Structure properties relationships of liquid crystal bent core organic semiconductors based on benzo[2,1-b:3,4-b′]dithiophene-4,5-dione

Article abstract of DOI:10.1039/c2jm34803a

We report the synthesis of a new diketone bridged dithiophene end-capped with the mesogenic functionalities: 2,7′-bis(alkoxy-biphenyl) and 2,7′-bis(alkoxy-phenyl)-benzo[2,1-b:3,4-b′]dithiophene-4,5-dione. Optical and electrochemical properties in solution were investigated by UV-visible absorption and cyclic voltammetry. Liquid crystal properties of these new materials were investigated by differential scanning calorimetry, optical polarizing microscopy, and X-ray diffraction studies. Both compounds exhibit layered phases though there are differences in the organisation of the phase structures. Thin films were implemented as active layers into organic thin-film transistors to evaluate the charge transport properties.

Full text of DOI:10.1039/c2jm34803a

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ISSN 0959-9428
PAPER
M. Vallet-Regí et al.
Supramolecular mechanisms in the synthesis of mesoporous magnetic
nanospheres for hyperthermia
0959-9428(2012)22:1;1-2
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Registered Charity Number 207890

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Journal of Materials Chemistry
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Cite this: DOI: 10.1039/c0xx00000x
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ARTICLE TYPE
Structure properties relationships of liquid crystal bent core organic
semiconductors based on benzo[2,1-b:3,4-b’]dithiophene-4,5-dione
Hecham Aboubakr,a M.-Gabriela Tamba,b Abdou Karim Diallo,a Christine Videlot-Ackermann,a Lénaïk
Belec,c Olivier Siri,a Jean-Manuel Raimundo,*a Georg H. Mehl*b and Hugues Brisset*c
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
We report the synthesis of new diketone bridged dithiophene endꢀcapped with the mesogenic
functionalities: 2,7’ꢀbis(alkoxyꢀbiphenyl) and 2,7’ꢀbis(alkoxyꢀphenyl)ꢀbenzo[2,1ꢀb:3,4ꢀb’]dithiopheneꢀ
4,5ꢀdione. Optical and electrochemical properties in solution were investigated by UVꢀvisible absorption
10 and cyclic voltammetry. Liquid crystal properties of these new materials were investigated by differential
scanning calorimetry, optical polarizing microscopy, and Xꢀray diffraction studies. Both compounds
exhibit layered phases though there are differences in the organisation of the phase structures. Thin films
were implemented as active layers into organic thinꢀfilm transistors to evaluate the charge transport
properties.
liquid cristal properties which effectively lead to an increase of
charge carrier mobilities in OTFTs.24,25 For example in
15 Introduction
oligothiophene series Funahashi et al. reported a hole mobility of
Research into organic thinꢀfilm transistors (OTFTs) based on
oligothiophenes is currently a topic of intensive investigations.1,2,3
In this context distyryloligothiophenes have been described as
new semiconductors with an exceptional stability for pꢀchannel
20 OTFTs.4,5,6,7,8 Based on these considerations some synthetic
strategies to modify the distyryloligothiophene backbone have
been developed in order to increase the hole charge carrier
mobility and solubility for investigation of solutionꢀprocessed
organic thin film transistors,9,10 sensors,11,12 solar cells.13 Recently
25 strategies have been reported to increase the electroaffinity by
grafting of fluoro,14,15 perfluoro16 or cyano17 groups on styryl part
of the semiconductor to facilitate electron injection and to turn
these compounds into a nꢀtype semiconductor with the aim to
develop complementary systems. Contrary to what has been
30 expected the incorporation of these electron withdrawing groups
has resulted only in a small decrease of the LUMO energies. On
the other hand we obtained a nꢀtype semiconductor only for one
2
–1 –1
50 0.02 cm V s under an ambient atmosphere for an OTFT based
on a smectic thin films prepared by spinꢀcoating with a liquidꢀ
crystalline phenylterthiophene.26 In this context we have decided
to replace styryl group grafted at the extremities of benzo[2,1ꢀ
b:3,4ꢀb’]dithiopheneꢀ4,5ꢀdione by mesogenic groups to combine
55 the effect on the electrowithdrawing group on the LUMO level
and the potential of the mesognenic group to increase the
cristallinity of the solid phase.
We report here the synthesis and characterizations of 2,7’ꢀ
bis(alkoxyꢀphenyl) and 2,7’ꢀbis(alkoxyꢀbiphenyl) benzo[2,1ꢀ
60 b:3,4ꢀb’]dithiopheneꢀ4,5ꢀdione, named LCꢀdiketone 1 and 2
respectively. Compared to parent molecule DSꢀdiketone, UV
spectra, oxidation and reduction potentials are quite similar for
LCꢀdiketone 1 and 2 which indicates that the replacement of
styryl groups by alkoxyꢀphenyl or biphenyl groups do not change
65 dramaticaly bandgap and HOMO/LUMO levels. On the other
hand structural modifications of benzo[2,1ꢀb:3,4ꢀb’]dithiopheneꢀ
4,5ꢀdione core with alkoxyꢀphenyl/biphenyl groups increase the
solubility and lead to liquid crystal properties that open the
possibility to increase the charge carrier mobilities in OTFT
70 devices.
ꢀ
4
perfluoro compound with a very little electron mobility (9.8 10
2
cm /Vs) under high vaccum.16 This is in marked contrast to the the
35 observations for structurally related compounds without ethylene
linkages where the presence perfluorene,18,19 perfluoroalkyl20 or
dicyanomethylene groups21,22 at the extremities of oligothiophene
led to nꢀtype channel organic semiconductors. This result
provided the reason why we have decided to replace bithiophene
40 core by benzo[2,1ꢀb:3,4ꢀb’]dithiopheneꢀ4,5ꢀdione named DSꢀ
diketone.23 As expected an important decrease of the LUMO
level was observed indicating an increase of the electroaffinity.
