Isomeric carbazolocarbazoles: Synthesis, characterization and comparative study in Organic Field Effect Transistors

Article abstract of DOI:10.1039/c2tc00363e

We report here the synthesis and characterization of a new family of isomeric carbazolocarbazole derivatives, namely carbazolo[1,2-a]carbazole, carbazolo[3,2-b]carbazole and carbazolo[4,3-c]carbazole. Thermal, optical, electrochemical, morphological and semiconducting properties have been studied to understand the influence of geometrical isomerism on the optoelectronic properties of these compounds. Different packing patterns have been observed by single crystal X-ray diffraction (XRD) which then correlate with the different morphologies of the evaporated thin films studied by XRD and Atomic Force Microscopy (AFM). The effect of N-substituents has also been evaluated for one of the isomers revealing a noticeable influence on the performance as organic semiconductors in Organic Field Effect Transistors (OFETs). A good p-channel field effect has been determined for N,N′-dioctylcarbazolo[4,3-c]carbazole with a mobility of 0.02 cm² V^-1 s^-1 and I _on/I_off ratio of 10⁶ in air. These preliminary results demonstrate the promising properties of molecular carbazolocarbazole systems which should be further explored in the area of organic semiconducting materials.

Full text of DOI:10.1039/c2tc00363e

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Materials Chemistry C
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Isomeric carbazolocarbazoles: synthesis,
characterization and comparative study in Organic Field
Cite this: J. Mater. Chem. C, 2013, 1,
1959
Effect Transistors†
a
bc
a
Miriam Mas-Montoya, Rocıo Ponce Ortiz, David Curiel, Arturo Espinosa,a
*
´
´
d
b
b
*
*
Magali Allain, Antonio Facchetti and Tobin J. Marks
We report here the synthesis and characterization of a new family of isomeric carbazolocarbazole
derivatives, namely carbazolo[1,2-a]carbazole, carbazolo[3,2-b]carbazole and carbazolo[4,3-c]carbazole.
Thermal, optical, electrochemical, morphological and semiconducting properties have been studied to
understand the influence of geometrical isomerism on the optoelectronic properties of these
compounds. Different packing patterns have been observed by single crystal X-ray diffraction (XRD)
which then correlate with the different morphologies of the evaporated thin films studied by XRD and
Atomic Force Microscopy (AFM). The effect of N-substituents has also been evaluated for one of the
isomers revealing a noticeable influence on the performance as organic semiconductors in Organic Field
Effect Transistors (OFETs). A good p-channel field effect has been determined for N,N0-dioctylcarbazolo
[4,3-c]carbazole with a mobility of 0.02 cm2 Vꢀ1 sꢀ1 and Ion/Ioff ratio of 106 in air. These preliminary
results demonstrate the promising properties of molecular carbazolocarbazole systems which should be
further explored in the area of organic semiconducting materials.
Received 8th October 2012
Accepted 29th December 2012
DOI: 10.1039/c2tc00363e
www.rsc.org/MaterialsC
organic thin lms are light, exible, and mechanically robust
which is important for certain opto-electronic device
Introduction
Research in organic/molecular electronics has advanced over
the past few years due to the cooperative contributions from
different disciplines in the areas of physics, chemistry, mate-
rials science and engineering.1–8 As far as synthetic chemistry is
concerned, the development of new organic semiconducting
materials, which could offer an alternative to the traditional
amorphous silicon-based devices, has produced abundant p-
conjugated polymers and low molecular weight molecules.9–18
The structural diversity of organic molecules enables tuning of
the materials properties depending on the specic device
requirements.19–26 Moreover, these systems can be processed
either by vacuum or solution methodologies. Furthermore,
applications.
Within the group of molecular materials, the family of
acenes has been extensively investigated in Organic Field Effect
Transistors (OFETs) with excellent results.27,28 However several
acenes suffer from instability under ambient conditions. This
problem is even more accentuated when increasing the length
of the aromatic fused system.29,30 Derivatization of the poly-
aromatic nucleus has been useful to enhance stability and to
inuence intermolecular interactions, which critically condi-
tion solid-state charge transport.31–35 Another important
strategy consists of incorporating heteroaromatic rings within
the fused polyaromatic structure. Several synthetic procedures
have described the preparation of heteroacenes with different
kinds of heteroatoms.36–38 The thiophene ring has been by far
the most studied heterocycle among the heteroacene deriva-
tives.39 Very good mobilities have been reported, which are
getting closer to the renowned pentacene.40 Nitrogenated het-
eroacenes have also offered promising results in thin lm
transistors, as has been elegantly demonstrated by Ong and
Leclerc research groups. Furthermore, the presence of pyrrolic
nitrogen atoms offers the possibility of using their reactivity to
access derivatives whose structure could be easily modied to
tune semiconducting properties.41–44 In this regard, we have
previously described the synthesis of a new hexacyclic system
consisting of two fused carbazole units.45 The reported carba-
zolo[1,2-a]carbazole displayed a low lying HOMO level, with a
aDepartment of Organic Chemistry, Faculty of Chemistry, University of Murcia,
Campus of Espinardo, 30100 Murcia, Spain. E-mail: davidcc@um.es; Fax: +34
968364149; Tel: +34 868888389
bDepartment of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston,
Illinois 60208, USA. E-mail: a-facchetti@northwestern.edu; t-marks@northwestern.
edu
c
´
´
Departamento de Quımica-Fısica, Facultad de Ciencias, Campus de Teatinos 29071,
´
Malaga, Spain
dLaboratory of Chemistry, Molecular Engineering of Angers (CIMA), UMR CNRS 6200 –
University of Angers, 2, Boulevard Lavoisier, 49045 Angers cedex, France
† Electronic supplementary information (ESI) available: NMR spectra, UV-vis
spectra, cyclic voltammetry, XRD, AFM images, output characteristics,
computational details and calculated structures. CCDC 905793–905795. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2tc00363e
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relatively wide HOMO–LUMO gap. These properties were
promising for potentially air stable p-type semiconductors,
which encouraged us to explore this new heterophenic
compound.46 Thus, in this study we have designed synthetic
routes to several carbazolocarbazole isomers (Fig. 1). These
molecules represent a new class of heterophenes with no
precedents in the literature. With a few exceptions, the effect of
core isomerism on the performance of a semiconductor has not
been commonly studied.47,48 Thus we focused our interest on
the comparative study of structural changes and hole transport
in carbazolocarbazole thin lm transistors.
Results and discussion
Synthesis
The synthesis of the carbazolo[1,2-a]carbazole derivative, 5, has
been recently described.45 It basically involved the use of
adequately tetrasubstituted naphthalenes as starting materials.
