Substitution Pattern Controlled Quantum Interference in [2.2]Paracyclophane-Based Single-Molecule Junctions

2.2]Paracyclophane-Based Single-Molecule Junctions

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Substitution Pattern Controlled Quantum Interference in

[

Ksenia Reznikova, Chunwei Hsu, Werner M. Schosser, Almudena Gallego, Katawoura Beltako,

Fabian Pauly,* Herre S. J. van der Zant,* and Marcel Mayor *

Cite This: https://doi.org/10.1021/jacs.1c06966

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sı Supporting Information

ABSTRACT: Quantum interference (QI) of electron waves passing

through a single-molecule junction provides a powerful means to

inﬂuence its electrical properties. Here, we investigate the correlation

between substitution pattern, conductance, and mechanosensitivity

in [2.2]paracyclophane (PCP)-based molecular wires in a

mechanically controlled break junction experiment. The eﬀect of

the meta versus para connectivity in both the central PCP core and

the phenyl ring connecting the terminal anchoring group is studied.

We ﬁnd that the meta-phenyl-anchored PCP yields such low

conductance levels that molecular features cannot be resolved; in the

case of para-phenyl-coupled anchoring, however, large variations in

conductance values for modulations of the electrode separation

occur for the pseudo-para-coupled PCP core, while this

mechanosensitivity is absent for the pseudo-meta-PCP core. The experimental ﬁndings are interpreted in terms of QI eﬀects

between molecular frontier orbitals by theoretical calculations based on density functional theory and the Landauer formalism.

INTRODUCTION

Relationships between substitution pattern and single-

molecule conductance were already theoretically predicted

and experimentally conﬁrmed in a variety of examples, ranging

■

In recent years, great advancements have been made in the

ﬁeld of molecular electronics toward single-molecule junction

from simple phenyl rings connected directly to the electrodes

studies. The visionary idea of Aviram and Ratner to proﬁt

from single molecules as functional units in electronic devices

led to the development of several proof-of-concept molecular

to more sophisticated oligo(phenylene vinylene) (OPV) and

oligo(phenylene ethynylene) (OPE)-based molecular wires.

5,6

Unanimously, these studies report a decrease in electronic

transparency upon shifting the anchoring groups from the para

to the meta position. This observation was rationalized by

devices, such as molecular wires,

switches,

rectiﬁers/

The design of

9,10

diodes, and thermo-electronic devices.

molecules incorporated in such electrode−molecule−electrode

junctions is guided by our understanding of charge transport

through the molecules. Indeed, even small structural

2−24

Yoshizawa and co-workers,

who considered frontier

orbital theory for simple organic molecules. Their set of

rules predicts for benzene the para connection to be the

symmetry-allowed one for charge transport, while charge

transport involving the meta connection is symmetry-

forbidden, resulting in high and low conductance, respectively.

Because the eﬀects of substitution patterns in planar π-

systems are well described and understood, our focus moved to

three-dimensional structures like [2.2]paracyclophane (PCP),

with two benzene systems facing each other interlinked by a

11,12

modiﬁcations such as substituent eﬀects,

conformational

ﬂexibility, and changes in the anchoring groups and their

4−18

positions

can result in large conductance variations.

Particularly strong variations are predicted for quantum

interference (QI) eﬀects originating from the interplay

between diﬀerent transport pathways. Destructive QI (DQI)

or constructive QI (CQI) between the pathways can occur,

reﬂected in a low or high conductance, respectively. QI

eﬀects thus become essential molecular design elements, on

the one hand enriching the variety of functionalities emerging

from the molecular structure but on the other hand making a

full comprehension of the molecule’s electronic transport

behavior more challenging. A detailed understanding of these

QI eﬀects and of their origin in the molecule’s structure is thus

crucial to realize their full potential in future electronic

components and devices.

pair of C H bridges. Initially, we considered the structure as

Received: July 5, 2021

XXXX The Authors. Published by

American Chemical Society

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

para -OPE PCP in Figure 1 ) displayed a sharp destructive QI

Article

a model to investigate the through-space coupling of the

thumb (para → good transport due to CQI; meta → bad

transport due to DQI) is inverted for the PCP subunit.

6,27

stacked π-systems

but realized quickly that their behavior

is much richer. Already the ﬁrst model compound (ps-para-

Excited by this hypothesis, we explored the eﬀects of the

substitution pattern in more detail and designed the four PCP

model compounds 1−4 (see Figure 1) consisting of

comparable subunits but with various substitution patterns.

