Molecular order of air-stable p-type organic thin-film transistors by tuning the extension of the π-conjugated core: The cases of indolo[3,2-b]carbazole and triindole semiconductors

Article abstract of DOI:10.1039/c4tc01692k

Charge transport in organic devices depends strongly on the molecular order and morphology of the organic semiconductor thin films. In the design of new organic semiconductors, the selection of the appropriate core plays a key role in the molecular packing and charge transport characteristics of the organic device. Four derivatives of carbazole that mainly differ in the extension of the π-conjugated core, including indolo[3,2-b]carbazole and triindole derivatives, exhibited hole mobilities ranging from 10^-5 to 10^-2 cm² V^-1 s^-1 as active layers in organic thin-film transistors (OTFTs). X-ray analyses of the single crystals and evaporated thin films gave insights into the molecular packing of the compounds that justified their OTFTs characteristics.

Full text of DOI:10.1039/c4tc01692k

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Materials Chemistry C
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Molecular order of air-stable p-type organic thin-
film transistors by tuning the extension of the p-
conjugated core: the cases of indolo[3,2-b]-
carbazole and triindole semiconductors†
Cite this: DOI: 10.1039/c4tc01692k
a
b
a
Marta Reig, Joaquim Puigdollers and Dolores Velasco*
Charge transport in organic devices depends strongly on the molecular order and morphology of the
organic semiconductor thin films. In the design of new organic semiconductors, the selection of the
appropriate core plays a key role in the molecular packing and charge transport characteristics of the
organic device. Four derivatives of carbazole that mainly differ in the extension of the p-conjugated
core, including indolo[3,2-b]carbazole and triindole derivatives, exhibited hole mobilities ranging from
Received 31st July 2014
Accepted 1st October 2014
ꢀ5
ꢀ2
2
ꢀ1 ꢀ1
10
to 10 cm
V
s
as active layers in organic thin-film transistors (OTFTs). X-ray analyses of the
DOI: 10.1039/c4tc01692k
single crystals and evaporated thin films gave insights into the molecular packing of the compounds that
justified their OTFTs characteristics.
www.rsc.org/MaterialsC
The search for high mobility organic semiconductors has
yielded pentacene as one of the most promising materials,
Introduction
2
ꢀ1 ꢀ1
Aer intensive research effort, p-type OTFTs have now achieving high hole mobilities of 3 cm V
s
for vacuum
1b,2
successfully achieved hole mobilities comparable to those deposited OTFTs.
Nevertheless, pentacene-based devices
obtained with amorphous silicon. The performances of OTFTs present several drawbacks for practical applications due to a
3
have achieved impressive progress in recent years, due to the lack of air stability and fast device degradation.
processing of new materials, device geometries, and
Carbazole derivatives have been widely studied as hole-
morphology control. However, the properties of the organic transporting semiconductors, due to their electron-donating
4
semiconductors still determine the performance of OTFT capabilities. In addition, they are recognised as reasonably
devices. In view of this, when designing new suitable organic stable compounds against oxidative doping by atmospheric
compounds for molecular electronics, the selection of an oxygen. The carbazole unit presents a series of advantages
appropriate core structure is a signicant aspect to take into ahead of other conventional organic semiconductors, such as
1
account, because it could modulate the optical and electronic its low cost, easy availability, and its high stability in environ-
properties and even determine the stability and durability of the mental conditions.
fabricated devices under ambient conditions. One strategy to
In this work, we deal with the study of 9-methyl-9H-carbazole
0
0
0
achieve materials with high air stability is to work with 1, 9,9 -dimethyl-9H,9H -[3,3 ]bicarbazole 2, 5,11-dimethy-
compounds that exhibit low-lying high occupied molecular lindolo-[3,2-b]carbazole 3, and N-trimethyltriindole 4 (Fig. 1) as
orbital (HOMO) levels, in order to obtain compounds stable active organic semiconductor layers in OTFTs. Within this
against oxidative doping by atmospheric oxygen.
a
2
Grup de Materials Org a` nics, Institut de Nanoci `e ncia i Nanotecnologia (IN UB),
Departament de Qu ´ı mica Org a` nica, Universitat de Barcelona, Mart ´ı i Franqu `e s 1,
E-08028, Barcelona, Spain. E-mail: dvelasco@ub.edu; Fax: +34 93 339 78 78; Tel:
+
34 93 403 92 60
b
Dept. Eng. Electr `o nica & Centre de Recerca en Nanoenginyeria, Universitat Polit `e cnica
de Catalunya, C/Jordi Girona, 1-3, 08034, Barcelona, Spain. E-mail: joaquim.
puigdollers@upc.edu; Tel: +34 93 401 10 02
†
Electronic supplementary information (ESI) available: Absorption spectra, cyclic
voltammograms, OTFTs characteristics, detailed crystallographic data and
powder X-ray diffraction patterns. CCDC 1017245. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c4tc01692k
Fig. 1 Molecular structure of carbazole-based derivatives 1–4.
