Oligoaryl cruciform structures as model compounds for coordination-induced single-molecule switches

Article abstract of DOI:10.1002/ejoc.200901150

The synthesis of two new cruciform structures 3 and 4 comprising an oligo(phenylene-vinylene) (OPV) and a perpendicular oligoaryl bar - namely an oligophenylene (OP) (3) and a naphthyl-phenyl-naphthyl system (4) - is reported. The OPV rod consists of two terminal pyridine units, whereas the oligoaryl rod bears two terminal acetylsulfanyl groups as protected anchor groups. The OPV bar was assembled via a Homer-Wadsworth-Emmons reaction, which directly led to the desired E,E isomers. The perpendicular oligoaryl bars were assembled with a Suzuki reaction using the corresponding boronic acids, which were already fitted with an ethyl-trimethylsilane (ethyl-TMS) sulfanyl group. In a last step, the ethyl-TMS-protected sulfur atoms were transpro-tected to the thioacetyl units. The cruciform structures 3 and 4 are model compounds to investigate a coordinationinduced single-molecule switch exploiting the potentialdependant different bonding strengths of the anchor groups to gold. Metal-molecule-metal junctions were formed using a mechanically controllable break junction (MCBJ) setup. Current traces of molecular junctions were statistically analyzed. Further investigations of model compounds consisting only of the single bar confirm that individual molecules carrying the required function for the switching experiments were trapped between two electrodes and were mainly immobilized via thiol-gold anchor bonds.

Full text of DOI:10.1002/ejoc.200901150

FULL PAPER
DOI: 10.1002/ejoc.200901150
Oligoaryl Cruciform Structures as Model Compounds for
Coordination-Induced Single-Molecule Switches
Sergio Grunder,[a] Roman Huber,[b] Songmei Wu,[b] Christian Schönenberger,[b]
Michel Calame,*[b] and Marcel Mayor*[a,c]
Dedicated to Giorgio Lazzarini
Keywords: Molecular electronics / Conducting materials / Arenes / Cross-coupling / Rearrangement
The synthesis of two new cruciform structures 3 and 4 com- tected to the thioacetyl units. The cruciform structures 3 and
prising an oligo(phenylene-vinylene) (OPV) and a perpen-
dicular oligoaryl bar – namely an oligophenylene (OP) (3)
and a naphthyl–phenyl–naphthyl system (4) – is reported.
The OPV rod consists of two terminal pyridine units, whereas
the oligoaryl rod bears two terminal acetylsulfanyl groups as
protected anchor groups. The OPV bar was assembled via a
Horner–Wadsworth–Emmons reaction, which directly led to
the desired E,E isomers. The perpendicular oligoaryl bars
were assembled with a Suzuki reaction using the corre-
sponding boronic acids, which were already fitted with an
ethyl-trimethylsilane (ethyl-TMS) sulfanyl group. In a last
step, the ethyl-TMS-protected sulfur atoms were transpro-
4 are model compounds to investigate a coordination-
induced single-molecule switch exploiting the potential-
dependant different bonding strengths of the anchor groups
to gold. Metal–molecule–metal junctions were formed using
a mechanically controllable break junction (MCBJ) setup.
Current traces of molecular junctions were statistically ana-
lyzed. Further investigations of model compounds consisting
only of the single bar confirm that individual molecules car-
rying the required function for the switching experiments
were trapped between two electrodes and were mainly im-
mobilized via thiol–gold anchor bonds.
molecular films,[3–8] small assemblies[9] and even single
molecules providing an experimental base of “molecular
electronics”.[10–14] Electronic transport through single mole-
cules was studied with various scanning probe setups rang-
ing from designed molecular structures diluted in a matrix
of a self-assembled monolayer (SAM)[15,16] to transient
scanning probe break junctions.[17,18] Attempts to obtain
more symmetric pairs of electrodes were based on mechani-
cally controllable break junctions (MCBJ)[19–24] or electro-
migration techniques.[25–27] Numerous molecular structures
have been integrated on a single-molecule level like alkane-
dithiols[28–32] as the commercially available working horse
of physicists to more rigid structures like for instance oligo-
(phenylene-vinylenes) (OPVs),[33,34] oligo(phenylene-ethyn-
ylenes) (OPEs)[33,35–37] or oligophenylenes.[38] Rectification
as first electronic function was demonstrated with suitably
designed molecular structures in monomolecular films[3,4,39]
as well as on a single-molecule level.[40] The next complex
electronic function of a two terminal single-molecule junc-
tion is switching, requiring a bistable molecular structure.
Several triggers to alter between the molecules state were
considered such as light, electrochemistry or the applied
electrical field. Variation of the transport current due to
single-molecule photoisomerization was observed for di-
arylethene[41] and azobenzene systems.[7] Transient electro-
Introduction
The visionary concept of integrating designed molecular
structures in electronic circuits as minute functional units is
today called “molecular electronics” and roots in H. Kuhn’s
ideas of “molecular engineering” proposed in the 1960s.[1]
Soon later, Aviram and Ratner discussed potential rectifica-
tion emerging from a molecular structure based on a theo-
retical model.[2] While this pioneering suggestion moved
molecules, as smallest unit providing the required structural
variation, further into the focus of interest, the concept re-
mained on a theoretical level due to the missing tools re-
quired to realize single-molecule transport experiments.
Only within the last two decades the tremendous improve-
ment of physical setups enabled the integration of mono-
[a] Department of Chemistry, University of Basel,
St. Johanns-Ring 19, 4056 Basel, Switzerland
Fax: +41-61-267-1016
E-mail: marcel.mayor@unibas.ch
[b] Department of Physics, University of Basel,
Klingelbergstrasse 82, 4056 Basel, Switzerland
E-mail: michel.calame@unibas.ch
[c] Karlsruhe Institute of Technology, Institute for Nanotechnol-
ogy,
P. O. Box 3640, 76021 Karlsruhe, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.200901150.
Eur. J. Org. Chem. 2010, 833–845
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
833

S. Grunder, R. Huber, S. Wu, C. Schönenberger, M. Calame, M. Mayor
FULL PAPER
chemically triggered single-molecule junctions were re-
Cruciform molecules are not only interesting model com-
ported for immobilized redox active species like e.g. violog- pounds for molecular electronics.[54,61] Related structural
ens[42–46] and ferrocenes.[47,48] Attempts to integrate electro- motives were already investigated as electrooptically active
chemically swithable supramolecular single-molecule sys- chromophores in self-assembled thin films,[62,63] as chromo-
tems were so far limited to a rotaxane in a platinum-based phores with tuneable band gaps,[64] as metal ion[65–67] or
break junction.[49] Recently, even an example of a single- pH[68]sensors,[69] and as building blocks of coordination
molecule switch based on the applied electric field was re- polymers.[70]
ported to display a weak hysteresis.[50] Unfortunately, the
The proposed switching mechanism relies on the dif-
paper reports exclusively the phenomenon without a
suggestion of the switching mechanism.
ferent electronic transparencies of the bar subunits and the
interplay of their anchor groups with the electrochemical
potential. In our parent design (cruciform 1) the sulfur an-
chor group was in meta-position to disfavour electronic
communication through the sulfur immobilized bar.[71]
However, the transport currents through this subunit were
too little to enable its immobilization in a MCBJ in a liquid
environment. Moving the sulfur anchor groups into para-
positions (cruciform 2) considerably increases the electronic
transport through the immobilized molecule allowing to
observe its presence within the junction. However, the in-
creased transport through the sulfur-functionalized OPV
bar considerably reduces the potential window to observe
the coordination dependent switching of the pyridine-func-
tionalized bar. We thus investigated systematically the sin-
gle-molecule transport currents through the corresponding
sulfur and pyridine-functionalized OPE and OPV rod sub-
units in the MCBJ setup in a liquid environment without
electrochemical control.[33] The observed results are
sketched in Figure 2.
We have synthesized numerous model compounds to in-
vestigate hypothesized switching mechanisms in monomole-
cular films and on a single-molecule level ranging from op-
tically addressed azobenzene systems[8,51,52] over electro-
chemically triggered viologens[44,46] and rigid rod cruci-
forms[53,54] to macrocyclic structures.[55,56] Recently we pro-
posed to profit from the potential dependent coordination
of pyridines to gold electrodes[17,57–59] as switching mecha-
nism in an electrochemically controlled junction. A molecu-
lar cruciform consisting of two bars with different transport
features and different terminal anchor groups was sug-
gested.[53] As displayed in Figure 1, the lower conducting
bar with sulfur anchor groups covalently immobilizes the
molecule within the junction and the potential dependent
coordination of the high conducting bar should provide a
switching mechanism on a single-molecule level. While elec-
trochemical control has already been achieved for transient
single-molecule junctions,[17,44,46,60] the combination of
stable single-molecule junctions at room temperature with
electrochemical control allowing to switch an immobilized
molecule remains challenging.
Figure 2. Measured trend in the conductivity of the OPV and OPE
rods with terminal sulfur or pyridine anchor groups.
While the increased electron transport through the OPV
structures with respect to the corresponding OPE structures
was expected due to the superior overlap of the olefinic π-
system with the phenyl π-system compared to the acetylene
π-system, the improved conductivity of the sulfur immobi-
lized rods compared to the pyridine-functionalized counter-
parts was surprising.[28,57] Consequently, the additional
conductivity due to the coordination of the pyridine sub-
units of 2 can not be observed at a lower applied potential
between the two electrodes than the transport current aris-
ing from the para-sulfur-functionalized OPV bar, making
its detection very challenging.
Figure 1. Coordination-induced switching principle: the coordina-
tion of the better conducting terminally pyridine-functionalized bar
subunit to both electrodes is controlled by the electrochemical po-
tential applied with respect to a reference electrode. Upon coordi-
nation (ON-state) the potential between both electrodes triggers
electron transport through the bar subunit.
834
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 833–845

