Molecular length dictates the nature of charge carriers in single-molecule junctions of oxidized oligothiophenes

Article abstract of DOI:10.1038/nchem.2160

To develop advanced materials for electronic devices, it is of utmost importance to design organic building blocks with tunable functionality and to study their properties at the molecular level. For organic electronic and photovoltaic applications, the ability to vary the nature of charge carriers and so create either electron donors or acceptors is critical. Here we demonstrate that charge carriers in single-molecule junctions can be tuned within a family of molecules that contain electron-deficient thiophene-1,1-dioxide (TDO) building blocks. Oligomers of TDO were designed to increase electron affinity and maintain delocalized frontier orbitals while significantly decreasing the transport gap. Through thermopower measurements we show that the dominant charge carriers change from holes to electrons as the number of TDO units is increased. This results in a unique system in which the charge carrier depends on the backbone length, and provides a new means to tune p- and n-type transport in organic materials.

Full text of DOI:10.1038/nchem.2160

ARTICLES
PUBLISHED ONLINE: 2 FEBRUARY 2015 | DOI: 10.1038/NCHEM.2160
Molecular length dictates the nature of charge
carriers in single-molecule junctions of
oxidized oligothiophenes
1
†
2†
1
2
*
1
*
Emma J. Dell , Brian Capozzi , Jianlong Xia , Latha Venkataraman and Luis M. Campos
To develop advanced materials for electronic devices, it is of utmost importance to design organic building blocks with
tunable functionality and to study their properties at the molecular level. For organic electronic and photovoltaic
applications, the ability to vary the nature of charge carriers and so create either electron donors or acceptors is critical.
Here we demonstrate that charge carriers in single-molecule junctions can be tuned within a family of molecules that
contain electron-deficient thiophene-1,1-dioxide (TDO) building blocks. Oligomers of TDO were designed to increase
electron affinity and maintain delocalized frontier orbitals while significantly decreasing the transport gap. Through
thermopower measurements we show that the dominant charge carriers change from holes to electrons as the number of
TDO units is increased. This results in a unique system in which the charge carrier depends on the backbone length, and
provides a new means to tune p- and n-type transport in organic materials.
rganic electronic materials have impacted the development tunnelling microscope (STM)-based break-junction (STM-BJ)
of semiconducting , photovoltaic2 and thermoelectric technique provides a means of probing the fundamental electronic
1
3
,4
O
devices . The precise control afforded over molecular properties at the single-molecule level, rather than in the bulk
1
8,19
design by organic synthesis allows device properties to be tuned material (Fig. 1a) . In this technique, a molecule is bridged
readily. For example, the ability to design and synthesize, from between two nanoscale gold electrodes and its conductance is
the principles of electronic structure, hole-/electron-transporting measured. The conductance depends on a number of factors,
p- and n-type, respectively) and ambipolar semiconductors has including the energetic alignment of the molecular frontier
led to significant advancements in organic electronics
(
5
–8
.
orbitals with the Fermi level of the gold electrodes. Whether the
Although design principles to create stable n-type organic materials molecule conducts through the highest occupied molecular
are evolving, to isolate the intrinsic, fundamental electronic- orbital (HOMO) or the LUMO depends on which of the two
structure contributions of the building blocks to bulk transport frontier orbitals is better aligned with the Fermi level and which is
remains challenging. Strategies to increase electron affinity, such better coupled to the metal. Generally, the dominant factor in
as incorporating electronegative functional groups onto conjugated this alignment is the nature of the linker groups that bind to
9–11
molecules, have been developed to facilitate electron transport
.
