New indolo[3,2-b]carbazole derivatives for field-effect transistor applications

Article abstract of DOI:10.1039/b900271e

The synthesis, characterization, and field-effect transistor (FET) properties of new indolo[3,2-b]carbazole (IC) based materials are reported. Instead of adding the long alkyl chains on the nitrogen atoms of the IC backbone like many other IC-based molecu

Full text of DOI:10.1039/b900271e

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
New indolo[3,2-b]carbazole derivatives for field-effect transistor applications†
b
c
b
a
Pierre-Luc T. Boudreault,ab Salem Wakim,ac Ming Lee Tang, Ye Tao, Zhenan Bao and Mario Leclerc
*
*
Received 8th January 2009, Accepted 18th February 2009
First published as an Advance Article on the web 20th March 2009
DOI: 10.1039/b900271e
The synthesis, characterization, and field-effect transistor (FET) properties of new indolo[3,2-
b]carbazole (IC) based materials are reported. Instead of adding the long alkyl chains on the nitrogen
atoms of the IC backbone like many other IC-based molecules, they were added at both ends of the
molecule (octylthiophene, p-octylbenzene). Also, the amine groups on the IC backbone were either free
or protected by methyl groups. The impact on the organization and thin-film morphology showed that
the molecules stand perpendicular to the surface as demonstrated by XRD and AFM. The highest hole
mobility obtained by these new p-type organic semiconductors was 0.22 cm2 Vꢀ1 sꢀ1 with an on/off ratio
of about 105. The best performance was obtained with 3,9-di(p-octylbenzene)-5,11-
dihydroxyindolo[3,2-b]carbazole. This performance is one of the best obtained by both IC derivatives
and materials containing a secondary amine on the backbone.
backbone and a favorable side chain conformation. Ong and
coworkers subsequently reported that the mobility could be
improved by 2 orders of magnitude if halogens were added at the
2,8-positions or if the linear alkyl chains on the amino group
were replaced by 4-octylphenyl substituents.10 More recently, we
reported new IC derivatives, with phenyl or thiophene groups at
the 2,8- or 3,9-positions and alkyl chains on the amine groups,
Introduction
Great attention has been paid to organic semiconductors (OSCs)
over the past two decades.1 This class of materials is promising
because of its great versatility and potential as organic field-effect
transistors (OFETs),2 photovoltaic cells (PCs),3 and organic
light-emitting diodes (OLEDs).4 Significant progress has been
made such that oligomers and polymers might be competitive
with amorphous silicon (a-Si) in FET and PC applications.
Organic semiconductors may be used for applications currently
using a-Si transistors for integrated large area, flexible, low cost
circuits such as radio frequency identification (RFID) tags, smart
cards, and active matrix displays.5 Although there are some
OSCs with charge carrier mobilities higher than those of a-Si,
most of them still have stability and solubility issues. For
instance, pentacene and rubrene6 are oxidized under ambient
conditions and present a lack of solubility and photostability.
A promising pathway to afford stable and soluble OSCs is the
synthesis of indolo[3,2-b]carbazole (IC) derivatives.7 It has been
shown for the past few years that carbazole- and indolocarba-
zole-based materials combine great stability, solubility, and good
performance in OFETs,8–11 PCs,12 and thermoelectric materials.13
Our group reported an IC derivative, i.e., the 5,11-dioctyl-6,12-
dimethylindolo[3,2-b]carbazole in an OFET application with
that show hole mobilities up to 0.20 cm2 Vꢀ1 sꢀ1 11
Crystallo-
.
graphic data of the phenyl-substituted indolo[3,2-b]carbazole
derivatives indicated that their aromatic backbones formed
layered packing with strong bidirectional face-to-face and edge-
to-face interactions within each layer. Thin-film X-ray diffrac-
tion analyses also indicated that the aromatic backbone adopted
an edge-on orientation relative to the substrate with the alkyl
chains pointed toward the surface. This kind of organization
where the long axis of the active semiconducting core is parallel
to the substrate surface is very interesting and explains the
high mobility values obtained with the phenyl-substituted IC
derivatives.
