Synthesis and thermal behaviour of α,α′-didecyloligothiophenes

Article abstract of DOI:10.1039/b209176c

α,α′-Didecylquater-, -quinque- and -sexi-thiophenes were synthesized by Kumada cross-coupling and oxidative coupling reactions. For the former reaction Pd(dppf)Cl₂ was found to be a more efficient catalyst than the usually applied Ni(dppp)Cl₂. Thermal behaviour of all new oligothiophenes was investigated by differential scanning calorimetry and polarizing optical microscopy. It was shown that all these compounds possess not only crystal phases but also high temperature ordered smectic mesophases and that the clearing point increases linearly with the number of conjugated thiophene rings. A degree of order in the crystal phase was estimated on the basis of thermodynamic data. The highest degree of order was proposed for α,α′-didecylquaterthiophene, which explains why the mobility of end-α,α′-capped quaterthiophenes in FET (field effect transistor) devices is comparable or sometimes better than those of corresponding quinque- and sexi-thiophene derivatives.

Full text of DOI:10.1039/b209176c

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Synthesis and thermal behaviour of a,a’-didecyloligothiophenes
Sergei Ponomarenko and Stephan Kirchmeyer*
H. C. Starck, Research Electronic Chemicals, Central Research and Development Division,
Bayerwerk G8, 51368 Leverkusen, Germany. E-mail: stephan.kirchmeyer.sk@hcstarck.de
Received 19th September 2002, Accepted 13th November 2002
First published as an Advance Article on the web 16th December 2002
a,a’-Didecylquater-, -quinque- and -sexi-thiophenes were synthesized by Kumada cross-coupling and oxidative
coupling reactions. For the former reaction Pd(dppf)Cl
2
was found to be a more efficient catalyst than the
usually applied Ni(dppp)Cl . Thermal behaviour of all new oligothiophenes was investigated by differential
2
scanning calorimetry and polarizing optical microscopy. It was shown that all these compounds possess not
only crystal phases but also high temperature ordered smectic mesophases and that the clearing point increases
linearly with the number of conjugated thiophene rings. A degree of order in the crystal phase was estimated
on the basis of thermodynamic data. The highest degree of order was proposed for a,a’-didecylquaterthiophene,
which explains why the mobility of end-a,a’-capped quaterthiophenes in FET (field effect transistor) devices is
comparable or sometimes better than those of corresponding quinque- and sexi-thiophene derivatives.
Introduction
Experimental
Oligothiophenes are a promising class of organic semiconduct-
ing materials, which are considered to be realistic candidates
for industrial use in the production of solution processed
organic field effect transistors (FETs) for cheap, large area
Materials
All solvents and reagents used were purchased from Aldrich
and were used as received unless otherwise stated.
1
smart electronic devices. It has already been shown that TFTs
(
thin film transistors) made from a,a’-oligothiophenes as the
active layer possess not only high field effect mobility [up to
Methods
2
.23 cm (Vs) ], but also small off currents (on/off ratios in
21
2
1
0
H NMR spectra were recorded on a Bruker DPX 400
spectrometer, equipped with a QNP Probehead, operating at
4
6 3
the range 10 –10 ). Moreover, it has been noted that end-
capped oligothiophenes usually show better electronic char-
acteristics than unsubstituted oligothiophenes due to their
better structural organisation in the crystal phase. However,
only a few examples of end-capped oligothiophenes have been
1
synthesized and investigated up to now. Since the first
1
400.13 MHz for H, in CDCl
solutions unless otherwise
3
stated. Phase transitions were studied by differential scanning
calorimetry (DSC) with a Mettler TA-4000 thermosystem at a
scanning rate of 10 K min and 1 K min . The polarizing
microscopic investigations were performed using a Mettler FP-
4
2
1
21
5
,6
9
0 central processor equipped with a hot stage Mettler FP-82
examples (i.e. a,a’-dihexylsexithiophene, a,a’-bidodecylsexi-
7
2,8
and control unit in conjunction with a Zeiss polarizing
microscope.
