Synthesis of low band gap [1,2,5]-thiadiazolo[3,4- g ]quinoxaline and pyrazino[2,3- g ]quinoxaline derivatives by selective reduction of benzo[1,2- c;4,5- c′]bis[1,2,5]thiadiazole

Article abstract of DOI:10.1021/ol102465a

The reduction of a dithienylbenzobisthiadiazole derivative TBBT can be performed selectively so as to afford either [1,2,5]-thiadiazolo[3,4-g] quinoxaline (TQ) or pyrazino[2,3-g]quinoxaline (PQ) derivatives. This approach offers a much milder, shorter, an

Full text of DOI:10.1021/ol102465a

ORGANIC
LETTERS
2011
Vol. 13, No. 1
46-49
Synthesis of Low Band Gap
[1,2,5]-Thiadiazolo[3,4-g]quinoxaline and
Pyrazino[2,3-g]quinoxaline Derivatives
by Selective Reduction of
Benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole
Hairong Li,# Teck Lip Tam,# Yeng Ming Lam, Subodh G. Mhaisalkar, and
Andrew C. Grimsdale*
School of Materials Science and Engineering, Nanyang Technological UniVersity,
50 Nanyang AVenue, Singapore 639798
acgrimsdale@ntu.edu.sg
Received October 12, 2010
ABSTRACT
The reduction of a dithienylbenzobisthiadiazole derivative TBBT can be performed selectively so as to afford either [1,2,5]-thiadiazolo[3,4-
g]quinoxaline (TQ) or pyrazino[2,3-g]quinoxaline (PQ) derivatives. This approach offers a much milder, shorter, and more efficient route to PQ
and TQ derivatives than current methods. It is further shown how the optical and electrochemical properties of PQ and TQ can be tuned by
choice of appropriate substituents.
Molecules and polymers based on 1,2,5-thiadiazolo[3,4-
g]quinoxaline (TQ) and pyrazino[2,3-g]quinoxaline (PQ)
Scheme 1
.
Current Synthetic Route to Dithienyl-TQ and
Dithienyl-PQ Derivatives
have received increasing attention since 2005 as n-type
materials due to their interesting electronic and photophysical
properties.1 Without exception, their synthesis has been based
on the method of Kitamura et al.2 illustrated in Scheme 1
for 4,9-dithienyl-TQ and 4,8-dithienyl-PQ derivatives. The
key intermediate is the dinitrobenzothiadiazole 5. Reaction
of 5 with zinc reduces both the benzothiadiazole ring and
the nitro groups to give a tetraamine intermediate that is
condensed with 2 equiv of a diketone to give PQ derivatives.
By contrast iron does not reduce the ring and so TQ derivatives
can be obtained by reduction of 5 under these milder conditions
followed by condensation of the resulting diamine with dike-
tones. The main disadvantages of these routes are that the
# These authors have contributed equally to this work.
10.1021/ol102465a  2011 American Chemical Society
Published on Web 11/30/2010

synthesis of 5 requires 4 steps, some using very strong
conditions, and proceeds in only low overall yield (17%).
equiv of a diketone affords PQ derivatives in only 3 steps
from a commercially available tetraaminobenzene. When zinc
was replaced by iron powder as the reducing agent under
the same reaction conditions, it was found that selective
reduction of only one thiadiazole ring in BBT occurred, thus
affording a route to TQ derivatives by condensation with 1
equiv of the diketone. A smaller excess of reducing agent is
required in the synthesis of TQ as compared to PQ deriva-
tives as only one ring is reduced. This selectivity is due to
the hypervalent sulfur in one ring in BBT making the
reduction of BBT to BT more facile than the reduction of
BT.
This synthetic inefficiency in the preparation of compounds
4 and 5, which had previously been used as intermediates
in the synthesis of benzobisthiadiazole (BBT) derivatives,
was a major motivation for us to develop our recently
reported efficient (87% yield) one-pot synthesis of 4,8-
dibromobenzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole (7).3 Having
developed a short and high yielding route to the dibromo-
BBT 7 and its dithienyl adduct (TBBT), we were then
interested to explore whether reduction of BBT derivatives
might offer an effective method for obtaining PQ and TQ
derivatives. As shown in Scheme 2, we now report that
To test the versatility of the reaction procedure and to
investigate how the properties of PQ and TQ molecules
could be tuned, we chose to make derivatives with biphenyl
(Figure 1, TQ1 and PQ1) and dithienyl (TQ2 and PQ2)
Scheme 2
.
Novel Synthetic Route to Dithienyl-TQ and
Dithienyl-PQ Derivatives
reduction of TBBT by stirring with a large excess of zinc
powder in acetic acid at 80 °C followed by addition of 2
Figure 1. Target molecules: PQ1-2 and TQ1-2, and literature
models PQ0 and TQ0 for comparison.
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substituents. Since TQ0 and PQ0,2 the reference molecules
chosen from the literature for comparison of properties, are
not very soluble, longer solubilizing alkyl or alkoxy chains
were incorporated.
As shown in Scheme 3, the biphenyl diketone 12 was
prepared in good yield by Suzuki coupling of 10 and 11.
The corresponding dithienyl diketone 15 was prepared by
Stille coupling of 13 and 14.
Reduction of TBBT with zinc followed by condensation
of the resulting crude tetraamine with 12 afforded PQ1 in
31% yield (22% overall yield). The condensation with 15
was less efficient producing PQ2 in only 17% yield (12%
overall). The condensations of the diketones with the crude
diamine from reduction of TBBT with iron proceeded in
much higher yields with TQ1 and TQ2 being obtained in
82% and 65% yield, respectively (58% and 46% overall
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Org. Lett., Vol. 13, No. 1, 2011
47

