Synthesis and characterization of new thieno[3,2-b]thiophene derivatives

Article abstract of DOI:10.3390/molecules171012163

Three derivatives of thieno[3,2-b]thiophene end-capped with phenyl units have been synthesized and characterized by MALDI TOF mass spectroscopy, elemental analysis, UV-vis absorption spectroscopy and thermogravimetric analysis (TGA). All compounds were prepared using Pd-catalyzed Stille or Suzuki coupling reactions. Optical measurements and thermal analysis revealed that these compounds are promising candidates for p-type organic semiconductor applications.

Full text of DOI:10.3390/molecules171012163

Molecules 2012, 17, 12163-12171; doi:10.3390/molecules171012163
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Synthesis and Characterization of New Thieno[3,2-b]thiophene
Derivatives
Moawia O. Ahmed 1,*, Wojciech Pisula 2 and Subodh G. Mhaisalkar 3,*
1
Department of Chemistry, Faculty of Science, Taibah University, P.O. Box 30003,
Medina Munawarah 41477, Saudi Arabia
2
Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany
3
School of Materials Science and Engineering, Nanyang Technological University,
50 Nanyang Avenue, 639798 Singapore
* Authors to whom correspondence should be addressed;
E-Mails: moahmed@taibahu.edu.sa (M.O.A.); subodh@ntu.edu.sg (S.G.M.).
Received: 12 September 2012; in revised form: 5 October 2012 / Accepted: 12 October 2012 /
Published: 16 October 2012
Abstract: Three derivatives of thieno[3,2-b]thiophene end-capped with phenyl units have
been synthesized and characterized by MALDI TOF mass spectroscopy, elemental
analysis, UV-vis absorption spectroscopy and thermogravimetric analysis (TGA). All
compounds were prepared using Pd-catalyzed Stille or Suzuki coupling reactions. Optical
measurements and thermal analysis revealed that these compounds are promising
candidates for p-type organic semiconductor applications.
Keywords: Thieno[3,2-b]thiophene; 2,5-dibromothieno[3,2-b]thiophene; 4-dodecyl-
phenylboronic acid; 4-(trifluoromethyl)phenylboronic acid
1. Introduction
In recent years, there has been growing interest in conjugated organic materials due to their
interesting electronic properties [1–8]. Many studies on small molecules have been focused on
pentacene, which has been used as an effective p-type semiconductor for organic field-effect
transistors (OFETs) [9–11]. However, pentacene suffers from the disadvantage of being easily
oxidized and it also presents extreme insolubility in common organic solvents. On the other hand,
devices based on oligothiophenes have poor air stability and show rapid device-performance

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degradation due to their low band gap and high-energy of the highest occupied molecular orbital
(HOMO) level [12–14]. Therefore, high stability as well as high charge carrier mobility of the OFET is
desirable. It was an aim of the present work to design and synthesize some new organic materials with
good air and thermal stability as semiconductors, using a straightforward synthetic route. We report the
synthesis and characterization of a series of small compounds based on thieno[3,2-b]thiophene for
thin-film trasistor (TFT) applications.
2. Results and Discussion
Thieno[3,2-b]thiophene-2-carboxylic acid (D) and thieno[3,2-b]thiophene (E) were prepared according
to the method of Fuller and co-workers from commercially available 3-bromothiophene (A) [15].
2,5-Dibromothieno[3,2-b]thiophene (1) was prepared according to the procedures reported by Xinnan
and co-workers (Scheme 1) [16]. Compounds D, E and 1 were characterized by 1H- and 13C-NMR and
GC-mass spectroscopic techniques (Figures S1 and S2).
Scheme 1. Preparation of D, E and 1.
Br
. LDA, THF
Br
CHO
ethyl-2-sulfanyl
acetate,
S
1
O
NaOH
S
S
2. -formylpiperidine
N
C
S
O
K2CO3, DMF
A
B
C
S
S
Cu
NBS
Br
DMF
S
COOH
S
Quinoline
Br
S
S
D
E
1
The syntheses of compounds 3, 5 and 6 were achieved as outlined in Schemes 2 and 3. Compound 3
was synthesized by utilizing a one-pot Stille coupling between bromobenzene and the freshly
prepared tri-n-butylstannyl derivative of the known compound 2,5-bis(5-(tributylstannyl)thiophen-2-
yl)thieno[3,2-b]thiophene (2) in the presence of catalytic PdCl2(PPh3)2 in refluxing THF (Scheme 2) [17].
Scheme 2. Synthesis of 3 by Stille coupling reaction.
S
SnBu
3
Br
S
S
Br
S
S
S
S
Pd(PPh )
3 4
S
1
DMF
2
Br
1. tert-BuLi,THF
3.
PdCl2(PPh )
S
2
S
2. Bu SnCl
3
3 2
S
3

