Dithiolodithiole as a building block for conjugated materials

Article abstract of DOI:10.1002/anie.201310290

The development of new conjugated organic materials for dyes, sensors, imaging, and flexible light emitting diodes, field-effect transistors, and photovoltaics has largely relied upon assembling π-conjugated molecules and polymers from a limited number of

Full text of DOI:10.1002/anie.201310290

Angewandte
Chemie
DOI: 10.1002/anie.201310290
Conjugated Materials
Dithiolodithiole as a Building Block for Conjugated Materials**
Derek J. Schipper, Lionel C. H. Moh, Peter Mꢀller, and Timothy M. Swager*
Abstract: The development of new conjugated organic mate-
rials for dyes, sensors, imaging, and flexible light emitting
diodes, field-effect transistors, and photovoltaics has largely
relied upon assembling p-conjugated molecules and polymers
from a limited number of building blocks. The use of the
dithiolodithiole heterocycle as a conjugated building block for
organic materials is described. The resulting materials exhibit
complimentary properties to widely used thiophene analogues,
such as stronger donor characteristics, high crystallinity, and
a decreased HOMO–LUMO gap. The dithiolodithiole (C4S4)
motif is readily synthetically accessible using catalytic process-
es, and both the molecular and bulk properties of materials
based on this building block can be tuned by judicious choice
of substituents.
T
he importance of conjugated organic materials can hardly
be overstated. They have proved invaluable to established
applications such as dyes,[1] sensors,[2] and imaging[3] as well as
emerging technologies including flexible light emitting
diodes,[4] field-effect transistors,[5] and photovoltaics.[6] The
development of new materials with desirable properties has
largely relied upon assembling p-conjugated molecules and
polymers from a limited number of building blocks.[7] We
describe herein the use of the dithiolodithiole heterocycle as
a conjugated building block for organic materials. The
resulting materials exhibit complimentary properties to
widely used thiophene analogues such as stronger donor
characteristics, high crystallinity, and a decreased HOMO–
LUMO gap.
Figure 1. Comparison of various conjugated building blocks.
resulting analogous materials, which is demonstrated by the
difference in lmax between p-terphenyl (274 nm) and 2,5-
diphenylthiophene (326 nm). Substitution with 2-benzothio-
phene leads due a further reduction in HOMO–LUMO gap
(369 nm), which can be attributed to the benzo-substitution
leading to partial retention of aromatic character in the
excited state. Additionally, (benzo)thiophenes can exhibit
À
S S contacts in the solid state, which can be crucial for
favorable change transport characteristics.[9]
As part of an ongoing research program dedicated to the
development of novel conjugated materials[8] we seek new
building blocks for conjugated materials which might serve to
not only widen the scope of available building blocks, but also
to introduce attractive properties to the resulting material. A
rudimentary analysis of some commonly employed conju-
gated units is shown in Figure 1. Phenylene and thiophene
units are aromatic in the ground state and upon absorption of
a photon populate a formally non-aromatic quinoidal excited
state. Thiophenes have a lower aromatic stabilization energy
which contributes to lower HOMO–LUMO gaps in the
We were inspired by the successful implementation
(benzo)thiophene-based materials for a variety of applica-
tions,[10] which are ascribable to the range of desirable
properties they exhibit. Given that these properties can be
attributed to the presence of a sulfur atom and/or fused ring
system, we hypothesized that a heterocycle that contains
multiple sulfur atoms in a fused ring systems may lead to
a material with enhanced properties. A survey of possible
atomic arrangements revealed that the dithiolodithiole (C4S4)
heterocycle could be a viable candidate. Examination of the
C4S4 unit reveals a 12eÀ (formally anti-aromatic by Hꢀckelꢁs
rule) p system in the ground state and a non-aromatic p
system in the quinoidal excited state, leading to potentially
interesting optical and electronic properties. Illustratively, the
[*] Dr. D. J. Schipper, L. C. H. Moh, Dr. P. Mꢀller, Prof. Dr. T. M. Swager
Department of Chemistry and Institute for Soldier Nanotechnolo-
gies, Massachusetts Institute of Technology
Cambridge, MA 02139 (USA)
absorption
maximum
of
3,6-diphenyldithiolodithiole
(458 nm) is at significantly longer wavelength than the
analogous (benzo)thiophene materials.
