Light‐Driven Reversible Intermolecular Proton Transfer at Single‐Molecule Junctions

All-in-One Photoacid Generators with Green/Red-light Responsiveness and Cooperative Functionality

Photoacid generators (PAGs) are invaluable molecular tools that exhibited tremendous potential in emerging interdisciplinary researches of life-science, nanotechnology and smart materials. However, current PAGs are primarily mono-functional in terms of acid generation and rely on UV/deep-blue light excitation, posing a fundamental hurdle to their broader adoption. Developing cooperatively functioned PAGs with long-wavelength light responsiveness presents a formidable challenge due to the absence of suitable molecular scaffolds. Here, we introduce a newly-developed perylene bisimides PAG motif (PBI-PAG) that integrates desired multi-functionality and visible-light photo-reactivity. Taking advantages of characteristic opto-electronic properties of PBI scaffold, PBI-PAGs are capable of quantitative releasing (>99 %) a palette of acids upon green/red light (560-605 nm) excitation. Concurrently, a photo-generated counterpart is functioned as a photo-sensitizer that could perform cooperatively with acid as an anti-metastasis cancer therapy agent. These two processes constitute the first example of a cooperatively functioned PAG operated at substrate-adaptive wavelengths.

Acid-Mediated Modulation of the Conductance of Diazapentalene Molecular Junctions

We report an acid-mediated regulation of single-molecule junction conductance achieved using an electron-deficient unit, diazapentalene, functionalized with thiophene extending units and thiomethyl aurophilic terminal groups. This diazapentalene derivative exhibits a protonation reaction in the presence of trifluoroacetic acid, as characterized by UV-vis absorption spectroscopy, and the protonated species shows a voltage-dependent single-molecule conductance, which is not observed for the pristine molecules. Specifically, under a high bias voltage of 850 mV, we observe a conductance value for the protonated molecule larger than that for the deprotonated one by a factor of 4. Density functional theory-based transport calculations show a slight broadening of the HOMO and LUMO frontier orbitals, as well as a reduced HOMO-LUMO gap when the molecule becomes protonated; this implies an increased conductance under protonation that is consistent with the experimental conductance data. Our work demonstrates a new molecular design for versatile control of molecular conductance through the use of acid in the solvent environment.

Linear Non-benzenoid Isomer of Acene Fusing Chrysene with Azulene Units

Non-benzenoid polycyclic aromatic hydrocarbons (PAHs) have received considerable attention owing to their distinctive optical and electrical properties. Nevertheless, the synthesis and optoelectronic application of non-benzenoid PAHs remain challenging. Herein, we present a facile synthesis of linear non-benzenoid PAH with an armchair edge, diACh, by fusing chrysene with two azulene units. We systematically investigated the optical and electrical properties, which were also compared to its isomers, including benzenoid and non-benzenoid zigzag edge isomers. diACh exhibits global aromaticity, good planarity, and suitable highest occupied molecular orbital/lowest unoccupied molecular orbital energy levels. The protonation of diACh in solution successively forms a stable tropylium cation and dication. Moreover, the neutral, cationic, and dicationic states of diACh can be transformed with remarkable reversibility during the protonation-deprotonation process, as confirmed by ultraviolet-visible absorptions, fluorescence spectra, 1H nuclear magnetic resonance, and theoretical calculations. Additionally, we fabricate p-type organic field-effect transistor (OFET) devices based on diACh with hole mobility up to 0.026 cm2 V-1 s-1, and we further develop OFET-based acid vapor sensors with good sensitivity, recyclability, and selectivity. These findings underscore the unique properties of linear non-benzenoid PAHs with an armchair edge engendered by the fusion of azulene with the acene backbone, showcasing prospective applications in organic optoelectronics.

Technologies for investigating single-molecule chemical reactions

Single molecules, the smallest independently stable units in the material world, serve as the fundamental building blocks of matter. Among different branches of single-molecule sciences, single-molecule chemical reactions, by revealing the behavior and properties of individual molecules at the molecular scale, are particularly attractive because they can advance the understanding of chemical reaction mechanisms and help to address key scientific problems in broad fields such as physics, chemistry, biology and materials science. This review provides a timely, comprehensive overview of single-molecule chemical reactions based on various technical platforms such as scanning probe microscopy, single-molecule junction, single-molecule nanostructure, single-molecule fluorescence detection and crossed molecular beam. We present multidimensional analyses of single-molecule chemical reactions, offering new perspectives for research in different areas, such as photocatalysis/electrocatalysis, organic reactions, surface reactions and biological reactions. Finally, we discuss the opportunities and challenges in this thriving field of single-molecule chemical reactions.

