Approaching the Integer-Charge Transfer Regime in Molecularly Doped Oligothiophenes by Efficient Decarboxylative Cross-Coupling

Article abstract of DOI:10.1002/anie.201914458

A library of symmetrical linear oligothiophene was prepared employing decarboxylative cross-coupling reaction as the key transformation. Thiophene potassium carboxylate salts were used as cross-coupling partners without the need of co-catalyst, base, or additives. This method demonstrates complete chemoselectivity and is a comprehensive greener approach compared to the existing methods. The modularity of this approach is demonstrated with the preparation of discreet oligothiophenes with up to 10 thiophene repeat units. Symmetrical oligothiophenes are prototypical organic semiconductors where their molecular electrical doping as a function of the chain length can be assessed spectroscopically. An oligothiophene critical length for integer charge transfer was observed to be 10 thiophene units, highlighting the potential use of discrete oligothiophenes as doped conduction or injection layers in organic electronics applications.

Full text of DOI:10.1002/anie.201914458

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Approaching the Integer-Charge Transfer Regime in Molecularly
Doped Oligothiophenes by Efficient Decarboxylative Cross-
Coupling
Authors: Jiang Tian Liu, Hannes Hase, Sarah Taylor, Ingo Salzmann,
and Pat Forgione
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201914458
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10.1002/anie.201914458
Angewandte Chemie International Edition
RESEARCH ARTICLE
Approaching the Integer-Charge Transfer Regime in Molecularly
Doped Oligothiophenes by Efficient Decarboxylative Cross-
Coupling
Jiang Tian Liu, [a] Hannes Hase,[b] Sarah Taylor,[a] Ingo Salzmann*,[a][b][d] and Pat Forgione*[a][c][d]
Dedication ((optional))
Abstract: A library of symmetrical linear oligothiophene has been
prepared employing decarboxylative cross-coupling reaction as the
key transformation. We utilized thiophene potassium carboxylate
salts as cross-coupling partners without the need of co-catalyst,
base, or additives. This method demonstrates complete
undergoes integer-charge transfer with molecular p-dopants
such as 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane
(F4TCNQ).[9] This process readily generates one electron-hole
in P3HT per dopant radical anion formed and increases its sheet
conductivity by >5 orders of magnitude at typical doping ratios
in the percent range.[10] Polythiophenes commonly contain a
mixture of various size and batch-to-batch differences that
make it difficult to reproduce the exact distribution. In turn,
long discrete oligothiophenes are valuable due to their uniform
size distribution and reproducibility that would have an impact
in a variety of applications.[11] However, despite the chemical
similarity of oligo- and polythiophene, they have been reported
to behave fundamentally different regarding their response to
electrical doping. In contrast to integer-charge transfer in
P3HT, only fractional charge transfer occurs for, e.g.
quaterthiophene (T4)[12] or sexithiophene (T6)[13] upon doping
with F4TCNQ, as the frontier molecular orbitals of OSC and
dopant hybridize to form ground-state charge transfer
complexes (CPXs).[8b] Instead of one electron being simply
transferred from the highest occupied molecular orbital
(HOMO) of the oligothiophene to the lowest unoccupied
molecular orbital (LUMO) of the p-dopant, both electrons
occupy the bonding hybrid orbital of the CPX, while its
antibonding orbital remains empty. Mediated by the coupling
strength of the CPX, these unoccupied states can lie well within
the fundamental gap of the pristine oligothiophene environment
and represent the effective acceptor level for doping-related
charge transfer (as opposed to the dopant LUMO in the case of
P3HT). Since all available states are occupied according to
Fermi-Dirac statistics, only a small fraction of these CPX states
is occupied at room temperature, generating holes in the OSC
matrix. Therefore, upon CPX formation the doping efficiency
becomes significantly reduced. Interestingly, however, despite
chemoselectivity and is
a comprehensive greener approach
compared to the existing methodologies. The modularity of this
approach is demonstrated with the preparation of discreet
oligothiophenes with up to 10 thiophene repeat units. Symmetrical
oligothiophenes represent prototypical organic semiconductors and
allow us to spectroscopically assess their response to molecular
electrical doping regarding the degree of charge transfer as a
function of the chain length. We find evidence that the critical length
for integer charge transfer as it is found in the polythiophene limit to
be not more than 10 thiophene units, which highlights the potential of
long discreet oligothiophenes for their use in organic electronic
applications as doped conduction or injection layers.
Introduction
Thiophene based-materials have attracted a tremendous amount
of attention in recent decades.[1,2] Among them -conjugated
poly/oligothiophene have been at the center of focus due to
their applicability as organic electronic materials in organic
photovoltaic cells (OPVCs),[1-4] organic light emitting diodes
(OLEDs),[2-4] organic field-effect transistors (OFETs),[1,5] liquid
crystal,[6] and fluorescent biomarkers.[7] In electronic
applications, conjugated organic molecules and polymers are
employed as organic semiconductors (OSCs) and, in analogy to
their inorganic counterparts, electrical doping is frequently
employed to tune both their conductivity and electronic
structure to meet application-specific demands.[8] In this
context, it has been found that the prototypical organic
semiconductor regio-regular poly(3-hexylthiophene) (P3HT)
the strong recent interest in the doping of OSCs and its
8c]
fundamental relevance for electronic applications,[8a,
the
parameters that govern the strikingly different doping behavior
of conjugated oligo- and polymers have not yet been
systematically explored.
[a]
[b]
Department of Chemistry and Biochemistry
Concordia University
7141 rue Sherbrooke O., Montréal, QC, Canada H4B 1R6
E-mail: pat.forgione@concordia.ca
Here, we aim to assess the transition between the two
fundamentally different doping phenomenologies of oligo- and
polythiophene by altering their most apparent difference, that
is, the number of repeat units with uninterrupted π-conjugation.
