High hole mobilities in the amorphous films of 2,7-di(9-carbazolyl)-9-(2- ethylhexyl)carbazole

Article abstract of DOI:10.1246/cl.2008.344

2,7-Di(9-carbazolyl)-9-(2-ethylhexyl)carbazole was synthesized and investigated as new effective hole-transporting material. The synthesized compound showed high thermal stability with the initial weight loss temperature of 400°C. It forms glass with glass-transition temperature of 88°C. Hole drift mobilities in the amorphous films of the newly synthesized compound exceeded 10^-2cm²V^-1 s^-1 at high electric field, as characterized by the xerographic time of flight technique. Copyright

Full text of DOI:10.1246/cl.2008.344

344
Chemistry Letters Vol.37, No.3 (2008)
High Hole Mobilities in the Amorphous Films of 2,7-Di(9-carbazolyl)-9-(2-ethylhexyl)carbazole
Ausra Tomkeviciene,1 Juozas V. Grazulevicius,ꢀ1 and Vygintas Jankauskas2
1Department of Organic Technology, Kaunas University of Technology, Radvilenu pl. 19, LT-50254, Kaunas, Lithuania
2Department of Solid State Electronics, Vilnius University, Sauletekio al. 9, LT-10222, Vilnius, Lithuania
(Received October 22, 2007; CL-071163; E-mail: juozas.grazulevicius@ktu.lt)
2,7-Di(9-carbazolyl)-9-(2-ethylhexyl)carbazole was synthe-
sized and investigated as new effective hole-transporting materi-
al. The synthesized compound showed high thermal stability
with the initial weight loss temperature of 400 ꢁC. It forms glass
with glass-transition temperature of 88 ꢁC. Hole drift mobilities
in the amorphous films of the newly synthesized compound ex-
ceeded 10ꢂ2 cm2 Vꢂ1 sꢂ1 at high electric field, as characterized
by the xerographic time of flight technique.
metries, and elemental analysis.7
The glass-forming capability and thermal stability of 2,7-
di(9-carbazolyl)-9-(2-ethylhexyl)carbazole (1) were estimated
by differential scanning calorimetry (DSC) and thermogravimet-
ric analysis (TGA). Compound 1 exhibited melting isotherm
during the first DSC heating scan, but cooling of the melt led
to the formation of a glassy state that persisted in the subsequent
DSC scans (Figure 1). Compound 1 appeared to have consider-
ably lower glass-transition temperature (Tg) than the earlier
synthesized 3,6-disubstituted carbazole derivative of very simi-
lar structure, i.e. 3,6-di(9-cabazolyl)-9-octylcarbazole.8 Tg of 1
was found to be 88 ꢁC while the 3,6-di(9-carbazolyl)-9-octylcar-
bazole showed Tg of 134 ꢁC. The 1% weight loss temperature
(Td) for the 2,7-disubstituted carbazole compound 1 (400 ꢁC)
was found to be higher than that for 3,6-di(9-cabazolyl)-9-
octylcarbazole (375 ꢁC).
UV absorption spectra of the dilute solution of compound 1
is given in Figure 2. For the comparison the spectrum of 3,6-
di(9-cabazolyl)-9-octylcarbazole is also shown. The lowest
energy absorption band of compound 1 is hyperchromically
and bathochromically shifted with respect to that of 3,6-di(9-
cabazolyl)-9-octylcarbazole.
