Mechanical Fixation by Porphyrin Connection: Synthesis and Transport Studies of a Bicyclic Dimer

Article abstract of DOI:10.1021/acs.joc.9b02327

The bowl-shaped, 3-fold interlinked porphyrin dimer 2 was obtained in respectable yields during macrocyclization attempts. Its bicyclic structure, consisting of a macrocycle made of a pair of acetylene interlinked tetraphenylporphyrins which are additionally linked by a C-C bond interlinking two pyrrole subunits, has been confirmed spectroscopically (2D-NMR, UV/vis, HR-MALDI-ToF MS). Late-stage functionalization provided the structural analogue 1 with acetyl-protected terminal thiol anchor groups enabling single molecule transport investigations in a mechanically controlled break junction experiment. The formation of single-molecule junctions was observed, displaying large variations in the observed conductance values pointing at a rich diversity in the molecular junctions.

Full text of DOI:10.1021/acs.joc.9b02327

Article
pubs.acs.org/joc
Cite This: J. Org. Chem. XXXX, XXX, XXX−XXX
Mechanical Fixation by Porphyrin Connection: Synthesis and
Transport Studies of a Bicyclic Dimer
†
†
†
‡
†
̌
̈
Patrick Zwick, Kevin J. Weiland, Juraj Malincík, Davide Stefani, Daniel Haussinger,
Herre S. J. van der Zant,‡ Diana Dulic, and Marcel Mayor*,†,∥,⊥
§
́
†Department of Chemistry, University of Basel, St. Johanns-Ring 19, 4056 Basel, Switzerland
‡Kavli Institute of Nanoscience, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands
§Department of Physics and Department of Electrical Engineering, Faculty of Physicaland Mathematical Sciences, University of
Chile, Avenida Blanco Encalada 2008, Santiago 8330015, Chile
∥Institute for Nanotechnology (INT), Karlsruhe Institute of Technology (KIT), P.O. Box 3640, 76021 Karlsruhe, Germany
⊥Lehn Institute of Functional Materials (LIFM), School of Chemistry, Sun Yat-Sen University (SYSU), 510275 Guangzhou, China
S
* Supporting Information
ABSTRACT: The bowl-shaped, 3-fold interlinked porphyrin dimer 2 was obtained
in respectable yields during macrocyclization attempts. Its bicyclic structure,
consisting of a macrocycle made of a pair of acetylene interlinked tetraphenylpor-
phyrins which are additionally linked by a C−C bond interlinking two pyrrole
subunits, has been confirmed spectroscopically (2D-NMR, UV/vis, HR-MALDI-ToF
MS). Late-stage functionalization provided the structural analogue 1 with acetyl-
protected terminal thiol anchor groups enabling single molecule transport
investigations in a mechanically controlled break junction experiment. The formation
of single-molecule junctions was observed, displaying large variations in the observed
conductance values pointing at a rich diversity in the molecular junctions.
junctions with mechanosensitive transport features10−12 or
INTRODUCTION
■
redox properties13 were reported. The variation of the single-
Molecules are the smallest building blocks still providing the
molecule transport features was either due to a mechanically
structural diversity and integrity required to integrate a
triggered alteration of the spin state14 or the manipulation of
particular function by the spatial arrangement of their atoms.
the spatial conformation of two neighboring π-systems in close
In combination with the skills of chemistry in molecular design
proximity. The latter has been rationalized by quantum
and synthesis making the targeted structures available in
interference phenomena and was observed in bimolecular
enormous numbers of identical copies, they became the object
junctions due to π-stacking15 as well as in covalently
of interest as potential functional subunit in electronics. While
interlinked π-systems like [2.2]paracyclophanes.16
the visionary concept was suggested almost 5 decades ago by
Porphyrins play a key role in nature’s machinery converting
the pioneering paper of Aviram and Ratner, hypothesizing the
sun light into chemical energy,17 and their adjustable electronic
rectifying behavior of a virtual single molecule junction,1
and optical properties18−20 make particularly appealing
mainly the developments in experimental physics enabled the
building blocks in molecular electronics and molecular devices.
realization of single molecule junctions.2 The increasing
Consequently, electronic transport studies of model com-
experimental skills moved the focus of interest from theoretical
pounds based on porphyrin subunits have been reported; e.g.,
molecular wires,21,22 tapes,23 and even the anchoring groups
hypotheses to electric circuits comprising single molecules.
for gold electrodes have been altered.24 Recently, the variations
The parallel blossom of the three required scientific disciplines
(synthetic chemistry,3 experimental physics,4 and theoretical
of transport paths in single porphyrin junctions recorded in a
simulation5) enables the structure−property relationships
mechanically controlled break junction (MCBJ) device were
required to evaluate both the potential and the limitation of
systematically analyzed by an unsupervised clustering algo-
molecular electronics.6
rithm,25 and the study revealed the potential of porphyrin
While fundamental transport−property correlations corro-
borated the single-molecule nature of the junction7 and first
electronic functions like rectifying8 or switching9 were
successfully implemented, we became recently interested in
mechanosensitive molecular junctions. First, single-molecule
Special Issue: Functional Organic Materials
Received: August 27, 2019
Published: November 5, 2019
© XXXX American Chemical Society
A
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Figure 1. (a) Porphyrin cyclophane T1 as initial target structure. (b) Hypothesized variation of the spatial arrangement of both porphyrins
triggered by the electrode displacement. In the oligophenylenethinylen macrocycle resembling a parallelogram, the increase in the distance of both
anchor groups translates in an increased distance of both porphyrin planes together with their lateral shift. As guide for the eye, the initial yellow
arrow and the touching points of the distance arrows are displayed in the bottom sketch of Figure 1b).
derivatives as well behaved and reproducible model com-
pounds for single molecule junctions.
touching points of the distance arrows are displayed in the
lower porphyrin plane).
In this work, we report our synthetic efforts toward the
macrocyclic porphyrin cyclophane T1, which we were not able
to isolate so far. However, in our attempts to form the
macrocycle of T1, we observed the formation of an unexpected
additional carbon−carbon bond between both porphyrin
planes, resulting in the 3-fold interbridged porphyrin
architecture 2, as a precursor of the anchor group decorated
bowl-shaped porphyrin dimer 1 (see Figure 2). The interesting
bicyclic porphyrin architecture 2 was isolated, and its structural
identity was corroborated by means of 2-dimensional nuclear
magnetic resonance spectroscopy (2D-NMR), high-resolution
mass spectrometry (HR-MS), and UV−vis spectroscopy.
