Synthesis and characterization of [1,2,5]chalcogenazolo[3,4-f]benzo[1,2,3] triazole and [1,2,3]triazolo[3,4-g]quinoxaline derivatives

Article abstract of DOI:10.1021/ol201829s

The synthesis and characterization is reported of low bandgap [1,2,5]chalcogenazolo[3,4-f]benzo[1,2,3]triazole and [1,2,3]triazolo[3,4-g] quinoxaline derivatives that display higher solubility and stability then their thiadiazole counterparts, [1,2,5]chal

Full text of DOI:10.1021/ol201829s

ORGANIC
LETTERS
2011
Vol. 13, No. 17
4612–4615
Synthesis and Characterization of
[1,2,5]Chalcogenazolo[3,4-f]-
benzo[1,2,3]triazole and
[1,2,3]Triazolo[3,4-g]quinoxaline
Derivatives
Teck Lip Tam, Hairong Li, Yeng Ming Lam, Subodh G. Mhaisalkar,
and Andrew C. Grimsdale*
School of Materials Science and Engineering and Energy Research Institute at NTU,
Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798
ACGrimsdale@ntu.edu.sg
Received July 7, 2011
ABSTRACT
The synthesis and characterization is reported of low bandgap [1,2,5]chalcogenazolo[3,4-f]benzo[1,2,3]triazole and [1,2,3]triazolo-
[3,4-g]quinoxaline derivatives that display higher solubility and stability then their thiadiazole counterparts, [1,2,5]chalcogenazolo-
[3,4-f]benzo[2,1,3]thiadiazole and [1,2,5]thiadiazolo[3,4-g]quinoxaline, respectively.
Recently developed benzo[1,2-c;4,5-c0]bis[1,2,5]thia-dia-
zole (BBT) derivatives have been shown experimentally to
be strong electron acceptors with high mobilities and
low bandgaps with low LUMO levels.1ꢀ8 Theoretical
studies suggest that these properties are a consequence of
their strong quinonoid contributions and hypervalent
sulfurs.1ꢀ3,5ꢀ7,9ꢀ11 In the case of benzo-2,1,3-chalcogen-
diazoles, density functional theory (DFT) B3LYP calcula-
tions have shown that the bandgap and LUMO trends are
Te < Se < S.12 Thus, extrapolating from these results, one
would expect that the chalcogendiazolo-benzothiadiazoles
(1) Kono, T.; Kumaki, D.; Nishida, J.-i.; Tokito, S.; Yamashita, Y.
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r
10.1021/ol201829s
Published on Web 08/03/2011
2011 American Chemical Society