Unfortunately DSꢀdiketone based films reveal only a low
microstructural order in the solid state preventing any efficient
45 charge transport in air.
Results and discussion
Synthesis
The synthetic route toward LCꢀdiketones
1 and 2 from
benzo[2,1ꢀb:3,4ꢀb’]dithiopheneꢀ4,5ꢀdione 427 is outlined in
75 scheme 1. A double iodation of compound 4 is made as described
in literature.28 Targets LCꢀdiketones 1 and 2 were obtained
classicaly by a Suzuki coupling with boronic acid and tetrakis
To increase microscopic ordering some researchers have used
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triphenylphosphine palladium.
phenyl/biphenyl groups.
O
S
O
O
O
S
O
O
S
S
S
4
S
C9H
19 O
C9H19
O
NIS,TFA
reflux, 3h
O
O
S
S
C8
H
C8H17
O
17 O
O
O
S
S
I
I
3
(HO)2B
OC8H17
400
600
(nm)
800
Toluene
Pd(Ph3)4
or
λ
(HO)2B
O
OC9H19
Figure 1. Absorption spectra of DSꢀdiketone, and LCꢀdiketones 1 and 2
O
35
at 10ꢀ5M in dichloromethane.
Cyclic voltammetry of diketones
S
S
1
Cyclic voltammograms (CV) of DSꢀdiketone reveals two
successive reversible oxidation waves at 0.82 and 1.00V vs
C8H17O
OC8H17
O
O
40 Fc/Fc+ corresponding to the formation of the radical cation and
dication and two successive reversible reduction waves at ꢀ1.00
and ꢀ1.73 V vs Fc/Fc+ corresponding to the formation of radical
anion and dianion.23 Moreover adsorption of the oxidized species
followed by cathodic desorption from the electrode surface was
45 reported for the DSꢀdiketone. For LCꢀdiketones 1 and 2, two
successive reversible oxidation waves at 0.70/0.87V and
0.85/0.95 vs Fc/Fc+ corresponding to the formation of the radical
cation and the dication are observed respectively. For both LCꢀ
diketones one reversible reduction wave at ꢀ0.99 and ꢀ0.96V vs
50 Fc/Fc+ corresponding to the formation of radical anion are
observed for LCꢀdiketones 1 and 2 respectively (Figure 2). In
contrast to DSꢀdiketone, no adsorption on electrode is observed
during the electrochemical experiments for LCꢀdiketones 1 and
2 due to the higher solubility of the mesogneic systems. A
55 comparison of the E1/2(ox2)ꢀE1/2(ox1) values for DSꢀdiketone and
the LCꢀdiketones 1 and 2 indicate that the replacement of styryl
group by phenyl or biphenyl groups does not influence the
formation of the dication state. This indicates a less marked effect
of the diketone groups compared to the DSꢀdiketone in good
60 agrement with a decrease of planarity of conjugated backbone.
Optical and electrochemical data of DSꢀdiketone, LCꢀdiketones 1
and 2 based solutions were used to establish the energy diagram
of LUMO and HOMO in Figure 3. EHOMO is estimated from
S
S
2
C9H19
O
OC9H19
Scheme 1. Synthetic scheme for LCꢀdiketones 1 and 2.
5
UV-vis spectra
It is known that the presence of double bonds in oligothiophenes
results in planar structures with reduced overall aromatic
character.29 In this context replacement of a styryl group by
10 phenyl or biphenyl groups is expected to lead to a hypsochromic
shift of the absorption maxima and a decrease of the resolution of
the vibronic structure. As expected UVꢀvis spectra of LC
diketones 1 and 2 in solution (CH2Cl2) reveal a hypsochromic
shifts of 20 and 40 nm respectively when compared to the DSꢀ
15 diketone indicating a reduction in the effective conjugation
length. Moreover, electronic interaction between carbonyl groups
and conjugated oligomer backbone can decrease the effective
conjugation as already observed for copolymers based on
cyclopentadithiophenone.30,31 The UVꢀvis spectrum of DSꢀ
20 diketone shows a good resolution of the vibronic structure with
maxima at 367, 386 and 404 nm. The UVꢀvis spectra of LCꢀ
diketones 1 and 2 show a decrease of the vibronic structure in
good agreement with a less planar geometry. Both LCꢀdiketones
exhibit an absorption band in the low energy range (around 620
25 nm) attributed to an intramolecular charge transfer (Figure 1).32
By extrapolation of the high λ range edge of the absorption
spectra values of 3 eV and 2.8 eV are obtained for the optical
bandgap (Eg(opt)) of LCꢀdiketones 1 and 2 respectively, which is
0.2ꢀ0.4 eV lower when compared to the DSꢀdiketone. This
30 increase of the bandgap is correlated to the decrease of
conjugated backbone planarity due to the replacment of styryl by
E1/2(ox1) and ELUMO from the relation ELUMO = EHOMO
+
65 Eg(opt).33,34 Compared to DSꢀdiketone (HOMO/LUMO: ꢀ
5.66/ꢀ3.06 eV) HOMO/LUMO energies increase for 1 (ꢀ5.54/ꢀ
2.54 eV) and are similar for 2 (ꢀ5.69/ꢀ2.80 eV) which is
consistent the less planar conjugated system.
2
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25
Figure 4. DFTꢀoptimized geometries, frontier orbitals, and orbital
energies (eV vs vacuum) of 1 and 2.