The correct arrangement of substituents on the naphthalene unit
determined the regioselectivity of the synthetic process, which
was completed by a Suzuki–Miyaura cross-coupling reaction, fol-
lowed by a nitrene insertion from a nitro group via a Cadogan
reaction. Following this general strategy carbazolo[3,2-b]carba-
zole, 10, and carbazolo[4,3-c]carbazole, 14, could be prepared as
depicted in Scheme 1.
Compound 10 was synthesised from 1,5-dihydroxynaph-
thalene, which was selectively brominated at positions 2 and 6.
The alkylation of hydroxyl groups played the double role of
improving the solubility of the molecules as well as blocking
those positions with the aim of getting a regiospecic evolution
of the route in subsequent steps. The transformation of the
bromo substituents into boronic acid groups enabled the
accomplishment of the Suzuki–Miyaura coupling with o-azi-
doiodobenzene. In this case the azide group offered better
results than the above-mentioned nitro group to thermally
generate the nitrene to be inserted. 7,14-Dioctyloxycarbazolo
[3,2-b]carbazole, 10, was isolated with a discrete nal yield. This
result might be caused by the reduced reactivity of the naph-
thalene b positions, accompanied by the electronic effects
exerted by the octyloxy groups and a possible steric hindrance of
these aliphatic chains. Regarding compound 14, 1,5-dia-
minonaphthalene was transformed into the corresponding
diiodonaphthalene 11, via a Sandmeyer-type reaction, and this
into diboronic ester 12. The cross-coupling reaction with
o-azidoiodobenzene led to 1,5-diarylated naphthalene 13.
Heating of the diazide regiospecically evolved through a
nitrene insertion into the adjacent position of the naphthalene
unit. This carbazolo[4,3-c]carbazole was further derivatised by
means of attaching different groups to its nitrogen atoms. An
Scheme 1 Synthetic routes to carbazolocarbazoles.
alkyl chain was connected by reacting the carbazolocarbazole
with 1-bromooctane under basic conditions to obtain
compound 15. Additionally, a copper-mediated N-arylation with
p-butylbromobenzene yielded N,N0-diarylcarbazolo[4,3-c]carba-
zole 16. All the products in the synthetic routes were charac-
terised by NMR spectroscopy and mass spectrometry.
Thermal properties
The thermal stability of these compounds was determined by
thermogravimetric analysis (TGA). All the carbazolocarbazoles
exhibited a signicant weight loss, related to thermal decom-
position, above 325 ꢁC. Melting points vary over a wide range of
temperatures depending on both the isomeric structure and the
substituent type, which could well be interpreted as a rst sign
of the dissimilar intermolecular interactions of these isomers.
Correspondingly, compounds 14–16, sharing the carbazolo[4,3-
c]carbazole skeleton exhibit signicantly different melting
points, which again highlights the striking inuence of
substituents on the molecular solid-state interactions (Table 1).
Electrochemical properties
Electrochemical characterization in solution was carried out
by cyclic voltammetry. The results are displayed in Fig. 2 and
Table 2.
Fig. 1 Isomeric carbazolocarbazole cores.
1960 | J. Mater. Chem. C, 2013, 1, 1959–1969
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Table 1 Thermal properties of carbazolocarbazoles
electrochemical stability of the resulting cationic carbazolo[1,2-
a]carbazole. HOMO energies were calculated from the onset of
the rst oxidation process (Table 2). Thus, compound 10 shows
a higher lying HOMO than the other isomers. According to an
extension of the Clar's theory for polyheteroaromatic systems,
the number of aromatic sextets can be related to the ionization
potential of the molecule.50 In this regard, all the carbazolo-
carbazole isomers are hexacyclic systems where three sextets
can be drawn. Nevertheless the number of Clar structures which
can be represented with the maximum number of sextets is
different in each case, with only one for the [3,2-b] isomer 10
and two structures for the [1,2-a], 5, and [4,3-c], 14–16, isomers.
Besides the full benzenoid structure of compounds 14–16 would
justify their lower HOMO energy. Thus, a good agreement is
found between the electrochemical potentials and the stability
of the isomeric carbazolocarbazoles. Within the series of car-
bazolo[4,3-c]carbazoles we can only observe very subtle changes.
When unsubstituted carbazolocarbazole 14 is compared to the
N-alkylated 15, and the N-arylated 16 derivatives a slight
destabilization of the HOMO is observed in 15 presumably due
to the weak electron releasing effect of the alkyl group which
increases the electron density on the nitrogen atom. Conversely,
the aryl substituent on 16 has a very little stabilizing effect
presumably due to the twisted arrangement with the carbazo-
locarbazole plane (Fig. 3 and 8). These data are in agreement
with the theoretical calculations, which show a general delo-
calization of the electron density over the surface of the carba-
zolocarbazole structure. Again it can be observed how the
isomerism inuences the electronic structure of closely related
molecules.
Compound
5
10
14
15
16
TGA (ꢁC)
Mp (ꢁC)
345
140
325
229
365
330
186
350
240
>330a
a
Melting point could not be detected before sample decomposition.
Fig. 2 Cyclic Voltammetry of (a) 10, (b) 5, (c) 15 and (d) 16, 10ꢀ3 M CH2Cl2, 0.1 M
TBAPF6, T ¼ 25 ꢁC, scan rate: 100 mV sꢀ1, reference electrode: SCE, working
electrode: Pt.
Compound 14 is poorly soluble which made difficult the
acquisition of a clear voltammogram. Nevertheless the electro-
chemical data were good enough to obtain the anodic peak
onset. The rest of the carbazolocarbazoles exhibited a rst
quasireversible wave whose potential could be related to the
HOMO energy (Fig. 2). A second quasireversible wave was also
registered for compounds 10, 15 and 16. This double wave
behavior is attributed to the oxidation of the two fused carba-
zole units as it could also be inferred from the analogous energy
differences between the Single Occupied Molecular Orbitals
(SOMOs) of the studied carbazolocarbazole radical-cation
models, which were determined by DFT calculations (Table 2).