To this end, we propose a novel, highly efficient and
sustainable synthetic route towards linear oligothiophenes of
different length employing a decarboxylative cross-coupling
strategy as a greener synthetic method for constructing these
molecules. The common synthetic pathway towards
oligothiophenes generally utilizes palladium-mediated cross-
coupling reactions such as Stille, Kumada, Neighshi, and
Suzuki.[14] These reactions are robust methodologies, but often
involve organometallic reagents and produce metallic waste
that can interfere in the electronic properties of the final
Department of Physics
Concordia University
7141 rue Sherbrooke O., Montréal, QC, Canada H4B 1R6
E-mail: ingo.salzmann@concordia.ca
[c]
[d]
Center for Green Chemistry and Catalysis
McGill University
801 rue Sherbrooke O., Montréal, QC, Canada H3A 0B8
Centre for Research in Molecular Modeling (CERMM), Centre for
NanoScience Research (CeNSR), Concordia University, 7141 rue
Sherbrooke O., Montreal, QC, Canada H4B 1R6
Supporting information for this article is given via a link at the end of the
document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201914458
Angewandte Chemie International Edition
RESEARCH ARTICLE
[15]
compounds.
Direct (hetero)arylation is a greener efficient
4
5
6
7
8
PdCl2/P(t-Bu3)2
PdCl2/dppf
54[c]
43[c]
[16]
approach for constructing conjugate-polymeric materials,
but for small discrete oligomers, some suffer from
regioselectivity issues and unwanted polymerization.
Alternatively, decarboxylative cross-coupling provides greater
DMA
190C, w, 8min
[17,18]
PdCl2/JohnPhos
PdCl2/PPh3
81
72
85
chemoselectvitiy since the coupling occurs directly at the
PdCl2/P(o-Toyl)2
[19]
carboxylic acid.
This coupling has been demonstrated as an
[a] 4 equiv. of carboxylic acid, 1 equiv. of TBAC, 1.5 equiv. of Cs2CO3. [b] 4 equiv. of
carboxylic acid, 1.5 equiv. of Cs2CO3 [c] Yield determined by 1H-NMR with 1,3,5-
Trimethoxybenzene as internal standard
excellent synthetic tool for bi-aryl arene-based substrates, [20] or
[21-23]
heteroaryl substrates for medicinal chemistry.
Initial effort was focused on exploring and optimizing reaction
conditions for the synthesis of symmetrical terthiophene 3a,
starting from two commercially available thiophene derivatives
1a and 2 (Table 1). Our first attempt was focused on conditions
previously disclosed
Additionally, the major by-product is CO2 gas that is easily
separable from the final mixture, in contrast to the
organometallic by-product obtained from other synthetic routes.
This would further facilitate application studies since these
metallic residues affect the device performance.
[22b, 23b]
that provided the corresponding
[15]
We
product 3a with a limited yield of 31% (Entry 1). This low
yield was not surprising since this condition only applied to
electron-deficient aryl halides and was not demonstrated for
electron-rich arenes like dibromothiophene.[22] Interestingly,
removing tetra-butyl ammonium chloride improved the yield to
51% (Entry 2), which is in contrast to previous reports that
indicated this additive was essential to obtain high yields.
Further optimization allowed us to determine that using the
potassium salt of 1 with higher temperature and dimethyl
acetamide (DMA) as solvent yielded a comparable yield of
49% (Entry 3). Subsequent atom efficiency could be achieved
by eliminating the need for Cs2CO3 as the base since we were
no longer employing the carboxylic acid but rather the
corresponding salt and reducing the carboxylate loading from 2
to 1.1 equivalent (Entries 3-8). Further optimization focused on
the palladium and ligand sources. Using palladium chloride
with tri-tertbutyl-phosphine ligand provided the product with a
similar yield of 54% (Entry 4). Employing a bidentate ligand
such as 1,1’Bis(diphenylphosphino)ferrocene (dppf) did not
further improved the yield (entry 5). The more sterically
hindered monodentate ligand JohnPhos improved the yield to
81% (entry 6) and with the less sterically hindered
triphenylphosphine (PPh3) had a reduced yield of 72%
(entry 7). Lastly, employing a moderately steric hindered
tris(o-tolyl) phosphine offered the best isolated yield of 85%
(entry 8). This new set of conditions offered a high yielding
route that avoids both the use of cesium carbonate as base and
an additive. Importantly for materials-based applications, the
only by-products are water-soluble potassium bromide and CO2
gas, that can be easily removed and therefore would not
interfere in the OSC properties.
envisioned utilizing decarboxylative coupling as an efficient
strategy for constructing a library of discreet oligothiophenes,
which can be complementary to existing methodologies, and
provides a greener approach for green energy applications. Our
linear oligothiophenes of different lengths allow us to
systematically study their charge-transfer behavior in thin films
upon doping with F4TCNQ. By increasing the oligomer length
our aim is to approach the polymer limit to connect the
fundamentally different doping phenomenologies of oligo- and
polythiophene and to determine the critical chain length for
integer rather than fractional charge transfer upon molecular
doping.
Results and Discussion
Our initial goal was to develop an efficient strategy for the
preparation of thiophene-based materials that is more
environmentally benign compared to the common alternatives.
Towards this end, we pursued a modified strategy based on
previously reported decarboxylative cross-couplings of
thiophene derivatives that preclude the use of co-catalysts and
avoids the generation of metallic waste that can interfere in
[22, 23]
electronic applications of oligothiophenes.
However,
among the many decarboxylative cross-coupling methods
reported, none have been utilized for the construction of
oligothiophenes while only one group describes the synthesis of
a unique pyrrole-based oligomer for electronic application.[24]
Furthermore, many developed methodologies require the use of
silver or copper as co-catalyst[20,21] that generate more metallic
waste. This monometallic approach has been developed for the
construction of bi-(hetero) aryls, [22,23] and applied to biomass
With these optimized conditions in hand, we subsequently
explored other thiophene-based substrates for applications in
materials chemistry (Table 2). The potassium 3-
hexylthiophene-2-carboxylate (1b) and potassium 3,4-
ethylenedioxylthiophene-5-carboxylate salts (1c) were prepared
derived furan carboxylic acid substrates. [23e]
Table 1. Optimization of condition for synthesis of terthiophene
from 3-hexyl thiophene and 3,4-ethylenedioxylthiophene
[25]
(EDOT) respectively from previous reports.