E-mail: tswager@mit.edu
[**] We thank the National Science Foundation Foundation for support.
D.J.S. thanks the National Science and Engineering Research
Council of Canada for a postdoctoral fellowship. L.C.H.M. thanks
the Agency for Science, Technology and Research (Singapore) for
a graduate scholarship.
Despite the potential advantages of the C4S4 system,
reports of examples are scant.[11] To study C4S4 derivatives in
more detail, synthetic accessibility is required. A suitable
approach was demonstrated by Blum and co-workers who
reported the synthesis of a single example of a C4S4 derivative
in 26% yield from 1,4-diphenylbutadiyne and elemental
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201310290.
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sulfur at 1508C for 52 h.[11] Although butadiynes and ele-
mental sulfur are convenient starting materials, a reduction of
reaction times and increase in yields would be beneficial.
Upon screening various reaction conditions, we identified two
sets of conditions, using either thermal (conditions A) or
microwave heating (conditions B), which improved both the
reaction time and product yield (Scheme 1). In most cases,
addition of a disulfide catalyst proved beneficial for achieving
shorter reaction times and/or lower reaction temperatures.
strong electron-donating group lead to a complex mixture of
products of which some have been completely character-
ized.[13] The C4S4 derivatives generally display a relatively
broad absorption band with absorption maxima (lmax) ranging
from 449 nm to 649 nm with molar extinction coefficients (e)
ranging from 5700–26300 LmolÀ1 cmÀ1. Compounds with
electron withdrawing groups generally exhibit a bathochromic
shift as well as an increase in e values, which is most likely due
to the increased donor–p-acceptor nature of the chromo-
phore. The ability of the C4S4 to act as a strong electron-
donating group was also confirmed by the relatively low
oxidation potential this family of compounds exhibits. Cyclic
voltammetry of this series of compounds reveals two rever-
sible oxidations with E1/2 values ranging from 0.01 V to 0.21 V
for the first oxidation and 0.62 V to 0.78 V for the second
oxidation relative to a Fc/Fc+ reference. The UV/Vis and CV
data was used to estimate Eg and HOMO/LUMO values,
which are also summarized in Table 1.
Materials that undergo significant redox induced changes
in their absorption properties have demonstrated utility in
electrochromic devices,[15] and electrochemical oxidation of
C4S4 derivatives were studied using spectroelectrochemistry.
An illustrative example using 7 is depicted in Figure 2. As
shown, the radical cation produces the longest wavelength
absorptions, whereas further oxidation to the closed shell
dication produces a shift to a larger bandgap. Apart from the
drastic change in UV/Vis absorption properties upon oxida-
tion, further evidence for oxidation of the p-system as
opposed to oxidation of sulfur atoms is provided by the fact
that 6 does not undergo oxidation within the solvent window.
Scheme 1. Reaction conditions.
The reaction likely proceeds through sulfur-centered radicals
with the disulfide catalyst acting as a radical mediator.[12]
The synthesis of a variety of different C4S4 derivatives was
carried out using the newly developed conditions. The
reaction is tolerant of a variety of functional groups, including
halogens, alkyl, ester, cyano, and boronic esters.[13] The
reaction yields are adequate (11–66%) to obtain material to
study the photophysical and electrochemical properties,
which are shown in Table 1.[14] Substrates containing only
Table 1: Dithiolodithiole scope and photo/electrochemical properties.
[a] Yield of isolated product. [b] Measured in CH2Cl2. [c] CV measured in CH2Cl2 with 0.1m nBu4NPF6 as supporting electrolyte using a Pt button
working electrode, Pt counter electrode with Fc/Fc+ reference. [d] Estimated from the absorption onset: Eg =1240/l. [e] Calculated from the first
oxidation E1/2: HOMO=À(E1/2ox+4.80); LUMO=HOMOÀEg. EH=2-ethylhexyl, Pin=pinacolato.
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Figure 2. Spectroelectrochemistry of a solution of 7 in 0.1m TBAPF6
dichloromethane solution. Time between each absorption measure-
ment is 15 s. a) At 450 mV. b) At 900 mV.
