Synthesis and optical properties of soluble sexithiophenes with one central head-to-head junction

Article abstract of DOI:10.1016/S0040-4020(02)00098-4

The regioselective synthesis of three sexithiophenes characterized by the presence of one central 3,3′-dimethyl-2,2′-bithiophene subsystem is described. One of these compounds was obtained under mild conditions by microwave-mediated synthesis. All sexithiophenes were soluble in organic solvents and displayed 30-40% fluorescence quantum yields in solution. In thin films the fluorescence quantum yields dropped to 1-2%, indicating conformational changes and strong intermolecular interactions in the solid state.

Full text of DOI:10.1016/S0040-4020(02)00098-4

TETRAHEDRON
Pergamon
Tetrahedron 58:2002) 2245±2251
Synthesis and optical properties of soluble sexithiophenes with one
central head-to-head junction
a
G. Sotgiu, M. Zambianchi, G. Barbarella and C. Botta
a
a,p
b
a
Consiglio Nazionale Ricerche, I.Co.C.E.A, Via Gobetti 101, 40129 Bologna, Italy
b
Consiglio Nazionale Ricerche, I.C.M., Via Bassini 15, 20133 Milano, Italy
Received 2 November 2001; revised 18December 2001; accepted 17 January 2002
0
0
AbstractÐThe regioselective synthesis of three sexithiophenes characterized by the presence of one central 3,3 -dimethyl-2,2 -bithiophene
subsystem is described. One of these compounds was obtained under mild conditions by microwave-mediated synthesis. All sexithiophenes
were soluble in organic solvents and displayed 30±40% ¯uorescence quantum yields in solution. In thin ®lms the ¯uorescence quantum
yields dropped to 1±2%, indicating conformational changes and strong intermolecular interactions in the solid state. q 2002 Elsevier
Science Ltd. All rights reserved.
It is known that thiophene oligomers are among the most
promising organic materials for electronic and electro-
ties of thiophene oligomers is the rationale underlying the
present work, which describes the synthesis and the optical
properties of the three sexithiophenes depicted in Fig. 1.
1
±3
optical applications. Sexithiophene is one of the best
organic semiconducting materials for the fabrication of
4
thin ®lm ®eld effect transistors :FETs). It has been demon-
strated that high purity and high molecular ordering in the
®
5
lm are essential to obtain high charge mobilities. The
highest charge mobilities, comparable to those of
amorphous silicon, have been obtained by using single
crystals for FET fabrication. Single crystals represent the
6
Figure 1. Molecular structure of the title compounds.
upper limit attainable in terms of purity and molecular
ordering for organic molecules. However, preparing single
crystals of thiophene oligomers of a suf®cient size for
device application is a very dif®cult task. Thus, an ambitious
goal is to take advantage of spontaneous self-assembly
properties of these oligomers and obtain in thin ®lms a
degree of molecular ordering comparable to that reached
in single crystals.
The ¯uorescence spectra of all compounds in solution and in
thin ®lms were investigated in order to get some insight into
their conformation and self-assembly modalities.
1. Results and discussion
Achieving this objective is hindered by synthetic dif®cul-
ties, solubility problems and by the ultrapurity standards
required for applications in microelectronics.
The synthesis of functionalized thiophene oligomers is
generally carried out by reacting thienyl bromides with
thienylstannanes or organoboron derivativesÐStille and
8
,9
1
0,11
Unsubstituted sexithiophene is scarcely soluble and dif®cult
Suzuki±Miyaura reaction,
respectivelyÐin the
4
,5
to obtain in a pure form. Solubility can be achieved by
inserting appropriate substituents into the aromatic skeleton
of sexithiophene but the substituents modify the self-
organization properties of the molecule in ways that are
presence of palladium:0) catalysts. Both reactions are
greatly dependent on the steric and electronic characteristics
of reagents and catalysts and require careful adjustment of
the experimental conditions case by case.
7
dif®cult to predict.
We have demonstrated that sexithiophene 4 is characterized
by high FET mobility. However, so far, we have only been
7
The search for structural elements helping solubility and
processability yet not altering the self-organization proper-
able to obtain this compound in low yield via the Stille
coupling. Given its good electrical characteristics, we
7
were then checking alternative ways to synthesize this
compound in better yields.
Keywordsꢀ microwave heating; thiophenes; optical properties.
Corresponding author; e-mailꢀ barbarella@area.bo.cnr.it
p
0040±4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PIIꢀ S0040-4020:02)00098-4

2246
G. Sotgiu et al. / Tetrahedron 58 +2002) 2245±2251
Scheme 1.
In the last few years, the use of microwaves has been found
to be expedient for a great variety of organic reactions both
in solution or in the absence of solvents.
