Syntheses and properties of 3,4-diaryldithieno [2,3- b;3′,2′- d ]thiophenes

Article abstract of DOI:10.1080/17415993.2013.774400

Dithieno[2,3-b;3′,2′-d]thiophenes (DTT), having substituted phenyl groups at 3- and 4-carbons, have been synthesized, applying 2,5-diketone ring closure reaction using P₄S₁₀. Cyclic voltammetry studies indicated that all the DTTs had

Full text of DOI:10.1080/17415993.2013.774400

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Syntheses and properties of
3,4-diaryldithieno [2,3-b;3′,2′-
d]thiophenes
Sule Taskiran Cankaya a , Asli Capan a , Mehmet Emin Cinar a , Ezgi
Akin a , Sebahat Topal a & Turan Ozturk a b
a Department of Chemistry, stanbul Technical University, Maslak,
34469, Istanbul, Turkey
b Chemistry Group, Organic Chemistry Laboratory, TUBITAK UME,
PO Box 54, 41470, Gebze-Kocaeli, Turkey
Version of record first published: 20 Mar 2013.
To cite this article: Sule Taskiran Cankaya , Asli Capan , Mehmet Emin Cinar , Ezgi Akin , Sebahat
Topal & Turan Ozturk (2013): Syntheses and properties of 3,4-diaryldithieno [2,3-b;3′,2′-
d]thiophenes, Journal of Sulfur Chemistry, DOI:10.1080/17415993.2013.774400
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Journal of Sulfur Chemistry, 2013
http://dx.doi.org/10.1080/17415993.2013.774400
Syntheses and properties of 3,4-diaryldithieno
[2,3-b;3ꢀ,2ꢀ-d]thiophenes
Sule Taskiran Cankayaa, Asli Capana, Mehmet Emin Cinara, Ezgi Akina, Sebahat Topala
and Turan Ozturka,b
*
aDepartment of Chemistry, Istanbul Technical University, Maslak, 34469 Istanbul, Turkey; bChemistry
Group, Organic Chemistry Laboratory, TUBITAK UME, PO Box 54, 41470 Gebze-Kocaeli, Turkey
(Received 30 December 2012; final version received 1 February 2013)
Dithieno[2,3-b;3ꢀ,2ꢀ-d]thiophenes (DTT), having substituted phenyl groups at 3- and 4-carbons, have
been synthesized, applying 2,5-diketone ring closure reaction using P4S10. Cyclic voltammetry studies
indicatedthatalltheDTTshadtheoxidationpotentialsbetween1.19and1.70V. Computationalcalculations
displayed positive radical spin densities at C2 positions for DTTs and negative for their dimers.
R
R
R
R
P4S10
O
O
Toluene
reflux
S
S
S
S
S
S
Keywords: dithienothiophenes; phosphorus decasulfide (P4S10); radical spin density; cyclic voltammetry;
electropolymerization
1. Introduction
Dithienothiophenes (DTT) are formed by three fused thiophenes, which have six isomers, depend-
ing on the orientations of the thiophenes (1, 2). They have been increasingly attractive due to their
interesting properties in organic electronic materials (3–5). Presence of three sulfur atoms makes
DTTs rich in electrons and good electron donor molecules, which are important properties for
building blocks of wide variety of organic materials, having electronic and optical applications
such as conducting polymers, electrochromic displays, electroluminescence, excited fluorescence,
photochromism, nonlinear optical chromophores, transistors with high mobilities of on/off ratios,
charge-transfer complexes and labeling for biological systems.
*Corresponding author. Email: ozturktur@itu.edu.tr
© 2013 Taylor & Francis

