New conjugated oligothiophenes containing the unique arrangement of internal adjacent [All]-S,S-oxygenated thiophene fragments

Article abstract of DOI:10.1002/chem.201203936

The quest for obtaining conjugated oligothiophene-containing molecules with narrower HOMO-LUMO gaps and higher oxidation and reduction potentials is the subject of this study. Molecules containing the bithiophene tetraoxide (2) and the terthiophene hexaox

Full text of DOI:10.1002/chem.201203936

FULL PAPER
DOI: 10.1002/chem.201203936
New Conjugated Oligothiophenes Containing the Unique Arrangement of
Internal Adjacent [All]-S,S-Oxygenated Thiophene Fragments
Shay Potash and Shlomo Rozen*[a]
Abstract: The quest for obtaining con-
jugated oligothiophene-containing mol-
ecules with narrower HOMO–LUMO
gaps and higher oxidation and reduc-
tion potentials is the subject of this
study. Molecules containing the bithio-
phene tetraoxide (2) and the terthio-
phene hexaoxide (3) moieties were
prepared and studied. They were ob-
tained by transferring oxygen atoms to
the corresponding dibromo oligothio-
phenes with the HOF·CH3CN complex
and then cross-coupling them with
either thiophene- or acetylene tin de-
rivatives. The photophysical and elec-
trochemical studies of the products re-
vealed that this particular class of
mixed thiophenes is characterized by
significantly smaller frontier orbital
gaps and higher oxidation and reduc-
tion potentials compared with any
other arrangement of oligothiophenes
including various [all]-S,S-oxygenated
thiophene derivatives.
Keywords: cross-coupling
voltammetry fluorine
·
·
cyclic
HOF·
·
CH3CN · oligothiophenes · oxygen
transfer · UV/Vis spectroscopy
Introduction
HOF·CH3CN,[8] readily prepared from dilute fluorine (10%
in N2) and aqueous acetonitrile.[9] It derives its oxygen-trans-
fer ability from the highly electrophilic oxygen atom it pos-
sesses. It was instrumental in quite a few difficult oxygen-
transfer reactions, such as in the oxygenation of either elec-
tron-depleted or sensitive sulfur-containing compounds,[10] in
the preparation of linear[11] and star-shaped[12] [all]-S,S-oxy-
genated oligothiophenes, in oxidizing thienopyrroles,[13] and
more.[14] Other previously unknown or very difficult reac-
Thiophene-based oligomers and polymers have attracted a
great deal of attention over the past two decades because of
their suitability to serve as active components for many po-
tential electronic applications, such as organic field-effect
transistors,[1] photovoltaic devices,[2] light-emitting diodes,[3]
and more.[4] A major issue in this field is the design and syn-
thesis of suitable materials, especially with regard to band-
gap control,[5] and regulation of electronic properties, such
as ionization potential and electron affinity.
One of the most attractive ways to control these proper-
ties by chemical manipulation is the transformation of the
sulfur atoms of the oligothiophenes into the corresponding
sulfones. This change destroys the aromaticity of the rings
and transforms the double bonds into more of a conjugated
polyene system with a good chance for a large band-gap re-
duction. As a matter of fact, this idea had been conveyed by
Tanaka already in the 1980s,[6] and consequently, many
works were designed to follow this prediction. The results of
these efforts, however, were limited either to one, or in a
very few cases, to two terminal S,S-dioxo rings (1).[7] At-
tempts to prepare other combinations of oxidized thio-
phenes were met with overwhelming synthetic difficulties.
