Synthesis,1 Hand 13C NMR assignment and electrochemical properties of novel thiophene-thiazolothiazole oligomers and polymers

Article abstract of DOI:10.1002/mrc.2593

Novel hexyl-substituted bisthiophene compounds containing a thiazolothiazole(5,4-d) unit have been explored. The molecules are soluble in common organic solvents, which would enhance their chance of possible integration in printable electronics. Synthesis and complete elucidation of the chemical structures by detailed 1D/2D NMR spectroscopy are described.This provides interesting input for chemical shift prediction software, because few experimental data on this type of compounds are available. Furthermore, the potential n-type character of these derivatives is verified using electrochemical measurements. In addition, the low-bandgap character of conjugated polymers containing the thiazolothiazole unit is demonstrated by performing an electropolymerization.

Full text of DOI:10.1002/mrc.2593

Research Article
Received: 14 January 2010
Revised: 15 February 2010
Accepted: 15 February 2010
Published online in Wiley Interscience: 18 March 2010
(www.interscience.com) DOI 10.1002/mrc.2593
Synthesis, 1H and 13C NMR assignment
and electrochemical properties of novel
thiophene–thiazolothiazole oligomers
and polymers
S. Van Mierloo,a S. Chambon,a A. E. Boyukbayram,a,b P. Adriaensens,a
L. Lutsen,c T. J. Cleija and D. Vanderzandea∗
Novel hexyl-substituted bisthiophene compounds containing a thiazolothiazole(5,4-d) unit have been explored. The molecules
are soluble in common organic solvents, which would enhance their chance of possible integration in printable electronics.
Synthesisandcompleteelucidationofthechemicalstructuresbydetailed1D/2DNMR spectroscopyaredescribed. Thisprovides
interestinginputforchemicalshiftpredictionsoftware, becausefewexperimentaldataonthistypeofcompoundsareavailable.
Furthermore, the potential n-type character of these derivatives is verified using electrochemical measurements. In addition,
the low-bandgap character of conjugated polymers containing the thiazolothiazole unit is demonstrated by performing an
c
electropolymerization. Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Keywords: NMR; 1H; 13C; thiazolothiazole derivatives; chemical shift; prediction software; electropolymerization
Introduction
solubility in common organic solvents. Hence, functionalization,
purification and characterization become more straightforward
and the molecules can be utilized in solvent-based processing,
such as printable electronics. Moreover, the alkyl chains might
improve stacking properties and, consequently, positively impact
the charge mobilities in OFETs.
In this paper, we report the synthesis, detailed NMR struc-
tural elucidation and electrochemical properties of the first series
of novel alkylthiophenylsubstituted thiazolothiazole oligomers.
Cyclic voltammetry (CV) was used to verify the potential n-type
character of these compounds and to estimate their highest occu-
pied molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) energy levels. In addition, the molecules were
used as monomers in an electropolymerization to demonstrate
the low-bandgap character of conjugated polymers containing
thiazolothiazole units.
Organic field-effect transistors (OFETs)[1] based on organic
semiconductors attract considerable attention for applications
such as flexible displays, low-cost electronic paper and smart
memory/sensor elements. There are many low molecular weight
hole-transporting (p-type) semiconductors, such as pentacene[2]
and oligothiophenes.[3] However, the number of n-type organic
semiconductors is still limited and the FET performances are not
satisfactory. In the literature, a series of small n-type bisthiophenes
bearing a thiazolothiazole unit have been reported, which have
highchargecarriermobilities.[4] Thethiazolothiazoleunithassome
excellent characteristics for use in electronic applications. First of
all, itselectron-acceptingpropertyisusefultoenhancethestability
toward oxygen. Consequently, these compounds are stable in
ambient atmospheres at room temperature over a period of
severalmonths.[5] Secondly, thethiazolothiazolemoietyhasarigid
planar structure due to the fused ring system, which can lead to
efficient intermolecular π –π interactions.[4,6,7] Finally, the 2,5-bis-
substituted thiazolothiazole is quite straightforward to prepare,
starting from the corresponding aldehyde and dithiooxamide.
