Single-molecule contact switching via electro-inductive effects

Abstract

The non-faradaic application of electric fields generated at the surface of charged electrodes to polarize bound molecules, also termed as electro-inductive effects, have recently attracted increasing attention in modifying the chemical reactivity of molecules in electrosynthesis. Herein, we applied this electro-inductive effect to control the Lewis adduct formation and dissociation between BF₃ and pyridine N of heterocycles to realize single-molecule contact switching. In situ single-molecule conductance measurements, in situ Raman analysis and theoretical calculations clearly show that the outward electric field along the positively-charged electrode surface polarizes adsorbed molecules to withdraw electron density from the terminal pyridine N, which weakens the N–BF₃ Lewis bond for dissociation upon applied positive potentials. The released unbounded pyridine N can connect the molecule into a molecular circuit for electron transfer (considered as the “ON” state). Meanwhile, the inward electric field along the negatively charged electrode surface promotes the formation of an N–BF₃ Lewis bond, leading to breaking of the molecular circuit (considered as the “OFF” state). Combined with the optimization of BF₃ concentration from the equilibrium BF₄⁻ ⇌ BF₃ + F⁻, the electro-inductive effect can reversibly switch single-molecule conductance in conductance measurements and tunnelling currents in I–V measurements.

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Introduction

Since the first prototype of a single-molecule rectifier proposed by Aviram and Ratner in 1974,¹ many studies have used various anchoring groups as “alligator clips” to connect single molecules into two metal electrode nanogaps,^2–4 which provide an ideal test platform for exploring electron transport at the single-molecule scale.^5–7 Using these chemical linkers like thiols,^8,9 thioethers,^10,11 amines^8,12 and pyridines,^8,13–16 much effort has been devoted to the design of stimuli-responsive functional molecules, and they have been integrated into electric circuits to show the possibility and capability of single-molecule devices.^17–22 However, the nature of molecule-metal contacts greatly determines the electron conduction and the current–voltage characteristics via modification of the molecular adsorption geometry and interfacial coupling between the discrete molecular orbitals and the electronic bands of the electrodes.^23–26 For example, recent experiments have demonstrated that tuning interfacial molecule-metal contacts via electrical and mechanical means can achieve the functionality of a molecular switch.^27–29 However, using external control to reversibly disconnect or connect molecule-metal contacts remains a challenge, and single-molecule circuits can be physically turned on and off, promising molecular switches with large ON/OFF ratios.^30–32

Electric fields have been shown to play a key role in many types of catalytic chemical reactions.^33,34 Very recently, the interfacial electric field near an electrode induced by an applied potential^35,36 has been shown to be analogous to the effect of electron withdrawing/donating substituents, which could continuously modulate the electronic properties of surface-bound molecules to change their chemical reactivity in electrosynthesis. This non-faradaic use of polarized electrochemical interfaces, termed as the electro-inductive effect in 2020, has attracted increasing attention.³⁵ For example, the electro-inductive effect has been shown to activate aromatic molecules toward redox neutral hydrolysis, crosscoupling, and amidation reactions, and control formation and dissociation of N-heterocycle-BF₃ Lewis adducts.^35,37,38 This electrochemical polarization may provide a new, yet unexplored, approach to control interfacial molecule-metal contacts in molecular junctions to achieve switching functionality.

In this work, we first used a scanning tunnelling microscope-based break junction (STM-BJ) method^4,39–41 to reveal the electro-inductive effect on molecule-metal contacts in single-molecule junctions. As schematically illustrated in Scheme 1, at a positively charged surface, the outward electric field along the electrode surface polarizes the adsorbed molecules to withdraw electron density from the pyridine nitrogen (denoted as N). This can weaken the Lewis bond of N–BF₃ and even make it dissociate. The released pyridine group can interact with the STM tip to form a molecular junction to allow electron transport (ON state). Conversely, at a negatively charged surface, the inward electric field pushes electron density toward the N, which promotes the formation of N-heterocycle-BF₃ Lewis adducts. This causes the molecules to be unable to connect with the STM tip, thereby achieving a circuit break (OFF state).