Today, the only tool for transferring oxygen atoms to
more than one or two of the sulfur atoms of the thiophene
is the acetonitrile complex of the hypofluorous acid,
tions involved the transfer of oxygen atoms to nitrogen
[15]
ꢀ
atoms forming various N O derivatives, reactions result-
ing in “impossible” epoxidations,[16] tertiary hydroxyla-
tions,[17] forming azulene derivatives,[18] and more, have also
been described.[19]
In her highly interesting chapter in the Handbook of Thi-
ophene-Based Materials,[20] Barbarella described some parti-
ally oxygenated oligothiophenes and stated that two or
three adjacent central S,S-oxygenated thiophene rings incor-
porated into a conjugated chain should have a much greater
effect on the electron affinity than that achieved up to that
time by any other combination of S,S-oxygenated and non-
oxygenated thiophenes. A theoretical paper by Casado et al.
states that some intramolecular charge-transfer from the
outer electron-rich thiophene rings to the central electron-
withdrawing ones might be expected and theoretical calcula-
tions actually predicted such polarization.[21] Unfortunately,
however, such hypothesis has never been verified because
the relevant materials could not be made due to insur-
mountable synthetic hurdles.
[a] S. Potash, Prof. S. Rozen
School of Chemistry
The present work deals with the successful overcoming of
such difficulties and as a result with the ability to examine
the above prediction. We describe the preparation of some
new oligothiophenes containing medial adjacent [all]-S,S-di-
oxido oligothiophene (moieties 2[OO] and 3[OOO])
flanked by lateral non-oxidized thiophene rings prepared by
Tel-Aviv University
Tel-Aviv 69978 (Israel)
Fax : (+972)3-6409293
E-mail: rozens@post.tau.ac.il
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203936.
Chem. Eur. J. 2013, 19, 5289 – 5296
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the HOF·CH3CN complex (as suggested by one of the refer-
ees, in addition to the compounds numbers, we denote de-
rivatives containing one S,S-dioxo ring by [O], two such
rings [OO], one thiophene ring [T], two of them as [TT],
and so on, see Scheme 1). Comparing the UV/Vis absorp-
tions of [all]-S,S-oxygenated oligomers and their electro-
chemical redox potentials with the respective non-oxygenat-
ed ones will also be presented.
yield. Similarly, by using the more substituted 6b,[25] com-
pound 5b could be successfully converted to the quarter-
thiophene tetraoxide 7b [TOOT] in 91% yield.
With the above quaterthiophenes ACHTNUTRGNEUNG[TOOT] in hand we
aimed at the next step, the preparation of an [all]-S,S-oxy-
genated quaterthiophene. Compound 7a was not suitable
for this purpose because its two lateral thiophene rings are
only monosubstituted.[26] The quaterthiophene 7b, however,
was successfully further oxidized by using the HOF·CH3CN
complex to form the quaterthiophene octaoxide 8 [OOOO]
in 82% yield after 15 min at 08C. Here as well, no other
oxygen-transfer agent such as meta-chloroperbenzoic acid
(mCPBA) or dimethyldioxirane (DMDO) could accomplish
such a task.
Compound 5,5’-dibromo-2,2’-bithiophene-S,S,S’,S’-tetraox-
ide (5a [OO]) was also treated with the stannyl derivative
6b under the usual reaction conditions, but only a complex
reaction mixture was formed, from which we were not able
to isolate any specific compound. However, when the reac-
tion was modified and a THF/PhMe solution of the reactive
dibromide 5a was added dropwise under an argon atmos-
phere to a boiling toluene solution of 6b (2.5 equiv) and
Scheme 1. Thiophene-S,S-dioxo moieties for incorporation in conjugated
oligothiophenes.
Results and Discussion
To synthesize the new series of oligomers containing the
moieties 2 [OO] or 3 [OOO], we started with the oxidized
[OO] dibromo oligothiophenes of type 5. The first in this
series, compound 5a[11] (made from 4a [TT]), is the unsub-
stituted bithiophene tetraoxide with low solubility in most
solvents, especially after polymerization, limiting its useful-
ness for most applications in which, for example, a spin coat-
ing is required. It is well-established though, that alkyl sub-
stituents at the 3 or 4 positions will increase solubility.[22] We
thus treated 5,5’-dibromo-4,4’-dihexyl-2,2’-bithiophene, 4b
[TT][23] with a slight excess of the HOF·CH3CN complex to
form the tetraoxide 5b [OO] in 98% yield in only 5 min at
08C (Scheme 2). Even when the sulfur atoms are somewhat
sterically hindered when adjacent to the alkyl group, as in
4c [TT],[24] the desired product 5c [OO] was formed in
96% in 10 min (Scheme 2).