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series, the extension of the p-conjugated core was progressively
Compounds 1–4 showed one oxidation process by cyclic
voltammetry, whereas no reduction processes were observed for
any of them. Table 1 collects the electrochemical characteris-
tics, the HOMO energy level estimated from cyclic voltammetry
ꢀ
2
2
varied, leading to improved OTFTs performances of 10 cm
ꢀ1
ꢀ1
V
s .
Indolo[3,2-b]carbazole and triindole show extended
p
systems, in which two or three carbazole units, respectively, and the energy band gap estimated from the UV-visible spectra
share an aromatic ring. Indolo[3,2-b]carbazole and triindole (the absorption spectra and cyclic voltammograms can be found
unit present a planar structure with two or three N–H units, in the ESI, Fig. S1 and S2†). In this series of carbazole-based
respectively, which allows easy introduction of alkyl chains, in compounds, when increasing the conjugation of the system,
order to increase the solubility of the systems and to promote the HOMO levels are slightly destabilized from ꢀ6.16 eV
the molecular order of the materials during the deposition for carbazole 1 to ꢀ5.45 eV corresponding to triindole
process. Both heterocycles display large energy band gaps and derivative 4 (Fig. 2). The relative low-lying HOMO levels and
low-lying HOMO energy levels. In the last few years, indolo- high HOMO–LUMO gaps (ꢁ3.7 eV) of these compounds are
carbazole has received special attention as a building block in indicative of their potential hole transporting properties.
the eld of organic electronics, and recent studies have shown Moreover, the energy levels found for the organic semi-
the great potential of indolo[3,2-b]carbazole-based derivatives to conductors 1–4 are perfectly suitable to Au electrode work
5
be used in OTFTs. The triindole core presents a larger molec- function (ꢀ5.1 eV) as it is displayed in Fig. 2.
ular structure than the indolocarbazole core. Accordingly, a
higher molecular order could be expected from it, and there-
fore, better charge carrier transport. To the best of our knowl-
Organic thin-lm transistors
edge, there are still no studies about the application of triindole
derivatives in OTFTs. Recently, triindole derivatives have been Charge carrier mobilities were measured by using the organic
described as suitable organic materials in Organic Solar Cells materials 1–4 as vacuum-deposited active layers in standard
6
7
(
OSCs) and OLEDs. The charge transport properties of liquid- bottom gate-top contact OTFTs.
8
crystal-based triindole derivatives and of triindole 4 have been
The fabrication of OTFTs with compound 1 as the organic
reported by the space-charge-limited-current method (SCLC), semiconductor layer was not successful, due to its low subli-
ꢃ
ꢀ6
yielding values of hole mobilities for the later ranging from ꢁ1 mation temperature (40 C at 10 mbar under the experi-
ꢀ
4
ꢀ3
2
ꢀ1 ꢀ1 6b
ꢂ 10 to ꢁ1 ꢂ 10 cm V
s . The easy preparation, high mental conditions during the deposition process). 2-based
thermal stability, p-stacking capability, high carrier mobilities, devices fabricated with 50 nm thick SiO dielectric presented a
2
and optoelectronic properties of such molecules make indolo- bad yield and poor reproducibility: only 17% of the devices
carbazole- and triindole-based materials very promising organic fabricated with polystyrene (PS) treated substrates presented
semiconductors for optoelectronic applications.
