Novel electron-deficient oligo(phenyleneethynylene) derivatives for molecular electronics

Article abstract of DOI:10.1080/00397911.2018.1434205

This work reports synthesis and characterizations of two new electron-poor “oligo(phenyleneethynylene) (OPE) type” molecular wires for fundamental studies of electron transport in molecular junctions. These OPE derivatives display three aromatic rings fun

Full text of DOI:10.1080/00397911.2018.1434205

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
Novel electron-deficient
oligo(phenyleneethynylene) derivatives for
molecular electronics
Rachid Kitouni, Pramila Selvanathan, Olivier Galangau, Lucie Norel, Brahmia
Ouarda & Stéphane Rigaut
To cite this article: Rachid Kitouni, Pramila Selvanathan, Olivier Galangau, Lucie Norel, Brahmia
Ouarda & Stéphane Rigaut (2018): Novel electron-deficient oligo(phenyleneethynylene) derivatives
for molecular electronics, Synthetic Communications, DOI: 10.1080/00397911.2018.1434205
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SYNTHETIC COMMUNICATIONS®
https://doi.org/10.1080/00397911.2018.1434205
Novel electron-deficient oligo(phenyleneethynylene)
derivatives for molecular electronics
Rachid Kitounia,b, Pramila Selvanathana, Olivier Galangaua, Lucie Norela,
Brahmia Ouardab, and Stéphane Rigauta
b
aUniv Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes), Rennes, France; Département de
Chimie, Faculté des Sciences Exactes, Laboratoire des Techniques Innovantes de Préservation de
l’Environnement, Université des Frères Mentouri, Constantine, Algeria
ABSTRACT
ARTICLE HISTORY
Received 5 January 2018
This work reports synthesis and characterizations of two new electron-
poor “oligo(phenyleneethynylene) (OPE) type” molecular wires for
fundamental studies of electron transport in molecular junctions. These
OPE derivatives display three aromatic rings functionalized (i) with NO2
(OPN) or fluorine (OPF) groups on the central aryl core and (ii) with the
requisite protected thiolate anchoring groups on the lateral rings at both
ends. We show that the moderately effective Sonogashira couplings can
give access to such rare electrodeficient molecules but are unfortunately
associated with significant side reactions. We detail the choice of
adequate reaction conditions to allow the recovery of suitable amounts
of compounds bearing several strongly electron-withdrawing substitu-
ents on their central ring for further study of the physical properties.
KEYWORDS
Dinitrobenzene; molecular
electronics; oligo
(phenylethylene);
Sonogashira coupling
tetrafluorobenzene
GRAPHICAL ABSTRACT
Introduction
During the past decades, oligo(phenyleneethynylene) (OPE) derivatives have emerged as
benchmark molecules for studying charge transport in molecular junctions.[1–3] Indeed,
following the “bottom-up” approach expressed by Feynman proposing that atoms can be
arranged in a manner to build circuits of a few thousands of Angströms across,[4] single
molecules as active elements have been considered as potential building blocks for future
nanoelectronic systems because of their advantages on cost, scalability, component density,
and power consumption.[1,5,6] In that context, the attractiveness of OPEs lies in their rigid,
rod-like structure with extended π-conjugation through the backbone leading to high con-
ductivity and other electrical properties such as negative differential resistance or molecular
CONTACT Stéphane Rigaut
stephane.rigaut@univ-rennes1.fr
Univ Rennes, CNRS, ISCR (Institut des Sciences
Chimiques de Rennes), UMR 6226, F-35000 Rennes, France.
Supplemental data for this article can be accessed on the publisher’s website.
© 2018 Taylor & Francis