The interesting parent compound thiazolothiazole(5,4-d) has
already been synthesized many years ago in moderate yield, by
combininganaldehydederivativewithdithiooxamideatveryhigh
temperature.[8,9] Unfortunately, non-alkylsubstituted derivatives
are very poorly soluble and require vacuum deposition techniques
for device fabrication. Here, alternatively, a series of analogous
thiazolothiazole containing molecules are presented, which do
not have the above-mentioned disadvantages. To this end, the
synthesized molecules, i.e. D1, D2 and D3 (cf Fig. 1) have been
functionalizedwithtwosubstituted3-hexylthiophenes.Thechoice
of substitution with alkyl chains leads to a significantly improved
∗
Correspondence to: D. Vanderzande, Hasselt University, Institute for Materials
Research(IMO),UniversitaireCampus,BuildingD,B-3590Diepenbeek,Belgium.
E-mail: dirk.vanderzande@uhasselt.be
a
HasseltUniversity,InstituteforMaterialsResearch(IMO),UniversitaireCampus,
Building D, B-3590 Diepenbeek, Belgium
b
c
Department of Chemistry, Karabuk University, 78050 Karabuk, Turkey
IMEC, Division IMOMEC, Universitaire Campus, Wetenschapspark 1, B-3590
Diepenbeek, Belgium
c
Magn. Reson. Chem. 2010, 48, 362–369
Copyright ꢀ 2010 John Wiley & Sons, Ltd.

Novel thiophene-thiazolothiazole oligomers and polymers
NMR characterization
In order to confirm the structure of the three thiazolothiazole
derivatives(D1, D2andD3), theywerestudiedextensivelybyNMR
1
in order to perform a complete assignment of their 13C and H
resonances. Table 1presentsanoverviewofallsignalassignments.
Such a detailed analysis is advantageous, because the obtained
chemicalshiftinformationcanbeveryusefulforimplementationin
NMR-based prediction software. The chemical structures of all the
three compounds differ only at the 5-position of the thiophene
rings: D1 bears two hydrogen atoms, D2 two bromine atoms
and D3 two unsubstituted thiophene rings. Figure 1 shows the
structures of D1, D2 and D3, together with an arbitrary numbering
given to their different carbons and hydrogens. APT, DEPT, T1C and
short-range/long-range HETCOR experiments were performed on
these compounds in order to obtain a complete chemical shift
assignment of the 13C and 1H resonances.
Figure 1. Chemical structures of the 2,5-bis(3-hexylthiophen-2-yl)thiazolo
[5,4-d] thiazole derivatives D1, D2 and D3. The different carbon atoms are
numbered from C1 to C16 and the different hydrogen atoms are labeled
from H-a to H-k.
Assignment of the resonances of the core of D1 and D2
APT, DEPT and short-range HETCOR. The core of the derivatives
is formed by six different carbons, C1 to C6. For D2, it appears
that the 160.9, 150.7, 144.0, 134.0 and 116.1 ppm signals are
quaternary carbons and only the 134.1 ppm signal is a methine
carbon. The latter, therefore, corresponds to C5. The short-range
HETCOR experiment indicates that the 134.1 ppm carbon signal is
connected (1J or direct coupling) to a proton singulet resonance
at 6.93, corresponding to H-b.
Scheme 1. Synthetic route toward D2.
In D1, the 162.2, 150.7, 143.8 and 132.5 ppm signals arise from
non-protonatedcarbons,whereasthe131.5and128.0 ppmsignals
arise from CH-carbons. The short-range HETCOR indicates that the
131.5 ppm signal is attached to a 1H doublet at 6.96/6.98 ppm and
the 128 ppm signal to another 1H doublet at 7.32/7.34 ppm. Based
ontheresultsobtainedforD2,the1Hdoubletat6.96/6.98 ppmcan
be legitimately attributed to H-b and the 131.5 ppm 13C resonance
toC5.Bydeduction,the128 ppmresonancecanthenbeattributed
to C6 and the 1H doublet at 7.32/7.34 ppm to H-a. Moreover, this
assignment is confirmed by the resonance frequency of H-a.