The rapid synthesis of 1,10-phenanthroline-5,6-diimine (Phendiimine) and its fascinating photo-stimulated behavior

Here, for the first time, we report synthesis of 1,10-phenanthroline-5,6-diimine (Phendiimine) based on an acid catalysed SN2 reaction of 1,10-phenanthroline-5,6-dione and 2-picolylamine in EtOH as a solvent. The synthesized Phendiimine molecule showed excellent photo-sensitivity against visible light, together with photoluminescence in both water and ethanol and also, it showed electrochemical activity with Fe electrode in ethanol and H2SO4 solution. Tauc plot also showed Phendiimine is a direct band-gap semiconductor. The hot-point probe test also showed that it is a n-type semiconductor. The UV-vis. absorption maximum shift in two solvents (water and ethanol) demonstrates the solvatochromism behavior of the molecule. The practical significance of this work and its guiding implication for future related research can be outlined as follows. Based on the results obtained, it appears that the Phendiimine molecule could revolutionize the medical field, potentially in the design of artificial eyes, increasing the yield of photovoltaic cells through enhanced heat transfer, improving computers and industrial photo-cooling systems, serving as photo-controller in place of piezoelectric devices, functioning as electronic opt couplers, controlling remote lasers, changing convection in photothermal heaters, designing miniaturized real photo-stimulated motors, creating photo or thermal switches through spin crossover complexes, developing electronic light-dependent resistance (LDR) devices, constructing X-ray and gamma-ray detectors, designing intelligent clothing, creating photo dynamic tumour therapy (PDT) complexes, singlet fission materials in solar cells and more.

Photo-Controllable Smart Hydrogels for Biomedical Application: A Review

Nowadays, smart hydrogels are being widely studied by researchers because of their advantages such as simple preparation, stable performance, response to external stimuli, and easy control of response behavior. Photo-controllable smart hydrogels (PCHs) are a class of responsive hydrogels whose physical and chemical properties can be changed when stimulated by light at specific wavelengths. Since the light source is safe, clean, simple to operate, and easy to control, PCHs have broad application prospects in the biomedical field. Therefore, this review timely summarizes the latest progress in the PCHs field, with an emphasis on the design principles of typical PCHs and their multiple biomedical applications in tissue regeneration, tumor therapy, antibacterial therapy, diseases diagnosis and monitoring, etc. Meanwhile, the challenges and perspectives of widespread practical implementation of PCHs are presented in biomedical applications. This study hopes that PCHs will flourish in the biomedical field and this review will provide useful information for interested researchers.

Theoretical Study on the Open-Shell Electronic Structure and Electron Conductivity of [18]Annulene as a Molecular Parallel Circuit Model

Herein, the electron conductivities of [18]annulene and its derivatives are theoretically examined as a molecular parallel circuit model consisting of two linear polyenes. Their electron conductivities are estimated by elastic scattering Green's function (ESGF) theory and density functional theory (DFT) methods. The calculated conductivity of the [18]annulene does not follow the classical conductivity, i.e., Ohm's law, suggesting the importance of a quantum interference effect in single molecules. By introducing electron-withdrawing groups into the annulene framework, on the other hand, a spin-polarized electronic structure appears, and the quantum interference effect is significantly suppressed. In addition, the total current is affected by the spin polarization because of the asymmetry in the coupling constant between the molecule and electrodes. From these results, it is suggested that the electron conductivity as well as the quantum interference effect of π-conjugated molecular systems can be designed using their open-shell nature, which is chemically controlled by the substituents.

Reversible electric switching of NDI molecular wires by orthogonal stimuli

The charge transport of 1,4,5,8-naphthalene diimide (NDI)-based molecules is explored. Experimental results show that the conductance of the TH-NDI molecular junction can be reversibly tuned by bias voltage and solvent, while the conductance of the PH-NDI junction is almost independent of the bias voltage and solvent. Based on these orthogonal stimuli, an AND logic gate of TH-NDI junction with an electric signal as the output is constructed. These results will advance the development of functional molecular devices.