Here, we report not only the OTFT performance of carbazole ꢂ 10 cm V
containing compounds 1–4, but also the determination of the Table 1 presents the device characterization for OTFTs
OTFT characteristics, with a maximum hole mobility around 2
ꢀ
5
2
ꢀ1 ꢀ1
s
and a threshold voltage of ꢀ16.0 V (Table 1).
molecular packing in the organic semiconductor layer of the fabricated with the more extended p-conjugated cores of
devices by means of X-ray diffraction (XRD) studies. Therefore, compounds 3 and 4. Devices fabricated with substrates of
the aim of this work is to determine the correlation of the device 50 nm of SiO2 presented similar hole mobilities, but lower
performance with the molecular order in the semiconducting threshold voltages, than those fabricated from substrates of
thin lms as a function of the p-conjugated core design of the 110 nm of SiO . The reproducibility of the measurements and
2
organic semiconductors.
the yield of the working devices were found to be independent
of the thickness of the SiO layer. The highest values of charge
2
mobilities were achieved by modifying the dielectric surface
with a polystyrene thin lm layer, which also resulted in lower
threshold voltages and a higher reproducibility. 3 and 4
Results and discussion
Synthesis and physical properties
Compound 2 was synthesized by oxidative dimerization of 9- PS-treated SiO
methyl-9H-carbazole with FeCl in anhydrous chloroform the average of ten devices (3: 89% and 4: 100%), exhibiting all of
following a previously reported procedure. Indolo[3,2-b]carba- the tested devices with similar performances. The highest hole
2
devices presented a superior yield, as given by
3
9
ꢀ
2
2
ꢀ1 ꢀ1
zole was synthesized by condensation of 1,4-cyclohexanodione mobility was registered for triindole 4, 3 ꢂ 10 cm V
s
in
for the indolocarbazole 3. The
tion. Triazatruxene was synthesized by the cyclocondensation OTFTs characteristics of 3- and 4-based devices fabricated on
ꢀ3
2
ꢀ1 ꢀ1
with phenylhydrazine, followed by a double Fischer indoliza- front of 7 ꢂ 10 cm V
s
10
11
2
of 2-oxindole. N-methyl derivatives 1, 3 and 4 of the corre- bare SiO surfaces showed lower reproducibility and lower
ꢀ
5
ꢀ3
2
ꢀ1 ꢀ1
sponding 9H-carbazole, indolo[3,2-b]carbazole, and triazatrux- charge mobilities of 7 ꢂ 10 and 5 ꢂ 10 cm
V
s ,
ene were obtained by methylation under standard conditions in respectively.
1
high yields. Compounds 1–4 were entirely characterized by H-
A representative output, transfer, and saturation character-
istics for 4-based OTFTs are shown in Fig. 3 (Fig. 3a and b; for
1
3
NMR, C-NMR and MS.
Indolocarbazole 3 and triindole 4 exhibited high thermal OTFT characteristics of compound 3-based devices see ESI,
stability by thermal gravimetric analysis (TGA), showing the Fig. S3a and b†). The output characteristics are indicative of the
ꢃ
ꢃ
onset of decomposition at 384 C and 461 C, respectively.
OTFT behaviour of the device in both the linear and saturated
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Table 1 Optical and electrochemical properties for compounds 1–4 and OTFT characteristics of devices based on 1–4 carbazole derivatives
with polystyrene treated c-Si/SiO substrates
2
a
ꢀ1
ꢀ1
ox
b
c
2
ꢀ1 ꢀ1
Compound
l
abs (3) nm (M cm
)
E
onset (V)
E
gap (eV)
EHOMO/ELUMO (eV)
m (cm V
s
)
Vth (V)
Ion/Ioff (A/A)
1
2
3
4
263 (32 314), 294 (29 722)
303 (76 660)
283 (52 268), 340 (67 146)
317 (79 023)
1.20
0.84
0.68
—
4.10
3.90
3.57
3.53
ꢀ6.16/ꢀ2.06
ꢀ5.80/ꢀ1.90
ꢀ5.64/ꢀ2.07
—
—
—
ꢀ
5
1
4
3
2 ꢂ 10
ꢀ16.0
ꢁ10
ꢁ10
ꢁ10
ꢀ3
ꢀ3 e
e
7 ꢂ 10 (1 ꢂ 10
)
ꢀ9.6 (ꢀ5.5)
d
ꢀ2
ꢀ2 f
f
ꢀ5.45 /ꢀ1.92
3 ꢂ 10 (1 ꢂ 10
)
ꢀ12.8 (ꢀ9.0)
a
b
c
Wavelength of the maximum absorption and molar extinction coefficient. Onset oxidation potential. Energy difference between the HOMO and
d
e
f
LUMO orbitals. Determined from data given in ref. 6b. Maximum value registered on the 8th day. Maximum value registered on the 5th day. The
results in parentheses are for the measurements aer ve months in air.