Indeed the neighboring sulfur atom has a deshielding effect on
the proton H-a, leading to a resonance signal at lower field as
compared with H-b.
Results and Discussion
Synthesis
The first molecule, i.e. 2,5-bis(3-hexylthiophene-2-yl)thiazolo(5,4-
d)thiazole D1, was synthesized via a condensation reaction
between 3-hexylthiophene-2-carbaldehyde and dithiooxamide.
3-Hexylthiophene-2-carbaldehyde was prepared according to a
modified literature procedure.[5,10] Instead of iodine-activated
magnesiumturnings, n-butyllithiumwasusedtocreateananionat
the 2-thiophenylposition, followed by a nucleophilic addition on
DMF resulting in 3-hexylthiophene-2-carbaldehyde. The resulting
compound D1 was purified by column chromatography and
two consecutive recrystallizations from ethanol and acetonitrile.
D1 was obtained as yellow crystals in 47% yield. In order to
functionalize compound D1, a bromination reaction with NBS was
carried out in DMF, giving 2,5-bis(5-bromo-3-hexylthiophene-2-
yl)thiazolo(5,4-d)thiazole D2 in 71% yield (Scheme 1).
Long-range HETCOR. For both derivatives D1 and D2, the signals
at 162.2 and 150.7 ppm/160.9 and 150.7 ppm show no coupling
with protons. Those signals therefore can be attributed to C2 and
C1 (Table 1; videinfra) because both of them are at least four bonds
away from a hydrogen atom.[11]
Functionalization of D2 was readily achieved by using a
Stille cross coupling with tributyl(thiophen-2-yl)stannane under
catalytic conditions with Pd(PPh3)4 resulting in 2,5-bis(4-hexyl-
2,2ꢁ-bithiophene-5-yl)thiazolo(5,4-d)thiazole D3 (Scheme 2). This
product was also purified by column chromatography followed
by recrystallization from ethanol resulting in D3 in 58% yield. The
purity of D1 and D3 was further confirmed by HPLC as being
higher than 99%.
Due to the substitution with hexyl side chains, all three
compounds D1, D2 and D3 are soluble in common organic
solvents such as chloroform, dichloromethane and ethers. To
confirm the identity of the synthesized structures, complete
NMR characterization was performed. To this end, in the
following section, the complete structural elucidation of the three
synthesized compounds D1, D2 and D3 will be presented.
In D1, the remaining carbon signals (143.8 and 132.5 ppm)
shouldthereforecorrespondtoC3andC4.Thelong-rangeHETCOR
experiment shows that the 132.5 ppm signal correlates only with
the 6.96/6.98 ppm 1H doublet, whereas the 143.8 ppm resonance
1
is coupled to both the 7.32/7.34 ppm H doublet (H-a) and the
tripletcenteredat2.86 ppm,correspondingtoH-c.Thefactthatthe
143.8 ppm signal is coupled with H-a indicates that it corresponds
to C4. Indeed, C3 is to far (four bonds) from H-a to have a coupling
with it. Hence, by deduction the 132.5 ppm signal corresponds
to C3.
In D2, three quaternary carbon signals have a long-range
coupling(2J or3J)withprotons, i.e. 116.1 ppmwithH-b, 134.0 ppm
with H-b and 144.0 ppm with H-a and H-c. The latter cannot
correspond to C6 as it is four bonds away from H-c. By comparison
with its hydrogenated equivalent (D1), it is possible to attribute
c
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S. Van Mierloo et al.
Scheme 2. Synthetic route toward D3.