5-
methylthiophene-2-carboxylate and thiophene-2-carboxylate
were prepared directly from the commercially available
carboxylic acids. The 3,3-hexyl substituted terthiophene 3b is a
common moiety featured in OPVCs or serve as building block
Entry
Pd
Solvent
Conditions
Yield (%)
[1-3]
for conjugate polymers.
Employing the decarboxylative
Sources/Ligand
cross-coupling provide the corresponding product (3b) in
excellent yield (83%) from 3-hexylthiophene carboxylate (1b).
Similarly, unsubstituted terthiophene product 3c is commonly
found as a fundamental building block in material chemistry. [1-
1
2
3
Pd(t-Bu3P)2
Pd(t-Bu3P)2
Pd(t-Bu3P)2
DMF
DMF
DMA
170C, w, 8min
170C, w, 8min
190C, w, 8min
31[a]
51[b], [c]
49[c]
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10.1002/anie.201914458
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RESEARCH ARTICLE
3] However, this route only affords a limited yield of 22% from
potassium 2-thiophene carboxylate. Terminal substituted
observed for both substrates. EDOT was also revisited as the
electrophilic
cross-coupling
partner.
2,5-dibromo-3,4-
oligothiophenes are also valuable in various material
ethylenedioxythiophene only offered a limited yield of 24%
with 3-methyl carboxylate salt (1a) for product 6a, and a slight
increased yield of 44% is obtained from 3-hexylthiophene
carboxylate salt (1b) for product 6b. Although, this method
seemed to be intolerant to electron rich substrates, interestingly,
the only decarboxylate cross-coupling report related to organic
[1-4]
applications.
We utilize potassium 5-methylthiophene
carboxylate to synthesize product 3d as an example, for this
class of molecules, yet it only offers moderate 44% yield for
terthiophene product 3d. Nevertheless, the lower yields of
compound 3c and 3d likely due to the fact that both thiophene
carboxylate salts lack a substituent at the C3-position as electronic materials solely utilize electron-rich pyrrole
previously observed. [22b] Furthermore, electron rich thiophene
substrates such 3,4-ethylenedioxylthiophene (EDOT) is also a
key structural feature for tuning the electronic properties of
material for different applications.[1,3] The decarboxylative
cross-coupling demonstrated the ability to incorporate EDOT
units into the corresponding product (3e), albeit in a moderate
yield of 31% from potassium 3,4-ethylenedioxylthiophene-5-
carboxylate salt, making the method described herein
complementary that this previous report.24
Table 3. Iterative synthesis of symmetric oligothiophene
carboxylate salts.
Table 2. Substrate scope for thiophene-based oligomers
R
R
R
PdCl2 (5 mol%)
P(o-Toyl)3 (10 mol%)
S
S
OK
Br
n
Br
+
S
S
S
n
DMA
mw, 190 °C, 8 min
O
R'
n=1,2
R'
2.2 equiv.
1.0 equiv.
R
R
S
S
S
R
S
S
R
R
S
R
S
S
S
S
R'
R'
S
S
7a R = CH3, R' = CH3 69%
7b R = CH3, R' = C6H13 75%
3b R = 3-C6H13
3c R = H
3d R = 5-CH3
83%
22%
39%
4a R = 3-CH3
4b R = 3-C6H13
82%
90%
R
3e R = 3,4-OCH2CH2O 31%
R
R
R
R
S
S
S
S
S
S
S
S
S
S
R = C6H13
91%
R
4b
1. Bromination
O
O
R'
5a R' = CH3, R = CH3
5b R' = C6H13, R = C6H13 30%
63%
6a R = CH3 24%
6b R = C6H13 44%
2. Cross-Coupling 61%
R
R'
S
S
S
Subsequently, two quaterthiophene derivatives with similar
feature are obtained through cross-coupling with 5,5’-dibromo-
2,2’-bithiophene. 3-substituted quaterthiophene product 4a and
4b are commonly used building blocks for poly/oligothiophene,
S
S
S
R = C6H13
R' = C6H13
R'
R
8
1. Bromination
85%
[1-3]
2. Cross-Coupling 75%
and have been used as model compounds in fundamental
research on molecular doping.[12] This method afford an
excellent yield of 82% for 3-methyl substituted product 4a with
3-methyl thiophene carboxylate salt 1a while product 4b gave
the highest yield of 90% from 3-hexyl thiophene carboxylate
salt 1b. Further effort focused on examining this approach with
different electrophilic coupling-partners, along with the most
compatible 3-substituted carboxylate thiophene substrate (1a
and 1b). Sterically hindered 2,5-dibromo 3-methylthiophene
with 3-methyl thiophene carboxylate salt (1a) afford product 5a
with 63% yield, which is also a preliminary example for ability
to synthesize regioirregular oligothiophenes. The increasingly
sterically hindered 2,5-dibromo 3-hethylthiophene with 3-
hexylthiophene carboxylate salt (1b) provided the
corresponding regioirregular terthiophene product 5b in
moderate yield (30%), which further supports that the
preparation of regioirregular oligothiophenes is not favored
employing this method. Moreover, mono-arylation is not
R'
R
R
S
S
S
S
S
S
S
S
R = C6H13
R' = C6H13
R
R
R'
9
1. Bromination
89%
2. Cross-Coupling 81%
R
R
R
R'
S
S
S
S
S
S
S
S
S
S
R'
R
R
R
10
R = C6H13
R' = C6H13
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10.1002/anie.201914458
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RESEARCH ARTICLE
This approach was subsequently applied to construct long,
discreet oligothiophenes for our molecular doping studies by
extension of terthiophene product (3a). We initiated the
As shown in Table 4, we compared our conditions with related
synthetic methods utilizing palladium catalysts through the
environmental-factor (E-factor) and atom economy metrics to
evaluate the sustainability of this decarboxylative cross-
synthesis by bromination of 3a following previously reported
[26]
conditions.
The subsequent cross-coupling followed the coupling method.[28] E-factor is a measure of the waste
same conditions employed for tri- and tertramers (Table 3).