2
S.T. Cankaya et al.
Among the six isomers, dithieno[3,2-b;2ꢀ,3ꢀ-d] 1 is the most studied one as the orientation
of the fused three thiophene rings leads to a better conjugation. Considering its suitability for
constructing π-conjugated systems, various synthetic methods have appeared in the literature
(1, 2), including the method developed by our group (6), which led to the development of some
interesting materials (7–10). It involves ring closure reactions of thiophenes containing 3-mono or
3,4-diketones to form thiophenes or dithiins with the use of Lawesson’s reagent (11) or phosphorus
decasulfide (P4S10) (12). In this study, as a continuation of our research line, the ring closure
reaction was performed on thiophene containing 2,5-diketone 5 to obtain 3,4-diaryldithieno[2,3-
b;3ꢀ,2ꢀ-d]thiophenes (DTT) 2.
S
S
S
S
S
S
2
1
2. Results and discussion
2.1. Syntheses
Syntheses of DTT 6a–6d were achieved applying the previously developed ring closure reaction
of 1,8-diketone (6). Treatment of 2,5-dibromothiophene 3 with two moles of n-BuLi at −78◦C was
followedbyadditionoftwomolesofelementalsufurandthentwomolesofdesired α-bromoketone
4 to obtain the corresponding 2,5-diketones 5a–5d (Scheme 1). The ring closure reaction of 5
was performed using P4S10 (12) in refluxing toluene to obtain the DTTs 6a–6d between 10% and
16%. It is important to note that when an impure P4S10 was used no product was obtained. The
easiest way of identifying pure and impure P4S10 is that while the impure P4S10 has yellow color
and bad smell, the pure one has very light yellow color and almost none or very slight smell.
However, the lower yields of the DTTs 2 is not related to the purity of P4S10, as the same P4S10
was used for the syntheses of the DTTs 1 (dithieno[2,3-b;2ꢀ,3ꢀ-d]thiophenes), which gave higher
yields up to 95% (8). It is known that carbons 3- and 4- of a thiophene ring are less nucleophile
compare with the carbons 2- and 5-, which could be the possible explanation for the lower yields
of the ring closure reactions of 5a–5d.
Scheme 1. Syntheses of the DTTs 6a–6d.
2.2. Electrochemistry
The oxidation reduction behaviors of the monomers 6a–6d were investigated by cyclic
voltammetry (CV). The voltammograms were recorded in acetonitrile/tetrabutylammonium

Journal of Sulfur Chemistry
3
Figure 1. Cyclic voltammograms of DTTs. 1 × 10−3 M solutions of 6a–6d in acetonitrile, 0.1 M Bu4NPF6; 100 mV/s
scan rate, Pt wires as working and counter electrodes and Ag/AgCl as a reference electrode.
hexafluorophosphate (0.1 M) solvent-electrolyte couple, using a CV cell consisting of Pt wire as
working and counter electrodes andAg/AgCl as a reference electrode. Solutions were prepared as
1 × 10−3 M, monomer concentrations and the experiments were conducted at room temperature.
The monomers 6a–6c, having R groups “H,” “MeO,” “Br” and “NO2,” respectively, displayed
oxidation potentials between 1.19 and 1.70V (Figure 1). Regarding the nature of the “R” groups
on the phenyl moiety, the oxidation potentials followed the order of 6d (1.70V) > 6c (1.51V) > 6a
(1.47V) > 6b (1.19V). While the DTT 6d, having strong electron withdrawing nitro group had the
highest oxidation potential, the DTT 6b, having strong electron donating methoxy group had the
lowest oxidation potential. As a next step, their electropolymerizations were attempted. However,
in spite of all the efforts, including different scan rates, altering the solvent-electrolyte couple,
applying constant potential, etc., it was not successful. Rather than obtaining an increase at wave
intensities of the CV curves, a decrease was obtained (Figure 2). In order to understand the behav-
iors of the monomers during the electropolymerization, a computational study was conducted.
2.3. Computational study
Geometry optimizations of the symmetric DTT molecules were performed within C2 symme-
try constraints at the DFT level, using the Gaussian 03 program (13). While the hybrid DFT
method B3LYP (14) with a conventional all-electron basis set 6-31+G(d) was applied on closed
shell molecules, the radical cations were located at unrestricted B3LYP/6-31 + G(d) level. The
minima were verified by analyzing the harmonic vibrational frequencies, using analytical sec-
ond derivatives, which have NIMAG = 0. Single point calculations, including solvent effects at
UB3LYP/6-311++G(d,p) level, were conducted on gas phase optimized geometries to obtain
spin densities. Solvent effects were incorporated by using self-consistent reaction field with a
united atom Hartree–Fock parametrization (15) of the polarizable continuum model (16) as imple-
mented in Gaussian 03 package. Acetonitrile (ε = 36.64) was employed as a solvent to mimic the
measurement and electro-polymerization conditions. The spin densities were visualized, using
GaussView 5.0 (Figure 3).