Table 1. Chemical shifts (ppm) and assignments of the proton and the carbon atoms in the thiazolothiazole derivatives D1–D3. The chemical shift
scales are calibrated to TMS at 0 ppm
D1
D2
D3
D1
D2
D3
Atom
R = H
R = Br
R = thiophen-2-yl
R = H
R = Br
R = thiophen-2-yl
Carbon
C1
Protons
–
150.7
162.2
132.5
143.8
131.5
128.0
30.8
30.7
30.0
32.3
23.3
14.8
–
150.7
160.9
134.0
144.0
134.1
116.1
30.9
30.4
30.0
32.3
23.3
14.8
–
150.7
161.5
131.0
144.5
127.6
137.3
31.1
–
–
C2
–
–
–
–
C3
–
–
C4
–
–
–
6.93 (s)
–
C5
H-b
H-a
H-c
H-d
H-e
H-f
H-g
H-h
6.97 (d)
7.33 (d)
2.95 (t)
1.7 (q)
1.42 (q)
1.3 (m)
1.3 (m)
0.88 (t)
–
7.00 (s)
–
C6
C7
2.84 (t)
1.66 (q)
1.42 (q)
1.32 (m)
1.32 (m)
0.89 (t)
–
2.88 (t)
1.71 (q)
1.45 (q)
1.34 (m)
1.34 (m)
0.90 (t)
–
C8
30.5
C9
30.1
C10
C11
C12
C13
C14
C15
C16
32.3
23.3
14.8
139.4
125.2
128.8
126.0
–
–
H-i
H-j
H-k
–
–
7.22 (dd)
7.02 (dd)
7.24 (dd)
–
–
–
–
–
–
–
–
144.0 ppm to C4 and 134.0 ppm to C3. By elimination, C6 in the
brominated sample has a resonance signal at 116.1 ppm.
Concerning the two remaining carbon signals of the core of D1,
one can observe that the resonance at 162.2 ppm shifts to 160.9
when the thiazolothiazole compound is brominated. However,
the other signal at 150.7 ppm remains unchanged. The latter can
be attributed to C1 as it is the carbon most remote from the
substituent on C6. Most conclusions can be confirmed by 13C
spin-lattice relaxation time (T1C) experiments (Table 2). Carbon
nuclei mainly relax through space via the surrounding magnetic
moments of protons (1/T1C has a 1/r6 dependency). These proton
magnetic moments induce local oscillating magnetic fields due to
molecularmotions;thesourceofT1 relaxation.[12] Asthe150.7 ppm
signal has the longest relaxation time, its corresponding carbon
atoms indeed should be the most remote from protons. C2, being
spatially more close to protons (e.g. H-c), therefore corresponds to
the resonance signal at 162.2 in D1 and 160.9 in D2. In addition,
the assignment of C3 and C4 in D1 can be confirmed by means of
the T1C relaxation decay times, being clearly longer (less efficient
relaxation) for C3. The same holds true for D2 sharing a decrease
in the T1C in going from C3 over C6 to C4.
Table 2. 13C spin-lattice relaxation decay times (T1C) for all carbon
signals of D1 and D3
D1
13C resonance
D3
13C resonance
frequency (ppm)
T1C (s)
frequency (ppm)
T1C (s)
162.2
150.7
143.8
132.5
131.5
128
14.2
21.1
6.6
161.5
150.7
144.5
139.4
137.3
131.0
128.8
127.6
126.05
125.2
32.3
6.7
9.3
3.3
15.9
0.66
0.72
2.01
0.74
1
4.0
6.1
7.8
32.3
0.63
0.40
0.41
0.63
1.67
0.50
0.71
1.00
2.32
3.12
30.8
30.7
30.0
1.3
23.3
2.9
14.8
3.2
31.1
30.5
30.1
23.3
Assignment of the resonances of the core of D3
14.8
The 1H spectrum presents four resonance patterns corresponding
to the four protons of the core: a singulet at 7.00 ppm and three
double doublets at 7.02 ppm (3J = 3.7 Hz, 3J = 5.1 Hz), 7.22 ppm
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Novel thiophene-thiazolothiazole oligomers and polymers
Figure 2. Long-range HETCOR (J = 8 Hz) of D3.
Figure 3. Short-range HETCOR (J = 140 Hz) of D3.