Synthesis of metalla-dual-azulenes with fluoride ion recognition properties

Azulene-based conjugated systems are of great interests due to their unusual structures and photophysical properties. Incorporation of a transition metal into azulene skeleton presents an intriguing opportunity to combine the dπ-pπ and pπ-pπ conjugated properties. No such metallaazulene skeleton however has been reported to date. Here, we describe our development of an efficient [5 + 2] annulation reaction to rapid construction of a unique metal-containing [5-5-7] scaffold, termed metalla-dual-azulene (MDA), which includes a metallaazulene and a metal-free organic azulene intertwined by sharing the tropylium motif. The two azulene motifs in MDA exhibit distinct reactivities. The azulene motif readily undergoes nucleophilic addition, leading to N-, O- and S-substituted cycloheptanetrienyl species. Demetalation of the metallaazulene moiety occurs when it reacts with nBu4NF, which enables highly selective recognition of fluoride anion and a noticeable color change. The practical [5 + 2] annulation methodology, facile functional-group modification, high and selective fluoride detection make this new π-conjugated polycyclic system very suitable for potential applications in photoelectric and sensing materials.

Local cation-tuned reversible single-molecule switch in electric double layer

The nature of molecule-electrode interface is critical for the integration of atomically precise molecules as functional components into circuits. Herein, we demonstrate that the electric field localized metal cations in outer Helmholtz plane can modulate interfacial Au-carboxyl contacts, realizing a reversible single-molecule switch. STM break junction and I-V measurements show the electrochemical gating of aliphatic and aromatic carboxylic acids have a conductance ON/OFF behavior in electrolyte solution containing metal cations (i.e., Na⁺, K⁺, Mg²⁺ and Ca²⁺), compared to almost no change in conductance without metal cations. In situ Raman spectra reveal strong molecular carboxyl-metal cation coordination at the negatively charged electrode surface, hindering the formation of molecular junctions for electron tunnelling. This work validates the critical role of localized cations in the electric double layer to regulate electron transport at the single-molecule level.

Stereoelectronic Modulation of a Single-Molecule Junction through a Tunable Metal-Carbon dπ-pπ Hyperconjugation

Conjugated molecules play a critical role in the construction of single-molecule devices. However, most conventional conjugated molecules, such as hydrocarbons, involve only a pπ-pπ conjugation of light elements. While the metal d-orbitals can introduce abundant electronic effects to achieve novel electronic properties, it is very scarce for the charge transport study of dπ-pπ conjugated pathways with a metal involved. Here, we employed the single-molecule break junction technique to investigate the charge transport through dπ-pπ conjugated backbones with metal-carbon multiple bonds integrated into the alternative conjugated pathways. The involved dπ-pπ conjugation not only supports high conductivity comparable to that of conjugated hydrocarbons but also significantly enhances the tunable diversity in electronic properties through the metal-induced secondary interaction. Specifically, the introduction of the metal brings an unconventionally stereoelectronic effect triggered by metal-carbon dπ-pπ hyperconjugation, which can be tuned by protonation taking place on the metal-carbon multiple bonds, collectively modulating the single-molecule rectification feature and transmission mechanism. This work demonstrates the promise of utilizing the diverse electronic effect of metals to design molecular devices.

Supramolecular Enhancement of Charge Transport through Pillar[5]arene-Based Self-Assembled Monolayers

Intermolecular charge transport is one of the essential modes for modulating charge transport in molecular electronic devices. Supermolecules are highly promising candidates for molecular devices because of their abundant structures and easy functionalization. Herein, we report an efficient strategy to enhance charge transport through pillar[5]arene self-assembled monolayers (SAMs) by introducing cationic guests. The current density of pillar[5]arene SAMs can be raised up to about 2.1 orders of magnitude by inserting cationic molecules into the cavity of pillar[5]arenes in SAMs. Importantly, we have also observed a positive correlation between the charge transport of pillar[5]arene-based complex SAMs and the binding affinities of the pillar[5]arene-based complexation. Such an enhancement of charge transport is attributed to the efficient host-guest interactions that stabilize the supramolecular complexes and lower the energy gaps for charge transport. This work provides a predictive pattern for the regulation of intermolecular charge transport in guiding the design of next generation switches and functional sensors in supramolecular electronics.