4
S.T. Cankaya et al.
Figure 2. Electropolymerization study of 6a. 1 × 10−3 M solution of 6a in acetonitrile, 0.1 M Bu4NPF6; 100 mV/s
scan rate, Pt wires as working and counter electrodes and Ag wire as a reference electrode.
Figure 3. Spin densities and the spin surfaces of the radical cations of 6a–6d.
Computational results indicated that the DTT 6a, having spin densities of 0.20 on thiophene
α-carbons (4 and 11), looked the best candidate for an electro-polymerization. Replacing the
hydrogen atom at the para position of the phenyl ring with Br, NO2 and MeO reduced the spin
densities slightly to 0.190, 0.187 and 0.175, respectively. Although the computational studies
demonstrated that the DTTs had potential for electropolymerization, all the attempts for 6a–6d
were failed. Then, the computational study was extended to the corresponding dimers of the
monomers. UB3LYP/6-31+G(d) level optimization with C2 symmetry constrain revealed the
fact that the dimer is not undergoing further polymerization due to the zero spin densities at
the corresponding carbon atoms (10 and 38) (Figure 4). When the dimer is closely examined,
accumulation of electron density through the atoms 1-4-45-42 could easily be observed. This

Journal of Sulfur Chemistry
5
Figure 4. Spin densities of the radical cation of the dimer.
explains that while the two thiophenes, connecting the DTTs, have a good deal of conjugation,
it cannot be extended to the other thiophenes as the thiophenes in the middle of each DTT break
the conjugation. Expected steric hindrance, emerging from the interaction of the closest hydrogen
atoms 51 located at the ortho position of the phenyl unit and 40, proximate to the sulphur atom
39, could also affect the polymerization, owing to the short distance of 2.72Å (17). According
to the reported C–H· · · S hydrogen bond distance of ∼2.7Å (18), predicted distance of 3.55Å
between sulphur (5) and hydrogen (62) is too long and points out the absence of hydrogen bonding.
Moreover, Van der Waals interactions (∼3.0Å) cannot be observed (19).
3. Conclusion
3,4-Diaryl substituted DTT were synthesized, applying the diketone ring closure reaction, using
P4S10. All attempts for their electrochemical polymerizations were failed. Computational chem-
istry studies revealed that although the DTTs had enough spin densities for electropolymerization
at carbon 2- of the peripheral thiophenes, their dimers had spin densities less than zero, which
prevented further chain elongation leading to their polymers.
4. Experimental
4.1. General
2,5-dibromothiophene (Across) phosphorus decasulfide, 2-bromoacetophenone, 2-bromo-4ꢀ-
methoxyacetophenone, 2-bromo-4ꢀ-nitroacetophenone, 2,4ꢀ-dibromoacetophenone, t-butyllithium,

