Regiochemical effects of furan substitution on the electronic properties and solid-state structure of partial fused-ring oligothiophenes

Article abstract of DOI:10.1021/jo301744s

Oligomers containing the new fused-ring heterocyclic conjugated building block thieno[3,2-b]furan were synthesized, and the effects associated with furan ring substitution into fused-ring oligothiophenes on the electronic properties and solid-state structure were assessed. A series of four-ring oligomers which vary in the degree of furan ring substitution and the regiochemistry of placement were synthesized via Stille cross-coupling and oxidative homocoupling strategies. The electronic properties of these oligomers were studied by UV-vis absorption and fluorescence spectroscopies. Substitution of furan rings at the terminal positions yields oligomers with a narrower HOMO-LUMO gap relative to the all-thiophene analogue 2,2′-bithieno[3,2-b]thiophene, and incorporation of furan rings at the interior positions results in oligomers with an increase in rigidity and a higher fluorescence quantum yield. Packing motifs of the oligomers were determined using single crystal X-ray diffraction. In contrast to the herringbone crystal packing observed for nonfused oligothiophenes, oligofurans, thiophene-furan hybrid oligomers, and the all-thiophene analogue 2,2′-bithieno[3,2-b]thiophene, all three regioisomers derived from the dimerization of thieno[3,2-b]furan arrange in a π-stacked packing motif in the solid state.

Full text of DOI:10.1021/jo301744s

Article
pubs.acs.org/joc
Regiochemical Effects of Furan Substitution on the Electronic
Properties and Solid-State Structure of Partial Fused-Ring
Oligothiophenes
John T. Henssler and Adam J. Matzger*
Department of Chemistry and the Macromolecular Science and Engineering Program, University of Michigan, 930 North University
Avenue, Ann Arbor, Michigan 48109-1055, United States
S
* Supporting Information
ABSTRACT: Oligomers containing the new fused-ring heterocyclic conjugated
building block thieno[3,2-b]furan were synthesized, and the effects associated with
furan ring substitution into fused-ring oligothiophenes on the electronic properties
and solid-state structure were assessed. A series of four-ring oligomers which vary in
the degree of furan ring substitution and the regiochemistry of placement were
synthesized via Stille cross-coupling and oxidative homocoupling strategies. The
electronic properties of these oligomers were studied by UV−vis absorption and
fluorescence spectroscopies. Substitution of furan rings at the terminal positions
yields oligomers with a narrower HOMO−LUMO gap relative to the all-thiophene
analogue 2,2′-bithieno[3,2-b]thiophene, and incorporation of furan rings at the
interior positions results in oligomers with an increase in rigidity and a higher
fluorescence quantum yield. Packing motifs of the oligomers were determined using
single crystal X-ray diffraction. In contrast to the herringbone crystal packing
observed for nonfused oligothiophenes, oligofurans, thiophene−furan hybrid oligomers, and the all-thiophene analogue 2,2′-
bithieno[3,2-b]thiophene, all three regioisomers derived from the dimerization of thieno[3,2-b]furan arrange in a π-stacked
packing motif in the solid state.
INTRODUCTION
■
Tailoring the electronic and solid-state properties of
oligothiophenes through structural modification has been an
area of intense research activity in recent years primarily due to
potential applications in the field of organic electronics.1−3 One
general strategy that has received substantial attention is the
replacement of a thiophene ring in α-oligothiophenes with
another five-membered heteroaromatic moiety such as
furan,4−12 pyrrole,10,13−16 silole,17−20 and phosphole.21−25
Thiophene−furan hybrid oligomers have displayed some
more desirable properties than the pure oligothiophene
analogues. For example, replacement of sulfur with oxygen
tends to reduce spin−orbit coupling and thus decreases
intersystem crossing, resulting in higher fluorescence quantum
yields.8,26 Furthermore, oxygen, which has a smaller van der
Waals radius than sulfur, is credited with thiophene−furan
hybrid oligomers achieving more densely packed herringbone
arrangements as compared to the corresponding oligothio-
phene.5,7 Altering the degree and regiochemistry of ring fusion
are also effective methods of manipulating the electronic and
solid-state properties of oligothiophenes.27−29 For example,
nonfused oligothiophenes assume a herringbone packing motif
in the solid state; upon introduction of ring fusion, some
oligothiophenes have been shown to adopt a π-stacked
arrangement.28,30 This change in packing motif is primarily
attributed to the elimination of two β-hydrogen atoms for each
degree of ring fusion (Figure 1) which reduces the number of
Figure 1. Structural comparison between nonfused α-linked and fused-
ring heterocycles involving thiophene and furan.