2-thiophene tributylstannyl using experimental conditions
typical of the Stille reaction.
1
2±15
In the ®rst case we succeeded in obtaining tetramer 2 in
the same yield :70%) by reacting 1 for a few hours with
2-thiophene boronic acid in re¯uxing tetrahydrofuran,
K CO and Pd:PPh ) . However, several attempts to react
Scheme 1 summarizes the synthetic method followed to
prepare sexithiophene 4 in good yield under mild conditions
through the use of microwaves. The yields reported in the
scheme are relative to isolated products after chromato-
graphy.
2
3
3 4
dibromo 3 with 2-thiophene boronic acid using the same
experimental conditions failed. Only traces of the desired
hexamer were detected by mass spectrometry and most of
the product recovered was made of polymeric chains.
Nevertheless we were able to identify :by mass
0
bithiophene 1 was coupled to 2-thiophene boronic acid
0
0
As shown in Scheme 1, 2,5 -dibromo-3,3 -dimethyl-2,2 -
7
1
under the action of microwaves using H O/toluene as the
2
spectrometry and H NMR) the formation of a dibromo-
solvent, a quaternary salt :Bu NBr), K CO and Pd:OAc)
2
octathiophene, a monobromo nonamer and of a quater-
thiophene having B:OH)2 groups at both the terminal
positions.
4
2
3
as the catalyst. The reaction afforded quaterthiophene 2 in
0% yield within a reaction time not exceeding 5 min.
Besides 2, only the starting material was present in the
7
reaction mixture, making the puri®cation procedures
straightforward. Bromination of
The formation of polymeric chains indicates that in the
Suzuki reaction metal±halogen exchange may become
competitive with the cross-coupling when the size of the
oligomer increases. By contrast, the fact that the use of
microwaves led to the formation of 4 in high yield suggests
that the use of microwaves causes a large acceleration of the
cross-coupling reaction with respect to that of metal±
halogen exchange.
2
with N-bromo-
succinimide afforded dibromo quaterthiophene 3 in high
yield. Compound 3 was then reacted with 2-thiophene
boronic acid in the same conditions as 1 under microwave
action. The reaction afforded sexithiophene 4 in high yield.
In this case, besides the starting material, the reaction
mixture also contained some monobromo quinquethiophene
that did not react further with 2-thiophene boronic acid even
on changing reaction conditions and times. In consequence,
for 4 the puri®cation procedures were slightly more
elaborate than for 2 and the yields of isolated product
lower :65%).
The results of the Stille coupling of dibromo 1 with
2-tributylstannyl thiophene are reported in Scheme 2. It is
seen that in this case the yield of quaterthiophene 2 is low
and there is the formation of longer oligomers containing
0
repeated 3,3-dimethyl-2,2 -bithiophene subunits that we
These results show that the use of microwaves is very
ef®cient in the synthesis of sexithiophene 4 via Suzuki
coupling under mild conditions. The reaction occurs in
very short times, with high yields and can be carried out
using the same experimental conditions for the preparation
of both tetramer 2 and hexamer 4.
were able to isolate and purify. Several attempts to improve
the yield of this reaction in quaterthiophene 2 by use of
microwaves were unsuccessful but we cannot exclude that
this is only due to our inability to ®nd the appropriate
conditions.
The formation of oligomers 5 and 6 containing repeating
0
For comparison, and with the aim of achieving a better
understanding of the reactions we were dealing with, we
reacted dibromo 1 in the absence of microwavesꢀ :a) with
2-thiophene boronic acid using experimental conditions
typical of the Suzuki±Miyaura reaction and :b) with
3,3-dimethyl-2,2 -bithiophene subunits can be explained by
assuming that the metal±halogen exchange reaction and the
subsequent coupling of the metallated±halogenated inter-
mediates :Scheme 3) are competitive with the cross-
coupling reaction leading to the desired quaterthiophene 2.
Scheme 2.

G. Sotgiu et al. / Tetrahedron 58 +2002) 2245±2251
2247
Scheme 3.
It is reasonable to assume that the degree of competition
between the halogen±metal exchange reaction and the
cross-coupling reaction depends on the reactivity of the
brominated±metallated intermediate species that is formed,
which in turn should depend on the oligomer size and on the
metal used. A better knowledge of these aspects of the Stille
and Suzuki reactions should make it possible to pilot these
reactions in the direction of the exclusive :or near so)
formation of short or, in alternative, of long oligomers or
polymers.
The crude reaction products contained large amounts of
byproducts which were separated from the desired hexamers
by silica gel chromatography. The reaction yields of isolated
16 and 17 were in consequence rather low, 25 and 16%,
respectively. Again we ascribed the low reaction yields to
the metal±halogen exchange reaction leading to a mixture
of longer oligomers, for which we had spectroscopic
evidence but that we did not try to isolate and purify.