First of all, the knowledge of the single crystal structure of
compounds 3 and 4 was crucial because it could provide the
molecular packing motif and indicate the main intermolecular
interactions of these compounds. The crystal structure of
12
triindole derivative 4 was previously reported to adopt a face-
to-face packing in an alternate arrangement due to the p–p
interactions between adjacent triindole molecules. For that
˚
˚
compound, two different distances (3.53 A and 3.68 A) between
two neighbouring molecules were observed due to the steric
hindrance induced by the methyl groups.
In order to examine the molecular packing and the inter-
molecular interactions in the indolocarbazole derivative 3, the
single crystal structure was determined by X-ray crystallography.
The compound crystallizes in space group C2/c of the mono-
Fig. 2 Energy levels (HOMO–LUMO) of materials 1–4 measured by
cyclic voltammetry. Gold is also shown for comparison.
˚
clinic system with unit-cell dimensions of a ¼ 16.8377(13) A, b ¼
˚
˚
ꢃ
5
1
.6889(4) A, c ¼ 15.3882(11) A, b ¼ 106.671 (2), and volume ¼
3
˚
412.05(18) A . The cif le and the detailed crystallographic
data are provided in the ESI (Table S1–S5 and Fig. S4†).
The X-ray structure of 3 is shown in Fig. 4. The molecular
structure of indolocarbazole 3 is nearly planar, with a small
regions. The hole mobility (m) in the saturated region and the
threshold voltage (Vth) were calculated from eqn (1):
ꢃ
WCox
m
ðV
torsion angle around 1.5 (Fig. 4a). The molecules exhibit a
2
I
D
¼
G
ꢀ Vth
Þ
(1)
ꢃ
2L
herringbone packing motif with a herringbone angle of 70.96
(
Fig. 4b). Every molecule is surrounded by six molecules
where W and L are the channel width and length, respectively,
ox is the unit dimensional dielectric capacitance of gate insu-
showing C–H/p interactions between one of the hydrogen
atoms of the methyl groups with the central benzene ring of an
adjacent indolocarbazole molecule with a distance of 2.68 A˚ .
Moreover, one of the hydrogen atoms of the methyl groups also
interacts with one peripheral benzene ring with a distance of
3
C
lator, m is the hole mobility and Vth is the threshold voltage.
A key requirement in the design of new semiconductors is
the stability under ambient conditions. OTFTs fabricated with
both types of semiconductors 3 and 4 operate well in air,
exhibiting remarkable ambient storage stability. Similar
threshold voltages, mobilities and Ion/Ioff ratios were obtained
aer 5 months of storage in air and in dark conditions,
demonstrating excellent air stability (Fig. 3c and d, the air-
stability experiments for 3-based devices can be found in the
ESI, Fig. S3c and d†). The increase in charge mobilities aer the
˚
.01 A, being responsible for the abovementioned herringbone
packing motif (Fig. 4c).
For indolo[3,2-b]carbazole derivatives, the packing motif in
the solid state strongly depends on the length of the alkyl chain
in the nitrogen position of the indolocarbazole derivatives.
Different packing motifs have been described in previously
reported methods, from the non-substituted indolo[3,2-b]

rst few days of exposure to air can be attributed to p-doping of
13
13,14
carbazole to alkylated derivatives.
5 nm thick lms were deposited by sublimation in a vacuum
system of compounds 3 and 4 on both bare and on polystyrene
treated c-Si/SiO surfaces. All the thin lms showed strong
For GIXRD studies,
the device by O
2
.
7
2
diffraction peaks, which suggested that both 3 and 4 produced
well-ordered lms under the experimental conditions (Fig. 5).
As shown in Fig. 5a, vacuum deposited 3 thin lms on bare and
on PS-treated c-Si/SiO2 surfaces presented identical X-ray
diffraction patterns, which means that the molecules are
Molecular order and morphology of the thin lms
For further exploration of the relationship between the OTFTs
performance and the role of the molecular order in the thin

lms, grazing incidence X-ray diffraction (GIXRD) and atomic
force microscopy (AFM) analysis were performed.
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Fig. 3 OTFTs characteristics of a device fabricated with 4 as the semiconductor layer and with PS-treated substrate. (a) Output characteristics at
different gate voltages (V ). (b) Transfer (V
¼ ꢀ20 V) and saturation characteristics. (c) Mobility and threshold voltage of one representative
device fabricated with 4 as the semiconductor layer and with PS-treated substrate as a function of storage time in the air. (d) Transfer char-
G
D
nd
acteristics on the 5th day and the 152 day of the experiment at V
selected device.