Scheme 1 Schematic diagram of STM-BJ methods used to probe the electro-induced effect of the N–BF₃ Lewis bond in LiBF₄ solution, which can regulate metal-molecule contacts in the single-molecule junction for switching.

A propylene carbonate (PC) solution containing LiBF₄ is used because BF₃ can be liberated from LiBF₄ based on the equilibrium BF₄⁻ ⇌ BF₃ + F⁻ in this system according to previous reports.³⁷ In this way, the electrochemical control of BF₃ + N ⇌ N–BF₃via electro-inductive effects can be used to reversibly tune the metal-molecule contacts in the molecular junctions. Electrochemical STM-BJ clearly shows the conductance peaks as a result of the formation of the single-molecule junction, dependent on the applied potential as well as the types and concentrations of electrolytes. In situ Raman spectroscopic evidence and theoretical calculations also confirm that the reversible single-molecule conductance switching can be ascribed to electro-inductive effects on this N-heterocycle–BF₃ Lewis adduct. Furthermore, I–V measurements performed in a fixed nanogap between the STM tip and substrate confirmed the feasibility of physically disconnecting or connecting single-molecule circuits, providing a new approach to achieve a reversible single-molecule switch via electro-inductive effects.

Results and discussion

The influence of Lewis bonds on single-molecule conductance

We first selected 1,4-bis(pyridin-4-yl)benzene (BPB) with pyridine groups as a target molecule, which has a very well-defined single molecule conductance peak in previous reports.^42,43 To obtain the intrinsic conductance information of molecular junctions, conductance measurements (see experimental details in ESI†) by using STM-BJ were carried out in a PC solution of 0.1 mM BPB, 0.1 mM BPB + 50 mM LiBF₄ or 0.1 mM BPB + 50 mM LiNO₃. As shown in Fig. 1a, the representative conductance-displacement traces show step features at about 10^−3.8G₀ (G₀ = 2e²/h, where e is the electron charge and h is Planck's constant) both in 0.1 mM BPB and 0.1 mM BPB + 50 mM LiNO₃ solution. Meanwhile, there are only decay traces observed in 0.1 mM BPB + 50 mM LiBF₄ solution. Typically, these step features are ascribed to the formation of single-molecule junctions after rupturing Au atomic contacts during the stretching process of the tip. This indicates that the formation of BPB molecular junctions could be obstructed in the solution containing 50 mM LiBF₄. 1000 individual conductance-displacement traces are used to construct 1D conductance histograms in Fig. 1b, showing a well-defined conductance peak at 10^−3.8G₀ in solution without LiBF₄, in contrast to only a blank background in LiBF₄ solution. This reveals that the BF₃ from BF₄⁻ dissociation forms a Lewis bond with the pyridine N in the molecule, and the N–BF₃ cannot bind to the Au electrodes to form a molecular junction.

Fig. 1 (a) Representative conductance-displacement traces and (b) 1D conductance histograms obtained in PC solution of 0.1 mM BPB (purple), 0.1 mM BPB + 50 mM LiBF₄ (blue) and 0.1 mM BPB + 50 mM LiNO₃ (yellow). (c) 2D conductance-displacement histograms of BPB molecular junctions. The inset panel shows the corresponding relative stretching distance distribution. (d) 1D conductance histograms of BPB molecular junctions obtained in LiBF₄ solutions with different concentrations. (e) UV-vis absorption spectra of 0.1 mM BPB solution with different concentrations of LiBF₄. (f) Plot of normalized Gaussian-fitted conductance peak intensity and UV-vis absorption peak intensity as a function of electrolyte concentration.