The absorption and photoluminescence spectra of
compounds 4, 16, 17 in THF are shown in Fig. 2.
The synthesis of sexithiophenes 16 and 17 was carried out
via conventional Stille coupling according to the method
shown in Scheme 4.
The maximum wavelength absorptions :lmax) of 4, 16, 17
are 390, 400 and 410 nm, respectively. These values are
markedly blue shifted with respect to the l
of unsubsti-
max
0
diiodo-3,3 dimethyl-2,2 -bithiophene 14 with the appro-
17
Compound 16 was synthesized through the coupling of 5,5 -
tuted sexithiophene :432 nm ) and are explained by the
lower p±p overlap associated with the presence of the
methyl groups, i.e. of a head-to-head `rotational defect'.
0
0
priate stannanes in the presence of Pd:AsPh ) prepared
3
4
1
in situ, while product 17 was obtained using 5,5 -bis-
6
0
0
0
:
tributylstannyl)-3,3 dimethyl-2,2 -bithiophene 15 with the
The photoluminescence wavelengths :l ) of 4, 16, 17 upon
PL
corresponding monobromo derivative under the same
conditions.
UV irradiation :lexcˆ370 nm) are in the range 500±510 nm
with Stokes shifts between absorption and emission
Scheme 4.

2248
G. Sotgiu et al. / Tetrahedron 58 +2002) 2245±2251
25
Figure 2. Absorption and photoluminescence spectra of sexithiophenes 4 :triangle), 16 :open circle) and 17 :solid circle), 10 M in THF. Absorbances are
measured for 1 cm optical path cells.
wavelengths amounting to more than 100 nm. These values
are similar to that measured for unsubstituted sexithiophene,
geometry and to the supramolecular structure. In unsubsti-
tuted thiophene oligomers, planar conformations and strong
intermolecular interactions in the solid state lead to photo-
1
8,19
for which a l ˆ508nm has been reported.
It is worth
PL
2
3
noting that while the absorption wavelengths are markedly
blue shifted with respect to that of unsubstituted sexi-
thiophene, the emission wavelengths are nearly the same,
suggesting that in spite of the head-to-head junction
luminescence quenching.
0
Recently, high level ab initio calculations of 3,3 -dimethyl-
2
4
2,2-bithiophene have been reported. According to the
calculations, the rotational pro®le is very ¯at indicating
the existence of a large number of equienergetic conforma-
tions with slightly different inter-ring rotation angles.
1
8±20
the excited state is planar. In agreement with this,
the
emission bands show some vibronic structure while the
absorption bands are shapeless.
The ¯uorescence quantum yields :QY, namely the ratio
between absorbed and emitted photons under UV irradia-
tion), in chloroform were 33, 42 and 34% for 4, 16, 17,
respectively. For unsubstituted sexithiophene, a QY value
It is probably the existence of a large family of rotational
conformers that explains the great solubility of these
compounds. The large entropy increase in solution
compared to the solid state together with the lack of
formation of aggregates for steric reasons explains why
sexithiophene 16 is very soluble while its non-methylated
1
8,20
of 40±42% in solution has been reported.
2
2
After deposition as thin ®lms by spin-coating from chloro-
form, the ¯uorescence QY of 4, 16, 17 dropped to 1±2%, i.e.
to values more than one order of magnitude smaller than
those in solution. Once again, this behaviour is in agreement
with what has been observed for unsubstituted thiophene
counterpartÐnamely :a,v-dihexyl sexithiophene, is even
less soluble than unsubstituted sexithiophene itself.
2
0
oligomers and indicates that in the solid state, the
molecules assume a planar conformation despite the head-
to-head `rotational' defect and give rise to strong
intermolecular interactions.
2. Conclusions
The remarkable improvement obtained in the synthesis of
sexithiophene 4 shows that there is room for developing
microwave-mediated synthetic methodologies leading to
the preparation of long thiophene oligomers rapidly and in
high yields. The comparison of the results obtained by the
use of microwaves and those obtained with conventional
heating suggests that the microwaves cause the acceleration
of the cross-coupling reaction compared to the metal±
halogen exchange reaction which is at the origin of the
formation of a variety of undesired products.
This conclusion is strongly supported by the single crystal
X-ray structure of sexithiophene 4 showing that the inner
head±head groups are in the trans-orientation and the
molecules have a planar conformation favoured by packing
7
forces. Measurements of the absolute QY of single crystals
2
1
of 4 using an integrated sphere gave a value of 3%. The
fact that in thin ®lm, a very similar QY value was measured
indicates that the arrangement of 4 is of the same type as that
that in the single crystal. By analogy, the same is likely to be
true for 16 and 17, the planarization process in this case
Finally, we have shown that the presence of one inner
dimethyl head-to-head `rotational defect' into the skeleton
of sexithiophene makes it very soluble and does not
prevent a planar arrangement in the solid state, which
leads to strong intermolecular interactions and ¯uorescence
quenching.