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Scheme 3. Synthesis of 5 and 6 by Suzuki coupling reaction.
S
NBS, DMF
S
S
S
Br
S
S
S
Br
S
2
4
S
C12H25
S
S
C12H25
S
(HO)2B
(HO)2B
C12H25
5
4
, Na CO
CF3
2
3
S
Pd(PPh ) , Toluene
3 4
F3C
S
S
CF3
S
6
Compounds 5 and 6 were synthesized by the organometallic Suzuki coupling of 2,5-bis(5-
bromo-thiophen-2-yl)thieno[3,2-b]thiophene (4) and the appropriate arylboronic acid [4-dodecyl-
phenylboronic acid for 5 or 4-(trifluoromethyl)phenylboronic acid for 6] using Pd(PPh3)4 in refluxing
THF (Scheme 3). The Stille coupling reaction was used instead of the Suzuki one to synthesize
compound 3 because our attempts to synthesize pure 5,5′-(thieno[3,2-b]thiophene-2,5-diyl)bis(thiophene-
5,2-diyl)diboronic acid (7, Figure 1) from compound 2 were unsuccessful. This was probably due to
the relatively poor stability of the thiophene boronic acid derivatives, which decreases as the number
of thienyl units increases [18]. Thanks to the one-pot Stille coupling reaction, which showed good
efficiency and with a proper mixing of bromobenzene and freshly prepared tri-n-butylstannyl
derivative of compound 2, we were able to obtain the desired compound 3 in good yield and with high
purity after vacuum sublimation.
Figure 1. 5,5′-(thieno[3,2-b]thiophene-2,5-diyl)bis(thiophene-5,2-diyl)diboronic acid (7).
S
(HO)2B
S
S
B(OH)2
S
7
All compounds are solid yellow in color and have very limited solubility in common organic
solvents. Therefore, it is hard to characterize them by NMR techniques. This is in sharp contrast to
their recently reported analogues with two alkyl chains introduced at the position 3- or 4-position
of the thiophene ring which possess good solubility in common organic solvents and are easily
1
13
characterized by H and C-NMR techniques [19,20]. Nevertheless, the solubility of compounds 3, 5
and 6 in THF was just high enough to investigate their optical properties. Compounds 3, 5 and 6 were
purified by vacuum sublimation and their masses were determined by MALDI TOF spectroscopy.
Prominent base peaks for the molecular ions were observed for all compounds in the mass spectra
(Figure S3).
To gain a better understanding of the various factors influencing the stability of the synthesized
molecules, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular

Molecules 2012, 17
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orbital (LUMO) of 3 (Figure 2) were determined by complete geometry optimization using the
gradient-corrected B3LYP density functional with the polarized 6-31G * basis set (B3LYP/6-31G *) [21].
The calculated HOMO and LUMO energies are listed along with the associated band gap in Figures 2
and S4. The calculated HOMO level of 3 is lower than those of most oligothiophenes and pentacene,
indicating a higher oxidative stability [11,14].
Figure 2. HOMO and LUMO Compound 3.
The optical properties of 3, 5 and 6 were investigated by UV-vis absorption spectroscopy. The
absorption spectra of the three compounds are depicted in Figure 3. Compounds 3, 5 and 6 exhibited
absorption maxima in THF solution at 415, 401, and 408 nm, respectively.
Figure 3. UV-Vis spectra of 3, 5 and 6.
The absorption maximum of unsubstituted compound 3 is red shifted compared to that of ,-dodecyl
and trifluoromethyl substituted derivatives 5 and 6, respectively. The estimated HOMO-LUMO gaps
obtained from the end absorptions are 2.50 eV for 3, 2.65 eV for 6 and 2.80 eV for 5. The lower gap
for 3 is probably due to the lesser steric hindrance between the two phenyl rings and thiophene units,
which leads to more effective -conjugation and stronger interactions between the molecules of 3. On
the other hand, the presence of substituents in 5 and 6 leads to the more steric hindrance which leads in

