NMR
spectrum using internal standard.
b
c
Second run. The 81% deuterated substrate (the product of entry 7)
was used as the starting material.
Estimation of Deuterated Ratio. D content(%) was calcu-
lated by the internal standard method and obtained from
relative proton (1H) integration ratio between deuterated
compounds (1 and 2) and 1,4-dioxane as an internal
Table 2. H/D exchange conditions of 2-phenylpyridine.a
Temperature
(ꢀC)
10% Pd/C
(wt %)
Time
(h)
Entry
D content (%)b
1
2
3
4
5
6
7
8
a
80
10
10
5
1
1
1
2
0.5
1
10
67
77
56
80
62
72
90
120
160
160
160
160
160
160
5
10
10
10
25
2
1
All data performed by microwave reactor.
b
The deuterium contents (D content) were determined by 1H NMR
spectrum using internal standard.
found that H/D exchange ratio at d-position (77%) of 9H-
carbazole and e, i-position (82%) of 2-phenylpyridine was
relatively low, which might be caused by steric
hindrance.5a
And high deuterium substitution rate at the a, b, c-
position of 9H-carbazole (approximately 96–97%) and pyri-
dyl group protons (a, b, c,and d) of 2-phenylpyridine
(approximately 95–98%) was observed.
These results indicate that deuterium exchange is more
advantageous in the vicinity of nitrogen, because metal cat-
alysts such as palladium and platinum have high affinity for
a lone pair of nitrogen atom.
In conclusion, we propose microwave reaction to enable
effective deuterium exchange of aromatic proton for the
application of organic light-emitting material without any
preference of Pt or Pd metal catalyst. And we provide opti-
mized experimental conditions for deuterium exchange,
based on temperature, amount of catalyst and reaction time
using microwave reactor.
Figure 1. 1H NMR spectra of (a) 9H-carbazole (1) and
(b) 2-phenylpyridine (2) obtained by H/D exchange (600 MHz,
CDCl3, 1,4-dioxane as an internal standard).
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Table 3. Each proton position D content(%).a
D content(%) substituted in each proton
Compoundsb
Catalyst
a
b
c
d
e
f
g
h
i
D content(%)
9H-Carbazole
2-Phenylpyridine
Pt/C
Pd/C
97
98
97
95
96
96
77
96
—
—
—
—
—
93
90
82
91
91
91
82
a
D content(%) was determined by 1H NMR spectrum using 1,4-dioxane internal standard.
Compounds were obtained by H/D exchange under microwave irradiation condition.
b
standard.8 D content(%) of deuterated compound can be
calculated using the following formula:
an internal standard) δ 8.70 (s), 7.99 (s), 7.74 (d), 7.44 (d),
7.24 (m); HR-MS (FAB, positive): mass calculated for
C11H1D9N1 [M + H]+, 165.1378; observed 165.1380 m/z,
int 64.7%, Err +1.2 ppm.
Starting material yieldð%Þ
1HNMR Integral of starting material
1HNMRIntegral of internal standard
¼
× n
Acknowledgments. This research was supported by the
Basic Science Research Program through the National
Research Foundation of Korea (NRF - 2017R1D1A1B0303
5388).
n: The ratio of the internal standard to the initial amount of
starting material.
D contentð%Þ ¼ 100%−starting material yieldð%Þ
References
Deuterated 9H-Carbazole. A mixture of 9H-carbazole
(30 mg, 0.18 mmol), 10% Pt/C (7.5 mg, 25 wt % substrate)
in D2O (2 mL) in a microwave vessel was bubbled with H2
for 10 min and charged with H2 for 10 min, followed by
microwave irradiation, using the following parameters
(mode: dynamic, power: 160 W, temperature: 160 ꢀC, time:
1.5 h, pressure: 200 psi, high stir, cooling off ). After cool-
ing, the reaction mixture was partitioned with dichloro-
methane and water. The organic extracts were filtered
through a membrane filter (Whatman™-PTFE, 0.20 μm) to
remove Pt/C catalyst. The filtrate was concentrated in vacuo
1. (a) T. Abe, A. Miyazawa, H. Konno, Y. Kawanishi, Chem.