The mostly reversible voltammograms of carbazolocarba-
zoles sharply contrast that of carbazole, which is oxidised at
much higher potential and consists of irreversible processes
subject to electropolymerization upon repeated scans.49 Con-
cerning compound 5, its second electrochemical process is
irreversible. This lack of reversibility might be caused by a poor
Optical properties
The absorption spectra acquired in solution display the
expected pattern for polyaromatic compounds. Three sets of
bands could be distinguished, namely a, p and b according to
Clar's model51 or 1Lb, 1La and 1Bb, respectively, according to
Platt's denition.52 All these bands are assigned to different p–
p* transitions which, in general, also display a sharp vibra-
tional structure. The intensity of these bands follows the usual
trend, with the a bands showing the lower absorptivity, fol-
lowed by the p bands and with the b bands having a higher
intensity (Fig. 4). A manifest red shi of the low energy bands is
detected, which correlates with the extended p-conjugation of
carbazolo[3,2-b]carbazole, 10, over carbazolo[4,3-c]carbazole,
14, and this over carbazolo[1,2-a]carbazole, 5. As it is
commonly accepted for heteroaromatic fused systems, carba-
zolocarbazoles could be considered as isoelectronic structures
of polycyclic aromatic hydrocarbons. Regarding the series of
compounds 5–16, the carbazolo[1,2-a]carbazole, carbazolo[4,3-
c]carbazole and carbazolo[3,2-b]carbazole would be isoelec-
tronic of naphtho[1,2-c]chrysene, dibenzo[c,l]chrysene and
dibenzo[a,j]tetracene respectively. Therefore, they could all be
classied as phene-like structures. However, the longer linear
core of compound 10 is consistent with the pronounced red-
shi of the absorption spectrum and consequently its narrower
HOMO–LUMO gap.53–55
Table 2 Electrochemical and spectroscopic characterization of carbazolocarba-
zoles in solution
Compound
5
10
0.43
[0.37]
1.07
14
15
0.81
[0.75]
1.41
[1.33]
2.90
ꢀ2.55
ꢀ5.45
—
16
0.92
[0.84]
1.40
[1.32]
2.95
ꢀ2.59
ꢀ5.54
—
E1/2 (V)
[Eonset_1] (V)
E1/2 (V)
0.72
[0.65]
—
—
[0.81]
—
[Eonset_2] (V)
Opt. gap (eV)
LUMO (eV)a
HOMO (eV)b
SOMO (eV)c
[1.21]
3.25
ꢀ2.10
ꢀ5.35
ꢀ6.08
[1.00]
2.56
ꢀ2.61
ꢀ5.07
ꢀ6.34
[1.29]
3.02
ꢀ2.49
ꢀ5.51
ꢀ6.38
a
b
ELUMO ¼ Eopt. gap ꢀ EHOMO
.
EHOMO ¼ ꢀ(4.7 + Eox. onset). c Determined
by computational methods.
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Fig. 3 DFT Frontier Molecular Orbitals energies (eV) and isosurfaces (0.04 isovalue). Compounds have been modeled bearing methyl groups instead of longer alkyl
chains.
Crystallographic characterization
The molecular structure and packing pattern of carbazolo-
carbazoles 5 and 14–16 were determined by X-ray diffraction
analysis. Compound 5 consists of a planar core which rules out
any distortion effect of the atoms located at the bay position
dened by the two carbazole units (Fig. 5). The alkyl chain
substituents are extended and remain approximately within the
plane of the aromatic core. This carbazolo[1,2-a]carbazole
crystallizes in parallel planes.45 Packing within each plane
seems to be stabilised by the interactions established with the
octyloxy substituents, which precludes any contact between the
aromatic units. These aromatic contacts are neither observed
between molecules in adjacent slipped-stack planes.
As far as the carbazolo[4,3-c]carbazole, 14, is concerned,
molecules are packed with their long axis aligned with the
crystallographic c-axis. Each molecule interacts with six
surrounding neighbour molecules through face-to-face and
edge-to-face contacts. A side view of the packing reveals a
Fig. 4 UV-vis spectra 10ꢀ5 M in CH2Cl2 (a) and (b); model spectra in solution and
thin film (c) and (d).
˚
herringbone pattern where face to face contacts at 3.21 A can be
measured for molecules packed in parallel planes. Besides edge
to face contacts are established between molecules located in
contiguous stacks (Fig. 6).
The crystal structure of compound 15 also shows a herring-
bone arrangement but with a very different packing mode from
the parent carbazolo[4,3-c]carbazole, clearly inuenced by the
alkyl chains (Fig. 7).
These aliphatic substituents are completely oriented out of
the plane dened by the carbazolocarbazole core which inter-
feres with the packing of the aromatic system. The expanded
unit cell of 15 does not show any face-to-face interaction
An analysis of the wavelengths of the p and b bands
evidences their direct dependence on the molecular geometry
dictated by the different angular ring fusions.56 The absorption
spectra were also recorded for thin lm samples prepared by
spin-coating. The appearance of the spectra is identical to those
obtained in solution but with the solid state spectra showing a
subtle bathochromic shi, presumably as a result of intermo-
lecular interactions.
These results show how carbazolocarbazoles have none or a
very weak absorption in the lower region of the visible spectrum
which makes them virtually transparent to the visible light.
These relatively wide band gap materials, provided that they
show semiconducting properties, may become interesting for
the fabrication of transparent optoelectronic devices.57,58
Moreover, their low lying HOMOs would confer stability
towards an oxidative environment. It is also worth highlighting
that concerning the electronic requirements for the fabrication
of OFETs, the carbazolocarbazoles show a rather good corre-
spondence of their HOMO energies with the gold workfunction
to be used as the source and drain electrodes.
Fig. 5 Top view (a) and crystal packing (b) of 5, hydrogen atoms have been
omitted for clarity.
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Fig. 8 Top and side views (a), crystal packing (b) of 16, hydrogen atoms have
been omitted for clarity.
Fig. 6 Top and side views (a) and crystal packing (b) of 14, hydrogen atoms have
been omitted for clarity.
Fig. 7 Top and side views (a), crystal packing (b) of 15, hydrogen atoms have
been omitted for clarity, and (c) crystal packing without alkyl chains.
between the aromatic rings. Instead, several edge-to-face
contacts can be measured for each molecule with four
surrounding carbazolocarbazoles. Different from compound
14, the stacked molecules in the N-alkylated derivative, 15, are
not perfectly aligned but slipped along their long axis due to the
presence of alkyl chains.
Fig. 9 Q/2Q X-ray diffraction scans of vapor-deposited 5 (Td ¼ 90 ꢁC), 10 (Td
¼
90 ꢁC), 14 (Td ¼ 90 ꢁC), 15 (Td ¼ 60 ꢁC), and 16 (Td ¼ 90 ꢁC) films grown on HMDS-
treated Si/SiO2.
The structure of compound 16 shows that the N-phenyl
substituents twisted 54.76ꢁ with respect to the carbazolocarba-
zole plane (Fig. 8). The molecules are packed in staggered
planes which tend to pile up along the crystallographic a-axis.
Face-to-face interactions are observed between carbazolo-
carbazole in adjacent stacks. Besides, the presence of phenyl
groups favors close edge-to-face phenyl-carbazolocarbazole
contacts.