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S. Van Mierloo et al.
(3J = 3.7 Hz, 4J = 1.0 Hz) and 7.24 ppm (3J = 5.1 Hz, 4J = 1.0 Hz).
According to the J-coupling pattern and the literature data,[12]
these four signals correspond to H-b, H-j, H-i and H-k, respectively.
Concerning the assignment of the carbons, the assignments
for D1 and D2 already allow the assignment of the core carbon
signals at 161.5, 150.7, 144.5 and 131.0 ppm to C2, C1, C4 and C3,
respectively. From the remaining six carbons of the core, the ones
resonating at 137.3 and 139.4 ppm are quaternary carbons and
correspond to C6 and C13. The long-range HETCOR experiment
(Fig. 2) showed that the 139.4 ppm signal couples with H-b, H-i
and H-j, whereas the 137.3 ppm signal has only a small coupling
with H-i. This allows to assign the 139.4 ppm signal to C13 as
C6 is separated by four bonds from H-j. C6 then corresponds
to the signal at 137.3 ppm. This attribution is further confirmed
by the T1C relaxation which shows a longer decay time for the
137.3 ppm signal as compared with the 139.4 ppm one (6.1 and
4 s, respectively).
According to the short-range HETCOR (Fig. 3), the four remain-
ing signals at 128.8, 126.0, 125.2 and 127.6 ppm are respectively
coupled with the double doublets centered at 7.02 (H-j), 7.24 (H-k),
7.22 (H-i) and the singulet at 7.00 (H-b) ppm. They correspond to
C15, C16, C14 and C5, respectively.[13]
Figure 4. Cyclic voltammograms of D1 (solid: 1st scan; dashed: 2nd scan;
dotted: 3rd scan).
Table 3. HOMO–LUMO and optical/electrochemical energy gap
values of D1 and D3 as well as the corresponding electropolymerized
conjugated polymers P1 and P3
Alkyl side chain resonance assignment
OP
OP
Eg (eV)a
Eg (eV)a
The attribution of the carbon and proton signals is only described
for D1 as the same procedure holds for D2 and D3.
HOMO (eV) LUMO (eV) EgEC (eV) (solution) (thin film)
The chemical shift and J-coupling pattern of the 2.95 and
0.88 ppm signals indicate that they correspond to H-c and
H-h, respectively. The short-range HETCOR experiment already
attributed their corresponding carbon signals to 30.8 and
14.8 ppm, respectively.
D1
P1
D3
P3
−5.74
−5.26
−5.54
−5.25
−2.91
−3.43
−3.14
−3.51
2.83
1.83
2.40
1.74
2.84
2.46
1.9
1.7
a Determined by means of UV–Vis spectroscopy.
In order to assign the four remaining side chain carbon signals
and their respective protons, 13C spin-lattice relaxation time (T1C
)
and COSY experiments were performed. Table 2 summarizes the
T1C values for the different carbon signals. For mobile side chains,
the closer the carbon is situated toward the side chain end, the
longer its relaxation decay time will be due to less restricted
conformational motions.[12] This is clear for C7 (30.8 ppm) and C12
(14.8 ppm) which have respectively the shortest and the longest
T1C time constant of the alkyl chain carbons. Following this theory,
we can attribute the 30.7, 30.0, 32.3 and 23.3 ppm signals to C8,
C9, C10 and C11, respectively. The short-range HETCOR spectrum
allows the assignment of their respective proton signals and the
COSY experiment confirmed these attributions.
The cyclic voltammogram of monomer D1 displays typical
oxidation and reduction processes as shown in Fig. 4. The
HOMO–LUMOgapwascalculatedat2.83 eVusingtheonsetvalues
of oxidation and reduction and corresponding HOMO–LUMO
values (Table 3). The sharp oxidation peak around 0.670 V in Fig. 4,
which occurs after the first scan, is a result of the formation of
an intermediate oligomer. Upon further electropolymerizing of
D1 (Fig. 5), the oxidation peak of the conjugated polymer (P1, cf
Fig. 6) appears at ca 0.5 V. The final onset values of the oxidation
and reduction of P1 are 0.325 and −1.505 V, from which the
electrochemical energy gap can be estimated at 1.83 eV (Table 3).