However, linear IC derivatives with alkyl chains at both ends
of the aromatic backbone have not been reported. Therefore, it
could be interesting to develop such molecules and evaluate their
performance in OFETs. Moreover, the organization of these
linear IC derivatives in thin-films may be completely different to
those with long side chains on the amino groups.11 Thus, the
development of these linear IC derivatives will provide additional
information about the relationships between molecular struc-
ture, solid state organization, and charge transport properties of
IC derivatives.
a hole mobility of 10ꢀ3 cm2 Vꢀ1 sꢀ1 9
. X-Ray crystallographic
analysis showed that the molecules possessed strong cofacial p–p
˚
stacking, with short intermolecular distances of 3.6 A. This kind
of organization was obtained due to the planar structure of the
aCanada Research Chair on Electroactive and Photoactive Polymers,
In this article, we report the synthesis and characterization of
six new linear IC derivatives with terminal alkyl side chains. We
introduce phenyl and thiophene units at the 3,9-positions (as
shown in Fig. 1), instead of the 2,8-positions, to allow the
formation of such linear structures. Moreover, 3,9-aryl-
substituted IC derivatives with alkyl chains on the amino groups
have shown better thin-film organization and higher perfor-
mances than their corresponding 2,8-aryl-substituted IC deriva-
tives.11 Since we used octyl side chains for the previously
ꢀ
ꢀ
Departement de Chimie, Universite Laval, Quebec City, Quebec, Canada
G1K 7P4. E-mail: mario.leclerc@chm.ulaval.ca
bDepartment of Chemical Engineering, Stanford University, 381 North
South Mall, Stanford, CA 94305-5025, U. S. A.. E-mail: zbao@stanford.
edu
cInstitute of Microstructural Sciences, National Research Council of
Canada, Ottawa, Ontario, Canada K1A 0R6
† Electronic
Characterization of materials, UV-vis spectra and AFM images. See
DOI: 10.1039/b900271e
supplementary
information
(ESI)
available:
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Results and discussion
Synthesis of the indolo[3,2-b]carbazole derivatives
The complete synthesis of the new IC-based materials is
described in Scheme 1. The starting compound for the synthesis
of the new IC derivatives is 3,9-dibromo-5,11-dihydroindolo[3,2-
b]carbazole, whose synthesis has been described in the litera-
ture.10 The next step is to add bulky t-butyloxycarbonyl (Boc) on
both amines of the IC backbone. Those groups will not only act
as protecting groups for further reactions but will also make the
framework more readily soluble in common solvents such as
dichloromethane and tetrahydrofuran (THF). Tholander and
Bergman17 have described an efficient way to add the Boc units
onto the amines of the IC. Using the same synthetic procedure
with 3,9-dibromo-5,11-dihydroindolo[3,2-b]carbazole, the pure
protected IC (2) was isolated in a 86% yield.
Fig. 1 Chemical structure of 3,9-disubstituted indolo[3,2-b]carbazole.
synthesized oligomers, we decided to continue with the same
trend by comparing different molecules with the same chain
length. For two of the IC derivatives, vinylene units were intro-
duced into the oligomeric structures and, hopefully, they will
result in coplanar molecules with low dihedral angles between the
vinylene and aryl groups.14 In addition, we studied the effect of
having two hydrophilic free amines on the molecule compared
with the amine protected with methyl side chains. Up to now,
only a few molecules containing free amines on their backbones
have been synthesized with limited success for OFET applica-
Afterwards, we were able to add aromatic units to both ends of
compound 2. 5-Octylthiophene units were added using Stille
coupling18 to afford compound 3 in a 59% yield. On the other
hand, a p-octylbenzyl group was introduced using the Suzuki
cross-coupling reaction reported by Buchwald and coworkers19
giving compound 4 in a 73% yield. It was also possible to perform
a Suzuki cross-coupling between compound 2 and compound 5,
affording compound 6 in a 76% yield. Compound 5 was obtained
from vinylboronic acid pinacol ester and 1-iodo-4-octylbenzene
tions.