2
2
being assisted by the long terminal alkyl chains. It is
known that in the solid state, strong intermolecular inter-
actions increase the probability of non-radiative decay and
that these processes are strictly correlated to the molecular

G. Sotgiu et al. / Tetrahedron 58 +2002) 2245±2251
2249
0
00
0
phene, 3, 0.056 g :0.44 mmol) of commercial 2-thiophene
00
0
0
00 00 000
3
. Experimental
5,5 -dibromo-4 ,3 -dimethyl-2,2 ꢀ5 ,2 ꢀ5 ,2 -quaterthio-
3
.1. General information
boronic acid, 0.131 g :0.95 mmol) of K
:0.19 mmol) of Bu NBr and 0.22 mL of water in the
CO , 0.064 g
2 3
4
2
acid, K CO , Pd dba , AsPh , Pd:OAc) , 2,2 -bithiophene,
-Tributylstannylthiophene, Pd:PPh ) , 2-thiophene boronic
0
presence of 2 mg :0.01 mmol) of Pd:OAc) . The reaction
2
3
4
mixture was stirred and was carried out under microwave
irradiation for 5 min at 25 W. After usual workup the
residue was chromatographed on silica gel with hexane as
eluent to give 0.067 g :yield 67%) of hexamer 4. Elemental
analysis, melting point, lmax, mass spectrum, NMR were in
agreement with those reported in Ref. 7.
2
3
2
3
3
2
hexyl bromide, dodecyl bromide, butyllithium, tributyltin
chloride, sulfur, potassium tert-butoxide, N-iodosuccini-
mide were available from Aldrich Chemical Co. Tetrabutyl-
ammonium bromide and N-bromosuccinimide were
obtained from Carlo Erba and Lancaster, respectively.
Organic solvents were dried by standard procedures. Flash
chromatographies were carried out using silica gel
0
00 000 0000 0000 0 0000 00
0
2 :5 ,2 octithiophene, 6.
0
00 00 000
3.1.4. 4 ,3 ,4 ,3 ,4 ,3 -Hexamethyl-2,2 :5 ,2 :5 ,2 :
0
00 0000 0000 0000 0 0000 0 0000 00 0000 00 0000 000
:
4
:
200±300 mesh ASTM) or LiChroprep RP-8and RP-18
0±63 mm :Merck). Analytical thin layer chromatographies
TLCs) with 0.2 mm silica gel plates :Merck). Stille and
5 ,2 :5 ,2 :5 A
40 mL toluene solution containing 85 mg :0.082 mmol) of
3
Pd dba and 202 mg :0.66 mmol) of AsPh was re¯uxed for
2 3 3
0
dimethyl-2,2 -bithiophene :4) dissolved in 30 mL of
0
Suzuki reactions with conventional heating were carried
out under nitrogen. Microwave assisted reactions were
carried out under air and they were performed using a
commercial system Synthewave 402 manufactured by
Prolabo.
10 min. Then 2.90 g :8.24 mmol) of 5,5 -dibromo-3,3 -
0
toluene was added. 5.50 mL :17.30 mmol) of commercial
2-tributylstannylthiophene was added dropwise to the
re¯uxed mixture. After 45 h the reaction mixture was
evaporated, and the residue was chromatographed on silica
gel with hexane as eluent to give 1.15 g :39% yield) of
The characteristics of compounds 1±5, 9, 13 have already
been reported.
7
7
7
tetramer 2, 0.81 g :36% yield) of hexamer 5 and 0.71 g
24% yield) of octamer 6 as dark red microcrystalline solid.
Anal. calcd for 6, C38 ꢀ C, 61.42; H, 4.07. Foundꢀ C,
61.37; H, 4.09. MS m/e 742 :M ); lmax :CH
:
0
00
0
0
00 00 000
, via microwave assisted Suzuki method. A 20 mL
3
.1.1. 4 ,3 -Dimethyl-2,2 :5 ,2 :5 ,2 -quaterthiophene,
H S
30 8
z1
2
reactor for a microwave oven was charged with 0.1 g
2
Cl
) 383 nm;
2
1
mp 1648C. H NMR :CDCl
J
J
, TMS/ppm) d 7.22 :dd,
3
3
3
3
3
0
0
phene :1), 0.08g :0.62 mmol) of commercial 2-thiophene
0
:
0.28mmol) of 5,5 -dibromo-3,3 -dimethyl-2,2 -bithio-
HHˆ5.0 Hz, JHHˆ1.0 Hz, 2H), 7.16 :dd, JHHˆ3.5 Hz,
13
HHˆ1.0 Hz, 2H), 7.01 :m, 8H), 2.22 :m, 18H); C NMR
boronic acid, 3 mg :0.013 mmol) of Pd:OAc) , 0.193 g
2
:CDCl , TMS/ppm) d 137.4, 137.3, 137.2, 136.7, 136.4,
128.2, 128.1, 127.8, 126.8, 126.6, 124.4, 123.6, 15.04,
15.01.