gold. For example, amine linker groups usually display HOMO
Although there is a wealth of organic electron-deficient building conducting behaviour, whereas pyridine linkers display LUMO
1
,2
20,21
blocks that may be used to synthesize conjugated molecules , it conducting behaviour . At the molecular level, the nature of
is a challenge to predict a charge-carrier type prior to making the the charge carriers in single-molecule junctions can be evaluated
3
,20
materials. For example, thiophene-1,1-dioxide (TDO, Fig. 1a), an by measuring the Seebeck coefficient (thermopower, S) . The
oxidized counterpart of thiophene, is unique in that it imparts value of S relates to the derivative of the transmission function at
2
2
vastly different electronic properties that arise from the break the electrode Fermi energy (EF) and hence its sign relates to the
3
,22
in aromaticity when the sulfur lone pairs are engaged in nature of the charge carriers (Fig. 1c)
bonding . This leads to electron-deficient building blocks with
.
12
Here we study the charge-transport characteristics of a family
strong dipole moments and oligoene-like backbones (Fig. 1b). of molecules that are composed of electron-deficient oligo-TDO
Interestingly, compounds that contain TDO moieties have building blocks coupled to unoxidized thiophenes on both sides
3
,18,20
low-lying lowest unoccupied molecular orbitals (LUMOs) and (Fig. 1a)
. This particular electron-rich/electron-poor combi-
n-type characteristics1
1,13–17
. However, the point at which these nation yields a drastic reduction in the transport gap of these
materials exhibit n-type behaviour as a function of conjugation molecules (as compared to their all-thiophene counterparts). We
length is unknown. As families of n-type materials are under- show two interesting results that impact molecular conductance.
developed in comparison to their p-type counterparts, to determine First, the conductances exhibit a very shallow decay with
this crossover point is an important fundamental development that molecular length (similar to that of oligoenes). This is a
could lead to the design of advanced materials for applications in manifestation of their small energy gaps and very-well-conjugated
organic electronics9
,11
.
backbones, with contributions from the quinoidal character of
To characterize electron-transport properties in these materials these systems. Second, through thermopower measurements with
can be challenging because bulk transport depends on both the two electrodes at different temperatures, we find that the
intramolecular and intermolecular orbital coupling, the latter dominant charge carriers switch from holes to electrons when the
11
being a difficult parameter to control . However, the scanning number of TDO monomers in the molecular backbone increases
1
2
Department of Chemistry, Columbia University, New York 10027, USA. Department of Applied Physics and Mathematics, Columbia University, New York
†
1
0027, USA. Contributed equally to this work. *e-mail: lv2117@columbia.edu; lcampos@columbia.edu
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.2160
ARTICLES
a
a
1.2
A
TDO1
Cold (TC)
Hot (TH)
1
0
.0
.8
TDOn:
n = 1–4)
TDO2
TDO3
TDO4
(
S
S
0.6
S
S
S
n
0
0
0
.4
.2
.0
O
O
b
From TDO4: oligoene-like backbone
300
400
500
600
700
800
O
O
O
O
Wavelength (nm)
S
S
S
X
X
S
S
S
b
c
O
O
O
O
c
TDO1
TDO2
TDO3
TDO4
E (eV)
Transmission function
3.0
LUMO
3.4
n-type
3
.7
EF
(–) Seebeck
coefficient
R
R
S
R
S
3.9
R
4
5
6
.0
.0
.0
4.2
S
S
(+) Seebeck
O
O
O
O
S
S
S
S
EF
coefficient
O
O
O
O
2
3
4
p-type
S
S
S
S
HOMO
R
R
R
R
5
.5
5.5
Eg 1.8
5.5
Figure 1 | Characterization of the nature of the charge carriers using the
STM-BJ technique. a, Schematic representation of the STM-BJ set-up to
measure the Seebeck coefficient of a family of molecules, TDOn (n = 1–4,
TDO4 is shown in the lower panel). The STM-BJ gold substrate is heated
with the gold tip maintained at room temperature. b, Diagrammatic
representation of the oligoene-like backbone in TDO4 that results from the
break in aromaticity of the thiophene ring on oxidation and therefore
increases the conjugation along the backbone. c, Example of a transmission
function that determines the probability that an electron of a given energy
will tunnel through the molecular junction. Highlighted are sections in which
5.6
Eg 2.1
Eg 1.6
Eg 1.4
Figure 2 | Energy-level characterization of the TDOn family. a, Normalized
UV-vis absorption spectra for the TDOn family measured in
dichloromethane. b, Colour images of the 0.2 M TDOn dichloromethane
solutions. c, CV-derived HOMO and LUMO levels (eV) and the HOMO–
g
LUMO gap (E ) for the TDOn family (R = SMe). These are determined by
first measuring the oxidation and reduction potentials (Eox1/2 and Ered1/2) by
CV carried out in dichloromethane solutions using a Pt working electrode
and an Ag/AgCl reference electrode (Supplementary Fig. 2 and
Supplementary Table 1). The HOMO and LUMO levels for TDOn are then
obtained using a HOMO level for ferrocene of −4.8 eV and Eox1/2 0.4 V
F
E aligns closer to the HOMO or LUMO of the molecule. The Seebeck
coefficient is proportional to the slope of the transmission function and thus
yields positive or negative values depending on the alignment.