5% [PdACHTNUTGRNEUG(N PPh3)4], the outcome of the reaction was drastically
changed and we were able to isolate the highly soluble 7c
[TOOT] in 71% yield (Scheme 3).
An additional important advantage of the above cross-
coupling approach over the direct oxygenation of thiophene
oligomers is the possibility to incorporate and preserve oxi-
dizable functional groups in the final products. Attaching
the acetylene moiety can serve as an example. Much like
thiophenes, alkynes are occasionally employed as monomers
in the production of polymers in the materials-science
field.[27] However, the preparation of bithiophenetetraoxide-
acetylene co-oligomers (compounds 9 [OO]) could not be
achieved through the direct oxygenation of the appropriate
non-oxygenated material because the HOF·CH3CN complex
readily oxidizes alkynes[28] as well as the sulfur atoms of the
thiophenes.
The tetraoxo compounds 5 [OO] were used then for cre-
ating the new quaterthiophene tetraoxides 7 [TOOT]. Thus
5b [OO] was reacted with 2.5 mol equivalents of 2-(tributyl-
To circumvent this difficulty, we attempted to couple tri-
AHCTUNGTRENNUNG
stannyl)thiophene (6a) in the presence of 5% [PdACTHUNTRGNE(UNG PPh3)4] in
toluene for 4.5 h forming the tetraoxide 7a [TOOT] in 70%
ACHTUNGTRENNUNG
This approach proved success-
ful as the resulting 9a and 9b
[OO] were formed in 65 and
82%
yield,
respectively
(Scheme 4). Such interesting
compounds cannot be made by
any other method.
To test the prediction con-
cerning the usefulness of the
combinations of oxidized- and
non-oxidized thiophene rings in
oligomers,[20] we also attempted
to introduce the terthiophene
hexaoxide moiety into the skel-
eton of a conjugated oligomer.
Scheme 2. Oxidation of bithiophenes.
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New Conjugated Oligothiophenes
FULL PAPER
in the [all]-oxygenated 11
[OOO] in 93% yield in
a
20 min reaction. This compound
was utilized as a building block
for the new quinquethiophene
hexaoxide
12
[TOOOT],
through a quantitative reaction
with 2.5 molar equivalents of 2-
(triACTHNUTRGNEUGbN utylstannyl)thiophene (6a)
(Scheme 5).
It is important to emphasize
that all the oligomers men-
tioned so far were highly solu-
ble in organic solvents, readily
isolated, and potentially easily
manipulated for building elec-
tronic devices.
Once the conjugated oligo-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
hand it was interesting to
review their electronic proper-
ties, most of which were de-
duced from the respective UV/
Vis spectra and electrochemical
behavior. The data of the dibro-
minated all-oxygenated oligo-
thiophene derivatives can be
Scheme 3. Coupling of bithiophene tetraoxides with thiophenes.
For that purpose six oxygen atoms were transferred to the
dibromo-terthiophene 10 [TTT] by using the HOF·CH3CN
complex, the only reagent able to perform the task, resulting
Scheme 4. Coupling of bithiophene tetraoxides with alkynes.
Scheme 5. Preparation of quinquethiophene hexaoxide.
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5291

S. Rozen et al.
Table 1. Absorption (lmax [nm]), molar absorptivity (e [mꢀ1 cmꢀ1]), and
carbons as in 9a thus enabling a more planar configuration
(see below). The forceful effect of the adjacent bithiophene-
tetraoxide (2 [OO]) on the frontier orbitals, which caused
an overall redshift of 140 nm (13 [TT]!9a [OO]), is felt
also when the acetylene moiety is replaced with thiophene
rings. The band-gap in quaterthiophene 14 [TTTT]
(Scheme 6) is reduced by 0.68 and 0.71 eV when all the
sulfur atoms or only the ones on the adjacent central rings
were oxidized to the sulfones 8 [OOOO] and 7a [TOOT]
correspondingly. A further 0.14 eV redshift was observed
when two terminal alkyl groups were added as in 7b
[TOOT] versus 7a [TOOT].