Pentathiophene derivatives from 3-methyl thiophene
carboxylate salt 1a provided the product 7a in a 59% yield for
3-methyl pentathiophene, and 75% yield for pentathiophene
product 7b from 3-hexyl thiophene carboxylate salt (1b). We
further aimed to extend quaterthiophene derivatives to obtain
long symmetrical oligothiophene more suitable for charge-
transfer studies. An iterative sequence (Table 3, bottom) with
production from each reaction, where our method only has 4.1
compared to 9.0 for the Suzuki cross-coupling[29] , and 5.1 from
[30]
the Stille cross-coupling
for model product 3b. Atom
economy reflects the conversion efficiency for a synthesis. It
depicts how efficiently the atoms of the starting material are
incorporated into the desired product. In this case,
decarboxylative coupling demonstrated a higher efficiency
[29]
(55%) compared to both the corresponding Suzuki (49%)
3-hexyl substituted quaterthiophene product 4b was and Stille (44%) [30] cross-couplings. Additionally, our method
undertaken. The synthesis starts with the bromination of the
core oligothiophene (4b) and followed by cross-coupling with
2.2 equivalent of 3-hexylthiophene carboxylate salt (1b). The
bromination reaction afforded the corresponding product with
yield 91%, and subsequent cross-coupling provided the
sexithiophene 8 in 61% yield. The octithiophene derivative (9)
was obtained from the same iterative reaction sequent, where
product 8 is brominated in 89% yield, then underwent cross-
coupling with the same carboxylate salt (1b) to afford product 9
in 81% yield. Further iteration of octithiophene derivative 9
afforded decithiophene derivative (10) with excellent yields of
89% for the bromination of 9 and 81% yield for subsequent
decarboxylative cross-coupling reaction. Interestingly, we did
not observe any homo-coupling product from the carboxylate
coupling partner throughout all of our synthesis, where similar
reports using a Stille reaction sequence often afforded a homo-
also provided further improvements based on the short reaction
time (8 mins) compared to the Suzuki (48h) [29] and Stille (16h)
[30] alternatives.
coupling by-product (5-15%) from the organotin functionalized
[27]
thiophene substrates.
The absence of homo-coupling by-
product provides a more efficient purification as the homo-
coupling impurities often have similar properties to the target
molecules.
Table 4. Comparison of green chemistry index of 3b with different synthetic
strategies
Decarboxylative Cross-Coupling:
E-Factor: 4.1. Atom Economy: 55 %.
Figure 1. UV/vis/NIR thin film spectra of pure 4b, 8, 9, 10 and P3HT together
with a plot of extrapolated transition onset versus the inverse number of
thiophene repeat units (top); the reference data for P3HT+F4TCNQ (F4) has
been taken from Ref. [31]. Thin film spectra upon doping with F4 at a ratio of
1:10 (bottom). Asterisks mark transitions related to the dopant radical anion
(asterisks); vertical bars mark transitions assigned to the ground-state charge
transfer complex (CPX) formed between the oligothiophene and F4; P1 and
P2 mark the polaronic transitions in charged P3HT, and vertical arrows in
10+F4 transitions discussed in detail in the text.
Suzuki Cross-Coupling [29]
:
E-Factor: 9.0. Atom Economy: 49 %.
Stille Cross-Coupling [30]
:
Subsequently, we employed this series of discrete
oligothiophenes as OSCs and investigated changes in their
doping phenomenology with increasing π-conjugation length.
Figure 1 (top) shows the ultraviolet/visible/near infrared
(UV/vis/NIR) absorption spectra of 4b, 8, 9, and 10 in thin
films established by drop casting on fused silica substrates from
chloroform solution, the corresponding spectrum of regio-
E-Factor: 5.1. Atom Economy: 44 %.
[a] For E-factor and atom economy calculation, see supporting information
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10.1002/anie.201914458
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RESEARCH ARTICLE
regular P3HT is shown as a reference. Fully in line with
respectively, and β is a resonance integral that takes into
previous reports, the energies of the fundamental transition
account the intermolecular electronic coupling energy;[8b, 12, 33]
β
[32]
S0→S1 decrease with increasing chain length.
We observe
strongly depends on the intermolecular arrangement[13, 34] and
has been determined to 0.42 eV in 1:1 co-crystals of T4 and
the expected 1/n dependency of the optical gap with the number
n of thiophene repeat units (inset in Figure 1), which both F4.[12] The trend we observe for the redshift of the CPX
indicates that the conjugation length increases with n and that absorption with increasing oligomer length beyond T4 is fully
the series does not significantly differ in planarity. [32d] A linear
fit to the absorption- onset values of our oligomer series
converges to 1.89 eV for n → ∞, which equals the onset of the
P3HT absorption in the error margin. To investigate the
response of this series to molecular doping, we added the
molecular p-dopant F4TCNQ (F4) at a ratio of 1:10 to the
solutions, the corresponding thin-film UV/vis/NIR spectra are
depicted in Figure 1 (bottom). The spectrum of 4b+F4 is very
similar to previously reported spectra of non-alkylated
quaterthiophene (T4) doped with F4,[12] showing a prominent
feature at 0.89 eV (peak) in the fundamental gap of the pristine
material. For the longer oligomers 8 (n = 6 thiophene units) and
9 (n = 8) we again find very similar spectra, now with the sub-
gap absorption feature lying at lower energy (in both cases
centered at 0.66 eV). In analogy to Ref. 12, these doping-
related optical transitions are assigned to the S0→S1 transition
in a CPX formed between the oligomer and the dopant. The
energies E1/2 of the CPX frontier molecular orbital levels
in line with a recent theoretical study on the density-functional
theory (DFT) level, where the electronic structure of CPXs
formed by T4, T6, T8, and T10 with F4 has been calculated.[35]
In contrast to this study, however, the absorption characteristics
of the doped films change fundamentally for our longest
oligomer 10 (n = 10). Two sharp transitions arise at sub-gap
energies (marked with dotted lines in Figure 1, bottom), which
are assigned to the radical anion of F4.[9, 33, 36] They are equally
observed in P3HT doped with F4 (black curve) where integer
charge transfer has been reported in numerous studies before.[9,
12, 37]
For doped P3HT, these peaks occur together with broad
transitions due to the positively charged polymer chain, which
are commonly referred to as polaronic transitions P1 and P2
(see labels in Figure 1).[39] In analogy, one would therefore
expect optical transitions that stem from the radical cation of 10
in our film as well. In the related spectrum we observe a
prominent feature at 1.04 eV and a weaker peak at 0.6 eV
(marked with vertical arrows in Figure 1, bottom). While the
between which this transition occurs can be modelled as: latter occurs at a position where one would indeed expect the
E1/2 = 0.5 (H + L) ± [(H – L)2 – 4β 2]0.5, where H, L are the
HOMO and LUMO energies of oligomer and dopant,
CPX peak for this film, the former is found at significantly
higher energy, which is clearly at odds with such an
interpretation. We note that in the DFT study on the CPX of
T10 and F4 in Ref. [35] no such dramatic changes are expected
whatsoever. Another theoretical study on the optical properties
of charged oligothiophenes including T10 reported the two
lowest energy transitions of its radical cation T10●+ to be
around 0.6 eV and 1.3 eV;[39] an experimental study on non-
alkylated T9●+ and T12●+ reported transitions (in solution at
room temperature) at 0.67 eV/1.46 eV and 0.59 eV/1.42 eV,
respectively,[8c] which clearly does not agree with the position
in the film of 10. Notably, however, the authors further report
spectra recorded at reduced temperature where they find new
peaks arising at 0.94 eV/1.94 eV instead, which they attribute
to dimerization of the T12 cations. Therefore, while the strong
peak we observe at 1.04 eV in the doped film of 10 is highly
unlikely to be due to the neutral CPX, it might well be due to
oligomer radical cations packed in the film.