Nuckolls
and
coworkers15
reported
dihy-
drodiazapentacene with a field-effect mobility of about 0.06 cm2
Vꢀ1 sꢀ1 in the saturation regime. Also, imidazolylquinoline
derivatives were synthesized by Chen et al. with a hole mobility
of 0.15 cm2 Vꢀ1 sꢀ1 16
.
Scheme 1 Synthesis of new indolo[3,2-b]carbazole derivatives. DMAP ¼ dimethylaminopyridine. SPhos ¼ 2-dicyclohexylphosphino-20,60-dimethoxy-
biphenyl. TFA ¼ trifluoroacetic acid. BTAC ¼ benzyltriethylammonium chloride.
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via a modified Heck coupling developed by Stewart and
Whiting.20 These conditions allow the Heck reaction product to
be produced at the expense of the Suzuki product. A depro-
tection reaction was performed on compounds 3, 4, and 6 by
treating them with a dichloromethane–TFA (1 : 1) mixture to
give the new materials 7, 8 and 9 in good yields. Finally,
compounds 10–12 were synthesized in good to moderate yields
by a double alkylation of the unprotected compounds using the
phase transfer condensation reaction in basic conditions.11 The
materials were characterized by elemental analyses and high
resolution mass spectroscopy. It was very difficult to obtain
NMR spectra from these molecules because of their very low
solubility. We believe that the purity should be high enough to be
tested in transistor devices. We hope to investigate the effect of
the structural modification on the organization of the molecules
when sublimated onto Si–SiO2 substrates.
Fig. 2 UV-vis absorption spectra of vacuum-deposited thin-films of
compound 10 at different deposition temperatures.
are located at around 370 nm and the longest wavelength
absorption maxima are at 460 nm. Above this deposition
temperature, the absorption band at 370 nm is considerably
smaller relative to the band left at around 460 nm. The same
phenomenon is observed for the band at 290 nm which greatly
diminishes at higher deposition temperatures leaving a relatively
strong band at 260 nm. Similar behaviours were also observed
for compounds 7, 8 and 11. This could be caused by a different
organization on the surface at higher deposition temperatures. It
will be shown later that this behaviour is in good agreement with
the atomic force microscopy (AFM) images, X-ray diffraction
(XRD) analyses and FET properties. Finally, the optical
bandgap (Eg) of the new materials estimated from the solid state
spectra range from 2.62 to 2.76 eV.
Thermal, optical, and electrochemical properties
Thermogravimetric analysis (TGA) showed that the new deriv-
atives are all thermally stable with decomposition temperatures
above 250 ꢁC. Differential scanning calorimetry (DSC)
measurements were performed on all new materials and revealed
quite surprising thermal properties. With the exception of
compound 10, all the other compounds did not show any thermal
transition before the decomposition temperature. For compound
10, we observed two transitions during the heating process; one
ꢁ
ꢁ
sharp peak at 195 C and a second small peak at 250 C. When
cooling the sample down, a crystallisation transition can be
observed at 244 ꢁC.
Cyclic voltammetry (CV) experiments showed irreversible
oxidation processes in the case of the IC derivatives containing
secondary amines (7–9) and a quasi-reversible oxidation wave for
their corresponding methylated compounds (10–12). The highest
occupied molecular orbital (HOMO) energy levels, estimated
from the oxidation onset,22 range from 5.3 to 5.6 eV (Table 1).