2
R. KITOUNI ET AL.
switching that can be systematically tuned by chemical design. Such a tuning is particularly
important to achieve controllable properties by choosing the appropriate chemical
functions, which allow infinite possibilities of developments toward the preparation of
molecular devices with efficient performances. In our group, we participated to such
studies by probing the electrical conductivity of metal-containing OPE derivatives and
by establishing that insertion of ruthenium atoms in the backbone provides interestingly
higher conductance over pure organic OPEs.[7]
Basically, a molecular wire consists in a molecular chain that provides strong electrode
coupling between two electrodes attached to its chain ends to promote electron transfer
through this bridge. The exploration of charge transport properties has been conducted
with a wide range of molecules using various techniques (mechanical break junctions,
scanning probe microscopies, nanogaps …) to understand the structure/property
relationship.[8] Various factors are known to affect the electrical properties of a typical
molecular junction such as the alignment between the Fermi level of the electrodes and
the frontier orbitals of the molecule. Recently, many efforts have been devoted to gain a
better understanding by rationalizing the impact of bridging molecule structures on the
resulting electronics and current–voltage characteristics of the molecular junctions by
revisiting simple molecular wires.[9,10] In that respect, εh, the key offset of the appropriate
molecular orbital energy relative to the electrode Fermi level used to simulate the I–V
curves can be tuned by changing (i) the contact terminal groups, (ii) the electrodes work
functions, and (iii) the electronic structure of the molecules.[9]
In this context and following the chemical potential equalization principle allowing to
establish the existence or not of a so-called Fermi level pinning phenomenon (i.e., same
alignment of relevant molecular orbitals in the junction in spite of a varying ionization
potentials in isolated compounds), the precise investigation of organic OPE derivatives
is particularly attractive.[9–11] Indeed, a minimum number of synthetic steps will give access
to a wide range of molecules likely to display different ionization potentials, to show that εh
for the molecule inserted between electrodes can indeed differ largely from the one of the
same molecules in its isolated state. This can be reached by varying the donor and/or
acceptor strengths on phenyl rings on the backbone. However, surprisingly, OPE
derivatives bearing strongly electron-withdrawing substituents and protected thiolate
anchoring groups are extremely rare,[12] owing to the modest reactivity of the involved
alkyne reagents and highly deleterious side reactions. Therefore, in the course of our
current work aiming at (re-)investigating the electrical conductivity of several OPEs with
three aryl groups and different HOMO levels,[13] we report the synthesis of two original
highly electron-poor OPE molecules incorporating NO2 groups or fluorine atoms on
the aryl central core and bearing protected thiolate anchoring groups at both ends of
the molecule.
Results and discussion
The two targeted compounds are derivative of the ubiquitous OPE compound constituted
of three aryl group. The central aromatic ring is substituted with four fluorine atoms or two
nitro groups in the ortho and para positions (Chart 1).
OPE was previously obtained by Tour et al.[14] from 1,4-diodobenzene by Sonogashira
coupling with S-(4-ethynylphenyl)ethanethioate. By adjusting the nature of this central

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3
Chart 1. Target compounds.
core, i.e., with 1,4-diiodo-tetrafluorobenzene in this case, this route presents the advantage
of potentially afford several OPE analogues with a different central core in one step only.
The modest ability of 1,4-diiodo-tetrafluorobenzene to undergo Sonogashira coupling with
electron-rich terminal alkynes was already highlighted in similar coupling involving
p-OMe- or p-NMe2-substituted phenyl acetylenes, and affording the corresponding OPEs
in ca. 41% yield.[15] To complement these results, the monocoupling compound obtained
from 1,4-diiodo-tetrafluorobenzene and S-(4-ethynylphenyl)ethanethioate was recently
reported in moderate yield (25% yield) again confirming this tendency.[16] Herein, the
preparation of OPF was similarly attempted from 1,4-diiodo-tetrafluorobenzene in the
presence of a twofold excess of S-(4-ethynylphenyl)ethanethioate[17] (2.2 Eq.). The reaction
was performed in degazed Et3N, at room temperature, in the presence of PdCl2(PPh3)2
catalyst (4 mol%) and CuI (20 mol%) (Scheme 1) for 2 days. Rerun of the reaction in
the crude was required with a second addition of equivalent amount of catalyst and CuI
after 24 h. TLC and NMR monitoring of the reaction clearly indicated the full consumption
of S-(4-ethynylphenyl)ethanethioate, whereas some diiodo-tetrafluorobenzenes were
recovered. They also revealed that at least two other products are present, the monosubsti-
tuted intermediate and the homocoupling derivative, respectively. The 19F NMR spectrum
of the crude shows a set of characteristic signals clearly indicating the presence of starting
aryl-iodide, monosubstituted compound, and OPF in a 2/3/3 ratio, respectively. Although
the first coupling takes place in a reasonable amount, the second one appears less effective
and the homocoupling process takes place at the expense of desired cross-coupling
reactions that seem kinetically disfavored, a fact we often observed with the electron-rich
S-(4-ethynylphenyl)ethanethiolate.
Indeed, with electron-rich aryl acetylenes, coupling reactions are slower than those with
precursors bearing electron-withdrawing substituents which are known to undergo faster
coupling reaction rate than oxidation of Pd0 by the surrounding medium.[18] Thus, since
the reaction setup is not completely dioxygen free, homocoupling might be highly pro-
moted with this electron-rich aryl acetylenes. In addition, those alkynes are also
Scheme 1. Synthesis path yielding OPF.

