Synthesis and characterization of thieno[3,4- b ]pyrazine-based terthienyls: Tunable precursors for low band gap conjugated materials

Article abstract of DOI:10.1021/jo400577q

Synthetic methods have been developed for the preparation of new 2,3-dihalo- and 2,3-ditriflato-5,7-bis(2-thienyl)thieno[3,4-b]pyrazines. From these reactive intermediates, a variety of new 2,3-difunctionalized 5,7-bis(2-thienyl)thieno[3,4-b]pyrazines hav

Full text of DOI:10.1021/jo400577q

Article
pubs.acs.org/joc
Synthesis and Characterization of Thieno[3,4‑b]pyrazine-Based
Terthienyls: Tunable Precursors for Low Band Gap Conjugated
Materials
Ryan L. Schwiderski and Seth C. Rasmussen*
Department of Chemistry and Biochemistry, North Dakota State University, NDSU Dept. 2735, P.O. Box 6050, Fargo, North Dakota
58108-6050, United States
S
* Supporting Information
ABSTRACT: Synthetic methods have been developed for the preparation of new 2,3-dihalo- and 2,3-ditriflato-5,7-bis(2-
thienyl)thieno[3,4-b]pyrazines. From these reactive intermediates, a variety of new 2,3-difunctionalized 5,7-bis(2-thienyl)-
thieno[3,4-b]pyrazines have been produced as precursors to conjugated materials. Structural, electronic, and optical
characterization of these new analogues illustrate the extent to which the electronic nature of the functional groups can be
used to tune the electronic properties of these thieno[3,4-b]pyrazine-based terthienyl units.
Scheme 1. Synthesis of Simple Thieno[3,4-b]pyrazine-Based
Terthienyls via Condensation
INTRODUCTION
■
The fused-ring thieno[3,4-b]pyrazine (TP) unit1 has become a
quite popular building block and has been shown to be very
successful in producing reduced band gap (Eg = 1.5−2.0 eV)2,3
and low band gap (Eg < 1.5 eV)2−4 conjugated organic
materials. As the material’s band gap determines a number of
the desirable properties of conjugated polymers, control of this
critical parameter is an important factor in the production of
technologically useful materials and the application of TPs has
been one of the more successful approaches to this goal.2
Although the simple TP unit has been frequently used as a
monomer in the production of numerous homo- and co-
polymeric materials,2−4 some materials scientists find these
monomeric units difficult to work with due to their somewhat
reactive nature. As a consequence, many favor the application
of the terthienyl analogues such as 5,7-bis(2-thienyl)thieno[3,4-
b]pyrazine5,6 (Scheme 1, 4a), in which the reactive α-positions
of the TP are substituted with 2-thienyl groups. While these
TP-based terthienyls actually undergo oxidation at lower
potentials than the simple TP monomers, the increased size
and conjugation length of the oligomeric analogues results in
slower reactivity and thus a lower chance of loss of product to
unwanted oxidative coupling or decomposition.
in a method analogous to the monomeric TPs as shown in
Scheme 1. Here, the bromo functionalities of the common
precursor 2,5-dibromo-3,4-dinitrothiophene (1)7 are utilized to
generate the 3′,4′-dinitro-2,2′:5′,2″-terthiophene (2) via Stille
coupling with 2-(tributylstannyl)thiophene. Reduction of 2 to
generate the diamine 3 then allows condensation with various
The first examples of TP-based terthienyls were reported by
Yamashita and co-workers in 1994,5 and these oligomeric
species have since become common precursors to TP-based
copolymeric materials.4 Such TP-based terthienyls are prepared
Received: March 19, 2013
© XXXX American Chemical Society
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α-diones to generate the central TP unit of the final terthienyl
4. These methods have since been used to produce a wide
variety of unfunctionalized,5,6 dialkyl,5,8−13 diaryl,14−22 and
even heteroaryl16a,17,23−25 TP-based terthienyls.
material isolated consisting of the noncyclized product 11 as
shown in Scheme 3. The addition of a catalytic amount of HCl
resulted in significantly higher yields, but now generated a
mixture of 10 and byproduct 12 (Scheme 3).
The choice of functional groups for either the monomeric
TPs or the terthienyl analogues here is dependent on the α-
dione employed as illustrated in Scheme 1. As a consequence,
this has limited the functional groups to either alkyl or aryl, thus
resulting in a very narrow range of electronic variance and
restricting the extent of possible tuning available in the
application of these fused-ring units to the production of low
band gap materials. In order to overcome this limitation, our
group has recently reported new synthetic methods for the
production of monomeric TPs which allows the incorporation
of a wide variety of side chains, including both strongly
electron-donating and -withdrawing groups.26 This current
report now expands this new synthetic approach to the TP-
based terthienyls through the synthesis of new 2,3-difunction-
alized 5,7-bis(2-thienyl)thieno[3,4-b]pyrazines containing facile
leaving groups (Chart 1) as precursors to a variety of new
Scheme 3. Synthesis of 5,7-Bis(2-thienyl)thieno[3,4-
b]pyrazine-2,3(1H,4H)-dione
Chart 1. Thieno[3,4-b]pyrazine-Based Terthienyls with
Good Leaving Groups
Although 10 could be selectively extracted from the mixture
by aqueous KOH, the ratio of the two species in the mixture
was approximately 2.8:1, resulting in a significant loss of yield.
Attempts to shift this ratio by treatment of the mixture with
aqueous HCl at reflux temperatures appeared to have no effect.
Finally, changing the solvent conditions of the reaction to a
water−ethanol mixture strongly enhanced the formation of 10
with only trace amounts of the byproduct 12 detected. Using
these modified conditions, the desired product 10 was
successfully isolated in yields of 85% and its structure was
confirmed by X-ray crystallography.
To further explore the formation of 12 in these reactions, a
pure sample of 10 was heated at reflux in absolute ethanol
containing a catalytic amount of HCl over a period of 24 h.
Under these conditions, ∼53% of the original sample of 10 was
converted to 12. This confirms that 12 is produced via
nucleophilic attack of ethanol on 10, rather than production
directly from diamine 3. It is thus believed that it is the
enhanced ethanol solubility of 10 in comparison to the
monomeric analogue 8, as well as the presence of catalytic HCl,
that is responsible for the difference in reactivity observed here.
