An Air-Stable Semiconducting Polymer Containing Dithieno[3,2-b:2′,3′-d]arsole

Article abstract of DOI:10.1002/anie.201602491

Arsole-containing conjugated polymers are a practically unexplored class of materials despite the high interest in their phosphole analogues. Herein we report the synthesis of the first dithieno[3,2-b;2′,3′-d]arsole derivative, and demonstrate that it is stable to ambient oxidation in its +3 oxidation state. A soluble copolymer is obtained by a palladium-catalyzed Stille polymerization and demonstrated to be a p-type semiconductor with promising hole mobility, which was evaluated by field-effect transistor measurements.

Full text of DOI:10.1002/anie.201602491

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201602491
German Edition: DOI: 10.1002/ange.201602491
Polymers Very Important Paper
An Air-Stable Semiconducting Polymer Containing
Dithieno[3,2-b:2’,3’-d]arsole
Joshua P. Green, Yang Han, Rebecca Kilmurray, Martyn A. McLachlan,
Thomas D. Anthopoulos, and Martin Heeney*
Abstract: Arsole-containing conjugated polymers are a practi-
cally unexplored class of materials despite the high interest in
their phosphole analogues. Herein we report the synthesis of
the first dithieno[3,2-b;2’,3’-d]arsole derivative, and demon-
strate that it is stable to ambient oxidation in its + 3 oxidation
state. A soluble copolymer is obtained by a palladium-
catalyzed Stille polymerization and demonstrated to be a
p-type semiconductor with promising hole mobility, which was
evaluated by field-effect transistor measurements.
Figure 1. Structures of pnictogen-containing dithienometalloles.
(X = N) and dithieno[3,2-b:2’,3’-d]phosphole (X = P, DTP)
have seen application in all major areas of plastic electron-
ics.[11–13] The interest in DTPs partially arises from their high
electron affinity, which results from the s–p hyperconjugation
exhibited by the phosphole ring, whereby the s* orbital of the
S
ince the first reports of the metallic behavior of conjugated
polymers (CPs), the development of new materials has been
at the forefront of research in plastic electronics.[1,2] The
enormous scope to tune their optoelectronic characteristics
and device performance by the copolymerization of different
monomeric units is appealing for many applications.[3] In
recent years, there has been a growing interest in the
incorporation of heavier elements—silicon and germanium
in Group 14, phosphorus in Group 15, selenium and tellurium
in Group 16—as replacements for lighter elements, such as
carbon, nitrogen, and sulfur, in CPs.[4–7] Their use has been
shown to have a significant impact on the polymer band gap
and energy levels, as well as the solid-state packing.[8] The
inclusion of heavy atoms can also facilitate intersystem
crossing, leading to the rapid conversion of singlet excitons
into triplets, and can lead to solid-state phosphorescence.[9,10]
Among the CPs containing Group 15 elements, those
incorporating a dithienometallole (Figure 1) have garnered
much attention. For example, dithieno[3,2-b:2’,3’-d]pyrrole
À
exocyclic P C bond is able to interact with the p* orbitals of
the fused heterocycle.[14] Furthermore, modification of the P
lone pair, through reactions with either Lewis acids or
oxidizing agents, can further tune the energy levels.[15]
However, as phosphole derivatives are prone to uncontrolled
oxidation in ambient atmosphere, the phosphole oxide is
often deliberately formed to prevent uncontrolled aging.[16]
As such, the properties of the unoxidized phospholes are
rarely reported.
Inspired by the fascinating properties of DTP, we were
interested in preparing the heavier analogue containing
a bridging arsenic atom. Of particular interest was the fact
that arsenic-containing compounds are typically more diffi-
cult to oxidize to the + 5 oxidation state than their P
analogues.[17–19] This is related to the poor shielding of the
filled 3d orbitals in As, such that the s electrons are relatively
tightly bound compared to those in the corresponding
phosphorus-based compounds. Therefore, arsole-containing
polymers may be intrinsically more resistant to ambient
oxidation than their phosphole equivalents, allowing the
properties of the heterocycle in the + 3 oxidation state to be
explored. As far as we are aware, there have been no reports
of the use of arsole-containing materials in organic electronics
and only limited examples of conjugated polymers containing
arsenic in the backbone, mostly related to poly(vinylene
arsine)s.[20–22]
[*] J. P. Green, Dr. Y. Han, Prof. M. Heeney
Department of Chemistry and Centre for Plastic Electronics
Imperial College London
London, SW7 2AZ (UK)
E-mail: m.heeney@imperial.ac.uk
Dr. Y. Han, Prof. T. D. Anthopoulos
Department of Physics and Centre for Plastic Electronics
Imperial College London (UK)
R. Kilmurray, Prof. M. A. McLachlan
Department of Materials and Centre for Plastic Electronics
Imperial College London (UK)
Herein, we present the first synthesis of a dithienoarsole
(DTAs) monomer and demonstrate that it is stable to
oxidation under ambient conditions. Significantly, we show
that it is possible to directly polymerize the DTAs derivative
in its + 3 oxidation state by a palladium-catalyzed Stille
polymerization. To the best of our knowledge, the equivalent
Pd-catalyzed polymerizations with DTP derivatives have only
been reported for the + 5 oxidation state (commonly DTP
oxides).[16] To demonstrate the utility of the DTAs building
block, we synthesized the vinylene copolymer PDTAsV (see
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201602491. Additional data can be found under doi.org/
10.6084/m9.figshare.3171586.
ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution License, which permits use, distribution and
reproduction in any medium, provided the original work is properly
cited.
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Scheme 1), in which the vinylene
comonomer plays an important role
in allowing the backbone to plana-
rize, thereby leading to a promising
organic field effect transistor
(OFET) performance.[23,24]
One issue in planning a synthetic
route to DTAs is the toxicity of
many arsenic-containing precursors.
We therefore utilized dichlorophe-
nylarsine as the arsenic building
block as its synthesis from a readily
available precursor in one step is
well established.[25] Although it is
a strong lachrymator and vesicant, it
has a history of safe use in synthetic
chemistry.[26] However, one potential
drawback of dichlorophenylarsine is
that the resulting 4-phenyldi-
Scheme 1. Synthesis and polymerization of the dithienoarsole monomer. Reaction conditions:
a) tetrabutylammonium fluoride (TBAF), THF, 08C; b) HCl, SO2, 558C; c) n-BuLi (2.2 equiv),
À788C!RT; d) lithium diisopropylamide (LDA), À78 to À30 to À788C, CBr4; e) Pd(PPh3)4,
chlorobenzene, microwave reactor.
thieno[3,2-b;2’,3’-d]arsole (DTAs) would only have a phenyl
substituent as the solubilizing sidechain, possibly resulting in
polymers with limited solubility. As such, we incorporated
additional dodecyl sidechains into the peripheral positions of
the DTAs to ensure good polymer solubility and process-
ability.
The synthesis of the DTAs monomer is shown in
Scheme 1. Compound 1[27] was desilylated with TBAF, and
the resulting compound 2 was reacted with n-BuLi at À788C,
followed by quenching of the resulting dianion with dichloro-
phenylarsine to afford dithienoarsole 5 in 57% yield.
Attempts to brominate 5 under electrophilic conditions
(NBS) resulted in some competing oxidization of the arsole
ring to the arsole oxide. However, 5 could be cleanly
brominated in 60% yield by dilithiation with LDA at low
temperature, followed by the addition of carbon tetrabromide
as a non-oxidizing halogen source.
With monomer 6 in hand, we investigated its oxidation to
the arsole oxide, initially using hydrogen peroxide, which
gives almost quantitative yields when used to oxidize
dithienophosphole.[13] However, the reaction was found to
be low-yielding when DTAs was used, with around 60% of
the unoxidized material being recovered. Using meta-chlor-
operoxybenzoic acid as a stronger oxidant resulted in an
improved yield of the oxide of 6, although starting material
still remained even with excess m-CPBA. The oxidation was
readily monitored by 1H NMR spectroscopy, with the phenyl
resonances shifting downfield and splitting upon oxidation
(Figure 2). In agreement with the reduced reactivity of the
monomer compared to DTP, samples of 6 stored under
ambient conditions displayed no signs of oxidation after six
months. The reluctance of the DTAs to oxidize is in agree-
ment with the poor shielding ability of the filled 3d orbitals,
and is in stark contrast to the DTP analogue. We note that 2,5-
diarylarsoles were very recently be reported to be more air-
stable than their phosphole analogues.[28]
Figure 2. Comparison of the 1H NMR spectra of 6 (middle), the oxide
of 6 (top), and PDTAsV (bottom) between 7.10–7.90 ppm.
binding to the Pd catalyst, did not seem to have any
detrimental effects. This finding is in line with the weaker
donicity of triphenylarsine versus triphenylphosphine as
ligands for palladium.[29] After precipitation and work-up,
PDTAsV was obtained as a dark blue polymer of reasonable
molecular weight (Mn = 12.3 kDa, “ = 1.9 by size exclusion
chromatography).
