Improved synthesis of the high-mobility organic semiconductor dithiophene-tetrathiafulvalene

Article abstract of DOI:10.1055/s-2007-966053

Dithiophene-tetrathiafulvalene (DT-TTF) has been shown to be a promising organic semiconductor for organic transistors, since it combines high mobility with processability. A novel synthesis to prepare this material is described here. It involves the coupling of a 1,3-dithiol-2-one. The yield of this route is higher than previously reported yields. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-966053

SHORT PAPER
1621
Improved Synthesis of the High-Mobility Organic Semiconductor
Dithiophene-Tetrathiafulvalene
I
N
mprove
d
Synthesi
ú
s
of
D
ithioph
r
ene-Te
t
i
rathiafu
a
lvalene Crivillers, Neil S. Oxtoby, Marta Mas-Torrent, Jaume Veciana, Concepció Rovira*
Institut de Ciència de Materials de Barcelona(CSIC), Campus Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
Fax +34(93)5805729; E-mail: cun@icmab.es
Received 14 February 2007; revised 11 April 2007
might inhibit employing these molecules in commercial
applications. Herein we report a significantly improved
high-yield route to prepare this extremely promising or-
Abstract: Dithiophene-tetrathiafulvalene (DT-TTF) has been
shown to be a promising organic semiconductor for organic transis-
tors, since it combines high mobility with processability. A novel
synthesis to prepare this material is described here. It involves the ganic semiconductor.
coupling of a 1,3-dithiol-2-one. The yield of this route is higher than
The published synthetic routes of DT-TTF are either long
previously reported yields.
or unreliable. All these synthetic routes are low yielding
and proceed via thieno[3,4-d]-1,3-dithiole-2-thione (1)
Key words: tetrathiafulvalene, 1,3-dithiol-2-one, coupling reac-
tions, cyclizations, sulfur heterocycles
(
Scheme 1). They involve the direct coupling of 1,3-dithi-
6
ole-2-thione 1 (31% yield), the conversion of the thione
into the toxic selone and subsequent coupling (yield from
thione 48%), or the coupling of the [3,4-d]-1,3-dithiole-
Tetrathiafulvalene (TTF) derivatives have been intensely
studied for over 30 years, since the discovery of the first
7
2
1
-carbene formed by deprotonation of the thieno[3,4-d]-
organic
metal
tetrathiafulvalene-tetracyanoquino-
,3-dithiolium cation with tertiary amine (yield from the
1
dimethane (TTF-TCNQ). The charge-transfer salts of
cationic radical TTF derivatives have generated a wide
variety of molecular conductors and superconductors.2
The driving force in the crystallization of these salts is the
p–p stacking, which permits, together with the S···S inter-
actions, intermolecular electronic transfer, which is re-
sponsible for their transport properties. TTF derivatives
have been used not only as building blocks of organic con-
ductors, but also as components of molecular machines,
organic magnets, electrochemical sensors, and solar
7
thione 31%) (Scheme 1). The novel route to synthesize
DT-TTF that we show here involves the coupling of the
precursor 1,3-dithiol-2-one with trimethyl phosphite, and
gives a significantly improved yield for the DT-TTF cou-
pling reaction (70%).8
S
S
S
S
1
S
S
S
S
3
cells.
S
S
Se
Recently, TTF derivatives have also proved to be excel-
lent semiconductors for use in organic field-effect transis-
S
S
S
S
4
tors (OFETs). It has been shown that the best performing
S
S
TTF OFETs have contained planar and symmetrical
TTFs, and field-effect mobilities of the order of amor-
phous silicon have been achieved with these materials.5
More importantly, one major advantage of TTF semicon-
ductors compared to acene and oligothiophene deriva-
tives, which have been the benchmark in the field of
OFETs, is that TTFs are generally soluble in various or-
ganic solvents. This not only allows for the synthesis of
tailored materials, but also makes these materials compat-
ible with low-cost processing techniques that are desirable
for potential applications. To date, the highest perfor-
mance TTF OFETs have been based on dithiophene-
tetrathiafulvalene (DT-TTF), which exhibits the highest
DT-TTF
Scheme 1 Previously reported routes to DT-TTF
The synthesis reported here was thus based on the cou-
pling of thieno[3,4-d]-1,3-dithiol-2-one (8) (Scheme 2).
The type of chalcogenone (ketone, thione, or selone) used
often has a great influence on the yield of the self- or
cross-coupling products. Some examples show that the
coupling of 1,3-dithiol-2-ones to form TTFs proceed in
higher yield than the coupling of the corresponding 2-
9
thiones. However, the yields of the formation of 2-ones
2
from the corresponding 2-thiones by the reaction with
field-effect mobility (1.4 cm /Vs) found for a solution-
5
a
mercury(II) acetate vary. In the case of thieno[3,4-d]-1,3-
processed material. However, the DT-TTF synthetic
routes found in the literature are low yielding, which
7
dithiole-2-thione (1), this is only 30%. Hence, to improve
the total reaction yield in the new synthesis reported here,
cyclic dithiocarbonate 8 was formed directly by the cy-
clization of the b-keto-O-alkyl precursor 4 in the presence
of sulfuric acid (Scheme 2). Similar reactions have been
used for the synthesis of bisethylenethio-tetrathiaful-
SYNTHESIS 2007, No. 11, pp 1621–1623
x
x.
x
x
.
2
0
0
7
Advanced online publication: 11.05.2007
DOI: 10.1055/s-2007-966053; Art ID: T02907SS
Georg Thieme Verlag Stuttgart · New York
©