potential electrostatic interactions between the hydrogen-
dominated edges and the face (π-system) of adjacent
molecules.28,31,32 The π-stacked packing motif is predicted to
be more favorable for charge carrier mobility due to the
increase in π-orbital overlap.33 Despite substantial interest, the
number of thiophene-based oligomers that display π-stacking
remains low, motivating our discovery of molecular design
strategies that favor π-stacking of conjugated oligomers.
The properties of nonfused thiophene−furan hybrid
oligomers have been reported for systems that vary in the
ratio of thiophene and furan rings,4−12 but the effects of
regiochemistry on the electronic properties and solid-state
Received: September 3, 2012
Published: September 17, 2012
© 2012 American Chemical Society
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a
Scheme 1. Synthetic Route to Oligomers 1−5
a
Yields represent the material isolated by column chromatography in >96% purity (<4% regioisomers).
Figure 2. UV−vis absorption and emission spectra for oligomers 1−5 and T2-T2 in CH2Cl2.
packing have not been systematically evaluated. Further, until
recently it was not possible to study the effect of ring fusion in
alternating thiophene−furan hybrid systems due to the
inaccessibility of the thiophene−furan fused-ring moiety
thieno[3,2-b]furan.34 In this investigation we utilized thieno-
[3,2-b]furan and thieno[3,2-b]thiophene to generate a series of
five four-ring oligomers in order to explore the effects of the
extent of furan ring substitution and the regiochemistry of furan
placement into the partial fused-ring oligothiophene 2,2′-
bithieno[3,2-b]thiophene (T2-T2) on the electronic properties
and solid-state packing arrangement. Specifically, all three
regioisomers of the thieno[3,2-b]furan dimer were synthesized
along with both regioisomers derived from the coupling of
thieno[3,2-b]furan and thieno[3,2-b]thiophene moieties.
(7)35 or 8 under Stille conditions to generate oligomers 2 and
3, respectively. Intermediate 8 is cross-coupled with tributyl-
(thieno[3,2-b]thiophene-5-yl)stannane (9)36 or homocoupled
under Stille type conditions to generate oligomers 4 and 5,
respectively.
Electronic Properties. The electronic properties of 1−5
were studies using UV−vis absorption and fluorescence
spectroscopies (Figure 2). These data are compared to T2-T2
because it is the all-thiophene analogue of the oligomers under
investigation and has been extensively studied.28,29 Oligomers 1
and 2, with furan rings at the periphery, yield a bathochromic
shift of the absorption spectrum compared to that of T2-T2,
indicating a narrower HOMO−LUMO gap. On the other hand,
4 and 5, which contain furan rings at the interior of the
structure, display a blue shift in the absorption spectrum
relative to that of T2-T2, which reflects the reduced electron
delocalization known to occur across furan rings.26 Oligomer 3,
which consists of two thieno[3,2-b]furan units linked in a head-
to-tail fashion and therefore is the only isomer with both an
interior and exterior furan ring, yields a small blue shift in the
absorption spectrum compared to T2-T2. The direction and
relative magnitudes of these shifts indicate that placement of a
furan ring at the terminal position acts to narrow the HOMO−
LUMO gap, and positioning a furan ring at the interior of the
molecule widens the optical band gap to a similar extent. For
nonfused thiophene−furan hybrid oligomers, the effect of
RESULTS AND DISCUSSION
■
Oligomer Synthesis. Oligomers 1−5 can be generated
from thieno[3,2-b]furan34 and thieno[3,2-b]thiophene34 in two
or fewer linear steps (Scheme 1). Oligomer 1 is derived directly
from thieno[3,2-b]furan upon treatment with 1 equiv of
butyllithium to generate the anion at the 5-position, followed
by oxidative homocoupling employing Fe(acac)3. The
syntheses of oligomers 2−5 utilize tributyl(thieno[3,2-b]-
furan-5-yl)stannane (6) and/or 2-bromothieno[3,2-b]furan
(8) which are derived from thieno[3,2-b]furan in one step.34
Intermediate 6 is coupled with 2-bromothieno[3,2-b]thiophene
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regiochemistry on the position of the absorption envelope has
not been previously evaluated but a compilation of the limited
reports on such systems reveals that shifts in the longest
wavelength absorption maximum (λmax) values are similar to