(refs 24,42). The specific relationships used are: EHOMO = −e[Eox1/2 + 4.4]
and ELUMO = −e[Ered1/2 + 4.4].
with the linker groups remaining constant. This is the first time
that changing the number of repeat units in an oligomer has been
shown to change the conducting orbital, that is, the charge carriers. Supplementary Fig. 1. The fully unoxidized trimer has the
This work, therefore, provides an additional handle with which to shortest onset of absorption wavelength (λonset = 425 nm) and
tune conductance in single-molecule measurements, and also thus has the largest optical energy gap (see Supplementary Fig. 1).
underlines the promise of TDO as a powerful electron-deficient The fully oxidized non-aromatic trimer shows a significant
motif with a low-lying LUMO to tune the electronic properties in red-shift (about 100 nm) at the onset of absorption. This is
bulk materials.
caused by a better electron delocalization along the molecular
backbone, stabilization of the LUMO and increased planarity,
which is inferred from the pronounced vibronic structure of its
Results and discussion
2
5
Tuning the frontier molecular orbitals. We first show that our absorption spectrum . The third trimer, TDO1, displays the
molecule design, which couples
a
central electron-deficient longest λonset (575 nm), even though only a single unit is
oligothiophene-1,1-dioxide section to two electron-rich thiophenes oxidized. This reduction in the optical gap of TDO1 compared to
at either end, does, indeed, achieve a fully conjugated system with that of the unoxidized trimer demonstrates the powerful donor–
a small HOMO–LUMO gap . To this end, we compared the acceptor hybridization in these systems26,27.
23
ultraviolet–visible (UV-vis) absorption spectra of three different
We now turn to UV-vis spectroscopy and cyclic voltammetry
trimers: (1) a fully unoxidized (ter-thiophene) trimer, (2) a fully (CV) measurements to determine the trends in the frontier energy
oxidized trimer and (3) a single-oxidized unit flanked by two levels for the family of TDOn oligomers (with n = 1 to 4).