the HOMO–LUMO energy gap (DEg [eV])[a] in solution.[b]
[c]
Compound
lmax (e)
DEg
DDEg
4a [TT]
5a [OO]
4b [TT]
5b [OO]
4c [TT]
5c [OO]
10 [TTT]
11 [OOO]
320[11]
3.87
3.20
3.80
3.15
4.40
3.64
3.60
3.14
388[11]
0.68
0.64
0.76
0.46
326 (19000)
393 (28000)
282[24]
341 (20000)
344[29]
395 (35000)
[a] DEg =hc/l. [b] In CH2Cl2. [c] DDEg =DEg(non-oxygenated compound)ꢀ-
DEg(oxygenated compound)
.
found in Table 1 listing the maximum wavelength absorption
(lmax), molar absorptivity (e) and the HOMO–LUMO
energy gap (DEg). The difference of the HOMO–LUMO
energy gap between the S,S-dioxide derivatives and their
parent oxygen-free compounds (DDEg) is also presented. It
was immediately clear that there is an impressive 0.65–
0.75 eV bathochromic shift in transforming compounds 4
[TT] to the oxidized 5 [OO]. The difference between the
absorption maxima of 5a,b and that of 5c could be ex-
plained by the presence of alkyl groups at position 3 in the
latter, which deviates the molecule from planarity, and re-
duces the p orbital overlap (see some relevant DFT calcula-
tions below). It should be noted that per-oxygenation of the
terthiophene 10 [TTT] to form 11 [OOO] also causes a no-
table redshift by same magnitude found in tetraoxygenated
bithiophenes 5a and 5b.
Conjugated oligomers derived from the alkyl-substituted
[all]-S,S-dioxo-oligothiophenes were also examined by UV/
Vis spectroscopy. The results are shown in Table 2. Despite
the already low band-gap of the non-oxidized materials de-
scribed in Schemes 3–6, a further considerable reduction in
the band-gap was still observed. Thus the alkyne containing
oligomer 9b [OO] shows 0.52 eV redshift compared with
the non-sulfonated 13 [TT].[24] An additional 0.56 eV red-
shift was found, when the two methyls at 3,3’ positions (9b)
were substituted with two hexyl groups located at the 4,4’
Scheme 6. Structures of quaterthiophene and quinquethiophenes.
Constructing a larger conjugated quinquethiophenes such
as the family of 15, revealed that the effect of three sulfo-
nated thiophene bridge (3 [OOO]) within oligomers is no
different than that of the impact of the 2 [OO] moiety. The
dioxo 15b [TTOTT] has been prepared and examined in the
past.[30] It displays a 79 nm redshift (0.52 eV) in comparison
to the non-oxygenated 15a [TTTTT]. Adding four more
oxygen atoms to 15b [TTOTT] to form 12 [TOOOT],
causes yet another redshift, and this hexaoxide absorbs at
111 nm higher wavelength (0.69 eV) compared with the
non-oxygenated analogue 15a [TTTTT], similar to the 14
[TTTT]!7a [TOOT] transformation.
Table 2. Absorption (lmax [nm]), molar absorptivity (e [mꢀ1 cmꢀ1]) and
HOMO–LUMO energy gap (DEg [eV])[a] in solution.[b]
[c]
Compound
lmax (e)
DEg
DDEg
13 [TT]
9b [OO]
9a [OO]
14 [TTTT]
7a [TOOT]
7b [TOOT]
8 [OOOO]
15a [TTTTT]
15b [TTOTT]
12 [TOOOT]
338 (34000)[24]
394 (60000)
478 (73000)
376 (17000)[31]
480 (44000)
507 (35000)
473 (41000)
396[32h]
3.67
3.15
2.59
3.30
2.58
2.45
2.62
3.13
2.61
2.45
0.52
1.08[d]
0.71
0.85[e]
0.68[f]
The data in Tables 1 and 2, clearly suggest that the pres-
ence of an alkyl substituent next to the sulfone group in an
adjacent ring results in smaller bathochromic shift compared
with the parent unsubstituted material. We believe that this
results from the deviation of planarity in the linear oligoene
chain as is also evident from DFT calculations (B3LYP/6-
31G+d) on the oxygenated bithiophene fragments, with
alkyl chains in different positions. Table 3 lists the HOMO–
475[30h]
0.52
507 (50000)
0.69[g]
[a] DEg =hc/l. [b] In CH2Cl2. [c] DDEg =DEg(non-oxygenated compound)ꢀ-
DEg(oxygenated compound). [d] Compared with 13; 0.55 eV compared with 9b.
