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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