24
gold-binding methyl-sulfide-bearing thiophenes , as shown in The absorption spectra in Fig. 2a show a clear reduction in the
2
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ARTICLES
a
c
b
100
1
0–2
00
20
10
0
1
25
20
15
10
5
10–3
10–4
TDO1
TDO2
TDO3
TDO4
1
0–1
0–1
0–2
0–3
0–4
1
1
1
1
1
0–5
1
10–2
0
1 2 3 4
TDO units
1
nm
10–3
10–4
0–5
0
10–5
Displacement (nm)
10–5 10–4 10–3 10–2 10–1 100
0.0
0.4
0.8
1.2
Conductance (G0)
Displacement (nm)
Figure 3 | Single-molecule conductance data. a, Sample conductance versus displacement traces, laterally offset, for the TDOn series. These traces display
molecule-specific conductance features, which persist for longer displacements as the molecular length increases. b, 1D log-binned conductance histograms,
each composed of 20,000 traces, for TDO1–TDO4 measured at a 10 mV bias in 1-octylbenzene. Arrows indicate the peak-conductance positions. Inset: plot
−βn
of the conductance as a function of the number of oxidized thiophene monomer units (n) in TDO1–TDO4. Using G ≈ e , we obtained a decay constant of
.04/n. c, 2D conductance–displacement histogram for TDO3 created by aligning all the traces at the point where the conductance crosses 0.5 G , and then
overlaying them. The colour bar indicates the number of counts per 1,000 traces. For other 2D histograms see Supplementary Fig. 3.
1
0
optical gap with increasing n, accompanied by a deepening of the increase in the molecular plateau length with increasing backbone
colours of the molecules in solution from red to dark blue, as length. This indicates that we have probed similar junction
3
4
shown in Fig. 2b. To understand how the gaps decrease, we structures for all the molecules in this series . The inset of Fig. 3b
looked at the CV measurements of the TDOn oligomers shows the peak conductance values on a semilogarithmic scale
(
Supplementary Fig. 2), which show both oxidation and reduction plotted against n and illustrates that the conductance decreases
−
βn
peaks, in contrast to the analogous series of all-unoxidized rings, exponentially with increasing n (G ≈ e ) with a decay constant
in which only oxidation peaks are seen in the electrochemical of β = 1.04/n. As these TDO units are non-aromatic, the
window of the solvent . The oxidation peaks for the TDOn conduction path follows that of a chain of alternating double and
28
family are around +1.1 V, indicating that the energy of the single bonds across the backbone, as shown in Fig. 1b. We
–
1
HOMO does not change significantly as additional TDO units are therefore used a through-bond distance of 5.4 Å n as opposed to
−1
added. The reduction peaks, however, contain a shift from −1 V a through-space distance of 3.8 Å n to convert the measured β
–
1
for TDO1 to −0.3 V for TDO4, which shows that the LUMO into a decay constant of 0.2 Å . Interestingly, this value compares
3
5,36
energy decreases as TDO units are added. In Fig. 2c, we show the well with β determined for oligoene systems , and shows that
HOMO and LUMO levels derived from the CV data (see the transport across TDO oligomers is similar to that across
Supplementary Information). We see that as the number of TDO alkene oligomers.
repeat units increases, the LUMO of the molecules drops from
3.4 eV in TDO1 to 4.1 eV in TDO4 (Fig. 2c). The reduction poten- Thermopower measurements. Although the narrow HOMO–
tial for TDO4 is similar to that of substituted fullerenes, which could LUMO gap of the isolated molecules is reflected in the relatively
imply an n-type behaviour1 , in stark contrast to the unoxidized high conductances and shallow decay of the TDOn series, small-
1,29
3
0
oligothiophenes, which all demonstrate p-type characteristics . bias conductance measurements alone do not provide any
Although these CV-derived values do not determine the exact align- information on the nature of the orbital that dominates
31
ment of the orbitals relative to a metal electrode , the trends should charge transport. To probe how such an orbital alignment is
be discernible in transport measurements.