Additionally, 2 was investigated with computational methods
at the B3LYP/LanL2DZ level of theory (for details regarding
the calculations, see the Supporting Information). The
calculation supports the bowl shape of the molecule. The
strain introduced by additional β−β linkage is accommodated
by bending of the porphyrins from their usual planar
conformation (additional information, e.g., calculated distances
and angles can be found in Figure S36).
Motivated by these promising results, our interest moved
toward macrocyclic architectures comprising porphyrins, with
the intention to manipulate the interaction between the
porphyrins in a junction mechanically. The first target structure
along these lines was the porphyrin cyclophane T1 displayed in
Figure 1a. The macrocyclic phenyl ethynylene framework
resembles a parallelogram with two zinc(II)porphyrins facing
each other in an offset cofacial orientation. Both porphyrin
levels of the macrocycle are decorated with an acetyl masked
thiol anchor group, with the intention to immobilize the
macrocycle in a MCBJ experiment with covalent S−Au bonds.
The chemically stable fixation of the structure inside the
electrodes’ gap is crucial to enable the mechanical manipu-
lation of the molecular junction. Upon immobilization, the
spatial arrangement of both cofacially arranged porphyrin
subunits, and thus also the fraction of the transport current
arising from the spatial proximity of both porphyrin systems,
are expected to depend on the electrode spacing. As sketched
in Figure 1b, an increase of the electrode spacing by Δd (green
arrow) should not only result in an increased spacing between
both porphyrin levels (difference between red and yellow
arrow) but also in the relative displacement of both porphyrin
subunits with respect to each other (as a guide for the eye the
The fixation by an additional C−C bond between both
porphyrin subunits in 1 was expected to handicap its
mechanosensitivity. It remained, however, an interesting
model compound to investigate the transport features and,
B
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RESULTS AND DISCUSSION
■
Retrosynthesis. The synthetic plan toward T1 is displayed
in Scheme 1, with the macrocyclization of two molecules of 3
in a 2-fold Sonogashira−Hagihara reaction as key step. The
macrocyclization was expected to be particularly challenging.
The search for suitable (pseudo) high dilution conditions
favoring macrocyclization over oligo- and polymerization
became challenging with palladium and copper catalysts
required to activate both functional groups engaged in the
coupling reaction. After successful cyclization, late-stage
liberation of the peripheral acetylenes and subsequent
decoration with subunits exposing the required anchor groups
was expected to provide the target porphyrin cyclophane T1.
The highly functionalized porphyrin monomer 3, exposing
with acetylene and bromine both functional groups for the
macrocyclization, was divided into literature known 2,2′-((4-
(tert-butyl)phenyl)methylene)bis(1H-pyrrole),27 commercially
available 2-bromobenzaldehyde and 4, which itself should be
available by a sequence of Sonogashira−Hagihara cross-
coupling reactions profiting from the variation in reactivity of
halide substituents from commercially available 4-bromo-3-
iodobenzaldehyde.
Synthesis. The diacetylene benzaldehyde 4 was found to
be easily accessible over two subsequent Sonogashira−
Hagihara cross-coupling reactions from 4-bromo-3-iodoben-
zaldehyde. First, the more reactive iodine was substituted by
trimethylsilyl (TMS) acetylene at room temperature, providing
5 in 85%. At an elevated temperature of 80 °C, the bromine
was substituted by triisopropylsilyl (TIPS) acetylene, providing
the benzaldehyde building block 4 with an overall yield of 69%
over both coupling steps on a 20 g scale.
The porphyrin 6 was condensed from 2,2′-((4-(tert-
butyl)phenyl)methylene)bis(1H-pyrrole),28 2-bromobenzalde-
hyde, and 4 (see Scheme 2). Trifluoroborane diethyl etherate
was used as a Lewis acid in a 10 mM solution of the starting
Figure 2. (a) Structures of the 3-fold interlinked polycyclic porphyrin
architectures 1 and 2. (b) Calculated (geometry optimized by
B3LYP/LanL2DZ)26 ball-and-stick model of the porphyrin bicyclic
dimer 2 displaying the bowl-shaped arrangement of the porphyrin
architecture.
thus, also the suitability of complex multilevel porphyrin
architectures for MCBJ experiments. Hence, the transport
properties of the bowl-shaped porphyrin bicycle 1 were
analyzed by immobilization in MCBJ experiments, applying
both fast- and self-breaking techniques.
Scheme 1. Retrosynthetic Strategy to the Porphyrin Cyclophane T1 over Key Intermediates T2, 3, and 4
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a
Scheme 2
a
Key: (i) TMS-CCH, Pd(PPh3)2Cl2, CuI, piperidine/THF (1:3), rt, 3 h; (ii) TIPS-CCH, Pd(PPh3)2Cl2, CuI, piperidine/C6H5CH3 (1:3), 80 °C, 2
h; (iii) (1) BF3·OEt2, CH2Cl2, −10 °C, 15 min, (2) DDQ, CH2Cl2, rt, 30 min; (iv) Zn(OAc)2, CH2Cl2/CH3OH (1:1), rt, 1 h; (v) K2CO3,
CH2Cl2/CH3OH (1:1), rt, 2 h.
1
Figure 3. H NMR spectrum of 2 recorded in C6D6 at room temperature with a 600 MHz proton frequency.
materials in CH2Cl2 at −10 °C (20 mM for 2,2′-((4-(tert-
butyl)phenyl)methylene)bis(1H-pyrrole)). The reaction mix-
ture was oxidized by 2,3-dichloro-5,6-dicyano-1,4-benzoqui-
none (DDQ) after 15 min. It is noteworthy that the yields
dropped significantly for longer reaction times and higher
temperatures (e.g., 2% for 1 h at rt), where increased
scrambling of the condensed intermediates prior to aromatiza-
tion was observed. The porphyrin 6 was isolated in 9% yield on
a 500 mg scale in a multistep protocol. The crude reaction
mixture was first filtered through a silica plug using CH2Cl2 as
eluent. The crude was further purified by a size exclusion
column (Bio Beads SX-3 in toluene) where the fractions
comprising the target structure were identified by thin layer
chromatography (TLC; Rf(6) = 0.55 for SiO2, ethyl acetate/
toluene/cyclohexane: 1/1/25) and collected. The analytically
pure 6 was finally obtained by precipitation from a
concentrated CH2Cl2 solution by the addition of CH3OH.