those observed for the partial fused-ring oligomers in this
study.8,9,29
Table 1. Electronic Properties for Oligomers 1−5 and 2,2′-
Bithieno[3,2-b]thiophene in CH2Cl2
Inspection of the absorption profiles reveals information
about the oligomer conformations in solution.27,29,37 In the
planar conformation orbital overlap and conjugative stabiliza-
tion are greatest but the heteroatoms can influence the ground-
state oligomer conformations and in many systems contribute
to an increase in the population of molecules with nonplanar
geometries. For example, α-linked thiophene rings experience
steric repulsion between the sulfur atom of one ring and a β-
hydrogen atom of an adjacent ring. Consequently α-
oligothiophenes typically display broad, unstructured absorp-
tion profiles which reflect the twisted conformations and
rotational freedom between adjacent rings in the aromatic
system. In this study 1, 2, and T2-T2, which have two thiophene
rings at the interior of the oligomer, display the broadest and
least structured absorption envelopes, suggesting a larger
population of oligomers in nonplanar conformations in
comparison to 3−5 (Figure 2). In contrast, 5, which contains
two furan rings at the interior of the oligomer, yields the
narrowest absorption profile with significantly more vibronic
structure than 1-4 and T2-T2, and these features reflect a more
rigid structure in solution (Figure 2). The higher degree of
conformational rigidity for 5 can be attributed to replacement
of sulfur with oxygen, having a smaller van der Waals radius,
thus circumventing the issue of steric repulsion between
adjacent rings.38 Instead attractive intramolecular interactions
between the oxygen of each interior furan ring and the β-
hydrogen atom of the adjacent interior furan rings may
contribute to the stabilization of the planar conformation.39
Oligomers 3 and 4 each contain one thiophene and one furan
ring at the central positions and show an intermediate
broadening of the absorption profile (Figures 2). Trends
associated with both the dihedral angle of the energy-
minimized structure and the energy barrier of rotation between
thiophene−thiophene, thiophene−furan, and furan−furan rings
have been computationally investigated and are in accord with
the assessment of the experimental absorption profiles
above.38−42 Further, extinction coefficients (ε) provide
experimental evidence regarding oligomer conformation in
the ground state. An increase in the extinction coefficient is
expected as the conformation of an oligomer becomes more
planar.37,43 Indeed, a trend of increasing extinction coefficient
from 1 to 5 is observed, coinciding with the above
conformational assessment based on the shape of the
absorption envelope.
Oligomers 1, 3, 4, and 5 exhibit higher fluorescence quantum
yields (ΦF) as compared to T2-T2 (Table 1). Most notably, 5
displays a 5-fold increase in the fluorescence quantum yield.
This result may be attributed to both increased planarization
and reduced influence of the heavy atom effect.26,44,45
Introduction of furan rings into oligothiophenes typically
results in an increase in the fluorescence quantum yield but
in a very small number of cases, when furan rings are located in
noncentral positions of the molecule, a decrease in emission
efficiency has been observed.8 Likewise, 2 displays a
fluorescence quantum yield slightly lower than that of T2-T2.
Despite reports aimed specifically at determining the cause of
photophysical phenomena such as reduced fluorescence
quantum yield in some thiophene−furan hybrid oligomers,
the explanation remains elusive.8,26
Solid-State Structure. The single crystal structures of 1, 3,
and 5 were determined to elucidate the role of intermolecular
interactions in the solid-state packing arrangements in this
series of thieno[3,2-b]furan dimers. The significant difference
between the propensity of 1, 3, and 5 versus the all-thiophene
analogue T2-T2 to form single crystals larger than 0.03 mm ×
0.03 mm × 0.01 mm should be noted. In comparison to
oligothiophenes, the thieno[3,2-b]furan dimers form signifi-
cantly larger single crystals (>0.5 mm × 0.3 mm × 0.1 mm by
slow evaporation from organic solvents). These observations
suggest that the thieno[3,2-b]furan moiety acts to enhance
solid-state ordering over longer distances in comparison to
thieno[3,2-b]thiophene.