thiopnene and a,a’-dihexylquaterthiophene ) have shown
very promising characteristics, it is worth synthesizing different
compounds of this type in order to optimise the properties and
to clarify the structure–property relationship. Since it is known
that the field effect mobility depends not only on the chemical
structure of the molecular semiconductor, but also on their
structural organisation in the bulk, i.e. on morphology and a
degree of order in the crystal phase, the relationship between
phase order and chemical structure needs to be further clarified.
Indeed, it is already known that oligothiophene derivatives
could form different mesophases.
viour has also been observed for several a,a’-dialkyloligothio-
phenes.
systematic investigation of the thermal behaviour of a,a’-
dialkyloligothiophenes have been published up to now.
Dependence of the phase behaviour of 5,5@-dialkyl-2,2’:5’,2@-
Synthesis
2
-Decylthiophene (1). Anhydrous THF (60 ml) was added
to a solution of 0.08 mol butyllithium in 32 ml of hexane
2.5 M solution) with cooling below 0 uC. Thiophene (8.08 g,
.096 mol) was added within 10 min and the temperature was
3
(
0
kept between 0 and 10 uC. The cooling bath was then removed
and the temperature was allowed to rise to 20 uC. Decyl
bromide (23.0 g, 0.104 mol) was added in one portion without
external cooling. The temperature of the yellow-orange
solution rose slowly to 32 uC within 2 h. The solution was
refluxed overnight and 200 ml of ice water were added with
vigorous stirring. After separation of the layers, the aqueous
layer was extracted twice with diethyl ether. The combined
organic solutions were dried over magnesium sulfate and
subsequently concentrated under vacuum to give 22.63 g of raw
product. The product was then purified by distillation in high
vacuum (0.2 mBar, 85 uC) to give 13.1 g (73%) of 99.9% pure
(GC–MS) product as a colorless liquid. Isolated yield: 73%;
9
–11
Complex phase beha-
7
,8
However, only a few papers devoted to the
1
2
terthiophenes and different alkyl-capped quaterthiophene
1
3
derivatives on the length of the terminal alkyl substituent
have been reported. In the present paper, for the first time a
relationship between the thermal behaviour of a,a’-didecyl-
oligothiophenes and the length of the central oligothiophene
unit, i.e. on the number of conjugated thiophene rings in the
molecule, is considered.
1
?
reaction yield 88%. MS m/z 224 (M ).
DOI: 10.1039/b209176c
J. Mater. Chem., 2003, 13, 197–202
This journal is # The Royal Society of Chemistry 2003
197

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2
-Bromo-5-decylthiophene (2). In the absence of light, a
solution of 7.93 g (45 mmol) N-bromosuccinimide (NBS) in
0 ml of anhydrous DMF was added dropwise at 0 uC to a
suspension was formed. It was stirred for 30 min at 278 uC,
and anhydrous powdered CuCl (440 mg, 3.3 mmol) was added
2
5
in one portion, upon which the colour of the solution changed
to black. The mixture was stirred until it returned to room
temperature (the colour changed from black to green and a
yellow precipitate formed gradually) and then for an additional
30 min at 30 uC. The mixture was poured into 200 ml of water,
containing 10 ml of 1 M hydrochloric acid. Then 300 ml of
diethyl ether were added and a yellow solid formed in the ether
phase, which was washed three times with 200 ml of water and
filtered to give a yellow precipitate, which was washed with
diethyl ether and dried under vacuum to give 669 mg (67%) of
solution of 10.0 g (45 mmol) of 2-decylthiophene in 50 ml of
anhydrous DMF within 30 min. The reaction mixture was
stirred for 4 h at room temperature. It was then poured on to
ice and extracted twice with dichloromethane. The organic
phases were combined, washed with water and dried over
sodium sulfate. The solvent was evaporated on a Rotavapor to
give 14.1 g of crude product. After removing the residual DMF
by heating at 40 uC under high vacuum (0.2 mBar) for 2 h,
1
3.32 g of the product as a yellowish liquid was obtained.
