Single Dynamic Covalent Bond Tailored Responsive Molecular Junctions

Article abstract of DOI:10.1002/anie.202106666

Responsive molecular devices are one of the core units for molecular electronics, and dynamic covalent bonds (DCBs) provide the opportunity for the fabrication of responsive molecular devices. Herein we employ a single dynamic acyl hydrazone bond to fabricate tailored molecular devices using the scanning tunneling microscopy break-junction technique (STM-BJ) and the eutectic Ga-In technique (EGaIn). We found that the single-DCB-tailored molecular devices exhibited acid-base and/or photo-thermal response with three well-defined molecular conductance states. The reversible switching has the ON/OFF ratio of ≈10 between each state for single-molecule junctions and ≈3 for the SAMs-based molecular junctions. Combined with the density functional theory calculations, we revealed that the multiple conductance states of these molecular junctions originate from the dynamic acyl hydrazone bond exchange and C=N isomerization. Our work opens the avenue towards the design of tailored single-molecule electrical devices by implanting dynamic covalent bonds in molecular architectures.

Full text of DOI:10.1002/anie.202106666

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Single dynamic covalent bond tailored responsive molecular
junctions
Authors: Yong Hu, Jin Li, Yu Zhou, Jie Shi, Guopeng Li, Hang Song,
Yang Yang, Jia Shi, and Wenjing Hong
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202106666
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10.1002/anie.202106666
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RESEARCH ARTICLE
Single dynamic covalent bond tailored responsive molecular
junctions
Yong Hu, [+] [a,b] Jin Li, [+] [a] Yu Zhou, [+] [a] Jie Shi,[+] [a] Guopeng Li, [a] Hang Song, [a] Yang Yang, [a] Jia Shi,
[a] Wenjing Hong*[a,b]
Dedicated to the 100th anniversary of the chemistry department in Xiamen University
[a]
Dr. Y. Hu[+], Dr. J. Li[+], Y. Zhou[+], J. Shi[+], G. Li, H. Song, Y. Yang, J. Shi, Prof. W. Hong
State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University
Xiamen 361005 (China)
E-mail: whong@xmu.edu.cn
[b]
[+]
Dr. Y. Hu, Prof. W. Hong
Shenzhen Research Institute of Xiamen University
Shenzhen 518000 (China)
These authors contributed equally to this work
Supporting Information for this article is given via a link at the end of the document.
Abstract: Responsive molecular devices are one of the core units for
molecular electronics, and dynamic covalent bonds (DCBs) provide
the opportunity for the fabrication of responsive molecular devices.
Herein we employ a single dynamic acyl hydrazone bond to fabricate
tailored molecular devices using the scanning tunneling microscopy
break-junction technique (STM-BJ) and the eutectic Ga-In technique
(EGaIn). We found that the single-DCB-tailored molecular devices
exhibited acid-base and/or photo-thermal response with three well-
defined molecular conductance states. The reversible switching has
the ON/OFF ratio of ~10 between each state for single-molecule
junctions and ~3 for the SAMs-based molecular junctions. Combined
with the density functional theory calculations, we revealed that the
multiple conductance states of these molecular junctions originate
from the dynamic acyl hydrazone bond exchange and C=N
isomerization. Our work opens the avenue towards the design of
tailored single-molecule electrical devices by implanting dynamic
covalent bonds in molecular architectures.
architecture, holds a great potential to construct tailored
molecular architectures with responsibility, reversibility and well-
defined conductance states.[8]
The dynamic covalent chemistry involves reversible chemical
reactions involving the formation/breaking of dynamic covalent
bonds (DCBs) under external stimuli. As one of the DCBs, acyl
hydrazine can be reversibly broken and regenerated in response
to different acid-base environments by acyl hydrazone
exchange[9], and the transformation of their C=N bonds occurs in
response to photo-thermal stimuli[8a], which provides unique
opportunities for fabricating well-defined multiple-state molecular
devices in response to different environmental variations.
However, it remains a challenge to incorporate the DCBs into the
metal-molecule-metal junctions since the molecule components
need to be connected between the two electrodes, but the length
of molecules changes dramatically before and after the dynamic
exchange of DCBs. Benefiting from the ability to regulate the size
of the nanogap, the single-molecule break junction technique
allows the measurement of DCBs with the different states in the
fine-tune nanogap at the nanometer scale, which is compatible
with the dramatic changes of anchor-to-anchor length resulting
from the dynamic exchange of DCBs.