impacted by the dramatic lowering of the LUMO with an
increasing number of oxidized thiophene units, we measured
Conductance measurements. Next, we performed single-molecule the Seebeck coefficient of these molecular junctions. The
conductance and thermopower measurements on the TDOn family Seebeck coefficient for a single-molecule junction is determined
to probe how their increased conjugation, smaller optical gaps and by heating the substrate although the STM tip kept at room
low-lying LUMOs impact charge transport through molecular temperature and measuring the thermoelectric current with no
junctions. For each molecule in the series, 20,000 conductance- external bias voltage applied (Fig. 1a, see Methods and the
2
0
versus-displacement traces were collected using the STM-BJ Supplementary Information for details) . With the molecular
technique (Fig. 3a; see Methods and the Supplementary conductance (G), thermoelectric current (I) and temperature
18,19
Information for details) . All the measured conductance difference (ΔT) known for each junction measured, S is given by
traces collected were compiled into one-dimensional (1D), S = I/(GΔT) (ref. 22). We compiled the Seebeck coefficients from
logarithmically binned conductance histograms3 (100 bins per hundreds of junctions for each molecule into the histograms shown
decade) without data selection and are shown in Fig. 3b. These in Fig. 4a; these were then fitted with a Gaussian function to
2
1D histograms display a clear peak that yields the most-probable determine the most-frequently measured Seebeck coefficient for
molecular conductance value, which decreases systematically with each molecule.
increasing numbers of TDO monomers (n) (Fig. 3b). The
In Fig. 4b we plot the Seebeck coefficient versus the molecular
relatively narrow breadth of the conductance histogram peaks is length. For TDO1–TDO3, this value is positive and decreases non-
2
8,33
indicative of a rather rigid backbone in this class of molecules
.
linearly with length, whereas for TDO4 it is negative and relatively
A 2D histogram for TDO3 that retains the displacement large in magnitude. This trend is contrary to that observed in amine-,
information and is created from the same data (Fig. 3c) shows thiol- and trimethyltin-linked oligophenyl series, in which the
that these junctions sustain a 1 nm elongation. Comparing 2D Seebeck coefficient maintains the same sign (positive) and increases
3
,21,37
histograms across the series (Supplementary Fig. 3) we see a clear in magnitude with molecular length
. Such measurements
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a
measurements for TDO1 (Supplementary Fig. 7, and the discussion
in the Supplementary Information).
S = 7.3 µV K–1
S = 6.4 µV K–1
S = 2.4 µV K–1
S = –22.1 µV K–1
TDO1
TDO2
TDO3
TDO4
0
0
.2
.1
The large, negative Seebeck coefficient measured for TDO4
indicates that electron transport is dominated by the LUMO. The
magnitude of this value is comparable to that of C , which is
6
0
3
8
among the highest values of organic compounds . Again, assuming
a single-Lorentzian transmission function for this system, we infer
0
.0
.2
that the LUMO is 0.7 eV from E , consistent with results from IV
F
0
measurements (Supplementary Fig. 7). The Seebeck coefficient for
TDO2 is smaller in magnitude than that for TDO1; if the transport
in TDO2 was dominated by the HOMO, we would expect a larger
Seebeck coefficient compared to that of TDO1. We thus conclude
that the transport in TDO2 is not dominated by a single
orbital. Finally, the slightly positive but small-magnitude Seebeck
coefficient measured for TDO3 suggests that the EF lies in a
relatively flat region of the transmission function, possibly in the
middle of the HOMO–LUMO gap with both orbitals contributing
to the charge transport. This implies that the transmission functions
for both TDO2 and TDO3 do not follow a Lorentzian shape and,
thus, a direct determination of the level alignment for these two
systems is not possible with these measurements. Taken together,
these measurements show that contributions from the LUMO to
charge transport become increasingly more important as the
number of oxidized thiophene units in the molecular backbone
increases. We showed additional evidence of a change in the con-
0
.1
0
.0
.2
0
0
.1
0
.0
.2
0
0
.1
0
.0
–
40
–20
0
20
40
39
ducting orbital by carrying out solvent-dependent measurements
Seebeck coefficient (µV K–1
)
on TDO1 and TDO4 (see Supplementary Fig. 6 for details). This
trend is in accord with the previously discussed UV-vis and CV
measurements, in which a decreasing optical gap and a lowering
of the reduction potential are observed with increasing molecular
length (Fig. 2).