1
3
Isolated yield: 98.5%; reaction yield (GC–MS): 99.9%. MS m/z
3
yellow crystals. H NMR (CDCl
7.1 Hz), 1.20–1.45 (overlapped peaks, 28 H), 1.68 (m, 4H), 2.79
3
, TMS/ppm): 0.88 (t, 6H, J ~
?
04 (M ).
1
3
t, 4H, J ~ 7.6 Hz), 6.68 (d, 2H, J ~ 3.9 Hz), 6.97 (d, 2H, J ~
3.4 Hz), 6.99 (d, 2H, J ~ 3.9), 7.03 (d, 2H, J ~ 3.4 Hz). Calc.
for C36 (693.2): C, 70.76; H, 8.25; S, 20.99. Found: C,
70.57; H, 8.40; S, 21.0%.
3
(
3
3
5-Decyl-2,2’-bithiophene (3). To a suspension of 0.793 g
33 mmol) of magnesium in 5 ml of anhydrous diethyl ether
a solution of 4.89 g (30 mmol) of 2-bromothiophene in 30 ml
of anhydrous diethyl ether was added dropwise under nitro-
gen. The solution of the Grignard reagent was refluxed for
(
50 4
H S
5,5’’’@-Didecyl-2,2’:5’,2@:5@,2’’’:5’’’,2@@:5@@,2’’’@-sexithiophene (7).
2
h, then cooled to room temperature and transferred to the
Synthesized was according to the procedure described for
compound 6 from 1.0 g (2.6 mmol) of 5-decyl-2,2’,5’,2@-
terthiophene (5), 1.55 ml of 2 M LDA solution (3.1 mmol)
dropping funnel of a second apparatus via syringe. In the
second apparatus the foregoing solution was added dropwise
to an ice-cooled suspension of 6.05 g (20 mmol) of 2-bromo-
-decylthiophene and 140 mg (0.2 mmol) of Pd(dppf)Cl2 in
0 ml of anhydrous diethyl ether. The reaction mixture was
stirred for 1 h at room temperature. It was then hydrolysed
with 20 ml of a saturated aqueous ammonium chloride solu-
tion followed by the addition of 100 ml of anhydrous pentane.
Then 500 ml of diethyl ether were added and the organic layer
was washed three times with 200 ml of water and dried over
sodium sulfate. After removal of the solvent, the residue
and 350 mg (2.6 mmol) of CuCl . The product was filtered
2
5
3
from the solution to yield orange-red crystals. Isolated yield:
1
4
95 mg (50%). H NMR (1,2-dichloroethane-d , 85 uC, TMS/
4
ppm): 0.88 (t, 6H), 1.20–1.45 (overlapped peaks, 28 H), 1.68 (m,
H), 2.78 (t, 4H), 6.69 (d, 2H), 7.02 (overlapped peaks, 4H), 7.08
overlapped peaks, 6H). Calc. for C44 (775.3): C, 68.17; H,
4
(
54 6
H S
7
.02; S, 24.81. Found: C, 67.90; H, 7.00; S, 25.0%. FD MS m/z:
?
1
774, 775, 776 (M , three most intense peaks of the isotope
pattern).
(
6.47 g, containing 99% of the product and 1% of bithio-
phene according to GC–MS analysis) was purified by column
chromatography (silica gel, hexane) to give 5.4 g of pure
material as a white solid. Reaction yield (GC–MS): 99%;
5,5@@-Didecyl-2,2’:5’,2@:5@,2’’’:5’’’,2@@-quinquethiophene (8).
n-Butyllithium (1.6 M in hexane, 2.4 ml, 3.9 mmol) was
added dropwise via syringe to a stirred solution of 3 (1.20 g,
?