In this regard, it is worth highlighting the detection of out-of-
plane sixth and h order diffraction peaks for compounds 10
and 15 respectively, which indicates a highly ordered molecular
structure in the sublimed lms, although there is an absence of
the second order peak for compound 15. Compound 14 also
showed good ordering as it reveals the recorded third order
diffraction peaks. The presence of a single set of Bragg reec-
tions in the diffraction patterns reveals that the lms exhibit
one phase with a preferential orientation.59 Contrastingly, only
two weak peaks were detected for the carbazolo[1,2-a]carbazole
Thin lm X-ray diffraction
Thin lms of the present semiconductors were deposited by 5 and no diffraction was observed in the lms of compound 16,
vacuum sublimation onto hexamethyldisilazane (HMDS) and presumably due to the twisting of N-phenyl substituents with
bare Si/SiO2 substrates, and their microstructures were analysed respect to the carbazolocarbazole skeleton found in the single
by wide-angle X-ray diffraction (WAXRD). The differences in the crystal, which may prevent close enough crystal packing. Since
structure of the isomeric carbazolocarbazoles evidenced a the morphology of organic molecules can be thermally affected,
dissimilar diffraction pattern which correlates with the sublimation experiments were carried out at different substrate
ꢁ
ꢁ
different crystallinity of the deposited lms (Fig. 9).
temperatures, namely, room temperature, 60 C and 90 C. In
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general better resolution of the higher order peaks was achieved
upon increasing the substrate temperature, except for
compound 15 which shows better crystallinity for the lms
deposited at room temperature (see ESI†). The experimental
diffraction pattern obtained for the lm of compound 5 does
not match with the simulated powder pattern from the single
crystal structure data, suggesting that the vapor-deposited thin
lm and the single crystal have a different phase. Regarding
compound 10, the periodicity of its diffraction peaks is
consistent with a regular molecular arrangement which corre-
˚
Fig. 10 AFM images of the present semiconductor vapor-deposited on HMDS-
treated Si/SiO2 substrates heated at either 25 ꢁC or 90 ꢁC.
sponds to a d-spacing of 18.8 A calculated from the primary
diffraction peak at 2q ¼ 4.70ꢁ. That distance is shorter than the
˚
length of the molecule (25.25 A) determined from the DFT
optimised structure, which shows extended octyloxy chains,
pointing in opposite directions, parallel to the plane dened by
the carbazolo[3,2-b]carabazole core. The shorter d-spacing
could then be ascribed to a tilted orientation of the molecule
relative to the substrate. Nevertheless, it cannot be ruled out
that the short d-spacing might also be due to the interdigitation
of alkyl chains from closely packed molecules.43
the presence of bigger grains is correlated with higher electrical
performance, due to larger crystalline domains. However, this is
not always the case.61 In particular, for semiconductors 5, 10,
and 14 a clear grain size increase with efficient interconnectivity
is found for higher temperatures (see ESI†), which could benet
charge transport. In the case of semiconductor 5, the grain
shape is also greatly modied by the temperature, varying from
round-shape small grains to long thread-shaped structures.
Different shapes are also found for semiconductor 10, where
small grains at low temperature are translated to big islands at
When the diffractogram of 14 was compared to the simu-
lated diffraction pattern obtained from the single crystal data, a
good correspondence was observed. This enabled the determi-
ꢀ
nation of (h0l) indices for the crystallite planes. The interplanar
ꢁ
˚
ꢀ
90 C. However, in the case of semiconductors 15 and 16 the
distance (8.9 A), calculated from the (101) reection, is shorter
than the molecule's length, suggesting a tilted alignment with
an edge-on orientation.
increased grain size with higher temperature is accompanied by
thin-lm dewetting, which is typically related to lower electrical
performance due to poor grain connectivity.
A suitable agreement was also found between the experi-
mental and the single-crystal simulated powder diffraction
patterns of compound 15. In this case the highly ordered
orientation of the crystal was dened by the (h00) planes (2q ¼
Field-effect transistor characterization
5.84ꢁ, 17.57ꢁ, 23.45ꢁ, 29.45ꢁ), although
a non-systematic
Top-contact/bottom-gate eld-effect transistors were fabricated
from sublimed lms of the present semiconductors deposited
at different substrate temperatures. Charge transport evalua-
tion was carried out via analysis of the OFET current–voltage
response in the saturation regime with the assumption of
conventional transistors theory, following eqn (1),
absence of the (200) peak was recorded. Accordingly the strong
˚
(100) reection corresponds to a d-spacing of 15.1 A shorter
than the a-axis. Thus, this indicates a tilted orientation of the
molecule with the alkyl chains pointing towards the substrate.
In this regard, a 36ꢁ tilt angle was calculated between the car-
bazolo[4,3-c]carbazole system in 15 and the normal to the cor-
responding Bragg plane.47 The higher ordering displayed by the
sublimed lms of compound 15, along with that previously
ꢀ
ꢁ
W
2
ðISD
Þ
¼
mCðVG ꢀ VTÞ
(1)
sat
2L
mentioned for 10, is in good agreement with the better perfor- where (ISD sat
)
is the drain current in the saturation regime, m is
mance detected for the transistors prepared with these the eld-effect carrier mobility of the semiconductor, W is the
molecules.
channel width, L is the channel length, C is the capacitance per
unit area of the insulator layer, VT is the threshold voltage, and
VG is the gate voltage. The calculated transistor parameters
Thin lm morphology by Atomic Force Microscopy
include charge carrier mobility (m), current on–off ratio (ION
/
In terms of charge transport, the most relevant interface in a IOFF), and threshold voltage (VT).62 For the calculation of the
eld-effect transistor is the dielectric–semiconductor interface. relevant OFET parameters, we used transfer plots of ID vs. VG at a
Although tapping mode AFM is unable to characterize those xed VD of ꢀ100 V to ensure that the device was operating in
buried interfaces, it is widely used to correlate the lm micro- the saturation regime. The obtained data are summarised in
structure with charge transport in the active region.5,60,61 Fig. 10 Table 3 and the selected output and transfer plots are shown
shows the AFM images of the different semiconducting lms in Fig. 11.
ꢁ
ꢁ
deposited at two extreme temperatures (25 C and 90 C).
The lowest electrical performance was found for the carba-
AFM images indicate that by increasing the substrate zolo[1,2-a]carbazole isomer, 5, with eld effect mobilities up to
temperature during thin-lm deposition the grain sizes are 2.3 ꢂ 10ꢀ5 cm2 Vꢀ1 sꢀ1 for lms deposited at 90 ꢁC and
greatly enhanced and different grain shapes are also found for measured under vacuum. This low mobility of 5 compared to the
some of the semiconductors at higher temperatures. In general, other semiconductors may be ascribed to the lack of linearity of
1964 | J. Mater. Chem. C, 2013, 1, 1959–1969
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Table 3 OFET electrical data for vapor-deposited films of the present semi-
conductors deposited at different substrate temperatures and measured in
vacuum and in air on HMDS-treated Si/SiO2 substrates
substitution with linear alkyl chains (15), and with bulky 4-
butylphenyl groups (16). As shown in a previous section, the
crystallographic characterization is indicative of
a quite
different crystal packing pattern for each semiconductor, from a
diverse herringbone structure for 14 and 15 (which is a common
molecular arrangement detected in fused polyaromatic systems
showing a good performance as organic semiconductors in
OFETs as it has been previously observed for pentacene63) to a
slipped stacked arrangement for semiconductor 16, where
bulky groups are present.