D
¼ ꢀ20 V. Inset figure shows the evolution with time of the Ion/Ioff ratio of the
Fig. 4 Single crystal structure of compound 3. (a) Molecular structure, (b) herringbone packing motif with its corresponding herringbone angle
and (c) short contacts given by C–H/p interactions.
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equally oriented, both on bare and on PS-treated SiO
2
surfaces. This improvement in the charge carrier mobility value can be
The combination of the GIXRD studies with the powder XRD related to the different packing motifs and the corresponding
data and the single crystal structures provides valuable infor- different intermolecular interactions, and with the different
13,15
mation about the molecular order in the thin lms.
molecular order in the thin lms. From the single crystal data
Thin lms based on indolocarbazole 3 presented only one reported above, it can be established that a change in the
ꢃ
strong diffraction peak in the GIXRD patterns at 2q ¼ 11.1 , in packing motif exists from the herringbone of compound 3 with
comparison to the more complex diffractogram spectra CH
3
/p interactions to
a
face-to-face arrangement of
obtained from the powder XRD data (Fig. S5a† in the ESI). From compound 4, dominated by p–p interactions, in which the
the single crystal data and the powder XRD pattern, the electronic coupling is maximized and the charge transport
ꢃ
diffraction peak at 2q ¼ 11.1 can be undoubtedly assigned to should be favoured. As a second important point, the GIXRD
the reection 200. The XRD study shows that the plane (100) is suggests that molecules in 4 are located completely perpen-
parallel to the c-Si/SiO substrate, i.e. with b- and c-axis also in dicular to the substrate, which seems to be the best molecular
2
the parallel plane. The molecular packing within the thin lm disposition for the charge transport through the semiconductor
corresponds to that of indolocarbazole 3 in the single crystal. active layer of an OTFT with a bottom-gate top-contact
ꢃ
This suggests that the molecules are tilted with an angle of 68
to the substrate (Fig. 5a and b).
geometry.
In addition to the local intermolecular interactions, the lm
For vacuum deposited 4 thin lms on PS-treated substrates, morphology and the grain sizes and boundaries strongly
one diffraction peak was mainly observed in the thin lm XRD inuence the charge transport properties, particularly at the
ꢃ
pattern at 2q ¼ 8.3 , together with a second order diffraction interface between the semiconductor and the dielectric. AFM
ꢃ
peak at 2q ¼ 16.6 , which is indicative of a higher crystalline images were taken from the 3 and 4 lms deposited by subli-
2
thin lm than that obtained for indolocarbazole 3. From the mation on both bare and polystyrene treated SiO surfaces
12
single crystal data, the powder XRD pattern (Fig. S5b† in the (Fig. 6). Remarkable morphology differences were observed
ꢃ
ESI) and the GIXRD data, the diffraction peaks at 2q ¼ 8.3 and between them.
ꢃ
2
q ¼ 16.6 were assigned to the corresponding reections 110
Thin lms based on indolocarbazole 3 deposited on bare
and on PS-treated surfaces showed similar topography
and 220, respectively, suggesting that they are parallel to the SiO
2
substrate with the c-axis also in the parallel plane. Conse- proles. Both lms present a similar average grain size, with an
quently, as it is collected in Fig. 5c, triindole molecules are average width of 600 nm and lengths that include from 600–
ꢃ
located totally perpendicular (90 ) to the substrate surface.
1400 nm. Only a slight enhancement of the interconnectivity
For the OTFTs performance, the best charge carrier mobility between the grains is observed for lms based on 3 deposited
was found for the triindole derivative 4-based devices in on PS-treated surfaces, which actually correspond to a higher
comparison with those obtained from the indolocarbazole 3. device performance (Fig. 6a and b, Table 1). Vacuum-deposited
Fig. 5 GIXRD patterns of vacuum-deposited 3 films on (a) bare and (b) polystyrene-treated c-Si/SiO
molecular packing of compound 3 with the (100) plane situated at 27 (above) and perpendicular (below) to the paper plane, respectively. (c)
2
surfaces. The inset image shows the
ꢃ
2
GIXRD pattern of vacuum-deposited 4 film on a polystyrene-treated c-Si/SiO surface. The inset images show the molecular packing in the thin
films seen from two different perspectives, being the (110) plane situated perpendicular to the paper. The hydrogen atoms were omitted for
clarity.