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turn to less effective -conjugation and weaker interactions between molecules. The absorption
maximium of compound 6 (408 nm) in THF solution is red-shifted with respect to that of , '-dodecyl
substituted derivative 8 (389 nm) recently reported by our group indicating the presence of strong
intermolecular interactions between 6 molecules. The estimated HOMO-LUMO gaps obtained from
the end absorptions for 6 (2.65 eV) is lower compared to that of 8 (2.71 ev). The variation of the
energy gaps may indicate a different influence of the dodecyl substituents on packing and device
performance of these materials.
Moreover, The HOMO level of unsubstitued derivative 3 (−4.94 eV) and 8 (−5.25 eV) match well
with the workfunction of metallic gold (−5.1 eV) and this may enhance the hole charge injection
between the electrode and the semiconductor and may greatly improve the device performance. The
presence of long alkyl chains in compound 8 (Figure 4) improves its solubility as well as its molecular
ordering, thus positively influencing its semiconducting properties.
Figure 4. Compound 8.
C12H25
S
S
S
S
8
C12H25
In our previous investigations we showed that OTFT devices based on compound 8 have good
stability and with a carrier mobilities up to 3.11 × 10−2 cm2 V−1 s−1 and on/off ratios up to 104 for a
top-contact OTFT made by spin coating. In contrast, compound 9 (Figure 5) showed extremely poor
FET performance, which may be due to a lower degree of molecular ordering caused by substituent
positions as evidenced by DSC data and two-dimensional wide-angle X-ray scattering results [19].
Figure 5. Compound 9.
C12H25
S
S
S
S
C12H25
9
Compounds 3, 5 and 6 possess limited solubility in common organic solvents, which makes them
promising candidates for organic electronic devices prepared by vapor evaporation. Thermogravimetric
analysis (TGA) reveals as all compounds being thermally stable, with decomposition temperatures
above 350 °C under nitrogen atmosphere. Figure 6 shows the TGA for 5.

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Figure 6. Thermal gravimetric analysis of 5.
3. Experimental
3.1. General
Reactions and manipulations were performed using standard Schlenk techniques under an
atmosphere of nitrogen or argon, unless otherwise stated. Solvents were purified, dried, and distilled
under an argon or nitrogen atmosphere prior to use. All new compounds were characterized by
MALDI TOF mass spectrometry and elemental analysis. 1H and 13C-NMR spectra were recorded on a
Bruker DPX 400 MHz spectrometer with chemical shifts referenced to CDCl3. Thermogravimetric
analyses (TGA) were conducted on a Du Pont Thermal Analyst 2100 system with a TGA 2950
thermogravimetric analyzer under a heating rate of 20 °C/min and a nitrogen flow rate of 70 mL/min.
3.2. Materials
Lithium diisopropylamide (LDA), N-formylpiperidine, ethyl-2-sulfanyl acetate, bromobenzene,
tributyl(thiophen-2-yl)stannane, thieno[3,2-b]thiophene, tert-butyllithium, tri-n-butyltin chloride,
PdCl2(PPh3)2, tetrakis(triphenylphosphine)palladium, N-bromosuccinimide (NBS), 4-(trifluoro-
methyl)phenylboronic acid and 4-dodecylphenylboronic acid, were all purchased from Aldrich
(Singapore). All reagents purchased commercially were used without further purification except for
tetrahydrofuran (THF), which was dried over sodium–benzophenone ketyl.
3.3. Synthesis
Thieno[3,2-b]thiophene-2-carboxylic acid (D), thieno[3,2-b]thiophene (E), 2,5-dibromothieno[3,2-
b]thiophene (1), 2,5-di(thiophen-2-yl)thieno[3,2-b]thiophene (2) and 2,5-bis(5-bromothiophen-2-
yl)thieno[3,2-b]thiophene (4) were prepared according to the procedures reported in the literature [15–17].
2,5-Di(thiophen-2-yl)thieno[3,2-b]thiophene (2). A mixture of 2,5-dibromothieno[3,2-b]thiophene (1)
(2.80 g, 9.33 mmol), tributyl(thiophen-2-yl)stannane (8.01 g, 21.46 mmol), and tetrakis(triphenyl-