Introduction
Responsive molecular devices, which employ covalent or non-
covalent interactions to construct functional molecular
architectures as building blocks, provide promising applications in
the sensing of environments.[1] Responsive molecular junctions
via the breaking and/or formation of covalent bonds, mimicking
solid-state switches and transistors, have been addressed using
a wide range of molecules triggered by pH[2], light[3], and
electrochemical potential[4], mechanical force[5]. However, the
responsibility and reversibility of molecular devices based on
covalent bonds are restricted due to the high reaction barrier of
breaking and forming the covalent bond,[6] while the responsibility
and reversibility achieved via non-covalent interactions have
limited stability due to their weak binding energy.[7] In this regard,
dynamic covalent chemistry, which allows the dynamic and
reversible exchange of molecular components within a molecular
Herein, we investigate the charge transport through DCB-
mediated responsive single-molecule junctions with acyl
hydrazone core using the scanning tunneling microscopy break-
junction (STM-BJ) technique and the complementary eutectic Ga-
In (EGaIn) technique. We characterized the reversible transitions
between multiple conductance states of the condensation of
isoniazid (IND) and pyridylaldehyde (PyA) during the dynamic
formation of the acyl hydrazone bond-conjugated (E) 4-
pyridinecarbaldehyde
isonicotinoyl
hydrazone
(E-PCIH)
molecules, as well as on dynamic photo-thermal isomerization of
C=N bond in the E-PCIH. We demonstrate a dynamic scenario
that can reversibly and repeatedly form three well-defined
conductance states using the STM-BJ technique, and we further
fabricate a responsive self-assembled monolayers (SAMs) device
using the EGaIn technique, which displays a distinguishable and
1
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10.1002/anie.202106666
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RESEARCH ARTICLE
reversible transition under the photo-thermal stimuli. The
combined density functional theory calculations revealed that the
multiple conductance states of these molecular junctions originate
from the dynamic acyl hydrazone bond exchange and C=N
isomerization.
Results and Discussion
The reversible nature of the acyl hydrazone bond allows the
condensation and hydrolysis of acyl hydrazone-type compounds
under mild conditions. Under the acid and base stimuli, the IND
and PyA (State 1) can be reversibly condensated into molecule
E-PCIH (State 2), and E-PCIH hydrolysis is back to the precursors
(Scheme 1).[10] Also, the acyl hydrazone bond of E-PCIH would
undergo an E→Z photoisomerization reaction of the C=N bond to
form Z-PCIH (State 3) in the presence of 365 nm irradiation and
reverse reaction when proceeding thermally.[8a] State 1 includes
PyA and IND, which is terminated with pyridine and amino groups,
which can efficiently connect to the gold electrodes forming
single-molecule junctions.[11] E-PCIH and Z-PCIH are terminated
with pyridine end-groups to build efficient charge transport
junctions.[12] The synthesis details and characterization data of E-
PCIH molecules are shown in Supporting Information
(Experimental Procedures Section).
Figure 1. a) Diagram of the multi-stimuli-responsive single-molecule junctions
of three conductance states measured by STM-BJ technique. b) all-data-point
1D conductance histograms constructed from E-PCIH + TFA, E-PCIH, and E-
PCIH + UV@365 nm
The three conductance states of dynamic transitions are
formed with the two-terminated anchors to gold electrodes, as
illustrated in Figure 1a. The charge transport properties of these
three states, including State 1, State 2 and State 3, are shown in
Figure 1b-d. Distinct conductance peaks were observed in the
one-dimensional (1D) conductance histograms of three states
than that of PC/DMF solvent (Figure S9). Upon the addition of 1%
(v/v) trifluoroacetic acid (TFA) in the E-PCIH (State 1), a well-
defined conductance peak was located at 10-3.6±0.25 G0 (19.5 nS)
(Figure 1b). Here, G0 = 2e2/h = 77.5 μS is the quantum of
conductance. The single-molecule conductance of the state
showed a pronounced conductance peak at 10-4.5±0.11 G0 (2.4 nS)
(Figure 1c) related to the junction formation of E-PCIH with the
electrodes, which shows almost an order of magnitude lower than
State 1. After undergoing UV@365 nm stimulus of the State 2, the
presence of remarkable low conductance peaks locating at 10-
5.8±0.23
G0 (0.1 nS) (Figure 1d), yielding an on/off ratio value ~20,
indicates an efficient electrical Z-isomer/electrodes coupling. The
typical individual conductance traces of them were shown in
corresponding insets. To create molecular switches with multi
states, Darwish et al. introduced the spiropyran derivatives as the
active components with an ON/OFF ratio of ~10, by controlling the
opening and closing of the spiro backbone under the light and
mechanical pulling[15], but the protonated and zwitterionic states
in opening forms of the spiro with have a limited conductance
difference due to the small geometrical changes. Introducing a
single acyl hydrazone bond, we demonstrate the fabrication of
molecular devices with stimuli-responsive, multi-transitions and
well-defined conductance states.