Single-molecule nano-optoelectronics: insights from physics

Single-molecule optoelectronic devices promise a potential solution for miniaturization and functionalization of silicon-based microelectronic circuits in the future. For decades of its fast development, this field has made significant progress in the synthesis of optoelectronic materials, the fabrication of single-molecule devices and the realization of optoelectronic functions. On the other hand, single-molecule optoelectronic devices offer a reliable platform to investigate the intrinsic physical phenomena and regulation rules of matters at the single-molecule level. To further realize and regulate the optoelectronic functions toward practical applications, it is necessary to clarify the intrinsic physical mechanisms of single-molecule optoelectronic nanodevices. Here, we provide a timely review to survey the physical phenomena and laws involved in single-molecule optoelectronic materials and devices, including charge effects, spin effects, exciton effects, vibronic effects, structural and orbital effects. In particular, we will systematically summarize the basics of molecular optoelectronic materials, and the physical effects and manipulations of single-molecule optoelectronic nanodevices. In addition, fundamentals of single-molecule electronics, which are basic of single-molecule optoelectronics, can also be found in this review. At last, we tend to focus the discussion on the opportunities and challenges arising in the field of single-molecule optoelectronics, and propose further potential breakthroughs.

Stable Universal 1- and 2-Input Single-Molecule Logic Gates

Controllable single-molecule logic operations will enable development of reliable ultra-minimalistic circuit elements for high-density computing but require stable currents from multiple orthogonal inputs in molecular junctions. Utilizing the two unique adjacent conductive molecular orbitals (MOs) of gated Au/S-(CH₂ )₃ -Fc-(CH₂ )₉ -S/Au (Fc = ferrocene) single-electron transistors (≈2 nm), a stable single-electron logic calculator (SELC) is presented, which allows real-time modulation of output current as a function of orthogonal input bias (V_b ) and gate (V_g ) voltages. Reliable and low-voltage (ǀV_b ǀ ≤ 80 mV, ǀV_g ǀ ≤ 2 V) operations of the SELC depend upon the unambiguous association of current resonances with energy shifts of the MOs (which show an invariable, small energy separation of ≈100 meV) in response to the changes of voltages, which is confirmed by electron-transport calculations. Stable multi-logic operations based on the SELC modulated current conversions between the two resonances and Coulomb blockade regimes are demonstrated via the implementation of all universal 1-input (YES/NOT/PASS_1/PASS_0) and 2-input (AND/XOR/OR/NAND/NOR/INT/XNOR) logic gates.

Photoconductance from the Bent-to-Planar Photocycle between Ground and Excited States in Single-Molecule Junctions

Single-molecule conductance measurements for 9,14-diphenyl-9,14-dihydrodibenzo[a,c]phenazine (DPAC) may offer unique insight into the bent-to-planar photocycle between the ground and excited states. Herein, we employ DPAC derivative DPAC-SMe as the molecular prototype to fabricate single-molecule junctions using the scanning tunneling microscope break junction technique and explore photoconductance dependence on the excited-state structural/electronic changes. We find up to ∼200% conductance enhancement of DPAC-SMe under continuous 340 nm light irradiation than that without irradiation, while photoconductance disappears in the case where structural evolution of the DPAC-SMe is halted through macrocyclization. The in situ conductance modulation as pulsed 340 nm light irradiation is monitored in the DPAC-SMe-based junctions alone, suggesting that the photoconductance of DPAC-SMe stems from photoinduced intramolecular planarization. Theoretical calculations reveal that the photoinduced structural evolution brings about a significant redistribution of the electron cloud density, which leads to the appearance of Fano resonance, resulting in enhanced conductance through the DPAC-SMe-fabricated junctions. This work provides evidence of bent-to-planar photocycle-induced conductance differences at the single-molecule level, offering a tailored approach for tuning the charge transport characteristics of organic photoelectronic devices.

Achieving Multiple Quantum-Interfered States via Through-Space and Through-Bond Synergistic Effect in Foldamer-Based Single-Molecule Junctions

The construction of multivalued logic circuits by multiple quantum-interfered states at the molecular level can make full use of molecular diversity and versatility, broadening the application of molecular electronics. Understanding charge transport through different conducting pathways and how they interact with each other in molecules with a secondary structure is an indispensable foundation to achieve this goal. Herein, we elucidate the synergistic effect from through-space and through-bond conducting pathways in foldamers derived from ortho-pentaphenylene by the separate modulation on these pathways. The shrinkage of central heterocycles' sizes allows foldamers to stack with larger overlap degrees, resulting in level-crossing and thus transformation from constructive quantum interference (CQI) to destructive quantum interference (DQI) in a through-space pathway. The alteration of central heterocycles' connection sites enhances through-bond conjugation, leading to amplified contribution from a through-bond pathway. The enhanced through-bond pathway destructively interferes with the through-space pathway, exerting a suppression effect on transmission. Therefore, four quantum-interfered states of through-space and through-bond combination are generated, including through-space CQI-dominated states, through-space DQI-dominated states, through-space CQI states with through-bond suppression, and through-space DQI states with through-bond suppression. These findings enable us to regulate charge transport within high-order structures via multiple conducting pathways and provide a proof of concept to construct multivalued logic circuits.