Models for Coordination-Induced Single-Molecule Switches
To enable the suggested switching mechanism we cur- natively, a perpendicular oligophenylene (OP) type struc-
rently follow two strategies. While from the side of the ture was considered. In particular OP structures comprising
transport experiment we are working on increasing the sen- sterically repelling substituents ortho to the interring bridge
sitivity to enable the controlled immobilization of low con- position displayed rather limited transport features in a
ducting rods like the meta-sulfur-functionalized OPV sub- UHV MCBJ setup.[38] In the cruciform structures 3 and 4
unit of the cruciform 1, the chemical design and synthesis the second OPV bar is placed in ortho-position with respect
side works on favouring the expected transport features of to the OP backbone and thus, a considerable increase in
the pyridine rod with respect to the sulfur-functionalized the interring torsion angle should result in a concomitant
subunit. Along these lines we would like to report here the decrease in its transport properties. In the case of the cruci-
synthesis, the structural analysis and first transport investi- form 3, the OP rod subunit is with about 1.5 nm between
gations of the new cruciform structures 3 and 4 (Scheme 1) both terminal sulfur atoms slightly shorter than the pyr-
consisting of terminally pyridine-functionalized OPV rod idine-functionalized OPV bar, which has about 1.6 nm be-
subunits and perpendicular sulfur-functionalized oligo- tween both terminal pyridine nitrogen atoms. To enlarge
phenylene (OP) type bar substructures.
the intramolecular sulfur–sulfur distance of the OP-type
bar both terminal phenyl units of 3 have been replaced by
naphthyl units in 4, expanding the length of the OP back-
bone to about 1.9 nm.
Synthetic Strategy
The synthetic strategy is to build up first the OPV system
by a Horner–Wadsworth–Emmons (HWE) reaction. The
perpendicular OP system will be assembled by a Suzuki re-
action with the corresponding boronic acid derivatives. As
the terminal sulfur anchor groups are already present dur-
ing the aryl coupling reaction, ethyl-TMS-protected sulfur
precursors are considered. The ethyl-TMS protection group
was already reported to maintain the basic conditions of a
Sonogashira reaction[72] and can be transprotected to the
desired acetyl protection group under rather mild reaction
conditions. Thus, an ethyl-TMS-protected thiophenol
seemed ideally suited to survive Suzuki cross coupling reac-
tion conditions as well.
Synthesis
The functional OPV building block 5 was synthesized
following a reported procedure.[73] With its two iodine
atoms as excellent leaving groups in palladium catalyzed
coupling reactions, compound 5 is an ideal candidate for
a Suzuki reaction to assemble the OP system. Thus, the
corresponding sulfur bearing boronic acid is required. The
corresponding boronic acid 8 was synthesized starting from
4-bromothiophenol (6) (Scheme 2). The thiol 6 reacts in a
radical addition reaction with vinyltrimethylsilane, using
Scheme 1. First generation of OPV/OPE cruciforms 1 and 2 and
the new OP/OPV cruciforms 3 and 4.
Results and Discussion
Molecular Design
To favour the transport current through the pyridine- tert-butyl peroxide as radical initiator, to the ethyl-TMS-
functionalized rod it consists of an OPV-type backbone in protected thiol 12 in 67% yield.[72] Subsequently, the bo-
3 and 4. A considerable spatial mismatch was expected for ronic acid was introduced by treating compound 7 with a
a terminally sulfur-functionalized OPE rod. Furthermore, 1.6  solution of n-butyllithium (nBuLi) in hexane to per-
the sulfur anchor groups should again be in meta-position form a bromine–lithium exchange.[72] The lithiated interme-
as the corresponding para-sulfur-functionalized OPE trimer diate is quenched by triisopropyl borate to provide boronic
displayed increased conductivity compared to the pyridine- acid 8 as a colourless solid after aqueous work up in 59%
functionalized OPV-type rod (see Figure 2). While we were yield.
able to measure electronic transport through individual
Having all building blocks for the assembly of the frame-
molecules consisting of a comparable structural motive in work for the new cruciform structure in hand, its synthesis
ultra high vacuum (UHV),[71] the structure would again be was investigated. Diiodo derivative 5 and boronic acid 8
clearly under our current detection limit in solution. Alter- were dissolved in a toluene/ethanol mixture to which cata-
Eur. J. Org. Chem. 2010, 833–845
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
835

S. Grunder, R. Huber, S. Wu, C. Schönenberger, M. Calame, M. Mayor
FULL PAPER
Scheme 2. Synthesis of target structure 3.
lytic amounts of tetrakis(triphenylphosphane)palladium ated pyridine units are fast and efficiently cleaved by their
and an excess (3.6 equiv.) of potassium carbonate (K2CO3) treatment in chloroform (CHCl3) with ethanol (EtOH).[74]
as base was added. After stirring for 7.5 h at 80 °C followed Thus, the ethyl-TMS-protected compound 9 was dissolved
by aqueous work up the crude was purified by column in tetrahydrofuran (THF) to which a TBAF solution was
chromatography (CC) to provide the cruciform structure 9 added. After stirring for 1 h at room temperature (room
as a yellow solid in 71% yield. The final step is the temp.), an excess of AcCl was added at 0 °C. The reaction
transprotection from the ethyl-TMS masked to acetyl-pro- mixture was stirred for 30 min while warming to room
tected sulfur anchor groups. While the ethyl-TMS group temp. Subsequently, CHCl3 and EtOH were added. After
was easily cleaved by tetrabutylammonium fluoride (TBAF) aqueous work up and CC the cruciform 3 was isolated in
the subsequent treatment with acetyl chloride (AcCl) re- 84% yield as a yellow solid.
sulted in the acetylation of both, the sulfur groups and the
For the assembly of target structure 4 boronic acid 16
pyridine nitrogen atoms. Fortunately, the intense study of was required (Scheme 3). Our strategy was to introduce the
Karl Freudenberg and Daniel Peters[74] revealed that acetyl- sulfur via a Newman–Kwart rearrangement (NKR) as ef-
Scheme 3. Synthesis of target structure 4. After deprotecting 2-bromo-6-methoxynaphthalene the phenol is converted to a thiophenol via
a Newman–Kwart rearrangement. Protection of the thiol and formation of the boronic acid leads to the building block 16 needed for
the Suzuki reaction to assemble the cruciform structure.
836
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 833–845

Models for Coordination-Induced Single-Molecule Switches
ficient method to convert phenols to thiophenols by the ing it to 100 °C in vinyltrimethylsilane and traces of 2,2Ј-
thermally activated rearrangement of an O-thiocarbamate azobis(2-methylpropionitrile) (AIBN) as radical starter in a
to the corresponding S-thiocarbamate.[75–77] The O-thiocarb- pressure tube. Upon cooling down, the ethyl-TMS-pro-
amate 12 was synthesized by treating 6-bromonaphthalen- tected thionaphthalene derivative 15 was obtained as a be-
2-ol (11), available after deprotection of 2-bromo-6-meth- ige precipitate in 96% yield. Treating 15 with nBuLi in dry
oxynaphthalene (10) with BBr3, with dimethylcarbamoyl THF at –78 °C and quenching with triisopropyl borate fol-
chloride using sodium hydride (NaH) as base.[78] The O- lowed by an aqueous work up and recrystallization from
thiocarbamate 12 was dissolved in diphenyl ether and hexane provided boronic acid 16 as a beige solid in 76%
heated to 259 °C. The extent of the rearrangement of 12 to yield. For the Suzuki coupling of the new boronic acid 16
the S-thiocarbamate 13 was monitored by thin-layer with diiodo derivative 5, similar reaction conditions were
chromatography (TLC). After 1.5 h the reaction mixture applied as described above for boronic acid 8. The new cru-
was cooled to room temp. and directly exposed to CC to ciform 17 comprising ethyl-TMS protection groups was iso-
provide the S-thiocarbamate 13 as a beige solid in 89% lated in 85% yield as a yellow solid after CC. For the
yield. After hydrolysis with KOH in methanol, 6-bromo- transprotection of the sulfur, similar reaction conditions as
naphthalene-2-thiol (14) was obtained as a beige solid in described above for the cruciform 9 were applied to 17.
97% yield. The thiol 14 was ethyl-TMS-protected by heat- Thus, after deprotection with TBAF and treating with
Scheme 4. Synthesis of control molecules 20 and 22.
Scheme 5. Synthesis of control molecules 25 and 27.
Eur. J. Org. Chem. 2010, 833–845
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
837

S. Grunder, R. Huber, S. Wu, C. Schönenberger, M. Calame, M. Mayor
FULL PAPER
AcCl, followed by hydrolysis in the CHCl3/EtOH, purifica- tion. During a typical experiment, the pushrod is moved at
tion by CC provided the acetyl-protected cruciform 4 as a a velocity of ca. 30 µm/s, so that the two gold leads separate
yellowish solid in 58% yield (Scheme 3).
at ca. 1 nm/s.
Both target cruciforms 3 and 4 were successfully synthe-
sized. To be able to relate particular transport characteris-
tics to the rod-like subunits of the cruciforms, several con-
trol compounds either consisting of a single bar of the cru-
ciform structure or lacking the coordinating pyridine nitro-
gen atom have been synthesized in addition. Thus, the OPV
with two pyridine anchor groups 18 was synthesized follow-
ing a reported procedure.[79] Furthermore, the correspond-
ing oligoaryl rods 20 and 22 were assembled to mimic the
terminal sulfur-functionalized bar substructure.
Figure 3. (a) Schematics of the MCBJ principle with a liquid cell.
(b) SEM image of the central part of a microfabricated Au junction
on top of a polyimide-coated stainless-steel substrate.
In order to measure in a liquid environment, a liquid cell
Therefore 20 and 22 were assembled via a Suzuki reac- (cell volume: 3 mL) is integrated into the setup. The cell
tion. The corresponding boronic acid was treated with 2,6- consists of a Viton tube placed on top of the break junction
diiodo-p-xylene to give the rods 19 and 21, respectively. sample and a larger glass reservoir, which allows an efficient
Transprotection of the sulfur using the same conditions as delivery of the molecules to the Au electrodes. Transient
described above yielded the acetylated control molecules 20 molecular junctions are formed by periodically opening and
and 22, respectively (Scheme 4).
closing the Au junction in the presence of molecules in solu-
Further the two cruciform structures without pyridine tion. Typically, target molecules are dissolved in a mixture
anchor groups but terminal benzene units at the transversal of THF and mesitylene (1:4 v/v, henceforth referred to as
bar substructure 25 and 27 were synthesized. Therefore the the solvent) to a final concentration of 0.25 m. In order
iodinated OPV bar 23 was synthesized by a reported pro- to remove the acetyl protection groups, a 50 m tetrabut-
cedure.[64] Having both building blocks, diiodide 23 and bo- ylammonium hydroxide (TBAH) deprotecting agent was
ronic acids 8 and 16, respectively, in hand, the assembly of added to the solution. During the measurements the solu-
the control cruciforms was achieved by using similar proto- tion was kept under an argon protection gas atmosphere to
cols as already described. Again, a Suzuki coupling reaction exclude oxygen, as molecular rods comprising two terminal
followed by a transprotection of the sulfur unit yielded the thiophenols are prone to polymerization by disulfide forma-
desired control molecules 25 and 27 (Scheme 5).
tion in the presence of oxygen. To determine the electrical
All new compounds were analyzed by determination of conductance G of the molecular junctions, a voltage bias of
the melting point (if solids), determination of the Rf value, 0.2 V is applied between the left and right contacts, and the
1H NMR and 13C NMR spectroscopy, mass spectrometry resulting current is measured with a custom made auto-
and elemental analysis (EA). Further UV/Vis spectra of all ranging IV converter. By recording hundreds of conduc-
cruciform structures 3, 4, 9, 17, 25 and 27 such as of the tance traces, a statistical analysis can be done and an
corresponding rods 20 and 22 were recorded. Interestingly, average conductance of the junction deduced.[32] The mea-
in spite of numerous different purification attempts, we sured conductance traces are converted to a logarithmic
were not able to record correct EA data of all target cruci- scale and the log(G) histogram is calculated using a con-
form structures. Gel permeation chromatography (GPC) in stant bin size. This representation gives a better overview
a HPLC setup was performed and the GPC chromatograms (over six orders of magnitude) of the molecular junction
of compound 3, 4, 20, 22, 25 and 27 are provided in the SI. conductance. Typically, a set of 100 single conductance
curves was used to calculate one histogram. The peak posi-
tion and shape of the conductance histogram are interpre-
ted as the typical signature of a particular molecule.
Single-Molecule Transport Investigations
First, cruciform 3 was exposed to the break junction
The above described target structures were investigated without adding any deprotection agent (Figure 4, a). Quite
in a mechanically controllable break junction (MCBJ) setup remarkably, the signature of compound 3 shows two dis-
described previously.[19,32,33,53] The principle of a MCBJ is tinct peaks.[80] In contrast, when compound 3 is deprotected
depicted in Figure 3. A gold lead with a free-standing con- in the liquid cell, then only the peak with higher conductiv-
striction in its middle (80–150 nm wide) is fabricated on an ity is observed (Figure 4, b). As the spontaneous deprotec-
electrically isolated, flexible spring-steel substrate. By tion of the sulfur anchor group in the presence of Au elec-
mounting the substrate in a three point bending mecha- trodes is expected, the observation of two peaks points
nism, the constriction can be stretched and eventually towards the formation of junctions bridged by the sulfur-
broken. This results in two atomic gold tips, which can be functionalized OP bar and by the pyridine-functionalized
brought back into contact by relaxing the bending tension OPV bar with comparable probabilities. Upon chemical de-
of the substrate. When broken in the presence of molecules protection of the sulfur anchor group in solution, the mo-
with anchoring groups, the two tips form ideal electrodes lecular junction bridged by the immobilization of the OP
to contact single molecules. The gap distance d between the rod with terminal sulfur groups seemed to be favoured. To
electrodes can be adjusted by controlling the pushrod posi- clarify this point, we further investigated the rod-like sub-
838
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 833–845