DSC, TGA measurements and LC properties
30 LCꢀdiketones 1 and 2 were investigated by Differential Scanning
Calorimetry (DSC), Thermal Gravimetric Analysis (TGA),
Optical Polarizing Microscopy (OPM) and XꢀRay Diffraction
(XRD) studies For thermal properties of LCꢀdiketones 1 and 2
explored by TGA results are summarrized in table 1 (TGA curves
35 and degradation temperatures of LCꢀdiketone 1 and 2 are
reported in Error! Reference source not found.).
Figure 2. CV of DSꢀdiketone (grey, intensity x10) and LCꢀdiketones 1
(blue) and 2 (red) at 10ꢀ3M in 0,1 M nꢀBu4NPF6/1,2ꢀdichlorobenzene,
v=250 mV.sꢀ1.
5
Weight changes
300°C
LCꢀdiketone 1
ꢀ2.2 %
LCꢀdiketone 2
+ 1.1%
400°C
ꢀ12.9 %
ꢀ4.4 %
600°C
ꢀ44.4 %
ꢀ38 %
800°C
ꢀ62.9 %
ꢀ45.8%
Table 1. Weight changes measured by TGA under nitrogen
40 As shown in Error! Reference source not found., both
compounds show a good thermal stability up to 250 °C. LCꢀ
diketone 1 shows three steps decomposition around 300°C,
380°C and 460° C with a total weight loss of 62.9% at 800°C.
LCꢀdiketone 2 exhibits a particular behavior above 200 °C, with
45 a slight mass increase up to 300°C which cannot be attributed to
oxidation phenomena under nitrogen atmosphere. Moreover, the
weight loss for this compound remains much lower than for the
first one. It is indeed below 5% at 400°C compared to 13 % for
LCꢀdiketone 1 at the same temperature, and only 46 % are
50 degraded at 800 °C, compared to 63 % for LCꢀdiketone 1. The
comparison between the degradation temperatures of the two
compounds (table in Figure S1) confirms that LCꢀdiketone 2 has
a higher thermal stability than LCꢀdiketone 1, especially for the
first two steps. This is consistent with the higher
55 aromatic/aliphatic proportion in LCꢀdiketone 2 structure.
Figure 3. Energy diagram of LUMO and HOMO levels for diketones.
The horizontal thick red line corresponds to the Fermi level of gold
electrode (~ 4.9 eV).
10
DFT calculation of HOMO/LUMO levels
The spatial representations of the HOMO and LUMO frontier
orbitals have been investigated by ab initio quantum calculations
15 with the Gaussian 03 package of programs at a hybrid Density
Functional Theory (DFT) level with B3LYP/6ꢀ31G(d,p)
procedure.35 HOMO and LUMO levels of neutral LC-diketones 1
and 2 are shown in Figure 4. For LCꢀdiketones 1 and 2 it is
expected that replacement of alkyl chains by methylene groups
20 does not change the results dramatically and permits to reduce the
computational costs. As expected, HOMOs and LUMOs of both
compounds are very similar and located over the fully conjugated
system and the central dithiopheneꢀdiketone core, respectively.
Compared to DSꢀdiketone no drastic change are observed.
For the DSC studies the materials were investigated using heating
rates of 5°C/min and 10 °C/min, in order to understand the rather
complex melting behaviour (Figure S2). The results are
60 summarized in table 2. The transitions are relatively broad which
is associated with the rigidity of the central bent shaped core and
the strong lateral dipole of the LCꢀdiketones unit and has been
observed in other materials.36
65
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LCꢀdiketones
1 (heat)
Transitions (°C)
Cr 115.1
Enthalpy [Jgꢀ1]
SmC [99.0] SmA 164.4
[5.10] Iso
SmA 158.1 [5.70] Iso
SmA 150.1 [5.80]
SmA 151.1 [5.70]
such a thermal history results in crystal formation on heating and
at higher temperatures the melting and SmA phase formation is
observed at similar temperatures as in previous cycles. The SmC
phase is a strictly monotropic, it is not a thermodynamically
45 stable phase.
2 (heat)
1 (cool)
2 (cool)
Cr 90.9
Tg 36.3
Tg 8.2
For the LCꢀdiketone 2 the first heating scan is characterized by
the formation of a of a liquid crystal phase at 90.9°C from the
crystalline state before the material clears at 158.1°C into an
isotropic liquid. This is associated with a phase transition
50 enthalpy of 5.70 Jgꢀ1. This temperature varies slightly with the
scan rate at a slower scan rate of 5°C/min an onset of 158.7°C is
detected. Cooling the sample form the liquid crystal phase results
in the formation of a glass transition at 8.2°C. On heating the
material crystallizes at 60.8°C, indicating that the glass transition
55 observed on cooling is metastable. The crystallisation on heating
is followed by a crystal transition at 78.7°C and by a crystal to
liquid crystal transition at 90.9°C in to LC phase, establishing
essentially the LC phase as the highest stable ordered phase.
Optical polarizing microscopy (OPM) studies confirm the
60 smectic structure of the LC phase. Figure 6 shows an optical
polarizing microscopy picture recorded on cooling at 78°C in the
supercooled smectic phase, recorded using untreated glass slides.
Table 2. Transition temperature as determined by OPM and DSC;
enthalpies as determined by DSC (2nd heat and cool ; scan rate 10 °C minꢀ
1; for the transitions the peak temperature is given; the SmC transition was
only observed by OPM).