The free-base porphyrin 6 was metalated to make it suitable
for standard Sonogashira−Hagihara cross-coupling reactions
toward T2. Treatment of a solution of 6 in CH2Cl2/CH3OH
(1:1) with an excess of Zn(OAc)2 (5 equiv) afforded 7 in 97%
yield. Subsequent deprotection of the TMS acetylene with
K2CO3 in CH2Cl2/CH3OH (1:1) gave 3 in 99% yield. All
three porphyrin monomers 6, 7, and 3 were characterized by
D
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Figure 4. (a) normalized UV−vis spectra of 3 (blue) and 2 (red) in CH2Cl2, displaying the substantial Soret-band splitting observed with 2. (b)
High-resolution MALDI mass spectrum of 2 (red, normalized to 1) and simulation of the expected isotopic distribution (blue, normalized to −1).
a
Scheme 3. Dimerization of the Precursor 3 to the Bowl-Shaped Porphyrin Dimer 2
a
Key: (vi) Pd(PPh3)4, dppf, CuI, NEt3/C6H5CH3 (1:3), 110°C, 2 d.
1
thin-layer chromatography (TLC), melting point, H NMR
and UV−vis spectroscopy, MALDI-ToF mass spectrometry, as
well as elemental analysis. However, due to the low solubility
of 3, 13C{1H}-NMR spectroscopy was only performed for 6
and 7.
1,1′-bis(diphenylphosphino)ferrocene (dppf) were added
together with 1.1 equiv of CuI and the reaction mixture was
stirred at 110 °C for 2 d in a closed Schlenk flask, reaction
products in a promising mass range were detected. However,
the parallel formation of oligomers and small amounts of
homocoupled side products rendered the isolation of the
compound comprising the interesting mass challenging. The
reaction mixture was filtered on a silica short plug using
toluene as eluent. Higher oligomers and monomeric porphyrin
species were removed by two subsequent size exclusion
columns (SX-1 and SX-3 Bio Beads in toluene). The remaining
mixture was subjected to automated GPC, and the separated
fractions were analyzed by MALDI-ToF-MS. The crude
product was subjected to column chromatography with freshly
distilled solvent, and the compound of interest was isolated in
5% yield.
The precursor 3 was exposed to a large variety of
Sonogashira−Hagihara conditions. Based on our experience,
we started with slightly elevated temperatures between 60 and
90 °C and with catalytic amounts (5 to 10 mol %) of Pd(0)
and Cu(I) salts with a variety of ligands. Neither variation of
the reaction solvents and bases nor the rigorous exclusion of
oxygen yielded promising results. In all attempts, exclusively
larger oligomers and/or debrominated precursors were
detected by matrix-assisted laser desorption time-of-flight
mass spectrometry (MALDI-ToF-MS) together with unidenti-
fied degradation products. Driven by the hope for finding of
suitable macrocyclization conditions, the harshness of the
reaction conditions was increased steadily. Indeed, when 3 was
dissolved as 500 μM solution in a mixture of toluene and
triethylamine (NEt3), equimolar amounts of Pd(PPh3)4 and
Surprisingly, the 1H NMR spectrum (Figure 3) of the
isolated compound disqualified the targeted cyclophane T2 as
its structure. Immediately apparent was the splitting of the tert-
butyl signals into two equally weighted singlets, instead of the
E
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a
Scheme 4
a
Key: (vii) (1) TBAF, THF, rt, 1 h, (2) S-(4-iodophenyl) ethanethioate, Pd(PPh3)4, CuI, NEt3/THF (1:3), rt, 2 h.
single peak expected for the symmetry of T2. In the aromatic
region, an odd number of downfield shifted signals of the β-
protons of the porphyrins’ pyrrole subunits were found. While
six signals displayed the doublet expected for the coupling with
their neighbors, the seventh signal was a singlet indicating the
absence of a proton-bearing next neighbor. The absence of a
larger than the rH of 3, resulting in a 2.48 times larger
molecular weight including the first solvation sphere which is
in reasonable agreement with the proposed dimeric structure.
By late-stage peripheral functionalization, a small sample of
the 3-fold interlinked porphyrin bicycle was optimized for
single-molecule junction experiments. The terminal acetylenes
of 2 were liberated by treatment with an excess of tetra-n-
butylammonium fluoride (TBAF) in wet THF. After 1 h, the
mixture was filtered over silica, and the solvent was evaporated
under reduced pressure. The remaining crude reaction product
was dried at high vacuum and redissolved in dry THF. The
degassed solution was transferred into a flask containing NEt3,
3 equiv of S-(4-iodophenyl)ethanethioate, 5 mol % Pd(PPh3)4,
and 6 mol % CuI. After the solution was stirred for 2 h at room
temperature, monitoring by TLC indicated spot to spot
conversion. The reaction mixture was filtered through a silica
short plug with CH2Cl2 as eluent. Size-exclusion chromatog-
raphy (Bio Beads SX-3 in toluene) provided the desired
porphyrin bicycle 1 in 84% yield over both steps (see Scheme
4). Due to both the minute dimension of the sample and the
limitations in solubility, only TLC, high-resolution MALDI-
ToF-MS, and UV/vis are available as data characterizing the 3-
fold interlinked bicycle 1 exposing acetyl-protected thiol
anchor groups.
1
coupling partner was further corroborated by H−1H COSY-
NMR spectroscopy (see Figure S25). The molecular signal of
the isolated compound recorded by high-resolution mass
spectrometry was with m/z = 1978.7912 (Figure 4b), two mass
units shorter than the value expected for T2 (m/z =
1980.8065), suggesting that the two missing β-protons were
not substituted by any NMR inactive nuclei, but pointing at a
newly formed intramolecular C−C bond interlinking the
pyrrole subunits of both porphyrins. In the UV/vis spectrum of
the isolated compound (Figure 4a), a Soret-band splitting of
50 nm (global maxima at 422 and 472 nm) was observed,
pointing at exciton coupling of a spatially fixed porphyrin
pair.29,30
The structure supported by the analytical data is the 3-fold
interlinked porphyrin dimer 2 (Scheme 3), in which an
additional C−C bond is formed between the β-positions of a
pyrrole subunit from each porphyrin. Such formations of β−β
linkages are known to form under strong Lewis acidic
conditions or, in rare cases, promoted by transition metals,
including palladium.31 They typically require high structural
preorganization as exemplified for porphyrin tapes and
sheets.32−35 We surmise that this additional linking occurs
immediately after closing of the cyclophane T2, promoted by
the proximity of the porphyrins. This might explain why we did
not find experimental evidence for T2. The additional C−C
bond breaks the symmetry of the compound and forces the
porphyrins to adapt to a bowl-shaped structure.