Interestingly, despite changing the conducting orbital, we still
found an exponential dependence of conductance on molecular
length with a well-defined decay constant. To understand how we
could observe this, we turned to an n-site tight-binding model in
which each site is assigned an occupied and an unoccupied
orbital (Supplementary Fig. 8). This model is described in detail
in the Supplementary Information. In general, we found that the
b
TDO4
TDO3
TDO2
TDO1
2
4
0
6
2
2
1
1
–
40
–20
0
20
40
conductance decays exponentially, as long as E is not close to
Seebeck coefficient (µV K–1
F
)
any resonance, even with a change in the dominant transport
channel with increasing length. However, for longer chain lengths,
we saw a deviation from this exponential dependence if an orbital
Figure 4 | Single-molecule thermopower data. a, STM-BJ measurements
taken with a temperature difference of 16 K between the gold tip and the
substrate. While the molecule bridges the gap between the electrodes,
the applied bias voltage is dropped to zero and the thermoelectric current is
measured. Seebeck coefficients for each junction are then calculated using
this current, as detailed in the text and Supplementary Information, and
compiled into the 1D histograms shown. The black curves are Gaussian fits
to the Seebeck coefficient distributions. b, Plot of Seebeck coefficients as a
function of molecular length for the TDOn family with error bars that reflect
the error in the Gaussian fits. A shift from positive to negative Seebeck
values with increasing length is seen, which indicates a change in the charge
carriers from holes to electrons.
was closer to EF.
We have used STM-BJ conductance and Seebeck-coefficient
measurements to illustrate the transport characteristics of
molecules that contain TDO, a poorly studied building block that
has tremendous impact on the electronic properties of both
polymers and small molecules. Our findings demonstrate that
these molecules constitute a unique system in which the charge
carrier in gold–molecule–gold junctions switches from holes
to electrons with an increasing number of monomers. Although
previous studies showed that the dominant conducting orbital is
2
0
generally dictated by the gold-binding linker units , control
through the backbone length is unprecedented. Moreover, TDO4
2
0,38
demonstrate that these oligophenyls conduct via hole transport is shown to have a relatively high Seebeck coefficient , which
through the HOMO. The increase in magnitude of S with increasing suggests that these systems may be potential candidates for
length in these oligophenyls is attributed to the HOMO moving thermoelectric devices. The conductance data also show that
closer to EF and simultaneously narrowing as the conducting the transport properties of oligo-TDOs are similar to those of
orbital delocalizes over a longer molecule, which results in a oligoalkenes, reinforcing the fully conjugated and non-aromatic
larger slope in the transmission function at E . Based on our nature of their backbones. These studies demonstrate that small
F
measurements on the TDOn series, we infer that transport in changes in molecular structure can have a major impact on
TDO1 is dominated by HOMO, which indicates a hole-type trans- charge-transport characteristics—a strategic molecular design pro-
port. Using the conductance and Seebeck coefficient determined for vides a method for engineering the carrier type in molecular
TDO1 and assuming the line shape of a single-Lorentzian trans- devices. Additionally, as the oxidized thiophene units have a
mission function, we found that the conducting orbital was −2 eV dramatic impact on the LUMO, these studies reveal a potentially
from E , in agreement with results from current and voltage (IV) new family of building blocks for n-type materials.
F
4
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ARTICLES
8
9
.
.
Meijer, E. J. et al. Solution-processed ambipolar organic field-effect transistors
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15, 33–38 (2003).
Methods
This section describes key experiments only; an extended experimental section is
provided in the Supplementary Methods.
Synthesis of the TDOn family. The general procedure to synthesize these
molecules is as follows (see the Supplementary Information for details).
First, a dibromo-terminated oligothiophene was treated with Rozen’s reagent .
Rozen’s reagent is a stabilized form of hypofluorous acid that can be used to
oxidize the thiophene units. Then, a Stille coupling was performed to add the
gold-binding linkers (with a flanking unoxidized thiophene moiety) to the
4
0
10. Katz, H. E. et al. A soluble and air-stable organic semiconductor with high
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organic semiconductors in organic electronics. Adv. Mater.