To further confirm the formation and breaking of DCBs, we
investigate the acid-base stimuli of E-PCIH using bulk
spectroscopic analysis. Figure 2a shows the UV-Vis absorption
spectra of E-PCIH under acid-base external stimuli that the UV
absorption of the State 2 of E-PCIH was located at 295 nm. After
adding 1% (v/v) TFA, the corresponding absorption peak shifted
to 278 nm, which is the characteristic absorption peak of IND.[16]
The obvious blue-shift is indicating the cleavage of the acyl
hydrazone bond to a hydrazide group. When we continue to add
1% (v/v) triethanolamine (TEA), the solution's absorption peak
Scheme 1. The schematic illustration for the dynamic-covalent-chemistry
transitions involves reversible breaking and reforming of acyl hydrazone
linkages under the acid and base stimuli (Left) and the isomerizations under the
light and heat (Right).
To determine the conductance of different states of the single-
molecule junctions, we carried out the STM-BJ measurements [13]
in our customized STM-BJ system (as shown in Figure S2). The
molecules were measured in solutions of 0.1 mM concentration of
target molecules with a mixed solvent of propylene carbonate/N,
N-Dimethylformamide (PC: DMF=49:1, v/v). Because the STM-
BJ measurements were carried out in a polar solvent environment,
the gold tips were covered with an apiezon wax to prevent the
leakage current from masking the molecular junction signal.[14]
A
fixed 100 mV bias was applied throughout the experiments. More
details regarding the STM-BJ measurements are described in the
Supporting Information: Experimental Procedures Section.
Typical individual conductance traces, 1D and 2D conductance-
displacement histograms are analyzed with homemade software,
which is available for research uses at https://github.com/Pilab-
XMU/XMe_DataAnalysis.
2
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10.1002/anie.202106666
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returns to State 2 (Figure 2a), suggesting regeneration of the acyl
hydrazone bond. Based on these observations, we can modulate
the different states within a molecular architecture by acid-based
stimuli based on reversible breaking and forming an acyl
hydrazone bond.
10-3.4±0.02 G0 (30.9 nS) as that of State 1 (as shown in Figure S11).
It is worth mentioning that the conductance of pure IND displays
a shoulder peak at ~10-5.0 G0 (0.8 nS), which may originate from
the isoniazid dimers forming by an intramolecular double
hydrogen bond of the hydrazide group.[18]
To understand the configuration of the junctions, statistical
analysis of the displacement distribution of the molecular
junctions (inset of Figure 2c and 2d) shows that the junction
elongations are determined to be 0.67 nm and 0.26 nm for E-
PCIH. After adding 0.5 nm, the snap-back of the gold
electrodes[19], the actual lengths of the molecular junctions for
them are ~1.17 nm and ~0.76 nm, which are well consistent with
the N-N theoretical length of E-PCIH (~1.15 nm) and IND (~0.65
nm), as shown in Figure S18. When added the 1% (v/v) TEA into
the E-PCIH + TFA solution, the appearance of low conductance
plateaus and complete disappearance of high conductance
plateaus (Figure S12) reinforce the reversible transitions of the
molecule under the acid-base stimuli, indicating that the dynamic
bonds in E-PCIH can be reversibly regenerated after adding the
TEA by acyl hydrazone exchange. These results demonstrate the
dynamic scenario that the fabrication of responsive molecular
devices can take advantage of dynamic exchange molecular
components in the acyl hydrazone bond.
The photo-thermal isomerization switching is also confirmed by
single-molecule conductance measurements. When the 365 nm
UV lamp is turned on for 30min, a remarkable low conductance of
State 3 at 10-5.8±0.23 G0 (0.1 nS) is obtained, and the 2D
conductance histogram of State 3 displays a different cloud-like
plateau compared with State 2 (Figure 3a). The E/Z isomerization
of E-PCIH attributes to the isomerize C=N bond in acyl
hydrazones.[8a] The low conductance level is assigned to the Z-
isomer state of E-PCIH, which is likely due to the twisted
configuration of Z-PCIH. And the end-to-end distance between
two pyridine groups for the junction of Z-PCIH was found to be
0.78±0.03 nm (Inset of Figure 3a), which is in good agreement
with the theoretical length ~ 0.83 nm (Figure S18). One of the
most significant advantages of DCBs is their stability and dynamic
reversibility. We further test the repeatability of the photo-thermal
conductance switching, and the low conductance peak
completely disappeared when State 3 is heated at 65° for 15 min.
It can be found that similar conductance value (located at ~10-4.6
G0 (~1.9 nS) in Figure S13a) and cloud-like plateau of State 3
(Figure 3b and Figure S13b) after heated as State 2 (Figure 2c),
indicating the reversible isomerization of the C=N bond under the
light and heat stimuli. The continuous conductance switching
between State 2 and State 3 was evaluated by operating with light
or heat stimuli for at least four cycles (Figure 3c).