The electrical characterization of these semiconductors also
reveals quite a different behaviour, with eld effect mobilities
ranging from 1.6 ꢂ 10ꢀ4 and 8.2 ꢂ 10ꢀ4 cm2 Vꢀ1 sꢀ1 for 14 and
16, respectively, to 0.06 cm2 Vꢀ1 sꢀ1, for semiconductor 15. The
much higher mobilities of the latter may be ascribed to the more
crystalline thin lms with Bragg reections up to the h order.
A lower but still good crystallinity is found for semiconductor
14; however, the different crystal packing induced by the
absence of alkyl chains is translated in more than two orders of
magnitude lower eld-effect mobility. The case of semi-
conductor 16 is intriguing, since it did not show any evidence of
thin-lm X-ray diffraction, which we previously related to the
twisting of N-phenyl substituents with respect to the carbazo-
locarbazole skeleton found in the single crystal. However,
characterization of the corresponding OFETs indicates a
modest hole mobility of 8.2 ꢂ 10ꢀ4 cm2 Vꢀ1 sꢀ1, higher than the
one recorded for semiconductor 14.
Vacuum
Comp.a (Td) mb
Air
c
c
VT
ION/IOFF mb
VT
ION/IOFF
5 (25 ꢁC)
1.4 ꢂ 10ꢀ5 ꢀ31 10+3
9.3 ꢂ 10ꢀ6 ꢀ7 10
NA
NA NA
5 (60 ꢁC)
1.4 ꢂ 10ꢀ6 ꢀ38 3 ꢂ 10+2
5 (90 ꢁC)
2.3 ꢂ 10ꢀ5 ꢀ26 2 ꢂ 10+3 8.0 ꢂ 10ꢀ6 ꢀ42 2 ꢂ 10+3
10 (25 ꢁC)
10 (60 ꢁC)
10 (90 ꢁC)
14 (25 ꢁC)
14 (60 ꢁC)
14 (90 ꢁC)
15 (25 ꢁC)
15 (60 ꢁC)
15 (90 ꢁC)
16 (25 ꢁC)
16 (60 ꢁC)
16 (90 ꢁC)
1.2 ꢂ 10ꢀ3 ꢀ28 10+5
1.1 ꢂ 10ꢀ3 ꢀ23 10+5
2.3 ꢂ 10ꢀ3 ꢀ23 2 ꢂ 10+3 8.7 ꢂ 10ꢀ4 ꢀ46 10+3
4.0 ꢂ 10ꢀ3 ꢀ25 10+5
5.0 ꢂ 10ꢀ6 ꢀ63 10+4
4.0 ꢂ 10ꢀ5 ꢀ63 10
3.3 ꢂ 10ꢀ3 ꢀ26 10+5
2.0 ꢂ 10ꢀ6 ꢀ64 6 ꢂ 10+2
1.5 ꢂ 10ꢀ5 ꢀ72 3 ꢂ 10+3
1.6 ꢂ 10ꢀ4 ꢀ67 3 ꢂ 10+4 7.3 ꢂ 10ꢀ5 ꢀ78 6 ꢂ 10+4
0.06
0.01
NA
ꢀ62 7 ꢂ 10+3 0.02
ꢀ64 10+6
ꢀ33 6 ꢂ 10+3 5.9 ꢂ 10ꢀ3 ꢀ47 7 ꢂ 10+5
NA NA
NA
NA NA
8.2 ꢂ 10ꢀ4 ꢀ15 10+4
8.2 ꢂ 10ꢀ4 ꢀ16 2 ꢂ 10+5
1.5 ꢂ 10ꢀ4 ꢀ24 5 ꢂ 10+3 9.3 ꢂ 10ꢀ5 ꢀ21 7 ꢂ 10+3
NA
NA NA
NA
NA NA
a
c
Substrate temperature. b Units are cm2 Vꢀ1 sꢀ1
.
Units are V.
The better performance of the semiconductor 15 with an
angular skeleton versus the linear isomer 10 is not surprising
since a similar trend was found previously by Takimiya et al. in a
series of naphthodithiophene semiconductor polymers.48 In that
case, the authors mainly ascribed the improved performance of
the angular-shape polymers to a highly ordered structure with a
very close p-stacking, whereas the linear-shape derivatives had a
very weak or no p-stacking order. This trend however differs for
small molecule derivatives.47 The effect of substrate temperature
during the semiconducting lm deposition was also analysed
and the corresponding data are shown in Table 3. Note that for
semiconductors 5, 10, and 14 the eld-effect mobility gradually
increases with substrate deposition temperature. This can be
easily understood if we analyse the trend in lm morphology. For
these three semiconductors, as the substrate temperature is
Fig. 11 Output (left) and transfer (right) characteristics for top-contact/bottom-
gate OFETs of compounds (a) 10 at 90 ꢁC and (b) 15 at 25 ꢁC, measured under
vacuum.
the p-conjugated skeleton, due to the U-shape of this molecule. raised the grain size (see Fig. 10) gradually increases while
Furthermore, XRD spectra show poor lm crystallinity, with the maintaining good interconnectivity. However, for semi-
presence of only two weak reections. The single crystal conductors 15 and 16, AFM images indicate (see ESI†) that as we
diffraction pattern indicates that even though the molecules are raise the substrate temperature the grain size increases but some
packed in parallel planes, these planes are staggered and no p–p cracks start to be noticeable at 60 ꢁC, followed by lm dewetting
stacking can be observed between adjacent planes.45 In contrast, at higher temperatures. This is in complete agreement with the
the carbazolo[3,2-b]carbazole isomer, 10, has a linear p-conju- electrical data, in which it can be observed that the best results
gated skeleton. In this case, we were not able to grow single are shown for room temperature devices, and the performance
crystals; however, thin lm X-ray diffraction shows highly crys- starts degrading at higher temperatures. Note that thin lms
talline lms with Bragg reections up to the sixth order. This deposited at 90 ꢁC become inactive in OFETs.
higher crystallinity is reected in a much more efficient charge
transport, with hole mobilities up to 4.0 ꢂ 10ꢀ3 cm2 Vꢀ1 sꢀ1 for
Conclusions
lms deposited at 90 ꢁC and measured under vacuum.