Compound 6 would be expected to have similar E1/2 values as
other C4S4 compounds if oxidation occurred at the sulfur atom
but very different E1/2 values if oxidation occurs at the p-
system given the disparate nature of the conjugated systems.
The X-ray crystal structures of compounds 1 and 12 are
shown in Figure 3 along with some elements of their packing
characteristics. The morphology of organic materials in the
solid state is an important parameter that strongly affects
performance in a variety of potential applications.[16] Thio-
phenes have been shown to exhibit favorable charge transport
À
properties, due in part to their ability of form S S con-
tacts.[9,17] The presence of multiple sulfur atoms in the C4S4
Figure 3. X-ray crystal structure of 1 and 12. Ellipsoids are set at 50%
probability. Characteristics of molecular packing for 1 and 12 are also
depicted with hydrogen atoms and some alkyl groups omitted for
clarity.[18]
À
heterocycle will likely lead to a higher degree of S S contacts
in the solid state and perhaps to favorable charge transport
properties. Indeed, X-ray crystallography of 1 reveals multi-
À
ple possible S S contacts for the sulfur atoms of the C4S4
À
heterocycle. “Internal” sulfurs exhibit two S S contacts,
À
whereas the “external” sulfurs exhibit one S S contact.
Overall, this leads to six S S contacts for a single molecule
angle between the C4S4 heterocycle and the phenyl rings of 12
is 0.49(8)8 which indicates a higher degree of p-orbital overlap
and increased effective conjugation length than that of 1,
which is due to enhanced electronic communication between
the electron-rich C4S4 heterocycle and electron-poor 2-
cyanoacrylate group. These altering characteristics demon-
strate the tunability of properties for this series of compounds
by varying substituents.
Given that many high performance organic electronic
materials include thiophene moieties, a direct comparison of
a thiophene- with C4S4-based material would be germane.
Figure 4 outlines a comparison of C4S4-based 7, 9, and 10 with
thiophene-based 14, 15, and 16, respectively. The compounds
compared have the same substitution pattern and functional
groups, the only difference being the central heterocycle.
Additionally, both chromophores have equal p-conjugation
lengths with both C4S4 and thiophene being a four carbon p-
conjugated unit. However, the disparity of the resulting
properties is striking. UV/Vis spectroscopy reveals that
compounds 7, 9, and 10 exhibit much broader absorption
than 14, 15, and 16. Moreover, the absorption maximum and
onset of absorption is, on average, red-shifted by 160 nm and
175 nm, respectively, when moving from thiophene to anal-
À
which compares favorably to analogous thiophene com-
pounds.[9,17] Additionally, the presence of a large number of
À
S S contacts imparts highly crystalline character, thus not
only aiding the ability to obtain suitable single crystals for X-
ray analysis but perhaps leading to favorable properties
attributed to crystalline organic materials. Moreover, the
crystal structure of 1 reveals there is a significant dihedral
angle of 24.54(4)8 between the planes defined by the central
C4S4 heterocycle and the pendant phenyl rings, presumably
limiting the effective conjugation length of the molecule.
However, other C4S4 derivatives can exhibit very different
molecular packing depending on the nature of the substitu-
ents employed. For example, the molecular packing of 12,
which contains strong electron-withdrawing groups (2-cya-
noacrylate), is dominated by intermolecular charge transfer
interactions between electron-rich C4S4 heterocycle and
electron poor 2-cyanoacrylate groups while not exhibiting
À
any close S S contacts. The intermolecular distance for 12 of
3.312 ꢂ indicates the close proximity of the conjugated
systems within the crystal packing which should be favorable
for charge transport properties. Furthermore, the dihedral
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the analogous and widely used thiophene derivatives. The
varying properties of C4S4 derivatives should lead to ability to
fine-tune both the molecular and bulk properties of materials
based on this building block by judicious choice of substitu-
ents. This work should contribute meaningfully to the
repertoire of available building blocks for conjugated organic
materials and lead to the discovery of new materials with
exciting properties.
Received: November 27, 2013
Published online: April 28, 2014
Keywords: conjugated materials · heterocycles · sulfur ·
.
synthetic methods · thiophenes
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