We demonstrate that the E-PCIH can tune the single-molecule
conductance switching with well-defined states forward or
backward through light or heat stimuli. Our previous work has
demonstrated dihydroazulene (DHA) / vinylheptafulvene (VHF)
system[20] as photo-thermal controllable switches. This work
displays a well-defined photo-thermal switching with a similar
ON/OFF ratio (~20), which is higher than those of azobenzene
derivatives isomerization.[21] Most importantly, the switching
undergoes an E→Z photoisomerization reaction of the C=N bond,
resulting in a large variation of anchor-to-anchor length to yield a
distinguishable 2D pattern, as shown in Figure 3a and 3b. To
understand the role of acyl hydrazone bond in the multi-
responsive molecular devices, we carried out the control
experiments by measuring the single-molecule conductance of
Figure 2. a) the UV-vis spectra of E-PCIH and E-PCIH + TFA + TEA, IND and
E-PCIH + TFA. b) reversible ON-OFF conductance transition of State 1 and
State 2 in a system of E-PCIH triggered by TFA and TEA. Error bars were
determined from the standard deviation of the Gaussian fit for the 1D
conductance histogram peaks. c) and d), the 2D histograms of conductance
versus relative distance (Δz), and relative distance distributions (inset) of E-
PCIH and E-PCIH + TFA.
To study the reversibility of the conductance transition, we
measured the single-molecule conductance of E-PCIH in the
continuous conductance switching between acid and base stimuli
(Figure 2b). The results display four switching cycles, in which the
conductance changed from 10-4.5 G0 (2.4 nS) for State 2 to 10-3.6
G0 (19.5 nS) for State 1 after the addition of 1% (v/v) TFA, and
then continue to add 1% (v/v) TEA, the conductance return to
State 2. These results show the reversibility of the responsive
molecular junctions in response to the acid-base stimuli. It's worth
noting that salt precipitation must consider if this reversible
conductance transition works as a practical acid-base switch.
Recurrently, adding TEA aqueous solution into the acidic organic
solution for stratification, then taking out the organic layer for
conductance measurements to avoid the salt precipitation.[17]
We constructed the 2D conductance histograms of State 1 and
State 2 by overlaying all the conductance traces for further
analysis. As shown in Figure 2c, it can be observed the distinct
cloud-like plateau of E-PCIH after the gold point contact ruptured.
Upon the addition of TFA, the 2D conductance histogram pattern
of E-PCIH + TFA has changed dramatically (Figure 2d) with a
shorter junction elongation, suggesting the broken of dynamic
acyl hydrazone bonds and produce of molecules IND and PyA.
Therefore, this high conductance value is assigned to the
molecule IND since the PyA without two anchors forms an
effective molecular junction (Figure S10). To confirm the
configuration of State 1, We also measured the conductance of
pure IND solution, which shows a similar conductance value at
3
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1,4-bis(4-pyridyl)butane (BPB), whose structure and length are
similar to the E-PCIH except that the acyl hydrazone bond is
replaced by alkane chain. No change of the conductance value
and plateau pattern was observed under acid and/or light stimuli
(Figure S14-16). Comparing the molecular structures, we found
that the dynamic acyl hydrazone bonds cause the conductance
switching behavior yet not originating from the protonation of
pyridine anchors under such mild conditions.
irradiation at 365 nm (for 30 min) to convert the molecules to the
cis isomer (Z-PCIH), which are constructed over 200 J(V) curves
with the ~95% yields of non-shorting junctions. It can be found
that low values of J for E-PCIH junctions and higher values of J
after irradiation at the range of ±0.5 V bias. Regardless of the
asymmetrical structure of the PCIH molecules, no rectification
was observed from the J(V) curves of the PCIH molecular
junctions.[24] The magnitude of J at 0.5
V
in E-PCIH
junctions increased from 10-3.83 A cm-2 to 10-3.30 A cm-2 after
irradiation, a ratio of J of 3.4. We also notice that the ON/OFF ratio
of the large-area junctions is smaller than that of the single-
molecule junctions, which may originate from the changes off
PCIH molecules' conformations in the SAMs. [25] Interestingly, the
ON/OFF states are reversed in EGaIn-based PCIH-SAMs
junctions compared with that of PCIH single-molecule junctions
under the photo-thermal stimuli. The mechanism of charge
transport across SAMs-based junctions is coherent tunneling (see
Supporting Information: Temperature dependence of the large-
area molecular junction), where the value of J at a given bias
decreases exponentially with the SAMs thickness (d) according to
the simplified Simmons equation given by 퐽 = 퐽0푒−훽푑 [26] Where β
.
is the tunneling decay constant, and J0 is the theoretical value of
J when d = 0. It was reasonable that the J increased as the PCIH
SAM thicknesses decreased from 1.15 nm to 0.90 nm (obtained
by the DFT calculations) after 365 nm UV irradiation. The
conductance characterization of the SAM further confirms the
transformation of PCIH in the photo-thermal stimuli.