Surprisingly, compounds 7 and 10 with thiophene moieties have
the lowest HOMO energy levels. Also, the addition of a methyl
group on the secondary amine of the IC derivatives increases
their HOMO energy levels. The HOMO energy levels are below
the air oxidation threshold (which is around 5.27 eV), indicating
good oxidation stability for all the IC derivatives.23
Furthermore, UV-vis absorption spectra of these molecules in
dichloromethane and N,N-dimethylformamide solutions and as
thin-films are reported in Table 1 and shown in Fig. 1S and 2S
(ESI†). A trend can be extracted from both sets of spectra: the
longest conjugation length was obtained with ICs disubstituted
with 5-octylthiophene (382 nm) and p-octylphenylenevinylene
(380 nm). The p-octylbenzene moieties induced the shortest
conjugation length among these 3 derivatives (364 nm). These
results are not surprising considering that thiophene units usually
produce more planar molecules with a more quinoid-like struc-
ture than phenyl units, increasing the conjugation with the IC
unit.11,21
In the case of compound 10, the UV-vis absorption properties
in the thin-films were studied as a function of the substrate
temperature applied during the deposition (Fig. 2). For substrate
Field-effect transistor performances
Top-contact field-effect transistors were made from the six
new materials by high-vacuum evaporation. Bare and
ꢁ
temperatures below 100 C, the main absorption band maxima
Table 1 Optical absorption maxima (lmax) and electrochemical properties of the new materials
Compound
lmax solution/nma
lmax thin-film/nmb
Eg thin-film/eVb
Eox vs. SCE/Vc
EHOMO/eVd
7
8
9
10
11
12
288, 382
300, 364
284, 380
290, 385, 430
288, 366, 425
290, 393, 450
343, 441
334, 436
351, 431
372, 438, 460
333, 435
347, 429
2.70
2.76
2.64
2.62
2.76
2.64
0.88
0.67
0.70
0.74
0.66
0.60
5.59
5.38
5.41
5.45
5.37
5.31
a
b
Measurements performed in DMF for 7, 8, 9 and in CH2Cl2 for 10, 11, 12. Measurements performed on 300 A thin-films vacuum-deposited on
˚
c
d
quartz. SCE: standard calomel electrode. The values correspond to the oxidation peak onset. Ionization potential (EHOMO; HOMO ¼ highest
occupied molecular orbital) measured from the cathodic onset.
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octadecyltrichlorosilane (OTS)-treated Si–SiO2 substrates, as
well as different substrate deposition temperatures (Td), were
used in this study in order to increase order within the deposited
thin-films and consequently to study the effect of the molecular
organization on the charge transport properties. The Si–SiO2
substrate functionalization with alkyl silanes has been proven to
be very useful in obtaining high performances in OFET devices.24
It decreases the number of traps located at the semiconductor–
dielectric interface.25 Moreover, it is interesting to compare the
performances of our ICs containing hydrophilic amines on the
framework with the corresponding hydrophobic ICs on both
bare and OTS-treated Si–SiO2 substrates.
The performances obtained with thiophene-substituted IC 7
are lower (about 3 times) than those obtained with phenyl-
substituted IC 8. As shown in Table 2, a hole mobility of 0.08 cm2
Vꢀ1 sꢀ1 and an on/off ratio of 106 were obtained when the thin-
film was deposited at Td ¼ 175 ꢁC. We note also that the
performances obtained with compound 7 on bare or OTS-treated
Si–SiO2 substrates are very close when the substrate deposition
temperature is equal to or higher than 75 ꢁC. Compound 9 with
vinylene groups shows relatively unchanged mobilities, about
4 ꢂ 10ꢀ3 cm2 Vꢀ1 sꢀ1, when the substrate temperature is varied
between 25 and 125 ꢁC. This mobility is at least one order lower
than the one measured with compound 8 having no vinylene
group.
As expected, all the IC derivatives behave as p-type organic
semiconductors with well-defined linear and saturation regimes.