The fluorescence λmax values span 39 nm across the series of
oligomers (Table 1), and comparison with the absorption λmax
values (the Stokes shift) also provides insight into molecular
conformation in solution. Smaller Stokes shifts tend to be
associated with molecules that undergo little change in
conformation between the ground and the excited state, for
example those which possess more rigid structures.27,29,37 In
agreement with the absorption profile analysis, 5 displays the
smallest Stokes shift (Table 1), thus supporting the notion that
it is the most rigid molecule in the series of oligomers in
solution. As the furan rings are migrated toward the periphery
of the oligomer, the Stokes shift increases and 1 has the largest
Stokes shift.
Oligomer 5 displays a slipped π-stacked arrangement with a
face-to-face intermolecular distance of 3.49 Å (Figure 3). The
distance between π-stacked oligomers was determined by a
perpendicular measurement between a plane defined by one
five-membered ring and the centroid of an overlapping five-
membered ring in an adjacent face-to-face oriented molecule.
The oligomer exists in a planar transoid conformation, and
short intramolecular Cβ−H···O distances (2.88 Å) are
observed. Both internal furan rings present close intermolecular
Cβ−H···O contacts (2.45 Å) between furan rings in
neighboring molecules. The α-hydrogen on each of the
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5, oligomer 1 arranges in a planar transoid fashion. Both
internal thiophene rings exhibit short Cβ−H···S contacts (2.89
Å) with thiophene rings in adjacent oligomers. Close Cα−H···O
(2.55 Å) and Cβ−H···O (2.50 Å) interactions are present
between the terminal furan rings of molecules in neighboring
columns.
The solid-state packing arrangements of 1, 3, and 5 all
display π−π interactions, which are markedly different from the
herringbone motif adopted by the oligothiophene analogue T2-
T2 (Figure 6).28 Furthermore, nonfused oligothiophenes,46−53
Figure 3. Crystal structure diagram of oligomer 5 illustrating (a) the
slipped π-stacked packing motif and (b) the orientation of oligomers
in adjacent columns.
terminal thiophene rings forms close interactions with the
thienyl sulfur and π-system (2.72 and 2.73 Å, respectively) of
the terminal thiophene ring in a molecule adjacent to the short
edge of the oligomer.
The structure of 3 reveals a slipped π-stacked motif with a
close face-to-face intermolecular distance of 3.33 Å, and the
oligomer exists in the cisoid conformation with a torsion angle
of 3° (Figure 4). The sulfur and oxygen in the central rings are
Figure 6. Crystal structure diagram of T2-T2 illustrating the
herringbone packing motif.28
oligofurans,40 and thiophene−furan hybrid oligomers5,7 arrange
in a herringbone fashion; neither adjusting the ratio of furan
and thiophene rings or the partial ring fusion in T2-T2 has been
demonstrated to yield oligomers with π-stacked packing
arrangements. The packing motifs of 1, 3, and 5 may be
attributed to a combination of the following features, which are
unique to these oligomers. First, the furan rings facilitate the
formation of an extensive network of intermolecular C−H···O
interactions as previously discussed. These interactions occur at
distances which are considerably closer than the sum of the van
der Waals radii for O and H (2.61 Å). Second, the condensed
ring approach eliminates β-hydrogen atoms, thereby increasing
the C/H ratio and favoring π−π interactions in the solid state.28
Notably, no close interactions exist between β-hydrogen atoms
and the π-system of neighboring molecules in the crystal
structures of 1, 3, or 5. Instead, short intermolecular Cβ−H···O
interactions are prevalent.