Figure 3. a) and b), the 2D histograms of conductance versus relative distance
(Δz), and relative distance distributions (inset) of E-PCIH + UV and E-PCIH +
UV + heat. c) reversible ON-OFF conductance transition of State 2 and State 3
in the system of E-PCIH triggered by UV at 365 nm and heat at 65°. Error bars
were determined from the standard deviation of the Gaussian fit for the 1D
conductance histogram peaks.
To show the versatility of the dynamic-covalent-chemistry
concept in molecular devices, we demonstrate the fabrication of
stimuli-responsive large-area junctions using the acyl hydrazone
bond and investigated the charge transport of them via a
homemade EGaIn instrument (Figure S3). Large-area molecular
junctions comprising self-assembled monolayers (SAMs) could
be used to study the reversible and reproducible conductance
switching at ambient conditions.[22] We fabricated SAMs of E-
PCIH as described in Supporting Information (Section: The
Fabrication of SAMs). And the characterizations of SAMs via
atomic force microscopy (AFM), and angle-resolved X-ray
photoelectron spectroscopy (ARXPS) are detailed in Supporting
Information (Section: SAMs Characterization). The conductance
of the PCIH SAMs was electrically characterized by contacting
them with conical EGaIn tips. Figure 4a shows a schematic
illustration of the large-area molecular junctions, which have an
Au substrate, a SAM of PCIH and a Schematic illustration of
GaOx/EGaIn top electrode. We measured statistically large
amounts of current density (J) curves of EGaIn/GaOx//PCIH-AuA-
Figure 4. a) Schematic illustration of EGaIn/GaOx//PCIH-AuA-DE junctions. b)
The J(V) curves of the monolayer of E-PCIH and followed by 365 nm light for
30 min (Z-PCIH) were measured using EGaIn top-contacts. The error bars are
the log-standard deviation of the Gaussian fit.
These results also confirmed that different coupling strength at
the electrodes (chemisorption or physisorption) upon trans/cis
isomerization of azobenzene derivatives affects the current
characteristics of their molecular junctions, in which larger
conduction for trans-isomer is found when the molecule is
chemisorbed at both electrodes. In contrast, the contrary result is
obtained in the presence of a physisorbed contact at one
electrode.[27] Regarding the EGaIn/GaOx//PCIH-AuA-DE junctions,
the GaOx/EGaIn top electrode contacts PCIH SAMs with
physisorption resulting in a higher J value for the cis-isomer (Z-
PCIH). Although the ON/OFF ratio of the SAMs-based device is
limited compared with that of the DCB-based single-molecule
device, its different transport mode provides an alternative for
designing DCB-based molecular devices. As a result, we
demonstrate that the construction of a large-area SAMs-based
DE
junctions (where "//" indicates physisorption, "/" indicates
chemisorption, AuA-DE indicates the anneal of Au electrodes after
directly evaporated), and determined the Gaussian log-average
value of J (<log|J|>) versus bias (V) along with the log-standard
deviation of log|J| following previously reported methods.[23] The
statistical log-average J(V) curves of EGaIn/GaOx//PCIH-AuA-DE
junctions are presented in Figure 4b for E-PCIH and after
4
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10.1002/anie.202106666
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RESEARCH ARTICLE
device introduces dynamic acyl hydrazone bonds, capable of
electrically switching between well-defined ON/OFF states under
photo-thermal stimuli on the SAMs scale.
junction length E-PCIH is longer than that of Z-PCIH, E-PCIH
states show higher transmission than Z-PCIH in almost all the
Fermi energy choices, which may originate from the twisted
configuration of Z-PCIH.[27] We speculate that the experimental
Fermi level should be located at a relatively flat position between
HOMO and LUMO in the transmission, for example, as indicated
by the cyan box at approximately -1.6 eV in Figure 5b, which is in
good agreement with experimental tendency. These findings
revealed that configuration changes from the formation of
dynamic covalent bonds lead to the significant conductance
difference of IND, E-PCIH and Z-PCIH in the single-molecule
junctions.