Current Transport Investigation. The suitability of the
bicyclic porphyrin dimer 1 for transport measurement in a
single-molecule junction was investigated in a MCBJ experi-
ment (Figure 5). Details of this technique have been published
previously,36−38 and the method is only presented briefly. A
lithographically fabricated thin gold wire was deposited on a
The topology of 2 was unambiguously determined by means
of 2D-NMR experiments. The full characterization with the
assignment of each proton and carbon signal can be found in
the Supporting Information (see Figure S30). A slight
concentration dependence of the proton chemical shifts
pointed at stacking behavior even in aromatic solvents like
deuterobenzene. To unambiguously prove the dimeric nature
of 2, its diffusion coefficient was determined (3.64(4) × 10−10
m2/s) by pulsed field gradient spin echo (PFGSE) experiment
and calibrated with the diffusion coefficient of compound 3
recorded under identical conditions (4.93(2) × 10−10 m2/s)
(see Figure S29 and Table S1). From the ratio of the two
diffusion coefficients the hydrodynamic radius rH of 2 was
calculated using the Stokes−Einstein equation to be 1.35 times
Figure 5. Sketch of a mechanically controlled break junction (MCBJ)
experiment displaying the multilayer architecture and the basic
components.
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Figure 6. Two-dimensional histograms built before (a) and after (b and c) drop-casting a CH2Cl2 solution containing 1. The insets show the one-
dimensional conductance histogram. The histogram in panel a was built from 2000 consecutive CDTs, while the one in panel b was built from the
10000 CDTs collected after the deposition of 1. The histogram in panel c was built from the 890 CDTs selected by the clustering algorithm. (d)
Two-dimensional histogram built from 704 consecutive conductance-time traces collected with the self-breaking method. Insets: one-dimensional
conductance histograms belonging to the panels.
flexible substrate. The center of the wire consisted of a 100 nm
wide constriction that was suspended after etching the
supporting polyimide, with an O2 plasma. The flexible
substrate was clamped between two lateral supports in a
three-point bending mechanism and deformed by driving a
pushing rod connected to a cantilever into it. Upon bending of
the substrate, the gold wire was stretched until the breaking of
the wire results in two sharp electrodes. The flexibility of the
substrate allowed the adjustment of the electrode separation by
the mechanical positioning of the pushing rod. The geo-
metrical design of the samples was such that the vertical
displacement of the cantilever was converted into a horizontal
electrode displacement, which resulted in a subnanometer
control over the electrodes separation. By relaxation of the
bending the two broken extremities could be fused again
reforming the gold wire. This process could be repeated several
thousand times without a noticeable aging of the wire,
providing the vast amount of data required for meaningful
statistical analyses. In this study, a 5 mM solution of 1 in
CH2Cl2 was drop casted on the junction after the character-
ization of the bare device. Once the solvent was evaporated,
the above-described breaking and making process was repeated
10000 consecutive times, and the breaking traces were
collected without any data selection giving a two-dimensional
conductance versus electrode displacement histogram. The
molecular junction formation has been operated in two
different modes. The fast-breaking mode39 forming quickly
one electrode pair after the other and the self-breaking mode,40
allowing the electrodes to relax screening for particular stable
configurations in the molecular junction.
In the fast-breaking mode, the conductance was measured as
the distance between the electrodes was progressively
increased until the conductance dropped below the detection
level. The electrode displacement speed was typically 3−9 nm/
s. In 9% of the 10000 recorded consecutive conductance-
displacement traces (CDTs), a plateau-like feature was
observed (Figure 6c). It is noteworthy that these 890 CDTs
reflecting a molecular feature have been selected by an
automated clustering algorithm40 which was dividing the
10000 CDTs (Figure 6b) into five classes. The plateaus
observed in the 9% of the CDTs were at various conductance
levels and occasionally longer than 2 nm, excluding through
space tunneling as potential origin of the CDTs. To
demonstrate the difference between these molecular junctions
(Figure 6c) and empty junctions in the histograms, the CDTs
of the same device before deposition of 1 are displayed in
Figure 6a). The remaining four classes displayed CDTs
without significant features and were attributed to empty
junctions. The one-dimensional conductance histogram (inset
of Figure 6c) displayed rather a continuous increase than a
well-defined peak representing the conductance of the
molecule in the junction. The absence of such a preferred
conductance value points at a large variety of molecular
configurations inside the junction with different conductance
characteristics.
In order to assess the most stable configurations in the
junction, self-breaking measurements were performed on the
same sample. In this method, the MCBJ junction was initially
stretched to a few-atom width and allowed to self-break by its
surface tension to form atomically sharp electrodes. The
electrodes were not moved during this self-relaxation process,
while the conductance was measured as a function of time. The
results are summarized in the conductance vs. time plot in
Figure 6d. The configurations having the longest lifetimes were
found between 10−5 and 10−6 G0, with many of these traces
reaching the maximum measuring time (120 s). Occasionally,
higher conductance values were also found and some of them
persist for time scales longer than the average fast-breaking
trace. Similar to the fast-breaking measurement method, these
observations indicate the presence of a variety of molecular
junction configurations, with those displaying the low-
conductance values being the most stable ones.
In spite of the fixation of the arrangement of both
porphyrins by the additional C−C bond which handicaps the
intended mechanical manipulation, the collected breaking
traces were analyzed for the appearance of electrode spacing
modulated conductance variations. Indeed, in about 2% of the
molecular traces recorded by the fast-breaking technique an
oscillation of the conductance with increasing electrode
distance was observed (Figure S35). However, the seldom
appearance of this feature pointing at a response to mechanical
manipulation does not yet allow for definite conclusions.
CONCLUSION
■
The bowl-shaped porphyrin dimer 2 has been synthesized by
serendipity, and its 3-fold interlinked bicyclic structure was
fully corroborated spectroscopically. Decoration with a pair of
peripheral anchor groups provided 1, making the interlinked
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The diffusion time was set to 80 ms, the Eddy current time to 5 ms
and the gradient length to 1.5 ms. Gradients with a smoothed square
shape (SMSQ10.100) were increased linearly in eight steps from 5 to
95% (2.41 to 45.74 G/cm). The sigmoidal intensity decrease was
fitted with a two-parameter fit (I0 and diffusion coefficient D) with the
dosy routine implemented in topspin 3.5 [Bruker Biospin GmbH,
2017]. Five peaks were analyzed for each compound, and the mean
and the 95% confidence interval are given in Table S1.
porphyrin architecture suitable for single molecule transport
studies. In spite the considerable dimension and the rigidity of
the structure, molecular junctions could be obtained in a
MCBJ experiment. The transport analyses displayed a large
spread of the recorded single molecule traces pointing at a rich
variety of configurations and electronic transparencies of the
single molecule junctions. Regardless of the fixed arrangement
of the interconnected porphyrin scaffold in 1, the transport
data even comprise first indications of mechanosensitive
molecular junctions.