5
For the LCꢀdiketone
1 on heating a number of crystal
transformations are observed before the system melts at 115.1°C
into a smectic A (SmA) phase, confirmed by optical polarizing
microscopy, where the formation of the homeotropic texture with
10 the formation of an oily streak texture on shearing was detected
(Figure S3). Due to the strong absorption of the material in the
visible, experiments had to carry out in very thin films. This
phase clears at 164.4°C into an isotropic liquid, a transition which
is associated with an enthalpy change of 5.10 Jgꢀ1. On cooling the
15 SmA phase is reformed. Careful investigation of the system
shows the formation of a SmC phase characterized by the
formation of a schlieren texture with typical 2 and 4 brush defects
(± ½ and ±1 disclinations) below 99.0°C, a typical texture is
shown in Figure 5.
no
P
A
200 µm
20 Figure 5. Optical polarizing microscopy of LCꢀdiketone 1. Monotropic
SmC phase at T= 73 °C.
It is noted that this transiton was not detectable using DSC,
pointing towards a second order phase transition, with a gradual
25 and small increase of the tilt with decreasing temperature. Such a
sample can be cooled quickly to room temperature and a glass is
formed. Due to the strong absorption of the material the transition
from a homeotropic to a schlieren texture could be detected easier
using the microscope with a polarizers and analyzers in a parallel
30 configuration. The texture of the SmC phase is of very low
birefringence. On shearing the sample, a nonspecific texture with
small defects could be observed, however no homeotropically
aligned regions could be detected. Dark and light domains
become visible on rotating one of the polarizers clockwise by a
35 small angle (5 ° to 10°) from the crossed position. Rotating the
polarizer anticlockwise by the same angle causes the reversal of
this effect, which results that the previously dark domains are
now appearing light and vice versa. This indicates that over the
whole temperature range the SmC phase chiral domains of
40 opposite handedness could be observed. Heating a sample with
65
Figure 6. Optical polarizing microscopy of LCꢀdiketone 2 (top) SmA at
T = 78 °C, between crossed polarizers, magnification 100x, on cooling
from the isotropic state. P, A = positions of the polarizer and analyzer, no=
direction of the ordinary refractive index’ (down) oily streak texture
70
formed after shearing of the sample.
A focal conic structure is clearly detectable, suggesting the
formation of a smectic A (SmA) phase with the molecules
4
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organized in layers and the molecular long axis perpendicular to 40 at 0.44 nm can be associated with fluid like hydocarbon chains.
the planes of the layers. Shearing of the sample, by moving the
top cover slip of the microscope slide resulted in the formation of
an oily streak type texture confirming the smectic character of the
phase structure. It is noted that thermal cycling of the sample in
The broad wide angle intensity at 0.66 nm which is related to the
LC phase, as it is disappears in the isotropic phase has broadly
the width of the central aromatic system. Thus some degree of
hindered rotation of the aromatic groups is likely to be
5
the LC phase improved the formation of the OPM texture 45 responsible. In conjunction with the weak shoulder that occurs
significantly, for further textures see the ESI, that suggesting the
formation of a smectic phase.
~0.37 nm, this suggests weak interactions of the planes of the
more board like mesogens. Indeed for many columnar systems πꢀ
π stacking of aromatic groups with distances of 0.3ꢀ0.36 nm is
often observed.37 In other words, the cylinder like symmetry of a
10 In order to elucidate the phase structure further the materials were
investigated by Xꢀray diffraction (XRD). The samples for the 50 typical rod – shaped molecule is distorted to a more ellipical
XRD measurements were sealed in glass capillaries (with an
outside diameter of 0.8 mm to 1 mm). Alignment was achieved
using a magnetic field (H ≈ 0.5 T) with samples placed in a
15 temperature controlled graphite furnace, using quartzꢀ
shape. This is reflected in the phase organisation with some local
alignment, the peaks are very broad, perpendicular to the long
axis of the moelcules, an interpretation which would be in line
with the modulation of the electron densities perpendicular to the
monochromatized CuKα radiation (CuꢀKα1ꢀradiation; λ = 0.154 55 layer normal.38,39,40 The absence of a sharp merdional reflections
nm). The incident Xꢀray beam was perpendicular to the glass
capillaries. The substances had to be heated, for orientation, into
the isotropic liquid during the sample preparation for the Xꢀray
20 investigations. However, measurements on cooling and on
suggest that this SmAb like ordering is only local. Numerous
attempts to characterize the structure of the monotropic SmC
phase by XRD studies failed, as even after short data recording
times, some sharp intensities in the wide angle region could be
heating could be conducted. Twoꢀdimensional patterns for 60 detected, indicative of a spatial very well defined organisation of
aligned samples were recorded with a 2D image plate detector
system.
the hydrocarbon chains, or in other words the formation of
crystallites.
The Xꢀray diffraction pattern of LCꢀdiketone 1 is characterized
25 by a sharp peak at 2Θ = 2.81(d=3.15nm) (meridional) and two
The XRD patterns of LCꢀdiketone 2 in the smectic phase are
characterized by a sharp small angle meridional intensities at
broad equatorial intensities at 2Θ = 13.15 (0.67nm) and 20.05 65 values of 2Θ = 2.81; d = 3.14 nm, 2Θ = 5.63; d = 1.57 nm (very
(0.44 nm) (both equatorial) and a very weak shoulder at centred
at 2Θ =24.0 (0.37 nm) (Figure 7).
weak) which change very little with temperature and equatorial
broad wide angle intensities at 2Θ values centred at 19.56 = 0.45
nm; a representative diffractogram is shown in Figure 8.