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precipitate was observed. The solid was isolated by vacuum

ltration, washed thoroughly with methanol and then dried.
The crude was puried by ash column chromatography using
a mixture of hexane and CH Cl (1 : 1 v/v) as eluent to give
2
2
1
6
compound 2 (4.3 g, 73%). H NMR (400 MHz, d -DMSO), d: 8.55
(
d, J ¼ 1.7 Hz, 2H), 8.26 (d, J ¼ 7.6 Hz, 2H), 7.89 (dd, J ¼ 8.5 Hz, J
¼
1.7 Hz, 2H), 7.67 (d, J ¼ 8.5 Hz, 2H), 7.58 (d, J ¼ 8.2 Hz, 2H),
) ppm.
-DMSO), d: 141.1, 139.7, 132.2, 125.8,
25.0, 122.7, 122.3, 120.4, 118.7, 118.2, 109.4, 109.2, 29.1 ppm.
7
.50–7.46 (m, 2H), 7.24–7.21 (m, 2H), 3.92 (s, 6H, N–CH
3
1
3
C NMR (100 MHz, d
6
1
+
HRMS (ESI-MS) (m/z): calcd for C H N (M + H) 361.1699,
2
6
21 2
found: 361.1684.
Synthesis of 5,11-dimethylindolo[3,2-b]carbazole (3)
NaH (12 mg, 0.52 mmol) was added to a solution of indolo[3,2-b]
carbazole (60 mg, 0.23 mmol) in anhydrous DMF under an inert
Fig. 6 AFM images of vacuum-deposited 3 films deposited on (a) bare
2
2
SiO
2
(5 ꢂ 5 mm ), (b) polystyrene treated SiO
2
(5 ꢂ 5 mm ) and (c)
2
polystyrene treated SiO
ited 4 films deposited on (d) bare SiO
2
(1 ꢂ 1 mm ). AFM images of vacuum-depos- atmosphere. The solution was stirred at room temperature for
2
2
(5 ꢂ 5 mm ) and (e) polystyrene 30 minutes. Then, methyl iodide (32 mL, 0.52 mmol) was added
2
treated SiO
2
(5 ꢂ 5 mm ).
and the mixture was stirred at room temperature for 30 minutes
2
and then treated with H O. The aqueous layer was extracted
with dichloromethane and the organic layer was dried over
Na SO . The solvent was removed by evaporation and the crude
was puried by ash column chromatography using a mixture
3
lms presented a terrace-like step morphology (Fig. 6c),
2
4
independent of the modication of the dielectric surface.
Vacuum-deposited 4 lms on bare and on PS-treated SiO2
of hexane and dichloromethane (4 : 1 v/v) to give compound 3
1
also presented identical morphologies, independently of the
(
2
7
55 mg, 83%). H NMR (400 MHz, CDCl
3
), d: 8.20 (d, J ¼ 8.0 Hz,
modication of the dielectric surface (Fig. 6d and e). AFM
images exhibited protrusions/hillocks, whose average thickness
was in the order of 130 nm and their height varied from 50 nm
to more than 100 nm.
Combining GIXRD and AFM data, it can be established that
vacuum deposited lms of indolocarbazole 3 and triindole 4
derivatives on bare and on PS-treated SiO2 surfaces, respec-
tively, presented identical X-ray diffraction patterns and
morphology. Therefore, the addition of a polystyrene SAM has a
null inuence on the growth of the molecular order and on the
morphology of the thin lm. The improvement of the electrical
characteristics of the OTFT devices observed in PS-treated c-Si/
H), 8.02 (s, 2H), 7.53–7.49 (m, 2H), 7.42 (d, J ¼ 8.0 Hz, 2H),
13
.26–7.22 (m, 2H), 3.96 (s, 6H, N–CH
), d ¼ 142.3, 136.9, 125.8, 123.0, 122.9, 120.3, 118.2, 108.3,
8.7, 29.5 ppm. HRMS (ESI-MS) (m/z): calcd for C H N (M +
3
) ppm. C NMR (100 MHz,
CDCl
9
3
2
0
17 2
+
H) 285.1386, found: 285.1382.