Scheme 3. Synthesis of the 1,2-Diketones
yields), thus providing a much shorter and more efficient
route to TQ derivatives than those previously reported. This
difference in yields may reflect either the greater stability
of the less electron-rich diamine intermediate compared to
the tetraamine, or the greater efficiency in the reduction. The
higher efficiency of the condensations of 12 compared to
15 may reflect the higher electron density of the thienyl
molecules making them less susceptible to nucleophilic
attack. While the yields from the final step in the synthesis
of the PQ derivatives are clearly capable of considerable
improvement through optimization of the reaction conditions,
the overall synthesis is still notably more efficient than the
existing route to PQ derivatives as TBBT can be obtained
in much higher yield than 5 (71% cf. 17%) and in 2 fewer
steps.
Most work on TQ and PQ derivatives has concentra-
ted on using them to make donor-acceptor copoly-
mers,1a,b,g-k,m-q but it is clear that the choice of substituents
R on TQ and PQ units can have profound effects on their
electronic structures, charge transfer abilities, solubility, and
film forming properties.1c-f,l,2b,4 We have investigated the
optical and electrochemical properties of our new molecules
TQ1-2 and PQ1-2 in order to gain some insight into their
structure-property relationships with a view to designing
materials with better performance in electronic devices. The
absorption maxima, both in solution and as thin films (Figure
2), and the estimated orbital energies for our new molecules
are tabulated in Table 1, with the values we measured for
TBBT and the values from literature for TQ0 and PQ0 being
given for comparison.
Figure 2. Absorption spectra of TBBT, TQ1-2, and PQ1-2 in
CHCl3 solution (a) and as thin films (b).
derivatives TQ0 and PQ0, due to the HOMO orbital energies
for our derivatives being significantly higher (∼0.4 eV) while
their LUMO energies are only slightly higher than those for
TQ0 and PQ0, though significantly higher than those for
TBBT. The thienyl-derivatives TQ2 and PQ2 had slightly
smaller bandgaps than the phenyl-substituted analogues TQ1
and PQ1. This seems to be due predominantly to a decrease
in their LUMO energies.
Theoretical studies on heterocyclic fused rings as acceptors
in donor-acceptor copolymers have predicted that the
strength of the acceptor has more effect on the LUMO than
HOMO of the polymer, with stronger acceptors lowering the
LUMO level more, and the order of electron-withdrawing
power is the following: BBT > TQ > PQ.5 This is consistent
with our measurements.
Comparing the absorption spectra for the PQ derivatives
shows that while PQ1 and PQ2 have similar absorption
edges, the thienyl-substituted derivative PQ2 absorbs much
more strongly in solution between 550 and 650 nm than the
phenyl analogue PQ1. By contrast the thienyl-substituted
TQ2 displays slightly weaker absorption in the red than the
phenyl-substituted TQ1. What is notable is that all the TQ
and PQ derivatives absorb much more strongly in the
400-600 nm range than TBBT.
We hypothesize that these results may be explicable in
terms of the relative contributions to the electronic structures
of these molecules of the quinoid component along the
thiophene-benzene-thiophene backbone of the molecule
and the cross-conjugation along the arms, as has been
observed in other systems.6 Modeling of the electronic
structures of these materials is underway in order to test this.
The optical bandgaps were calculated from the onset of
absorption in solution (Figure 2) and matched well with the
electrochemical bandgaps obtained from CV. The oxidation
and reduction processes for all our new compounds appear
to be reversible.
It is clear that the bandgaps for both TQ and PQ derivatives
are larger than those for TBBT, with those for the TQ
derivatives being lower than those for the corresponding PQ
derivatives. The bandgaps of our aryl-substituted molecules
TQ1-2 and PQ1-2 are smaller than those for the alkyl
(5) Kitamura, C. Studies on New Narrow-Bandgap Polymers Composed
of Aromatic-Donor and o-Quinoid-Acceptor Segments; Ph.D. Thesis, The
Graduate University for Advanced Studies, 1996.
(4) (a) Fu, Y.; Shen, W.; Li, M. Polymer 2008, 49 (10), 2614. (b) Wu,
W.-C.; Chen, W.-C. J. Polym. Res. 2006, 13 (6), 441
.
48
Org. Lett., Vol. 13, No. 1, 2011