1
622
N. Crivillers et al.
SHORT PAPER
valene (BET-TTF)9b 0and bisethylenedithio-tetrathiaful-
1
Br
Br
SLi
SLi
N CO
2
valene (BEDT-TTF).
N
S
i
S
S
S
O
The synthesis of DT-TTF via 8 was carried out in seven
high-yielding steps (Scheme 2). Although commercially
available, potassium O-isopropyl dithiocarbonate (3) is
easily formed by the reaction of propan-2-ol with carbon
disulfide and potassium hydroxide as previously de-
8
%
S
9
8
Scheme 3 One-pot synthesis of ketone 8 in tetrahydrofuran.
Reagents and conditions: (i) 1. n-BuLi (1 equiv); 2. S (1 equiv); 3. n-
8
BuLi (1 equiv); 4. S (1 equiv).
1
1
8
scribed. The reaction of 3 with 3-bromobutan-2-one led
to the formation of O-isopropyl S-3-oxobutan-2-yl dithio-
carbonate (4) through the elimination of potassium bro-
In conclusion, the route presented for the formation of
DT-TTF is reproducible and proceeds in high yield. The
development of novel and easier TTF synthetic routes is
vital if these materials are to be commercially exploited as
organic electronic components.
1
2
mide. The preparation of 4,5-dimethyl-1,3-dithiol-2-one
5) was achieved by the cyclization of 4. Next, the bromi-
(
nation of 5 with N-bromosuccinimide and white light (500
W) in carbon tetrachloride afforded 4,5-bis(bromometh-
yl)-1,3-dithiol-2-one (6) as previously described.13 The
cyclic dithiocarbonate 7 was formed by the reaction of the All compounds were purchased from Aldrich and used without fur-
dibromo-substituted cyclic dithiocarbonate 6 with sodium ther purification, except for 3-bromobutan-2-one, which was pur-
chased from Avocado. Melting points were determined on a Stuart
Scientific BIBBY SMP10 apparatus. IR spectra were measured on
sulfide. The aromatization of 7 to form 8 was carried out
by refluxing 7 in 2,3-dichloro-5,6-dicyano-1,4-benzo-
a Perkin-Elmer Spectrum One Fourier transform spectrometer;
quinone. These two steps are similar to those reported for
1
13
samples were dispersed in KBr pellets. H (250 MHz) and C NMR
1
4
the formation of thieno[3,4-d]-1,3-dithiole-2-thione. Fi-
nally, the self-coupling of cyclic dithiocarbonate 8 by re-
(63 MHz) spectra were recorded on a Bruker Avance 250 spectrom-
eter. MALDI-TOF MS was carried out in positive mode on a Maldi
fluxing in trimethyl phosphite for eight hours gave DT- 2K-probe mass spectrometer from Kratos Analytical.
8
TTF in high yield (70%) (Scheme 2). The yield of this
O-Isopropyl S-3-Oxobutan-2-yl Dithiocarbonate (4)
coupling reaction is much higher than that in any of the
previously reported routes.
1
1
A soln of 3 (4.53 g, 26 mmol) in anhyd acetone (130 mL) was add-
ed slowly to a soln of 3-bromobutan-2-one (4.0 g, 26 mmol) in an-
hyd acetone (50 mL). The resulting soln was stirred for 30 min and
S
then poured into H O (150 mL). The mixture was extracted with
S
2
OH
i
ii
Et O (3 × 50 mL), and the organic portions were combined, dried
K
2
S
O
S
O
(MgSO ), and concentrated on a rotary evaporator to an emulsion,
78%
91%
4
O
2
3
4
which was extracted with CH Cl (50 mL). Evaporation of the sol-
2
2
9
3% iii
vent yielded a yellow oil. Yield: 4.87 g (91%).
1
3
Br
Br
H NMR (250 MHz, CDCl ): d = 1.35 (d, J = 6 Hz, 6 H, i-Pr CH ),
3
3
3
3
1
.42 (d, J = 7 Hz, 3 H, CH ), 2.27 (s, 3 H, CH CO), 4.33 (q, J = 7
S
S
3
3
S
S
v
iv
58%
3
Hz, 1 H, CH), 5.68 (sept, J = 6 Hz, 1 H, i-Pr CH).
O
O
S
O
82%
S
S
4
,5-Dimethyl-1,3-dithiol-2-one (5)
7
6
5
Compound 4 (5.76 g, 28 mmol) was slowly added to a soln of concd
8
2% vi
H SO (160 mL) that had been pre-cooled in an ice–salt bath. The
2
4
resulting soln was stirred for 15 min, the ice–salt bath was removed,