Azulene-Embedded [n]Helicenes (n=5, 6 and 7)

Azulene is a non-benzenoid aromatic building block with unique chemical structure and physicochemical properties. By using the "bottom-up" synthetic strategy, we synthesized three azulene-embedded [n]helicenes ([n]AzHs, n=5, 6 and 7), in which one terminal azulene subunit was fused with n-2 benzene rings. P- and M-enantiomers were observed in the packing diagrams of [5]-, and [6]AzHs. However, P- and M-[7]AzHs could be isolated by recrystallization of the racemic mixture. These [n]AzHs were endowed with new properties through the azulene moiety such as low-lying first electric state (S₁ ), small optical energy gap and anti-Kasha emission. [6]-, and [7]AzHs exhibit strong chiroptical responses with high absorption dissymmetry factor (g_abs ) maxima of about 0.02, which is among the highest |g_abs | values of helicenes in the visible range. These azulene-embedded [n]helicenes contribute to the non-benzenoid helicene library and allow the structure-property relationships to be better understood.

Reversible photo control of proton chemistry

Spatial, temporal, and remote control of proton chemistry can be achieved by using photoacids, which are molecules that transform from weak to strong acids under light. Most of proton chemistry is driven by a high concentration of protons ([H⁺]), which is difficult to obtain using excited-state photoacids. Metastable-stable state photoacids (mPAHs) can reversibly generate a high [H⁺] under visible light with a moderate intensity. It has been widely applied in different fields, e.g. fueling dissipative assemblies, driving molecular machines, controlling organic reactions, powering nanoreactors, curing diseases, manipulating DNA and proteins, developing smart materials, capturing carbon dioxide in air etc. This article compares mPAH with excited-state photoacid as well as common acids e.g. HCl to explain its advantages. Recent studies on the thermal dynamics, kinetics, and photoreaction of mPAHs are reported. The advantages and disadvantages of the three types of mPAHs, i.e. merocyanine, indazole, and TCF mPAHs, are compared with regard to photo-induced [H⁺], switching rate, and other properties. The mechanisms of controlling or driving functional systems, which involve acid-base reactions, acid catalyzed reactions, ionic bonding, coordination bonding, hydrogen bonding, ion exchange, cation-π interaction, solubility, swellability, permeability, and pH change in biosystems, are described. Applications of mPAHs in the chemical, material, energy, biotechnology and biomedical fields published in the past 5 years are reviewed. Prospects in the development and application of mPAHs are discussed.

Recent Advances in Photochemical Reactions on Single-Molecule Electrical Platforms

The photochemical reaction is a very important type of chemical reactions. Visualizing and controlling photo-mediated reactions is a long-standing goal and challenge. In this regard, single-molecule electrical detection with label-free, real-time and in situ characteristics has unique advantages in monitoring the dynamic process of photoreactions at the single-molecule level. In this Review, we provide a valuable summary of the latest process of single-molecule photochemical reactions based on single-molecule electrical platforms. The single-molecule electrical detection platforms for monitoring photoreactions are displayed, including their fundamental principles, construction methods and practical applications. The single-molecule studies of two different types of light-mediated reactions are summarized as below: i) photo-induced reactions, including reversible cyclization, conformational isomerization and other photo-related reactions; ii) plasmon-mediated photoreactions, including reaction mechanisms and concrete examples, such as plasmon-induced photolysis of S-S/O-O bonds and tautomerization of porphycene. In addition, the prospects for future research directions and challenges in this field are also discussed. This article is protected by copyright. All rights reserved.