By moving to the more polar ethanol−water mixture, the
solubility of 10 is reduced and therefore has less opportunity to
react further to produce the unwanted byproduct 12.
With the successful production of 10 accomplished, efforts
turned to its conversion to the corresponding 2,3-dibromo-5,7-
bis(2-thienyl)thieno[3,4-b]pyrazine (5) via the same conditions
previously used for the conversion of 8 to 9.26 However, as with
the production of 10 discussed above, the conditions that were
successful for monomeric TPs did not translate well to the
corresponding terthienyl analogues. As shown in Scheme 4,
attempts to convert 10 to 5 resulted in very low yields with
significant amounts of black insoluble material indicative of
oxidative polymerization. Although the conditions applied in
Scheme 4 had been previously optimized to reduce the
decomposition of thiophene species via oxidative coupling and
polymerization processes,26 it is nearly impossible to eliminate
such processes in the oxidative conditions of the PBr5 mixtures
analogues. Initial efforts in the application of terthienyl 7 to
tunable functionalized TP-based terthienyls have been recently
reported as part of a study on side chain tuning of TP-based
materials.27 The synthetic versatility of the precursor species 5−
7 has been demonstrated here with the production of new TP-
terthienyls containing electron-donating and -withdrawing
groups. The characterization of these new molecular precursors
provide further insight to the extent that the electronic
properties of these species can be tuned through varying the
respective TP functional groups.
RESULTS AND DISCUSSION
■
Synthesis. 2,3-Dibromothieno[3,4-b]pyrazine (9), the
monomeric TP analogue of terthienyl 5, was previously
prepared via the intermediate thieno[3,4-b]pyrazine-2,3-
(1H,4H)-dione (8)26 as illustrated in Scheme 2. Using this
synthetic methodology as a starting point, this same basic
approach was then applied to the generation of 5. However,
treatment of the diamine 3 with diethyl oxalate using the
previously successful conditions resulted in very low yields
(<10%) of the desired product 10, with the majority of the
Scheme 2. Previous Synthesis of 2,3-Dibromothieno[3,4-
b]pyrazine
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Scheme 4. Conversion of 10 to TP-Based Terthienyls with Good Leaving Groups
Scheme 5. Proposed Mechanism for the Formation of Terthienyl 6
used. As terthienyls undergo oxidation at potentials nearly 1.0 V
(∼0.80−0.90 V)2 lower than their corresponding monomeric
analogues, decomposition via oxidative pathways becomes a
much more significant problem in the case of the terthienyls 5
and 10, which limits the successful isolation of the desired
dihalo product 5 under these bromination conditions.
Scheme 6. Generation of New TP-Based Terthienyls via
Simple Substitution of 6
Due to the difficulties encountered with the isolation of 5,
alternate methods for the generation of TP-based terthienyls
containing good leaving groups were then investigated. As a
number of groups had reported the activation of amides with
triflic anhydride to generate triflatoiminium triflate salts,28−31
these conditions were then applied to 10 in an attempt to
produce the ditriflato analogue 6 (Scheme 4). Typical
conditions for such reactions utilize various pyridine bases to
neutralize the triflic acid generated in this process, although the
application of less bulky pyridines have been reported to
undergo substitution reactions with the triflatoimine product to
generate the pyridinium species.30,31 In order to avoid such
secondary reactions with the pyridine base, 2,6-lutidine was
chosen as a less nucleophilic, yet cost-efficient base. Treatment
of 10 with 2.2 equiv of triflic anhydride and 2.6 equiv of 2,6-
lutidine successfully produced the desired ditriflato species 6 as
a reactive, yet stable, purple solid in near-quantitative yields.
Based on previous mechanistic studies of such amide
activations,30 a proposed mechanism for the formation of 6
from 10 is given in Scheme 5. It is possible that the lutidine
reacts first with the triflic anhydride to generate 2,6-dimethyl-
N-(trifluoromethylsulfonyl)pyridinium triflate, by analogy to
previously reported methods utilizing pyridine.30 However, the
reduced nucleophilic nature of the lutidine should inhibit or at
least limit this process. If this pyridinium intermediate is
produced, the proposed mechanism would deviate only in the
exact identity of the electrophile in the initial step.
In order to explore the application of 6 to the production of
new difunctionalized TP-based terthienyls, new analogues were
generated as shown in Scheme 6. New TP-based terthienyls
include the diethylamino species 13, dihexyloxy species 14, and
the phenylalkynyl analogue 15. All of these new TP-based
terthienyls were produced in moderate to good yields by simple
nucleophilic aromatic substitution of the triflate groups of 6.
Lastly, to further expand the variety of functionality possible
in TP-based terthienyls, the bis(bromomethyl) analogue 7
(Scheme 7) was produced via the room temperature
condensation of diaminoterthiophene 3 with the commercially
available 1,4-dibromo-2,3-butanedione. As in the case of 6
Scheme 7. Synthesis of New 2,3-Difunctionalized TP-Based
Terthienyls from Precursor 7
above, ether (16a,b) or amine (17) functionalities are then
produced via simple substitution reactions.