and the soln was stirred for a further 90 min. This soln was cooled
S
vii
S
S
S
in an ice bath and slowly poured into ice-cooled H O (400 mL). Af-
S
O
S
S
2
70%
S
S
ter stirring for 15 min, the soln was extracted with CH Cl (3 × 50
2
2
8
mL) to yield a dark red oil. The oil was purified by flash chromatog-
raphy (silica gel, CH Cl ). Yield: 3.90 g (93%); mp 46–47 °C.
DT-TTF
2
2
Scheme 2 The novel synthesis of DT-TTF. Reagents and condi-
tions: (i) KOH, CS ; (ii) 3-bromobutan-2-one, acetone; (iii) H SO ;
iv) NBS, CCl ; (v) Na S·9H O; (vi) DDQ, toluene; (vii) P(OMe) .
IR (KBr): 2942, 2914, 1754, 1655, 1598, 1434, 1188, 1092, 940,
2
2
4
–
1
8
86, 756, 576, 425 cm .
(
4
2
2
3
1
H NMR (250 MHz, CDCl ): d = 2.14 (s, 6 H, CH ).
3
3
1
3
C NMR (63 MHz, CDCl ): d = 191.9, 122.6, 13.5.
We also attempted to synthesize 8 directly in a one-flask
3
7
reaction, adapting methods from the literature, and fol- MALDI-TOF MS (positive mode): m/z = 146.5.
lowing the route shown in Scheme 3. 3,4-Dibromo-
4
,5-Bis(bromomethyl)-1,3-dithiol-2-one (6)
A soln of 5 (1.79 g, 12 mmol) and NBS (4.32 g, 24 mmol) in CCl4
40 mL) was refluxed by use of white light (500 W) for 4.5 h. After
the mixture had cooled to r.t., H O (150 mL) and CH Cl (50 mL)
thiophene was treated twice with n-butyllithium (1 equiv)
and elemental sulfur (1 equiv), to generate intermediate 9.
Without isolation of this compound, the reaction mixture
was allowed to react with 1,1¢-carbonyldiimidazole, to
give 8 (Scheme 3). However, this reaction gave several
side products and gave a maximum yield of 8%.
(
2
2
2
were added. The organic portion was separated, dried (MgSO ), and
concentrated to dryness on a rotary evaporator. After purification of
the residue by chromatography (silica gel, CH Cl ) a white solid
product was obtained. The analytical data of 6 were in accord with
literature values. Yield: 2.13 g (58%).
4
2
2
1
3
Synthesis 2007, No. 11, 1621–1623 © Thieme Stuttgart · New York

SHORT PAPER
,6-Dihydrothieno[3,4-d][1,3]dithiol-2-one (7)
Improved Synthesis of Dithiophene-Tetrathiafulvalene
1623
4
References
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(
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2
2
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2
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4
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1
after characterization by H NMR spectroscopy, it was quickly used
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1
H NMR (250 MHz, CDCl ): d = 4.08 (s, CH ).
3
2
Thieno[3,4-d][1,3]dithiol-2-one (8)
(
(
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mol) in anhyd toluene (100 mL) was refluxed under argon for 3 h.
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(
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4
was filtered on Celite, and the filtrate was concentrated on a rotary
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(
82%). Spectroscopic data were in accordance with those previous-
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7
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1
.4 M soln of n-BuLi in hexane (3.2 mL, 4.5 mmol) was added by
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8
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8
ture had stirred for another 1 h, a suspension of carbonylbis(imida-
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stirred for 1 h, the acetone–dry ice bath was removed, and the soln
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3
2
6
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Synthesis 2007, No. 11, 1621–1623 © Thieme Stuttgart · New York