The oligo(phenylene)-type PCP structures combine the

compactness favoring electronic transparency on a detectable

level with straightforward synthetic accessibility. Terminal

acetyl masked thiol anchor groups enable their immobilization

in a mechanically controlled break junction (MCBJ) by

covalent S−Au bonds, guaranteeing both electronic coupling

and mechanical stability. The latter is of particular importance

to enable subtle mechanical manipulation of the integrated

single molecule. The investigation of their transport properties

and the inﬂuence of mechanical manipulations are studied with

MCBJ experiments. The ﬁndings are rationalized by QI eﬀects

emerging from the interplay of frontier molecular orbitals,

discussed with a theoretical model based on density functional

theory (DFT). The electronic transport is described in terms

of the Landauer formalism, expressed through nonequili-

brium Green’s function (NEGF) methods.

RESULTS AND DISCUSSION

■

The PCP-based model compounds were assembled from the

corresponding para/meta building blocks by Suzuki−Miyaura

cross-coupling reactions. The literature known pseudo (ps)-

para- or ps-meta-dibromo[2.2]paracyclophane and either 4-

or 3-(tert-butylthio)phenyl boronic acid provided the PCP

derivatives with four diﬀerent combinations of substitution

Figure 1. Schematic representation of target structures 1−4 together

with the already reported model compound. The four molecules

include either para or meta connection patterns in both the central

PCP subunit and the peripheral phenyl subunits labeled in red and

blue, respectively. For simplicity, the following text refers to the

structures by their preﬁxes, with ps as an abbreviation for pseudo,

followed by the preﬁx referring to the substitution pattern of the

central PCP, and ﬁnally the preﬁx referring to the substitution pattern

of the thiol anchor group in the phenyl subunits.

patterns. The adaption of a protocol from Jevric et al.

enabled the subsequent transprotection and provided the

target structures 1a−4a in reasonable isolated yields ranging

from 63 to 85%. The identity of the new PCP derivatives was

corroborated by H NMR, C{ H} NMR spectroscopy, and

high-resolution mass spectrometry (HR-MS). A detailed

description of the synthetic protocols and the analytical data

of all new compounds are provided in the Supporting

Information (section 1).

The single-molecule electronic transport properties of PCPs

1−4 were investigated by integrating them into an electronic

circuit using an MCBJ setup operated at ambient conditions.

Two types of measurements were performed: fast-breaking and

modulation experiments (see below). Details of the technique

providing a pair of mechanically adjustable electrodes with a

distance resolution of atomic dimensions have been reported

feature close to the Fermi level, but the phenomenon also

depended substantially on the mechanical stress exposed to the

molecule. It thus seemed that the rigid but squeezable PCP

subunit provided mechanosensitivity to the molecular junction.

Interestingly, Yoshizawa and co-workers already provided

their orbital view of PCP subunits and predicted that the

charge transport through the pseudo-para-substituted PCP

should be suppressed due to DQI, while CQI would prevail for

pseudo-meta -substituted PCP. It thus seems that the rule-of-

Figure 2. Two-dimensional conductance vs electrode displacement density histograms. The ﬁrst (from the left) and second histograms are built up

from 6 834 and 9 638 traces at 100 mV with ps-para-para- and ps-meta-para-PCP molecules, respectively. The third and fourth 2D histograms

correspond to 3 780 and 10 000 traces at a bias voltage at 250 mV with ps-para-meta- and ps-meta-meta-PCP molecules, respectively.

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Article

Figure 3. (a−c) Conductance traces of modulation experiments with ps-para-para-PCP (1), displaying (a) in-phase, (b) double frequency, and (c)

anti-phase modulations. (d) Conductance traces of the modulation experiments with ps-meta-para-PCP (2). The red line in the distance−

modulation traces represents the voltage applied to the piezoelectric stack, whereas blue represents the measured conductance. The total

modulation time of the experiments is 15 s; for better visibility, only 5 s are displayed. (e) Calculated conductance of the ps-para-para-PCP

molecular junction during the gap opening. The conductance displacement data is extracted from Figure 5b by evaluating the transmission function

at the Fermi energy. The sketches rationalize the variety of conductance vs electrode displacement modulation behaviors observed for ps-para-para-

PCP in dependence of the position of the DQI dip with respect to the trapping state of the molecule in the individual junction; see panels a−c and

e. In particular, light background colors (yellow, red, and blue) relate the diﬀerent situations of junctions, including molecules in prestretched,

relaxed, or precompressed states, respectively, to the behavior in modulation experiments. (f) Same plot as in panel e but for ps-meta-para-PCP,

where mechanosensitivity is basically absent. The green background color connects the conductance that is rather insensitive to electrode

displacements to the observations made in the modulation measurements in panel d.