(XBTs) will show the same trend. However, it is well-
known that BBT has poor solubility. The fact that
heavy main-group elements often display short intera-
tomic contacts that are attributed to secondary bonding
interactions,13 would tend to make the selenium and
tellurium analogues of BBT even less soluble. [1,2,5]-
Thiadiazolo[3,4-g]quinoxalines (TQs) are also strong
electron acceptors and low bandgap materials.9,14ꢀ17
While TQs can be made more soluble via functionalization
at the 6 and 7 positions, higher solubility might also be
achieved by changing the sulfur in the thiadiazole ring to a
nitrogen, as the resulting [1,2,3]triazolo[3,4-g]quinoxaline
(TaQ) structure allows a solubilizing group to be attached
to the triazole ring and thus increase solubility. Using the
same argument, higher solubility could be attained if
one changed the thiadiazole ring in XBT to triazole.
The resulting chalcogena-diazolobenzotriazoles (XBTa)
should be low bandgap and low LUMO materials with
good solubility (Figure 1). In this paper, the synthesis and
characterization of the new XBTa and TaQ molecules is
introduced, and theyare comparedwiththeir XBT and TQ
counterparts.
Scheme 1. Synthesis of SeBT, SBTa, SeBTa, and TeBTa
Figure 1. Structures of XBT, XBTa, TQ, and TaQ derivatives.
BBT was synthesized according to our previously re-
portedmethod.18 SeBThas notbeenreportedpreviously in
literature, so for comparison purposes, its synthesis was
carried out (Scheme 1). Condensation of the known dia-
mine 1 with SeO2 in refluxing ethanol gives SeBT which, as
expected, had very poor solubility and was unstable in air.
Cyclic voltammetry and solution UVꢀvis absorption
spectrometry were therefore carried out immediately after
purification.
Scheme 2. Synthesis of TaQ1 and TaQ2
In view of the instability and poor solubility of SeBT,
TeBT was not prepared as it was expected to be even less
(13) Cozzolino, A. F.; Vargas-Baca, I.; Mansour, S.; Mahmoudkhani,
A. H. J. Am. Chem. Soc. 2005, 127, 3184–3190.
(14) Zoombelt, A. P.; Fonrodona, M.; Wienk, M. M.; Sieval, A. B.;
Hummelen, J. C.; Janssen, R. A. J. Org. Lett. 2009, 11, 903–906.
(15) Lee, Y.; Russell, T. P.; Jo, W. H. Org. Electron. 2010, 11, 846–
853.
(16) Fu, Y.; Shen, W.; Li, M. Polymer 2008, 49, 2614–2620.
(17) Dallos, T.; Hamburger, M.; Baumgarten, M. Org. Lett. 2011, 13,
1936–1939.
(18) Tam, T. L.; Li, H.; Wei, F.; Tan, K. J.; Kloc, C.; Lam, Y. M.;
Mhaisalkar, S. G.; Grimsdale, A. C. Org. Lett. 2010, 12, 3340–3343.
(19) Li, H.; Tam, T. L.; Lam, Y. M.; Mhaisalkar, S. G.; Grimsdale,
A. C. Org. Lett. 2011, 13, 46–49.
stable. The synthesis of TQ1 and TQ2 were reported by us
recently.19 The synthesis of XBTa and TaQ proceeds from
2, which was prepared according to literature.11 The sub-
sequent nitration, Stille coupling, reduction, and ring-
closure to form SBTa and SeBTa (Scheme 2) were analo-
gous to those used to make BBT and SeBT.7 It was found
that concentrated sulfuric acid with fuming nitric acid was
Org. Lett., Vol. 13, No. 17, 2011
4613