Models for Coordination-Induced Single-Molecule Switches
units of compound 3, namely the sulfur-functionalized OP transformed to a linear scale. Due to the width of the peaks
rod 20 and the pyridine OPV bar 18 separately (Figure 4, c and the log scale, the Glin values are slightly shifted to lower
and d, respectively). These experiments support the view conductances.[33]
that the two peaks observed in part a of Figure 4 for the
Table 1. Conductance values of the measured compounds. Glog de-
still protected compound 3 stem from the two rods forming
note the values derived from a Gaussian fit of the molecular signa-
the cruciform structure. We expect that the pyridine OPV
ture in the log(G) histogram. Glin denotes the corresponding maxi-
rod of compound 3 can be immobilized between the two
mum in a linear representation, which results slightly shifted to
electrodes, giving rise to the peak at lower conductance. lower values, depending on the width of the peak in log scale.[33]
However, in this situation, the protected sulfur end comes
Molecule
Glog-low [G0] Glog-high [G0] Glin-low [G0] Glin-high [G0]
close to the Au electrodes as well. Because the OP rod can-
not diffuse freely in solution, the probability to have a spon-
taneous deprotection of the sulfur groups increases: we will
3 – protected
3 – deprotected
4 – protected
–5.1
–4.8
–3.8
–3.6
–4.3
–4.5
3.3 ϫ 10–6 9.8 ϫ 10–5
1.5 ϫ 10–4
9.9 ϫ 10–6 2.6 ϫ 10–5
1.9 ϫ 10–5
sometimes form a sulfur–gold covalent bond at both ends 4 – deprotected
9.2 ϫ 10–6
6.2 ϫ 10–5
1.6 ϫ 10–5
of the OP rod. If this spontaneous deprotection happens
frequently enough, we expect the system to present two pos-
sible stable local conformations for the molecular junction:
one where the pyridine OPV rod dominates transport and
one where the thiolated OP rod dominates. By deprotecting
18
–4.7
20 – deprotected
22 – deprotected
–3.8
–4.4
Cruciform 4 was also investigated in the MCBJ and a
the sulfur end groups, we simply shift the equilibrium of similar pattern in the histogram is observed. When no de-
the system towards the situation dominated by the OP rod. protection agent is added (Figure 4, e), two peaks appear,
This interpretation is qualitatively supported by the control while when the cruciform is first deprotected, only one dis-
measurements performed for the rods taken separately. In- tinct peak is observed (Figure 4, f). Again, comparison with
deed, the peak for the pyridine OPV 18 appears clearly at the parent rod subunits 22 and 18 displayed that the lower
a substantially lower conductance than that for the depro- peak corresponds well to that of the pyridine OPV 18 (Fig-
tected OP rod 20, the latter being close to the peak exhib- ure 4, d) and the higher peak to that of the single rod 22
ited by the deprotected compound 3. Note also that mea- (Figure 4, g). However, the two peaks of 4 in Figure 4, e,
surements of the OP rod 20 when still protected do not are less clearly separated than what was observed for com-
exhibit a molecular signature: the probability for spontane- pound 3, pointing at the lower conductance of the sulfur
ous deprotection is here too low to result in a clear signal. terminated rod subunit of the cruciform 4 compared to 3
The conductance histogram in this case is similar to that (see Table 1). It is therefore not surprising that cruciform
for the pure solvent (Figure 4, h). An overview of the peak 4 shows a slightly lower conductance than the previously
conductance values in the log(G) histograms is provided in investigated cruciforms 2 and 3.
Table 1. Glog-low and Glog-high are obtained by a Gaussian fit
For both cruciform structures 3 and 4 the sulfur termin-
of the molecular peak in the log(G)–conductance histo- ated OP-type rod is better conducting than the pyridine-
grams. Glin-low and Glin-high are the same conductance values functionalized OPV rod, making the observation of the co-
Figure 4. Measured conductance histograms for compounds 3 and 4 without and with deprotection agent (a), (b) and (e), (f). The
measured reference conductance histograms for compounds 20, 18, 22, as well as for the solvent alone are shown in (c), (d), (g) and (h),
respectively.
Eur. J. Org. Chem. 2010, 833–845
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
839