3,34

previously

and are thus discussed in the Supporting

injection into the molecules’ π-systems without controlled

sulfur-to-sulfur immobilization.

Information (section 2.1).

In the fast-breaking experiments, several thousand con-

ductance traces for each investigated molecule were collected

and plotted as two-dimensional (2D) histograms displayed in

Figure 2. For the case of ps-para-para- and ps-meta-para-PCPs

with a constant bias voltage of 100 mV, clear conductance

plateaus with a length of ∼12.5 Å were observed. Through a

Of particular interest are the distance-dependent single-

molecule junction charge-transport studies of ps-para-para-

and ps-meta-para-PCPs, as substantial diﬀerences in the

transport behavior under mechanical stress are expected

between the central ps-para- and ps-meta-PCP subunits.

While the mechanosensitivity of the ps-para-PCP subunit

was already evidenced in modulation experiments for the ps-

reference-free clustering method on the unﬁltered data, the

pure gold-to-gold tunneling traces and the molecular traces

were separated. The molecular conductances of ps-para-para-

and ps-meta-para-PCPs were obtained through a log-normal ﬁt

para-para-OPE PCP (Figure 1) and rationalized as being due

to a distance-dependent conductance dip close to the Fermi

level originating from DQI, similar behavior is not expected for

−

−5

the ps-meta-PCP subunit, as CQI has been predicted.

distribution yielding values of 1.3 × 10 G and 2.2 × 10 G ,

To study the presence of DQI in more detail, modulation

experiments are useful. The modulation procedure consists of

respectively, where G = 2e /h is the quantum of conductance,

as shown in the corresponding one-dimensional (1D)

conductance histograms (see Supporting Information (section

constricting the gold wire down to a conductance of 2 G with

the piezo control. The junction will then break by itself due

to its surface tension. The electrodes are then separated by

.2)).

For the molecules with the meta-phenyl anchoring (ps-para-

7.5 Å, and a pulse of a triangular waveform with an amplitude

meta-PCP and ps-meta-meta-PCP) the bias voltage in the

transport experiment was increased to 250 mV in order to

bring molecular levels closer to resonance, as the conductance

of these PCPs was below the detection limit for a bias of 100

mV. However, even at this increased bias voltage, no clear

conductance plateaus were detected, even when using the

earlier-mentioned clustering method. Whether the lack of clear

plateaus is due to molecular conductance below the detection

threshold of the experimental setup of 10⁻^G0 or the

molecules’ inability to form stable molecular junctions cannot

be distinguished. The very short-breaking traces visible in

Figure 2 at higher conductance values for both ps-para-meta-

and ps-meta-meta-PCP are most likely due to direct electron

of 20 V_p_e_a_k_‑_t_o_‑_p_e_a_kis applied to the piezoelectric element, which

corresponds to a modulation amplitude of 5 Å between the

electrodes. The conductance is continuously monitored, and

the modulation is applied at a frequency of 5 Hz for 15 s.

Hereafter, the junction is fused again, and a new modulation

trace is recorded. The results of electrode-modulation

experiments on PCPs 1 and 2 are summarized in Figure 3.

Similar to ps-para-para-OPE PCP, a rich variety of

distance-modulated conductance responses has been observed

for ps-para-para-PCP (1). The periodic electrode displacement

caused a substantial conductance modulation, which was either

in-phase (Figure 3a), anti-phase (Figure 3c), or even two times

the frequency (Figure 3b) of the voltage applied to the piezo

Journal of the American Chemical Society

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GFs for each recording, lasting 15 s.

Article

Figure 4. Gauge factors obtained from (a) 1 442 ps-para-para-PCP traces and (b) 2 236 ps-meta-para-PCP traces in modulation experiments. The

counts are collected from fast Fourier transform (FFT) spectra over 1 s intervals of the traces, which means that there are 15 counts, with diﬀerent

Figure 5. (a) Illustration of PCP derivatives 1−4 immobilized in junctions between two gold electrodes. (b) Transmission maps of the four types

of PCP single-molecule junctions. The horizontal red resonances in the maps arise from molecular frontier orbitals. For the ps-para-para-PCP

molecule, an antiresonance is observed inside the HOMO−LUMO gap that shifts in energy as the displacement is varied. Similar tunable DQI

eﬀects are absent for all other molecules, in particular for ps-meta-para-PCP junctions and ps-meta-meta-PCP junctions with central ps-meta-PCP

systems. The position of the Fermi energy is indicated as a horizontal dashed line.