To understand the conductance of IND, E-PCIH and Z-PCIH
molecules in single-molecule junctions, we turn to density
functional theory (DFT) based calculations for these molecules
bridging Au electrodes. We first calculated the optimized
geometries and the HOMO-LUMO gap for molecules IND, E-
PCIH and Z-PCIH. In agreement with experiments, DFT
calculations provide the N-N theoretical length of 0.65nm, 1.15nm
and 0.83nm for IND, E-PCIH and Z-PCIH, respectively (Figure
S18). As shown in Figure S19, the LUMOs of IND, E-PCIH and Z-
PCIH are delocalized on the entire backbone. However, the
HOMOs of these molecules locate at the acyl hydrazone or acyl
hydrazone bond. The HOMO of E-PCIH displays more
delocalized than that of Z-PCIH upon the pyridine core, indicating
the stronger conjugation of E-PCIH. The HOMO-LUMO gap (as
shown in Figure S19) increases from 4.49 eV for E-PCIH to 4.71
eV for Z-PCIH, resulting from the twisted configuration of Z-
PCIH.[28]
Conclusion
In summary, we demonstrate the responsive molecular devices
based on a single dynamic covalent bond using the STM-BJ
technique and the EGaIn technique. The results of charge
transport investigation for the responsive single-molecule
junctions show that three well-defined conductance states were
observed with an ON/OFF ratio ~10 in response to acid-based or
photo-thermal stimuli of dynamic acyl hydrazone bond. The multi-
responsibility and reversibility of the DCB-based molecular device
are evaluated by operating the dynamic covalent processes with
acid-based and photo-thermal stimuli. We further find that the
ON/OFF ratio of the SAMs-based device is ~3 for the dynamic
isomerization with the control of photo-thermal stimuli. The DFT-
based theoretical calculation provides insights for rationalizing the
observed conductance trends in the single-molecule junctions of
dynamic covalent processes. These observations highlight the
opportunity to access unique molecular devices by introducing the
concept of dynamic covalent chemistry, which is of significant
potential to design responsive molecular electronics and
nanomachines at the single-molecule scale.
Acknowledgements
This work was supported by the National Key R&D Program of
China (2017YFA0204902), the National Natural Science
Foundation of China (21973079 and 21933012), China
Postdoctoral Science Foundation (2020M682082),
and
Figure 5. a) the molecular junction configurations of IND, E-PCIH, and Z-PCIH
used to compute transmission functions. b) Zero-bias transmission curves of E
- EF for IND, E-PCIH, and Z-PCIH, respectively.
Guangdong Basic and Applied Basic Research Foundation
(2020A151511106). We thank Prof. H. L. Chen for the insightful
discussions and suggestions on the characterization of self-
assembled monolayers.
We further employed a combined DFT and non-equilibrium
Green's function (NEGF) approach to obtain the transmission
spectra for the three systems. As shown in Figure 5, IND shows
a higher transmission probability than E-PCIH in almost all the
energy choices. Due to the uncertainty of the gold electrode's
contact configuration and solution environment, as well as
inherent limitations in DFT calculations, the calculated Fermi level
often deviates from the experimental value.[29] However, since
these molecules are neither oxidized nor reduced, the Fermi level
lies between the HOMO and the LUMO. As shown in Figure 5,
IND shows a higher transmission probability than E-PCIH in
almost all the Fermi energy choices. The high conductance of the
IND is likely attributed to the short molecular length. Although the
Keywords: dynamic covalent chemistry • responsive molecular
device • scanning tunneling microscopy break-junction technique
• acyl hydrazone bond • molecular junction
[1]
a) D. Xiang, X. L. Wang, C. C. Jia, T. Lee, X. F. Guo, Chem. Rev. 2016,
116, 4318-4440; b) C. Huang, A. V. Rudnev, W. Hong, T. Wandlowski,
Chem. Soc. Rev. 2015, 44, 889-901; c) T. A. Su, M. Neupane, M. L.
Steigerwald, L. Venkataraman, C. Nuckolls, Nat. Rev. Mater. 2016, 1;
d) P. Gehring, J. M. Thijssen, H. S. J. van der Zant, Nature Reviews
Physics 2019, 1, 381-396; e) N. Xin, J. Guan, C. Zhou, X. Chen, C. Gu,
Y. Li, M. A. Ratner, A. Nitzan, J. F. Stoddart, X. Guo, Nat. Rev. Phys.
2019, 1, 211-230; f) S. Fujii, S. Marques-Gonzalez, J.-Y. Shin, H.
Shinokubo, T. Masuda, T. Nishino, N. P. Arasu, H. Vazquez, M.
Kiguchi, Nat. Commun. 2017, 8.
5
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RESEARCH ARTICLE
[2]
Z. Li, M. Smeu, S. Afsari, Y. Xing, M. A. Ratner, E. Borguet, Angew.
Chem. Int. Ed. 2014, 53, 1098-1102.
[27] S. Osella, P. Samori, J. Cornil, J. Phys. Chem. C 2014, 118, 18721-
18729.
[3]
a) C. C. Jia, A. Migliore, N. Xin, S. Y. Huang, J. Y. Wang, Q. Yang, S.