We also statistically analyze stretching displacements (z) in each conductance-displacement trace from 10^−6.0 to 10^−0.3G₀, consistent with previous reports.^40,44,45 The two-dimensional (2D) conductance histogram in Fig. 1c clearly shows an obvious stretching state centered around 10^−3.8G₀ after rupturing Au atomic contacts, and the Δz of 0.6 nm is found by Gaussian fitting. By adding the snapback distance (0.5 nm) of breaking Au atomic contacts, the most probable absolute displacement for molecular junctions is about 1.1 nm, comparable to the BPB molecular length of 1.15 nm. This proves the formation of single-molecule junctions.

To further clarify the Lewis bond effect on the formation of molecular junctions, single-molecule conductance measurements were also carried out in 0.1 mM BPB solution with different concentrations of LiBF₄. It can be expected that the equilibrium BF₄⁻ ⇌ BF₃ + F⁻ shifts in the opposite direction when decreasing the concentration of BF₄⁻, resulting in less BF₃ in solution to interact with BPB molecules. The conductance histograms in Fig. 1d clearly show that the conductance peak appears at 30 mM LiBF₄, and becomes more intense as the concentration of LiBF₄ is further decreased. Finally, the conductance peak at 10 mM LiBF₄ is comparable with that in a pure molecule solution. Controlled experiments using the target molecule 1,4-bis((2,3-dihydrobenzo[b]thiophen-5-yl)ethynyl) benzene without pyridine groups do not show such a conductance peak transition (Fig. S1†). This strongly supports the idea that the molecules almost turn into an N–BF₃ Lewis adduct when the concentration of LiBF₄ is greater than 40 mM. Then, the N–BF₃ cannot bind to the Au electrode to form molecular junctions.

The UV-vis spectra in Fig. 1e also reveal that the absorption peak of the 0.1 mM BPB solution red shifts when changing the concentration of LiBF₄ from 10 mM to 50 mM. The absorption peaks can be Gaussian fitted into two peaks centred at 280 nm and 312 nm, corresponding to BPB and BPB-BF₃ species, respectively (Fig. S2†). An obvious peak transition state can be observed at 20 mM. Such an absorption peak transition state can be observed for these diluted samples (Fig. S3†). This suggests that the formation of an N–BF₃ Lewis bond can be completed in bulk PC solution. Previous reports^37,38 found that when one pyridine group forms a Lewis bond with BF₃, the UV-vis absorption peak undergoes a 70 nm red shift. In addition, theoretical simulations in Fig. S4† show that when the single-terminal pyridine group of the BPB molecule is combined with BF₃, its absorption peak is red-shifted by 20 nm, consistent with experimental results. Fig. 1f compares the Gaussian-fitted conductance peak intensity and UV-vis absorption peak intensity as a function of electrolyte concentration. This nonlinear correlation between conductance peak intensity and molecular concentration in solution is consistent with previous reports.^46,47 Notably, the most significant conductance peak intensity reduction occurs at 20 mM LiBF₄ when most of the BPB molecules turn into BPB-BF₃ in the solution, indicated by the absorption peak variation. This proves that the N–BF₃ Lewis bond at one terminal pyridine group of the molecule suppresses the molecular junction formation.