Such an XRD pattern is typical for a liquid crystal which aligns
70 parallel to the magnetic field. 2Θ scans, resulting in pseudo
powder pattern of the diffractograms in the smectic A phase at
100 °C, 120 °C and 190 °C shown in Figure 8 (centre) confirm
that the position of the small angle reflections does not change
significantly in the smectic phase, but are not present in the
75 isotropic phase and that the wide angle intensities are present in
both phases, however a shift to larger angular values in the liquid
crystal phases can be detected. The χ –scans at 100 and 120 °C
with maxima at 0° and 180° conform that the parallel orientation
does not change significantly with temperature and confirm thus
80 the smectic A character of the LC phase. Considering the overall
length of the molecule of approximately 4 nm made up of two
nonyloxy chains with a length of about 1 nm if a mostly all trans
confirmation is considered and a length of the aromatic core of
approximately 2 nm (19.9 Å from molecular estimation of the
85 two carbons connected to the ether oxygens) the formation of a
simple SmA phase is suggested with a full interdigitation of the
adjacent hydrocarbon chains. Such a phase structure would result
in a layer spacing of 3 nm (0.5 nm + 2nm + 0.5nm) which is very
close to the observed layer spacing of 3.14nm; the difference to
90 the observed layer spacing could be due to an incomplete
interdigitation of the alkyl chains. The wide angle values of 0.45
nm are typical for molten chains in the liquid crystal state. The
smaller 2Θ values in the isotropic state indicate that the
molecules are further separated in the isotropic state and more
95 ordered in the SmA phase, a feature fully in line with the
expectation for a SmA phase. Though some meridional intensity,
as observed for LCꢀdiketone 1 is still detectable (Figure 8 and SI)
30
Figure 7. (top) XRD pattern of 1 at 140 °C, in order to enhance the
contrast the intensities in the isotropic phase have been subtracted;
(down) 2Θ scans at 190, 140 and 100 °C, wide angle region magnified..
35
Considering the estimated length of the whole molecule of 3.45
nm, the observed layer reflection is within 10% of the value of
calculated molecular length, thus a simple monolayer SmA phase
phase organisation seems to be most likely. The broad intensities
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these intensities are are much weaker than in the LCꢀdiketone 2,
and this reflects the more rodꢀlike structure of LCꢀdiketone 2
when compared to LCꢀdiketone 1 and gives thus the reason for
the more typical SmA like phase structure of 2.
nominal thickness of 47 and 96 nm, respectively. Interdigitated
source and drain electrodes structured by liftꢀoff technique were
used in BGBC with channel lenghts (L) ranging from 2.5 up to 20
ꢁm (Figure S4a). Longer channel lengths (L = 25 or 50 ꢁm) were
30 realized by vacuum evaporation through a shadow mask in BC
(Bottom Contact) and TC (Top Contact) configuration to form
linear channels (Figure S4b). All measurements were performed
in air at room temperature. By applying either positive or
negative voltages, any fieldꢀeffect activity was measured
35 probably due to a low microstructural order as observed by AFM.
Figure 9 shows the surface morphology of a 96 nm thick LCꢀ
diketone 2ꢀbased thin film vacuumꢀdeposited on Si/SiO2 substrate
(Figure S5 for LCꢀdiketone 1). Numerous fused small grains
appeared with identical sizes, height and shape homogeneously
40 deposited on the surface together with no well defined grain
boundary. Such unfavorable thin film microstructure to charge
transport was observed on a large scale as shown on Figure S5.
5
45 Figure 9. AFM picture of LCꢀdiketone 2 based thin film deposited by
vacuum evaporation on Si/SiO2 substrate without annealing as postꢀ
treatment.
Due to the LC properties of both LCꢀdiketones 1 and 2
50 compounds, a annealing at 90°C for 90 min and for 1 and at
140°C for 40 min for 2 was applied, which are relatively low
temperatures. Figure 10 shows the optical microscopy image of a
LCꢀdiketone 2 based OTFT in BGTC configuration after a
annelaing at 90°C for 90 min postꢀtreatment. While the edges of
55 the channel shown have the same structure with numerous fused
small grains as vacuum evaporated thin films prior the postꢀ
treatment, the annealing has caused the formation of novel thin
and elongated structures in the center of the channel while
leaving some substrate surface visible. Such structures were
60 formed independently of each other without providing an
interconnection in the channel from source to drain electrodes
even in some channel lengthꢀbased OTFTs. A postꢀtreatment
consisting of annealing of vacuum evaporated of LCꢀdiketone 1
and 2 based thin films failed to form well defined interconnected
65 structures needed to ensure an effective charge transport in OTFT
devices.
10 Figure 8. LCꢀdiketone 2. (top) recorded XRD pattern for the LC phase at
120 °C; (centre) Θ scans of in the SmA phase 100°C (black) and 120°C
(red) and in isotropic phase at 190°C (green), insert magnification of the
wide angle region; (down) χ –scans of the angular interval 16 ꢀ 26 (2 Θ) at
100°C (red) and 120°C (back); for enhancement of the contrast the data
15 has been divided for the scattering found in the isotropic phase at 190°C.
OTFTs devices
Bottomꢀgate thinꢀfilm transistors with both topꢀ and bottomꢀ
electrode contact were fabricated in the manner described in the
20 Experimental part to realize soꢀcalled BGTC (BottomꢀGate Topꢀ
Contact) and BGBC (BottomꢀGate BottomꢀContact) devices,
respectively. The depostion of the active layer was achieved by
either vacuum evaporation or dropꢀcasting. Vacuum evaporated
thin films based on LCꢀdiketones 1 and 2 were realized with a
25 low rate (< 1 nm/min) under a pressure of 1ꢀ4×10ꢀ6 mbar to a
6
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45 leads to liquid crystal properties with interesting phase behaviour.
Changing the length to width ratio of the molecules modulates
not only the transition temperatures but also the type of self
assembly, going from a SmA monolayer phase with short range
SmAb characteristics and a monotropic SmC phase to an
50 interdigitated SmA phase. Quite surpisingly assembling these
materials in OTFTs devices lead to the formation of complex
three dimensional structures, which need to be investigated
further before the charge transport properties can be evaluated
fully.