3
:
1.4 mmol) of K CO , 0.09 g :0.28mmol) of Bu NBr, and
2
3
4
0
.31 mL of water in 5 mL of toluene as the solvent. The
reaction mixture was stirred and then irradiated for 5 min
at 25 W. After cooling to room temperature the mixture was
quenched with 1 mL of 2 M HCl, washed twice with brine,
0
0
3.1.5. 5-Hexyl-2,2 -bithiophene, 7. To a solution of 2,2 -
bithiophene :2.07 g, 0.012 mol) in THF :50 mL) was added
a 2.5 M solution of n-BuLi :5.0 mL, 0.012 mol) in hexane at
2708C. After 30 min, the mixture was heated to 2158C and
1-bromohexane :2.06 g, 0.012 mol) was added. The mixture
was heated at room temperature and stirred for 2 h. Then it
dried with Na SO4 and evaporated. The residue was
2
chromatographed on silica gel with petroleum ether/
CH Cl 95ꢀ5 as eluent and 0.072 g :72% yield) of 5 as
2
2
yellow microcrystalline solid was recovered. Elemental
analysis, melting point, lmax, mass spectrum, NMR were
in agreement with those reported in Ref. 7.
was hydrolyzed with a saturated solution of NH Cl and
4
extracted with diethyl ether. The organic layer was dried
and evaporated, the residue was chromatographed on RP-
0
, via conventional Suzuki method. A mixture of 0.1 g
00
0
0
00 00 000
3
2
:
.1.2. 4 ,3 -Dimethyl-2,2 :5 ,2 :5 ,2 -quaterthiophene,
18silica gel column with MeOH/CH
Cl :9ꢀ1) as eluent to
2 2
z1
give 1.96 g :63%) of a yellow oil. MS m/e 250 :M ); lmax
1
0
0
0
phene :1), 0.08g :0.62 mmol) of commercial 2-thiophene
0.28mmol) of 5,5 -dibromo-3,3 -dimethyl-2,2 -bithio-
:CH Cl ) 324 nm; H NMR :CDCl , TMS/ppm) d 7.19 :m,
2 2 3
3
2H), 7.04 :m, 2H), 6.73 :m, 1H), 2.85 :t, 2H, JHHˆ7.5 Hz),
3
13
boronic acid, 0.014 mmol of commercial Pd:PPh ) in 5 mL
3
1.75 :m, 2H), 1.41 :m, 6H), 0.98:t, 3H, JHHˆ5.5 Hz);
C
4
of dry THF was re¯uxed for 0.5 h. Then 0.31 mL of H O
2
NMR :CDCl , TMS/ppm) d 145.2, 137.9, 134.7, 127.6,
3
and 0.193 g :1.4 mmol) of K CO were added to the
2
124.6, 123.6, 123.3, 122.9, 31.5, 30.1, 28.7, 22.5, 14.0.
Proton and carbon-13 NMR spectra of 7 are given as supple-
mentary material.
3
reaction mixture. The reaction was followed by thin layer
chromatography :TLC) using petroleum ether/CH Cl 95ꢀ5
2
2
as the eluent. The mixture was re¯uxed for 3 h, and
quenched with 1 mL of 2 M HCl. After the usual workup
the residue was chromatographed on silica gel using the
same eluent. A total of 0.071 g :71% yield) of 5 was
recovered. Elemental analysis, melting point, lmax, mass
spectrum, NMR were in agreement with those reported in
Ref. 7.
0
a solution of 1.82 g :7.3 mmol) of 5-hexyl-2,2 -bithiophene
in 40 mL of dry ether at room temperature was added a
2.5 M solution of BuLi in hexane :3.2 mL, 8.0 mmol).
After 30 min, 2.60 g :8.0 mmol) of tributyltin chloride
was added dropwise. The reaction was stirred for 2 h,
hydrolyzed with water, extracted with ether, dried over
0
3.1.6. 5-Tributylstannyl-5 -hexyl-2,2 -bithiophene, 8. To
0
0
0
000
sexithiophene, 4, via microwave assisted Suzuki method.