It has been found that the charged state of F4 can be readily
assessed by characteristic frequency shifts (Δν) of its cyano
vibrational modes. For the present case, this allows estimating
the degree of charge transfer (δ) between the oligomer and the
dopant via: δ = 2Δν/ν0 [1-(ν1/ν0)2]-1, where ν0 and ν1 are the
frequencies in neutral F4 and in its radical anion,
respectively.[40] To this end, we performed Fourier-transform
infrared spectroscopy (FTIR) on our doped oligomer films, the
results are shown in Figure 2 together with data of neutral F4
and F4-doped P3HT as references. For 4b we find a shift by
6 cm-1 from the neutral position (2226 cm-1), which is identical
to the softening of this mode reported for T4;[12] this then
translates into δ = 0.2. For increasing oligomer length, δ
increases as well because the ionization potential of the
oligomer is reduced with its conjugation length.[35, 41] However,
for the most interesting case of the longest oligomer 10 where
we found spectral evidence for the presence of dopant radical
Figure 2. FTIR data in the region of the C≡N vibrations of 4b, 8, 9, and 10
films doped with F4 (1:10 ratio); δ denotes the degree of charge transfer to the
dopant, the vertical line gives the position of δ = 1 as found for P3HT; the
reference data for P3HT+F4 has been taken from Ref. [31].
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anions in UV/vis/NIR (cf. Figure 1), we now observe two peaks
at 2206 cm-1 and 2194 cm-1 in FTIR; the former occurs at the
same position as in 9 (δ = 0.6), and the latter at the same
position as in the P3HT reference (δ = 1.0). Note that for P3HT
the neighbouring mode at 2187 cm-1 has been assigned to F4
embedded into the alkyl-chain region of the polymer, while that
at 2194 cm-1 is due to F4 stacked with the polymer
backbone.[31] As the former is absent in our doped film of 10,
we therefore conclude that all dopant radical anions are closely
attached to the conjugated core of the oligomer.
Experimental Section
General procedure for decarboxylative cross-coupling: dibromo
(hetero)aryl (0.15 mmol), potassium (alkyl)thiophene carboxylate (0.33
mmol), PdCl2 (0.0075 mmol), and Tris(o-tolyl)phosphine (0.015 mmol)
were added to a 2.0 mL conical microwave vial equipped with a spin-vein.
Anhydrous DMA (1.5 mL) were added to the mixture, and the vial was
sealed. The mixture was heated in a microwave at 190 °C for 8 min. The
reaction was mixture was cooled to 22 °C, and the crude product was
diluted with EtOAc or chloroform (20 mL) and saturated NaCl (25mL), the
two phases was separated, and the aqueous phase was extracted with
EtOAc or chloroform (20ml) The combined organic phases were washed
with saturated NaCl (3 x 40ml), distill water(1 x 40ml), saturated NaHCO3
(1 x 40mL), and saturated NaCl (3 x 40ml). The organic phases were
dried over Na2SO4 and after filtration the solvent was evaporated under
reduced pressure. The crude mixture was purified by column
chromatography to yield the desired product.
The most remarkable finding from FTIR is the fact that in the
film of 10 dopants appear to be in two differently charged
states, one partially charged, as exclusively found for all shorter
oligomers (4b, 8, 9), and one state of δ = 1.0 as in P3HT. On
the basis of these data we tend to assign the absorption peak
found at 0.6 eV in UV/vis/NIR (blue curve in Figure 1, bottom)
to be due to the neutral CPX formed by 10 and F4, and that at
1.04 eV to some species carrying integer charge. Most likely,
this feature is due to the cation of 10 packed within the film,
however, alternative explanations for its nature like the
negatively charged CPX upon charge transfer with the organic
semiconductor host matrix cannot be ruled out, as its optical
signature has not yet been reported to date.[8b] Overall, our
present observations clearly indicate the critical length of
thiophene oligomers for integer charge transfer with the
common molecular dopant F4 to be not longer than ten
thiophene units. For even longer oligothiophenes, the doping
efficiency is therefore expected to approach that of P3HT while
retaining the advantages brought about by the well-defined
molecular length of oligomers in practical applications. Future
work will be dedicated to the synthesis of even longer discreet
thiophene oligomers following our synthesis approach and to
the electrical characterization of model devices based thereon in
order to fully understand the phenomenology of the transition
between the fundamentally different charge-transfer behavior of
thiophene oligomers and polythiophene.