1
1
isolated yield: 88%. MS m/z 306 (M ). H NMR (CDCl3,
3
TMS/ppm): 0.88 (t, 3H, J ~ 6.9 Hz), 1.20–1.45 (overlapped
peaks, 14 H), 1.67 (m, 2H), 2.78 (t, 2H, J ~ 7.8 Hz), 6.67 (d,
3
.9 mmol) in absolute THF (15 ml) at 0 uC under nitrogen. Upon
completion of the addition, the solution was stirred at 0 uC for
0 min, then it was allowed to warm to room temperature and
stirred for 30 min. Then it was cooled again to 0 uC, 1.03 g of
MgBr ?Et O were added in one portion and the solution was
3
3
H, J ~ 3.4 Hz), 6.97 (d, 1H, J ~ 3.4 Hz), 6.98 (dd, 1H, J ~
3
3
1
3
7
4
.4 Hz, J ~ 5.9 Hz), 7.089 (dd, 1H, J ~ 3.4 Hz, J ~ 1.0 Hz),
3
4
3
4
.156 (dd, 1H, J ~ 4.9 Hz, J ~ 1.0 Hz).
2
2
stirred at 0 uC for 30 min and for an additional 30 min without
a cooling bath. The solution of the Grignard reagent was
transferred to the dropping funnel of a second apparatus via
syringe. The previous solution was added dropwise to an ice-
cooled solution of 2,5-dibromothiophene (363 mg, 1.5 mmol)
and Pd(dppf)Cl2 (21 mg, 0.03 mmol) in 15 ml of absolute
THF. After stirring for 16 h at room temperature the reaction
mixture was hydrolysed with 1 M hydrochloric acid. Then it
was poured on to 100 ml of ice water and extracted twice with
5
-Bromo-5’-decyl-2,2’-bithiophene (4). Synthesis was as
described for compound (2) using 4.53 g (25 mmol) NBS and
.79 g (25 mmol) of 3 to give 8.57 g of the product as a yellow
solid. Isolated yield: 88%; reaction yield (GC–MS): 99.9%. MS
7
1
1
3
?
m/z 385 (M ). H NMR (CDCl , TMS/ppm): 0.88 (t, 3H, J ~
3
6
(
.9 Hz), 1.20–1.45 (overlapped peaks, 14 H), 1.67 (m, 2H), 2.77
3
t, 2H, J ~ 7.6 Hz), 6.66 (d, 1H, J ~ 3.4 Hz), 6.82 (d, 1H, J ~
3
3
.9 Hz), 6.91 (d, 1H, J ~ 3.4 Hz), 6.93 (d, 1H, J ~ 3.4 Hz).
3
3
2
00 ml of diethyl ether. The combined organic layers were
washed with water and filtered on glass filter G4 to yield the
5-Decyl-2,2’:5’,2@-terthiophene (5). Synthesis was as des-
cribed for compound (3) using 0.65 g (27 mmol) of magnesium,
1
product as orange crystals. Yield: 0.98 g (94%). H NMR
1
4
3
(
500 MHz, CDCl , TMS/ppm): 0.88 (t, 6H, J ~ 7.0 Hz),
3
3
5
.98 g (24 mmol) of 2-bromothiophene, 8.57 g (22 mmol) of
-bromo-5’-decyl-2,2’-bithiophene and 154 mg (0.22 mmol)
1
J ~ 7.5 Hz), 6.68 (d, 2H, J ~ 3.6 Hz), 6.97 (d, 2H, J ~
.20–1.45 (overlapped peaks, 28 H), 1.68 (m, 4H), 2.79 (t, 4H,
3
3
of Pd(dppf)Cl
(
2
. After recrystallisation from hexane 5.61 g
65%) of pure product was obtained as a yellow solid. MS m/z
3
.6 Hz), 6.99 (d, 2H, J ~ 3.6), 7.04 (d, 2H, J ~ 3.6 Hz), 7.05
3
3
(s, 2H). Calc. for C H S (693.2): C, 69.31; H, 7.56; S, 23.13.