0
0
00 00 000 000 0000 0000 0000 0
3
.1.3.
4 ,3 -Dimethyl-2,2 :5 ,2 :5 ,2 :5 ,2 :5 ,2 -
Na
as a yellow oil was isolated. MS m/e 539 :M ); H NMR
SO and evaporated. A total of 3.68g :94%) of product
2 4
z1
1
3
A 5 mL toluene solution containing 0.1 g :0.19 mmol) of
:CDCl
3
, TMS/ppm) d 7.27 :d, JHHˆ3.5 Hz, 1H), 7.11 :d,

2
3
250
G. Sotgiu et al. / Tetrahedron 58 +2002) 2245±2251
3
JHHˆ3.5 Hz, 1H), 7.02 :d, J ˆ3.5 Hz, 1H), 6.72 :d,
of NIS at 2208C. Then the mixture was stirred at room
temperature overnight and treated with 10% solution of
KOH :1£20 mL) and water :1£20 mL). After usual workup
the residue was chromatographed on silica gel with pentane
as eluent to give 4.37 g :yield 98%) of a white powderꢀ mp
HH
3
3
JHHˆ3.5 Hz, 1H), 2.82 :t, J ˆ7.5 Hz, 2H), 1.68:m,
HH
1
3
H8 ), 1.40 :m, 12H), 1.18:m, 6H), 0.98:m, 12H); C
NMR :CDCl , TMS/ppm) d 145.5, 143.8, 136.6, 136.3,
3
1
2
35.5, 125.2, 124.6, 123.5, 32.1, 30.6, 29.5, 29.3, 27.8,
3.1, 14.4, 14.0, 11.3. Proton and carbon-13 NMR spectra
z1
1
94±968C. MS m/e 446 :M ); H NMR :CDCl , TMS/ppm)
3
d 7.07 :s, 2H), 2.14 :s, 6H); C NMR :CDCl , TMS/ppm) d
3
1
3
of 8 are given as supplementary material.
1
39.8, 138.8, 134.1, 73.6.
0
tion of 5-bromo-2,2 -bithiophene :5.22 g, 0.021 mol) in dry
3
.1.7. 5-Dodecylsulfanyl-2,2 -bithiophene, 11. To a solu-
0
ether :30 mL) was added a 2.5 M solution of n-BuLi
0
0
0
3.1.10.
5,5 -Bisꢀtributylstannyl)-3,3 -dimethyl-2,2 -bi-
0
0
thiophene, 15. To a solution of 3,3 -dimethyl-2,2 -bithio-
phene :1.94 g, 0.01 mol) in dry ether :20 mL) at room
temperature was added a 2.5 M solution of BuLi in hexane
:8mL, 0.02 mol). After 1 h, 6.50 g :0.02 mol) of tributyltin
chloride was added dropwise. The reaction mixture was
stirred 2 h, hydrolyzed with water, extracted with diethyl
:
8.4 mL, 0.021 mol) in hexane at 2708C. After 20 min,
.68g :0.021 mol) of sublimed sulfur was added and the
mixture was stirred at 2708C for 2 h, then the reaction
0
mixture was hydrolyzed with a 1 M solution of NaOH
:
120 mL). The aqueous layer was acidi®ed with a 2 M solu-
tion of HCl and extracted with ether :3£50 mL). The
ether, dried over Na SO and evaporated. The crude
2 4
organic layer was dried over Na SO and evaporated to
2
compound was chromatographed on silica gel column
with petroleum ether as eluent to give 6.35 g :yield 82 %)
4
give 3.80 g :91%) of a red brown oil identi®ed as being
0
z1
-mercapto-2,2 -bithiophene :10). MS m/e 198:M ); H
1
z1
of a yellow oil. MS m/e 774 :M ); H NMR :acetone-d
1
5
NMR :CDCl , TMS/ppm) d 7.24 :m, 2H), 7.06 :m, 3H),
,
6
TMS/ppm) d 7.03 :s, 2H), 2.18:s, 6H), 1.61 :m, 12H), 1.32
3
1
3
.55 :s, 1H); C NMR :CDCl , TMS/ppm) d 144.1, 140.6,
36.7, 136.6, 134.6, 127.8, 124.6, 123.9. Proton and carbon-
3 NMR spectra of 10 are given as supplementary material.
13
3
1
1
:m, 12H), 1.12 :m, 12H), 0.89 :m, 18H); C NMR :acetone-
, TMS/ppm) d 143.5, 139.4, 137.4, 128.5, 29.6, 27.7, 14.3,
13.8, 11.3.