Conductance modulation of metal-molecule-metal junction via extra acid addition and its mechanism investigation

The advance of single molecular device fabrication strongly relies on the understanding of the metal-molecule-metal junction that can response to the external stimulus. A model Lewis basic molecule DBP which can react with Lewis acid and protic acid was synthesized, then the molecular conducting behavior of the original molecule and the resulted Lewis acid-base pair were researched. Allowing for their identical physical paths for charge conducting, these results indicated that adjusting the molecular electronic structure, even not directly changing the conductive molecular backbone, could also tune the charge transporting ability by nearly one order of magnitude. Furthermore, the addition of another Lewis base - Triethylamine to Lewis acid-base pair brought the electrical properties back to that of single DBP junction, which establishs a basic understanding in the design and construction of reversible and controllable molecular device based on pyridine derived molecule.

Substitution pattern controlled charge transport in BN-embedded aromatics-based single molecule junctions

The understanding of charge transport at a single molecule level is a prerequisite for the fabrication of molecular devices. Here, the relationship between molecular conductance, substitution pattern and stimuli response in BN-embedded aromatics was systematically investigated using the break junction technique. It was found that the para-phenylthioether-anchored BN molecule (p-BN-p) shows the highest conductance of 10^-4.86G₀, and the meta-phenylthioether-anchored BN molecule (m-BN-m) exhibits the lowest conductance which is lower than the instrument detection limit (<10^-6.0G₀). The m-BN-p and p-BN-m molecules, with both para- and meta-substituted anchor groups on two termini, show moderate conductances of 10^-5.50G₀ and 10^-5.45G₀, respectively. The conductance difference is interpreted as a distinct quantum interference effect caused by the substitution pattern of the anchoring groups. Notably, their conductance changes slightly upon coordination with a fluoride ion, in spite of the distinct change of their frontier orbital energy levels. These results demonstrate that, in addition to the frontier orbital energy levels, the anchors play an important role in the design of stimuli-responsive molecular electronic devices with a high on/off current ratio.

Gating the Conductance of Single - Molecule Junction with Ion-π Interaction

The single molecular conductance of viologen derivative VSMe and supramolecular compound VSMe-PA[5] (pillararene[5]) was investigated. The difference of their conductance demonstrated the gating effect of cation-π interaction. Theoretical calculations showed...

Single-atom control of electrical conductance and thermopower through single-cluster junctions

The control of single atoms offers fundamental insight into understanding the charge transport through single clusters, and the atomic precision of the clusters provides the opportunity to manipulate the charge transport even at the single-atom level. Herein, we designed and investigated the electrical conductance and thermopower of Anderson-type polyoxometalate (POM) clusters with single-atom variation using the scanning tunneling microscopy break-junction (STM-BJ) technique. Our results show the electrical conductance of single clusters can be changed by an order of magnitude by substituting different center-metal atoms, and the electrical conductance of clusters shows different bias-dependence. Furthermore, the Seebeck coefficients of the POM clusters also can be significantly changed by the center-metal atoms. The non-equilibrium quantum transport calculations reveal that the electrostatic potential profile is non-uniformly dependent on the center-metal atoms. This leads to gating of electrical conductance by bias voltage. This supports the tuning of thermopower and bias dependent transmission spectra. This work provides the fundamental understanding of single-atom control of charge transport in POM single-cluster junctions.

Single Dynamic Covalent Bond Tailored Responsive Molecular Junctions

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.

Electron-Catalyzed Dehydrogenation in a Single-Molecule Junction

Investigating how electrons propagate through a single molecule is one of the missions of molecular electronics. Electrons, however, are also efficient catalysts for conducting radical reactions, a property that is often overlooked by chemists. Special attention should be paid to electron catalysis when interpreting single-molecule conductance results for the simple reason that an unexpected reaction mediated or triggered by electrons might take place in the single-molecule junction. Here, we describe a counterintuitive structure-property relationship that molecules, both linear and cyclic, employing a saturated bipyridinium-ethane backbone, display a similar conductance signature when compared to junctions formed with molecules containing conjugated bipyridinium-ethene backbones. We describe an ethane-to-ethene transformation, which proceeds in the single-molecule junction by an electron-catalyzed dehydrogenation. Electrochemically based ensemble experiments and theoretical calculations have revealed that the electrons trigger the redox process, and the electric field promotes the dehydrogenation. This finding not only demonstrates the importance of electron catalysis when interpreting experimental results, but also charts a pathway to gaining more insight into the mechanism of electrocatalytic hydrogen production at the single-molecule level.