6
S.T. Cankaya et al.
dichloromethane (Aldrich), diethyl ether, toluene and sodium bicarbonate (Merck) were used
without further purification except for diethyl ether, which was dried over metallic sodium. CH-
Instruments Model 400A was used as a potentiostat for the CV studies. Fourier Transform Infrared
Spectrometer (FTIR) spectra were recorded on a Thermo Nicolet 6700 FT–IR spectrometer. 1H
and13CNMRspectrawererecordedonaVarianmodelNMR(500 MHz). Protonandcarbonchem-
ical shifts are reported in ppm downfield from tetramethylsilane. Mass spectra were recorded on
Thermo LCQ-Deca ion trap mass instruments.
4.2. 2,2ꢀ-(Thiophene-2,5-diylbis(sulfanediyl))bis(1-phenylethanone) 5a
To a solution of 2,5-dibromothiophene (5.4 mmol, 1.3 g), dissolved in dry diethyl ether (50 ml),
was added tert-butyllithium (5.9 mmol, 3.1 ml, 1.9 M) at −78◦C under nitrogen atmosphere,
and the mixture was stirred for 1 h at the same temperature. Then, S8 (5.4 mmol, 0.173 g) was
added and the mixture was stirred for 1 h. This process was repeated with the same quantities of
tert-butyllithium and S8 in the same conditions. The mixture was placed into an ice bath and 2-
bromoacetophenone (10.8 mmol, 2.1 g) was added portion wise. Stirring was continued overnight
at room temperature under nitrogen atmosphere. The reaction was quenched with water (10 ml).
The solution was extracted with dichloromethane and the organic layer was dried over Na2SO4,
filtered and the solvent was evaporated under reduced pressure. The crude product was separated
by column chromatography eluting with n-hexane/CH2Cl2: 3/1 to give the title compound 5a
(0.73 g, 35%) as a light yellow powder, mp 75–77◦C. IR (Attenuated Total Reflectance (ATR),
diamond) 3084, 3060, 2948, 2904, 1906, 1814, 1673, 1595, 1578, 1498, 1404, 1319, 1182,
1159, 1075, 1012, 964, 882, 749, 686, 521 cm−1; 1H NMR (500 MHz, CDCl3) δ (ppm) 7.90 (dd,
J = 7.4 Hz, 1.2 Hz, 4H), 7.58 (dt, J = 7.4 Hz, 1.2 Hz, 2H), 7.46 (t, J = 7.4 Hz, 4H), 6.95 (s, 2H),
4.19 (s, 4H); 13C NMR (125 MHz, CDCl3) δ (ppm) 193.6, 136.9, 135.3, 135.1, 133.6, 128.7,
128.6, 45.1; APCIMS: m/z = 385 (M+ + 1); Anal. Calcd. for C20H16O2S3(384): C, 62.44; H,
4.19%; found: C, 62.02; H, 4.25%.
The following were similarly prepared.
4.3. 2,2ꢀ-(Thiophene-2,5-diylbis(sulfanediyl))bis(1-(4-methoxyphenyl)ethanone) 5b
The crude product was separated by column chromatography eluting with n-hexane/CH2Cl2:
2/1 to give the title compound 5b (0.558 g, 21%) as a light brown solid, mp 76–77◦C. IR (ATR,
diamond) 3075, 3004, 2965, 2934, 2838, 1669, 1597, 1573, 1509, 1420, 1310, 1282, 1170, 1026,
993, 829, 772, 602, 557 cm−1; 1H NMR (500 MHz, CDCl3) δ (ppm) 7.87 (d, J = 8.4 Hz, 4H), 6.94
(s, 2H), 6.33 (d, J = 8.4 Hz, 4H), 4.14 (s, 4H), 3.87 (s, 6H); 13C NMR (125 MHz, CDCl3) δ (ppm)
192.3, 163.8, 137.0, 134.9, 130.0, 128.3, 113.9, 55.5, 44.8;APCIMS: m/z = 445 (M+ + 1);Anal.
Calcd. for C22H20O4S3 (444): C, 59.43; H, 4.50%; found: C, 59.55; H, 4.68%.
4.4. 2,2ꢀ-(Thiophene-2,5-diylbis(sulfanediyl))bis(1-(4-bromophenyl)ethanone) 5c
The crude product was separated by column chromatography eluting with n-hexane/CH2Cl2: 3/1
to give the title compound 5c (1 g, 31%) as a light brown powder, mp 104–106◦C. IR (ATR,
diamond) 3099, 3056, 2935, 2897, 1922, 1666, 1582, 1482, 1396, 1288, 1191, 1102, 1008, 992,
1
809, 795, 667, 562, 488 cm−1; H NMR (500 MHz, CDCl3) δ (ppm) 7.76 (d, J = 8.6 Hz, 4H),
7.61 (d, J = 8.6 Hz, 4H), 6.92 (s, 2H), 4.12 (s, 4H); 13C NMR (125 MHz, CDCl3) δ (ppm) 192.6,
136.7, 135.4, 133.9, 132.1, 130.1, 128.9, 44.7; APCIMS: m/z = 543 (M+ + 1); Anal. Calcd. for
C20H14O2S3 (542): C, 44.30; H, 2.59%; found: C, 43.63; H, 2.72%.