We are currently optimizing the molecular design of dimeric
porphyrin cyclophanes as model compounds to investigate the
interplay between mechanical manipulation and transport
behavior in a single molecule junction.
2,2′-((4-(tert-Butyl)phenyl)methylene)bis(1H-pyrrole). According
to ref 28, an oven-dried two-necked flask with reflux condenser was
charged with freshly distilled pyrrole (250 mL, 3.58 mol, 100 equiv)
and 4-tert-butylbenzaldehyde (6.00 mL, 35.8 mmol, 1.0 equiv). The
mixture was degassed with argon for 20 min. The reaction mixture
was heated to 55 °C under the absence of light in an oil bath. InCl3
(792 mg, 3.58 mmol, 0.1 equiv) was added in one portion, and the
mixture was stirred at 55 °C for 3 h. NaOH (4.30 g, 107 mmol, 3.0
equiv) was added, and the resulting mixture was stirred at 55 °C
overnight. The mixture was filtered over a plug of silica, which was
washed with pyrrole. The solvent was removed via distillation. The
crude product was purified via recrystallization (ethanol 7:1 water) to
obtain the title compound (8.04 g, 28.8 mmol, 81%) as a white
powdered solid: TLC (SiO2, CH2Cl2/cyclohexane (1:1), UV254 nm) Rf
= 0.13; 1H NMR (400 MHz, CD2Cl2, 298 K, δ/ppm) 8.02 (br s, 2H),
7.39−7.31 (m, 2H), 7.19−7.09 (m, 2H), 6.85−6.51 (m, 2H), 6.28−
5.99 (m, 2H), 5.99−5.81 (m, 2H), 5.42 (s, 1H); 13C{1H}-NMR (101
MHz, CD2Cl2, 298 K, δ/ppm) 150.4 (Cq), 139.9 (Cq), 133.3 (Cq),
128.4 (Ct), 126.1 (Ct), 117.6 (Ct), 108.8 (Ct), 107.3 (Ct), 44.1
(Cmeso), 34.9 (Cs, tBu), 31.6 (Cp, tBu); GC−MS (EI+, 70 eV) m/z 278
(68) [M+], 145 (100) [M+ − C10H14].
EXPERIMENTAL SECTION
■
General Remarks. Reagents and Solvents. All commercially
available compounds were purchased from Sigma-Aldrich, Acros,
Apollo Scientific, Alfa Aesar, and Fluorochem and used without
further purification. Anhydrous solvents were purchased from Sigma-
Aldrich and stored over molecular sieves (4 Å). Column
chromatography was performed on silica gel P60 (40−63 μm) from
Silicycle, and the solvents were technical grade. TLC was performed
with silica gel 60 F254 aluminum sheets with a thickness of 0.25 mm
purchased from Merck.
Synthesis. All reactions with reagents that are easily oxidized or
hydrolyzed were performed under argon using Schlenk techniques
with anhydrous solvents in glassware, which was dried prior to use.
Analytics and Instruments. Recycling gel permeation chromatog-
raphy (GPC) was performed on a Shimadzu Prominence System
equipped with SDV preparative columns from Polymer Standards
Service (two Showdex columns in series, 20 × 600 mm each,
exclusion limit: 30000 g/mol) with chloroform as solvent. NMR
experiments were performed on Bruker Avance III NMR
spectrometers operating at 250, 400, 500, or 600 MHz proton
frequencies. The instruments were equipped with a direct-observe 5
mm BBFO smart probe (250, 400, and 600 MHz), an indirect-
detection 5 mm BBI probe (500 MHz), or a five-channel cryogenic 5
mm QCI probe (600 MHz). All probes were equipped with actively
shielded z-gradients (10 A). The chemical shifts are reported in ppm
relative to tetramethylsilane or referenced to residual solvent peak and
the J values are given in Hz ( 0.1 Hz). Standard Bruker pulse
sequences were used, and the data was processed on Topspin 3.2
(Bruker) using 2-fold zero-filling in the indirect dimension for all 2D
experiments. GC-MS was performed on a Shimadzu GC-MS-2020-SE
instrument equipped with a Zebron 5 MS Inferno column, with a
temperature range of up to 350 °C. MALDI-TOF mass spectra were
recorded on a Bruker MicroFlex LRF spectrometer using trans-2-[3-
(4-tert-butylphenyl)-2-methylpropenylidene]malononitrile (DCTB)
as a matrix. High-resolution mass spectra (HRMS) were measured
on a Bruker solariX spectrometer with a MALDI source and a
Micromass (Waters) AutoSpec Ultima−EI-Sector-MS. Elemental
analyses were recorded at room temperature on an Elementar Vario
4-Bromo-3-((trimethylsilyl)ethynyl)benzaldehyde (5). 4-Bromo-3-
iodobenzaldehyde (27.3 g, 87.8 mmol, 1.0 equiv), Pd(PPh3)2Cl2
(3.08 g, 4.39 mmol, 0.05 equiv), and CuI (1.02 g, 5.27 mmol, 0.06
equiv) were dissolved in piperidine and THF (200 mL, 1:3 (v:v)),
and the mixture was purged with argon for 15 min in a heat gun dried
Schlenk tube. A solution of ethynyltrimethylsilane (15.0 mL, 105
mmol, 1.2 equiv) in THF (20 mL) was purged with argon for 5 min
and was subsequently transferred into the Schlenk tube via syringe.
The reaction mixture was stirred at rt for 3 h. The reaction mixture
was filtered over a plug of silica, eluted with CH2Cl2. The solvent was
removed under reduced pressure, and the residue was subjected to
column chromatography (SiO2, CH2Cl2/cyclohexane (2:1) yielding 5
(21.2 g, 75.5 mmol, 86%) as a yellowish oil: TLC (SiO2, CH2Cl2/
1
cyclohexane (2:1), UV254 nm) Rf = 0.30; mp 57 °C; H NMR (250
4
MHz, CD2Cl2, 298 K, δ/ppm) 9.93 (s, 1H), 7.96 (d, JHH = 2.0 Hz,
1H), 7.78 (d, 3JHH = 8.3 Hz, 1H), 7.66 (dd, 3JHH = 8.3 Hz, 4JHH = 2.0
Hz, 1H), 0.29 (s, 9H); 13C{1H}-NMR (63 MHz, CD2Cl2, 298 K, δ/
ppm) 190.9 (CC=O), 135.9 (Cq), 135.1 (Ct), 133.9 (Ct), 132.7 (Cq),
130.0 (Ct), 126.9 (Cq), 102.2 (CCC), 102.1 (CCC), −0.1 (Cp, TMS);
HRMS (ESI, +) m/z calcd for C13H17BrNaO2Si [M + Na +
CH3OH]+ 335.0073, found 335.0074.