The ionization potential (Ip) was measured by electron
photoemission in air.9 The Ip value for the film of compound 1
was found to be 5.83 eV. 3,6-Di(9-carbazolyl)-9-octylcarbazole
showed very similar Ip value of 5.80 eV. Using the value of
optical band gap (ꢀE) which can be estimated by the absorption
spectroscopy, a LUMO value of 2.45 eV was calculated for 1 by
the method described before.10
Amorphous film-forming hole-transporting materials are
known for various electronic and optoelectronic applications.1–3
Glass-forming semiconductors with high charge mobilities
are of particular interest for the application in organic-thin-film
transistors and solar cells. Compounds containing 2,7-substitut-
ed carbazole unit represent a class of effective hole-transporting
materials.4 The aim of this work was synthesis of new 2,7-dicar-
bazolyl-substituted carbazole derivative and estimation of its
charge-transport properties. There is a substantial number of
studies on 3,6-disubstituted carbazole compounds, including
small molecules, oligomers, and polymers for optoelectronic
applications. However, there are only few studies on low-
molecular-weight 2,7-disubstituted carbazole compounds so
far. The main obstacle was the lack of an efficient synthesis
procedure for these compounds. Leclerc and co-workers5 and
Mullen and co-workers6 reported convenient synthetic pathways
towards 2,7-dihalocarbazoles, which are useful precursors for
carbazole-based optoelectronic materials.
2,7-Di(9-carbazolyl)-9-(2-ethylhexyl)carbazole (1) was ob-
tained as described in Scheme 1 by the synthetic route,
comprising N-alkylation of 2,7-dibromo-9H-carbazole with 2-
ethylhexyl bromide to form 2,7-dibromo-9-(2-ethylhexyl)-
carbazole and a palladium-catalyzed aromatic C–N coupling
reaction of the later with carbazole. The synthesized compound
1 was purified by column chromatography followed by crystal-
lization from hexane to obtain pure and well-defined compound.
Hole-transport properties of compound 1 were studied by
the xerographic time-of-flight method. Representative dU=dt
transient for the amorphous films of 2,7-disubstituted carbazole
compound 1 is shown in Figure 3. It shows dispersive hole
transport. The dispersive charge transport was also observed
for 3,6-di(9-cabazolyl)-9-octylcarbazole.8
The hole-transit times (tT) needed for the estimation of hole
mobilities were established from intersection points of two
1
Compound 1 was characterized by H NMR, IR, mass spectro-
Acetic acid,
HNO3
1
st heating
P(OC2H5
)
3
Br
Br
Br
Br
Br
Br
Tm = 190 °C
N
H
NO2
2nd heating
Tg = 88 °C
Br
NaH
H
N
T
g
N
Br
N
Br
Pd2(dba)3, P(t-Bu)3
t-BuONa,Toluene
,
70
80
90
100
110
N
N
Temperature/°C
50
100
150
200
250
Temperature/°C
1
Figure 1. DSC curves of compound 1, at the heating/cooling
rate of 10 ꢁC minꢂ1, N2 atmosphere.
Scheme 1. Outline of the synthetic route.
Copyright ꢀ 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.3 (2008)
345
follows the nearly universal Poole–Frenkel relationship: ꢀ ¼
pffiffiffi
6x105
5x105
4x105
3x105
2x105
3,0x104
2,5x104
2,0x104
1,5x104
1,0x104
5,0x103
0,0
1
1A
ꢀ0 expðꢁ EÞ, which is usually observed for disordered organic
systems, where ꢁ is the Poole–Frenkel factor.11 Hole mobilities
in the amorphous film of compound 1 well exceed 10ꢂ2
cm2 Vꢂ1 sꢂ1 at high electric fields. They are by almost one order
of magnitude higher than those observed for 3,6-di(9-cabazolyl)-
9-octylcarbazole. The more extended conjugation network in
2,7-disubstituted carbazole may result in stronger electronic
coupling between molecules, thereby facilitating hole hopping.
It is also possible that different linking topologies (and thus
conformations) in 2,7- and 3,6-disubstituted carbazoles lead to
significant differences in molecular packing, which in turn
affect the molecular packing density and the positional disorder
in thin films, which are critical for hole transport.
300
320
340
360
380
Wavelength, [nm]
1x105
0
200
250
300
350
400
Wavelength, [nm]
Figure 2. UV absorption spectra of the dilute THF solutions
(10ꢂ5 M) of compound 1 and 3,6-dicabazolyl-9-octylcarbazole.