Electro-inductive effects of the Lewis bond in single-molecule junctions

To investigate the electro-inductive effects of the Lewis bond in single-molecule junctions, the four-electrode setup of electrochemical STM is used to perform conductance measurements in 0.1 mM BPB + 30 mM LiBF₄ solution. In this configuration, the potentials of the Au (111) substrate (E_substrate) could be adjusted relative to the reference electrode. Cyclic voltammograms (CVs) of BPB on an Au electrode in Fig. S5† do not show any redox peak in the potential window of −0.8 V to 0.2 V. In line with the CV profile, conductance measurements were carried out in the same potential range with a 0.2 V interval. As shown in Fig. 2a, there is no conductance peak observed until the applied potentials of the Au(111) substrate (E_substrate) increase to −0.2 V and above. The measurements of the potential of zero charge (PZC) in Fig. S6† exhibited a −0.22 V value for Au (111) in 0.1 mM BPB + 30 mM LiBF₄ solution. This reveals that the Au electrode surface is positively charged above −0.22 V. It generates an electric field outward along the electrode surface to withdraw the electron density from the terminal N of adsorbed molecules and makes the N–BF₃ Lewis bond dissociate. Thus, the unbonded N can bind to the Au tip to form a molecular junction, resulting in the appearance of a conductance peak. Fig. S7† also shows that the distinct conductance peaks attributed to BPB molecular junctions were consistently observed in LiNO₃ solution, indicating that the interfacial electric fields generated under applied potentials cannot completely suppress pyridine–Au interaction.

Fig. 2 (a) 1D conductance histograms obtained at different applied E_substrate values in 0.1 mM BPB + 30 mM LiBF₄. (b) Plot of the conductance peak intensities of BPB molecular junctions against the applied E_substrate in LiBF₄ solution with different concentrations. (c) Binding energies of BF₃ on N and ICOHP values for the N–BF₃ Lewis bond under different electric fields. A more negative ICOHP indicates a lower energy bonding state and higher bond strength. The inset panels show electron density difference (Δρ = ρ_{(+0.5/−0.5)} − ρ₀) diagrams for BPB adsorption on Au(111), where ρ_{(+0.5/−0.5)} and ρ₀ are the electron density with and without the electric field, respectively. The red region represents the accumulation of electrons while the blue one represents depletion. Isovalue = 0.005 e Å⁻³.

Furthermore, we also changed the LiBF₄ concentration in the solutions to investigate the electro-inductive effects of the N–BF₃ Lewis bond. Detailed conductance histograms at different E_substrate values can be found in Fig. S8.†Fig. 2b summarizes the conductance peak intensities of BPB molecular junctions against the applied E_substrate in LiBF₄ solution with different concentrations. In 10 mM LiBF₄ solution, the conductance peak exists in the electrochemical window from 0.2 V to −0.8 V, but its intensity weakens as the potential decreases. This means that when the BF₃ concentration in the solution is low, the inductive effect at negative potentials cannot convert adsorbed molecules into enough Lewis adducts, thereby reducing the probability of forming a molecular junction to a low enough level that no conductance peak can be observed.

Interestingly, in the LiBF₄ solution with concentration ranges of 20–40 mM, as the potential decreases from 0.2 V, the conductance peak intensity decreases, and the potential at which it disappears is significantly advanced as the concentration increases. When the electrolyte concentration reaches 50 mM, no conductance peak can be detected in the entire electrochemical window. This suggests that when the electrode surface carries more negative charges, a stronger electron-donating inductive effect will occur, promoting the formation of N–BF₃ Lewis bonds. However, the BF₃ concentration in solution is also crucial to achieve the complete suppression of the molecular junction formation. The formation of a single-molecule junction requires only a certain surface coverage of the unbonded molecules, in line with the previous reports.^43,46,48 Utilizing the electro-inductive effects can also regulate the interaction between Lewis basic pyridyl N and Lewis acid Al(CH₃)₃, which enables controllable modulation of single-molecule contacts in molecular junctions (Fig. S9†).

To gain a greater understanding of the electro-inductive effects on the N–BF₃ Lewis bonds, we further compared their behaviour under opposite electric field directions. The electric field strength was set to 0.5 V Å⁻¹.^49,50 The results show that the N–BF₃ bond (Fig. S10†) elongates and the binding energy (Fig. 2c) of BF₃ on N becomes less negative (−1.43 to −0.42 eV) as the electric field switches from −0.5 to 0.5 V Å⁻¹, suggesting that the Lewis acid–base interaction between BF₃ and N is suppressed by an outward electric field (positively charged surface). Further analyses of the electronic structures of N–BF₃ bonds were conducted by utilizing crystal orbital Hamiltonian population (COHP). As shown in Fig. S11,† a shift in the occupied bonding states towards the Fermi level was observed when the field was switched from −0.5 to 0.5 V Å⁻¹. This suggests an increase in bonding state energy, leading to reduced N–BF₃ stability.