Found: C, 69.20; H, 7.65; S, 23.0%.
1
88 (M ). H NMR (CDCl
1
3
, TMS/ppm): 0.88 (t, 3H, J ~
.9 Hz), 1.20–1.45 (overlapped peaks, 14 H), 1.67 (m, 2H),
?
3
6
2
2
3
40 52 5
3
.79 (t, 2H, J ~ 7.6 Hz), 6.68 (d, 1H, J ~ 3.9 Hz), 6.99 (dd,
3
4
3
4
H, J ~ 3.4 Hz, J ~ 5.4 Hz), 7.02 (dd, 1H, J ~ 4.2 Hz, J ~
3
.2 Hz), 7.05 (d, 1H, J ~ 3.4 Hz), 7.15 (dd, 1H, J ~ 3.4 Hz,
3
4
Results and discussion
4
3
4
J ~ 1.0 Hz), 7.20 (dd, 1H, J ~ 5.1 Hz, J ~ 1.2 Hz).
Synthesis
5
,5’’’-Didecyl-2,2’:5’,2@:2’’’-quaterthiophene (6). 5-Decyl-2,2’-
Choice of synthetic strategy (see Scheme 1) for the synthesis of
the target compounds was based on the ‘minimal price–highest
yield’ approach, implying that the minimal price for initial
reagents, which could be converted through a number of (not
necessarily minimal) high yield reactions to the desired products.
bithiophene (1.0 g, 3.3 mmol) in anhydrous THF (10 ml) was
added dropwise to a solution of lithium diisopropylamide
LDA; 1.95 ml of 2 M solution, 3.9 mmol) in 10 ml of
(
anhydrous THF at 278 uC under nitrogen. A white-yellowish
1
98
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Scheme 1
Thiophene, 2-bromothiophene, 2,5-dibromothiophene and 10-
bromodecane were selected as the starting reagents.
coupling of 5-decyl-2,2’-bithiophene (3) and 5-decyl-2,2’:5’,5@-
terthiophene (5), respectively, in the presence of equimolar
amount of CuCl . Isolated yields were 67 and 50%, respec-
tively. In this case the reaction conditions were not optimised,
so it is believed that careful optimisation of them allows further
increase of the reaction yield.
a,a’-Didecylquinquethiophene [Dec-5T-Dec, (8)] was synthe-
sized by the Kumada cross-coupling reaction between 2,5-
dibromothiophene and a Grignard reagent, freshly prepared
from 5-decyl-2,2’-bithiophene (3) by lithiation with n-butyl-
lithium followed by lithium exchange with a magnesium
dibromide etherate complex in THF, using the Pd(dppf)Cl2
catalyst. This reaction led to formation of Dec-5T-Dec in 94%
isolated yield. Such a high reaction yield can be explained by
the fact that the product is the most insoluble compound in the
reaction mixture and it precipitates during the reaction in its
pure form since the initial and all intermediate compounds and
by-products remain dissolved in the reaction solvent (THF at
room temperature).
Lithiation of thiophene followed by its reaction with 10-
bromodecane, adapted from the synthesis of 2-buthylthio-
2
1
5
phene, led to 2-decylthiophene (1) in 88% reaction yield
(
2
73% isolated yield) after 2 h of stirring. Bromination of
1
6
-decylthiophene with NBS in DMF in the absence of light
was quantitative, leading to formation of 2-bromo-5-decylthio-
phene (2) after stirring at room temperature for 4 h in 98%
isolated yield.