Table 1. Optical and Electrochemical Properties of TQ0-2, PQ0-2, and TBBT
TBBT
TQ0
TQ1
TQ2
PQ0
PQ1
3.12
5.10
1.98
PQ2
3.26
5.15
1.89
1.86
607 (4.61)
LUMO (-eV)
HOMO (-eV)
CV Eg (eV)
UV Eg (eV)
λmax (nm) in CHCl3
3.95
5.42
1.47
1.49
699
350
333
3.55
5.54
1.99
N/A
591
320
285
3.43
5.11
1.68
1.70
3.52
5.08
1.56
1.65
3.23
5.52
2.29
N/A
491
385
294
1.93
620 (4.00)
650 sa (4.01)
543 sa (3.99)
(log εmax
)
477 (4.23)
383 (4.32)
323 (4.85)
695 sa
655
492
394
343
242
567 (4.41)
418 (4.55)
331 (4.69)
713 sa
596
490 (4.78)
382 (4.90)
324 (5.03)
555 sa
500
575 (4.58)
447 (4.84)
339 (4.65)
643
597
459
λmax (nm) thin film
830 sa
699
N/A
N/A
N/A
385 sa
360
437
351
399
335
347
Tm (°C)
N/A
258
152
>300
170
a s: onset of a shoulder or tail down to longer wavelength.
The thin film absorption spectra generally resemble the
ones obtained from solution with a red shift of absorption
edge and peak broadening observed for all compounds due
to aggregation. The most notable changes are that the
absorption intensity from 543 nm onward significantly
increases for TQ1 and a shoulder emerges between 550 and
700 nm for PQ1. We have not yet succeeded in growing
crystals suitable for X-ray diffraction to study the packing
of the molecules in the solid state, but comparisons of the
melting points suggest that the biphenyl-substituted mol-
ecules PQ1 and TQ1 pack better in the solid state as they
melt at significantly higher temperatures than the thienyl-
substituted analogues PQ2 and TQ2. Since biphenyl is
known to have higher torsion angle than bithiophene due to
stronger repulsion between neighborhood R-protons, there
might be a dramatic conformational change in the biphenyl-
substituted TQ1 and PQ1 between solution and solid state,
which could account for the change in the shape of the
absorption as the arms become flatter and so have longer
effective conjugation. The PQ molecules all possess higher
melting points than the corresponding TQ derivatives, but
the differences are smaller than those within each class
suggesting the arms have a greater effect on the melting
points than do the cores.
The new molecules PQ1-2 and TQ1-2 are all freely
soluble in organic solvents and so it should be relatively
straightforward to incorporate them as electron-accepting
units into donor-acceptor oligomers and polymers. In view
of their relatively low bandgaps we believe such materials
should be promising candidates for organic photovoltaic
devices, organic transistors, or red or infrared emitting
OLEDs.
To sum up, we have demonstrated a new short and
relatively efficient synthesis of PQ and TQ derivatives which
makes these classes of molecules much more readily avail-
able for use as components in new materials for organic
electronics applications. We have shown that their optical
and electronic properties can be controlled by suitable choice
of solubilizing side arms on the heterocyclic cores, thus
enabling the potential performance of the final materials to
be optimized.
Acknowledgment. We thank Ms. Jane Pang (School of
Materials Science and Engineering, NTU) for helping
characterize the molecules. This work was supported finan-
cially by Robert Bosch (SEA) Pte Ltd.
Supporting Information Available: Detailed synthetic
procedures and spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
(6) Ortiz, R. P.; Facchetti, A.; Marks, T. J.; Casado, J.; Zgierski, M. Z.;
Kozaki, M.; Herna´ndez, V.; Navarrete, J. T. L. AdV. Funct. Mater. 2009,
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OL102465A
Org. Lett., Vol. 13, No. 1, 2011
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