Synthesis of N-trimethyltriindole (4)
NaH (98 mg, 4.05 mmol) was added to a solution of triaza-
truxene (0.28 g, 0.81 mmol) in anhydrous DMF under an inert
atmosphere. The solution was stirred at room temperature for
3
0 minutes. Then, methyl iodide (166 mL, 2.67 mmol) was added
and the mixture was stirred at room temperature for 30 minutes
and then treated with H O. The aqueous layer was extracted
with dichloromethane and the organic layer was dried over
Na SO . The solvent was removed by evaporation and the crude
2
SiO substrates appears to be related to the improvement of the
interface contact between the dielectric and the semiconductor
layer.
2
2
4
was puried by ash column chromatography using a mixture
Experimental section
Materials
of hexane and dichloromethane (5 : 1 v/v) as eluent to give
1
compound 4 (0.26 g, 84%). H NMR (400 MHz, CDCl ), d: 8.47
3
(
7
d, J ¼ 8.0 Hz, 3H), 7.59 (d, J ¼ 8.0 Hz, 3H), 7.49–7.45 (m, 3H),
All reagents for syntheses were obtained from commercial
sources and were used as received without further purication.
Indolo[3,2-b]carbazole was synthesized according to previously
13
.37–7.33 (m, 3H), 4.48 (s, 9H, N–CH
3
) ppm. C NMR (100 MHz,
CDCl
3
), d ¼ 142.1, 139.2, 123.0 (2C), 121.9, 120.0, 109.8, 102.7,
+
3
3
22 3
6.1 ppm. HRMS (ESI-MS) (m/z): calcd for C27H N (M + H)
88.1808, found: 388.1810.
10
reported method. Triazatruxene was synthesized according to
11
a previously reported method.
Instrumentation and methods
Flash column chromatography was carried out over silica gel
0
0
0
Synthesis of 9,9 -dimethyl-9H,9H -[3,3 ]bicarbazole (2)
1
13
9
-Methyl-9H-carbazole 1 (5.6 g, 31.1 mmol) was dissolved in (SDS, 230–240 mesh). H NMR (400 MHz) and C NMR
anhydrous CHCl (70 mL) under an inert atmosphere. Subse- (100 MHz) spectra were collected on a Varian Mercury spectro-
3
quently, FeCl (13.7 g, 84.5 mmol) was added and the reaction photometer. NMR spectra were processed with the MestRec
3
mixture was stirred at room temperature for 30 minutes. Then, Nova soware. HRMS was performed using a LC/MSD-TOF
the mixture was poured into methanol and the formation of a Agilent Technologies apparatus by electrospray (ESI-MS)
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technique. UV-Vis spectra were registered using a Varian Cary self-assembled monolayer to improve the device performance. A
ꢀ1
UV-Vis-NIR 500E spectrophotometer. TGA were performed with solution of PS in toluene (4 mg mL ) was deposited drop wise
a Mettler Toledo TGA/SDTA 851e instrument at a heating rate of on the substrate. The substrate was spun at 500 rpm for 5 s and
ꢀ
1
1
0 K min . Differential scanning calorimetry (DSC) thermo- 2500 rpm for 30 s with a P6700 spin coater. Organic materials 2–
grams were recorded using a Mettler–Toledo DSC821 module 4 were deposited by sublimation in a vacuum system with a base
ꢀ
6
under a nitrogen ow. Cyclic voltammograms were carried out pressure below 10 mbar. The sublimation temperature for the
ꢃ
using
a
microcomputer-controlled potentiostat/galvanostat organic compounds was regulated from 152 to 160 C, 120 to
ꢃ
ꢃ
Autolab with PGSTAT30 equipment and GPES soware. A 130 C and 160 to 170 C, respectively, to maintain a stable
ꢀ
1
˚
cylindrical three-electrode cell was used. The reference elec- deposition rate around 0.3 A s to obtain a 75 nm thickness
trode was a Metrohm Ag/AgCl/KCl (3 M) mounted in a Luggin layer. Then, they were transferred to a different vacuum
capillary containing a 0.1 M solution of tetrabutylammonium chamber to evaporate the metallic contacts. Gold was used for
perchlorate (TBAP) in dichloromethane. The counter and the source and drain electrodes, which were dened using a
working electrodes were a platinum spiral and a platinum wire, metallic mask that denes a channel length (L) and width (W) of
respectively. All voltammetric curves were recorded under 80 mm and 2 mm, respectively. The fabricated OTFTs were
quiescent conditions, at a scan rate of 100 mV s and under an electrically characterized in the dark and under air conditions.
argon atmosphere. All solutions were prepared in dichloro- The electrical characteristics were measured using an Agilent
methane (1 mM). Tetrabutylammonium perchlorate (Aldrich, 4156C parameter analyser.