stack. As sketched in Figure 3e, this observed mechanosensi-

tivity is caused by the DQI dip in the conductance versus

molecular length relation (insets in Figure 3e), with the variety

of observed behaviors reﬂecting the exact position of the

conductance dip in the particular molecular junction. We take

the in-phase case as an example to explain the observed

behavior. This corresponds to a starting position in which the

molecule is prestretched, as depicted in Figure 3e (top, light

yellow background). In this case, whenever the molecule is

stretched by increasing the voltage applied to the piezoelectric

element, the conductance goes up, and the conductance goes

down when the piezo voltage decreases, i.e., the conduction

follows the applied piezo-voltage modulation in-phase.

The striking similarity of the mechanosensitivity of ps-para-

para-PCP (1) and ps-para-para-OPE PCP, both comprising a

central para-PCP subunit, not only points at this structural

motif as the origin of the phenomenon but also further

corroborates its rationalization based on the presence of a DQI

dip in proximity of the Fermi level. Equally interesting are the

modulation experiments performed with the ps-meta-para-PCP

junctions with this structure do not feature signiﬁcant

conductance changes during the modulation experiments.

Remarkably, the absence of mechanosensitivity of the ps-meta-

para-PCP has not been reported experimentally before.

Particularly, this behavior indicates the absence of a trans-

mission dip as a function of electrode displacement (within the

displacement window probed by the experiment), suggesting

that DQI does not occur in the case of ps-meta-PCP cores.

This is in agreement with previous predictions of π-stacked

26,27

systems by the groups of Solomon

and Yoshizawa.

To quantify the mechanosensitivity of the molecule under

investigation, the gauge factor (GF) was determined as the

ratio between logarithmic conductance variation and linear

electrode displacement (see the Supporting Information

(

section 2.4) for more information). Figure 4 displays the

GFs for (a) ps-para-para-PCP and (b) ps-meta-para-PCP.

Indeed, the GFs of the measurements of both structures

visualize their diﬀerence. While the absence of mechanosensi-

tivity of ps-meta-para-PCP results in a sharp peak close to zero

(Figure 4b), the GF values recorded for ps-para-para-PCP

(2). As displayed in parts d and f of Figure 3, single-molecule

Journal of the American Chemical Society

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Article

Figure 6. HOMO and LUMO molecular frontier orbitals of the ps-para-para-, ps-meta-para-, ps-para-meta-, and ps-meta-meta-PCPs. The

anchoring sulfur atoms are marked i and j, respectively. Extended representations also including GPH−1 and GPL+1 for each molecule can be

found in the Supporting Information (section 3.3).

(

Figure 4a) display a broad distribution with a minimum at GF

0, documenting its pronounced mechanosensitivity.

To rationalize the experimental observations described

meta- and ps-meta-meta-PCP. Interestingly, for this pair of

model compounds with terminal meta-benzene linkers, ∼1

order of magnitude lower transmission values were calculated

compared to the pair with terminal para-benzene linkers. This

is also in line with the absence of measurable conductance

plateaus for the meta-phenyl-connected PCPs in Figure 2.

The conductance that we compute within the DFT-NEGF

earlier, extensive DFT calculations were performed. First, the

four model compounds ps-para-para-PCP (1b), ps-meta-para-

PCP (2b), ps-para-meta-PCP (3b), and ps-meta-meta-PCP

(4b) were optimized in the gas phase. The hydrogen atoms of

the terminal thiol groups were removed, and the molecules

were placed in model junctions, consisting of pairs of

tetrahedral gold leads (Figure 5a). Optimizing the junction

geometry, only the top three gold atoms in the ﬁrst layer of

each tip were allowed to relax, while the rest of the gold cluster

remained ﬁxed. Then, the systems were stretched in steps of

formalism at the Fermi energy is plotted in parts e and f of

Figure 3 for ps-para-para- and ps-meta-para-PCP derivatives,

respectively. Considering the example of the ps-para-para-PCP

single-molecule junction, the conductance features a dip that is

shifted toward negative displacements and is ∼2 orders of

magnitude lower than the base value. Molecular contacts

constructed from ps-meta-para-PCP show instead a rather

constant behavior in the studied displacement range without a

DQI dip.