3
d
6
To a solution of t-BuOK :2.60 g, 0.023 mol) in ethyl alcohol
10 mL) was added dropwise a solution of 10 :2.97 g,
.015 mol) in CH Cl :7 mL) at 08C. After 20 min, 3.73 g
:0.015 mol) of dodecyl bromide was added. Then the reac-
tion mixture was re¯uxed for 2 h and treated by water
0000 0 00 000 0 0 00 00 000
3.1.11. 5,5 -Dihexyl-4 ,3 -dimethyl-2,2 :5 ,2 :5 ,2 :
:
0
000 0000 0000 0000 0
5 ,2 :5 ,2 -sexithiophene, 16. To a 20 mL toluene
solution containing 0.03 mmol of :Ph As) Pd prepared in
2
2
3
4
situ was added 0.45 g :1 mmol) of 14 and 1.08g :2 mmol)
of 8. The mixture was re¯uxed for 15 h. After evaporation
of toluene the residue was chromatographed on RP-18silica
:
3£30 mL) and extracted with CH Cl :2£30 mL), dried
2
2
and evaporated. The crude product was chromatographed
on RP-8silica gel column with hexane/CH Cl :9ꢀ1) as
eluent to give 3.73 g :68%) of a deep yellow oil. MS m/e
3
7
gel column with CH
172 mg :yield 25 %) of a deep red polycrystalline solidꢀ
Cl /MeOH :6ꢀ4) as eluent to give
2 2
2
2
z1
1
z1
66 :M ); H NMR :CDCl , TMS/ppm) d 7.21 :m, 1H),
3
mp 123±1248C. MS m/e 690 :M ); lmax :CH
414 nm; H NMR :CDCl
3
2
Cl
, TMS/ppm) d 7.00 :m, 8H),
6.68:m, 2H), 2.79 :t, 4H), 2.21 :s, 6H), 1.68:m, 4H),
2
)
3
1
.13 :m, 1H), 7.01 :m, 3H), 2.77 :t, J ˆ7.5 Hz, 2H), 1.60
HH
1
m, 2H), 1.25 :m, 18H), 0.95 :m, 3H); C NMR :CDCl ,
3
:
TMS/ppm) d 140.4, 137.1, 134.0, 133.9, 127.8, 124.5,
3
1
3
1.33 :m, 12H), 0.89 :m, 6H); C NMR :CDCl , TMS/
3
1
2
23.8, 38.9, 31.9, 29.6, 29.57, 29.5, 29.4, 29.3, 29.1, 28.4,
2.7, 14.1. Proton and carbon-13 NMR spectra of 11 are
ppm) d 145.7, 137.4, 136.9, 136.5, 135.2, 134.5, 128.0,
126.5, 124.8, 124.1, 123.6, 123.4, 31.6, 30.2, 28.7, 22.6,
given as supplementary material.
15.0, 14.1. Anal. calcd for C38
Foundꢀ C, 65.87; H, 6.11.
H S ꢀ C, 66.04; H, 6.13.
42 6
0
solution of 5-dodecylsulfanyl-2,2 -bithiophene
0
3
To
.1.8. 5-Bromo-5 -dodecylsulfanyl-2,2 -bithiophene, 12.
0000 0 00 000 0
3.1.12. 5,5 -Bisꢀdodecylsulfanyl)-4 ,3 -dimethyl-2,2 :
0
a
0
0 mL toluene solution containing 0.045 mmol of
00 00 000 000 0000 0000 0000 00
5
2
,2 :5 ,2 :5 ,2 :5 ,2 -sexithiophene, 17. To
a
:
1
2.20 g, 0.006 mol) in DMF :50 mL) was added stepwise
.10 g :0.006 mol) of NBS at room temperature. Then the
mixture was stirred for 2 h and treated with ice. The organic
:Ph As) Pd prepared in situ was added 1.33 g :3 mmol) of
3 4
12 and 1.16 g :1.5 mmol) of 15. The mixture was re¯uxed
compound was extracted with CH Cl :1£30 mL) and
2
2
for 16 h. After evaporation of toluene the residue was chro-
matographed on silica gel column with cyclohexane/CH Cl
washed with saturated solution of NaCl :3£30 mL). The
organic layer was dried and evaporated and the residue
was chromatographed on silica gel using cyclohexane/
CH Cl :9ꢀ1) as eluent. The product was isolated as a yellow
2
2
:9ꢀ1) as eluent. A total of 221 mg :yield 16%) of pure 17 as a
deep red polycrystalline solidꢀ mp 103±1058C. MS m/e 922
2
2
z1
M ); l :CH Cl ) 400 nm; H NMR :CDCl , TMS/ppm)
1
z1
1
:
d 7.03 :m, 8H), 7.01 :s, 2H), 2.82 :t, 4H), 2.22 :s, 6H), 1.62
oil :2.40 g, yield 90%). MS m/e 445 :M ); H NMR
:
max 2 2 3
3
CDCl , TMS/ppm) d 6.97 :d, J ˆ3.8Hz, 1H), 6.94 :d,
3
HH
1
m, 4H), 1.30 :m, 36H), 0.88 :m, 6H); C NMR :CDCl3,
3
3
3
:
TMS/ppm) d 140.0, 137.5, 136.3, 136.1, 135.9, 134.5,
JHHˆ3.6 Hz, 1H), 6.92 :d, J ˆ3.8Hz, 1H), 6. 85 :d,
HH
3
3
JHHˆ3.6 Hz, 1H), 2.75 :t, J ˆ7.5 Hz, 2H), 1.58:m,
HH
1
3
1
2
33.9, 128.3, 126.8, 124.4, 124.2, 123.7, 39.0, 31.9, 29.64,
9.58, 29.5, 29.4, 29.3, 29.1, 28.4, 22.7, 15.0, 14.1. Anal.