S. Grunder, R. Huber, S. Wu, C. Schönenberger, M. Calame, M. Mayor
FULL PAPER
ordination-induced switching challenging. In particular for corded. Detailed investigations of the behaviour of the cru-
the cruciform 4, the proximity of the signature of both rods ciform in the presence of a control potential within an elec-
will make the detection of the coordination-induced switch- trochemical break junction setup are currently underway.
ing of its OPV subunit very demanding. We expect that ap-
The switching concept originally developed aimed at an
plying a potential to the liquid cell in the appropriate condi- increase in conductance upon coordination of the pyridine
tions shall enable us to modulate the conductance of the bar. We now observed experimentally that the pyridine rod
pyridine-functionalized OPV (18) by adjusting the pyr- exhibits a lower conductance than the sulfur bar. The mo-
idine–gold coordination.[17,57–59] Ultimately a binding-un- lecular system will therefore not result in an overall increase
binding control of the pyridine rod shall be possible. By in conductance of the molecular junction upon coordina-
doing this, the signature of the molecular junction shall tion. This, however, does not prevent the switching of the
alternate between a single and a double peak, thereby re- cross to be detected. As indeed demonstrated above, the
vealing a switching mechanism between a state where both signature of the cross can be tuned to present a stark con-
rods can bind (as shown in Figure 4, a and e) or where trast between its two possible conformational states be-
exclusively the sulfur-terminated rod binds (see Figure 4, b tween the electrodes. By choosing rods with sufficiently sep-
and f).
arated conductances, as is the case for compound 3, we ex-
Further experiments are now underway where the pos- pect to be able to unambiguously determine the binding
sible switching effects of the cruciform molecules are being status of the cross within the junction. Current efforts aim
investigated. For this purpose, a modified MCBJ setup in- at synthesizing alternative cruciform structures as well as
cluding a potentiostat for proper electrochemical control improving the detection electronics to improve the signal-
has been developed. Detailed results of these experiments to-noise ratio between the conductances of the different
will be published elsewhere.
binding states of the cross compound.
Conclusions
Experimental Section
Two new families of cruciforms, 3 and 4, based on a pyr-
idine-functionalized OPV rod and transversal sulfur-func-
tionalized OP-type rod were designed and synthesized as
potential coordination-induced single-molecule switches.
Their assembly was based on Horner–Wadsworth–Emmons
reactions and Suzuki cross-coupling chemistry. For the OP-
type substructure, the required boronic acid derivatives
comprising ethyl-TMS-protected sulfur groups were synthe-
sized. Interestingly, the ethyl-TMS-protected thiophenols
were stable under the basic reaction conditions of the Su-
zuki reaction even at elevated temperature and were ef-
ficiently transprotected to the acetyl-protected thiophenols
required for the immobilization strategy. In addition, sev-
Reagents and Solvents: All chemicals were used for the synthesis
without further purification unless otherwise noted. The solvents
for chromatography and crystallization were distilled once before
use, the solvents for extraction were used in technical grade. Dry
tetrahydrofuran (THF) and dry dichloromethane (DCM) were dis-
pensed from a Pure Solv MD Solvent Purification System. Dry
toluene was distilled from Na/benzophenone. Dry DMF was pur-
chased over molecular sieves from Fluka.
Synthesis: All reactions with reagents, which are easily oxidized or
hydrolyzed, were performed under argon using Schlenk technique,
only dry solvents were used and the glassware was heated out.
Analytics and Instruments: Bruker DPX NMR (400 MHz) and
Bruker BZH NMR (250 MHz) instruments were used to record the
eral control molecules like the pyridine-functionalized OPV 1H NMR spectra. Chemical shifts (δ) are reported in parts per mil-
lion (ppm) relative to residual solvent peaks or trimethylsilane
(TMS), and coupling constants [J] are reported in Hertz [Hz].
NMR solvents were obtained from Cambridge Isotope Laborato-
ries, Inc. (Andover, MA, USA). The measurements were done at
room temperature. Bruker DPX NMR (101 MHz) instruments
were used to record the 13C NMR spectra. Chemical shifts (δ) are
reported in parts per million (ppm) relative to residual solvent
peaks. The measurements were done at room temperature. Mass
spectra were recorded with a Bruker esquire 3000 plus (for ESI), a
Finnigan MAT 95Q (for EI), a Finnigan MAT 8400 (for FAB),
bar 18, both acetylsulfanyl-functionalized OP-type rods 20
and 22, together with cruciform structures 25 and 27 lack-
ing the coordinating pyridine nitrogen atoms, were synthe-
sized.
Transport investigations through these new cruciforms
and the corresponding control molecules were performed in
a MCBJ setup in a liquid environment. All cruciforms and
control structures were immobilized at a single-molecule
level, and their transport signature was recorded. We could
demonstrate that both rods of the cruciform structures are or a Voyger-De Pro (for MALDI-TOF) instrument. Elementary
analyses were performed with a Perkin–Elmer Analysator 240. UV/
Vis spectroscopy was measured in DCM with an Agilent 8453 in-
strument. Gel permeation chromatograms were recorded with a
Shimadzu LC-8A chromatograph. The measurements were done at
room temperature with an OligoPore 300ϫ7.5 mm column (par-
ticle size 6 µm) from Polymer Laboratories, by eluting with toluene
with a flow rate of 0.5 mL/min at λ = 220 nm. For column
chromatography, silica gel 60 (40–63 µm) from Merck or silica gel
60 (40–63 µm) from Fluka was used. For TLC silica gel 60 F254
glass plates with a thickness of 0.25 mm from Merck were used.
functional and can be used for immobilization. Control ex-
periments of the separate building blocks of the cruciform
were also performed to carefully characterize the functional
compound and confirm the immobilization of the cruci-
form. In spite of the new design, the para-sulfur-function-
alized OP-type rod remained the transport-dominating sub-
structure. However, when the coordination of the sulfur to
the gold electrode is handicapped by acetyl protection
groups, the signature of both bar substructures (the OP-
type rod and the pyridine-functionalized OPV rod) were re- UV light (254 or 366 nm) was used for detection.
840 Eur. J. Org. Chem. 2010, 833–845
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Models for Coordination-Induced Single-Molecule Switches
4-[2-(Trimethylsilyl)ethylthio]phenylboronic Acid (8):[81] Bromide 7
(10.0 g, 34.6 mmol) was dissolved in dry THF (40 mL) in a 100 mL
Schlenk flask and treated with nBuLi (1.6  in hexane, 34.6 mL,
55.4 mmol) at –78 °C. It was stirred for 30 min before triisopropyl
borate (41.4 mL, 180 mmol) was added. The reaction mixture was
then stirred for 2 h at –78 °C, 2 h at –20 °C and 17 h at room temp.
and then extracted with TBME (200 mL) and water (200 mL). The
aqueous phase was washed with TBME (100 mL), the combined
organic phases were washed with brine (2ϫ100 mL), dried with
MgSO4, filtered and the solvents evaporated. The crude was recrys-
tallized from hexane to give a colourless solid (5.20 g, 20.5 mmol,
59%); m.p. 144.3–145.5 °C. 1H NMR (400 MHz, CDCl3, 25 °C): δ
diiodo-p-xylene (232 mg, 0.648 mmol), Pd(PPh3)4 (160 mg,
0.139 mmol) and K2CO3 (850 mg, 6.15 mmol) were added. The re-
action mixture was then stirred for 14 h at 80 °C, cooled to room
temp. and extracted with TBME and water. The aqueous phase
was washed with TBME. The combined organic phases were
washed with water and brine, dried with MgSO4, evaporated and
purified by CC (silica gel, 3ϫ12 cm, hexane/EtOAc = 20:1) to give
19 as a colourless solid (331 mg, 0.633 mmol, 98%); m.p. 116.6–
117.4 °C. TLC Rf = 0.40 (hexane/EtOAc = 20:1). 1H NMR
3
(400 MHz, CDCl3, 25 °C): δ = 7.36 (d, JH,H = 8.5 Hz, 4 H), 7.30
(d, 3JH,H = 8.5 Hz, 4 H), 7.15 (s, 2 H), 3.03 (m, 4 H, CH2), 2.29 (s,
6 H, Me), 0.99 (m, 4 H, CH2), 0.07 (s, 18 H, SiMe3) ppm. 13C
NMR (101 MHz, CDCl3, 25 °C): δ = 140.2, 139.0, 135.8, 132.6,
3
3
= 8.10 [d, J(H,H) = 8.2 Hz, 2 H, CH], 7.36 (d, JH,H = 8.2 Hz, 2
H, CH), 3.05 (m, 2 H, CH2), 1.00 (m, 2 H, CH2), 0.09 (s, 9 H, 131.8, 129.6, 128.3, 29.5, 19.9, 16.9, –1.7 ppm. MS (EI): m/z (%) =
SiMe3) ppm. 13C NMR (101 MHz, CDCl3, 25 °C): δ = 143.7, 135.9,
126.4, 28.1, 16.6, –1.8 ppm. MS (MALDI-TOF): found m/z =
254.94, 254.10 calculated for C11H19BO2SSi.
522.3 (33) [M+], 73.0 (100).
2,5-Bis(4-thioacetylphenyl)-p-xylene (20): The starting material 19
(41 mg, 0.0784 mmol) was dissolved in 5 mL of THF. The mixture
1,4-Bis{4-[2-(trimethylsilyl)ethylthio]phenyl}-2,5-bis[(E)-pyridin-4-yl- was degassed for 15 min before TBAF (1  in THF, 3.00 mL,
vinyl]benzene (9): Iodide 5 (99.0 mg, 0.185 mmol) and boronic acid
8 (103 mg, 0.406 mmol) were dissolved in toluene (7 mL) and eth-
anol (2 mL). The mixture was degassed for 10 min, before K2CO3
(205 mg, 1.48 mmol) and Pd(PPh3)4 (20.0 mg, 0.0173 mmol) was
added. It was then stirred for 7.5 h at 80 °C. The hot reaction mix-
ture was filtered, washed with toluene, evaporated and purified by
column chromatography (silica gel, 2ϫ12 cm, CH2Cl2, 1% MeOH)
to give compound 9 as a colourless solid (92.0 mg, 0.131 mmol,
71%); m.p. 158.5 –160.0 °C. TLC Rf = 0.34 (CH2Cl2, 5% MeOH).
3.00 mmol) was added at room temp. The reaction mixture was
stirred for 45 min, then acetyl chloride (1.50 mL, 21.0 mmol) was
added at 0 °C. After stirring for 15 min at 0 °C, the reaction mix-
ture was quenched with ice and extracted with DCM/water. The
aqueous phase was washed twice with DCM. The combined or-
ganic phases were washed twice with water, dried with MgSO4 and
the solvents evaporated. The crude was purified by column
chromatography (silica gel, 1ϫ12 cm, hexane/EtOAc = 20:1) to
give 20 as a colourless solid (15 mg, 0.0369 mmol, 47%); m.p.
1
1H NMR (400 MHz, CDCl3, 25 °C): δ = 12.80 (m, 4 H), 7.74 (s, 2
190.5–195.8 °C. TLC Rf = 0.10 (hexane/EtOAc = 20:1). H NMR
3
3
H), 7.43–7.33 (m, 10 H), 7.22 (m, 4 H), 7.02 (d, JH,H = 16.2 Hz, (400 MHz, CDCl3, 25 °C): δ = 7.48 (d, JH,H = 8.4 Hz, 4 H), 7.42
3
2 H), 3.06 (m, 4 H, CH2), 1.02 (m, 4 H, CH2), 0.09 (s, 18 H, SiMe3)
ppm. 13C NMR (101 MHz, CDCl3, 25 °C): δ = 150.2, 144.5, 140.5,
137.5, 136.9, 134.4, 131.3, 130.2, 128.1, 128.0, 127.4, 120.9, 29.1,
(d, JH,H = 8.4 Hz, 4 H), 7.17 (s, 2 H), 2.47 (s, 6 H, CH3), 2.29 (s,
6 H, Me) ppm. 13C NMR (101 MHz, CDCl3, 25 °C): δ = 194.1,
142.7, 140.1, 134.1, 132.7, 131.9, 130.0, 126.3, 30.2, 19.9 ppm. MS
16.8, –1.7 ppm. MS (EI): m/z (%) = 700.3 (24) [M+], 672.3 (17), (EI): m/z (%) = 406.1 (39) [M+], 364.1 (26), 322.1 (100), 43 (14).
644.3 (6), 73.0 (100). GPC (oligopore 6 µm, toluene, UV/Vis photo-
diode array detector) area 100%. UV/Vis: λmax 280, 310, 342 nm.
GPC (oligopore 6 µm, toluene, UV/Vis photodiode array detector)
area 99.8%. UV/Vis: λmax = 273 nm.
1,4-Bis(4-thioacetylphenyl)-2,5-bis[(E)-pyridin-4-ylvinyl]benzene (3): O-(6-Bromonaphthalene-2-yl) Dimethylcarbamothioate (12): 6-Bro-
The starting material 9 (122 mg, 0.174 mmol) was dissolved in dry
THF (20 mL) in a 100 mL flask and the solution was saturated
with argon. Then TBAF (1  in THF, 3 mL, 3 mmol) was added
and it was stirred for 1 h at room temperature. To the pinkish solu-
tion was then added acetyl chloride (0.4 mL, 4.6 mmol) at 0 °C.
The reaction mixture turned yellow and was stirred for 30 min at
room temperature. Then chloroform (15 mL) and ethanol (15 mL)
were added. The reaction mixture was extracted with dichlorometh-
ane and water, the aqueous phase was washed three times with
dichloromethane, the combined organic phases were washed with
brine, dried with MgSO4 and the solvents evaporated. The crude
was purified by column chromatography (silica gel, 1ϫ12 cm,
CH2Cl2, 1% MeOH) to give the product 3 as a yellow solid (85 mg,
0.145 mmol, 84%); m.p. 208.4–210.9 °C. TLC Rf = 0.18 (CH2Cl2,
monaphthalen-2-ol (500 mg, 2.24 mmol) was dissolved in DMF
(10 mL) and added to a solution of sodium hydride (55% moist-
ened with oil, 193 mg, 6.72 mmol) in DMF (8 mL) at 0 °C. The ice
bath was taken away and the mixture stirred for 30 min. Then di-
methyl carbamoyl chloride (830 mg, 6.72 mmol) was added and it
was stirred for 2 h at 80 °C and 15 h at room temperature. The
reaction mixture was then extracted with 1% aqueous NaOH
(100 mL) and TBME (100 mL). The aqueous phase was washed
with TBME (2ϫ50 mL). The combined organic phases were
washed with brine and 5% aqueous HCl (100 mL), dried with
MgSO4 and the solvents evaporated. The crude was purified by
column chromatography (silica gel, 3ϫ12 cm, CH2Cl2/hexane =
5:1) to give the thiocarbamate 12 as a colourless solid (695 mg,
2.24 mmol, 100%); m.p. 126.9–131.9 °C. TLC Rf = 0.43 (CH2Cl2/
1
1
5% MeOH). H NMR (400 MHz, CDCl3, 25 °C): δ = 8.54 (m, 4
hexane = 5:1). H NMR (400 MHz, CDCl3, 25 °C): δ = 8.00 (s, 1
3
3
3
3
H), 7.78 (s, 2 H), 7.56 (d, JH,H = 8.2 Hz, 4 H), 7.51 (d, JH,H
=
H), 7.75 (d, JH,H = 8.9 Hz, 1 H), 7.67 (d, JH,H = 8.7 Hz, 1 H),
3
3
8.2 Hz, 4 H), 7.32 (d, JH,H = 16.3 Hz, 2 H), 7.23 (m, 4 H), 7.03
7.54 (d, JH,H = 8.8 Hz, 1 H), 7.46 (s, 1 H), 7.27 (m, 1 H), 3.48 (s,
(d, JH,H = 16.2 Hz, 2 H), 2.50 (s, 6 H, CH3) ppm. 13C NMR
3 H, CH3), 3.39 (s, 3 H, CH3) ppm. 13C NMR (101 MHz, CDCl3,
25 °C): δ = 188.0, 152.3, 133.0, 132.5, 130.3, 129.8, 128.5, 124.1,
120.0, 43.8, 39.2 ppm. MS (EI): m/z (%) = 310.9 (17) 309.0 (16)
[M+], 88.0 (100). C13H12BrNOS (310.21): calcd. C 50.33, H 3.90,
N 4.52; found C 50.10, H 3.80, N 4.30.
3
(101 MHz, CDCl3, 25 °C): δ = 193.8, 150.3, 144.3, 140.9, 140.4,
134.5, 134.4, 130.7, 130.5, 128.2, 128.1, 127.6, 120.9, 30.4 ppm. MS
(MALDI-TOF): found m/z
= 584.91, 584.16 calculated for
C36H28N2O2S2. GPC (oligopore 6 µm, toluene, UV/Vis photodiode
array detector) area 99.9%. UV/Vis: λmax = 291, 342 nm.
S-(6-Bromonaphthalene-2-yl) Dimethylcarbamothioate (13): Com-
pound 12 (4.70 g, 15.2 mmol) and phenyl ether (20.0 g) were placed
in a 100 mL two-necked flask with condenser (dry, Ar) and it was
heated to reflux (259 °C) for 1.5 h. The reaction mixture was then
2,5-Bis{4-[2-(trimethylsilyl)ethylthio]phenyl}-p-xylene (19): Boronic
acid 8 (389 mg, 1.53 mmol) was dissolved in toluene (20 mL) and
ethanol (5 mL). The mixture was degassed for 10 min before 2,5-
Eur. J. Org. Chem. 2010, 833–845
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
841