[e] Compared with 14. [f] Compared with 14; ꢀ0.18 eV compared with
7b. [g] Compared with 15a; 0.16 eV compared with 15b. [h] Molar ab-
sorptivity (e) for compounds 15a,b is not available in the literature.
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New Conjugated Oligothiophenes
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Table 3. Calculated dihedral angles and HOMO–LUMO gaps (DEg
Table 4. Absorption (lmax [nm]), molar absorptivity (e [Mꢀ1 cmꢀ1]), and
[eV]) for selected alkylbithiophenes.[a]
the HOMO–LUMO energy gap (DEg [eV])[a] in solution.[b]
[c]
Compound
C3-C2-C2’-C3’ angle
DEg
Compound
lmax (e)
DEg
DDEg
16a
16b
16c
16d
179.9
180
180
3.39
3.34
3.49
3.90
17 [TTTT]
400[32]
3.10
2.77
2.58
2.41
2.58
17a [OTTT]
17b [OTTO]
17c [TOOT]
18 [OOOO]
448[33]
0.33[d]
0.52[d]
0.69[d]
0.52[e]
480[33]
128.8
514 (57000)
480[11]
[a] For the sake of calculation economy, a propyl group was used as a
representative alkyl chain.
[a] DEg =hc/l.
DEg(oxygenated compound)
[b] In
CH2Cl2.
[c] DDEg =DEg(non-oxygenated compound)
ꢀ
. [d] Compared with 17. [e] Compared with 17;
ꢀ0.17 eV compared with 17c.
when compared with quaterthiophene 17 [TTTT]. The bis-
dioxide 17c (=7c) [TOOT], however, with its two central
adjacent oxidized rings, displays a much larger and impres-
sive 114 nm redshift absorbance when compared with the
starting non-oxidized oligothiophene 17 [TTTT].
Electrochemical measurements were also informative
about the differences between bithiophene tetraoxide (2)
and terthiophene hexaoxide (3) and those of other conjugat-
ed oligothiophenes.[20] Cyclic voltammetric measurements
were carried out on glassy carbon electrodes in CH3CN
using Et4NBF4 (0.1m) as an electrolyte with the substrate
(1 mm) at 20 mVsꢀ1. The voltage was measured versus an
Ag/AgCl electrode. The oxidation (EI p,a) and reduction
(EI p,c) potentials of the relevant oligomers are listed in
Table 5. In most cases, the electrochemical gaps are in good
correlation with the HOMO–LUMO gaps revealed by the
optical spectroscopy. Some oligothiophene S,S-dioxides have
been examined in the past as potential model compounds
for n-type semiconductors.[34] It has been shown that the
transformation of the oxygen-free 19a [TTTT] to mono-di-
oxide 19b [TTTO] causes a large positive effect on the re-
duction potential (ꢀ2.12 V!ꢀ1.28 V), whereas the transfor-
mation of 19b [TTTO] to the separated bis-dioxide 19c
[OTTO] results in additional, although weaker, shift
(ꢀ1.28 V!ꢀ1.18 V). However, when the two S,S-dioxide
rings are conjugated to one another, the reduction potential
grows quite substantially, as is evident from compound 7c
[TOOT] (EI p,c =ꢀ0.56 V). This phenomenon can be ex-
plained by the fact that the electron-accepting bis-sulfone
moiety 2 [OO], can stabilize negative charge more efficient-
ly than mono-sulfone 1 [O]. The higher reduction potential
is associated with lower LUMO levels indicating the gaps
between the frontier molecular orbitals HOMO–LUMO are
reduced.[35] This is a very important and advantageous fea-
ture, especially for organic n-type semiconductors, since it
reflects the ability of the molecule to accept electrons.[20,34a]
A similar phenomenon was observed with the terthio-
phene hexaoxide (3 [OOO]) moiety, in which one can see
that the EI p,c for compound 12 [TOOOT] (ꢀ1.05 V) is
higher than the already easily reducible S,S-dioxide 15b
[TTOTT] (ꢀ1.24 V).[35a] In both cases of the 2 [OO]- and 3
[OOO]-type of oligothiophenes, alkyl chains cause some de-
viation from planarity, hence decreasing the conjugation be-
tween the oxygenated rings, a fact responsible for a some-
what lower reduction potential. This is well demonstrated
Scheme 7. Geometry of substituted bithiophene [all]-S,S-dioxides.