Crystallography. The X-ray quality crystals of the
precursor dione 10 were grown from the slow evaporation of
methanol solutions, and selected bond angles are given in Table
1. The pyrazine-2,3-dione ring of 10 represents a cyclic diamide
which could potentially exist in multiple tautomeric forms as
shown in Scheme 8. For typical amides, however, the
equilibrium favors the amide tautomer almost exclusively over
the iminol (K = 108),32,33 regardless of the nature of any
functional group bound to the carbonyl. For cyclic systems such
as 4-hydroxypyrimide, in which the iminol form represents an
aromatic system, one would expect greater iminol content. In
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Table 1. Experimental Bond Lengths of 5,7-Bis(2-thienyl)thieno[3,4-b]pyrazine-2,3(1H,4H)-dione (10) and 2,3-
Difunctionalized 5,7-Bis(2-thienyl)thieno[3,4-b]pyrazines 4b, 4e, 7, and 14
a
b
bond
10
4b
7
14
4e
S(1)−C(4)
C(1)−C(2)
C(3)−C(4)
C(2)−C(3)
C(4)−C(5)
S(2)−C(5)
C(5)−C(6)
C(6)−C(9)
N(1)−C(6)
N(1)−C(7)
C(7)−C(8)
C(7)−X(1)
1.699(3)
1.360(4)
1.414(4)
1.443(3)
1.456(3)
1.727(3)
1.376(2)
1.420(4)
1.394(3)
1.355(2)
1.537(4)
1.222(3)
1.719(5)
1.34(1)
1.695(3)
1.343(4)
1.371(7)
1.488(8)
1.438(3)
1.733(2)
1.394(3)
1.426(3)
1.370(3)
1.311(3)
1.458(3)
1.489(3)
1.727(2)
1.359(2)
1.422(2)
1.428(2)
1.445(2)
1.735(2)
1.380(2)
1.419(2)
1.382(2)
1.291(2)
1.469(2)
1.341(2)
1.712(7)
1.32(1)
1.39(1)
1.41(1)
1.45(1)
1.713(6)
1.399(9)
1.427(9)
1.371(7)
1.313(9)
1.47(1)
1.489(7)
1.424(7)
1.417(8)
1.442(7)
1.719(5)
1.406(7)
1.418(6)
1.381(6)
1.298(7)
1.469(7)
1.495(8)
a
b
Reference 5a. Reference 16a.
The X-ray quality crystals of the TP-based terthienyls 7 and
14 were also obtained, and selected bond lengths are given in
Table 1, along with previously reported data from the analogues
4b5a and 4e16a for comparison. The bond lengths and angles of
all four TP-based terthienyls are in fairly good agreement, and
for the most part, the terthienyl backbones adopt a
predominately s-trans configuration analogous to α-terthio-
phene.36 Although the previously reported TP-based terthienyls
exhibit nonplanar conjugated backbones in which the external
thiophenes are rotated ∼10°−25° out of the plane of the
central TP,5a,16a both 7 and 14 show near-planar geometries
consistent with the structure of α-terthiophene. Terthienyl 14
exhibits torsion angles of 7.9° and 8.9°, similar to that of α-
terthiophene (6°−9°),36 while 7 exhibits a very planar
backbone with torsion angles of only 0.7° and 1.2°.
Scheme 8. Possible Tautomeric Forms of Pyrazine-2,3-dione
fact, this does seem to be true in the gas phase, where the
equilibrium has been reported to be near unity, although the
oxo form is again thought to predominate in either solution or
the crystalline state.34 For compound 10, the combination of a
cyclic system capable of an aromatic form and two different
amide groups both able to undergo tautomerism could further
favor the iminol form. However, this does not seem to be the
case, and the determined solid-state structure of 10 appears to
consist exclusively of the dione tautomer. The C−O bond
lengths here are 1.222 and 1.226 Å, both slightly shorter than
the typical CO bond length of 1.23 Å.35 While the C−N
bond lengths (1.355 and 1.351 Å) exhibit some shortening in
comparison to a pure CN single bond (1.47 Å),35 they are
still longer than that reported for formamide (1.32 Å).35 As
such, the dione structure given in Schemes 3−5 seems to
accurately represent the crystalline structure of 10.
Typical of monomeric TPs,1,26 all four TP-based terthienyls
exhibit some bond fixation with the fused pyrazine ring. For
example, while the delocalized structure of the parent pyrazine
heterocycle results in four equivalent CN bonds, the fused
pyrazine of TPs exhibits elongation of the thiophene-N bonds
and shortening of the exterior CN bonds of the pyrazine
a
Table 2. UV−visible Absorption Data for a Series of TP-Based Terthienyls
S0→S1 (ICT)
S0→S2 (π→π*)
terthienyl
λmax (nm)
ε (M−1 cm−1
)
f
λmax (nm)
ε (M−1cm−1
)
f
4b
4f
492
540
542
556
544
404
444
586
511
500
12100
7800
7400
b
0.242
0.123
0.134
b
339
338
313
337
312
348
328
361
339
338
21000
45100
35400
b
0.268
0.908
1.391
b
5
6
7
7100
11800
10200
4400
9600
9600
0.146
0.225
0.193
0.065
0.192
0.197
33300
12800
19000
58000
21400
19300
1.012
0.163
0.390
1.327
0.349
0.321
13
14
15
16a
17
a
b
In CH3CN. Accurate determination not possible due to reaction with solvent.
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ring, i.e. N(1)C(7). In the examples here, these exterior C
N bond lengths have an average length of 1.30 Å, nearly
equivalent to that of localized CN bonds (1.28 Å).37
Comparison of the TP of 14 with the analogous bonds of 10
further illustrates the structural distinction between the diamide
form of 10 and the dialkoxyaromatic ring of 14. As expected,
the N(1)C(7) bond of 14 exhibits significant shortening in
comparison to 10 (1.291 vs 1.355 Å). In addition, the CO
bond of 14 exhibits notable elongation relative to the CO
bond of 10 (1.341 vs 1.222 Å).
The external thiophenes of the TP-based terthienyls exhibit
deviations from typical thiophene geometry due to positional
disorder resulting from a portion of these heterocycles that are
rotated 180° around the interannular bond, resulting in some
mixing of the S1/C3 and S3/C12 positions. This is most
pronounced in the structure of 7, where positional disorder is
observed in both external rings, resulting in approximately
17.32% mixing of the S1/C3 positions and 31.53% mixing of
the S3/C12 positions. Terthienyl 14 exhibits disorder in only
one of the two external thiophenes, resulting in mixing of
approximately 7.43% for the S3/C12 positions. Other
deviations include shortened external α,β-C−C bonds (i.e.,
C(1)−C(2) = 1.32−1.36 Å) in comparison to the parent
thiophene (1.370 Å).38 Both the positional disorder of the
external thiophenes and the shortening of external α,β-C−C
bonds have been previously observed for other oligothio-
phenes,39−41 as well as the previous TP-based terthienyls 4b
and 4e.5a,16a
As can be seen in Table 2, the extinction coefficients and
oscillator strengths of these transitions are somewhat low. This
is due to a reduced “allowedness” of the transition as a result of
limited spatial overlap of the orbitals involved in the ICT
transition.44
Two stronger sets of bands are exhibited in the higher-energy
region of 250−350 nm, which are assigned as two π→π*
transitions localized primarily on the terthienyl backbone. As
would thus be expected, the energies of these transitions agree
well with the absorption energies reported for α-terthiophene.43
For the most part, the extinction coefficients for these π→π*
bands fall roughly between 20 × 103 and 58 × 103 M−1 cm−1,
with corresponding oscillator strengths of approximately 0.30−
1.30 and ranging from moderately allowed to fully allowed
transitions.