Quantitatively, under an inward electric field (negatively charged surface), the integrated COHP (ICOHP) value becomes more negative (Fig. 2c) which indicates a stronger N–BF₃ bond. This trend can be attributed to the distinctly different electron transfer under various electric fields. Specifically, the direction of electron transfer under the two opposite electric fields aligned with the charge transfer direction of the bare Au(111) surface (Fig. S12†). As shown in the inset panels of Fig. 2c, under an inward electric field, electrons transfer from the surface to the BF₃ termination (in line with the N–B dipole), while under an outward electric field, electrons are transferred from the BF₃ termination to the Au surface (opposite to the N–B dipole). The distinctly different electron transfer results in a strengthening and weakening of the polar B–N bond, respectively. The adsorption strength of BPB-BF₃ on Au(111) is much weaker than that of BPB (Fig. S13†), suggesting its difficulty to form molecular junctions.

Interfacial molecular evidence

To investigate and elaborate the interfacial molecular structures upon different electro-inductive effects, we performed in situ Raman analysis by using shell-isolated nanoparticle enhanced Raman spectroscopy^51–53 (SHINERS, See the experimental details in the ESI†). This typically used Au core–SiO₂ shell nanoparticles as Raman-signal amplifiers (Fig. S14†) and could effectively work on a single-crystal surface. Fig. 3a shows the potential-dependent Raman spectra in 0.1 mM BPB + 30 mM LiBF₄ solution from 0.2 V to −0.8 V with 0.1 V intervals. It is clear that the intense Raman bands at 1031, 1225, 1291, and 1609 cm⁻¹ can be ascribed to the pyridyl ring breathing mode, in-plane CH bending, inter-ring stretching and parallel CC stretching of the BPB molecule, consistent with previous reports.^51,54 The Raman signals of the BPB molecule can be observed throughout the entire electrochemical window, indicating that the molecule is stably adsorbed on the electrode surface via one of its pyridine groups. This further supports the idea that in the bulk solution, only one end of the molecular pyridine group forms a Lewis bond with BF₃, while the other pyridine group is uncoordinated and can bind to the gold electrode.

Fig. 3 (a) In situ Raman spectra obtained at Au(111) in 0.1 mM BPB + 30 mM LiBF₄ upon applied E_substrate from 0.2 V to −0.8 V. (b) Theoretical simulated Raman spectra of τ_cc of the BPB–BF₃ Lewis adduct compared to that of isolated BPB. The illustration depicts a molecular structure, where red dots represent atoms vibrating outward, and green crosses indicate atoms vibrating inward. (c) Plot of the normalized intensities of the Raman band of τ_cc and conductance peak of the molecular junctions under different applied potentials.

Interestingly, when the applied potential is reduced to −0.1 V and below, there is a sudden and significant increase in the intensities of the Raman bands associated with the molecules. At the same time, a new band appears at 403 cm⁻¹. According to the DFT simulations in Fig. 3b, compared with the isolated molecule, for the Lewis adduct with BF₃, a new Raman peak appears at 404 cm⁻¹, which is attributed to the CC out-of-plane twisting (τ_cc) vibration of the benzene ring and pyridine rings. In contrast, in the control experiment in LiNO₃ electrolyte solution (Fig. S15†), this Raman signal of τ_cc does not appear, and the other peak intensity of the molecule does not change significantly upon applied potential. This molecular evidence directly reveals that when the potential is reduced below the PZC, the molecules on the electrode surface form Lewis adducts with BF₃.