Phys. Lett. 2010, 491, 199. (b) M. A. Baldo, S. Lamansky,
P. E. Burrows, M. E. Thompson, S. R. Forrest, Appl. Phys.
Lett. 1999, 75, 4. (c) M. A. Baldo, M. E. Thompson,
S. R. Forrest, Nature 2000, 403, 750. (d) C. Adachi,
M. A. Baldo, S. R. Forrest, M. E. Thompson, Appl. Phys. Lett.
2000, 77, 904. (e) E. Holder, B. M. W. Langeveld,
U. S. Schubert, Adv. Mater. 2005, 17, 1109. (f ) B. Minaev,
V. Minaeva, H. Ågren, J. Phys. Chem. A 2009, 113, 726.
2. H. Tsuji, C. Mitsui, E. Nakamura, Chem. Commun. 2014, 50,
14870.
3. P. Wang, F. F. Wang, Y. Chen, Q. Niu, H. M. Wang,
X. C. Gao, B. Wei, H. W. Wu, X. Cai, D. C. Zou, J. Mater.
Chem. C 2013, 1, 4821.
4. T. A. Darwish, A. R. G. Smith, I. R. Gentle, P. L. Burn,
E. Luks, G. Moraes, M. Gillon, P. J. Holden, M. James, Tetra-
hedron Lett. 2012, 53, 931.
5. (a) A. M. Krause-Heuer, N. R. Yepuri, T. A. Darwish,
P. J. Holden, Molecules 2014, 19, 18604. (b) V. Derdau,
J. Atzrodt, J. Zimmermann, C. Kroll, F. Brückner, Chem.
Eur. J. 2009, 15, 10397. (c) J. A. McEwan, A. J. Clulow,
A. Nelson, N. R. Yepuri, P. L. Burn, I. R. Gentle, ACS Appl.
Mater. Interfaces 2017, 9, 14153. (d) J. Atzrodt, V. Derdau,
J. Label. Compd. Radiopharm. 2010, 53, 674. (e) H. Sajiki,
N. Ito, H. Esaki, T. Maesawa, T. Maegawa, K. Hirota, Tetrahe-
dron Lett. 2005, 46, 6995.
6. S. Horikoshi, N. Serpone, Catal. Sci. Technol. 2014, 4, 1197.
7. M. Yamamoto, K. Oshima, S. Matsubara, Heterocycles 2006,
67, 353.
8. M. Oromí-Farrús, G. Villorbina, J. Eras, F. Gatius, M. Torress,
R. Canela, Fuel 2010, 89, 3489.
1
(28 mg, yield 94%, D content 81%). H NMR (600 MHz,
CDCl3, 1,4-dioxane as an internal standard) δ 8.08 (d),
7.43 (d), 7.25 (d); HR-MS (FAB, positive): mass calculated
for C12H1D8N1 [M + H]+, 175.1237; observed
175.1236 m/z, int 90.0%, Err −0.5 ppm.
Deuterated 2-Phenylpyridine.
A
mixture of
2-phenylpyridine (50 mg, 0.32 mmol), 10% Pd/C
(12.5 mg, 25 wt % substrate) in D2O (2 mL) in a micro-
wave vessel was bubbled with H2 for 10 min and charged
with H2 for 10 min, followed by microwave irradiation,
using the following parameters (mode: dynamic, power:
ꢀ
160 W, temperature: 160 C, time: 1 h, pressure: 200 psi,
high stir, cooling off ). After cooling, the reaction mixture
was partitioned with dichloromethane and water. The
organic extracts were filtered through membrane filter
(Whatman™-PTFE, 0.20 μm) to remove Pd/C catalyst. The
filtrate was concentrated in vacuo (29.9 mg, yield 60%, D
1
content 90%). H NMR (600 MHz, CDCl3, 1,4-dioxane as
Bull. Korean Chem. Soc. 2019
© 2019 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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