ꢀ
1
electrochemical grade) was used as a supporting electrolyte. The
energy of the HOMO level was estimated from the onset of the
Conclusions
ox
0
+

rst oxidation peak as E
¼ ꢀE
+ E (Fc/Fc ) – 5.39,
onset
HOMO
0
+
where E (Fc/Fc ) ¼ +0.19 V vs. Ag/AgCl/KCl (3 M), and where All the carbazole-based compounds 1–4 presented low HOMO
+
ꢀ
5.39 eV corresponds to the formal potential of the Fc /Fc redox levels and high HOMO–LUMO gaps, indicating their potential
16
couple in the Fermi scale. The energy of the LUMO level was hole-transporting properties and air-stability. However, only
calculated as ELUMO ¼ EHOMO + Egap. Single-crystal analysis of compounds 3 and 4, with extended p-conjugated cores with two
material 3 was performed on a D8 Venture system equipped and three fused carbazole rings, presented good device perfor-
with a multilayer monochromator and a Mo high brilliance mances and air-stability and durability under air conditions, for
˚
Incoatec Microfocus Source (l ¼ 0.71073 A). The frames were longer periods of up to 5 months, with hole mobility values
ꢀ3
ꢀ2
2
ꢀ1 ꢀ1
integrated with the Bruker SAINT soware package using a ranging from 10 to 10 cm V
s . For indolocarbazole
narrow-frame algorithm. The structure was solved and rened derivative 3, a herringbone packing motif with CH /p inter-
3
using the Bruker SHELXTL Soware Package. Powder X-ray actions has been described in the semiconductor layer, whereas
diffraction (XRD) measurements were obtained using a PAN- a cofacial molecular packing is observed for triindole 4. For the
alytical X'Pert PRO MPD q/q powder diffractometer, in a latter, it has been found that in the crystal structure and in the
conguration of a convergent beam with a focalizing mirror and thin lm, it presents a face-to-face molecular packing, and
transmission geometry with a spinner glass capillary sample exhibits the best OTFT performance. For this material, an
holder, a PIXcel detector and with Cu Ka radiation at 45 kV and optimal perpendicular molecular disposition to the substrate
40 mA. Samples were prepared by introduction of the powder surface was determined by GIXRD. Indolo[3,2-b]carbazole and
materials 3 and 4 in Lindemann glass capillaries of 0.5 milli- specially triindole units are shown to be promising p-conju-
metres of diameter. Grazing incidence XRD (GIXRD) measure- gated cores for new organic semiconductors for electronic
ments of the thin-lms of the sublimated organic materials devices.
were performed using a PANalytical X'Pert PRO MRD diffrac-
tometer with a PIXcel detector, a parabolic G ¨o bel mirror at the
incident beam and a parallel plate collimator at the diffracted
Acknowledgements
beam, and with Cu Ka radiation at 45 kV and 40 mA. The angle Financial support from the Ministerio de Econom ´ı a y Com-
ꢃ
of incidence used was u ¼ 0.17 . Thin lms (75 nm) were petitividad (CTQ2012-36074 and TEC2011-27859) is gratefully
prepared from compounds 3 and 4 by sublimation on a vacuum acknowledged. M. Reig is grateful for the grant ADR from Uni-
system on a c-Si/SiO
2
substrate and on polystyrene-treated c-Si/ versitat de Barcelona (UB). We also thank Xavier Alcob ´e and
ꢀ
1
a
˚
SiO substrates with a deposition rate of 0.3 A s under a Josep M Bassas from the X-ray diffraction unit of the Scientic
2
ꢀ6
pressure below 10 mbar. AFM experiments were conducted and Technological Centers of the University of Barcelona (CCi-
using an AFM Dimension 3100 system attached to a Nanoscope TUB) for the realization of the X-ray diffraction measurements
IVa electronics unit (Bruker).
and their helpful discussion.
Notes and references
OTFT device fabrication
OTFTs devices were fabricated using the bottom-gate top-
contact geometry. The substrate present in a thermally oxidized
crystalline silicon wafer provided a gate dielectric (SiO ) of a
2
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E. Reinold, M. Weil, C. K ¨o rner, H. Ziehlke, C. Elschner,
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