The LUMO of P1 can be readily determined at −3.43 eV (Table 3),
whichismoreaccuratethanapreviouslyreportedvalue,whichwas
indirectly obtained from the oxidation potential and the optical
bandgap.[5]
The onset values of the oxidation and reduction for D3 give
an electrochemical HOMO–LUMO gap of 2.40 eV (Fig. 7). After
several scans, the oxidation peak of the corresponding conjugated
polymer (P3, cf Fig. 6) appears around 0.6 V (Fig. 8). The final onset
valuesoftheoxidationandreductionofP3are0.315and−1.425 V,
from which the electrochemical energy gap can be estimated at
1.74 eV (Table 3).
Electrochemical characterization
As NMR has confirmed that compounds D1 and D3 were available
in excellent purity, the electrochemical properties, which are
important toward potential application, can now be investigated.
CV was employed to investigate the electrochemical behavior of
D1 and D3 and to estimate their HOMO and LUMO energy levels.
In addition, the compounds were electropolymerized under the
same conditions. Upon electropolymerization, a dark red polymer
was obtained with D1 as monomer and a grayish-blue polymer
was obtained with D3 as monomer. The intensity of the color and
the film thickness increased with polymerization time. However,
after 1 h of polymerization, no further increase in the thickness
was observed. In addition, for both monomers, no bulk polymer
was observed in solution. For the CV of the polymers, the formed
polymer films were first rinsed with CH3CN, after which a fresh
electrolyte solution was added.
It is noteworthy that both D1 and D3 exhibit a distinct reversible
reduction peak in their cyclic voltammograms. This confirms the
potential n-type character of these compounds. However, a more
thorough mobility study has to be performed to confirm the
n-type character. Furthermore, the HOMO–LUMO energy gap
c
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Magn. Reson. Chem. 2010, 48, 362–369

Novel thiophene-thiazolothiazole oligomers and polymers
Figure 5. Electropolymerization of D1. Every 10th of 80 consecutive scans
is presented.
Figure 8. Electropolymerization of D3. Selective scans of a total of 103
consecutive scans are presented.
are fully confirmed by NMR spectroscopy. A complete assignment
of the proton and carbon resonances was accomplished by
means of 1D- and 2D-NMR experiments. As it concerns rather
newly developed compounds, such a complete signal assignment
will be highly useful for further progression in NMR prediction
software. CV was used to estimate the HOMO and LUMO energy
levels of D1 and D3. These values, in combination with the
reversible character of the reduction processes, indicate that these
compounds potentially have an n-type character. In addition, D1
and D3 were used as monomers in an electropolymerization and
it was demonstrated that the conjugated polymers containing
the thiophene–thiazolothiazole core as building block can have a
low-bandgap character.
Figure 6. Chemical structures of polymers P1 and P3.
Experimental
Synthesis
Tributyl(thiophen-2-yl)stannane was synthesized according to the
literature procedure.[14]
3-Hexylthiophene-2-carbaldehyde
A solution of 2-bromo-3-hexylthiophene (5.7 g, 0.023 mol) in THF
(150 ml) was cooled down to −78 ◦C, after which n-butyllithium
(14 ml, 0.023 mol, 1.6 M in hexane) was slowly added under N2
atmosphere. After the mixture was stirred for 15 min, dry DMF
(2.40 ml, 0.031 mol) was added dropwise at the same temperature.
Subsequently, the mixture was warmed up to room temperature
and stirred for an additional 2 h. Afterward the reaction mixture
was treated with a HCl-solution (0.5 M) and extracted with diethyl
ether (3 × 50 ml). The combined organic layers were washed with
saturated bicarbonate solution and brine, dried over magnesium
sulfate and concentrated by evaporation in vacuo. The crude
product was purified by column chromatography using silica
gel (eluent: hexane/ethylacetate 95 : 5) giving 3.20 g of a viscous
yellow oil (71% yield).