Journal of Sulfur Chemistry
7
4.5. 2,2ꢀ-(Thiophene-2,5-diylbis(sulfanediyl))bis(1-(4-nitrophenyl)ethanone) 5d
The crude product was separated by column chromatography eluting with n-hexane/CH2Cl2:
2/1 to give the title compound 5d (0.84 g, 26%) as a brown powder, mp 162–165◦C. IR (ATR,
diamond) 3357, 3109, 3075, 2918, 2860, 2781, 2701, 2451, 2290, 1681, 1602, 1514, 1342,
1268, 1185, 1110, 998, 853, 758, 683, 541, 435 cm−1; 1H NMR (500 MHz, CDCl3) δ (ppm) 8.30
(d, J = 9.0 Hz, 4H), 7.90 (d, J = 9.0, 4H), 7.03 (s, 2H), 4.15 (s, 4H); 13C NMR (125 MHz, CDCl3)
δ (ppm) 194.6, 153.2, 142.3, 139.1, 138.5, 132.4, 126.6, 47.2; APCIMS: m/z = 474 (M+); Anal.
Calcd. for C20H14N2O6S3 (474): C, 50.60; H, 2.96%; found: C, 50.95; H, 2.63%.
4.6. 3,4-Bisphenyldithieno[2,3-b;3ꢀ,2ꢀ-d]thiophene 6a
A mixture of P4S10 (1 g, 2.25 mmol) and 5a (0.29 g, 0.75 mmol) was refluxed in toluene (70 ml)
for 4 h. The mixture was then extracted with dichloromethane and water. The organic layer was
dried over Na2SO4, filtered and the solvent was evaporated under reduced pressure. The crude
product was separated by column chromatography eluting with hexane/CH2Cl2: 8/1 to give
the title compound 6a (26 mg, 10%) as a yellow powder, mp 108–109◦C. IR (ATR, diamond)
3024, 2957, 2920, 2850, 1938, 1884, 1678, 1596, 1540, 1486, 1443, 1369, 1327, 1275, 1206,
1183, 1156, 1076, 1029, 903, 804, 748, 687 cm−1; 1H NMR (500 MHz, CDCl3) δ (ppm) 7.64 (d,
J = 8.0 Hz, 4H ), 7.39 (t, J = 8.0 Hz, 4H), 7.29 (m, 4H); 13C NMR (125 MHz, CDCl3) δ (ppm)
143.6, 134.3, 128.8, 127.5, 125.6, 123.9; APCIMS: m/z = 348 (M+); Anal. Calcd. for C20H12S3
(348): C, 69.90; H, 3.46%; found: C, 69.58; H, 3.72%.
The following were similarly prepared.
4.7. 3,4-Bis(4ꢀ-methoxyphenyl)dithieno[2,3-b;3ꢀ,2ꢀ-d]thiophene 6b
The crude product was separated by column chromatography eluting with n-hexane/CH2Cl2:
6/1 to give the title compound 6b (38 mg, 12%) as a yellow powder, mp 126–128◦C. IR (ATR,
diamond) 3090, 2997, 2956, 2834, 1891, 1673, 1604, 1519, 1501, 1456, 1291, 1174, 1026, 827,
795, 575 cm−1; 1H NMR (500 MHz, CDCl3) δ (ppm) 7.23 (s, 2H), 7.12 (d, J = 8.7 Hz, 4H), 6.80
(d, J = 8.7, 4H), 3.79 (s, 6H); 13C NMR (125 MHz, CDCl3) δ (ppm) 158.5, 141.3, 130.1, 129.1,
126.8, 123.0, 114.2, 113.5, 55.2; APCIMS: m/z = 409 (M+ + 1); Anal. Calcd. for C22H16O2S3
(408): C, 64.66; H, 3.96%; found: C, 64.20; H, 4.28%.
4.8. 3,4-Bis(4ꢀ-bromophenyl)dithieno[2,3-b;3ꢀ,2ꢀ-d]thiophene 6c
The crude product was separated by column chromatography eluting with n-hexane/CH2Cl2:
6/1 to give the title compound 6c (28 mg, 16%) as a yellow powder, mp 193–195◦C. IR (ATR,
diamond) 3077, 3037, 3018, 2970, 1902, 1770, 1650, 1587, 1555, 1479, 1447, 1333, 1273, 1207,
1117, 1103, 1006, 935, 825, 794, 627, 560, 466 cm−1; 1H NMR (500 MHz, CDCl3) δ (ppm) 7.51
(d, J = 8.8 Hz, 4H), 7.48 (d, J = 8.8 Hz, 4H), 7.27 (s, 2H); 13C NMR (125 MHz, CDCl3) δ (ppm)
142.7, 133.0, 132.0, 127.0, 124.5, 122.6, 121.5;APCIMS: m/z = 507 (M+ + 1);Anal. Calcd. for
C20H10Br2S3 (506): C, 47.46; H, 1.98%; found: C, 47.14; H, 2.46%.
4.9. 3,4-Bis(4ꢀ-nitrophenyl)dithieno[2,3-b;3ꢀ,2ꢀ-d]thiophene 6d
The crude product was separated by column chromatography eluting with n-hexane/CH2Cl2:
8/1 to give the title compound 6d (24 mg, 14%) as a yellow powder, mp 195–197◦C. IR (ATR,
diamond) 3077, 3037, 3018, 2970, 1902, 1770, 1650, 1587, 1479, 1447, 1333, 1273, 1117,

8
S.T. Cankaya et al.
1
1075, 1006, 935, 825, 794, 627, 560, 466 cm−1; H NMR (500 MHz, CDCl3) δ (ppm) 7.51 (d,
J = 8.8 Hz, 4H), 7.48 (d, J = 8.8 Hz, 4H), 7.27 (s, 2H); 13C NMR (125 MHz, CDCl3) δ (ppm)
152.9, 146.9, 135.5, 129.8, 126.0, 124.5, 124.3, 111.5; APCIMS: m/z = 439 (M+ + 1); Anal.
Calcd. for C20H10N2O4S3 (438): C, 54.77; H, 2.30%; found: C, 54.12; H, 2.78%.
Acknowledgements
We thank Istanbul Technical University, Turkey, for a grant to Sule Taskiran Cankaya (Ph.D.), National Center for High
Performance Computing (UYBHM) for the computer time provided (d0001), Unsped Global Logistic for financial support
and TUBITAK for a Fellowship (2216) to Dr M. Emin Cinar.
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