S. Grunder, R. Huber, S. Wu, C. Schönenberger, M. Calame, M. Mayor
FULL PAPER
cooled to room temperature and directly chromatographed (silica
gel, 4ϫ12 cm, hexane/EtOAc = 5:1 to 3:1) to give 13 as a beige
solid (4.20 g, 13.5 mmol, 89%); m.p. 106.5–108.2 °C. TLC Rf =
0.20 (hexane/EtOAc = 5:1). 1H NMR (400 MHz, CDCl3, 25 °C): δ
8.6 Hz, 1 H), 3.11 (m, 2 H, CH2), 1.04 (m, 2 H, CH2), 0.11 (s, 9
H, SiMe3) ppm. 13C NMR (101 MHz, CDCl3, 25 °C): δ = 137.2,
137.5, 136.2, 131.4, 130.8, 129.3, 126.6, 126.3, 124.8, 28.7, 16.6,
–1.7 ppm. MS (MALDI-TOF): found m/z = 299.13, 304.11 calcu-
2
3
= 8.00 (d, JH,H = 1.7 Hz, 1 H), 7.98 (s, 1 H), 7.75 (d, JH,H
=
lated for C15H21BO2SSi.
3
2
8.6 Hz, 1 H), 7.68 (d, JH,H = 8.8 Hz, 1 H), 7.57 (dd, JH,H = 1.8,
1,4-Bis{6-[2-(trimethylsilyl)ethylthio]naphthalen-2-yl}-2,5-bis[(E)-
pyridin-4-ylvinyl]benzene (17): Iodide 5 (87 mg, 0.162 mmol) and
boronic acid 16 (110 mg, 0.362 mmol) were suspended in toluene
(8 mL) and ethanol (2 mL). The mixture was degassed, before
Pd(PPh3)4 (20 mg, 0.017 mmol) and K2CO3 (185 mg, 1.34 mmol)
were added. The reaction mixture was exposed to microwaves
(500 W, 80 °C) for 140 min. The solid material was filtered off, the
filtrate was evaporated and chromatographed (silica gel, 2ϫ12 cm,
CH2Cl2, 1% MeOH to 2% MeOH) to give the cruciform structure
17 as a yellow solid (110 mg, 0.137 mmol, 85 %); m.p. 221.0–
2
3
3JH,H = 8.6 Hz, 1 H), 7.56 (dd, JH,H = 2.0, JH,H = 8.7 Hz, 1 H),
3.13 (s, 3 H, CH3), 3.04 (s, 3 H, CH3) ppm. 13C NMR (101 MHz,
CDCl3, 25 °C): δ = 166.6, 135.1, 134.2, 133.3, 131.8, 129.8, 129.7,
129.5, 127.4, 126.8, 121.0, 36.9 ppm. MS (EI): m/z (%) = 311.0 (8)
309.0 (8) [M+], 158.0 (12), 72.0 (100). C13H12BrNOS (310.21):
calcd. C 50.33, H 3.90, N 4.52; found C 50.08, H 3.83, N 4.32.
6-Bromonaphthalene-2-thiol (14): A solution of the 13 (3.48 g,
11.2 mmol) in MeOH (250 mL) was saturated with Ar. Then solid
KOH (5.22 g, 93.0 mmol) was added and it was heated to 80 °C for
2.5 h. The reaction mixture was then quenched with 1  aqueous
223.0 °C. TLC Rf = 0.13 (CH2Cl2, 2 % MeOH). 1H NMR
3
HCl (250 mL) at 0 °C and it was extracted with CH2Cl2 (300 mL). (400 MHz, CDCl3, 25 °C): δ = 8.49 (d, JH,H = 6.1 Hz, 4 H), 7.90–
3
The aqueous phase was washed with CH2Cl2 (2ϫ100 mL). The
combined organic phases were washed with brine, dried with
MgSO4 and evaporated to give the thiol 14 as a beige solid (2.60 g,
7.87 (m, 6 H), 7.83 (d, JH,H = 8.7 Hz, 2 H), 7.79 (s, 2 H), 7.59
3
2
3
(dd, 2JH,H = 1.7, JH,H = 8.4 Hz, 2 H), 7.49 (dd, JH,H = 1.8, JH,H
3
3
= 8.6 Hz, 2 H), 7.40 (d, JH,H = 16.3 Hz, 2 H), 7.18 (d, JH,H
6.2 Hz, 4 H), 7.07 (d, JH,H = 16.2 Hz, 2 H), 3.13 (m, 4 H, CH2),
=
3
10.9 mmol, 97%); m.p. 175.6–178.6 °C. TLC Rf = 0.21 (hexane/
1
EtOAc = 5:1). H NMR (400 MHz, CDCl3, 25 °C): δ = 7.92 (s, 1 1.04 (m, 4 H, CH2), 0.10 (s, 18 H, SiMe3) ppm. 13C NMR
3
3
H), 7.69 (s, 1 H), 7.61 (d, JH,H = 8.6 Hz, 1 H), 7.55 (d, JH,H
=
(101 MHz, CDCl3, 25 °C): δ = 150.2, 144.4, 141.1, 136.9, 136.0,
134.6, 133.0, 131.4, 131.3, 128.53, 128.52, 128.4, 128.3, 127.8,
127.5, 127.0, 125.6, 120.8, 29.2, 16.8, –1.7 ppm. MS (EI): m/z (%)
8.8 Hz, 1 H), 7.52 (dd, 2JH,H = 1.8, 3JH,H = 8.8 Hz, 1 H), 7.34 (dd,
2JH,H = 1.9, 3JH,H = 8.6 Hz, 1 H), 3.60 (s, 1 H, SH) ppm. 13C NMR
(101 MHz, CDCl3, 25 °C): δ = 132.8, 132.6, 130.5, 130.2, 129.5,
129.2, 128.8, 128.1, 127.3, 119.8 ppm. MS (EI): m/z (%) = 239.9
= 801.2 (21) 800.3 (32) [M+], 772.3 (24) 73.0 (100). UV: λmax
=
325 nm. GPC (oligopore 6 µm, toluene, UV/Vis photodiode array
(100) 237.9 (98) [M+]. C10H7BrS (239.13): calcd. C 50.23, H 2.95; detector) area 99.1%.
found C 50.14, H 2.96.
1,4-Bis(6-thioacetylnaphthalen-2-yl)-2,5-bis[(E)-pyridin-4-ylvinyl]-
6-Bromonaphthalene-2-[2-(trimethylsilyl)ethyl]thiol (15): The thiol
14 (2.05 g, 8.57 mmol), vinyl-trimethylsilane (5.5 mL, 37.9 mmol)
benzene (4): The ethyl-TMS-protected thiol 17 (190 mg, 0.237
mmol) was dissolved in THF (25 mL) in a 100 mL flask. The mix-
ture was degassed before TBAF (1  in THF, 4.00 mL, 4.00 mmol)
was added. It was stirred for 1 h, then acetyl chloride (0.600 mL,
8.45 mmol) was added and it was stirred for 30 min. The reaction
mixture was quenched with ethanol (25 mL) and chloroform
(25 mL). After stirring for 15 min, the reaction mixture was ex-
tracted with water (200 mL) and CH2Cl2 (100 mL). The aqueous
phase was washed with CH2Cl2 (3ϫ50 mL). The combined organic
and
2,2Ј-azobis(2-methylpropionitrile)
(AIBN)
(70.0 mg,
0.426 mmol) were placed in a 25 mL pressure tube. The tube was
sealed and it was stirred for 24 h at 100 °C. The reaction mixture
was then evaporated. A precipitate was formed which was filtered
off and washed with chloroform. The filtrate was evaporated and
dried at high vacuum to give compound 15 as a beige solid (2.78 g,
8.19 mmol, 96%); m.p. 39.3–40.3 °C. TLC Rf = 0.29 (hexane/
1
CH2Cl2 = 10:1). H NMR (400 MHz, CDCl3, 25 °C): δ = 7.94 (d, phases were washed with brine, dried with MgSO4 and the solvents
3
2JH,H = 1.7 Hz, 1 H), 7.65 (m, 2 H), 7.61 (d, JH,H = 8.8 Hz, 1 H),
evaporated. The crude was purified by column chromatography
(silica gel, 2ϫ12 cm, CH2Cl2, 1% MeOH to 2% MeOH) to give
the acetylated cruciform 3 as a yellowish solid (94.0 mg,
0.137 mmol, 58%). TLC Rf = 0.25 (CH2Cl2, 5% MeOH); m.p. (de-
2
3
2
7.53 (dd, JH,H = 1.9, JH,H = 8.7 Hz, 1 H), 7.41 (dd, JH,H = 1.9,
3JH,H = 8.6 Hz, 1 H), 3.06 (m, 2 H, CH2), 0.55 (m, 2 H, CH2), 0.07
(s, 9 H, SiMe3) ppm. 13C NMR (101 MHz, CDCl3, 25 °C): δ =
135.7, 132.5, 132.2, 129.82, 129.75, 128.6, 128.0, 127.3, 125.7, comp.) above 250 °C. 1H NMR (400 MHz, CDCl3, 25 °C): δ = 8.49
3
119.1, 29.1, 16.7, –1.7 ppm. MS (EI): m/z (%) = 340.0 (11) 338.0 (d, JH,H = 5.9 Hz, 4 H), 8.06 (s, 2 H), 7.98–7.95 (m, 6 H), 7.89 (s,
(11) [M+], 73.0 (100). C15H19BrSSi (339.37): calcd. C 53.09, H 5.64; 2 H), 7.63 (dd, JH,H = 1.6, JH,H = 8.5 Hz, 2 H), 7.55 (dd, JH,H
2
3
2
3
3
found C 53.23, H 5.65.
= 1.7, JH,H = 8.5 Hz, 2 H), 7.38 (d, JH,H = 16.3 Hz, 2 H), 7.21
3
3
(d, JH,H = 6.2 Hz, 4 H), 7.09 (d, JH,H = 16.2 Hz, 2 H), 2.51 (s, 6
H, CH3) ppm. 13C NMR (101 MHz, CDCl3, 25 °C): δ = 194.2,
150.2, 144.3, 141.0, 138.7, 134.6, 134.3, 133.3, 132.8, 131.8, 130.9,
129.0, 128.6, 125.5, 128.3, 128.1, 127.8, 126.0, 120.9, 30.4 ppm. MS
(MALDI-TOF): found m/z = 684.07, 684.19 calculated for
C44H32N2O2S2. UV/Vis: λmax = 316 nm. GPC (oligopore 6 µm, tol-
uene, UV/Vis photodiode array detector) area 99.1%.
6-[2-(Trimethylsilyl)ethylthio]naphthalen-2-ylboronic Acid (16): Bro-
mide 15 (1.12 g, 3.29 mmol) was dissolved in THF (15 mL) in a
25 mL Schlenk flask. nBuLi (1.6  in hexane, 3.50 mL, 5.60 mmol)
was added at –78 °C and it was stirred for 25 min at this tempera-
ture. Then triisopropyl borate (3.50 mL, 15.2 mmol) was added.
The reaction mixture was stirred for 1.7 h at –78 °C, 2 h at –20 °C
and 3 h at room temperature. The reaction mixture was then
quenched with water and extracted with TBME. The aqueous
phase was washed with TBME (3ϫ50 mL). The combined organic
phases were washed with brine (2ϫ50 mL), dried with MgSO4 and
the solvents evaporated. The crude was recrystallized from hexane
to give boronic acid 16 as a beige solid (757 mg, 2.49 mmol, 76%);
2,5-Bis{6-[2-(trimethylsilyl)ethylthio]naphthalen-2-yl}-p-xylene (21):
2,5-Diiodo-p-xylene (91.0 mg, 0.254 mmol) and boronic acid 16
(170 mg, 0.557 mmol) were dissolved in toluene (6 mL) and ethanol
(1.5 mL). The mixture was argon-saturated before Pd(PPh3)4
(66 mg, 0.057 mmol) and K2CO3 (285 mg, 2.06 mmol) was added.
m.p. 169.8–172.7 °C. 1H NMR (400 MHz, CDCl3, 25 °C): δ = 8.72 The reaction mixture was then stirred for 15 h at 80 °C. The insolu-
(s, 1 H), 8.24 (d, JH,H = 8.2 Hz, 1 H), 7.93 (d, JH,H = 8.6 Hz, 1 ble material was filtered off and washed with toluene. The filtrate
3
3
3
3
H), 7.83 (d, JH,H = 8.2 Hz, 1 H), 7.67 (s, 1 H), 7.43 (d, JH,H
=
was evaporated and purified by column chromatography (silica gel,
842
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 833–845