For example, Fig. 3 shows the source–drain current (IDS) versus
the source–drain voltage (VDS) for an OFET made with
compound 8 at various gate voltages (VG). The hole mobilities
calculated in the saturation regime, the on/off current ratios and
the threshold voltages obtained at different Td are summarized in
Table 2. It is worth noting that all the devices were stored and
tested under ambient conditions.
The results obtained with the methylated ICs 10–12 are also
very interesting. First, these ICs clearly have lower performances
than their corresponding free amine ICs 7–9. For example,
compound 10 shows a hole mobility of 9 ꢂ 10ꢀ4 and 0.02 cm2 Vꢀ1
sꢀ1 at Td ¼ 25 ꢁC and 125 ꢁC, respectively. These values are about
one order of magnitude lower than those obtained with IC 7.
Moreover, as shown in Table 2, the hole mobility of IC 10
increased by one order of magnitude when the substrate depo-
sition temperature was changed from 75 to 100 ꢁC. These results
are in good agreement with the UV-vis results showing a signifi-
cant change in the molecular arrangement (orientation) at Td ¼
100 ꢁC. As we will see later, the AFM images and XRD analysis
clearly show this modification in the thin-film organization and
consequently explain this improvement.
The compound leading to the highest hole mobility among the
new linear ICs was found to be the free amine phenyl-substituted
IC 8. Indeed, compound 8 deposited at Td ¼ 25 ꢁC on OTS-
treated substrates shows a hole mobility and an on/off ratio of
0.03 cm2 Vꢀ1 sꢀ1 and 7 ꢂ 103, respectively. As shown in Table 2,
when the substrate deposition temperature was increased, both
the hole mobility and the on/off ratio improved. At Td ¼ 125 ꢁC,
the maximum hole mobility and the on/off ratio are 0.22 cm2 Vꢀ1
sꢀ1 and 3 ꢂ 105, respectively. This particular film was tested in
two different devices and the mobility average was 0.21 cm2 Vꢀ1
sꢀ1. The performances were also tested over two months and the
mobility was still 0.11 cm2 Vꢀ1 sꢀ1 indicating a good stability of
the thin-film. Higher Tds were also used, but the FET perfor-
mance began to decrease and the devices were difficult to
reproduce probably due to the presence of voids and discon-
nected films as seen by optical microscopy. Interestingly, good
performances were also obtained with compound 8 using bare
SiO2 substrates. For example, at Td ¼ 125 ꢁC, the hole mobility
and the on/off ratio are 0.12 cm2 Vꢀ1 sꢀ1 and 2 ꢂ 106, respectively.
Secondly, as observed for compounds 7 and 8, the hole
mobility of thiophene-substituted IC 10 is lower than the one
measured for phenyl-substituted IC 11 when the substrate was
heated during the deposition. This goes along with the trend
reported by our group in 2007 for other indolocarbazoles.11 The
phenyl units on both ends of the IC unit lead to higher mobilities
than the thiophene units even though the latter usually provides
more planar p-conjugated molecules.14 In fact, it has recently
been shown that oligomers containing phenyl functional groups
can achieve high performance.26 Finally, as shown in Table 2,
compound 12 exhibits very low hole mobilities, no matter what
substrate temperature was used during the deposition. The
addition of vinylene units between the indolocarbazole and the
Fig. 3 (a) Source–drain current (IDS) versus source–drain voltage (VDS) at various gate voltages (VG) for top-contact field-effect transistors using
compound 8 on OTS-treated SiO2 (Td ¼ 125 ꢁC). (b) The transfer characteristics in the saturation regime at a constant source–drain voltage (ꢀ100 V) are
also shown.
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phenyl (compounds 9 and 12) does not seem to be a good
strategy to improve the IC performance in OFETs.
Morphological and structural characteristics
The morphology and structure of vacuum-deposited thin-films
were studied by AFM and XRD in order to explain the OFET
results, and to understand the relationship between molecular
structures, thin-film properties and device performances. First,
as shown in Fig. 4, the deposition of compound 8 at Td ¼ 25 ꢁC
leads to thin-films with an irregular granular morphology and
high roughness. By increasing the substrate deposition
temperature, the grains become larger and the surface is
smoother which corresponds to improved OFET performances.