4
1
oxidized thiophene oligomers . As an example, the synthesis of TDO1 is
described here. A mixture of 40 ml acetonitrile and 4 ml water was cooled to −10°C.
22, 3876–3892 (2010).
12. Chen, W. et al. Aromaticity decreases single-molecule junction conductance.
J. Am. Chem. Soc. 136, 918–920 (2014).
A mixture of 20% F
generate HOF·CH CN (Rozen’s reagent). A 1 ml aliquot was removed, added to a
saturated KI solution and the liberated I was titrated against Na to
determine the concentration of Rozen’s reagent. 2,5-Dibromothiophene
400 mg, 1.65 mmol, 1 equiv.) was dissolved in 10 ml CH Cl and cooled to 0 °C.
HOF·CH CN (55 ml, 0.15 M, 8.27 mmol, 5 equiv.) was added dropwise and the
solution was stirred at room temperature for two hours (for each oxidation,
–3 equiv. Rozen’s reagent per thiophene moiety were used). The reaction was
quenched by adding a saturated NaHCO solution dropwise. After a standard
2 2
in N was bubbled through this solution for an hour to
3
1
3. Barbarella, G., Pudova, O., Arbizzani, C., Mastragostino, M. & Bongini, A.
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2
2 2 3
S O
(
2
2
1
4. Camaioni, N., Ridolfi, G., Fattori, V., Favaretto, L. & Barbarella, G.
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arrangement of internal adjacent [all]-S,S-oxygenated thiophene fragments.
Chem. Eur. J. 19, 5289–5296 (2013).
3
2
1
1
3
aqueous workup, 2,5-dibromothiophene-1,1-dioxide was obtained in 78% yield.
Next, 2,5-dibromothiophene-1,1-dioxide (30 mg, 0.11 mmol, 1 equiv.) and
tetrakis(triphenylphosphine)palladium (6.4 mg, 0.0055 mmol, 0.05 equiv.)
were dissolved in 2 ml dimethylformamide and sparged with Ar for 20 minutes.
1
1
1
2
7. Dell, E. J. & Campos, L. M. The preparation of thiophene-S,S-dioxides and their
role in organic electronics. J. Mater. Chem. 22, 12945–12952 (2012).
8. Xu, B. Q. & Tao, N. J. J. Measurement of single-molecule resistance by repeated
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.2160
ARTICLES
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3
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Author contributions
chemistry has to offer. Acc. Chem. Res. 47, 2378–2389 (2014).
1. Stille, J. K. The palladium-catalyzed cross-coupling reactions of organotin
reagents with organic electrophiles. Angew. Chem. Int. Ed. Engl.
B.C., E.J.D., L.V. and L.M.C. conceived and designed the experiments. B.C. performed
the conductance and thermopower measurements. E.J.D. and J.X. synthesized and
characterized the molecules. All authors discussed the results. E.J.D. and B.C.
contributed equally to this work. E.J.D., B.C., L.V. and L.M.C. wrote the paper with
contributions from all authors.
4
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dependence of electronic and electrochemical properties of conjugated systems:
polyacetylene, polyphenylene, polythiophene, and polypyrrole. J. Am. Chem.
Soc. 105, 6555–6559 (1983).
Additional information
Supplementary information and chemical compound information are available in
the online version of the paper. Reprints and permissions information is available online
at www.nature.com/reprints. Correspondence and requests for materials should be
addressed to L.V. and L.M.C.
Acknowledgements
This work was supported primarily by the National Science Foundation under award
DMR-1206202. E.J.D. thanks the Howard Hughes Medical Institute, American
Australian Association and Dow Chemical Company for International Research
Fellowships.
Competing financial interests
The authors declare no competing financial interests.
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