The behavior of another geometry that features a more

complex stick−slip motion is discussed in the Supporting

Information (section 3.2). In that case, remnants of trans-

mission valleys are visible for the ps-para-meta-PCP molecular

junction, further consolidating the hypothesis of the central ps-

para-PCP subunit as the origin of the DQI phenomenon.

Note, however, that we argue here on a qualitative level

because we did not correct DFT quasiparticle energies and

.1 Å, and a geometry optimization was performed at every

single step. A detailed description of the established

approach and explanations of the calculations are provided

in the Supporting Information (section 3.1).

A closer look at the two-dimensional contour plots of

transmission in dependence of energy and electrode displace-

ment in Figure 5b reveals important information about the

transmission behavior of the molecular junctions inside the

electronic gap between the highest occupied molecular orbital

(

HOMO) and the lowest unoccupied molecular orbital

LUMO). In the case of the ps-para-para-PCP, a transmission

valley (blue diagonal trace) with transmission values lower

therefore expect uncertainties with respect to absolute

−

17,38−41

than 10 is observed between the molecular frontier orbitals

red horizontal traces). The transmission valley corresponds to

conductance values,

as a comparison of experimental

(

and theoretical data in Figures 3 and 5 conﬁrms.

the DQI conductance dip, as shown in Figure 3e. The energy

position of the transmission minimum can be tuned by

mechanical manipulation of the junction. It should be noted

that the DQI crosses the complete HOMO−LUMO gap,

making it a robust feature for experimental detection, because

it does not depend on the precise position of the Fermi energy.

A similar valley is not present for the ps-meta-para-PCP

junction. Instead, the transmission stays rather constant in the

While the calculated transmission plots perfectly support the

hypothesis that mechanosensitivity can exclusively be observed

for structures with a central ps-para-PCP subunit providing

DQI, another qualitative argument is provided by considering

orbital symmetry rules, as suggested by Yoshizawa and co-

29,42

workers.

The qualitative prediction of QI phenomena is

based on the interplay of molecular frontier orbitals, especially

the HOMO and LUMO, in transport models using Landauer−

−

−4

range of ∼10 −10 inside the molecule’s HOMO−LUMO

gap. Rather uniform transmission values are furthermore

predicted in the molecules’ electronic gap for both ps-para-

Buttiker scattering theory and Green ’s function methods

(details are provided in the Supporting Information (section

3.3)). Thus, the gas-phase frontier orbitals of the model

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

https://pubs.acs.org/doi/10.1021/jacs.1c06966.

Article

compounds with terminal thiol groups were calculated and are

displayed in Figure 6.

quantum sensor operating at ambient conditions, i.e., at room

temperature.

Assuming that the Fermi energy of the electrodes lies

between the molecule’s HOMO and LUMO energies due to

charge neutrality, two orbital rules apply for the electronic

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

■

29,42

transport properties.

(1) The weights of the HOMO and

LUMO wave functions on the anchoring atoms i and j (see

Figure 6) need to be of decent size to yield a fair contribution

to the transmission. (2) If the parities of the molecule’s

HOMO and LUMO on the anchoring atoms are diﬀerent in

sign, transport is symmetry-supported through CQI, which is

typically reﬂected in a high transmission inside the HOMO−

LUMO gap. If HOMO and LUMO parities are the same

instead, the related molecular orbital resonances cancel each

other out at a certain energy inside the HOMO−LUMO gap.

As a consequence, transport is symmetry-inhibited, resulting in

a DQI dip in the energy-dependent transmission function,

typically leading to a reduced conductance of the single-

molecule junction. This argument assumes that HOMO and

LUMO orbital wave functions are of similar character on the

termini i and j.

Synthesis and characterization of all molecules, and

details concerning transport measurements and calcu-

lations (PDF)

AUTHOR INFORMATION

Corresponding Authors

■

Fabian Pauly − Institute of Physics, University of Augsburg,

6159 Augsburg, Germany; orcid.org/0000-0001-8017-

2379; Email: fabian.pauly@uni-a.de

Herre S. J. van der Zant − Kavli Institute of Nanoscience,

Delft University of Technology, 2628 GJ Delft, The

Netherlands; orcid.org/0000-0002-5385-0282;

Email: h.s.j.vanderzant@tudelft.nl

Marcel Mayor − Department of Chemistry, University of Basel,

The analysis of the molecules started by deﬁning the

terminal sulfur atoms as anchoring sites i and j (see Figure 6).