55
Experimental
Synthesis. Ethanol, methylene chloride, toluene were purchased
from CarloErba. Tetrabutylamonium hexafluorophosphate
(TBHP) was purchased from Fluka and silica gel (240ꢀ400 mesh)
60 from Merck. 1,2ꢀdichlorobenzene, trifluoroacetic acid (TFA), Nꢀ
iodosuccinimide (NIS) were purchased from SigmaꢀAldrich.
Melting points are uncorrected and were obtained from an
Figure 10. Optical microscopy image of an OTFT in BGTC configuration
based on LCꢀdiketone 2 deposited by vacuum evaporation on Si/SiO2
substrate. The complete OTFT device was exposed to an annelaing at
90°C during 90 min as postꢀtreatment.
5
LCꢀdiketones 1 and 2 are dark blue powders readily soluble in
common organic solvents. The limit of solubility of both LCꢀ
diketones was evaluated to be 16.7 and 12.5 mg/mL in
1
Electrothermal 9100 apparatus. H NMR spectra were recorded
on Bruker AC 250 at 250 MHz.
65
10 dichloromethane (CH2Cl2) and in chlorobenzene (CB),
respectively. A systematic study of the influence of concentration
on drop casted thin film properties was realized by using different
solutions. The highest concentration of both LCꢀdiketones 1 and
2 based solutions in CH2Cl2 was 16.7 mg/mL. By dilution, four
15 others concentrations were prepared with 8.35, 4.17, 2.08 and
1.04 mg/mL. In CB, the same procedure gave rise to
concentrations at 12.5, 8.35, 4.17, 2.08 and 1.04 mg/mL. The
active layers were realized by drop casting of solutions prepared
in CH2Cl2 or in CB onto the surface of linear BGBC devices. A
20 first annealing as described in the experimental part was applied
in order to remove residual solvent. While numerous OTFTs were
prepared for each solutions, any fieldꢀeffect activity was
observed. As vacuum evaporated thin film, a postꢀtreament
(110°C during 3h) was initiated to reach the LC properties of
25 both diketones. While the optical microscopy images highlighted
the presence of wellꢀdefined thin films (Figure S6), any charges
were collected and transported in the channel. Despite different
OTFT configrations, BC and TC, with channel lengths ranging
from 2.5 to 50 ꢁm, any fieldꢀeffect activity either under positive
30 or negative voltages was measured. The low microstructural
order of thin films as well as before as after annealing failed to
lead to charge transport properties in LCꢀdiketones 1 and 2 based
thin films. One possibilty to increase the structural order is to use
spinꢀcoating technique to prepare OTFTs and control the
35 temperature of solution and substrate before spinꢀcoating and the
film temperature after spinꢀcoating.41
Synthesis of 2,7’-diiodo-benzo[2,1-b:3,4-b’]dithiophene-4,5-
dione(3). To a 250 mL round bottom flask compound 4 (0.05 g,
0.22 mmol), TFA (1.4 mL), and NIS (2.07 g, 18.15 mmol) were
added. After stirring for several min at room temperature (RT),
70 the reaction mixture was heated to reflux for 3 h. The mixture
was poured into ice water (300 mL), then CH2Cl2 (100 mL) was
added. The organic layers were separated and the aqueous phase
was extracted with CH2Cl2 (3×30 mL). The combined organic
phase was washed with 1M Na2S2O3 solution and dried over
75 MgSO4. Removal of solvent in vacuo followed by precipitation in
ethanol gave 3 (92%) as dark purple powder. Mp 315ꢀ316 °C; 1H
NMR (250 MHz, CDCl3) δ: 7.64 (s, 2H). MS (EI): m/z (%):
471.8 (100) [M+]. Elemental analysis (%) calcd C10H2O2S2I2: C,
25.44; H, 0.43; found: C, 25.36; H, 0.49.
80
Synthesis of 2,7’- bis(4-(4'-(nonyloxy)biphenyl))-benzo[2,1-
b:3,4-b’]dithiophene-4,5-dione (2). In a solution containing 3
(90 mg, 0.19 mmol) and 10 mL of a mixture toluene/water (5:1)
under argon. The boronic acid, Na2CO3 and a catalytic quantity of
85 copper iodide was added and next the tetrakis triphenylphosphine
palladium (6 mol %).The solution was refluxed during 24 hours
then addition of water and filtration on silica gel. The filtrate is
extracted with CH2Cl2. The organic phases are combined, washed
with a solution saturated by NaCl and finally dried on MgSO4
90 and then concentrated. A purification by chromatographic on
silica column with dichloromethane is necessary to give a robust
1
blue solid with a 32 % yield. Mp 142 °C. H NMR (250 MHz,
3
3
CDCl3) δ: 7.66 (d, 4H, J=7.1 Hz, Hbenz), 7.63 (d, 4H, J=7.1 Hz,
3
Conclusion
Hbenz), 7.62 (s, 2H, Hthio), 7.54 (d, 4H, J=8.7 Hz, Hbenz), 6.98 (d,
95 4H, J=8.7 Hz, Hbenz), 4.04ꢀ3.98 (m, 4H, OꢀCH2), 1.84ꢀ1.76 (m,
3
In conclusion two novel semiconductors based on benzo[2,1ꢀ
b:3,4ꢀb’]dithiopheneꢀ4,5ꢀdione endꢀcapped alkoxyꢀbiphenyl or
40 alkoxyꢀphenyl groups have been synthesized and characterized. A
comparative study with the parent compound endꢀcapped by
stryryl group shows that these materials have almost similar UVꢀ
visible and electrochemical properties. On other hand using of
alkoxyꢀbiphenyl/alkoxyꢀphenyl groups to replace styryl group
4H, CH2), 1.49ꢀ1.29 (m, 24H, CH2), 0.89 (t, 6H, 3J=7.1 Hz, CH3).