4-((Triisopropylsilyl)ethynyl)-3-((trimethylsilyl)ethynyl)-
benzaldehyde (4). 4-Bromo-3-((trimethylsilyl)ethynyl)benzaldehyde
(5, 21.2 g, 75.4 mmol, 1.0 equiv), Pd(PPh3)2Cl2 (2.65 g, 3.77 mmol,
0.05 equiv), and CuI (879 mg, 4.52 mmol, 0.06 equiv) were dissolved
in NEt3 and toluene (200 mL, 1:3 (v:v)), and the mixture was purged
with argon for 15 min in a heat gun dried Schlenk tube. A solution of
ethynyltriisopropylsilane (20.3 mL, 90.5 mmol, 1.2 equiv) in toluene
(20 mL) was purged with argon for 5 min and was transferred into the
Schlenk tube via syringe, subsequently. The reaction mixture was
stirred at 80 °C oil bath temperature for 2 h. The reaction mixture
was filtered over a plug of silica, eluted with CH2Cl2. The solvent was
removed under reduced pressure, and the residue was subjected to
column chromatography (SiO2, ethyl acetate/cyclohexane (1:5)
yielding 4 (23.3 g, 80.7 mmol, 81%) as a yellowish oil: TLC (SiO2,
Micro Cube instrument. Melting points were measured on a Buchi M-
̈
565 melting point apparatus and are uncorrected. UV/vis absorption
spectra were recorded at 20 °C on a JASCO V-770 Spectropho-
tometer.
NMR Experiments 600 MHz Cryo. All NMR experiments were
performed on Bruker Avance III HD four-channel NMR spectrom-
eters operating at 600.13 MHz proton frequency. The instruments
were equipped with a cryogenic 5 mm four-channel QCI probe (H/
C/N/F) or an RT BBI probe, both with self-shielded z-gradient. The
experiments were performed at 298 K, and the temperature was
calibrated using a methanol standard showing accuracy within 0.2 K.
The compound was assigned using standard HSQC, HMBC, COSY,
and ROESY (mixing time 200 ms) experiments. The diffusion
coefficients were determined in an PFGSE (pulsed field gradient spin
echo) diffusion experiment using a bipolar gradient pulse sequence.
1
ethyl acetate/cyclohexane (1:5), UV254 nm) Rf = 0.53; H NMR (400
4
MHz, CD2Cl2, 298 K, δ/ppm) 9.95 (s, 1H), 7.96 (d, JHH = 1.6 Hz,
1H), 7.75 (dd, 3JHH = 8.0 Hz, 4JHH = 1.7 Hz, 1H), 7.62 (d, 3JHH = 8.0
Hz, 1H), 1.18−1.14 (m, 21H), 0.26 (s, 9H); 13C{1H}-NMR (101
MHz, CD2Cl2, 298 K, δ/ppm) 191.2 (CCHO), 135.9 (Cq), 134.7 (Ct),
134.0 (Ct), 131.6 (Cq), 128.7 (Ct), 126.9 (Cq), 104.9 (CCC), 102.5
H
DOI: 10.1021/acs.joc.9b02327
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
(CCC), 100.7 (CCC), 100.6 (CCC), 19.1 (Cp, TIPS), 11.8 (Cs, TIPS),
0.1 (Cp, TMS); HRMS (EI, +) m/z calcd for C23H34OSi2 [M]+
382.2148, found 382.2134 (16); calcd for C20H27OSi2 [M-iPr]+
339.1600, found 339.1591 (66); calcd for C17H21OSi2 [M − 2iPr +
H]+ 297.1131, found 297.1137 (100).
aqueous layer was extracted with CH2Cl2. The combined organic
phases were dried over anhydrous Na2SO4 and concentrated under
reduced pressure yielding 3 (139 mg, 129 mmol, 99%) as a pinkish
microcrystalline solid: TLC (SiO2, EtOAc/toluene/cyclohexane
(1:1:25)) Rf = 0.30; mp > 240 °C dec; 1H NMR (250 MHz,
5-(2-Bromophenyl)-10,20-bis(4-(tert-butyl)phenyl)-15-(4-
((triisopropylsilyl)ethynyl)-3-((trimethylsilyl)ethynyl)phenyl)-
porphyrin (6). A solution of 4-((triisopropylsilyl)ethynyl)-3-
((trimethylsilyl)ethynyl)benzaldehyde (4, 459 mg, 1.20 mmol, 1.0
equiv), 2-bromobenzaldehyde (141 μL, 1.20 mmol, 1.0 equiv), and
2,2′-((4-(tert-butyl)phenyl)methylene)bis(1H-pyrrole) (668 mg, 2.40
mmol, 2.0 equiv) in CH2Cl2 (125 mL) was purged with argon for 20
min in a heat gun dried Schlenk flask at −10 °C which was shielded
from light. Trifluoroborane diethyl etherate (0.03 mL, 250 μmol, 0.2
equiv) was added, and the reaction mixture was stirred for 15 min.