4
R. KITOUNI ET AL.
hypothesized to induce gentle formation of Pd-alkyne complexes that could further favor
homocoupling in slow reactions.[19] Finally, the deprotonation of the alkyne can also be
critical to slow down the overall process.[20]
Hence, after optimization, the targeted OPF was obtained in 27% isolated yield. The
moderate solubility of OPF in common organic solvents is worth noting, as it probably also
contributes to the poor isolated yield displayed in this study.
The structure of OPF was ascertained by NMR spectroscopy. The 19F NMR spectrum
shows a singlet at −136.7 ppm corresponding to the four equivalent fluorine atoms. The
1H NMR spectrum displays two characteristic deshielded doublets[16] at δ ¼ 7.63 ppm
and 7.45 ppm (3JHH ¼ 8.20 Hz) for the aryl moieties bearing the thioacetate groups that
resonate as a singlet at δ ¼ 2.44 ppm, all consistent with the OPF molecular structure.[16]
Finally, in the 13C NMR spectrum, the characteristic C‒F carbon atom resonance is
observed at 146.6 ppm (1JCF ¼ 250 Hz). The compound was also unambiguously identified
by high-resolution mass spectra (HRMS) techniques.
Sonogashira reactions with aryl nitro units are also known to be problematic, and
several side products are often observed.[21] Best conditions for these aromatic compounds
require moderate temperatures, short reaction times, and, in particular, the use of a base in
low concentration.[22] For example, substitution of nitro groups by a base has been
reported,[12] especially with two nitro groups on the same aromatic ring, and very recently
it has been shown that Pd0 catalyzes in fact the cleavage of Ar−NO2 bonds.[23] Also, the
proximity of the acetylene in ortho position of the nitro group assists the formation of
2-aryl-3H-indol-3-one N-oxides, a nitrogen heterostructure.[24]
At first glance, in the light of this facts, we planned to obtain OPN using the previous
method, i.e., from S-(4-ethynylphenyl)ethanethioate and 1,4-dibromo-2,3-dinitrobenzene
by trying different Pd catalysts (PdCl2(Ph3)2, Pd(Ph3)4, Pd2(dba)3) and different bases
(diisopropylethyl amine, triethylamine) in a mixture with THF. Unfortunately, the
1
formation of OPN was not observed from the thin-layer chromatography and from H
NMR spectroscopy. After workup, only few starting 1,4-dibromo-2,3-dinitrobenzene was
recovered along with the compound resulting from homocoupling of the terminal alkyne.
Unidentified products probably resulting from side reactions mentioned above were also
present. This evidences again the relatively low efficiency of the coupling, which is not
compatible with the low temperatures and short reaction time requirements for nitro
aromatics to avoid degradation products.
We then turned into the strategy consisting in preparing first the diethynyl central core
for a further coupling reaction with 1-iodo-4-thioacetyl benzene (Scheme 2). This strategy
Scheme 2. Synthesis path yielding OPN.