Overall, the effect of the side chains in the TP-based
terthienyls is similar to that previously reported for the
monomeric analogues,26 although with some significant
differences. As shown in both Table 2 and the absorption
spectra in Figure 2, the HOMO−LUMO energy, and thus the
UV−vis Spectroscopy. The spectral data of various TP-
based terthienyls are given in Table 2, and a representative
UV−vis spectrum of 4b is shown in Figure 1. The absorption
Figure 2. Normalized visible spectra of a series of TP-based
terthienyls.
energy of the ICT transition, decreases as the electron-
withdrawing nature of the side chain increases. Thus, by simply
changing the identity of the side chains of the TP-based
terthienyls reported here, it is possible to shift the λmax of the
ICT transition by over 180 nm. When presented as such in
terms of wavelength, this extent of tuning appears even greater
than that previously reported for the simple TP monomeric
analogues.26 However, when viewed in terms of a linear
electronvolt scale, the changes in energy are actually similar
(∼0.9 vs 1.1 eV).
Electrochemistry. The electrochemical data of the TP-
based terthienyls are given in Table 3, and a representative
cyclic voltammogram is shown in Figure 3. Typical of
monomeric TPs,26,45 the TP-based terthienyls exhibit a well-
defined irreversible oxidation assigned to the oxidation of the
terthiophene backbone, as well as a quasireversible pyrazine-
based reduction. As for the majority of thiophene species, the
irreversible nature of the oxidation here is due to the formation
of thiophene-based radical cations. The rapid coupling of these
radical cations then results in higher oligomeric and polymeric
species, which accounts for the observed irreversibility. It
should also be noted that, for terthienyls 13 and 17, the
alkylamino side chains are also redox active, and thus oxidations
of both the amine functionality and the thiophene are observed.
Figure 1. UV−visible spectrum of TP-based terthienyl 4b in CH3CN.
properties of the TP-based terthienyls are characterized by a
broad, low energy transition, which is formally assigned as an
intramolecular charge transfer (ICT) band. This assignment is
consistent with previous photophysical studies that have shown
that the lowest energy absorption of monomeric TPs is a broad
ICT band resulting from a transition between a predominately
thiophene-localized HOMO and a LUMO of greater pyrazine
contribution.1,26,42 For the TP-based terthienyls, the addition of
the 2-thienyl groups to the 5- and 7-positions of the TP unit
results in a HOMO which is now delocalized across the
terthienyl backbone, while still retaining a LUMO dominated
by pyrazine contributions (Figure 1). The ICT nature of this
low energy transition also accounts for the significant red shift
observed for dialkyl-functionalized TP-based terthienyls in
comparison to the parent α-terthiophene (492 vs 354 nm).43
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same exact change in the TP-based terthienyls results in only a
shift of 170 mV. This difference is due to the fact that the effect
of the two additional 2-thienyl groups in the terthienyls
contributes more significantly to the HOMO than the
functional groups at the 2- and 3-positions of the TP unit.
This results in delocalization of the HOMO across the
terthienyl backbone and significant destabilization of the
HOMO energy, as illustrated by a shift in the potential of
oxidation by 850 mV from monomeric TP to TP-based
terthienyl.26,45 Thus the side chain effects of the central TP unit
are diluted across the large conjugated backbone such that the
destabilization of the HOMO by the added external thiophenes
effectively overpowers the weaker contribution of the TP side
chains.
Table 3. Electrochemical Data for a Series of TP-Based
Terthienyls
a
oxidation
reduction
a
terthienyl
Ep , V
E1/2, V
ΔE, mV
4b
4f
0.50
−1.68
−1.46
−1.14
−0.99
−1.26
nwsw
100
60
−
−
160
0.54
0.70
0.80
0.62
b
5
b
6
7
c
13
14
15
16a
16b
17
−0.06, 0.24
0.45
−
90
−
110
120
90
−2.03
−0.96
−1.48
−1.48
−1.65
b
0.59
0.57
0.57
The variations in the potentials of both oxidation and
reduction of the TP-based terthienyls correlate well with the
c
0.32, 0.62
+
−
a
All potentials vs Ag/Ag+. nwsw = not within solvent window.
Irreversible; value corresponds to Ep . Nitrogen-based oxidation.
respective Hammett substituent constants (σp or σp ) as
b
c
c
shown in Figure 4. The shifts in redox potentials of comparable
Figure 3. Cyclic voltammogram of terthienyl 7.
In these cases, the initial oxidation is that of the amine, as
previously observed in the electrochemistry reported for 2,3-
di(N-decylamino)thieno[3,4-b]pyrazine26 and the related 3-(N-
alkylamino)thiophenes.46
As previously seen for monomeric TPs,26 the potential of
oxidation becomes more positive as the electron-withdrawing
nature of the side chain is increased, indicative of a stabilization
of the HOMO. With the exception of terthienyls 5 and 6,
however, all of the TP-based terthienyls reported here still
undergo oxidation at lower potentials than the parent α-
terthiophene (0.65 V vs Ag/Ag+).47 As might be expected, the
effect on the reduction potentials follows the opposing trend,
with electron-withdrawing groups reducing the potential to as
low as −0.99 V. Likewise, electron-donating groups increase the
potential to the extent that the reduction of TP-based
terthienyls with strongly donating groups such as dialkylamino
are no longer within the measurable solvent windows of
CH3CN. For terthienyl 15, an additional sharp irreversible
oxidation was observed at −60 mV after cycling through the
reduction at ∼ −1 V. While the nature of this additional redox
process is currently unknown, it is believed that this may be the
result of a redox-initiated 1,5-cyclization of the enediyne
structure. Similar cyclizations of enediynes via both oxida-
tion48,49 and reduction50 processes have been previously
reported, and a separate study of the electrochemical,
photochemical, and thermal cyclization of 15 is planned.