2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (599 mg, 2.64 mmol, 2.2
equiv) was added, and the mixture was stirred for 1 h at rt. The
reaction mixture was filtered over a plug of silica, and was
concentrated under reduced pressure. The residue was subjected to
size-exclusion column chromatography (Bio Beads SX3 in toluene)
and the collected fractions were analyzed by thin-layer chromatog-
raphy (SiO2, ethyl acetate/toluene/cyclohexane (1:1:25)). Crystal-
lization from a concentrated solution of the product comprising
fractions in CH2Cl2 by top layering with methanol gave 6 (116 mg,
107 μmol, 9%) as a microcrystalline purple solid: TLC (SiO2, EtOAc/
CD2Cl2, 298 K, δ/ppm) 9.00 (d, 3JHH = 4.7 Hz, 4H), 8.92 (d, 3JHH
=
3
3
4.8 Hz, 2H), 8.79 (d, JHH = 4.7 Hz, 2H), 8.36 (dd, JHH = 6.6 Hz,
4JHH = 1.7 Hz, 1H), 8.22−8.15 (m, 4H), 8.14−8.07 (m, 2H), 8.05
3
4
3
4
(dd, JHH = 7.3 Hz, JHH 2.0 Hz, 1H), 7.90 (dd, JHH = 7.8 Hz, JHH
1.5 Hz, 1H), 7.83−7.76 (m, 4H), 7.76−7.68 (m, 2H), 3.41 (d, J = 0.9
Hz, 1H), 1.62 (s, 18H), 1.27 (s, 21H); No 13C{1H}-NMR spectrum
was recorded due to the low solubility of 3; MALDI-TOF m/z = 1071
(100); UV/vis (CH2Cl2): λmax[nm] (ε [L·cm−1·mol−1]) = 422 (5.17
× 105), 507 (2.52 × 102), 550 (1.82 × 104), 589 (1.80 × 103). Anal.
Calcd for C65H63BrN4SiZn:C, 72.72; H, 5.91; N, 5.22. Found: C,
73.06; H, 6.26; N, 4.97;
[03](13,10-(3-(2-)),20-(2-))-5,15-Bis(4-(tert-butyl)phenyl)-10-(3-
e t h y n y l - 4 - ( ( t r i i s o p r o p y l s i l y l ) e t h y n y l ) p h e n y l ) - 2 0 -
phenylporphyrinato]zinc(II) (2). A dry and degassed solution of 3
(60.1 mg, 56.0 μmol, 1.0 equiv) in toluene (50 mL) was added to a
dry and degassed solution of Pd(PPh3)4 (64.7 mg, 56.0 μmol, 1.0
equiv), dppf (32.0 mg, 56.0 μmol, 1.0 equiv), and CuI (11.7 mg, 61.6
μmol, 1.1 equiv) in NEt3 (30 mL) and toluene (50 mL), and the
mixture was placed in a preheated oil bath and was stirred vigorously
at 110 °C for 2 d. The mixture was filtered over silica eluted with
toluene. The solvent was removed under reduced pressure and the
residue subjected to size-exclusion column chromatography (Bio
Beads SX3 in toluene), followed by size-exclusion column
chromatography (Bio Beads SX1 in toluene) followed by automated
GPC in chloroform. Finally, the compound was subjected to column
chromatography using freshly distilled solvents (SiO2, ethyl acetate/
toluene/cyclohexane (1:1:15) yielding 2 (3.21 mg, 1.62 μmol, 5.7%)
as a green solid: TLC (SiO2, EtOAc/toluene/cyclohexane (1:1:10))
Rf = 0.50.; 1H NMR (600 MHz, benzene-d6, 298 K, δ/ppm) 9.08 (d,
3JHH = 4.8 Hz, 1H, H-11), 8.88 (d, 3JHH = 4.5 Hz, 1H, H-12), 8.74 (d,
1
toluene/cyclohexane (1:1:25)) Rf = 0.55; mp > 300 °C; H NMR
(500 MHz, CD2Cl2, 298 K, δ/ppm) 8.91 (d, 3JHH = 4.9 Hz, 4H), 8.83
(d, 3JHH = 4.8 Hz, 2H), 8.70 (d, 3JHH = 4.8 Hz, 2H), 8.34 (dd, 3JHH
=
4.7 Hz, 4JHH = 1.8 Hz, 1H), 8.21−8.16 (m, 2H), 8.15−8.09 (m, 4H),
8.05 (dd, 3JHH = 8.0 Hz, 1H), 7.89 (d, 3JHH = 7.8 Hz, 1H), 7.83−7.77
(m, 4H), 7.76−7.68 (m, 2H), 1.61 (s, 18H), 1.29 (s, 21H), 0.24 (s,
9H), −2.80 (s, 2H); 13C{1H}-NMR (126 MHz, CD2Cl2, 298 K, δ/
ppm) 151.4, 143.2, 142.8, 139.3, 139.0, 135.8, 135.8, 134.9, 134.6,
134.6, 132.5, 131.8, 131.8, 130.6, 128.0, 126.6, 125.6, 124.6, 124.4,
124.4, 121.3, 119.0, 118.7, 105.8, 100.6, 99.3, 96.9, 35.4, 31.9, 19.3,
12.0, 1.3; MALDI-TOF m/z = 1081 (100); UV/vis (CH2Cl2):
λmax[nm] (ε [L·cm−1·mol−1]) = 420 (1.06 × 105), 518 (7.44 × 103),
549 (3.38 × 103), 592 (1.73 × 103), 647 (1.42 × 103). Anal. Calcd for
C68H73BrN4Si2:C, 75.45; H, 6.80; N, 5.18. Found: C, 75.71; H, 6.80;
N, 5.16.