ꢁ
At Td ¼ 125 C a lamellar-like morphology appears with the
presence of isolated grains on the top of the film. One can
presume that this kind of morphology leads to fewer grain
boundaries in the film and consequently the hole mobility
increases. Similar behaviour was observed for compound 7;
Fig. 4d shows the morphology of a thin-film deposited at Td ¼
ꢁ
175 C.
Amine-methylated ICs, 10 and 11, reveal interesting features
by AFM. For example, IC 10 exhibits an irregular granular
morphology when deposited at Td ¼ 25 ꢁC (Fig. 5). However, as
shown in Fig. 5, a more crystalline morphology appears starting
from Td ¼ 100 ꢁC with a terrace-like layered structure. The
stacked terrace layers lay parallel to the Si–SiO2 surface with the
˚
step height of the terrace layers about 39 A. Interestingly,
the apparition of this new organization corresponds well with
the increase in mobility by one order observed at Td ¼ 100 ꢁC. A
similar trend was also observed in the case of IC 11, where
a multilevel island-like structure appeared at Td ¼ 75 ꢁC
(Fig. 3S, ESI†). At this substrate temperature, the mobility is
one order higher than the one measured at Td ¼ 25 ꢁC. The
Fig. 4 AFM images of vacuum-deposited thin-films (40 nm) of
compound 8 at (a) room temperature (b) 75 ꢁC (c) 125 ꢁC and (d) 175 ꢁC.
The size of all the images is 1 mm2.
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in the XRD spectrum. On the other hand, this oligomer deposited
at Td ¼ 100 ꢁC exhibits a sharp primary diffraction peak at 2q ¼
ꢁ
ꢁ
˚
2.36 , corresponding to a layer distance of 37.4 A. At Td ¼ 125 C,
ꢁ
˚
the primary peak moves to 2q ¼ 2.23 (d-spacing ¼ 39.5 A) and its
intensity increases. Interestingly, the d-spacing of compound 10 at
Td ¼ 125 ꢁC corresponds exactly to the step height of the terrace
layers measured by AFM at this temperature. Moreover, the
˚
length of compound 10 is estimated to be around 39 A in an
extended linear conformation. Therefore, on the basis of the XRD
and AFM results, we suppose here that compound 10 adopts
a vertical orientation relative to the substrate. This organization is
completely different to the one observed with our previous IC
derivatives with alkyl chains on the nitrogen where the IC core is
quasi-parallel to the substrate and the alkyl chains are pointed
towards the surface. This example shows how the position of the
alkyl side chains can have a major effect on the molecular orga-
nization of these ICs. It also shows that thin-films, where the IC
molecules have a parallel or vertical orientation relative to the
substrate, could be obtained by simple modification of the alkyl
side chain position on the IC core.
Fig. 5 AFM images of vacuum-deposited thin-films (40 nm) of
compound 10 (a) at room temperature (b) at 75 ꢁC (c) at 100 ꢁC and (d) at
125 ꢁC. The size of all the images is 1 mm2.
As shown in Fig. 7, the XRD patterns of compound 8 were not
that clear. At Td ¼ 25 ꢁC, the XRD spectrum did not exhibit any
diffraction peaks, indicating an amorphous structure. When the
film was deposited at 75 ꢁC, a small peak could be observed at 2q
crystallites become more defined at Td ¼ 125 ꢁC, which could
explain the mobility enhancement observed in OFETs. The AFM
images of compounds 9 and 12 show irregular granular
morphologies with a high film roughness (Fig. 4S, ESI†). The
introduction of vinylene units between the IC and the phenyl
groups produces amorphous films with poor organization which
could be a reason for the low mobilities measured in OFETs.