MS (EI): m/z (%)[M+H]+: calcd 809.3692; found 809.3691.
Elemental analysis (%) calcd C52H56O4S2.CH2Cl2 (1/1): C, 71.64;
H, 6.78; S, 6.95; found: C, 71.60; H, 7.12; S, 7.93.
100
Synthesis of 2,7’- bis(4-octyloxyphenyl)-benzo[2,1-b:3,4-
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7

Journal of Materials Chemistry
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b’]dithiophene-4,5-dione (1). In a solution containing 3 (90 mg,
Thermal gravimetric analysis (TGA)
0.19 mmol) and 10 mL of a mixture toluene/water (5:1) under 60 TGA was performed on a Q600 from TA instruments under 100
argon. The boronic acid, Na2CO3 and a catalytic quantity of
copper iodide was added. And next the tetrakis
triphenylphosphine palladium was added (6 mol %).The solution
was refluxed during 24 hours then addition of water and filtration
on silica gel. The filtrate is extracted with CH2Cl2. The organic
phases are combined, washed with a solution saturated by NaCl
and finally dried on MgSO4 and then concentrated. A purification
mL/min nitrogen flow with a heating rate of 20°C/min from room
temperature to 800°C. All residual oxygen was removed by
applying a 400 mL/min nitrogen flow for 20 min inside the oven
before starting the experiment.
5
65
Optical polarizing microcopy (OPM). Initial studies of the
thermal properties and the mesophase behaviour of the samples
were carried out using OPM using an Olympus BX51 polarising
microscope. The microscope was equipped with a MettlerꢀToledo
10 by chromatographic on silica gel with dichloromethane is
necessary to give a robust blue solid with a 60 % yield. Mp 156
1
°C. H NMR (250 MHz, CDCl3) δ: 7.53 (s, 2H, Hthio), 7.50 (d, 70 FP900 heating stage.
3
3
4H, J=8.7 Hz, Hbenz), 6.94 (d, 4H, J=8.7 Hz, Hbenz), 4.02ꢀ3.97
(m, 4H, OꢀCH2), 1.83ꢀ1.75 (m, 4H, CH2), 1.53ꢀ1.30 (m, 20H,
OTFTs fabrication.
OTFTs fabrication. Both BottomꢀGate TopꢀContact (BGTC) and
BottomꢀGate BottomꢀContact (BGBC) configurations were used
3
15 CH2), 0.90 (t, 6H, J=7.1 Hz, CH3). MS (EI): m/z (%)[M+H]+:
calcd 629.2744; found 629.2769. Elemental analysis (%) calcd
C38H44O4S2: C, 72.57; H, 7.05; S, 10.20; found: C, 72.33; H, 75 for the OTFT devices. For BGTC, highly nꢀdoped silicon wafers
7.22; S, 10.46.
(gate), covered with thermally grown silicon oxide SiO2 (300 nm,
insulating layer), were purchased from Vegatec (France) and used
as device substrates. The capacitance per unit area of such silicon
dioxide dielectric layers was 12ꢀ13×10ꢀ9 F/cm2. Linear channels
80 were formed by the vacuum evaporation of Au source and drain
electrodes on top of the organic thin film through a shadow mask
with a channel width W = 1 mm and specific channel lengths (L
= 25 or 50 ꢁm). For BGBC, both linear and interdigitated source
and drain electrodes were used. Interdigitated source and drain
20 Physicochemical measurements in solution. UVꢀvisible
absorption spectra were obtained on
spectrophotometer. The electronic absorption maximum (λmax
and the optical band gap (Eg) are directly extracted from
absorption spectra of 1 and 2 based solution. Cyclic voltammetry
25 (CV) data were acquired using
(Bioanalytical Systems) and
a Varian Cary 1E
)
a
BAS 100 Potentiostat
PC computer containing
a
BAS100W software (v2.3). A threeꢀelectrode system based on a 85 electrodes as 30 nm Au with 10 nm high work funtion adhesion
platinum (Pt) working electrode (diameter 1.6 mm), a Pt counter
electrode and an Ag/AgCl (with 3 M NaCl filling solution)
layer (ITO) structured by liftꢀoff technique were deposited on nꢀ
doped silicon wafers covered with thermally grown SiO2 (230
nm). Such interdigitated OTFT structures were purchased from
Frauhofer (Germany) with a channel width W of 10 mm and
30 reference
electrode
was
used.
Tetrabutylammonium
hexafluorophosphate (TBHP) (Fluka) was used as received and
served as supporting electrolyte (0.1 M). All experiments were 90 different channel lengths L ranging from 2.5, 5, 10 and 20 ꢀm.
carried out in anhydrous 1,2ꢀdichlorobenzene (electronic grade
purity) at 20°C. Ferrocene was used as internal standard.