Molecules 2012, 17
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phosphine)palladium(0) (0.33 g, 0.467 mmol) in dry DMF (50 mL) was heated under nitrogen at 90 °C
overnight. After the reaction was cooled to room temperature, water (100 mL) was added to the solution
and the yellow precipitate was collected by filtration, washed with hexanes and ether, and then purified
by vacuum sublimation to afford 1.80 g of the tile compound as a yellow solid (63%). MALDI
TOF-MS (dithranol) m/z: 303.904 (100, M+), calcd for C14H8S4 = 309.950. Calcd for C14H8S4: C
55.23, H 2.65; S 42.13; Anal. found: C 55.87, H 2.54; S 42.07.
2,5-Di(thiophen-2-yl)thieno[3,2-b]thiophene (3). Compound 2 (2.00 g, 6.58 mmol) was dissolved in
dry THF (50 mL) and cooled in an acetone/dry ice bath for 20 minutes. n-Butyllithium (1.60 M
solution in hexanes, 8.22 mL, 13.16 mmol) was then added dropwise and the mixture was allowed to
stir 1 h. At this point the mixture was removed from the ice bath, allowed to warm to room
temperature, and then returned to the ice bath. Tributyltin chloride (3.57 mL, 13.16 mmol) was then
added dropwise and the mixture was allowed to warm to room temperature and stirred for 1 h, the
resulting 2,5-bis(5-tributylstannylthien-2-yl)thieno[3,2-b]thiophene solution was added by cannula
under nitrogen to a solution PdCl2(PPh3)2 (0.40 g, 0.57 mmol) and bromobenzene (1.77 g, 11.33 mmol) in
THF (20 mL). The mixture was heated to reflux under argon for 20 h. The solution was then allowed
to cool, quenched with water, and the orange solid was isolated by filtration and successively washed
with water, 1 N HCl, and acetone (25 mL each). The material was purified by vacuum sublimation
(p < 10−5 Torr), giving a bright yellow solid 1.20 g (40%). MALDI TOF-MS (dithranol) m/z (relative
intensity): 455.955 (100, M+), 456.971 (30%), 457.937 (20%); calcd. for C26H16S4 = m/z: 456.01
(100.0%), 457.02 (28.3%), 458.01 (1 8.1%). Anal. calcd for C26H16S4: C 68.38, H 3.53, S 28.09;
found: C 68.57, H 3.39, S 27.84.
2,5-Bis(5-bromothiophen-2-yl)thieno[3,2-b]thiophene (4). N-Bromosuccinimide (1.75 g, 9.87 mmol) was
added in portions over 20 min to a solution of 2 (1.50 g, 4.93 mmol) in DMF (20 mL) at 0 °C. After stirring
the solution for 3 h at ambient temperature, water (50 mL) was added, and the aqueous layer was extracted
with ethyl acetate (50 mL). The organic layer was dried (anhydrous Na2SO4) and the solvent removed
in vacuo. The residue was purified by chromatography on silica gel eluting with hexanes to give 4 as a
yellow solid (1.90 g, 83% yield). MALDI TOF-MS (dithranol) m/z: 461.797 (100%) 463.797 (62.00%)
and 459.797 (47.00%) (M); calcd. for C14H6Br2S4 = 461.770 (100.0%), 463.770 (61.50%), 459.770
(46.90%). Anal. Calcd for C14H6Br2S4: C, 36.38; H, 1.31; S, 27.75. Found: C, 36.69; H, 1.18; S, 27.84.
3.4. General Method for the Preparation of 5 and 6
To a deoxygenated solution of 4-dodecylphenylboronic acid (1.35 g, 4.50 mmol) or
4-(trifluoromethyl)phenylboronic acid (0.86 g, 4.50 mmol) and Na2CO3 (20.00 mL; 2.00 M), a
deoxygenated solution 4 (0.80 g, 1.73 mmol) in toluene (60.00 mL) and tetrakis(triphenylphosphine)-
palladium(0) (0.10 g, 0.087 mmol) was added. The reaction mixture was refluxed for 24 h under argon
atmosphere. The cooled solution was poured into a flask containing water (150 mL) mixed with 1.00 M
HCl (30 mL) and toluene (200 mL). The aqueous and organic phases were separated; the orange solid
was isolated by filtration and washed with water. The product was washed with toluene and dried
under vacuum. The product was further purified by sublimation to yield 5 (0.95 g, 69%) as a yellow
crystals and 6 (0.76 g, 74%) as orange crystals.

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(5) MALDI TOF-MS (dithranol) m/z: 792.389(M); calcd. for C50H64S4 = 792.390. Anal. Calcd for
C50H64S4: C, 75.70; H, 8.13; S, 16.17. Found: C, 75.96; H, 8. 05; S, 16.10.
(6) MALDI TOF-MS (dithranol) m/z: 591.791(M); calcd. for C28H14F6S4 = 591.990. Anal. Calcd for
C28H14F6S4: C, 56.74; H, 2.38; S, 21.64. Found: 56.55; H, 2.52; S, 21.49.
4. Conclusions
In summary, we have demonstrated the synthesis and characterization of a new family of
thieno[3,2-b]thiophene derivatives. Their photophysical and thermal analysis were investigated. All
compounds exhibited good thermal stability and appear therefore to be promising candidates for p-type
organic semiconductor applications. Further studies on the solid state packing of compounds 3, 5 and 6
using two-dimensional wide-angle X-ray scattering (2D-WAXS) and device performance in OFETs
are under way.
Supplementary Materials
Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/17/10/12163/s1.
Acknowledgments
This work was supported by an A*STAR grant (Research Grant No. 052 117 0032).
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