Comparing orbital wave functions on the terminal sulfur atoms

shows that gas-phase HOMO (GPH) and gas-phase LUMO

4056 Basel, Switzerland; Institute for Nanotechnology,

Karlsruhe Institute of Technology (KIT), 76021 Karlsruhe,

Germany; Lehn Institute of Functional Materials, School of

Chemistry, Sun Yat-Sen University, Guangzhou 510274, P.

R. China; orcid.org/0000-0002-8094-7813;

Email: marcel.mayor@unibas.ch

(

GPL) of both model compounds comprising a central para-

PCP (ps-para-para- and ps-para-meta-PCP) are of similarly

oriented π-character at the sulfur atoms and have the same

parities; thus, these structures should show DQI. In contrast,

both molecules with a ps-meta-PCP subunit have diﬀerent

parities in their GPH and GPL on the terminal sulfur atoms

and thus exhibit CQI. The orbital symmetry rules thus

rationalize the experimental observations and numerical

computations reported earlier, which identify the central ps-

para-PCP subunit as the origin of DQI.

Authors

Ksenia Reznikova − Department of Chemistry, University of

Basel, 4056 Basel, Switzerland

Chunwei Hsu − Kavli Institute of Nanoscience, Delft

University of Technology, 2628 GJ Delft, The Netherlands

Werner M. Schosser − Institute of Physics, University of

Augsburg, 86159 Augsburg, Germany

Almudena Gallego − Department of Chemistry, University of

Basel, 4056 Basel, Switzerland

Katawoura Beltako − Institute of Physics, University of

Augsburg, 86159 Augsburg, Germany

Let us point out that ferrocene recently emerged as a related

D system to the PCP, where the angle between two

cyclopentadienyl decks can be tuned rather continuously

around the central Fe core atom. The torsion can be

compared to meta or para connection to the PCP. The

mechanical distortion that explains the experimental results

here is mainly the displacement of two benzene rings as

compared to a rotation.

Complete contact information is available at:

https://pubs.acs.org/10.1021/jacs.1c06966

Author Contributions

K.R., C.H., and W.M.S. contributed equally.

Notes

CONCLUSION

■

We have studied the electronic transport properties of single-

molecule junctions based on π-stacked hydrocarbons. Our

study conﬁrms previous theoretical predictions for model

compounds in terms of molecular orbital symmetry rules with

regard to the suppression of electronic transport in PCP

subunits when contacted in ps-para geometry as compared to

ps-meta geometry. While the ps-meta subunit generally shows

high conductance in comparison to the ps-para geometry (still

relatively low compared to other conjugated molecular wires

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

This work was supported by a Horizon 2020 FET open project

QuIET; no. 767187). H.S.J.v.d.Z. and C.H. acknowledge

support from The Netherlands Organisation for Scientiﬁc

Research (NWO-Vrij program). M.M. acknowledges support

from the Swiss National Science Foundation (SNF Grant no.

■

(

00020-178808) and the 111 project (90002-18011002).

such as OPE3-dithiol ) and little sensitivity to mechanical

manipulation, the ps-para subunit oﬀers an exceptional

mechanoelectric sensitivity. Notably, our theoretical calcula-

tions predict that the DQI can be tuned through the complete

electronic gap region, explaining the experimental robustness

of the feature, as observations should be largely independent of

the precise location of the Fermi energy. Similar to an optical

ABBREVIATIONS

■

QI, quantum interference; DQI, destructive quantum interfer-

ence; CQI, constructive quantum interference; OPE, oligo-

(phenylene ethynylene); PCP, [2.2]paracyclophane; MCBJ,

mechanically controlled break junction; DFT, density func-

tional theory; NEGF, nonequilibrium Green’s function; ps,

pseudo; GF, gauge factor; FFT, fast Fourier transform;

Fabry−Per

ot interferometer, the DQI feature can be used to

detect minute displacement changes and hence serves as a

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Woerde, C. H. M.; Eelkema, R.; Grozema, F. C.; van der Zant, H. S. J.

Article

HOMO, highest occupied molecular orbital; LUMO, lowest

unoccupied molecular orbital; GPH, gas-phase HOMO; GPL,

gas-phase LUMO.

Cyanide and Isocyanide. J. Phys. Chem. C 2012, 116 (38), 20607−

0616.

17) Arroyo, C. R.; Tarkuc, S.; Frisenda, R.; Seldenthuis, J. S.;

Signatures of Quantum Interference Effects on Charge Transport

Through a Single Benzene Ring. Angew. Chem., Int. Ed. 2013, 52 (11),

(

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