Doping studies of linear oligothiophenes: 1 g/l F4TCNQ (TCI, used as
received) was dissolved in CF by stirring and heating at ~60°C for >12 h
and <48 h; 3 g/l of each oligothiophene species was dissolved in CF
directly prior to usage; doped solutions were also mixed directly prior to
usage. To facilitate structural comparison between doped systems with
differently large oligothiophenes, molar ratios given refer to the total
number of thiophene rings per dopant molecule. 20 µl of the respective
solution was then drop-casted onto 1 cm x 1 cm quartz and Si (undoped,
native oxide layer) for UV/vis/NIR and FTIR, respectively. No more than
10 min passed between deposition and measurement, all steps including
dissolution and spectral characterization (both UV/vis/NIR and FTIR)
have been performed under inert nitrogen atmosphere (from a N2
glovebox). UV/vis/NIR has been performed using an Agilent
Technologies Cary 5000 Spectrophotometer, range 2500-320 nm,
averaging time 0.033 s, data interval 1 nm, no source changeover,
detector and grating changeover at 800 nm. FTIR has been done on a
Thermo Electron Nicolet 6700 FT-IR Spectrometer, liquid N2-cooled
MCT/A detector, KBr beam splitter, range 4000-650 cm⁻¹, 256 scans,
0.964 cm⁻¹ data spacing.
Acknowledgements
Conclusions
This work was funded by the Natural Sciences and Engineering
Research Council (NSERC) and Le Fonds de Recherche du
Québec, Nature et Technologies (FRQNT). Support was also
kindly provided by the Centre for Green Chemistry and Catalysis
(CGCC), and Concordia University. Special thanks to Franklin
Chacón-Huete, Fadil Taç, and Dr. Mohammad Askari for their
valuable advice and to Dr. Georg Heimel for his valued
contributions in the conceptualization of the doping experiments.
In summary, we have developed a new method for the iterative
synthesis of symmetrical oligothiophenes. This methodology
has higher efficiency with improved E-factor and higher atom
economy compared to related routes. The scope of this method
has been extended to a library of thiophene based materials. We
further demonstrated a modular route to construct a discrete
library of oligothiophenes with various chain lengths. Based
thereon, we have synthesized linear oligothiophenes up to a
length of ten thiophene units and investigated their response to
molecular electrical doping using a prototypical dopant. From
the transition between fractional and integer charge transfer as
observed for the longest oligomer (10) we identify a critical
chain length of ten repeat units to be necessary for optimizing
the doping efficiency of oligothiophene. Overall, our work
highlights the utility of decarboxylative cross-coupling
methodologies and its applications in the ongoing development
of organic electronic materials and underlines the key role the
chain length of symmetrical discreet oligothiophenes plays for
organic electronic applications.
Keywords: Oligothiophene • Decarboxylative Cross-coupling •
Organic Semiconductors • Doping • Charge Transfer •
[1]
[2]
Perepichka, I.; Perepichka, D. Handbook of thiophene based materials:
Applications in organic electronics and photonics, Vol. 1, John Wiley &
Sons, Ltd: New York, 2009.
Zhang, L.; Colella, N. S.; Cherniawski, B. P.; Mannsfeld, S. C. B.;
Briseno, A. L. ACS Appl. Mater. Interfaces 2014, 6, 5327–5343.
Mishra, A.; Ma, C.-Q.; Bäuerle, P. Chem. Rev. 2009, 109, 1141–1276
Mazzio, K. A.; Luscombe, C. K. Chem. Soc. Rev. 2015, 44, 78–90.
a) Facchetti, A. Mater. Today. 2007, 10, 28–37. b) Larik, F. A.; Faisal,
M.; Saeed, A.; Abbas, Q.; Kazi, M. A.; Abbas, N.; Thebo, A. A.; Khan, D.
[3]
[4]
[5]
This article is protected by copyright. All rights reserved.

10.1002/anie.201914458
Angewandte Chemie International Edition
RESEARCH ARTICLE
M.; Channar, P. A. J. Mater. Sci. Mater. Electron. 2018, 29, 17975–
Spitz, C.; Lefèvre, J.; Dupas, G.; Fruit, C.; Hoarau, C. Chem. – A Eur. J.
2014, 20, 3610–3615. e) Chennamaneni, L. R.; William, A. D.;
Johannes, C. W. Tetrahedron Lett. 2015, 56, 1293–1296. f) Wang, S.;
Lu, H.; Li, J.; Zou, D.; Wu, Y.; Wu, Y. Tetrahedron Lett. 2017, 58, 2723–
2726. g) Hackenberger, D.; Weber, P.; Blakemore, D. C.; Goossen, L. J.
J. Org. Chem. 2017, 82, 3917–3925.
18010.
[6]
a) Leroy, J.; Boucher, N.; Sergeyev, S.; Sferrazza, M.; Geerts, Y. H.
European J. Org. Chem. 2007, 2007, 1256–1261. b) Cheng, X.; Dong,
X.; Wei, G.; Prehm, M.; Tschierske, C. Angew. Chemie Int. Ed. 2009,
48, 8014–8017.
[7]
[8]
a) Wong, B. M.; Piacenza, M.; Sala, F. Della. Phys. Chem. Chem. Phys.
2009, 11, 4498–4508. b) Fin, A.; Vargas Jentzsch, A.; Sakai, N.; Matile,
S. Angew. Chemie Int. Ed. 2012, 51, 12736–12739.
[22] a) Forgione, P.; Brochu, M.-C.; St-Onge, M.; Thesen, K. H.; Bailey, M.
D.; Bilodeau, F. J. Am. Chem. Soc. 2006, 128, 11350–11351. b)
Bilodeau, F.; Brochu, M.-C.; Guimond, N.; Thesen, K. H.; Forgione, P.
J. Org. Chem. 2010, 75, 1550–1560. c) Daley, R. A.; Liu, E.-C.;
Topczewski, J. J. Org. Lett. 2019, 21, 4734–4738.
a) B. Lüssem, C. M. Keum, D. Kasemann, B. Naab, Z. Bao, K. Leo,
Chem Rev 2016, 116, 13714-13751; b) I. Salzmann, G. Heimel, M.
Oehzelt, S. Winkler, N. Koch, Acc. Chem. Res. 2016, 49, 370-378; c) B.