Substitution of furan into nonfused oligothiophenes has been
reported to result in more densely packed herringbone
structures.5,7 A direct comparison cannot be made among 1,
3, and 5 versus T2-T2 due to the distinct difference in packing
motifs. Instead, pentathienoacene is the most appropriate
comparison compound because, like 1, 3, and 5, it also contains
six double bonds and presents a π-stacked arrangement in the
solid state. All three thieno[3,2-b]furan dimers display shorter
π-stacked distances (3.33−3.49 Å) than the reported value for
pentathienoacene (3.52 Å),54 which may be attributed to the
smaller van der Walls radius of oxygen in comparison to sulfur.
For example, 3 exhibits S···O contact between π-stacked
oligomers at a distance of 3.38 Å, which is shorter than those
observed for short S···S contacts.
Figure 4. Crystal structure diagram of oligomer 3 illustrating (a) the
slipped π-stacked packing motif and (b) the orientation of oligomers
in adjacent columns.
observed at a distance of 2.94 Å, which is considerably shorter
than the sum of the van der Waals radii for the two atoms (3.32
Å). A close Cβ−H···S interaction (2.87 Å) is present between
the internal furan and thiophene rings of molecules in
neighboring columns. Short Cβ−H···S and Cβ−H···O distances
(2.92 and 2.49 Å, respectively) are found between terminal
thiophene and furan rings in molecules that adjoin the long
edge of the structure. Oligomers also interact along the short
edge of the structure; the terminal thiophene ring forms a close
Cα−H···O contact (2.50 Å) with a terminal furan ring, which
displays close Cα−H···S and Cα−H···π interactions (2.80 and
2.68 Å, respectively) with a terminal thiophene ring in another
molecule adjacent to the short edge of the oligomer.
Oligomer 1 adopts a slipped π-stacked motif with a short
face-to-face intermolecular distance of 3.45 Å (Figure 5). Like
CONCLUSION
■
In summary, a series of oligomers containing the fused-ring unit
thieno[3,2-b]furan were synthesized. It was found that
migration of the furan ring toward the terminal position of
the oligomer results in a narrowing of the HOMO−LUMO
gap, whereas placement of furan at the interior position yields
an increase in planarization, extinction coefficient, and
fluorescence quantum yield in solution. In the solid state, all
three regioisomer dimers of thieno[3,2-b]furan present π−π
interactions which are absent in nonfused oligothiophenes,
oligofurans, thiophene−furan hybrid oligomers, and the
Figure 5. Crystal structure diagram of oligomer 1 illustrating (a) the
slipped π-stacked packing motif and (b) the orientation of oligomers
in adjacent columns.
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157.4, 145.8, 139.8, 139.7, 138.0, 137.6, 127.2, 122.4, 119.4, 115.4,
107.8, 106.2. IR (KBr): 3109 (w), 3074 (w), 2922 (vw), 2852 (vw),
1606 (w), 1502 (w), 1477 (w), 1454 (w), 1398 (w), 1348 (m), 1194
(w), 1160 (w), 1130 (w), 1055 (s), 1005 (w), 926 (w), 897 (m), 797
(vs), 764 (m), 743 (m), 733 (m), 723 (s), 702 (vs), 677 (m), 630 (m),
552 (m) cm−1. MS (EI, 70 eV) m/z (relative intensity): 265.0 (2.0),
264.0 (14.4), 263.0 (15.8), 262.0 (100, M+), 232.9 (57.3), 200.9
(32.7), 68.9 (54.2). Anal. Calcd for C12H6OS3: C, 54.93; H, 2.31.
Found: C, 55.08; H, 2.21.
thiophene analogue 2,2′-bithieno[3,2-b]thiophene. Therefore,
the combination of ring fusion and furan incorporation into
oligothiophenes serves as a new approach to engineer the
electronic and solid-state properties of conjugated materials.