While the side chain effect on the reduction potentials of the
TP-based terthienyls is essentially the same as that previously
seen with the monomeric TPs, the effect on the potential of
oxidation is significantly attenuated in the terthienyl analogues.
For example, changing the functional groups of monomeric
TPs from the hexyloxy to methylbromo results in nearly a 300
mV shift in the potential of oxidation.26 In comparison, the
Figure 4. Hammet plot of TP-based terthienyls.
molecular species are dependent on the polar, steric, and
mesomeric effects exerted by the substituents as described by
the Hammett−Taft equation:
E = ρ σ + S
π
where ρπσ describes the polar-mesomeric parameters and S
accounts for the steric factors.26,46,51−53 In cases of low or
comparable steric interactions as a result of the added
functional groups (i.e., S is constant), linear relationships are
+
found between the potential of oxidation and σp (or the
−
reduction potential and σp ).
+
As seen in Figure 4, both plots (oxidation vs σp and the
−
reduction vs σp ) give linear relationships with respective R
values of 0.900 and 0.953. In terms of the potential of
oxidation, the plot gives a ρπ value of 0.113, exactly one-third of
that previously found for the corresponding monomeric
thieno[3,4-b]pyrazines (ρπ= 0.340)26 and significantly lower
than that for typical thiophenes (ρπ = 0.80).51 The lower ρπ
values for both sets of thieno[3,4-b]pyrazine compounds is
indicative of the diminished effect of the functional group on
the potential of oxidation in these species. This lower effect is
expected as the functional groups are spatially removed from
the site of oxidation localized either on the thiophene ring of
the TP unit or across the terthienyl backbone. As the oxidation
F
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3′,4′-Diamino-2,2′:5′,2″-terthiophene (3). The following is a
modification of previously reported methods.5b Terthiophene 2 (13.5
g, 40.0 mmol) was suspended in a mixture of ethanol (340 mL) and
concentrated HCl (280 mL). Tin metal (45.6 g, 384 mmol) was then
added slowly in small portions, and the mixture stirred at room
temperature for 72 h. The mixture was then cooled to −25 °C and
filtered, and the isolated solid was washed with diethyl ether. The light
green diammonium salt was added to 400 mL of cold water and made
basic with 1 M KOH. The basic solution was then extracted with
CH2Cl2 until no color was observed in the extract. The combined
organics were dried with MgSO4, filtered, and concentrated under
vacuum. The crude product was purified by silica chromatography
(gradient CHCl3 to 5% triethylamine in CHCl3) to give 8.4−8.9 g of a
yellow-brown powder (75−80%). Mp 90−92 °C (lit.5b96.0−96.5 °C).
1H NMR: δ 7.25 (dd, J = 1.2, 3.6 Hz, 2H), 7.07 (m, 4H), 3.71 (s, 4H).
13C NMR: δ 136.2, 133.8, 128.0, 124.2, 124.1, 110.4. All NMR values
of the TP-terthienyls is delocalized across the terthienyl
backbone, the one-third reduction in the side chain effects is
perfectly consistent with the fact that only one-third of that
backbone is comprised of a functionalized TP.
The plot of the reduction potentials of the TP-based
terthienyls gives a higher value of ρπ = 0.694, indicating a higher
effect of the functional group on the TP reduction as expected.
What was unexpected, however, is that this value is
considerably larger than that previously found for the
monomeric thieno[3,4-b]pyrazines (ρπ = 0.463).26 As the side
chain contribution to the pyrazine ring should be identical
between the monomers and the TP-based terthienyls, the ρπ
values should be fairly consistent. Re-evaluation of the original
thieno[3,4-b]pyrazine data26 indicates that one outlying data
point for the 2,3-dicyanothieno[3,4-b]pyrazine monomer is
responsible for the skewing of the determined slope. Removal
of this data point results in a newly calculated ρπ value of 0.613
for the TP monomers, which is now in good agreement with
the TP-based terthienyls studied here. The cause of the poor
correlation for the 2,3-dicyanothieno[3,4-b]pyrazine26 mono-
mer is currently unknown, and its electrochemistry may need to
be restudied in greater detail.
agree with previously reported values.5b
5,7-Bis(2-thienyl)thieno[3,4-b]pyrazine-2,3(1H,4H)-dione
(10). Diamine 3 (1.39 g, 5.0 mmol) was added to 120 mL of a 1:1
ethanol/water mixture under a nitrogen atmosphere. Diethyl oxalate
(5.98 mL, 44 mmol) and concentrated HCl (0.20 mL, 6.5 mmol) were
then added and heated at reflux for 24 h. The mixture was then cooled
to room temperature, and the ethanol was removed under vacuum.
The remaining mixture was then cooled in a freezer, after which the
resulting precipitate was collected via vacuum filtration and washed
with water (300 mL). The collected solid was then dissolved in 1 M
aqueous KOH (500 mL) and filtered to remove any undissolved
material. The KOH solution was then made acidic, and the resulting
precipitate was collected via vacuum filtration. The solid was washed
with water (∼100 mL) and then diethyl ether until dry to produce
1.33−1.41 g of a yellow powder (80−85%). Mp 216 °C dec. 1H NMR:
δ 7.17 (m, 4H), 7.46 (dd, J = 2.0, 4.4 Hz, 2H), 8.68 (br, 2H); 13C
NMR (d-DMSO): δ 159.1, 136.4, 133.9, 133.2, 132.6, 129.3, 115.6;
HRMS m/z 354.9642 [M + Na]+ (calcd for C14H8N2NaO2S3
354.9640).
CONCLUSIONS
■
New classes of TP-based terthienyls containing electron-
donating and -withdrawing groups have been prepared from
terthienyls containing central 2,3-ditriflato- and 2,3-bis(bromo-
methyl)thieno[3,4-b]pyrazines. As previously demonstrated for
thieno[3,4-b]pyrazine monomers,26 the ability to vary the
functional groups allows significant tuning of the electronic and
optical properties of these popular conjugated building blocks.
The additional terminal thiophenes of the TP-based terthienyls
reduce the effect of the functional groups on the modulation of
the HOMO energies by approximately one-third in comparison
to that of the TP monomers, while having essentially no effect
on the modulation of the corresponding LUMO energies. As
such, the tuning ability of these TP-based terthienyls
complements that of the TP monomers and provides a
promising new class of building blocks for the production of
new low band gap materials.