For the next step – Kumada cross-coupling between 2-bromo-
-decylthiophene and the Grignard reagent made from 2-bromo-
5
thiophene and magnesium in diethyl ether – the Ni(dppp)Cl
2
1
6
catalyst, used usually in the similar synthesis, was found to be
1
7
less efficient than Pd(dppf)Cl2. In the former case, even a two-
fold excess of the Grignard reagent did not lead to complete
consumption of the 2-bromo-5-decylthiophene after 48 h under
reflux. In the latter case, reaction was usually complete within
3
0 min of stirring at room temperature, using only a 10% excess
of Grignard reagent, leading up to 83% isolated yield of 5-decyl-
,2’-bithiophene (3). This compound was used as a key precursor
As expected, the solubility of the oligothiophenes synthe-
sized (Dec-4T-Dec, Dec-5T-Dec and Dec-6T-Dec) decreases
from Dec-4T-Dec to Dec-6T-Dec, i.e. with increasing the
number of conjugated thiophene rings. Even Dec-4T-Dec is
already almost insoluble in diethyl ether and dichloromethane
2
for synthesis of the target compounds: a,a’-didecylquater- and
a,a’-didecylquinque-thiophenes.
For the synthesis of a,a’-didecylsexithiophene (6) another
precursor, namely 5-decyl-2,2’:5’,5@-terthiophene (5) was syn-
thesized. For this purpose the same reaction sequence, includ-
ing bromination of 5-decyl-2,2’-bithiophene with NBS in
DMF followed by Kumada cross-coupling with 2-thienylmag-
2
2
21
(solubility v10 mg ml ), but its solubility is quite good in
chloroform, THF and toluene (solubility ca. 1 mg ml at room
2
1
2
1
temperature and w10 mg ml at 50 uC). Dec-5T-Dec is almost
insoluble in all of these solvents at room temperature (solubility
2
2
21
nesium bromide using the Pd(dppf)Cl catalyst, was continued.
Isolated yields were 87 and 65%, respectively. The chemical
structures of both precursors as well as all intermediate
2
v10 mg ml ), but it is soluble at elevated temperatures
21
in chloroform, THF and toluene (solubility ca. 1 mg ml at
50 uC). Dec-6T-Dec is the most insoluble compound. It is only
soluble at elevated temperatures (above 50 uC) in chloroform,
dichloroethane and other chlorinated solvents, as well as in
toluene, chlorobenzene, dichlorobenzene, etc. concentrations
1
compounds were confirmed by GC–MS analysis as well as H
NMR spectrometry (see Experimental part).
a,a’-Didecylquater- and a,a’-didecylsexi-thiophenes [Dec-
T-Dec (6) and Dec-6T-Dec (7)] were synthesized by oxidative
2
1
21
4
of ca. 10 mg ml
.
J. Mater. Chem., 2003, 13, 197–202
199

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Table 1 Transition temperatures and enthalpies of transitions for a,a’-
oligothiophenes
a
Transition temperatures (uC) and enthalpies
2
1
)
Oligothiophene of transitions (in parentheses, kJ mol
Dec-4T-Dec
Dec-5T-Dec
Dec-6T-Dec
a
K
K
98 (41.8) SmX 168 (36.9) I
72 (10.7) SmX 225 (41.5) I
K 108 (21.9) SmX 281 (34.0) SmX1 290 (17.3) I
Note: K ~ crystalline phase; SmX and SmX1 ~ undefined smectic
ordered phases; I ~ isotropic liquid. Onset of the transition tem-
peratures from the second heating curves are shown.
Thermal behaviour
The thermal behaviour of all the oligothiophenes synthesized
was investigated by means of polarizing optical microscopy
(POM) and differential scanning calorimetry (DSC) methods.
The results are summarized in Table 1.
Fig. 2 Molecular models of oligothiophenes based on Molecular
Mechanic Calculations (HyperChem, MM1Force Field): (a) Dec-4T-
Dec, (b) Dec-5T-Dec, and (c) Dec-6T-Dec.