In the case of the carbazolo[4,3-c]carbazole, three different A series of carbazolo[1,2-a]carbazole, carbazolo[3,2-b]carbazole
substitution patterns are analysed: absence of alkyl chains (14), and carbazolo[4,3-c]carbazole have been synthesised and
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Journal of Materials Chemistry C
Paper
characterised. The crystal structure of several derivatives was Reichert instrument and are not corrected. Unless stated
1
presented. These systems exhibit a relatively wide HOMO– otherwise, all spectra were recorded at room temperature. H
LUMO gap covering the range of 2.56–3.25 eV with a low lying NMR and 13C NMR spectra were recorded on Bruker AV300,
HOMO energy which makes them almost completely trans- Bruker AV400 or Bruker AV600 spectrometers having frequen-
parent to the visible radiation and stable towards ambient cies of 300 MHz, 400 MHz or 600 MHz, for proton nuclei and
oxidation. These electronic properties are affected by the 75 MHz, 100 MHz or 150 MHz, respectively, for carbon nuclei.
different fused ring geometry of the isomers which determines Chemical shis are referred to tetramethylsilane (d ¼ 0 ppm) or
the benzenoid character of the aromatic system. Thin lms the residual peak from the deuterated solvent. Mass spec-
deposited by thermal evaporation revealed a different degree of trometry was recorded on an HPLC-MS TOF 6220 instrument.
crystallinity on moving from a highly crystalline structure in 10 Absorption spectra were recorded on a Cary 5000 UV-vis-NIR
to an amorphous lm in 16, as it could be inferred from XRD. spectrophotometer. Electrochemistry was recorded utilizing a
The nanomorphological differences between the carbazolo- Bass potenciostate and using platinum electrodes as working
carbazole isomer were also evident from the AFM images. and counter-electrodes and SCE as the reference electrode.
Moreover, a noticeable change in the morphology is detected by Compounds 6 (ref. 64) and 11 (ref. 65) were synthesised as
changing the temperature of the substrate prior to evaporation reported in the literature.
of the organic lm. Although the semiconductor lm grain
shape and size clearly changed upon heating, this does not
always correlate with the changes of FET performance. A slight
2,6-Dibromo-1,5-dioctyloxynaphthalene (7)
mobility enhancement was detected for samples 5 and 10 on To a solution of 2,6-dibromo-1,5-dihydroxynaphthalene (6, 4 g,
moving from 25 ꢁC to 90 ꢁC. This improvement in charge 12.6 mmol) and 1-bromooctane (6.6 mL, 37.8 mmol) in ethanol
mobility was more important for compound 14 which improved (100 mL) at room temperature, potassium hydroxide (2.1 g, 37.8
by a hundred fold upon raising the substrate temperature. mmol) was added and the mixture was reuxed for 8 hours.
Conversely, compounds 15 and 16 displayed larger mobility for Aer cooling at room temperature, the solvent was removed by
room temperature-deposited lms but no eld effect was rotary evaporation and the residue was puried by column
ꢁ
detected for transistors based on 90 C-deposited lms. The chromatography on silica gel (dichloromethane : hexane, 1 : 3)
inuence of substituents on the fused polyheteroaromatic to obtain 7 as a yellow solid (3.4 g, 50%). Mp 48–49 ꢁC. 1H NMR
system was also studied for the carbazolo[4,3-c]carbazole core. (300 MHz, CDCl3, ppm) d 0.88–0.93 (m, 6H), 1.26–1.40 (m, 16H),
OFETs fabricated with the N-octylated derivative 15 showed 1.53–1.60 (m, 4H), 1.92–1.97 (m, 4H), 4.07 (t, J ¼ 6.6 Hz, 4H),
much better results than those prepared with either the 7.60 (d, J ¼ 9.0 Hz, 2H), 7.74 (d, J ¼ 9.0 Hz, 2H); 13C NMR
unsubstituted carbazolocarbazole 14 or the N-arylated system (75 MHz, CDCl3, ppm) d 14.1, 22.7, 26.0, 29.3, 29.5, 30.3, 31.8,
16. Thus, alkyl chains seem to induce a favorable ordering of the 74.7, 113.8, 119.4, 130.1, 131.0, 152.8. HRMS-(m/z) for
molecular material which behaves as a better organic semi-
conductor. OFETs measured under vacuum exhibit a hole
mobility of 0.06 cm2 Vꢀ1 sꢀ1 and an Ion/Ioff ratio of 7 ꢂ 105, very
similar to the data obtained in ambient conditions (hole
C
26H39Br2O2, calculated: 541.1311, found: 541.1303 (M + H)+.
1,5-Dioctyloxynaphthyl-2,6-diboronic acid (8)
mobility of 0.02 cm2 Vꢀ1 sꢀ1 and an Ion/Ioff ratio of 106). These To a solution of 7 (3 g, 5.5 mmol) in anhydrous diethyl ether
preliminary results have shown the potential of the carbazolo- (120 mL) at ꢀ20 ꢁC under nitrogen atmosphere, n-BuLi (2.5 M,
carbazole skeleton for molecular electronics. Further work on 27.7 mmol) was added dropwise, immediately followed by the
these heteroaromatic systems, which reinforces our conclu- addition of freshly distilled N,N,N0,N0-tetramethylethylenedi-
sions, is currently being developed and results will be reported amine, TMEDA, (5 mL, 33.2 mmol). The mixture was stirred at
in due course.
this temperature for 2 hours and then at room temperature for 2
more hours. The temperature was again lowered to ꢀ20 ꢁC and
triethylborate (9.4 mL, 55.4 mmol) was added all at once. The
reaction mixture was allowed to reach room temperature and
stirred overnight. Hydrochloric acid (15%, 100 mL) was added
Experimental
General
Reagents used as starting materials were purchased from and the organic layer was washed with water and brine, dried
commercial sources and were used without further purication. over anhydrous Na2SO4 and the solvent was evaporated under
Solvents were dried following the usual protocols (THF, Et2O reduced pressure. The residue was precipitated with dichloro-
and Toluene were distilled from sodium wire with the benzo- methane and dried under vacuum to afford the pure expected
ꢁ
phenone indicator; CH3CN and CH2Cl2 were distilled from compound as a pale yellow solid (1.8 g, 68%). Mp 170–172 C.