2,3-Dibromo-5,7-bis(2-thienyl)thieno[3,4-b]pyrazine (5). The
reagents PBr5 (0.32 g, 0.75 mmol), Bu4NBr (0.19 g, 0.60 mmol), and
Na2CO3 (0.06 g, 0.6 mmol) were added to 50 mL of xylenes in a 100
mL round-bottom flask equipped with a Vigreaux column. This
column was then fitted with a condenser, with an outlet submerged in
2.5 M aqueous KOH solution. The entire assembly was maintained
under a gentle stream of nitrogen. The reaction mixture was then
heated to reflux, which resulted in a color change from deep red to
light yellow-orange. Compound 10 (0.10 g, 0.30 mmol) was then
added, and the reaction was allowed to stir at reflux overnight. The
reaction was quenched with a saturated aqueous NH4Cl solution (100
mL) and extracted with CH2Cl2 (3 × 50 mL). The combined organic
layers were dried with anhydrous MgSO4, concentrated, and purified
by column chromatography (2% CH2Cl2 in hexanes) to give ∼10 mg
EXPERIMENTAL SECTION
■
Materials. 2,5-Dibromo-3,4-dinitrothiophene (1),33 tributyl(2-
thienyl)stannane,54 2,3-dimethyl-5,7-bis(2-thienyl)thieno[3,4-b]-
pyrazine (4b),5b 2,3-diphenyl-5,7-bis(2-thienyl)thieno[3,4-b]pyrazine
1
of a purple solid (<10% yield). H NMR: δ 7.64 (dd, J = 1.1, 3.7 Hz,
2H), 7.42 (dd, J = 1.1, 5.1 Hz, 2H), 7.13 (dd, J = 3.7, 5.1 Hz, 2H).
5,7-Bis(2-thienyl)-2,3-bis(trifluoromethanesulfonato)thieno-
[3,4-b]pyrazine (6). Dione 10 (1.10 g, 3.3 mmol) was added to a
flask under a nitrogen atmosphere, which was then charged with 2,6-
lutidine (0.92 g, 8.6 mmol) and dry CH2Cl2 (150 mL) and cooled to
−10 °C in an ice/salt bath. Once cooled, trifluoromethanesulfonic
anhydride (7.26 mL, 1.0 M in CH2Cl2, 7.26 mmol) was added
dropwise over 1 h to limit heat evolution. The reaction was then
quenched with saturated aq. NaHCO3 (200 mL) and extracted with
CH2Cl2 (3 × 100 mL). The combined organics were dried with
MgSO4, filtered, and concentrated. The crude product was purified by
silica chromatography (1:1 CH2Cl2/hexanes) to give 1.87−1.95 g of a
purple solid (95−99%). Mp 174 °C dec. 1H NMR: δ 7.63 (dd, J = 1.2,
3.6 Hz, 2H), 7.47 (dd, J = 1.2, 5.2 Hz, 2H), 7.15 (dd, J = 3.6, 5.2 Hz,
2H); 13C NMR: δ 139.8, 133.1, 132.7, 128.4, 126.7, 128.1, 127.5,
126.6, 118.7 (q, JC−F = 300 Hz); HRMS m/z 618.8601 [M + Na]+
(calcd for C16H6F6N2O6S5Na 618.8626).
55
(4f),14a and PBr5 were all prepared as previously reported. Xylenes
and THF were distilled from sodium/benzophenone prior to use.
CH2Cl2 and CH3CN were dried over CaH and distilled prior to use.
DMF was dried over MgSO4 prior to use. All other materials were
reagent grade and used without further purification.
3′,4′-Dinitro-2,2′:5′,2″-terthiophene (2). The following is a
modification of previously reported methods.5b Compound 1 (17.2 g,
51.8 mmol) and tributyl(2-thienyl)stannane (42.51 g, 114 mmol) were
added to dry THF (250 mL), followed by the addition of
PdCl2(PPh3)2 (0.364 g, 1 mol %). The mixture was heated to reflux
for 16 h. After cooling, the reaction mixture was concentrated under
vacuum to approximately 50 mL. Hexanes were then added, and the
resulting orange precipitate was filtered and washed with hexanes. The
solid was recrystallized in methanol and purified by silica
chromatography (gradient of hexanes to 20% CH2Cl2 in hexanes) to
give 12.3−13.1 g (70−75%). Mp 144−145 °C (lit.5b149−151 °C). 1H
NMR: δ 7.61 (dd, J = 1.2, 5.2 Hz, 2H), 7.55 (dd, J = 1.2, 3.6 Hz, 2H),
7.18 (dd, J = 3.6, 5.2 Hz, 2H); 13C NMR: δ 136.2, 134.1, 131.5, 131.4,
128.6, 128.3.
2,3-Bis(N,N-diethylamino)-5,7-bis(2-thienyl)thieno[3,4-b]-
pyrazine (13). Ditriflate 6 (0.190 g, 0.32 mmol) was added to a flask
and brought under a nitrogen atmosphere. Dry CH2Cl2 (40 mL) was
G
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The Journal of Organic Chemistry
Article
added, followed by the addition of N,N-diethylamine (10.0 mL, 193
mmol). The reaction was allowed to stir at room temperature
overnight, after which it was quenched with saturated aq. NaHCO3
(100 mL) and extracted with CH2Cl2 (3 × 50 mL). The combined
organics were dried with MgSO4, filtered, and concentrated. The crude
product was purified by basic silica chromatography (prepared with
80:20 hexanes/CH2Cl2 containing 3% triethylamine, eluted with
hexanes/CH2Cl2) to give 0.13−0.14 g of a shiny red-orange solid
Hz, 2H), 4.86 (s); 13C NMR: δ 150.1, 137.5, 134.1, 127.7, 127.4,
126.3, 125.7, 31.6. NMR values agree with previously reported
values;27 HRMS m/z 506.8290 [M
+
Na]+ (calcd for
C16H10Br2N2S3Na 506.8265).