SYNTHETIC COMMUNICATIONS®
5
allowed Tour’s group to obtain the mono nitro analog[22] and Mayor’s group to achieve the
synthesis of macrocyclic molecular rods from macrocycles incorporating ortho-dinitro
phenyl units.[21] Compound 1 bearing two protected alkyne functions was then prepared
according to the Mayor’s procedure from 1,4-dibromo-2,3-dinitrobenzene by Sonogashira
coupling reaction with trimethylsilylacetylene, in the presence of Pd(PPh3)4/CuI and
diisopropylethylamine.[21] We noticed that the choice of diisopropylethylamine as the
base is crucial. With different amines such as diisopropylamine or triethylamine, severe
degradation of the central aromatic core is observed (vide supra). Thus, in these conditions,
product 1 was isolated in 73% yield and the TMS groups were readily removed using 2 Eq.
of potassium fluoride in a mixture of methanol/dichloromethane to afford 1,4-diethynyl-
2,3-dinitrobenzene (2) in 97% isolated yield.
Compound 2 was then further reacted with an excess of 1-iodo-4-thioacetyl benzene in
another Sonogashira cross-coupling reaction using a high loading of 50% Pd(PPh3)4
catalyst with respect to the aryl iodide and in the presence of a few amount of base. This
palladium loading was used to accelerate the reaction and promote the coupling reaction
over side reactions including homocoupling reaction and degradations of 2. Similar degra-
dations than those observed with the first strategy and involving nitro groups are however
observed but to a lesser extent here. Therefore, within 4 h, alkyne precursor 2 is consumed
and a large amount of unreacted 1-iodo-4-thioacetyl benzene is observed in the crude
1
along with the unidentified degradation products detected in the aromatic region of H
NMR spectrum. These reaction conditions represent the best compromise we could find,
and the target molecular wire OPN was obtained with a moderate 18% isolated yield
(see “Experimental” section). OPN was fully characterized by NMR spectroscopy. The
1H NMR spectrum shows a singlet at δ ¼ 2.45 ppm for the methyl groups and a singlet
at δ ¼ 7.77 ppm integrating for two hydrogen atoms corresponding to the two central
aromatic hydrogen atoms. The signals for the other aromatic protons are seen in OPN
as two separated slightly upfield doublets at δ ¼ 7.57 ppm (J ¼ 8.12 Hz) and 7.46 ppm
(J ¼ 8.16 Hz).
Preliminary studies on the optical properties of OPE and OPF show that the
electron-withdrawing character of fluoro and nitro groups clearly influences the energy
levels of the frontier orbitals with respect to OPE, as expected. Indeed, in the UV–vis
spectra (see Supplementary information), the observed shifts of the main electronic
transition of OPE from 333 to 340 and 348 nm in OPF and OPN, respectively, and their
shape modifications are characteristic of the lower energy of the transitions to the first
singlet excited states in OPF and OPN. A full rationalization of the physical and electrical
properties of these new electron-poor OPEs will be reported in due course.[13]
Conclusion
In this report, we present the synthesis and characterization of two new electron-poor
“OPE-type” molecular wires substituted with NO2 (OPN) and fluoro (OPF) groups on
the central aromatic core. The moderately efficient Sonogashira couplings giving
access to these molecules associated to various significant side reactions are unfortunately
detrimental to the reaction efficiency. Nevertheless, we found that an accurate choice of
the reaction conditions allows to perform the synthesis and to obtain such very rare
thiolate-terminated compounds bearing several strongly electron-withdrawing groups.