CaCl2; EtOH and MeOH were distilled from magnesium and 1H NMR (200 MHz, DMSO-d6, ppm) d 0.83–0.88 (m, 6H), 1.26–
stored with molecular sieves). Unless stated otherwise, all 1.39 (m, 16H), 1.41–1.55 (m, 4H), 1.77–1.84 (m, 4H), 4.03 (t, J ¼
reactions were carried out under nitrogen atmosphere. Column 6.4 Hz, 4H), 7.49 (d, J ¼ 8.4 Hz, 2H), 7.76 (d, J ¼ 8.4 Hz, 2H), 8.12
chromatography was run with silica gel 60 A CC 70–200 mm as (s, 4H); 13C NMR (50 MHz, DMSO-d6, ppm) d 14.0, 22.1, 25.6,
the stationary phase, purchased from Carlo Erba-SDS, and 28.7, 28.8, 29.8, 31.2, 74.6, 116.2, 122.7, 129.1, 130.1, 158.9.
using HPLC grade solvents purchased from either Panreac, or HRMS-(m/z) for
C
26H43B2O6, calculated: 473.3255, found:
Lab-Scan, or Scharlab. Melting points were measured in a 473.3253 (M + H)+.
1966 | J. Mater. Chem. C, 2013, 1, 1959–1969
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Journal of Materials Chemistry C
2,6-Di(2-azidophenyl)-1,5-dioctyloxynaphthalene (9)
1,5-Di(2-azidophenyl)naphthalene (13)
A stirred solution of Pd(PPh3)4 (0.22 g, 0.2 mmol) and 2-azi- A degassed solution of Pd(PPh3)4 (0.09 g, 0.08 mmol) and 2-
doiodobenzene (3.9 g, 16 mmol) in dimethoxyethane (80 mL) azidoiodobenzene (1.6 g, 6.6 mmol) in dimethoxyethane
was degassed at room temperature for 15 minutes. Then, 8 (3 g, (40 mL) was stirred at room temperature for 15 minutes. Then,
6.4 mmol), ethanol (50 mL) and aqueous Na2CO3 (2 M, 25.6 12 (1 g, 2.7 mmol), ethanol (30 mL) and aqueous Na2CO3 (2 M,
mmol) were sequentially added and the resulting mixture was 10.6 mmol) were added and the resulting mixture was reuxed
reuxed overnight. The reaction was cooled to room tempera- for 8 hours under nitrogen atmosphere. The reaction was
ture, concentrated under vacuum and the crude product was cooled to room temperature, concentrated under vacuum and
puried by column chromatography on silica gel eluting with the residue was puried by column chromatography on silica
dichloromethane : hexane (1 : 4) to obtain 9 as a white solid (2.6 gel eluting with dichloromethane : hexane (1 : 3) to isolate 13 as
g, 66%). Mp 95–96 ꢁC. 1H NMR (400 MHz, CDCl3, ppm) d 0.86– a white solid (0.67 g, 70%). Mp 182–183 ꢁC. 1H NMR (300 MHz,
0.90 (m, 6H), 1.16–1.29 (m, 20H), 1.50–1.56 (m, 4H), 3.66 (t, J ¼ DMSO-d6, 60 ꢁC) d 7.4–7.6 (m, 14H); 13C NMR (75 MHz, DMSO-
6.4 Hz, 4H), 7.22–7.31 (m, 4H), 7.38 (d, J ¼ 8.4 Hz, 2H), 7.43–7.46 d6, 60 ꢁC) d 118.7, 118.9, 124.8, 125.2, 125.3, 127.1, 129.2, 131.2,
(m, 4H), 8.00 (d, J ¼ 8.4 Hz, 2H); 13C NMR (100 MHz, CDCl3, 131.6, 135.9, 137.7. HRMS-(m/z) for C22H18N7, calculated:
ppm) d 14.1, 22.7, 25.9, 29.2, 29.3, 30.1, 31.8, 74.4, 117.9, 118.5, 380.1618, found: 380.1619 (M + NH4)+.
124.6, 127.2, 128.9, 129.0, 129.8, 130.8, 132.2, 138.2, 153.1.
HRMS-(m/z) for
C38H47N6O2, calculated: 619.3755, found:
Carbazolo[4,3-c]carbazole (14)
619.3753 (M + H)+.
A solution of 13 (2 g, 5.5 mmol) in freshly distilled o-dichloro-
benzene (50 mL) was reuxed for 8 hours. Aer cooling the
reaction mixture, a pale green precipitate was formed. This was
ltered, washed with hexane and dried under vacuum to afford
7,14-Dioctyloxycarbazolo[3,2-b]carbazole (10)
Small portions of 9 (0.4 g, 0.6 mmol) were added over reuxing
nitrobenzene (20 mL). Then, the reaction mixture was allowed to
cool to room temperature. The solvent was removed under
reduced pressure and the residue was puried by silica gel
column chromatography using ethyl acetate : hexane (1 : 3) as
the eluent to isolate 10 (0.1 g, 26%) as a light yellow solid. Mp
228–230 ꢁC. 1H NMR (600 MHz, DMSO-d6, ppm) d 0.84–0.91 (m,
6H), 1.29–1.31 (m, 8H), 1.38–1.40 (m, 4H), 1.43–1.47 (m, 4H),
1.66–1.71 (m, 4H), 2.07–2.11 (m, 4H), 4.22 (t, J ¼ 6.6 Hz, 4H),
7.18–7.21 (m, 2H), 7.43–7.47 (m, 4H), 7.94 (s, 2H), 8.23 (d, J ¼ 7.8
Hz, 2H), 11.08 (s, 2H); 13C NMR (150 MHz, DMSO-d6, ppm) d
14.5, 22.6, 26.3, 29.2, 29.5, 30.7, 31.8, 73.6, 95.4, 110.5, 116.9,
119.0, 121.1, 123.3, 123.4, 127.4, 139.0, 142.9, 149.0. HRMS-(m/z)
for C38H47N2O2, calculated: 563.3632, found: 563.3629 (M + H)+.
1
the pure title compound (1.2 g, 71%). Mp > 330 ꢁC. H NMR
(600 MHz, DMSO-d6) d 7.31 (t, J ¼ 7.5 Hz, 2H), 7.44 (t, J ¼ 7.5 Hz,
2H), 7.66 (d, J ¼ 7.8 Hz, 2H), 7.95 (d, J ¼ 8.7 Hz, 2H), 8.66 (d, J ¼
7.8 Hz, 2H), 8.88 (d, J ¼ 8.7 Hz, 2H), 11.69 (s, 2H); 13C NMR
(150 MHz, DMSO-d6) d 111.4, 113.4, 115.8, 119.1, 121.7, 121.8,
122.8, 124.1, 135.7, 138.9. HRMS-(m/z)for C22H15N2, calculated:
307.1230, found: 307.1226 (M + H)+.