Figure 7. Cyclic voltammogram of D3.
values of both monomers and polymers are in good agreement
withtheircorresponding opticalenergygaps(Table 3). Inaddition,
the low-bandgap character of conjugated polymers containing
thiazolothiazole units is demonstrated by the electrochemical
energy gap values observed for P1 and P3, which are both below
2 eV.
1H NMR (CDCl3): δ = 9.95 (s, 1H), 7.55 (d, J = 5.1 Hz, 1H), 6.93 (d,
Conclusion
J = 5.1 Hz, 1H), 2.88 (t, 2H), 1.58 (m, 2H), 1.21 (m, 6H), 0.80 (t, 3H).
Substitution of thiophene–thiazolothiazole derivatives D1, D2
and D3 with hexyl side chains noticeably improved their solubility
in common organic solvents. As a result, D1 and D2 could
readily undergo functionalization reactions to become suitable for
printable electronics. The proposed chemical structures of D1–D3
2,5-Bis(3-hexylthiophene-2-yl)thiazolo(5,4-d)thiazole D1
A mixture of 3-hexylthiophene-2-carbaldehyde (3.02 g, 0.015 mol)
and dithiooxamide (0.29 g, 2.42 mmol) was stirred at 200 ◦C for
c
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S. Van Mierloo et al.
2 h. Subsequently, the reaction mixture was cooled down to
room temperature and diluted with water (50 ml) and diethyl
ether (50 ml). The organic layer was separated and the aqueous
layer was extracted with diethyl ether (3 × 50 ml). The combined
organic layers were washed with a saturated bicarbonate solution
and brine, dried over magnesium sulfate and concentrated by
evaporation in vacuo. The resulting brown oil was purified by
column chromatography using silica gel (eluent: hexane/ethyl
acetate 95 : 5) followed by subsequent recrystallization from
acetonitrile and ethanol giving 0.54 g of yellow crystals (47%
yield).
an Ag/AgNO3 reference electrode (0.1 M AgNO3 and 0.1 M Bu4NPF6
in CH3CN), a platinum counter electrode and an indium tin
oxide (ITO)-coated glass substrate as the working electrode.
The respective monomers were dissolved to their maximum
solubility in the electrolyte solution. Cyclic voltammograms were
recordedatascanrateof50 mV/s.Oxidativeelectropolymerization
was achieved by scanning between 0 and 0.9 V at the same
scan rate for a large number of consecutive scans. For the
conversion to electron volts, all electrochemical potentials have
been referenced to a known standard (ferrocene/ferrocenium
in CH3CN, 0.05 V vs Ag/AgNO3), which in acetonitrile solution
is estimated to have an oxidation potential of −4.98 V versus
vacuum. UV–Vis spectra were recorded on a Varian CARY 500
UV–Vis–NIR spectrophotometer from 200 to 800 nm at a scan
rate of 600 nm/min.
1H NMR (CDCl3): δ = 7.33 (d, J = 5.1 Hz, 2H), 6.97 (d, J = 5.1 Hz,
2H), 2.95 (t, 4H), 1.7 (q, 4H), 1.42 (q, 4H), 1.3 (m, 4H), 1.3 (m, 4H), 0.88
(t, 6H); MS: m/z = 474 (M+); UV–Vis (solution chloroform) (λmax
392 nm, m.p. = 71 ◦C.
)
2,5-Bis(5-bromo-3-hexylthiophene-2-yl)thiazolo(5,4-
d)thiazole D2
NMR characterization
All 1H and 13C liquid-state NMR experiments on D1, D2 and
D3 were performed at room temperature on a Varian Inova 300
spectrometer in a 5-mm four-nucleus PFG probe. Solutions were
prepared in CDCl3 with concentrations of 3 and 50 mg/ml for
the 1H and 13C spectra, respectively. The chemical shift scales are
calibrated to TMS at 0 ppm. The proton spectra were acquired
with a 90◦ pulse of 4.3 µs, a spectral width of 4500 Hz, an
acquisition time of 3.5 s, a preparation delay of 8 s and 20
accumulations, processed with a linebroadening of 0.2 Hz and
a data matrix of 65k. The carbon spectra were acquired with a 90◦
pulse of 12 µs, a spectral width of 8500 Hz, an acquisition time
of 0.8 s, a preparation delay of 60 s and 2500 accumulations,
processed with a linebroadening of 3 Hz and a data matrix
of 32k.