2
TMS/ppm) d 139.1, 138.6, 135.1, 133.7, 130.6, 124.0,
H), 1.25 :m, 18H), 0.85 :m, 3H); C NMR :CDCl ,
3
calcd for C H S ꢀ C, 65.02; H, 7.20. Foundꢀ C, 64.93; H,
50 66 8
1
2
23.8, 111.2, 38.9, 31.9, 29.6, 29.56, 29.5, 29.4, 29.3,
9.1, 28.4, 22.7, 14.1.
7.22.
0
0
0
3
a
0
.1.9. 5,5 -Diiodo-3,3 -dimethyl-2,2 -bithiophene, 14. To
0
3.2. Optical measurements
0
solution of 3,3 -dimethyl-2,2 -bithiophene :1.94 g,
.01 mol) in CH Cl :30 mL) was added 4.5 g :0.02 mol)
2
2
Photoluminescence measurements were performed with a

G. Sotgiu et al. / Tetrahedron 58 +2002) 2245±2251
2251
SPEX 270 M polychromator equipped with a cooled CCD
detector by exciting the sample with a monochromated Xe
6. Sch oÈ n, J. H.; Kloch, C.; Laudise, R. A.; Batlogg, B. Appl.
Phys. Lett. 1998, 73, 3574.
2
5
lamp. PL QY were determined for 10 M solutions in dry
THF by using quinine sulfate monohydrate as the
7. Barbarella, G.; Zambianchi, M.; Antolini, L.; Ostoja, P.;
Maccagnani, P.; Bongini, A.; Marseglia, E. A.; Tedesco, E.;
Gigli, G.; Cingolani, R. J. Am. Chem. Soc. 1999, 121, 8920.
8. Stille, J. K. Angew. Chem. Int. Ed. Engl. 1986, 25, 508.
2
4,25
reference, exciting at 370 nm. Absorption spectra were
obtained with a Lambda 20 UV/vis spectrometer.
9
. Farina, V.; Krishnamurthy, V.; Scott, W. J. The Stille
Reaction; Wileyꢀ Chichester, 1998.
0. Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.
3
.3. Supplementary material
1
0
phene :7); 5-tributylstannyl-5 -hexyl-2,2 -bithiophene :8);
11. Suzuki, A. J. Organomet. Chem. 1999, 576, 147.
12. Caddick, S. Tetrahedron 1995, 38, 10403±10432.
13. Badone, D.; Baroni, M.; Cardamone, R.; Ielmini, A.; Guzzi,
U. J. Org. Chem. 1997, 62, 7170.
Proton and carbon-13 NMR spectra of 5-hexyl-2,2 -bithio-
0
-mercapto-2,2 -bithiophene :10); 5-dodecylsulfanyl-2,2 -
0
0
bithiophene :11).
0
5
1
4. Blettner, C. G.; K oÈ nig, W. A.; Stenzel, W.; Schotten, T.
J. Org. Chem. 1999, 64, 3885.
1
5. Larhed, M.; Hallberg, A. J. Org. Chem. 1996, 61, 9582.
6. Barbarella, G.; Zambianchi, M.; Sotgiu, G.; Bongini, A.
Tetrahedron 1997, 53, 9401.
Acknowledgements
The present work was partially supported by the project
1
1
7. Van Pham, C.; Burkhardt, A.; Shabana, R.; Cunningham,
D. D.; Mark Jr., H. B.; Zimmer, H. Phosphorus Sulfur Silicon
`
:
Nuovi emettitori di luce a semiconduttore organico'
CNR-5% Nanotecnologie).
1
989, 46, 153.
1
1
2
8. Chosrovian, H.; Rentsch, S.; Grebner, D.; Dahm, U.; Birckner,
E. Synth. Met. 1993, 60, 23.
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