The X-ray data obtained from deposited thin-films of
compounds 8 and 10 provide important information about the
film structures at different substrate deposition temperatures. As
shown in Fig. 6, the XRD pattern of compound 10, deposited
without any substrate heating, indicates the formation of an
amorphous structure. When deposited at Td ¼ 75 ꢁC, only a small
ꢁ
˚
¼ 3.04 corresponding to a d-spacing of 29.1 A. Even with this
small peak, it is believed that the heating of the substrate induced
some long range organization of the molecules on the OTS-
treated SiO2. Furthermore, by heating the film at 125 ꢁC, the
XRD pattern is clearly different. The peak that was observed at
75 ꢁC has completely disappeared and instead there are two new
ꢁ
˚
peaks. First, at 2q ¼ 2.87 corresponding to a d-spacing of 30.8 A
ꢁ
˚
and a second peak at 2q ¼ 3.19 (d-spacing ¼ 27.7 A). This could
mean that there are two different types of packing within the
same film.
These data are not enough to reveal whether the molecules
stand perpendicular or lay parallel to the substrate. The amine
groups on the backbone of the molecule change the organization
within the thin-film. The addition of new interactions between
the molecules probably leads to hydrogen bonding or other
intermolecular bonds that enhance the charge transport
ꢁ
˚
diffraction peak was observed at 2q ¼ 5.56 (d-spacing ¼ 15.9 A)
Fig. 7 XRD patterns of an evaporated thin-film of compound 8 on
Fig. 6 XRD patterns of an evaporated thin-film of compound 10 on
OTS-treated SiO2 at different substrate temperatures.
OTS-treated SiO2 at different substrate temperatures.
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properties. Further work on these IC derivatives will help to
understand the organization in three dimensions and the differ-
ence between the packing of the ICs containing secondary and
tertiary amines.
a saturated atmosphere of NH4OH overnight. 40 nm organic
films were vacuum-deposited on the treated Si–SiO2substrates at
room temperature and elevated temperatures, as described in the
text, at a deposition rate of 0.4–0.5 A sꢀ1 and 10ꢀ6 Torr. The thin-
ꢁ
film thickness of each derivative was determined in situ using
a quartz-crystal microbalance (QCM). A top-contact thin-film
transistor was used for measuring the charge mobility of the
indolo[3,2-b]carbazole derivatives: gold source and drain elec-
trodes (channel length (L) and width/length (W/L) of 50 mm and
ca. 20 respectively) were vacuum-deposited through a shadow
mask. The electrical characteristics of the organic field-effect
transistor (OFET) devices were measured under an accumulation
mode using a Keithley 4200-SCS semiconductor parameter
analyzer.
Experimental
Instrumentation
1H and 13C NMR spectra were recorded on a Varian AS400
apparatus in the appropriate deuterated solvent solution at 298
K. Chemical shifts were reported as d values (ppm) relative to the
internal tetramethylsilane standard. Differential scanning calo-
rimetry (DSC) analyses were performed on a Mettler Toledo
DSC823e instrument, calibrated with ultrapure indium. UV-vis
absorption spectra were taken using a Varian Cary 500 UV-VIS-
NIR spectrophotometer. Cyclic voltammetry (CV) measure-
ments were recorded on a Solartron 1287 potentiostat and C3
Cell Stand using platinum electrodes at a scan rate of 50 mV sꢀ1
against a Ag/Ag+ (0.1 M of AgNO3 in acetonitrile) reference
electrode in an anhydrous and argon saturated solution of 0.1 M
tetrabutylammonium perchlorate (TBAP) in CH2Cl2. Under
these conditions, the oxidation potential (EOx) of ferrocene was
0.053 V versus Ag/Ag+, whereas the EOx of ferrocene was 0.343 V
versus SCE. Top-contact devices were made according to a liter-
ature procedure.27 The Philips PANalytical X’Pert diffractom-
eter with a PreFIX X-ray mirror at the incident beam, and
a parallel plate collimator at the diffracted beam was used on the
thin-films of the evaporated molecules. U/2q scans were per-
formed with Cu Ka1 radiation at a power of 45 mV and 40 mA,
with a step size of 0.02, and a step time of 1.0 s. A Digital
Instruments MMAFM-2 scanning probe microscope was used to
perform tapping mode AFM on the samples with a silicon tip of
300 kHz frequency.