General Synthesis of 2,3-Dialkoxymethyl-5,7-bis(2-thienyl)-
thieno[3,4-b]pyrazine. Sodium alkoxide was prepared by addition of
NaH (0.40 g, 10.0 mmol, 60−65% in oil) to 20 mL of the specified
alcohol, and the mixture was stirred for 2 h to ensure complete NaH
consumption. The alkoxide solution was then added dropwise to 7
(0.49 g, 1.0 mmol) in 50 mL of dry CH2Cl2, and the mixture allowed
to stir for 6 h. Saturated aq. NH4Cl was then added, and the CH2Cl2/
alcohol solvents were removed by rotary evaporation and vacuum
distillation as needed. The aqueous mixture was then extracted with
CH2Cl2, after which the combined organic fractions were dried with
MgSO4 and concentrated.
1
(90−95%). Mp 106−109 °C. H NMR: δ 7.43 (dd, J = 1.2, 3.6 Hz,
2H), 7.27 (dd, J = 1.2, 5.2 Hz, 2H), 7.06 (dd, J = 3.6, 5.2 Hz, 2H), 3.64
(q, J = 7.1 Hz, 8H), 1.17 (t, J = 7.1 Hz, 12H); 13C NMR: δ 12.9, 43.9,
118.3, 122.2, 124.8, 126.9, 135.8, 136.3, 148.6; HRMS m/z 443.1393
[M + H]+ (calcd for C22H26N4S3H 443.1392).
2,3-Dihexyloxy-5,7-bis(2-thienyl)thieno[3,4-b]pyrazine (14).
Sodium hexyloxide was prepared by the addition of NaH (0.20 g, 5.0
mmol, 60−65% in oil) to a flask under a nitrogen atmosphere. The oil
was removed by washing with hexanes (2 × 10 mL), and the
remaining NaH was brought to dryness under vacuum. The flask was
then equipped with an addition funnel charged with ditriflate 6 (0.298
g, 0.5 mmol) in 50 mL of dry CH2Cl2, after which dry DMF (50 mL)
was added to the reaction flask. The reaction was cooled to −15 °C in
an ice/salt bath, and 1-hexanol (1.0 mL, 8 mmol) was added to the
DMF solution and allowed to stir for 1 h. The solution of 6 was then
added dropwise while maintaining a temperature below −10 °C. Once
the addition was complete, the reaction was allowed to warm to room
temperature and stirred overnight. Saturated aqueous NH4Cl (100
mL) was then added, and the mixture was extracted with CH2Cl2 (3 ×
100 mL). The combined organics were dried with MgSO4, filtered, and
concentrated. The crude product was purified by silica chromatog-
raphy (hexanes/CH2Cl2, 95:5) to give 0.18−0.20 g of an orange solid
2,3-Bis(ethoxymethyl)-5,7-bis(2-thienyl)thieno[3,4-b]-
pyrazine (16a). Purified by silica chromatography (100% CH2Cl2) to
1
give 0.24−0.26 g of a red/purple solid (55−60%). Mp 84−86 °C. H
NMR: δ 7.64 (dd, J = 1.2, 3.6 Hz, 2H), 7.36 (dd, J = 1.2, 5.2 Hz, 2H),
7.09 (dd, J = 3.6, 5.2 Hz, 2H), 4.86 (s, 4H), 3.67 (q, J = 6.8 Hz, 4H),
1.27 (t, J = 6.8 Hz, 6H); 13C NMR: δ 152.3, 137.7, 134.6, 127.5, 126.8,
125.4, 125.0, 72.6, 66.7, 15.5; HRMS m/z 439.0581 [M + Na]+ (calcd
for C20H20N2O2S3Na 439.0579).
2,3-Bis(hexyloxymethyl)-5,7-bis(2-thienyl)thieno[3,4-b]-
pyrazine (16b). Purified by silica chromatography (50% CH2Cl2/
1
hexanes) to give 0.29−0.32 g of a red-purple solid (55−60%). H
NMR: δ 7.64 (dd, J = 1.2, 3.6 Hz, 2H), 7.36 (dd, J = 1.2, 5.2 Hz, 2H),
7.09 (dd, J = 3.6, 5.2 Hz, 2H), 4.86 (s, 4H), 3.62 (t, J = 6.8 Hz, 4H),
1.68 (p, J = 6.8 Hz, 4H), 1.39 (m, 4H), 1.30 (m, 8H), 0.89 (t, J = 6.8
Hz, 6H); 13C NMR: δ 152.4, 137.7, 134.6, 127.5, 126.7, 125.4, 125.0,
72.8, 71.6, 31.9, 30.0, 26.1, 22.8, 14.3; HRMS m/z 551.1846 [M +
Na]+ (calcd for C28H36N2NaO2S3 551.1831).
1
(70−75%). Mp 113−115 °C. H NMR: δ 7.44 (dd, J = 1.2, 3.0 Hz,
2H), 7.30 (dd, J = 1.2, 4.5 Hz, 2H), 7.05 (dd, J = 3.0, 4.5 Hz, 2H), 4.55
(t, J = 7.0 Hz, 4H), 1.91 (p, J = 7.0 Hz, 4H), 1.49 (p, J = 7.0 Hz, 4H),
1.36 (m, 8H), 0.90 (t, J = 7.0 Hz, 6H); 13C NMR: δ 150.3, 135.4,
134.1, 127.1, 125.5, 123.3, 120.5, 68.1, 31.8, 28.5, 26.0, 22.8, 14.2. All
NMR values agree with previously reported values;27 HRMS m/z
551.1846 [M + Na]+ (calcd for C28H36N2O2S3Na 551.1831).