EXPERIMENTAL SECTION
■
General. Thieno[3,2-b]furan,34 2-bromothieno[3,2-b]furan,34
tributyl(thieno[3,2-b]furan-5-yl)stannane,34 2-bromothieno[3,2-b]-
thiophene,35 and tributyl(thieno[3,2-b]thiophen-2-yl)stannane36 were
prepared according to literature procedures. 2-Bromothieno[3,2-
b]furan and tributyl(thieno[3,2-b]furan-5-yl)stannane were each
>96% pure, with the impurity being the regioisomers tributyl(thieno-
[3,2-b]furan-2-yl)stannane and 5-bromothieno[3,2-b]furan, respec-
tively.34 THF was dried by passage through a column packed with
activated alumina. UV−vis spectroscopy was performed in CH2Cl2
solution (>10−5 M) at room temperature. Emission spectra were
collected in CH2Cl2 solution (>10−8 M) at room temperature, and
samples were excited at the wavelength corresponding to the longest
wavelength absorption maximum (Table 1). Infrared absorption
spectra were collected on a FT-IR instrument. Melting point ranges
were determined using a melting point apparatus equipped with a
thermocouple. Crystals of 1, 3, and 5 suitable for single crystal X-ray
diffraction were obtained by slow evaporation from a mixture of 3%
CH2Cl2 and 97% hexanes.
2,5′-Bithieno[3,2-b]furan (3). The title compound was prepared
according to a procedure analogous to that used for the synthesis of 2,
except using tributyl(thieno[3,2-b]furan-5-yl)stannane (6, 413 mg,
1.00 mmol), 2-bromothieno[3,2-b]furan (8, 203 mg, 1.00 mmol), and
Pd(PPh3)4 (23.1 mg, 20.0 μmol) to yield 3 (130 mg, 52%) as the
major product (>96% pure), with the impurity being the regioisomers.
A portion of pure 3 was isolated for characterization. mp 114−115 °C.
UV−vis (CH2Cl2): λ (log ε) = 360 (4.34, sh), 349 (4.45), 335 (4.38,
1
sh), 261 (3.86) nm. H NMR (500 MHz, CDCl3, δ): 7.55 (d, J = 2.1
Hz, 1H), 7.31 (s, 1H), 7.18 (d, J = 5.3 Hz, 1H), 7.07 (dd, J = 5.3, 0.6
Hz, 1H), 6.86 (s, 1H), 6.75 (dd, J = 2.1, 0.6 Hz, 1H). 13C NMR (125
MHz, CDCl3, δ): 157.6, 157.2, 152.9, 146.0, 133.4, 125.5, 125.1, 122.5,
110.8, 106.7, 106.3, 100.9. IR (KBr): 3136 (w), 3111 (w), 3091 (w),
2953 (vw), 2922 (vw), 2850 (vw), 1560 (w), 1500 (w), 1479 (w),
1458 (w), 1410 (w), 1402 (w), 1352 (s), 1211 (w), 1130 (w), 1088
(w), 1059 (m), 1034 (w), 1005 (w), 972 (m), 895 (m), 862 (w), 804
(m), 785 (s), 733 (vs), 706 (vs), 681 (m), 656 (s), 646 (m), 554 (m),
521 (m) cm−1. MS (EI, 70 eV) m/z (relative intensity): 249.0 (1.2),
248.0 (10.0), 247.0 (14.9), 246 (100, M+), 217.0 (48.3), 192.0 (43.2),
148.0 (33.0), 95.1 (23.1), 93.0 (30.8), 82.1 (22.9), 69.0 (47.3). Anal.
Calcd for C12H6O2S2: C, 58.52; H, 2.46. Found: C, 58.59; H, 2.28.