Protected from light, a solution of NBS (0.61 g, 3.44 mmol)
in DMF (50 ml) was added dropwise to solution of
a
2,5-bis(3-hexylthiophene-2-yl)thiazolo(5,4-d)thiazole D1 (0.51 g,
1.08 mmol) in DMF (50 ml), after which the mixture was stirred
for 48 h. Subsequently, the mixture was poured onto ice and ex-
tracted with diethyl ether (3×50 ml). The combined organic layers
were washed with a saturated bicarbonate solution and brine,
dried over magnesium sulfate and concentrated by evaporation
in vacuo. The crude solid was purified by column chromatography
usingsilicagel(eluent:hexane/ethylacetate95 : 5). Thecompound
was obtained as an orange solid (0.48 g, 71% yield).
1H NMR (CDCl3): δ = 6.93 (s, 2H), 2.84 (t, 4H), 1.66 (q, 4H), 1.42
(q, 4H), 1.32 (m, 4H), 1.32 (m, 4H), 0.89 (t, 6H); MS: m/z = 632(M+);
UV–Vis (solution chloroform) (λmax) 393 nm.
Acknowledgements
2,5-Bis(4-hexyl-2,2ꢀ-bithiophene-5-yl)thiazolo(5,4-d)thiazole
We gratefully acknowledge the EU for the FP6-Marie Curie-
RTN ‘SolarNtype’ (MRTN-CT-2006-035533), the IWT (Institute for
the Promotion of Innovation by Science and Technology in
Flanders) for the financial support via the SBO-project 060843
‘PolySpec’. We also want to thank BELSPO in the frame of
network IAP P6/27 and for a post-doc fellowship (A. E. B.).
Furthermore, the support of the Fund for Scientific Research-
Flanders (FWO projects G.0161.03N, G.0252.04N and G.0091.07N)
is acknowledged.
D3
A solution of tributyl(thiophen-2-yl)stannane (0.47 g, 1.26 mmol)
in dry THF (50 ml) was added dropwise to a stirred mixture
of 2,5-bis(5-bromo-3-hexylthiophene-2-yl)thiazolo(5,4-d)thiazole
D2 (0.32 g, 0.51 mmol) and Pd(PPh3)4 (0.003 g, 2.60 µmol) in
dry THF (50 ml) at ambient temperature. After stirring for 12 h
at reflux temperature, the reaction mixture was diluted with
water (50 ml). The organic layer was separated and the aqueous
layer was extracted with diethyl ether (3 × 50 ml). The combined
organic layers were washed with a saturated bicarbonate solution
and brine, dried over magnesium sulfate and concentrated by
evaporation in vacuo. The reaction product was purified by
column chromatography (eluent: hexane/ethyl acetate 95 : 5) and
recrystallized from ethanol resulting in 0.19 g of red crystals (58%
yield).
1H NMR (CDCl3): δ = 7.24 (dd, J = 5.1 Hz, J = 1.0 Hz, 1H), 7.22
(dd, J = 3.7 Hz, J = 1.0 Hz, 1H), 7.02 (dd, J = 5.1 Hz, J = 3.7 Hz,
1H), 7.00 (s, 2H), 2.88 (t, 4H), 1.71 (q, 4H), 1.45 (q, 4H), 1.34 (m, 4H),
1.34 (m, 4H), 0.90 (t, 6H); MS: m/z = 638 (M+); UV–Vis (solution
chloroform) (λmax) 449 nm; m.p. = 153 ◦C
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