2,3-Bis(N,N-diethylaminomethyl)-5,7-bis(2-thienyl)thieno-
[3,4-b]pyrazine (17). Terthienyl 7 (0.24 g, 0.5 mmol) was added to
dry CH2Cl2 (25 mL), followed by the dropwise addition of N,N-
diethylamine (25 mL, 242 mmol). The reaction was heated at reflux
for 5 h. It was then quenched with distilled H2O and extracted with
CH2Cl2 (3 × 50 mL). The combined organics were washed with
saturated aq. NaHCO3 (2 × 100 mL) and brine (2 × 100 mL), dried
with MgSO4, filtered, and concentrated. The crude product was
purified by basic silica chromatography (prepared with 3% triethyl-
amine/CH2Cl2, eluted with 100% CH2Cl2) to give 0.13−0.14 g of a
dark red solid (55−60%). Mp 63−66 °C. 1H NMR: δ 7.63 (dd, J = 1.2,
3.6 Hz, 2H), 7.36 (dd, J = 1.2, 5.2 Hz, 2H), 7.09 (dd, J = 3.6, 5.2 Hz,
2H), 4.10 (s, 4H), 2.73 (q, J = 7.2 Hz, 8H), 1.06 (t, J = 7.2 Hz 12H);
13C NMR: δ 154.6, 137.7, 135.0, 127.3, 126.5, 124.6, 124.5, 57.5, 46.6,
2,3-Bis(phenylethynyl)-5,7-bis(2-thienyl)thieno[3,4-b]-
pyrazine (15). Phenylacetylene (0.064 g, 0.625 mmol) was added to a
flask under nitrogen and cooled to 0 °C. Once cool, butyl lithium
(0.25 mL, 0.625 mmol, 2.5 M in hexanes) was added dropwise and the
reaction was allowed to stir for 30 min. In a separate flask, ditriflate 6
(0.149 g, 0.25 mmol) was dissolved in 50 mL of THF and cooled to 0
°C in an ice bath. The lithium phenylacetylide solution was then added
dropwise to the solution of 6, after which the mixture was allowed to
warm to room temperature and stirred overnight. The THF was then
removed under vacuum, 100 mL of saturated aq. NH4Cl were added,
and the mixture was extracted with CH2Cl2 (3 × 50 mL). The
combined organics were dried with MgSO4, filtered, and concentrated.
The crude product was purified by silica chromatography (prepared
with 100% hexanes, eluted with 70:30 hexanes/CH2Cl2) to give 69−
11.4; HRMS m/z 471.1707 [M + H]+ (calcd for C24H31N4S3
471.1705).
UV−visible Spectroscopy. UV−visible spectra were measured on
a dual beam scanning spectrophotometer using samples prepared as
dilute CH3CN solutions in 1 cm quartz cuvettes. Oscillator strengths
were determined from the visible spectra via spectral fitting to
accurately quantify the area of each transition and then calculated
using literature methods.56
Electrochemistry. All electrochemical methods were performed
utilizing a three-electrode cell consisting of a platinum disc working
electrode, a platinum wire auxiliary electrode, and a Ag/Ag+ reference
electrode (0.251 V vs SCE).57 The supporting electrolyte consisted of
0.10 M tetrabutylammonium hexafluorophosphate (TBAPF6) in dry
CH3CN. Solutions were deoxygenated by sparging with argon prior to
each scan and blanketed with argon during the measurements. All
measurements were collected at a scan rate of 100 mV/s.
1
75 mg of a dark blue solid (55−60%). Mp 206−209 °C. H NMR: δ
7.72 (d, J = 3.6 Hz, 2H), 7.68 (d, J = 6.5 Hz, 4H), 7.41 (m, 8H), 7.14
(dd, J = 3.6, 5.2 Hz, 2H); 13C NMR: δ 140.0, 136.6, 132.6, 129.9,
128.7, 127.8, 127.3, 125.9, 125.8, 122.1, 96.5, 87.9; HRMS m/z
523.0383 [M + Na]+ (calcd for C30H16N2S3Na 523.0368).
2,3-Bis(bromomethyl)-5,7-bis(2-thienyl)thieno[3,4-b]-
pyrazine (7). The following is a modification of previously reported
methods.27 Diamine 3 (1.39 g, 5.0 mmol) was added to absolute
ethanol (80 mL) and gently heated with stirring until completely
dissolved, after which it was allowed to cool to room temperature. In a
similar manner, 1,4-dibromo-2,3-butanedione (1.89 g, 7.5 mmol) was
added to absolute ethanol (40 mL), gently heated with stirring until
completely dissolved, and allowed to cool to room temperature. The
dione solution was then added dropwise to the solution of 3, and the
mixture was allowed to stir for 6 h, during which precipitation of the
product occurred. The mixture was then cooled to −25 °C in a freezer
(ca. 1 h), filtered, and washed with cold ethanol to give 2.03−2.23 g of
ASSOCIATED CONTENT
■
S
* Supporting Information
NMR spectra for 5−7, 10, 13−17; crystallographic data for 7,
10, 14; and full UV−visible data. This material is available free
of charge via the Internet at http://pubs.acs.org.
1
a purple solid (84−92%). Mp 160 °C dec. H NMR: δ 7.66 (dd, J =
1.2, 3.6 Hz, 2H), 7.39 (dd, J = 1.2, 5.2 Hz, 2H), 7.11 (dd, J = 3.6, 5.2
H
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Article
(18) Beaupre, S.; Breton, A.; Dumas, J.; Leclerc, M. Chem. Mater.
2009, 21, 1504.
AUTHOR INFORMATION
Corresponding Author
*E-mail: seth.rasmussen@ndsu.edu.
Notes
■
(19) Janietz, S.; Krueger, H.; Schleiermacher, H.; Wurfel, U.;
Niggemann, M. Macromol. Chem. Phys. 2009, 210, 1493.
(20) Yen, W.; Pal, B.; Yang, J.; Hung, Y.; Lin, S.; Chao, C.; Su, W. J.
Polym. Sci., Part A: Polym. Chem. 2009, 47, 5044.
(21) Mohamad, D.; Johnson, R. G.; Janeliunas, D.; Kirkus, M.; Yi, H.;
Lidzey, D. G.; Iraqi, A. J. Mater. Chem. 2010, 20, 6990.
(22) Lai, M.; Tsai, J.; Chueh, C.; Wang, C.; Chen, W. Macromol.
Chem. Phys. 2010, 211, 2017.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the National Science Foundation (DMR-
0907043) and North Dakota State University for support of
this research and NSF-CRIF (CHE-0946990) for the purchase
of the departmental XRD instrument. We wish to thank Dr.
Angel Ugrinov for assistance with the collection of the X-ray
crystal data.
(23) Mak, C. S. K.; Cheung, W. K.; Leung, Q. Y.; Chan, W. K.
Macromol. Rapid Commun. 2010, 31, 875.
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