LUMO gap and the twist angles of four oxygenated com-
pounds (Scheme 7). From this table, one can conclude that
when such alkyls are located in position 3 and 3’, they con-
tort the bithiophene-tetraoxide out of planarity. Unlike the
calculated structures of bithiophenes 16a–c [OO], which are
almost perfectly planar, compound 3,3’-dipropyl-2,2’-bithio-
phene-1,1,1’,1’-tetraoxide (16d [OO]) has a twist angle of
over 508 between the planes of the two rings resulting in
elevated HOMO–LUMO gap by about 0.4–0.5 eV.
Oligothiophenes containing the tetraoxide 2 moiety and
are unsubstituted in the inner rings, exhibit a similar trend
to the substituted derivatives described above (Scheme 8).
The results, listed in Table 4, show that the oxidation of a
single sulfur atom of the parent oxygen-free 17 (e.g., 17
[TTTT]!17a [OTTT]) causes a bathochromic shift of
about 0.3 eV.[33] With more than one sulfur atom being oxi-
dized, the position of the sulfones in the oligomeric chain
becomes important. The bis-dioxide 17b [OTTO], with its
two lateral oxygenated rings, as well as the [all]-S,S-oxidized
octaoxide 18 [OOOO][11] display a substantial 80 nm redshift
Scheme 8. Various combinations of oxygenated and non-oxygenated tet-
rathiophenes.
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5293

S. Rozen et al.
Table 5. Reduction and oxidation potentials[a] and electrochemical [V] and optical [eV] gaps.
Compound[b]
Red.
pot.
Ox.
pot.
EC Opt.
gap gap
by comparing the two compounds 9 [OO].
When the alkyls are positioned away from
the sulfone group in the adjacent ring (9b)
the reduction potential is relatively high
(ꢀ0.54 V), whereas in compound 9a, the de-
viation from planarity is more substantial
and the EI p,c is considerably lower
(ꢀ0.86 V).
19a
19b
ꢀ2.12[34] 0.95[34] 3.07 3.01
ꢀ1.28[34] 1.04[34] 2.32 2.64
Another beneficial factor found in all
cases is revealed by the oxidation potentials
of the tetra- and hexa-oxygenated com-
pounds. These are higher than in the di- or
non-oxygenated derivatives indicating that
oligomers and polymers containing one or
more SO2 moiety are more stable toward air
and oxygen damage.[20]
19c
7a
ꢀ1.18[34] 1.30[34] 2.48 2.53
ꢀ0.71
1.76
2.47 2.58
Conclusion
A systematic method for the insertion of the
2 and 3 moiety types into the backbone of
conjugated oligomers was developed by first
7b
7c
13
ꢀ0.91
ꢀ0.56
1.37
2.28 2.45
2.18 2.41
3.67
oxygenating
transformation
a
dibromo-oligothiophene (a
possible only with
HOF·CH3CN) followed by a Stille reaction
with tin derivatives of type 6. The products
constitute a previously unknown family of
materials. From their optical and electro-
chemical properties it is obvious that they
display preferable electronic properties,
namely considerably reduced band-gaps,
higher electron affinities and higher stabili-
ties towards air compared with the corre-
sponding non-oxygenated and even [all]-S,S-
oxygenated equivalents. This effect has been
proposed, but never proven before.[20] More-
over, these materials exhibit thermal stabili-
ty as is evident from their lack of decompo-
sition beneath their melting point tempera-
tures (see the Supporting Information).[2a]
They also have good solubility and processa-
bility in organic solvents, in contrast to the
poorer solubility of the per-oxygenated oli-
gothiophenes.