too harsh a condition to use for nitration of 2, but nitration
in fuming nitric acid gave 3 in 27% yield. An alternative
nitration procedure for benzotriazole was recently re-
ported after completion of the current synthesis of 3.20
For TeBTa, ring closure was carried using tellurium tetra-
bromide. The product, whose identity was confirmed
only by MALDI-TOF, appears to be soluble in the reac-
tion mixture but was extremely unstable when exposed to
air and moisture. No further characterization was attain-
able. For TaQ1 and TaQ2, condensation with diketones 6
and 7 in ethanol/acetic acid mixture give the desired
molecules. The resulting molecules SBTa, SeBTa, TaQ1,
and TaQ2 are highly soluble and stable low bandgap
materials.
As can be seen from the UVꢀvisible spectra presented in
Figure 2a, the effect of changing the chalcogen from sulfur
to selenium in both the XBT and XBTa series is to red-shift
the absorption and reduce the optical bandgaps (Eg, UV)
as measured by onset of absorption. By contrast, as can be
seenin Figure 2b, the effectof replacing the sulfurinthe BT
and TQ series with a nitrogen in BTa and TaQ is to
increase the bandgap slightly. The bandgaps of the new
materials still remain below 2 eV.
Table 1. Electrochemical and Photophysical Properties of
XBTa, XBT, TaQ, and TQ Series
HOMO LUMO Eg (CV) Eg (UV)
(eV)
(eV)
(eV)
(eV)
λ
max (nm)a
SBTa ꢀ5.15
SeBTa ꢀ5.12
ꢀ3.48
ꢀ3.59
ꢀ3.94
ꢀ3.60
ꢀ3.48
1.67
1.53
1.43
1.32
1.83
1.80 337, 618
1.60 322, 363, 700
1.49 333, 350, 696
1.37 344, 377, 797
1.95 308, 365, 440, 553,
597s
BBT
ꢀ5.37
SeBT ꢀ4.92
TaQ1 ꢀ5.31
TaQ2 ꢀ5.25
ꢀ3.55
1.70
1.86 340, 377, 428s, 516,
626s
TQ1
TQ2
ꢀ5.11
ꢀ5.08
ꢀ3.43
ꢀ3.52
1.68
1.56
1.70 323, 383, 477, 620
1.65 331, 418, 567, 650s
a s, shoulder.
Electrochemical characterization of SBTa and SeBTa
(Table 1) revealed that replacement of sulfur with selenium
slightly increases the HOMO value (HOMO and LUMO
values were calculated from onset of oxidation and reduc-
tionrespectively). TheLUMOontheotherhand, decreases.
The experimental trends for both HOMO and LUMO
follow that of benzo-2,1,3- chalcogenadiazoles reported in
literature.12,21 From these trends, we believe the instability
of TeBTa arises from it having an even higher HOMO
above ꢀ5 eV.
The decreases in electrochemical bandgap (Eg, CV) from
SBTafSeBTa and from BBTfSeBT follow the expected
trends, but the magnitude of change in HOMO energy for
BBTfSeBT is larger than for SBTafSeBTa. The changes
in LUMO energy show the reverse pattern. These differ-
ences are believed to be a consequence of resonance
symmetry stabilization of BBT, leading to delocalization
and thus stabilization of the HOMO and LUMO. The
HOMO, LUMO, and bandgap trends of TaQ1fTaQ2
follow that of TQ1fTQ2 due to better conjugation be-
tween the bithienyls and the core than the biphenyls.
With the exception of symmetrical BBT, substitution of
sulfur with nitrogen in both SeBTfSeBTa and TQfTaQ
produces a slightly lower LUMO and a much lower
HOMO. These are probably attributable to the higher
electron affinity and ionization potential of nitrogen as
compared to sulfur. Lower HOMO energies enhance sta-
bility of the materials and also have the potential to
increase open circuit voltage in organic photovoltaic de-
vices using such materials as donors.22
Differential scanning calorimetry (DSC) showed that
both SBTa and SeBTa have low melting temperatures of
134 and 156 °C, respectively. On the other hand, BBT and
SeBT do not melt but decompose. The low melting tem-
peraturefor theXBTa islikelyduetothe alkyl grouponthe
triazole ring. First, the flexible alkyl group is likely to
prevent adjacent molecules on the triazole side coming
close to one another. Second, secondary bonding
Figure 2. Solution UVꢀvis absorption spectra of (A) XBTa and
XBT series and (B) TaQ and TQ series in dichloromethane.
(21) Suzuki, T.; Tsuji, T. J. Org. Chem. 2001, 66, 8954–8960.
€
(20) Pasker, F. M.; Blanc, S. M. L.; Schnakenburg, G.; Hoger, S.
Org. Lett. 2011, 13, 2338–2341.
(22) Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.;
Waldauf, C.; Heeger, A. J.; Brabec, C. J. Adv. Mater. 2006, 18, 789–794.
4614
Org. Lett., Vol. 13, No. 17, 2011

interaction on the triazole side is no longer possible,
effectively reducing the secondary bonding interaction
probability by half. On the other hand, the lack of alkyl
groups on BBT allows the molecules to come close
together, at the same time allowing secondary bonding
interactions to occur. Such decrease in intermolecular
interactions may also explain why both SBTa and SeBTa
have good solubility whereas BBT and SeBT do not. TaQ1
and TaQ2 also show lower melting temperatures (168 and
117 °C, respectively) than TQ1 and TQ2 (242 and 152 °C,
respectively) and also much higher solubility.
In conclusion, the triazole analogues of chalcogenadia-
zolo-benzothiadiazoles and thiadiazolo-quinoxalines have
been synthesized and characterized. These triazole analo-
gues have much higher solubilities than the parent com-
pounds, and lower HOMO energies were also achieved,
which enhances their stability. While their bandgaps are
higher as a result, they remain low bandgap materials.
These qualities of higher solubility and stability are gen-
erally beneficial in the field of solution processable organic
electronics, suggesting thatmaterials containingtheseunits
may show promising properties for such applications.
Acknowledgment. This work was supported by Robert
Bosch (SEA) Pte Ltd.
Supporting Information Available. Detailed descrip-
tions of experimental procedures, plus mass spectra and
NMR spectra for all newly synthesized compounds.
CV and DSC data for the final products. This material
is available free of charge via the Internet at http://
pubs.acs.org.
Org. Lett., Vol. 13, No. 17, 2011
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