35 Electrochemical reduction/oxidation potential vs Fc/Fc+
(E1/2(red1) and E1/2(ox1)) values are determined from the cyclic
The capacitance per unit area of 230 nm thick silicon dioxide
dielectric layers was 14.6×10ꢀ9 F/cm2. Linear channels for BGBC
were realized with the same shadow mask as BGTC i.e. with W =
1 mm and L = 25 or 50 ꢁm. In linear channelꢀbased BGBC,
voltammogram at a concentration of 1×10ꢀ3 M with a scan rate of 95 source and drain electrodes were deposited prior the depostion of
250 mV.sꢀ1.
the active layers by vacuum evaporation. The organic active
layers were depositted by either vacuum evaporation or dropꢀ
casting. The vacuum thermal evaporation is a common method
for thin film deposition. This technique consists of heating until
40 Computational Methodology. DFT calculations were carried
out using the Gaussian 03 program. Becke´s threeꢀparameter
exchange functional combined with the LYP correlation 100 evaporation of the material. The vapor finally condenses in the
functional (B3LYP) was employed.42 We also made use of the
standard 6ꢀ31G** basis set.43
form of a thin film on a cold substrate. High vacuum with
pressures of 1ꢀ4×10ꢀ6 mbar was used. The semiconductor layers
were vacuum deposited onto the substrates to a thickness
controlled with an in situ quartz crystal monitor. Substrate
45
X-ray diffraction
Xꢀray diffraction experiments were performed on a MAR345 105 temperature (Tsub) during deposition was the temperature of block
diffractometer with a 2D image plate detector (CuKα radiation,
graphite monochromator, = 1.54 Å). The samples were heated
50 in the presence of a magnetic field using a homeꢀbuilt capillary
furnace.
on which the substrates were mounted, i.e. typically 30°C. For
drop casting, organic compounds were dissolved in
dichloromethane (CH2Cl2) stabilized by ethanol and in
chlorobenzene (CB). The organic solutions were added dropwise
110 at volumes of 20ꢀ40 ꢁL onto the surface of devices. Such volume
covered entirely the channel of one BGBC OTFT and beyond.
After the organic solution deposition, different postꢀtreatments
were realized to eliminate the solvent: vacuum (1.3×10ꢀ4 Pa)
during 3 hours or heating to 30°C during 1 hour in air for
115 dichloromethaneꢀbased solutions and heating to 110°C during 3 h
in air for chlorobenzeneꢀbased solutions. Currentꢀvoltage
λ
Differential scanning calorimetry (DSC)
Phase transitions were determined using a Mettler DSC822e with
55 STARe software calibrated with indium in a nitrogen atmosphere
calibrated against an indium standard (156.6°C 28.45J/g; reported
transition temperatures are the onset of the endotherm).
8
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16 Y. Didane, R. Ponce Ortiz, J. Zhang, K. Aosawa, T. Tanisawa, H.
Aboubakr, F. Fages, J. Ackermann, N. Yoshimoto, H. Brisset, C.
VidelotꢀAckermann, Tetrahedron, 2012, 68, 4664.
17 Y.Didane, P. Marsal, F. Fages, A. Kumagai, N. Yoshimoto, H.
Brisset, C. VidelotꢀAckermann, Thin Solid Films, 2010, 519, 578.
18 A. Facchetti, M.H. Yoon, C.L. Stern, H.E. Katz, T.J. Marks, Angew.
Chem., Int. Ed., 2003, 42, 3900.
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2006, 128, 5792.
20 S. Ando, J. Nishida, H. Tada, Y. Inoue, S. Tokito, Y. Yamashita, J.
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characteristics were obtained with a HewlettꢀPackard 4140B
picoꢀamperemeterꢀDC voltage source at room temperature in air
The sourceꢀdrain current (ID) in the saturation regime is governed
by the following equation:
5
(ID)sat = (W/2L) Ciꢀ (VGꢀVt)2
(1)
where Ci is the capacitance per unit area of the gate insulator
layer, VG is the gate voltage, Vt is the threshold voltage, and ꢁ is
the fieldꢀeffect mobility.
21 T.M. Pappenfus, R.J. Chesterfield, C.D. Frisbie, K.R. Mann, J.
Casado, J.D. Raff, L.L. Miller, J. Am. Chem. Soc., 2002, 124, 4184.
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M.H. Haukaas, K.R. Mann, L.L. Miller, C.D. Frisbie, Adv. Mater.,
2003, 15, 1278.
Notes and references
a
10 Centre Interdisciplinaire de Nanoscience de Marseille (CINaM), CNRS
UMR 7325, Aix Marseille Université, 13288 Marseille cedex 09, France.
Phone +33.6.17.24.81.93, Fax. +33.4.91.41.89.16, Eꢁmail: jeanꢁ
manuel.raimundo@univꢁamu.fr.
23 Y. Didane, A. Kumagai, N. Yoshimoto, C. VidelotꢀAckermann, H.
Brisset, Tetrahedron, 2011, 67, 1628.
b
Department of Chemistry, University of Hull, Hull, HU6 7RX, UK,
15 Phone +44(0)1482 465590 ; Fax +44 (0)1482 466410
, Eꢁmail:
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g.h.mehl@hull.ac.uk
c Laboratoire MAPIEM (EA 4323),Université du SUD ToulonꢁVar, ISITV,
BP 56, 83162 La Valette du Var cedex, France. Phone
+33.4.94.14.67.24, Fax. +33.4.94.14.27.86, Eꢁmail: brisset@univꢁtln.fr.
20 Electronic Supplementary Information (ESI) available: Figure S1, TGA
and degradation temperatures of LCꢁdiketones 1 and 2 ; Figure S2, DSC
of LCꢁdiketones 1 and 2; Figure S3, OPM of LCꢁdiketone 1, Figure S4,
Optical microscopy images of a LCꢁdiketone 2 thin films, Figure S5, AFM
pictures and corresponding crossꢁsection of LCꢁdiketones 1 and 2.
25 Figure S6, Optical microscopy images of thin films deposited by drop
casting in BGBC configuration of LCꢁdiketone 2, Figure S7, 1H NMR
spectra of LCꢁdiketones 1 and 2 See DOI: 10.1039/b000000x/
1
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