Conclusion
We reported in this study the synthesis, characterization and
FET performance of new indolo[3,2-b]carbazole derivatives. 3,9-
Di(p-octylbenzene)-5,11-dihydroindolocarbazole (8) exhibits
a good hole mobility (0.22 cm2 Vꢀ1 sꢀ1). Compound 7 also led to
a mobility close to 0.1 cm2 Vꢀ1 sꢀ1. It was demonstrated that the
new materials organized quite differently depending on the
substrate temperature using UV-vis spectroscopy, AFM and
X-ray diffraction. We were also able to show that the alkylation
on both ends of the molecule leads to a clearly different film
morphology. The molecules tended to stand perpendicular to the
surface instead of laying down parallel to it. Furthermore, the
performances for the free amine compounds were almost
the same with and without surface modification. The new
methylated compounds did not demonstrate good performances
compared to their unprotected amine counterparts. This may be
due to the steric hinderance created by the methyl groups on each
side of the IC framework. Future work will focus on synthesizing
new polymers and oligomers based on the indolo[3,2-b]carbazole
backbone that can be solution processed. This could lead to very
promising materials if the crystallinity observed in previous
compounds can be maintained.
Material synthesis and characterization
2-Octyl-5-(tributyltin)thiophene,28 1-iodo-4-octylbenzene,29 4-
octylbenzeneboronic acid30 and 3,9-dibromo-5,11-dihydroxy-
indolo[3,2-b]carbazole10 were synthesized according to already
published procedures. All starting organic compounds were
purchased from Aldrich, Alfa Aesar and TCI America and used
without further purification. All reactions were carried out under
argon at 1 atm unless mentioned otherwise. Some reaction
solvents were distilled before use (THF from potassium–benzo-
phenone; acetonitrile from CaH2). Column chromatography was
Moreover, the application of these molecules in sensing
devices is under way. It is believed that they may be efficient in
detecting different kinds of analytes because of the reactive
amines on the IC. By comparing compounds 8 and 11 in sensor
devices, we have already found that ICs containing secondary
amines were 20–30 times more sensitive in the vapour sensing of
solvents such as acetone than the tertiary amines. This highlights
the versatility and the potential of indolocarbazole derivatives
for applications in electronic devices.
ꢁ
carried out on silica gel (size 40–63 mm, pore size 60 A, Silicycle).
All the other compounds have been synthesized following
procedures described in the ESI†.
Device fabrication
Acknowledgements
Highly doped n-type (100) Si wafers (<0.004 U cm) were used as
substrates for the devices. SiO2 layers (unit area capacitance, C ¼
This work was supported by discovery and strategic grants from
the Natural Sciences and Engineering Research Council
(NSERC) of Canada. P.L.T. Boudreault thanks FQRNT (B2)
for a PhD Scholarship and NSERC for a PhD Canada Graduate
Scholarship (CGS D–Alexander-Graham Bell). M. L. Tang
acknowledges a Kodak Graduate Fellowship. The authors
gratefully acknowledge Dr Anatoliy Sokolov for preliminary
sensor studies.
10 nF cmꢀ2), as gate dielectric, were thermally grown to 3000 A
˚
thickness onto the Si substrates. The Si–SiO2 substrates were
then cleaned with acetone and isopropyl alcohol. The wafers
were then treated with a 3 mM octadecyl trimethoxysilane
(C18H37Si(OCH3)3, OTS) solution in trichloroethylene, which
was spin-coated onto the substrates before treating with
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M. Belletete, G. Durocher, Y. Tao and M. Leclerc, J. Am. Chem.
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