Models for Coordination-Induced Single-Molecule Switches
3ϫ12 cm, CH2Cl2/hexane = 1:1) to give 21 as a colourless solid 0.0480 mmol, 36%). TLC Rf = 0.41 (CH2Cl2/hexane = 1:1); m.p.
(158.3 mg, 0.254 mmol, 100 %); m.p. 138.0–141.0 °C. TLC Rf = 237–239 °C followed by decomp. 1H NMR (400 MHz, CDCl3,
1
3
0.49 (CH2Cl2/hexane = 1:1). H NMR (400 MHz, CDCl3, 25 °C):
25 °C): δ = 7.76 (s, 2 H), 7.55 (s, 8 H), 7.40 (d, JH,H = 7.3 Hz, 4
δ = 7.82–7.77 (m, 8 H), 7.54 (d, 3JH,H = 8.4 Hz, 2 H), 7.46 (d, 3JH,H H), 7.32 (t, JH,H = 7.5 Hz, 4 H), 7.24 (t, JH,H = 7.2 Hz, 2 H),
3
3
= 8.6 Hz, 2 H), 7.28 (s, 2 H), 3.10 (m, 4 H, CH2), 2.35 (s, 6 H, 7.12 (m, 4 H), 2.49 (s, 6 H, CH3) ppm. 13C NMR (101 MHz,
Me), 1.01 (m, 4 H, CH2), 0.08 (s, 18 H, SiMe3) ppm. 13C NMR
(101 MHz, CDCl3, 25 °C): δ = 140.7, 138.9, 134.8, 132.9, 132.6,
132.1, 131.5, 128.4, 127.66, 127.65, 126.6, 126.3, 29.5, 20.0, 16.9,
–1.7 ppm. MS (EI): m/z (%) = 623.3 (26), 622.3 (49) [M+], 566.2
(37), 73.1 (100).
CDCl3, 25 °C): δ = 194.0, 141.6, 139.7, 137.2, 134.7, 134.2, 130.7,
130.3, 128.7, 127.8, 127.7, 127.0, 126.6, 126.4, 30.3 ppm. MS (EI):
m/z (%) = 582.2 (100) [M+], 540.2 (96), 498.1 (38). UV/Vis: λmax
=
295, 347 nm. GPC (oligopore 6 µm, toluene, UV/Vis photodiode
array detector) area 99.2%.
2,5-Bis(6-thioacetylnaphthalen-2-yl)-p-xylene (22): The starting ma-
terial 21 (104 mg, 0.167 mmol) was dissolved in THF (8 mL). The
solution was degassed, before TBAF (1  in THF, 1.70 mL,
1.70 mmol) was added. After stirring for 45 min at room tempera-
ture more TBAF was added (1  in THF, 1.00 mL, 1.00 mmol). It
was stirred for 2 h, then more TBAF (1  in THF, 0.60 mL,
0.60 mmol) was added and it was stirred for another 30 min. Then
acetyl chloride (0.5 mL, 7.04 mmol) was added dropwise. The reac-
tion mixture was poured onto ice and extracted with CH2Cl2. The
aqueous phase was washed with CH2Cl2 (2ϫ50 mL). The com-
bined organic phase was washed with brine, dried with MgSO4 and
the solvents evaporated. The crude was purified by column
chromatography (silica gel, 2ϫ12 cm, CH2Cl2/hexane = 1:1) to
give the acetylated rod 22 as a colourless solid (39.0 mg,
0.0770 mmol, 46%); m.p. 197.6–200.2 °C. TLC Rf = 0.55 (CH2Cl2).
1,4-Bis{6-[2-(trimethylsilyl)ethylthio]naphthalen-2-yl}-2,5-bis[(E)-2-
phenylethenyl]benzene (26): Iodide 23 (91.0 mg, 0.170 mmol) and
boronic acid 16 (107 mg, 0.352 mmol) were dissolved in toluene
(8 mL) and ethanol (2 mL). The mixture was degassed, before
Pd(PPh3)4 (25 mg, 0.0217 mmol) and K2CO3 (188 mg, 1.36 mmol)
were added. The reaction mixture was exposed to microwaves
(500 W, 85 °C) for 3 h and then evaporated, absorbed on silica gel
and chromatographed (silica gel, 2ϫ12 cm, hexane/CH2Cl2 = 5:1
to 1:1) to give 26 as a colourless solid (73.0 mg, 0.0913 mmol,
54%). TLC Rf = 0.60 (CH2Cl2/hexane = 1:1). 1H NMR (400 MHz,
CDCl3, 25 °C): δ = 7.94 (s, 2 H), 7.87–7.83 (m, 6 H), 7.80 (s, 2 H),
2
3
2
7.64 (dd, JH,H = 1.7, JH,H = 8.4 Hz, 2 H), 7.48 (dd, JH,H = 1.8,
3
3
3JH,H = 8.5 Hz, 2 H), 7.35 (d, JH,H = 7.2 Hz, 4 H), 7.27 (t, JH,H
= 7.4 Hz, 4 H), 7.24–7.20 (m, 4 H), 7.15 (d, 3JH,H = 16.2 Hz, 2 H),
3.13 (m, 4 H, CH2), 1.04 (m, 4 H, CH2), 0.10 (s, 18 H, SiMe3) ppm.
1H NMR (400 MHz, CDCl3, 25 °C): δ = 8.01 (s, 2 H), 7.93–7.89 13C NMR (101 MHz, CDCl3, 25 °C): δ = 140.4, 137.7, 137.4, 135.4,
3
(m, 4 H), 7.87 (s, 2 H), 7.59 (dd, 2JH,H = 1.4, JH,H = 8.4 Hz, 2 H), 134.8, 132.9, 131.5, 129.8, 128.9, 128.6, 128.5, 128.4, 127.8, 127.7,
2
3
7.51 (dd, JH,H = 1.5, JH,H = 8.5 Hz, 2 H), 7.29 (s, 2 H), 2.49 (s,
6 H, CH3), 2.34 (s, 6 H, Me) ppm. 13C NMR (101 MHz, CDCl3,
25 °C): δ = 194.4, 140.7, 140.6, 134.2, 133.3, 132.9, 132.4, 132.1,
131.3, 128.9, 128.5, 127.7, 121.7, 125.2, 30.3, 20.0 ppm. MS (EI):
m/z (%) = 506.1 (31) [M+], 464.1 (35), 422.2 (100). UV/Vis: λmax
231, 261, 301 nm. GPC (oligopore 6 µm, toluene, UV/Vis photodi-
ode array detector) area 99.0%.
127.6, 127.0, 126.8, 126.6, 125.9, 29.3, 16.8, –1.7 ppm. MS (EI):
m/z (%) = 798.3 (100) [M+].
1,4-Bis(6-thioacetylnaphthalen-2-yl)-2,5-bis[(E)-2-phenylethenyl]-
benzene (27): The ethyl-TMS-protected cruciform 26 (55 mg,
0.070 mmol) was dissolved in 10 mL of THF. It was degassed for
10 min before TBAF (1  in THF, 2.00 mL, 2.00 mmol) was added
at room temperature. The reaction mixture was stirred for 55 min,
1,4-Bis{4-[2-(trimethylsilyl)ethylthio]phenyl}-2,5-bis[(E)-2-phenyl- then acetyl chloride (0.40 mL, 4.61 mmol) was added at 0 °C. The
ethenyl]benzene (24): Iodide 23 (116 mg, 0.217 mmol) and boronic
acid 8 (121 mg, 0.476 mmol) were dissolved in toluene (8 mL) and
ethanol (2 mL). The mixture was degassed, before Pd(PPh3)4
(20 mg, 0.0173 mmol) and K2CO3 (239 mg, 1.74 mmol) were
added. The reaction mixture was exposed to microwaves (500 W,
85 °C) for 4 h. The reaction mixture was then evaporated, absorbed
on silica and chromatographed (silica gel, 2ϫ12 cm, CH2Cl2/hex-
ane = 1:4 to 1:2) to give 24 as a colourless solid (110 mg,
mixture was stirred for 40 min. Then 5 mL of ethanol was added
and it was extracted with water and DCM. The aqueous phase was
washed twice with DCM. The combined organic phases were
washed with water, dried with MgSO4, evaporated and purified by
column chromatography (silica, 2ϫ12 cm, CH2Cl2/hexane = 1:3 to
1:1) to give the product 27 as a colourless solid (9.00 mg,
0.0132 mmol, 19%). TLC Rf = 0.37 (CH2Cl2/hexane = 1:1); m.p.
301–304 °C. 1H NMR (400 MHz, CDCl3, 25 °C): δ = 8.05 (s, 2 H),
2
0.157 mmol, 73 %). TLC Rf = 0.61 (CH2Cl2/hexane = 1:1). 1H 8.01 (s, 2 H), 7.96 (m, 4 H), 7.86 (s, 2 H), 7.68 (dd, JH,H = 1.7,
NMR (400 MHz, CDCl3, 25 °C): δ = 7.42 (m, 12 H), 7.31 (t, 3JH,H
3JH,H = 8.5 Hz, 2 H), 7.53 (dd, JH,H = 1.7, JH,H = 8.5 Hz, 2 H),
2
3
3
3
3
= 7.5 Hz, 4 H), 7.23 (t, JH,H = 7.2 Hz, 2 H), 7.16 (d, JH,H
=
7.35 (d, JH,H = 7.2 Hz, 4 H), 7.28 (m, 4 H), 7.21 (m, 2 H), 7.17
16.3 Hz, 2 H), 7.08 (d, JH,H = 16.2 Hz, 2 H), 3.06 (m, 4 H, CH2), (s, 4 H), 2.51 (s, 6 H, CH3) ppm. 13C NMR (126 MHz, CDCl3,
3
1.02 (m, 4 H, CH2), 0.09 (s, 18 H, SiMe3) ppm. 13C NMR 25 °C): δ = 194.3, 140.3, 139.5, 137.3, 134.8, 134.3, 133.3, 132.7,
(101 MHz, CDCl3, 25 °C): δ = 139.8, 137.7, 137.4, 136.7, 134.6,
130.3, 129.7, 128.7, 128.2, 127.62, 127.57, 127.0, 126.6, 29.3, 16.9,
–1.7 ppm. MS (EI): m/z (%) = 698.3 (95) [M+], 73.0 (100).
131.5, 130.1, 129.1, 129.0, 128.7, 128.5, 127.9, 127.7, 126.6 (6 C),
125.6 ppm. MS (EI): m/z (%) = 682.2 (7) [M+], 640.2 (41), 598.2
(100). UV/Vis: λmax 317, 354 nm. GPC (oligopore 6 µm, toluene,
UV/Vis photodiode array detector) area 99.6%.
1,4-Bis(4-thioacetylphenyl)-2,5-bis[(E)-2-phenylethenyl]benzene (25):
Compound 24 (92.0 mg, 0.132 mmol) was dissolved in THF Supporting Information (see also the footnote on the first page of
(10 mL). The mixture was degassed, before TBAF (1  in THF,
this article): The synthesis of compound 11 as well as the 1H NMR
3.00 mL, 3.00 mmol) was added. After stirring for 55 min at room and 13C NMR spectra for compounds 3, 4, 8, 9, 11–22, 24–27 are
temperature, acetyl chloride (0.500 mL, 7.04 mmol) was added at
0 °C. The reaction mixture was stirred for 40 min at room temp.
and then quenched with water at 0 °C. The mixture was then ex-
tracted with CH2Cl2. The aqueous phase was washed with CH2Cl2.
The combined organic phase was dried with MgSO4, evaporated
given.
Acknowledgments
and chromatographed (silica gel, 2ϫ12 cm, CH2Cl2/hexane = 1:1) Financial support by the Swiss National Science Foundation
to give the cruciform 25 as a colourless solid (28.0 mg, (SNSF), the National Center of Competence in Research “Nano-
Eur. J. Org. Chem. 2010, 833–845
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
843