6
R. KITOUNI ET AL.
Importantly, even if the yields are modest, these reaction conditions allow the recovery of
an accurate amount of compounds for further physical studies to rationalize an operating
pinning of the relevant orbital of these molecules (HOMO) to the Fermi level of the
electrodes in molecular junctions.
Experimental part
General
All reactions were achieved under an inert atmosphere, using the Schlenk techniques. Sol-
vents were freshly distillated under argon using standard procedures. S-(4-ethynylphenyl)
ethanethioate) and S-(4-iodophenyl)ethanethioate were prepared as previously reported.[17]
NMR spectra were recorded on a Bruker AVANCE I 500 MHz spectrometer or a Bruker
AVANCE III 400 MHz spectrometer. IR spectra were recorded on a IFS 28 Brucker
spectrometer using KBr pellets or ATR. HRMS were recorded in Rennes at the Centre
Régional de Mesures Physiques de l’Ouest (CRMPO) on a Bruker MicrO-Tof-Q II
spectrometer.
Procedure for OPF
1,2,4,5-tetrafluoro-3,6-diiodobenzene (125 mg, 0.309 mmol, 1 Eq.), S-(4-ethynylphenyl)
ethanethioate) (120 mg, 0.682 mmol, 2.2 Eq.), PdCl2(PPh3)2 (9 mg, 4 mol%), CuI (12 mg,
20 mol%) were added in a Schlenk tube and were pumped 45 min under vacuum. Then,
distilled and thoroughly degassed Et3N (6 mL) was added to the Schlenk tube at 0 °C.
The suspension was then allowed to stir vigorously for 24 h at 50 °C. The reaction was
quenched by removing the solvent under vacuum and the residue was taken up in diethyl
ether and filtered over a pad of celite. The solvents were removed. The crude was pumped
under vacuum 45 min and the previous synthetic procedure was repeated again on the
crude with the same amount of catalysts: PdCl2(PPh3)2 (9 mg, 4 mol%), CuI (12 mg,
20 mol%). After 24 h stirring at 50 °C, the solvent was removed and the residue was taken
up and filtered with diethylether over a pad of celite. Purification by column chromato-
graphy with silica gel (petroleum ether/dichloromethane (8:2)) afforded several fractions.
The third one contained a mixture of OPF and the homocoupling adduct. A second
purification on this fraction with silica gel (diethyl ether/pentane (4:96) to MeOH/diethyl
1
ether (1:10)) afforded 42 mg (27%) of OPF as a white solid. H NMR (400 MHz, ppm,
3
3
CDCl3): δ ¼ 2.44 (s, 6H, CH3), 7.45 (d, JHH ¼ 8.2 Hz, 4H, C6H4), 7.63 (d, JHH ¼ 8.2 Hz,
4H, C6H4). 19F NMR (376.5 MHz, ppm, CDCl3): δ ¼ −136.78. 13C NMR (100 MHz, ppm,
1
=
CDCl3): δ ¼ 192.9 (C O), 146.6 (d, JCF ¼ 250 Hz, C‒F), 134.3 (o-C6H4SCOCH3), 132.5
(m-C6H4SCOCH3), 130.0 (C‒S), 122.6 (C‒C≡C), 102.2 (‒C≡C‒C6F4), 76.1 (‒C≡C‒C6F4),
30.4 (SCOCH3). UV-vis (CH2Cl2): λmax ¼ 339 nm. FTIR (cm−1, ATR) ¼ 2967, 2916, 2847
(υC‒H); 2226, 2207 (υC≡C); 1695 (υC O); 1483 (υC ¼ C). HRMS FABþ (m/z): 521.0264 (calcd:
=
521.02636, ([M þ Na]þ).
Procedure for OPN
1,4-diethynyl-2,3-dinitrobenzene (125 mg, 0.578 mmol), S-(4-iodophenyl)ethanethioate
(803.5 mg, 2.891 mmol), Pd(PPh3)4 (1.336 mg, 1.156 mmol), and CuI (11 mg, 0.057 mmol)
were added in a Schlenk tube and pumped under vacuum. Then, distilled diisopropylethy-
lamine (0.8 mL) and toluene (16 mL) were added to the reaction mixture. The reaction was
allowed to stir for 4 h at room temperature. The mixture was evaporated to dryness, the

SYNTHETIC COMMUNICATIONS®
7
residue was taken up with diethylether and the solution was filtered. After evaporation and
purification with silica gel (pentane/dichloromethane (1:1)), 55 mg (18%) of OPN was
recovered as a white solid.1H NMR (400 MHz, ppm, CDCl3): δ ¼ 2.45 (s, 6H, CH3), 7.45
3
3
(d, JHH ¼ 8.2 Hz, 4H, C6H4), 7.58 (d, JHH ¼ 8.2 Hz, 4H, C6H4), 7.80 (s, 2H, C6H2(NO2)2).
13
=
C NMR (400 MHz, ppm, CDCl3): δ ¼ 192.80 (C O), 135.05 (CH, C6H2), 134.30
(o-C6H4SCOCH3), 132.66 (m-C6H4SCOCH3), 130.60 (C‒S), 121.96 (Cq, C6H4), 118.30 (C‒
C≡C), 99.69 and 82.83 (C≡C), 30.39 (CH3). UV-vis (CH2Cl2): λmax ¼ 348 nm. FTIR (cm−1,
ATR) ¼ 2958, 2922, 2851 (υC‒H); 2212 (υC≡C); 1694 (υC O); 1547 (υC ¼ C). HRMS FABþ
=
(m/z): 539.0342 (calcd: 539.0345, ([M þ Na]þ).
Funding
This work was supported by the Université de Rennes 1, the CNRS, and the Agence Nationale de la
Recherche (RuOxLux—ANR-12-BS07-0010-01). RK thanks the Algerian Ministry of High Education
and Scientific Research for his Grant (Profas). We also thank Gilles Alcaraz for discussions.
ORCID
Stéphane Rigaut
http://orcid.org/0000-0001-7001-9039
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