Lüssem, M. Riede, K. Leo, physica status solidi (a) 2013, 210, 9-43. d)
Jacobs, I. E.; Moulé, A. J. Adv. Mater. 2017, 29, 1703063.
[23] a) Mitchell, D.; Coppert, D. M.; Moynihan, H. A.; Lorenz, K. T.; Kissane,
M.; McNamara, O. A.; Maguire, A. R. Org. Process Res. Dev. 2011, 15,
981–985. b) Hassanpour, A.; De Carufel, C. A.; Bourgault, S.; Forgione,
P. Chem. – A Eur. J. 2014, 20, 2522–2528. b) Chen, F.; Wong, N. W.
Y.; Forgione, P. Adv. Synth. Catal. 2014, 356, 1725–1730. c) Pandey,
G.; Bhowmik, S.; Batra, S. Org. Lett. 2013, 15, 5044–5047. d) Castro,
M. C. R.; Raposo, M. M. M. Tetrahedron 2016, 72, 1881–1887. e)
Chacón-Huete, F.; Mangel, D.; Ali, M.; Sudano, A.; Forgione, P. ACS
Sustain. Chem. Eng. 2017, 5, 7071–7076.
[9]
a) P. Pingel, D. Neher, Phys. Rev. B 2013, 87, 115209. b) Kiefer, D.;
Kroon, R.; Hofmann, A. I.; Sun, H.; Liu, X.; Giovannitti, A.; Stegerer, D.;
Cano, A.; Hynynen, J.; Yu, L.; Zhang, Y.; Nai, D.; Harrelson, T. F.;
Sommer, M.; Moulé, A. J.; Kemerink, M.; Marder, S. R.; McCulloch, I.;
Fahlman, M.; Fabiano, S.; Müller, C. Nat. Mater. 2019, 18, 149–155.
[10] a) D. T. Duong, C. Wang, E. Antono, M. F. Toney, A. Salleo, Org.
Electron. 2013, 14, 1330-1336; b) K. H. Yim, G. L. Whiting, C. E.
Murphy, J. J. M. Halls, J. H. Burroughes, R. H. Friend, J. S. Kim, Adv.
Mater. 2008, 20, 3319-3324.
[24] a) Arroyave, F. A.; Reynolds, J. R. Org. Lett. 2010, 12, 1328–1331. b)
Arroyave, F. A.; Reynolds, J. R. J. Org. Chem. 2011, 76, 8621–8628.
[25] a) Michael, R, D.; Hays, D. S. Heterocycles 1995, 40, 925–937. b) Cao,
X.; Xu, Y.; Cao, Y.; Wang, R.; Zhou, R.; Chu, W.; Yang, Y. Eur. J. Med.
Chem. 2015, 102, 471–476.
[11] a) Bäuerle, P.; Fischer, T.; Bidlingmeier, B.; Rabe, J. P.; Stabel, A.
Angew. Chemie Int. Ed. 1995, 34, 303–307. b) Tao, T.; Qian, H. F.;
Zhang, K.; Geng, J.; Huang, W. Tetrahedron 2013, 69, 7290–7299. c)
Liang, L.; Chen, X.-Q.; Xiang, X.; Ling, J.; Shao, W.; Lu, Z.; Li, J.; Wang,
W.; Li, W.-S. Org. Electron. 2017, 42, 93–101. d) Manninen, V. M.;
Heiskanen, J. P.; Kaunisto, K. M.; Hormi, O. E. O.; Lemmetyinen, H. J.
RSC Adv. 2014, 4, 8846–8855.
[26] a) Jones, A. L.; Gish, M. K.; Zeman, C. J.; Papanikolas, J. M.; Schanze,
K. S. J. Phys. Chem. A 2017, 121, 9579–9588. b) Arsenyan, P.; Paegle,
E.; Belyakov, S. Tetrahedron Lett. 2010, 51, 205–208.
[27] Koch, F. P. V; Smith, P.; Heeney, M. J. Am. Chem. Soc. 2013, 135,
13695–13698.
[12] H. Méndez, G. Heimel, S. Winkler, J. Frisch, A. Opitz, K. Sauer, B.
Wegner, M. Oehzelt, C. Rothel, S. Duhm, D. Többens, N. Koch, I.
Salzmann, Nature Commun. 2015, 6, 8560.
[28] a) Trost, B. M. Angew. Chemie Int. Ed. 1995, 34, 259–281. b)
Constable, D. J. C.; Curzons, A. D.; Cunningham, V. L. Green Chem.
2002, 4, 521–527.
[13] I. Salzmann, G. Heimel, S. Duhm, M. Oehzelt, P. Pingel, B. M. George,
A. Schnegg, K. Lips, R. P. Blum, A. Vollmer, N. Koch, Phys. Rev. Lett.
2012, 108, 035502.
[29] Lim, H. C.; Kim, J.-J.; Jang, J.; Hong, J.-I. New J. Chem. 2018, 42,
11458–11464.
[30] Grand, C.; Zajaczkowski, W.; Deb, N.; Lo, C. K.; Hernandez, J. L.;
Bucknall, D. G.; Müllen, K.; Pisula, W.; Reynolds, J. R. ACS Appl.
Mater. Interfaces 2017, 9, 13357–13368.
[14] Johansson Seechurn, C. C. C.; Kitching, M. O.; Colacot, T. J.; Snieckus,
V. Angew. Chemie Int. Ed. 2012, 51, 5062–5085.
[15] a) Bannock, J. H.; Treat, N. D.; Chabinyc, M.; Stingelin, N.; Heeney, M.;
de Mello, J. C. Sci. Rep. 2016, 6, 23651. b) Estrada, L. A.; Deininger, J.
J.; Kamenov, G. D.; Reynolds, J. R. ACS Macro Lett. 2013, 2, 869–873.
[16] a) Leclerc, M.; Brassard, S.; Beaupré, S. Polym. J. 2019. b) Zhang, J.;
Kang, L. J.; Parker, T. C.; Blakey, S. B.; Luscombe, C. K.; Marder, S. R.