Stille polymerization of 6 with trans-1,2-bis(tributylstan-
nyl)ethene using catalytic Pd(PPh3)4 was performed in
a microwave reactor to yield PDTAsV (Scheme 1). Gratify-
ingly, the presence of the arsenic atom, a possible site for
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The chemical structure of the polymer was confirmed by
The XRD data and UV/Vis spectra suggest that the
polymer is able to planarize and crystallize in the solid state.
To further probe the monomer and polymer geometries, DFT
calculations were performed at the B3LYP level of theory
with the 6-311G(d,p) basis set. Initially considering the
monomer, the calculations predicted the bridged bithiophene
to be perfectly co-planar whilst the phenyl group attached to
the bridging arsenic atom is twisted orthogonally to the fused-
ring structure, and is almost perpendicular to the molecular
plane (Figure 4a). The predicted frontier molecular orbitals
(Supporting Information, Figure S11) show that the highest
occupied molecular orbital (HOMO) has a nodal plane
through the arsenic atom whereas the lowest unoccupied
molecular orbital (LUMO) has some contribution from the
1
a combination of elemental analysis and H NMR spectros-
1
copy. Although the H NMR resonances were significantly
broadened in comparison to the monomer material, no signals
attributable to uncontrolled oxidation to the arsole oxide
were observed (Figure 2). Together with the matching ele-
mental analysis, these results provide good evidence for the
stability of the polymer under the polymerization conditions.
À
s* orbital of the As Ph bond.
Moving to the polymeric system, minimum energy models
of the trimer predict a highly planar backbone with twists of
< 18 between the DTAs and adjacent double bonds (Fig-
ure 4b). The insertion of the double bond between the head-
to-head dodecyl sidechains clearly reduces any undesirable
Figure 3. a) UV/Vis absorption spectra of PDTAsV in chlorobenzene
solution (black), as a thin film on a glass slide (dashed), and after
annealing at 2008C for 30 min (gray). b) XRD patterns of PDTAsV
films drop-cast onto silicon wafers, as-cast (black) and after annealing
at 2008C for 30 min (gray). Traces offset for clarity.
The absorption spectra of PDTAsV in chlorobenzene
solution and as a thin film are shown in Figure 3a. In solution,
PDTAsV shows an absorption maximum (lmax) at 616 nm
with a pronounced shoulder (lsh) at 652 nm. Upon film
formation, the lmax value of PDTAsV was red-shifted to
638 nm, whereas lsh was shifted to 682 nm. The red shift is
suggestive of enhanced molecular ordering compared to the
solution state. Further evidence of solid-state ordering was
obtained by X-ray diffraction (XRD) of drop-cast films
before and after thermal annealing for 30 min at 2008C
(Figure 3b). The as-cast sample displayed a broad, low-
intensity peak with a d-spacing of 21.0 Š. Upon annealing,
this peak became sharper and significantly enhanced in
intensity, which is indicative of an increase in molecular
ordering. The d-spacing increased to 22.6 Š, which is very
similar to the value observed for other bridged bithiophene–
vinylene co-polymers with dodecyl sidechains, and attributed
to the distance between conjugated backbones in a lamellar-
type arrangement.[30]
Figure 4. a) Predicted geometry of the dithienoarsole monomer and
b) frontier molecular orbitals of PDTAsV, as calculated by DFT [B3LYP,
6-311G(d,p)]. c) Representative transfer characteristics measured with
a top-gate/bottom-contact architecture in nitrogen atmosphere at
room temperature.
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torsional twisting between the DTAs units. Furthermore, the
Keywords: arsenic · conjugated polymers ·
side-on view clearly shows that the phenyl rings protrude
perpendicularly to the conjugated backbone. Note that in the
minimum-energy conformer shown, the phenyl groups are
predicted to alternate above and below the plane of the
backbone, but there is no energy difference between trimers
with alternative arrangements, and we therefore expect
a random arrangement in the actual polymer. It is also
worth noting that the barrier to inversion of the phenyl group
in arsole has been predicted to be large owing to the limited
p character of the lone pair,[31] and we would expect similar in
DTAs. Finally, the frontier molecular orbital calculations
(Figure 4b) show that whereas both the HOMO and LUMO
are extensively delocalized over the polymer backbone, only
the LUMO has a significant contribution on the As bridge,
similar to the monomer.
field-effect transistors · molecular electronics · semiconductors
How to cite: Angew. Chem. Int. Ed. 2016, 55, 7148–7151
Angew. Chem. 2016, 128, 7264–7267
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Acknowledgements
We thank the UKꢁs Engineering and Physical Sciences
Research Council (EPSRC) for financial support through
the Doctoral Training Centre in Plastic Electronics (EP/
G037515/1).
Received: March 10, 2016
Published online: April 27, 2016
Angew. Chem. Int. Ed. 2016, 55, 7148 –7151
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