1.62
1.22[24]
9a
ꢀ0.54
ꢀ0.86
1.80
1.80
2.34 2.59
9b
2.66 3.15
One more conclusion that could be de-
duced from this work is that alkyl groups, in-
corporated in the S,S-oxidized oligothio-
phenes for the sake of improving solubility,
should be placed, whenever possible, in sites
that will have as little steric interaction with
a neighboring SO2 group. This will minimize
the deviation from planarity and reduce the
HOMO–LUMO gap, providing this is a
main goal.
15b
12
ꢀ1.24[35a] 1.33[35a] 2.57 2.61
ꢀ1.05[35a] 1.58[35a] 2.63 2.45
Finally, we would like to clarify some of
the widely spread legends and prejudices as-
[a] EI p,c for compound 13 is not available in the literature.[24] [b] R1 =C6H13; R2 =SiMe2tBu.
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New Conjugated Oligothiophenes
FULL PAPER
0.93 (12H, m), 1.26–1.46 (24H, m), 1.61–1.74 (8H, m), 2.681 (4H, t,
8 Hz), 2.868 (4H, t, 7.6 Hz), 6.875 (2H, d, 4 Hz), 7.163 (2H, s),
7.554 ppm (2H, d, 4 Hz); 13C NMR: d=14.31, 14.76, 18.01, 23.24, 27.50,
27.97, 29.01, 29.47, 30.00, 30.96, 31.02, 32.17, 126.37, 126.97, 128.56,
128.73, 130.30, 133.39, 134.40, 151.87 ppm; lmax (e)=507 nm
(35000mꢀ1 cmꢀ1); HRMS (APPI) m/z calcd for C40H58O4S4Na: 753.3120
[M+Na]; found: 753.3116; elemental analysis calcd (%) for C40H58O4S4:
C 65.71, H 8.00, S 17.54; found: C 65.23, H 7.93, S 17.32.
sociated with elemental fluorine, an essential ingredient for
producing HOF·CH3CN. It is true that pure fluorine may
destroy most organic substances (as had been shown already
by Moissan more than a century ago), but if it is diluted
(with N2 or He) it is much less dangerous and easier to
work with than chlorine, for example (it is also less toxic
than Cl2[36]). Diluted fluorine is commercially available or
technical >95% F2 could be diluted on the spot to whatever
degree desired by using a simple vacuum line.[19b]
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Experimental Section
General experimental procedures: 1H NMR and 13C NMR spectra were
obtained at 400 MHz by using CDCl3 (unless mentioned otherwise) as a
solvent and Me4Si as an internal standard. MS spectra were measured
under MALDI, EI, or APPI conditions. UV spectra were recorded in
CH2Cl2.
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General procedure for working with fluorine: Fluorine is a strong oxidant
and a corrosive material. It should be used only with an appropriate
vacuum line.[19b] For the occasional user, however, various premixed mix-
tures of F2 in inert gases are commercially available, thereby simplifying
the process. Unreacted fluorine should be captured by a simple trap con-
taining a base such as soda lime located at the outlet of the glass reactor.
If elementary precautions are taken, the work with fluorine is simple,
and we have never experienced difficulties working with it.
[5] J. Roncali, Chem. Rev. 1997, 97, 173.
[6] K. Tanaka, S. Wang, T. Yamabe, Synth. Met. 1989, 30, 57.
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Int. Ed. Engl. 1986, 25, 554.
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[11] E. Amir, S. Rozen, Angew. Chem. 2005, 117, 7540; Angew. Chem.
Int. Ed. 2005, 44, 7374.