S. Grunder, R. Huber, S. Wu, C. Schönenberger, M. Calame, M. Mayor
FULL PAPER
[34]
[35]
D. S. Seferos, S. A. Trammell, G. C. Bazan, J. G. Kushmerick,
Proc. Natl. Acad. Sci. USA 2005, 102, 8821.
J. G. Kushmerick, D. B. Holt, S. K. Pollack, M. A. Ratner, J. C.
Yang, T. L. Schull, J. Naciri, M. H. Moore, R. Shashidhar, J.
Am. Chem. Soc. 2002, 124, 10654.
L. T. Cai, H. Skulason, J. G. Kushmerick, S. K. Pollack, J. Na-
ciri, R. Shashidhar, D. L. Allara, T. E. Mallouk, T. S. Mayer,
J. Phys. Chem. B 2004, 108, 2827.
J. Liao, M. A. Mangold, S. Grunder, M. Mayor, C. Schönen-
berger, M. Calame, New J. Phys. 2008, 10, 065019.
E. Lörtscher, M. Elbing, M. Tschudy, C. von Hänisch, H. B.
Weber, M. Mayor, H. Riel, ChemPhysChem 2008, 9, 2252.
J. G. Kushmerick, C. M. Whitaker, S. K. Pollack, T. L. Schull,
R. Shashidhar, Nanotechnology 2004, 15, S489.
M. Elbing, R. Ochs, M. Koentopp, M. Fischer, C.
von Hänisch, F. Weigend, F. Evers, H. Weber, M. Mayor, Proc.
Natl. Acad. Sci. USA 2005, 102, 8815.
D. Dulic, S. J. van der Molen, T. Kudernac, H. T. Jonkman,
J. J. D. de Jong, T. N. Bowden, J. van Esch, B. L. Feringa, B. J.
van Wees, Phys. Rev. Lett. 2003, 91, 207402/1.
D. I. Gittins, D. Bethell, D. J. Schiffrin, R. J. Nichols, Nature
2000, 408, 67.
W. Haiss, H. Van Zalinge, S. J. Higgins, D. Bethell, H. Hoeben-
reich, D. J. Schiffrin, R. J. Nichols, J. Am. Chem. Soc. 2003,
125, 15294.
Z. Li, B. Han, G. Meszaros, I. Pobelov, T. Wandlowski, A.
Błaszczyk, M. Mayor, J. Chem. Soc. Faraday Trans. 2006, 131,
121.
B. Han, Z. Li, T. Wandlowski, A. Błaszczyk, M. Mayor, J.
Phys. Chem. C 2007, 111, 13855.
Z. Li, I. Pobelov, B. Han, T. Wandlowski, A. Błaszczyk, M.
Mayor, Nanotechnology 2007, 18, 044018/1.
X. Xiao, D. Brune, J. He, S. Lindsay, C. B. Gorman, N. Tao,
Chem. Phys. 2006, 326, 138.
F. Chen, N. J. Tao, Acc. Chem. Res. 2009, 42, 429.
H. Yu, Y. Luo, K. Beverly, J. F. Stoddart, H.-R. Tseng, R.
Heath James, Angew. Chem. Int. Ed. 2003, 42, 5706.
E. Lörtscher, J. W. Ciszek, J. Tour, H. Riel, Small 2006, 2, 973.
V. Ferri, M. Elbing, G. Pace, M. D. Dickey, M. Zharnikov, P.
Samorì, M. Mayor, M. A. Rampi, Angew. Chem. Int. Ed. 2008,
47, 3407.
M. Elbing, A. Błaszczyk, C. von Hänisch, M. Mayor, V. Ferri,
C. Grave, M. A. Rampi, G. Pace, P. Samorì, A. Shaporenko,
M. Zharnikov, Adv. Funct. Mater. 2008, 18, 2972.
S. Grunder, R. Huber, V. Horhoiu, M. T. González, C.
Schönenberger, M. Calame, M. Mayor, J. Org. Chem. 2007, 72,
8337.
scale Science” and the Gebert Rüf Foundation is gratefully ac-
knowledged.
[1] H. Kuhn, Struct. Chem. Mol. Biol. 1968, 566.
[2] A. Aviram, M. A. Ratner, Chem. Phys. Lett. 1974, 29, 277.
[3] R. M. Metzger, Acc. Chem. Res. 1999, 32, 950.
[4] M.-K. Ng, L. Yu, Angew. Chem. Int. Ed. 2002, 41, 3598.
[5] J. G. Kushmerick, D. B. Holt, J. C. Yang, J. Naciri, M. H.
Moore, R. Shashidhar, Phys. Rev. Lett. 2002, 89, 086802.
[6] R. E. Holmlin, R. F. Ismagilov, R. Haag, V. Mujica, M. A.
Ratner, M. A. Rampi, G. M. Whitesides, Angew. Chem. Int. Ed.
2001, 40, 2316.
[7] J. M. Mativetsky, G. Pace, M. Elbing, M. A. Rampi, M. Mayor,
P. Samorí, J. Am. Chem. Soc. 2008, 130, 9192.
[8] G. Pace, V. Ferri, C. Grave, M. Elbing, C. von Hänisch, M.
Zharnikov, M. Mayor, M. A. Rampi, P. Samorí, Proc. Natl.
Acad. Sci. USA 2007, 104, 9937.
[9] J. Chen, M. A. Reed, A. M. Rawlett, J. M. Tour, Science
(Washington DC) 1999, 286, 1550.
[10] A. Wassel Ronald, B. Gorman Christopher, Angew. Chem. Int.
Ed. 2004, 43, 5120.
[11] M. Mayor, H. B. Weber, Chimia 2003, 56, 494.
[12] C. Joachim, J. K. Gimzewski, A. Aviram, Nature 2000, 408,
541.
[13] N. J. Tao, Nat. Nanotechnol. 2006, 1, 173.
[14] N. Weibel, S. Grunder, M. Mayor, Org. Biomol. Chem. 2007, 5,
2343.
[15] L. A. Bumm, J. J. Arnold, M. T. Cygan, T. D. Dunbar, T. P.
Burgin, L. Jones II, D. L. Allara, J. M. Tour, P. S. Weiss, Sci-
ence (Washington DC) 1996, 271, 1705.
[16] X. D. Cui, A. Primak, X. Zarate, J. Tomfohr, O. F. Sankey,
A. L. Moore, T. A. Moore, D. Gust, G. Harris, S. M. Lindsay,
Science (Washington DC) 2001, 294, 571.
[17] B. Xu, N. J. Tao, Science (Washington DC) 2003, 301, 1221.
[18] L. Venkataraman, J. E. Klare, I. W. Tam, C. Nuckolls, M. S.
Hybertsen, M. L. Steigerwald, Nano Lett. 2006, 6, 458.
[19] L. Grüter, M. T. González, R. Huber, M. Calame, C. Schönen-
berger, Small 2005, 1, 1067.
[20] J. Reichert, R. Ochs, D. Beckmann, H. B. Weber, M. Mayor,
H. von Lohneysen, Phys. Rev. Lett. 2002, 88, 176804/1.
[21] M. A. Reed, C. Zhou, C. J. Muller, T. P. Burgin, J. M. Tour,
Science (Washington DC) 1997, 278, 252.
[22] C. Kergueris, J. P. Bourgoin, S. Palacin, D. Esteve, C. Urbina,
M. Magoga, C. Joachim, Phys. Rev. B 1999, 59, 12505.
[23] R. H. M. Smit, Y. Noat, C. Untiedt, N. D. Lang, M. C.
van Hemert, J. M. van Ruitenbeek, Nature 2002, 419, 906.
[24] C. A. Martin, D. Ding, J. K. Sørensen, T. Bjørnholm, J. M.
van Ruitenbeek, H. S. J. van der Zant, J. Am. Chem. Soc. 2008,
130, 13198.
[25] J. Park, A. N. Pasupathy, J. I. Goldsmith, C. Chang, Y. Yaish,
J. R. Petta, M. Rinkoski, J. P. Sethna, H. D. Abruna, P. L.
McEuen, D. C. Ralph, Nature 2002, 417, 722.
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[49]
[50]
[51]
[52]
[53]
[54]
[55]
[56]
[57]
[58]
A. Błaszczyk, M. Fischer, C. von Hänisch, M. Mayor, Eur. J.
Org. Chem. 2007, 2630.
A. Błaszczyk, M. Chadim, C. von Hänisch, M. Mayor, Eur. J.
Org. Chem. 2006, 3809.
N. Weibel, A. Mishchenko, T. Wandlowski, M. Neuburger, Y.
Leroux, M. Mayor, Eur. J. Org. Chem. 2009, 6140.
X. Li, J. Hihath, F. Chen, T. Masuda, L. Zang, N. J. Tao, J.
Am. Chem. Soc. 2007, 129, 11535.
D. Mayer, T. Dretschkow, K. Ataka, T. Wandlowski, J. Elec-
troanal. Chem. 2002, 524–525, 20.
T. Wandlowski, K. Ataka, D. Mayer, Langmuir 2002, 18, 4331.
B. Q. Xu, X. L. Li, X. Y. Xiao, H. Sakaguchi, N. J. Tao, Nano
Lett. 2005, 5, 1491.
J. E. Klare, G. S. Tulevski, K. Sugo, A. de Picciotto, K. A.
White, C. Nuckolls, J. Am. Chem. Soc. 2003, 125, 6030.
H. Kang, P. Zhu, Y. Yang, A. Facchetti, T. J. Marks, J. Am.
Chem. Soc. 2004, 126, 15974.
[26] Z. M. Wu, M. Steinacher, R. Huber, M. Calame, S. J.
van der Molen, C. Schönenberger, Appl. Phys. Lett. 2007, 91,
053118/1.
[27] E. A. Osorio, K. O’Neill, M. Wegewijs, N. Stuhr-Hansen, J.
Paaske, T. Bjørnholm, H. S. J. van der Zant, Nano Lett. 2007,
7, 3336.
[28] F. Chen, X. Li, J. Hihath, Z. Huang, N. Tao, J. Am. Chem.
Soc. 2006, 128, 15874.
[29] X. Li, J. He, J. Hihath, B. Xu, S. M. Lindsay, N. Tao, J. Am.
Chem. Soc. 2006, 128, 2135.
[30] W. Wang, T. Lee, M. A. Reed, Phys. Rev. B 2003, 68, 035416.
[31] W. Wang, T. Lee, M. A. Reed, J. Phys. Chem. B 2004, 108,
18398.
[32] M. T. González, S. Wu, R. Huber, S. J. van der Molen, C.
Schönenberger, M. Calame, Nano Lett. 2006, 6, 2238.
[33] R. Huber, M. T. González, S. Wu, M. Langer, S. Grunder, V.
Horhoiu, M. Mayor, M. R. Bryce, C. Wang, R. Jitchati, C.
Schönenberger, M. Calame, J. Am. Chem. Soc. 2008, 130, 1080.
[59]
[60]
[61]
[62]
[63]
[64]
[65]
H. Kang, G. Evmenenko, P. Dutta, K. Clays, K. Song, T. J.
Marks, J. Am. Chem. Soc. 2006, 128, 6194.
J. N. Wilson, M. Josowicz, Y. Wang, U. H. F. Bunz, Chem.
Commun. 2003, 2962.
J. N. Wilson, U. H. F. Bunz, J. Am. Chem. Soc. 2005, 127, 4124.
844
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 833–845