2-(Thieno[3,2-b]thiophen-2-yl)thieno[3,2-b]furan (4). The
title compound was prepared according to a procedure analogous to
that used for the synthesis of 2, except using tributyl(thieno[3,2-
b]thiophen-2-yl)stannane (9, 429 mg, 1.00 mmol), 2-bromothieno-
[3,2-b]furan (8, 203 mg, 1.00 mmol), and Pd(PPh3)4 (23.1 mg, 20.0
μmol) to yield 4 (166 mg, 63%) as the major product (>96% pure),
with the impurity being the regioisomers. A portion of pure 4 was
isolated for characterization. mp 178−179 °C. UV−vis (CH2Cl2): λ
(log ε) = 359 (4.33, sh), 347 (4.46), 335 (4.40, sh), 269 (3.70) nm. 1H
NMR (500 MHz, CDCl3, δ): 7.51 (s, 1H), 7.38 (d, J = 5.2 Hz, 1H),
7.25 (dd, J = 5.2, 0.6 Hz, 1H), 7.20 (d, J = 5.3 Hz, 1H), 7.08 (dd, J =
5.3, 0.6 Hz, 1H), 6.89 (s, 1H). 13C NMR (125 MHz, CDCl3, δ): 157.4,
152.6, 139.8, 138.2, 135.2, 127.6, 125.6, 125.1, 119.5, 115.1, 110.8,
101.4. IR (KBr): 3126 (w), 3107 (w), 3078 (w), 2918 (vw), 2848
(vw), 1556 (w), 1502 (w), 1466 (w), 1452 (w), 1439 (m), 1414 (w),
1350 (m), 1215 (w), 1190 (w), 1157 (w), 1082 (w), 1036 (w), 989
(m), 918 (m), 814 (m), 787 (s), 739 (m), 700 (vs), 658 (m), 644 (s),
633 (m), 527 (m) cm−1. MS (EI, 70 eV) m/z (relative intensity):
265.0 (2.1), 264.0 (14.2), 263.0 (16.5), 262.0 (100, M+), 233.0 (41.2),
207.9 (51.1), 163.9 (44.7), 95.1 (25.6), 82.1 (28.6), 69.0 (39.3). Anal.
Calcd for C12H6OS3: C, 54.93; H, 2.31. Found: C, 54.97; H, 2.21.
2,2′-Bithieno[3,2-b]furan (5). The title compound was prepared
according to a procedure analogous to that used for the synthesis of 2,
except using hexamethylditin (328 mg, 1.00 mmol), 2-bromothieno-
[3,2-b]furan (8, 406 mg, 2.00 mmol), and Pd(PPh3)4 (23.1 mg, 20.0
μmol) to yield 5 (176 mg, 71%) as the major product (>96% pure)
with the impurity being the regioisomers. A portion of pure 5 was
isolated for characterization. mp 225−226 °C. UV−vis (CH2Cl2): λ
(log ε) = 353 (4.45), 342 (4.44, sh), 335 (4.56), 328 (447, sh), 320
(4.42, sh), 225 (4.40) nm. 1H NMR (500 MHz, CDCl3, δ): 7.22 (d, J
= 5.3 Hz, 1H), 7.09 (dd, J = 5.3, 0.5 Hz, 1H), 6.98 (d, J = 0.5 Hz, H).
13C NMR (125 MHz, CDCl3, δ): 157.6, 149.4, 126.0, 124.9, 110.8,
101.7. IR (KBr): 3134 (w), 3122 (w), 3107 (vw), 3093 (w), 1599 (w),
1444 (s), 1360 (w), 1178 (m), 1088 (m), 1043 (s), 1026 (s), 893 (s),
858 (s), 827 (m), 798 (vs), 735 (s), 704 (vs), 654 (vs), 548 (m) cm−1.
MS (EI, 70 eV) m/z (relative intensity): 249.1 (1.25), 248.1 (9.7),
247.1 (15.0), 246.1 (100, M+), 217.1 (26.2), 192.0 (75.6), 148.0
(23.9), 69.0 (30.4). Anal. Calcd for C12H6O2S2: C, 58.52; H, 2.46.
Found: C, 58.52; H, 2.43.
5,5′-Bithieno[3,2-b]furan (1). In a dry two-neck flask fitted with a
reflux condenser, thieno[3,2-b]furan (208 mg, 1.68 mmol) was
dissolved in THF (30 mL), and the solution was sparged with N2
gas. A 1.6 M butyllithium solution in hexanes (1.00 mL, 1.60 mmol)
was added to the solution at 0 °C under a N2 atmosphere. After 30
min at this temperature, the reaction mixture was allowed to warm to
rt over 15 min. Fe(acac)3 (1.13 g, 3.20 mmol) was then added to the
flask in one portion, and the solution was heated to reflux for 12 h.