As can be seen from the data obtained, all three
oligothiophenes in question (Dec-4T-Dec, Dec-5T-Dec and
Dec-6T-Dec) possess not only crystal phases, but also high
temperature liquid crystal, most probably ordered smectic,
mesophases. Isotropisation temperature increases linearly with
the number of conjugated thiophene rings, while the enthalpy
of this transition has the reverse tendency.
The most interesting results were obtained for oligothio-
phene Dec-4T-Dec. POM showed that cooling from the
isotropic melt to 170 uC leads to formation of a mosaic texture
ordered smectic mesophases. It is worth noting the good
reproducibility of the values of the temperatures and enthalpies
of both transitions at all heating and cooling curves even at
2
1
rather high scanning rates (10 K min ). It indicates a fast
speed of the phase transitions as well as good thermal stability
of the compound (all of the measurements were made at
normal atmospheric conditions). Both of these findings are
very important for the application of this compound as a
FET. Moreover, the rather large interval of the crystal phase
(from room temperature to 98 uC) makes a,a’-didecylquater-
thiophene a more promising material for FET application as
[
Fig. 1(a)], characteristic for either columnar or for ordered
1
8
smectic mesophases. Since the chemical structure of this
molecule does not favour the formation of columnar phases
1
Fig. 2(a)], it is meaningful to suggest the formation of one of
9
[
2
,8
the ordered smectic mesophases in this temperature region.
Further cooling to 92 uC leads to crystallisation [Fig. 1(b)].
Data of DSC measurements confirm these observations. DSC
curves for a sequence of heating, cooling and second heating of
this compound are shown in Fig. 3. The high value of the
compared with the a,a’-dihexylquaterthiophene
with the
crystal-to-mesophase transition at 84 uC, since it was shown
that the field effect mobility in the liquid crystal mesophase is
2
0
lower than in the crystal phase.
DSC of the quinquethiophene derivative Dec-5T-Dec
2
1
enthalpy of transition at 168 uC (37 kJ mol ) is typical for
showed two phase transitions: at 72 uC with the enthalpy
2
1
21
1
(
7
(
0.7 kJ mol , and at 225 uC with the enthalpy of 41.5 kJ mol
see Table 1). However, reproducibility of the transition at
2 uC was achieved only at rather slow scanning rates
2
1
1 K min ) that indicate some hindering during the crystal-
lisation process. Polarizing optical microscopy of Dec-5T-Dec
does not show formation of any typical textures that could also
be attributed to hindered phase transitions. Nevertheless,
mechanical shear in the upper phase showed its fluidity, so it
was referred to a liquid crystal mesophase. Bearing in mind the
high value of isotropisation enthalpy (see Table 1), which is
Fig. 3 DSC thermogram of Dec-4T-Dec. A sequence of heating,
21
cooling and second heating at the rate of 10 K min is shown. Peaks of
the transition temperatures together with the enthalpies of transition
are indicated.
Fig. 1 Polarizing optical micrograph of Dec-4T-Dec on cooling: (a)
70 uC, and (b) 90 uC.
1
2
00
J. Mater. Chem., 2003, 13, 197–202

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very close to that of Dec-4T-Dec, as well as molecular
symmetry of Dec-5T-Dec (Fig. 2b), it was also attributed to
an ordered smectic mesophase.
series of oligothiophenes under consideration. Such findings
might give an explanation as to why similar quaterthiophene
derivatives show comparable or even better values of field effect
2
2
DSC of the sexithiophene derivative Dec-6T-Dec
showed three phase transitions: at 108 uC with the enthalpy
mobility than the corresponding sexithiophene compounds
by the following way. On the one hand, one should expect a
higher value of the mobility with an increasing the number of
thiophene rings in oligothiophenes due to the larger con-
jugation length of the molecules. This could be true for
molecular semiconductors, i.e. if one measures the properties of
individual molecules or at least a monolayer. However, a usual
FET consists of a thin film of the material (at least several
monolayers), so intermolecular interactions and the order
within the molecular layer rather than the properties of single
molecules are responsible for the mobility. That is why a
decrease of the degree of order in the crystal phase compensates
or even exceeds the gain connected with the increase in the
conjugation length.