Fig. 3c compares the average normalized intensities of the Raman band of τ_cc from three independent experiments and conductance peak of molecular junctions under different applied potentials. The Raman intensity is normalized by the maximum peak area at 404 cm⁻¹ in each dataset. Clearly, when the applied potential is lower than the PZC of −0.22 V, the intensity of the Raman peak of τ_cc increases abruptly, while the intensity of the conductance peak decreases sharply. This proves that the inward electric field generated at the negatively charged electrode surface promotes the formation of N–BF₃ Lewis adducts, preventing the formation of molecular junctions and causing the conductance peak to disappear. In addition, in situ Raman experiments in 50 mM LiBF₄ solution clearly show strong τ_cc in the potential range of 0.2 V to −0.8 V (Fig. S16†), because the increase in BF₃ concentration promotes the formation of N–BF₃ Lewis bonds. This further proves that the formation of N–BF₃ Lewis bonds prevents the Au-molecule contact from forming molecular junctions, consistent with the results of no conductance peak observed in the conductance measurement experiments.

Single-molecule contact switch

We next sought to further confirm our hypothesis of the electro-inductive effect controlled single-molecule contact switches. Conductance measurements of BPB were carried out upon cycling E_substrate between 0 V and −0.6 V in 0.1 mM BPB + 30 mM LiBF₄ solution. The conductance histograms in Fig. S17† clearly show that a well-defined conductance peak at 10^−4.0G₀ repeatably appears at 0 V, but disappears at −0.6 V. Furthermore, 1,2-bis(4-pyridyl)ethylene (BPY-EE) and 4’-(pyridin-4-yl)-[1,1′-biphenyl]-4-carboxylic acid (PBBA) also show the same interesting switching phenomena upon changing the applied E_substrate to generate different electro-inductive effects near the electrode surface. Detailed conductance histograms can be found in Fig. S18.† The different concentrations of LiBF₄ for BPY-EE and PBBA to achieve complete suppression of the conductance peaks might arise from their electronic structures, which might alter the strength of Lewis acid–base interactions between pyridinic nitrogen atoms and BF₃. The conductance peak disappears due to the formation of N–BF₃ Lewis bonds preventing the formation of molecular junctions, which means that the molecular circuit is disconnected (OFF state). Meanwhile, the positively charged surface generates the electron-withdrawing effects to break N–BF₃ Lewis bonds. Then, it promotes the formation of a molecular junction to allow electron tunnelling via the single-molecules, considered as a connection of the molecular circuit (ON state). Fig. 4a–c summarizes the results of repeatedly cycling the applied potentials followed by single-molecule conductance measurements. The conductance peaks at 0 V or −0.6 V appear and disappear repeatedly, clearly indicating that this single-molecule contact control is reversible. This demonstrates the good stability of single-molecule switches upon different electro-inductive effects.

Fig. 4 Sequential cycles of conductance switching of (a) BPB, (b) BPY-EE and (c) PBBA between “ON” and “OFF” states. The purple dots are the conductance values obtained at 0 V after the different potential cycles, and the blue dots are positioned at 10⁻⁶G₀ (the detection limit of the current signal amplifier) to indicate the disappearance of conductance peaks at −0.5 V or −0.6 V. The molecular structures are shown in the inset panels. (d) 2D I–V histogram of BPB obtained from dozens of data points when forming a single-molecule junction and sweeping the E_substrate from 0 V to −0.6 V.