Models for Coordination-Induced Single-Molecule Switches
[66] M. Hauck, J. Schoenhaber, A. J. Zucchero, K. I. Hardcastle,
T. J. J. Mueller, U. H. F. Bunz, J. Org. Chem. 2007, 72, 6714.
[67] J. Tolosa, A. J. Zucchero, U. H. F. Bunz, J. Am. Chem. Soc.
2008, 130, 6498.
[68] P. L. McGrier, K. M. Solntsev, J. Schoenhaber, S. M. Brom-
bosz, L. M. Tolbert, U. H. F. Bunz, Chem. Commun. 2007,
2127.
[74] K. Freudenberg, D. Peters, Ber. Dtsch. Chem. Ges. [Abteilung]
B: Abhandlungen 1919, 52, 1463.
[75] M. S. Newman, H. A. Karnes, J. Org. Chem. 1966, 31, 3980.
[76] H. Kwart, E. R. Evans, J. Org. Chem. 1966, 31, 410.
[77] G. C. Lloyd-Jones, J. D. Moseley, J. S. Renny, Synthesis 2008,
661.
[78] F. Beaulieu, V. Snieckus, Synthesis 1992, 112.
[79] H. Detert, O. Sadovski, E. Sugiono, J. Phys. Org. Chem. 2004,
17, 1046.
[69] A. J. Zucchero, J. N. Wilson, U. H. F. Bunz, J. Am. Chem. Soc.
2006, 128, 11872.
[70] W. W. Gerhardt, A. J. Zucchero, J. N. Wilson, C. R. South,
U. H. F. Bunz, M. Weck, Chem. Commun. 2006, 2141.
[71] M. Mayor, B. Weber Heiko, J. Reichert, M. Elbing, C.
Von Hänisch, D. Beckmann, M. Fischer, Angew. Chem. Int. Ed.
2003, 42, 5834.
[72] C. J. Yu, Y. Chong, J. F. Kayyem, M. Gozin, J. Org. Chem.
1999, 64, 2070.
[80] Note that, for compound 1, no molecular signature was ob-
served in such experiments. This probably stems from the fact
that this cross has a conductance located below the detection
limit of the setup due to the meta position of the anchor
groups. Compound 2 provided a clear signal with a well-de-
fined single peak in the log(G) histogram (see ref.[53]).
[81] A. Brikh, C. Morin, J. Organomet. Chem. 1999, 581, 82.
Received: October 12, 2009
[73] J. N. Wilson, P. M. Windscheif, U. Evans, M. L. Myrick,
U. H. F. Bunz, Macromolecules 2002, 35, 8681.
Published Online: January 4, 2010
Eur. J. Org. Chem. 2010, 833–845
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
845