[5-(2-Bromophenyl)-10,20-bis(4-(tert-butyl)phenyl)-15-(4-
((triisopropylsilyl)ethynyl)-3-((trimethylsilyl)ethynyl)phenyl)-
porphyrinato]zinc(II) (7). A solution of 6 (149 mg, 138 μmol, 1.0
equiv) and zinc acetate (127 mg, 690 μmol, 5 equiv) in CH2Cl2 and
methanol (50 mL, 1:1) was stirred at rt for 1 h. The mixture was
eluted with CH2Cl2 and was washed with satd aq NaHCO3 solution,
water, and brine. The aqueous layer was extracted with CH2Cl2. The
combined organic phases were dried over anhydrous Na2SO4 and
concentrated under reduced pressure yielding 7 (153 mg, 134 μmol,
97%) as microcrystalline pinkish solid: TLC (SiO2, EtOAc/toluene/
cyclohexane (1:1:25)) Rf = 0.36; mp > 300 °C; 1H NMR (600 MHz,
3JHH = 4.4 Hz, 1H, H-44), 8.69 (d, 3JHH = 4.6 Hz, 1H, H-22), 8.51 (d,
3JHH = 4.6 Hz, 1H, H-23), 8.46 (d, 3JHH = 4.2 Hz, 1H, H-45), 8.46 (s,
1H, H-31), 8.16 (m, 2H, H-16/H-38), 8.14 (m, 2H. H-20/H-42),
8.13 (s, 1H, H-33), 7.99 (d, 3JHH = 7.4 Hz, 1H, H-7), 7.76 (dd, 3JHH
=
7.7 Hz, 4JHH = 1.9 Hz, 1H, H-27), 7.69 (m, 1H, H-17), 7.68 (m, 1H,
3
H-39), 7.65 (m, 1H, H-19), 7.64 (m, 1H, H-41), 7.56 (d, JHH = 7.7
3
Hz, 1H, H-28), 7.16 (d, JHH = 7.0 Hz, 1H, H-4), 6.87 (t, 3JHH = 7.1
3
Hz, 1H, H-6), 6.78 (t, JHH = 7.4 Hz, 1H, H-5), 1.51 (s, 9H, H-48),
1.49 (s, 9H, H-54), 1.30 (d, 3JHH = 6.9 Hz, 16H, H-51), 1.23 (h, 3JHH
= 7.4 Hz, 3H, H-52).; 13C{1H}-NMR (151 MHz, over 2D-NMR
[HMBC, HSQC], benzene-d6, 298 K, δ/ppm)161.5 (C-35), 155.2
(C-32), 152.5 (C-21), 152.2 (C-8), 151.9 (C-43), 151.4 (C-13),
150.6 (C-40), 150.6 (C-18), 150.3 (C-46), 149.4 (C-24), 148.5 (C-
10), 147.3 (C-34), 143.2 (C-26), 142.7 (C-33), 140.3 (C-15), 139.0
(C-37), 138.4 (C-31), 135.6 (C-12), 134.8 (C-38), 134.7 (C-42),
134.64 (C-27), 134.60 (C-16), 134.5 (20), 132.3 (C-23), 132.02 (C-
22), 131.95 (C-44), 131.4 (C-28), 130.8 (C-45), 129.9 (C-6), 128.4
(C-5), 126.9 (C-29), 126.8 (C-11), 126.1 (C-14/C-36), 125.2 (C-7),
124.7 (C-4), 124.29 (C-41), 124.28 (C-39), 124.26 (C-30), 124.19
(C-19), 124.18 (C-17), 121.4 (C-14/C-36), 121.3 (C-25), 114.0 (C-
9), 105.8 (C-49), 97.7 (C-2/C-50), 95.6 (C-2/C-50), 82.4 (C-1),
35.0 (C-47/C-53), 31.81 (C-54), 31.82 (C-48), 19.1 (C-51), 11.8 (C-
52); HR-MALDI-TOF m/z [M+] Calcd for C130H122N8Si2Zn2
1978.7908; Found 1978.7912; UV/vis (CH2Cl2) λmax[nm]) = 420,
447, 470, 508, 550, 593, 689.
CD2Cl2, 298 K, δ/ppm) 9.00 (d, 3JHH = 4.7 Hz, 4H), 8.91 (d, 3JHH
=
3
3
4.7 Hz, 2H), 8.78 (d, JHH = 4.7 Hz, 2H), 8.34 (dd, JHH = 7.0 Hz,
4JHH = 1.8 Hz, 1H), 8.21−8.08 (m, 6H), 8.07−8.02 (m, 1H), 7.88
3
4
(dd, JHH = 7.9 Hz, JHH = 1.7 Hz, 1H), 7.83−7.76 (m, 4H), 7.74−
7.68 (m, 2H), 1.62 (s, 18H), 1.28 (s, 21H), 0.23 (d, J = 0.7 Hz, 9H);
13C{1H}-NMR (126 MHz, CD2Cl2, 298 K, δ/ppm) 151.3, 151.1,
150.3, 150.2, 143.8, 143.5, 140.0, 139.0, 135.7, 134.8, 134.5, 133.2,
132.9, 132.4, 132.0, 131.7, 131.6, 130.4, 128.0, 126.5, 125.4, 124.9,
124.2, 122.2, 119.9, 111.0, 105.9, 104.0, 96.7, 0 35.4, 32.0, 19.3, 12.0,
1.3; MALDI-TOF m/z = 1144 (100); UV/vis (CH2Cl2) λmax[nm] (ε
[L·cm−1·mol−1]) = 422 (1.55 × 106), 505 (2.93 × 104), 549 (8.21 ×
104), 588 (2.82 × 104). Anal. Calcd for C68H71BrN4Si2Zn:C, 71.28;
H, 6.25; N, 4.89; found: C, 71.43; H, 6.44; N, 4.82;
[5-(2-Bromophenyl)-10,20-bis(4-(tert-butyl)phenyl)-15-(3-ethyn-
yl-4-((triisopropylsilyl)ethynyl)phenyl)porphyrinato]zinc(II) (3). A
solution of 7 (150 mg, 131 μmol, 1.0 equiv) and K2CO3 (181 mg,
1.31 mmol, 10 equiv) in CH2Cl2 and methanol (50 mL, 1:1) was
stirred at rt for 1 h. The mixture was eluted with CH2Cl2 and was
washed with satd aq NaHCO3 solution, water, and brine. The
I
DOI: 10.1021/acs.joc.9b02327
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
was done at the Kavli Nanolab at Delft. Generous financial
support by the Swiss National Science Foundation (SNF Grant
No. 200020-178808) is gratefully acknowledged. M.M.
acknowledges support by the 111 project (90002-18011002).
D.D. acknowledges support by Fondecyt Regular Project
(Grant No. 1181080). We thank Chuwei Hsu and Alfredo
Rates Soriano for their help with the measurements and the
clustering, respectively.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.joc.9b02327.
■
S
Synthetic overview, NMR spectra, HR-MS reports, UV−
vis spectra, elemental analysis reports, example single
trace of conductance experiments, and calculation details
(PDF)
́
́
̀
(12) Stefani, D.; Perrin, M.; Gutierrez-Ceron, C.; Aragones, A. C.;
Labra-Munoz, J.; Carrasco, R. D. C.; Matsushita, Y.; Futera, Z.;
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AUTHOR INFORMATION
Corresponding Author
*E-mail: marcel.mayor@unibas.ch. Phone: +41 207 10 06.
ORCID
■
Davide Stefani: 0000-0002-9406-9265
̈
Daniel Haussinger: 0000-0002-4798-0072
Herre S. J. van der Zant: 0000-0002-5385-0282
́
Diana Dulic: 0000-0002-8302-5507
Marcel Mayor: 0000-0002-8094-7813
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H. S. J.; Renaud, N. Mechanically Controlled Quantum Interference
in Individual π-Stacked Dimers. Nat. Chem. 2016, 8 (12), 1099−
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Mayor, M.; Pauly, F.; van der Zant, H. S. J. Large Conductance
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This study was supported by the EU through a RISE(DAF-
NEOX) project, SEP 210165479, and partially funded by the
FET open project QuIET (no. 767187). The device fabrication
J
DOI: 10.1021/acs.joc.9b02327
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