Financial support of this research by the Lithuanian Science
and Studies Foundation is gratefully acknowledged.
107
106
105
tT
U = 320 V
References and Notes
1
2
3
4
5
a) Y. Shirota, J. Mater. Chem. 2005, 15, 75. b) Y. Shirota,
H. Kageyama, Chem. Rev. 2007, 107, 953.
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107, 1324.
P. M. Borsenberger, D. S. Weiss, Organic Photoreceptors
for Xerography, Dekker, New York, 1998.
J.-F. Morin, M. Leclerc, D. Ades, A. Siove, Macromol.
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1
d = 4 µm
10-8
10-7
10-6
Time/s
Figure 3. Time-of-flight transient for amorphous 4 mm thick
layer of 1 (U is the surface voltage).
a) G. Zotti, G. Schiavon, S. Zecchin, J.-F. Morin, M. Leclerc,
Macromolecules 2002, 35, 2122. b) J.-F. Morin, M. Leclerc,
Macromolecules 2002, 35, 8413.
6
7
F. Dierschke, A. C. Grimsdale, K. Mullen, Synthesis 2003,
2470.
0,1
0,01
1E-3
Compound 1: mp. 188–189 ꢁC. A 1H NMR spectrum yielded
the following chemical shifts (300 MHz, CDCl3), ꢂ 0.83
(t, J ¼ 6:45 Hz, 3H, CH3), 0.95 (t, J ¼ 7:30 Hz, 3H, CH3),
1.25–1.51 (m, 8H, CH2), 2.11–2.19 (m, 1H, CH), 4.23 (d,
J ¼ 7:34 Hz, 2H, NCH2), 7.37 (d, J ¼ 1:10 Hz, 1H), 7.39
(d, J ¼ 1:46 Hz, 2H), 7.42 (d, J ¼ 1:10 Hz, 1H), 7.48–7.60
(m, 10H), 7.69 (d, J ¼ 1:84 Hz, 2H), 8.27 (d, J ¼ 7:33 Hz,
4H), 8.40 (d, J ¼ 8:02 Hz, 2H). IR (KBr), ꢃ (cmꢂ1); (arene
C–H) 3042, 3021; (aliphatic C–H) 2952, 2925, 2853;
(C=C in Ar) 1496, 1459. MS (APClþ, 20 V), m=z (%) 610
([M + H]þ, 100). Anal. Calc. for C44H39N3: C, 86.66; H,
6.45; N 6.89%. Found: C, 86.70; H 6.50; N 6.92%.
S. Grigalevicius, J. V. Grazulevicius, V. Gaidelis, V.
Jankauskas, Polymer 2002, 43, 2603.
2,7-Dicarbazolyl-9-(2-ethylhexyl)carbazole
3,6-Dicarbazolyl-9-octylcarbazole
1E-4
600
800
1000
1200
E
1/2, [V/cm]1/2
Figure 4. The electric field dependencies of the hole drift
mobilities in the amorphous films of compound 1 and 3,6-
di(9-cabazolyl)-9-octylcarbazole.
8
9
asymptotes from the double-logarithmic plots. The obtained
hole mobilities are shown in Figure 4 as a function of the square
root of electric field (E1=2). For the comparison electric field
dependence of hole mobility is shown also for the amorphous
film of 3,6-di(9-cabazolyl)-9-octylcarbazole.
E. Miyamoto, Y. Yamaguchi, M. Yokohama, Electro-
photography 1989, 28, 364.
10 A. Dijken, J. J. A. M. Bastiaansen, N. M. M. Kiggen,
B. M. W. Langeveld, C. Rothe, A. Monkman, I. Bach, P.
Stossei, K. Brunner, J. Am. Chem. Soc. 2004, 126, 7718.
¨
The field dependence of the hole mobilities (ꢀ)
11 H. Bassler, Int. J. Mod. Phys. B 1994, 8, 847.
¨
Published on the web (Advance View) February 23, 2008; doi:10.1246/cl.2008.344