Molecules 2018, 23, 922. c) Bohra, H.; Wang, M. J. Mater. Chem. A
2017, 5, 11550–11571. d) Pouliot, J.-R.; Grenier, F.; Blaskovits, J. T.;
Beaupré, S.; Leclerc, M. Chem. Rev. 2016, 116, 14225–14274. e) Bura,
T.; Beaupré, S.; Légaré, M.-A.; Quinn, J.; Rochette, E.; Blaskovits, J.
T.; Fontaine, F.-G.; Pron, A.; Li, Y.; Leclerc, M. Chem. Sci. 2017, 8,
3913–3925. f) Bura, T.; Blaskovits, J. T.; Leclerc, M. J. Am. Chem. Soc.
2016, 138, 10056–10071.
[31] H. Hase, K. O’Neill, J. Frisch, A. Opitz, N. Koch, I. Salzmann, J. Phys.
Chem. C. 2018, 122, 25893-25899.
[32] a) S. S. Zade, M. Bendikov, Org. Lett. 2006, 8, 5243-5246; b) A.
Facchetti, M. H. Yoon, C. L. Stern, G. R. Hutchison, M. A. Ratner, T. J.
Marks, J. Am. Chem. Soc. 2004, 126, 13480-13501; c) J. A. E. H. van
Haare, E. E. Havinga, J. L. J. van Dongen, R. A. J. Janssen, J. Cornil,
J.-L. Brédas, Chem. – A Eur. J. 1998, 4, 1509-1522; d) W. R. Salaneck,
O. Inganas, J. O. Nilsson, J. E. Osterholm, B. Themans, J. L. Bredas,
Synth. Met. 1989, 28, C451-C460.
[33] H. Méndez, G. Heimel, A. Opitz, K. Sauer, P. Barkowski, M. Oehzelt, J.
Soeda, T. Okamoto, J. Takeya, J.-B. Arlin, J.-Y. Balandier, Y. Geerts, N.
Koch, I. Salzmann, Angew. Chem., Int. Ed. 2013, 52, 7751-7755.
[34] V. Coropceanu, J. Cornil, D. A. da Silva Filho, Y. Olivier, R. Silbey, J. L.
Brédas, Chem. Rev. 2007, 107, 926-952.
[17] a) Schipper, D. J.; Fagnou, K. Chem. Mater. 2011, 23, 1594–1600 b)
Wang, X.; Wang, K.; Wang, M. Polym. Chem. 2015, 6, 1846–1855.
[18] Blaskovits, J. T.; Johnson, P. A.; Leclerc, M. Macromolecules 2018, 51,
8100–8113
[35] A. M. Valencia, C. Cocchi, J. Phys. Chem. C 2019, 123, 9617-9623.
[36] D. A. Dixon, J. C. Calabrese, J. S. Miller, J. Phys. Chem. 1989, 93,
2284-2291.
[19] Rodríguez, N.; Gooßen, L. J. Chem. Soc. Rev. 2011, 40, 5030–5048.
b) Perry, G. J. P.; Larrosa, I. European J. Org. Chem. 2017, 2017,
3517–3527.
[37] a) F. Ghani, A. Opitz, P. Pingel, G. Heimel, I. Salzmann, J. Frisch, D.
Neher, A. Tsami, U. Scherf, N. Koch, Journal of Polymer Science Part
B-Polymer Physics 2015, 53, 58-63; b) P. Pingel, L. Y. Zhu, K. S. Park,
J. O. Vogel, S. Janietz, E. G. Kim, J. P. Rabe, J. L. Bredas, N. Koch, J.
Phys. Chem. Lett. 2010, 1, 2037-2041.
[20] a) Gooßen, L. J.; Deng, G.; Levy, L. M. Science. 2006, 313, 662–664.
b) Becht, J. M.; Le Drian, C. Org. Lett. 2008, 10, 3161–3164. c) Shang,
R.; Xu, Q.; Jiang, Y.-Y.; Wang, Y.; Liu, L. Org. Lett. 2010, 12, 1000–
1003. d) Tang, J.; Biafora, A.; Goossen, L. J. Angew. Chemie Int. Ed.
2015, 54, 13130–13133.
[38] G. Heimel, ACS Cent. Sc. 2016, 2, 309-315.
[39] U. Salzner, J. Chem. Theory Comput. 2007, 3, 1143-1157.
[40] a) J. S. Chappell, A. N. Bloch, W. A. Bryden, M. Maxfield, T. O. Poehler,
D. O. Cowan, J. Am. Chem. Soc. 1981, 103, 2442-2443; b) M.
Meneghetti, C. Pecile, J. Chem. Phys. 1986, 84, 4149-4162; c) E.
Kampar, O. Neilands, Russ. Chem. Rev. 1986, 55, 334-342.
[21] a) Messaoudi, S.; Brion, J.-D.; Alami, M. Org. Lett. 2012, 14, 1496–
1499. b) Li, X.; Zou, D.; Leng, F.; Sun, C.; Li, J.; Wu, Y.; Wu, Y. Chem.
Commun. 2013, 49, 312–314. c) Haley, C. K.; Gilmore, C. D.; Stoltz, B.
M. Tetrahedron 2013, 69, 5732–5736. d) Rouchet, J.-B.; Schneider, C.;
This article is protected by copyright. All rights reserved.

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[41] S. S. Zade, M. Bendikov, Chem. – A Eur. J. 2008, 14, 6734-6741.
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Entry for the Table of Contents (Please choose one layout)
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RESEARCH ARTICLE
Symmetrical linear oligothiophenes
are important model organic
semiconductors. A library of such
oligothiophenes has been
Jiang Tian Liu, Hannes Hase, Sarah
Taylor, Ingo Salzmann*, and Pat
Forgione*
synthesized through an efficient
decarboxylative cross-coupling
route. These compounds enable us
to explore the critical oligomer
length for integer-charge transfer
upon molecular doping thereby
connecting the fundamentally
different doping phenomenologies
of oligo- and polythiophenes.
Page No. – Page No.
Approaching the Integer-Charge
Transfer Regime in Molecularly
Doped Oligothiophenes by Efficient
Decarboxylative Cross-Coupling
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RESEARCH ARTICLE
Author(s), Corresponding Author(s)*
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Title
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