To further test the molecule-contact switching in a fixed nanogap, we first constructed a single-molecule junction at 0 V in 0.1 mM BPB + 30 mM LiBF₄ solution. When a single-molecule junction was formed, we hovered the STM tip above the substrate and performed I–V measurements. The experimental details can be found in the ESI.† The applied E_substrate is swept from 0 V to −0.6 V, during which the electrode surface becomes negatively charged below the PZC and the initial electric field direction can be reversed to promote BF₃–pyridine interactions. Fig. S19† shows the typical I–V curve recorded upon the formation of molecular junctions. It is clear that the current jumps to the low current baseline during a negative potential sweep. This confirms that the electro-inductive effect induces the breaking of Au–N contacts in the molecular junction and forms N–BF₃ Lewis bonds. Fig. 4d shows the statistical results of the I–V curves collected at the formation of molecular junctions. The relative current difference is about 0.43 nA, which is comparable to single-molecule conductance in the break junction measurements. In addition, I–t tests according to previous reports⁷ in 0.1 mM BPB + 30 mM LiBF₄ at an E_substrate of 0 V confirm that the current jumps to the ON state due to junction formation, in contrast to only the base current observed at −0.6 V (Fig. S20†). These results further confirm that the electro-inductive effects at different applied potentials can modulate the Lewis chemistry between N and BF₃, leading to conductance ON/OFF switching.

Conclusions

We have successfully demonstrated that electro-inductive effects can be used to control the metal-molecule contact in a single-molecule junction, thus forming a type of connection-controlled molecular switch. Conductance measurements using the STM-BJ method clearly show that the single-molecule conductance peaks of heterocyclic molecules containing pyridyl groups become significantly weaker or even disappear when decreasing applied potentials to negatively charge an electrode surface in LiBF₄ solutions with different concentrations. Upon changing the electrolyte to LiNO₃ or using no pyridine groups, there is almost no change in the single-molecule conductance peak. Molecular evidence obtained by using UV-visible spectroscopy and in situ Raman spectroscopy reveals that the inward electric field along the negatively charged surface promotes the formation of the N–BF₃ Lewis bond between the BF₃ (liberated from LiBF₄ based on the equilibrium BF₄⁻ ⇌ BF₃ + F⁻) and molecular pyridyl group. In contrast, the outward electric field along the electrode surface polarizes the adsorbed molecules to withdraw electron density from the terminal pyridine N, which weakens the N–BF₃ Lewis bond so it can dissociate upon applied positive potentials. With appropriate concentrations of LiBF₄ (e.g. 10–40 mM), the electro-inductive effect can lead to the single-molecule conductance peaks of BPY-EE and PBBA reversibly disappearing at low potentials and appearing at high potentials. This can be analogous to physically disconnecting and connecting molecule-metal contacts in single-molecule junctions to tune electron tunnelling. Furthermore, such metal-molecule contact switching is also demonstrated in a fixed nanogap between two electrodes in I–V measurements. This work sheds light on the electro-inductive effects on interfacial Lewis chemistry and opens a new avenue for the design of high-performance single-molecule switches.

Data availability

The data that support the findings of this study are available from the corresponding authors upon reasonable request.

Author contributions

Z. Y. L.: Conceived the theme of the paper and completed most of the experiments, discussions, data analysis, and wrote the manuscript; B. T. H.: Conducted surveys, completed part of the experiments, and participated in discussions and data interpretation; Y. J. T.: Theoretical aspect investigation, completed theoretical calculations, and interpreted the data; L. L. N.: Completed part of the experiments and participated in data analysis and discussions; W. Q.: Guided the theoretical calculations, participated in data analysis, and contributed to writing, reviewing, and editing the manuscript; Z. J. F.: Guided the paper, interpreted data, and reviewed the manuscript; S. Y.: Guided the paper, interpreted data, and reviewed the manuscript; W. Y. H.: Conceptualized and directed the project, conducted surveys, supervised, managed the project, secured funding, and wrote the manuscript; Z. X. S.: Conceptualized the study, conducted surveys, developed methodology, performed formal analysis, secured funding, and provided comments and edits.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge financial support from the National Natural Science Foundation of China (No. 22102150, 22172146, 22303085, and 21872126), and the Zhejiang Provincial Natural Science Foundation of China (No. LZ25B030003, LQ21B030010, LQ24B030014).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5sc02252e

‡ Ya-Li Zhang, Tian-Hang Bai, Jing-Tao Ye, contributed equally to this work.

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