General procedure for the formation of HOF·CH3CN: A mixture of 10–
20% F2 in nitrogen was used throughout this work. The gas mixture was
prepared in a secondary container prior to the reaction and passed at a
rate of about 400 mL per minute through a cold (ꢀ158C) mixture of
CH3CN (30 mL) and H2O (3 mL) in a regular glass reactor. The develop-
ment of the oxidizing power was monitored by reacting aliquots with an
acidic aqueous solution of KI. The liberated iodine was then titrated with
thiosulfate. Typical concentrations of the oxidizing reagent were around
[12] S. Potash, S. Rozen, J. Org. Chem. 2011, 76, 7245.
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1992, 57, 7342; d) S. Rozen, A. Bar-Haim, E. Mishani, J. Org. Chem.
1994, 59, 1208; e) E. Golan, S. Rozen, J. Org. Chem. 2003, 68, 9170.
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2011, 133, 10046.
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M. Kol, S. Rozen, Synthesis 1999, 1427; c) M. Carmeli, S. Rozen, J.
Org. Chem. 2005, 70, 2131; d) T. Harel, S. Rozen, J. Org. Chem.
2010, 75, 3141; e) S. Rozen, Acc. Chem. Res. 1996, 29, 243.
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0.4–0.6 molLꢀ1
.
Typical oxygenation with HOF·CH3CN: A dibromo oligothiophene (4a–c
or 10) was dissolved in CH2Cl2, and the mixture was cooled to 08C. The
oxidizing agent was then added in one portion to the reaction vessel. The
reaction was stopped after a few minutes, and the excess of HOF·CH3CN
was quenched with saturated sodium bicarbonate. The mixture was
poured into water and extracted with CH2Cl2, the organic layer dried
over MgSO4, and the solvent evaporated. The crude product was usually
purified by recrystallization or flash chromatography.
Typical Stille cross-coupling: A dibromo oligothiophene derivative (5b,c
or 11), an organo-tin compound (6a–c) and [PdACHTNURTGNEUNG(PPh3)4] were dissolved in
dry toluene. The mixture was degassed for 20 min and heated under
argon. The solvent was washed with water, dried over MgSO4 and evapo-
rated. The crude product was purified by flash chromatography, usually
followed by recrystallization.
5,5’-Dibromo-4,4’-dihexyl-2,2’-bithiophene-1,1,1’,1’-tetraoxide (5b): Com-
pound 5b was prepared from 4b[23] (4.8 g, 9.9 mmol) by using 4.5 equiv of
the oxidizing agent for 5 min. The product was recrystallized from
hexane. A yellow solid (5.4 g, 98%) was obtained. M.p. 181–1828C;
1H NMR: d=7.126 (2H, s), 2.47 (4H, t, J=7.6 Hz), 1.61–1.56 (4H, m),
1.34 (12H, m), 0.904 ppm (6H, t, J=6.8 Hz); 13C NMR: d=14.70, 23.16,
27.09, 29.58, 30.73, 32.05, 118.32, 128.41, 131.12, 142.92 ppm; lmax (e)=
393 nm (28000mꢀ1 cmꢀ1); HRMS (EI) m/z calcd for C20H28O4S2Br2:
553.9796 [M+Na], found: 553.9800; elemental analysis calcd for
C20H28O4S2Br2Na: C 43.14, H 5.07; found: C 42.91, H 4.85.
3’,4’’,5,5’’’-Tetrahexyl-2,2’:5’,2’’:5’’,2’’’-quaterthiophene-1’,1’,1’’,1’’-tetraox-
ide (7b): Compound 7b was prepared from a dry toluene solution
(40 mL) of 5b (750 mg, 1.35 mmol), and 6b[25] (1.54 g, 3.37 mmol) in the
presence of [PdACHTUNGTRENNUNG(PPh3)4] (78 mg, 5%) as described above. The mixture
was heated at reflux for 4 h, the product chromatographed and a red-
violet solid (900 mg, 91%) was obtained. M.p. 1048C; H NMR: d=0.86–
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1
Chem. Eur. J. 2013, 19, 5289 – 5296
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5295

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Received: November 3, 2012
Revised: February 9, 2013
Published online: March 27, 2013
5296
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 5289 – 5296