P. Wang, H. L. Chen, D. M. Wang, B. Y. Feng, Z. R. Liu, G. Y. Zhang,
D. H. Qu, H. Tian, M. A. Ratner, H. Q. Xu, A. Nitzan, X. F. Guo, Science
2016, 352, 1443-1445; b) S. Cai, W. Deng, F. Huang, L. Chen, C. Tang,
W. He, S. Long, R. Li, Z. Tan, J. Liu, J. Shi, Z. Liu, Z. Xiao, D. Zhang,
W. Hong, Angewandte Chemie-International Edition 2019, 58, 3829-
3833.
[28] Z. Chen, L. Chen, J. Liu, R. Li, C. Tang, Y. Hua, L. Chen, J. Shi, Y.
Yang, J. Liu, J. Zheng, L. Chen, J. Cao, H. Chen, H. Xia, W. Hong, The
Journal of Physical Chemistry Letters 2019, 10, 3453-3458.
[29] N. Ferri, N. Algethami, A. Vezzoli, S. Sangtarash, M. McLaughlin, H.
Sadeghi, C. J. Lambert, R. J. Nichols, S. J. Higgins, Angew. Chem. Int.
Ed. 2019, 58, 16583-16589.
[4]
[5]
a) S. Afsari, P. Yasini, H. Peng, J. P. Perdew, E. Borguet, Angew Chem
Int Ed Engl 2019, 58, 14275-14280; b) J. He, Q. Fu, S. Lindsay, J. W.
Ciszek, J. M. Tour, JACS 2006, 128, 14828-14835.
a) S. Y. Quek, M. Kamenetska, M. L. Steigerwald, H. J. Choi, S. G.
Louie, M. S. Hybertsen, J. B. Neaton, L. Venkataraman, Nat.
Nanotechnol. 2009, 4, 230-234; b) R. Frisenda, V. A. E. C. Janssen, F.
C. Grozema, H. S. J. van der Zant, N. Renaud, Nat. Chem. 2016, 8,
1099-1104; c) E. Leary, C. Roche, H.-W. Jiang, I. Grace, M. Teresa
Gonzalez, G. Rubio-Bollinger, C. Romero-Muniz, Y. Xiong, Q. Al-
Galiby, M. Noori, M. A. Lebedeva, K. Porfyrakis, N. Agrait, A. Hodgson,
S. J. Higgins, C. J. Lambert, H. L. Anderson, R. J. Nichols, JACS 2018,
140, 710-718.
[6]
[7]
[8]
B. Shao, I. Aprahamian, Chem 2020, 6, 2162-2173.
H. Chen, J. Fraser Stoddart, Nat. Rev. Mater. 2021.
a) I. Cvrtila, H. Fanlo-Virgos, G. Schaeffer, G. Monreal Santiago, S.
Otto, J Am Chem Soc 2017, 139, 12459-12465; b) Y. Ren, L. You, J
Am Chem Soc 2015, 137, 14220-14228.
[9]
J. Holub, G. Vantomme, J.-M. Lehn, JACS 2016, 138, 11783-11791.
[10] Y. Zheng, J. Ren, Y. Wu, X. Meng, Y. Zhao, C. Wu, Bioconjugate
Chem. 2017, 28, 2620-2626.
[11] F. Chen, X. L. Li, J. Hihath, Z. F. Huang, N. J. Tao, J. Am. Chem. Soc.
2006, 128, 15874-15881.
[12] E. Leary, A. La Rosa, M. T. González, G. Rubio-Bollinger, N. Agraït, N.
Martín, Chem. Soc. Rev. 2015, 44, 920-942.
[13] a) B. Xu, N. Tao, Science 2003, 301, 1221-1223; b) L. Venkataraman,
J. E. Klare, C. Nuckolls, M. S. Hybertsen, M. L. Steigerwald, Nature
2006, 442, 904-907; c) C. Tang, L. J. Chen, L. Y. Zhang, Z. X. Chen, G.
P. Li, Z. W. Yan, L. C. Lin, J. Y. Liu, L. F. Huang, Y. L. Ye, Y. H. Hua, J.
Shi, H. P. Xia, W. J. Hong, Angewandte Chemie-International Edition
2019, 58, 10601-10605.
[14] L. Nagahara, T. Thundat, S. J. R. o. s. i. Lindsay, 1989, 60, 3128-3130.
[15] a) N. Darwish, A. C. Aragones, T. Darwish, S. Ciampi, I. Diez-Perez,
Nano Lett. 2014, 14, 7064-7070; b) M. C. Walkey, C. R. Peiris, S.
Ciampi, A. C. Aragones, R. B. Dominguez-Espindola, D. Jago, T.
Pulbrook, B. W. Skelton, A. N. Sobolev, I. D. Perez, M. J. Piggott, G. A.