2
1
21
2
1.9 kJ mol , at 281 uC with the enthalpy 34 kJ mol , and
21
at 290 uC with the enthalpy of 17.3 kJ mol (see Table 1).
Again, as in the case of Dec-5T-Dec, reproducible values of
the transition enthalpies were obtained at low scan rates only
2
1
(
1 K min ). Polarizing optical microscopy showed melting
at 281 uC and isotropisation at 290 uC. This observation
allowed us to attribute the upper phase between 281 and
2
90 uC to a liquid crystal mesophase. Unfortunately, the high
temperature of this mesophase accompanied by sublimation
of the chemical did not allow us to obtain any pronounced
texture, therefore it was not possible to make a more exact
attribution of this mesophase on the basis of POM. How-
ever, the relatively high value of the isotropisation enthalpy
2
1
(
17.3 kJ mol ) allowed us to suggest that it is smectic rather
Conclusions
than a nematic mesophase. Moreover, this value is more than
twice lower than those for the transitions SmX-I for Dec-4T-
Dec and Dec-5T-Dec (see Table 1) which also indicate its
A new series of oligothiophenes, namely a,a’-didecylquater-,
a,a’-didecylquinque- and a,a’-didecylsexi-thiophenes, has been
synthesized and characterized. The phase behaviour of these
compounds was investigated by combination of POM and
DSC methods. It was shown that all of these compounds
possess not only crystal phases, but also high temperature
liquid crystal, most probably ordered smectic, mesophases.
Combination of the results obtained, including solubility
properties, ease of LC mesophase and crystal phase formation
as well as estimation of the degree of order in the crystal phase,
indicate that a,a’-didecylquaterthiophene has the highest order
in its crystalline phase and therefore might be the most
promising material for solution processed FET application as
compared with the corresponding quinque- and sexi-thiophene
derivatives. Measurements of field effect mobility for all the
2
1
different ordering. That is why it was marked as ‘SmX1’. As
far as the mesophase in the temperature region between 108
and 281 uC is concerned, ii is reasonable to suggest that it is
similar to SmX found for Dec-4T-Dec and Dec-5T-Dec. It
follows from the fact that not only are the transitions from the
K phase to the SmX mesophase close for all three compounds
in question (98, 72 and 108 uC), but also the enthalpies of their
2
1
‘
melting’ are similar (36.9, 41.5 and 34 kJ mol ).
In order to estimate an average degree of order in the
crystalline phase (or how well crystals could form a particular
compound) for all three oligothiophenes under study, we
compared the total values of entropies (DS
chemical during the transitions from the isotropic melt to the
low temperature crystal phase. These are in particular: DS
SDS , where DS are entropies of the transitions from the
S
) lost by each
2
3
new compounds synthesized will be published elsewhere.
S
~
i
i
isotropic melt to the LC mesophase, from the LC mesophase to
the crystal phase as well as from one mesophase to another. Of
course, such an approach could not give us exact quantitative
results since the dependence of the entropy DS(T) on the
temperature within each phase should be taken into account.
But since such changes are much smaller within the
temperature interval under consideration than the changes
that happen during the phase transitions, it could give us at
least qualitative results for comparison reasons. Such depen-
dence is presented as a diagram in Fig. 4. One can see that the
Acknowledgements
The authors would like to thank Dr H. van Well, Dr R. Friebe
and Dr J. Wesener (Bayer AG, BSD-ZA) and their co-workers
for help in the characterization of the new materials synthesized
as well as B. Luettge and E. Voss (Bayer AG, BTS-PT-GP) for
PO microscopy facilities and PO microphotographs of the
samples.
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S
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