1,8-Dioctylcarbazolo[4,3-c]carbazole (15)
To a solution of 14 (0.5 g, 1.6 mmol) and potassium hydroxide
(0.6 g, 9.8 mmol) in freshly distilled dimethylformamide, at
room temperature, 1-bromooctane (2.8 mL, 16 mmol) was
added dropwise and the reaction mixture was then stirred at
60 ꢁC for 8 hours. The precipitate formed over this time, which
corresponds to the pure product, was ltered off, washed with
water and dried under vacuum (0.78 g, 90%). Mp 185–186 ꢁC. 1H
NMR (400 MHz, CDCl3) d 0.84–0.88 (m, 6H), 1.26–1.47 (m, 20H),
1.92–2.01 (m, 4H), 4.50 (t, J ¼ 6.9 Hz, 4H), 7.40 (t, J ¼ 7.8 Hz, 2H),
7.50–7.61 (m, 4H), 7.89 (d, J ¼ 9.0 Hz, 2H), 8.70 (d, J ¼ 7.8 Hz,
2H), 8.98 (d, J ¼ 9.0 Hz, 2H); 13C NMR (100 MHz, CDCl3) d 14.0,
22.6, 27.4, 29.2, 29.4, 29.5, 31.8, 43.2, 109.2, 110.9, 116.4, 119.2,
122.1, 122.3, 123.4, 124.1, 124.9, 136.5, 139.5. HRMS-(m/z) for
1,5-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)
naphthalene (12)
n-BuLi (2.5 M, 5.3 mmol) was added dropwise to a solution of 11
(0.5 g, 1.3 mmol) in anhydrous diethyl ether (20 mL) at ꢀ78 ꢁC
under nitrogen atmosphere. Aer the addition was completed,
the solution was stirred at this temperature for 1 hour, warmed
at ꢀ10 ꢁC for 6 hours and cooled again to ꢀ78 ꢁC. Then, 2-
isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (0.94 mL,
4.6 mmol) was added all at once. The mixture was allowed to
reach room temperature and stirred overnight. The reaction
mixture was quenched with HCl (15%). The organic layer was
C
38H47N2, calculated: 531.3734, found: 531.3736 (M + H)+.
1,8-Di(4-butylphenyl)carbazolo[4,3-c]carbazole (16)
washed with water and brine, dried over anhydrous Na2SO4 and A round-bottomed ask was charged with 14 (0.4 g, 1.3 mmol),
the solvent was evaporated under reduced pressure. The residue 1-bromo-4-butylbenzene (1.0 g, 4.7 mmol), copper powder
was treated with methanol to obtain a pure light yellow solid (8.3 mg, 10 mol%), potassium carbonate (1.1 g, 7.8 mmol) and
(0.3 g, 56%) which was used without further purication. Mp dimethylacetamide (15 mL). The reaction mixture was reuxed
273–275 ꢁC. 1H NMR (300 MHz, CDCl3) d 1.43 (s, 24H), 7.52 (dd, for 5 days with vigorous stirring. The mixture was ltered
J1 ¼ 6.9 Hz, J2 ¼ 8.4 Hz, 2H), 8.07 (d, J ¼ 6.9 Hz, 2H), 8.89 (d, J ¼ through a Celite pad, the solvent was removed by rotary evap-
8.4 Hz, 2H); 13C NMR (75 MHz, CDCl3) d 24.9, 83.7, 125.4, 131.9, oration and the remaining residue was puried by column
135.3. HRMS-(m/z) for C22H31B2O4, calculated: 381.2416, found: chromatography using dichloromethane : hexane (1 : 2) as the
381.2411 (M + H)+.
eluent to provide the pure compound (0.36 g, 48%) as a pale
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. C, 2013, 1, 1959–1969 | 1967

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Journal of Materials Chemistry C
Paper
yellow solid. Mp 239–240 ꢁC. 1H NMR (400 MHz, THF-d8) d 1.03
(t, J ¼ 7.2 Hz, 6H), 1.45–1.54 (m, 4H), 1.73–1.79 (m, 4H), 2.81 (t,
J ¼ 7.6 Hz, 4H), 7.36–7.41 (m, 4H), 7.49 (d, J ¼ 7.2 Hz, 2H), 7.53
(d, J ¼ 8.4 Hz, 4H), 7.59 (d, J ¼ 8.4 Hz, 4H), 7.82 (d, J ¼ 8.8 Hz,
2H), 8.71 (d, J ¼ 7.2 Hz, 2H), 8.98 (d, J ¼ 9.2 Hz, 2H); 13C NMR
8 H. S. Nalwa, Handbook of Organic Electronics and Photonics,
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¨
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121.1, 123.0, 123.2, 124.8, 125.2, 126.3, 128.4(ꢂ2), 130.7(ꢂ2),
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571.3108, found: 571.3105 (M + H)+.
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Field-effect transistors were fabricated by vapour deposition of
ꢃ50 nm thick layers of semiconductors at preselected temper-
atures (ꢃ6 ꢂ 10ꢀ6 Torr, 0.2 A sꢀ1) on p-doped Si (001) wafers
˚
with a 300 nm thermally grown SiO2 dielectric layer. Prior to
deposition, the wafers were cleaned by rinsing twice with EtOH
followed by a 5 min plasma cleaning. Field-effect transistors on
hexamethyldisilazane (HMDS)-treated substrates were also
fabricated in order to reduce charge trapping and thus enhance
n-type mobility. Trimethylsilation of the Si/SiO2 surface was
carried out by exposing the silicon wafers to HMDS vapour at
room temperature in a closed air-free container under nitrogen
´
20 P. M. Beaujuge and J. M. J. Frechet, J. Am. Chem. Soc., 2011,
for more than 1.0 week. Top contact OTFTs were fabricatꢀe1d
133, 20009–20029.
by vapour deposition of gold electrodes (ꢃ10ꢀ7 Torr, 0.2 A s
,
˚
21 Y. Wen and Y. Liu, Adv. Mater., 2010, 22, 1331–1345.
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ꢃ50 nm thick) onto the semiconductor thin lms through a
shadow mask to obtain devices with deliberately varied channel
widths and lengths. The capacitance of the 300 nm SiO2 gate
insulator is 10ꢀ8 F cmꢀ2. Characterization of the devices was
performed under vacuum in a customised high-vacuum probe
station (ꢃ10ꢀ6 Torr) and in ambient atmosphere using a sub-
femtoamp meter and a source meter.
24 B. J. Jung, N. J. Tremblay, M.-L. Yeh and H. E. Katz, Chem.
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25 M. E. Roberts, A. N. Sokolov and Z. Bao, J. Mater. Chem.,
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´
D. C. and M. M.-M. are grateful to the Consejerıa de Uni-
´
´
versidades, Empresa e Investigacion de la Region de Murcia for
the nancial support through the Regional Plan of Science and
Technology. M. M.-M. acknowledges the Ministry of Education
for a FPU fellowship. We also wish to thank the Super-
computation Center at "Fundacion Parque Cientıco de Mur-
cia" for their technical support and the computational
´
´
´
resources used in the supercomputer Ben-Arabı.
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