Koutsantonis, N. Darwish, ACS Appl. Mater. Interfaces 2019, 11,
36886-36894; c) S. Kumar, J. T. Van Herpt, R. Y. N. Gengler, B. L.
Feringa, P. Rudolf, R. C. Chiechi, JACS 2016, 138, 12519-12526.
[16] A. Favila, M. Gallo, D. J. J. o. M. M. Glossman-Mitnik, 2007, 13, 505-
518.
[17] C. Tang, J. Zheng, Y. Ye, J. Liu, L. Chen, Z. Yan, Z. Chen, L. Chen, X.
Huang, J. Bai, Z. Chen, J. Shi, H. Xia, W. Hong, iScience 2020, 23,
100770.
[18] A. R. Owen, J. W. Golden, A. S. Price, W. A. Henry, W. K. Barker, D. A.
Perry, The Journal of Physical Chemistry C 2014, 118, 28959-28969.
[19] a) W. Haiss, C. Wang, I. Grace, A. S. Batsanov, D. J. Schiffrin, S. J.
Higgins, M. R. Bryce, C. J. Lambert, R. J. Nichols, Nat. Mater. 2006, 5,
995-1002; b) M. S. Hybertsen, L. Venkataraman, Acc Chem Res 2016,
49, 452-460; c) M. Kamenetska, M. Koentopp, A. C. Whalley, Y. S.
Park, M. L. Steigerwald, C. Nuckolls, M. S. Hybertsen, L.
Venkataraman, Phys. Rev. Lett. 2009, 102, 126803.
[20] C. Huang, M. Jevric, A. Borges, S. T. Olsen, J. M. Hamill, J.-T. Zheng,
Y. Yang, A. Rudnev, M. Baghernejad, P. Broekmann, A. U. Petersen, T.
Wandlowski, K. V. Mikkelsen, G. C. Solomon, M. B. Nielsen, W. Hong,
Nat. Commun. 2017, 8.
[21] a) S. Martin, W. Haiss, S. J. Higgins, R. J. Nichols, Nano Lett. 2010, 10,
2019-2023; b) Y. Kim, A. Garcia-Lekue, D. Sysoiev, T. Frederiksen, U.
Groth, E. Scheer, Phys. Rev. Lett. 2012, 109, 226801.
[22] a) Y. Han, C. Nickle, Z. Zhang, H. P. A. G. Astier, T. J. Duffin, D. Qi, Z.
Wang, E. del Barco, D. Thompson, C. A. Nijhuis, Nat. Mater. 2020, 19,
843-848; b) X. Qiu, S. Rousseva, G. Ye, J. C. Hummelen, R. C.
Chiechi, Adv. Mater. 2021, 33, e2006109.
[23] a) C. A. Nijhuis, W. F. Reus, J. R. Barber, G. M. Whitesides, J. Phys.
Chem. C 2012, 116, 14139-14150; b) L. Yuan, L. Jiang, B. Zhang, C. A.
Nijhuis, Angewandte Chemie-International Edition 2014, 53, 3377-3381;
c) C. S. S. Sangeeth, A. T. Demissie, L. Yuan, T. Wang, C. D. Frisbie,
C. A. Nijhuis, JACS 2016, 138, 7305-7314.
[24] X. P. Chen, M. Roemer, L. Yuan, W. Du, D. Thompson, E. del Barco, C.
A. Nijhuis, Nat. Nanotechnol. 2017, 12, 797-803.
[25] a) Y. Liu, L. Ornago, M. Carlotti, Y. Ai, M. El Abbassi, S. Soni, A.
Asyuda, M. Zharnikov, H. S. J. van der Zant, R. C. Chiechi, The Journal
of Physical Chemistry C 2020, 124, 22776-22783; b) S. Soni, G. Ye, J.
Zheng, Y. Zhang, A. Asyuda, M. Zharnikov, W. Hong, R. C. Chiechi,
Angew. Chem. Int. Ed. 2020, 59, 14308-14312.
[26] a) J. G. Simmons, J Journal of applied physics 1963, 34, 2581-2590; b)
J. G. Simmons, J Journal of applied physics 1963, 34, 1793-1803.
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This article is protected by copyright. All rights reserved.

10.1002/anie.202106666
Angewandte Chemie International Edition
RESEARCH ARTICLE
Entry for the Table of Contents
The responsive molecular devices based on the concept of dynamic covalent chemistry were fabricated, and the conductance
measurement shows that three well-defined conductance states were observed with an ON/OFF ratio ~10 in response to acid-based
or photo-thermal stimuli of dynamic acyl hydrazone bond, while the ON-OFF ratio is ~3 in self-assembled monolayer devices, which
originate from the dynamic acyl hydrazone bond exchange and C=N isomerization.
7
This article is protected by copyright. All rights reserved.