Upon cooling to rt, the reaction mixture was poured into stirring
ethanol and cooled to −78 °C. The resulting precipitate was collected
by cold filtration on a fritted funnel and rinsed with cold ethanol. The
crude solid was purified by column chromatography on silica gel (3%
CH2Cl2 in hexanes) to yield 1 (73.0 mg, 37%) as the major product
(>96% pure) with the impurity being the regioisomers. A portion of
pure 1 was isolated for characterization. mp 137−138 °C. UV−vis
1
(CH2Cl2): λ (log ε) = 370 (4.19, sh), 354 (4.32), 259 (3.93) nm. H
NMR (500 MHz, CDCl3, δ): 7.53 (d, J = 2.1 Hz, 2H), 7.17 (d, J = 0.6
Hz, 2H), 6.73 (dd, J = 2.1, 0.6 Hz, 2H). 13C NMR (125 MHz, CDCl3,
δ): 157.4, 145.7, 138.1, 122.2, 107.3, 106.2. IR (KBr): 3144 (vw), 3120
(w), 3070 (w), 1680 (vw), 1597 (w), 1479 (m), 1373 (m), 1336 (m),
1165 (w), 1132 (m), 1063 (m), 1003 (m), 897 (m), 847 (w), 795
(vs), 733 (s), 719 (vs), 675 (m), 577 (m), 552 (m) cm−1. MS (EI, 70
eV) m/z (relative intensity): 249.0 (1.4), 248.0 (9.9), 247.0 (14.5),
246.0 (100, M+), 217.0 (32.8), 189.9 (27.9), 185.0 (19.2), 69.0 (59.8).
Anal. Calcd for C12H6O2S2: C, 58.52; H, 2.46. Found: C, 58.56; H,
2.32.
5-(Thieno[3,2-b]thiophen-2-yl)thieno[3,2-b]furan (2). In a
pressure vessel, tributyl(thieno[3,2-b]furan-5-yl)stannane (6, 413 mg,
1.00 mmol) and 2-bromothieno[3,2-b]thiophene (7, 219 mg, 1.00
mmol) were combined in toluene (10 mL), and the solution was
sparged with N2 gas. Pd(PPh3)4 (23.1 mg, 20.0 μmol) was added to
the mixture all at once, and the vessel was quickly sealed. The reaction
mixture was stirred for 12 h at 130 °C. The solution was then diluted
with hexanes and passed through a silica plug. Solvent was removed
from the filtrate in vacuo, and the residue was recrystallized from
methanol (−78 °C). The solid was collected by cold filtration on a
fritted funnel, rinsed with cold methanol, and then purified by column
chromatography on silica gel (3% CH2Cl2 in hexanes) to yield 2 (150
mg, 57%) as the major product (>96% pure) with the impurity being
the regioisomers. A portion of pure 2 was isolated for characterization.
mp 170−171 °C. UV−vis (CH2Cl2): λ (log ε) = 368 (4.20, sh), 353
(4.34), 345 (4.33, sh), 269 (3.83, sh) nm. 1H NMR (500 MHz,
CDCl3, δ): 7.54 (d, J = 2.1 Hz, 1H), 7.37 (d, J = 0.6 Hz, 1H), 7.36 (d,
J = 5.3 Hz, 1H), 7.23 (dd, J = 5.3, 0.6 Hz, 1H), 7.20 (d, J = 0.6 Hz,
1H), 6.73 (dd, J = 2.1, 0.6 Hz, 1H). 13C NMR (125 MHz, CDCl3, δ):
9302
dx.doi.org/10.1021/jo301744s | J. Org. Chem. 2012, 77, 9298−9303

The Journal of Organic Chemistry
Article
(27) Henssler, J. T.; Zhang, X.; Matzger, A. J. J. Org. Chem. 2009, 74,
9112.
(28) Zhang, X.; Johnson, J. P.; Kampf, J. W.; Matzger, A. J. Chem.
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and NMR spectra for 1−5, as well as
CIF files and crystallographic information for 1, 3, and 5. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
Mater. 2006, 18, 3470.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: matzger@umich.edu.
■
1988, 44, 427.
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(33) Bredas, J. L.; Calbert, J. P.; da Silva, D. A.; Cornil, J. Proc. Natl.
Notes
Acad. Sci. U.S.A. 2002, 99, 5804.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation
(CHE 0957591). The authors thank Dr. William W. Porter III
for X-ray structure determination.
(39) Balbas
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