Controlling Light-Induced Proton Transfer from the GFP Chromophore

Green Fluorescent Protein (GFP) is known to undergo excited-state proton transfer (ESPT). Formation of a short H-bond favors ultrafast ESPT in GFP-like proteins, such as the GFP S65T/H148D mutant, but the detailed mechanism and its quantum nature remain to be resolved. Here we study in vacuo, light-induced proton transfer from the GFP chromophore in hydrogen-bonded complexes with two anionic proton acceptors, I^- and deprotonated trichloroacetic acid (TCA^- ). We address the role of the strong H-bond and the quantum mechanical proton-density distribution in the excited state, which determines the proton-transfer probability. Our study shows that chemical modifications to the molecular network drastically change the proton-transfer probability and it can become strongly wavelength dependent. The proton-transfer branching ratio is found to be 60 % for the TCA complex and 10 % for the iodide complex, being highly dependent on the photon energy in the latter case. Using high-level ab initio calculations, we show that light-induced proton transfer takes place in S₁ , revealing intrinsic photoacid properties of the isolated GFP chromophore in strongly bound H-bonded complexes. ESPT is found to be very sensitive to the topography of the highly anharmonic potential in S₁ , depending on the quantum-density distribution upon vibrational excitation. We also show that the S₁ potential-energy surface, and hence excited-state proton transfer, can be controlled by altering the chromophore microenvironment.

In situ photoconductivity measurements of imidazole in optical fiber break-junctions

We developed a method based on the mechanically controllable break junction technique to investigate the electron transport properties of single molecular junctions upon fiber waveguided light. In our strategy, a metal-coated tapered optical fiber is fixed on a flexible substrate, and this tapered fiber serves as both the optical waveguide and metal electrodes after it breaks. For an imidazole bridged single-molecule junction, two probable conductance values below 1G0 are observed. The higher value shows an approximately 40% enhancement under illumination, while the lower one does not show distinguishable difference under illumination. Theoretical calculations reveal these two conductance values resulting from the imidazole monomer junction and the imidazole dimer junction linked via a hydrogen bond, respectively. In imidazole monomer junctions, the absorption of a single photon strongly shifts the transmission function resulting in optical-induced conductance enhancement. In contrast, the transmission function of imidazole dimer junctions remains at the same level in the bias window despite the light illumination. This work provides a robust experimental framework for studying the underlying mechanisms of photoconductivity in single-molecule junctions and offers tools for tuning the optoelectronic performance of single-molecule devices in situ.

Enhanced Gating Performance of Single-Molecule Conductance by Heterocyclic Molecules

Enhancing the gating performance of single-molecule conductance is significant for realizing molecular transistors. Herein, we report a new strategy to improve the electrochemical gating efficiency of single-molecule conductance with fused molecular structures consisting of heterocyclic rings of furan, thiophene, or selenophene. One order magnitude of gating ratio is achieved within a potential window of 1.2 V for the selenophene-based molecule, which is significantly greater than that of other heterocyclic and benzene ring molecules. This is caused by the different electronic structures of heterocyclic molecules and transmission coefficients T(E), and preliminary resonance tunneling is achieved through the highest occupied molecular orbital at high potential. The current work experimentally shows that electrochemical gating performance can be significantly modulated by the alignment of the conducting orbital of the heterocyclic molecule relative to the metal Fermi energy.

Single-Molecule Sensing of Interfacial Acid-Base Chemistry

Bronsted acid and base interactions are a cornerstone of chemistry describing a wide range of chemical phenomena. However, probing such interaction at the solid-liquid interface to extract the elementary and intrinsic information at a single-molecule level remains a big challenge. Herein, we employ an STM break junction (STM-BJ) technique to investigate the acid-base chemistry of carboxylic acid-based molecules at a Au (111) model surface and propose a prototype of a single-molecule pH sensor for the first time. The single-molecule measurements in different environmental conditions verify that the formation probability of molecular junctions is determined by the populations of deprotonated -COO^- form in a self-assembled monolayer. Furthermore, the variation of the intensity of the conductance peaks (i.e., junction-forming probability) with the pH of the bulk solution fits well to the Henderson-Hasselbalch type equation. From the equation, a good linear relation is found between the degree of dissociation of the immobilized -COOH group and the environmental pH, providing a feasible way to design chemicals and biosensors and a detector at the single-molecule scale.

Proton-Gated Ring-Closure of a Negative Photochromic Azulene-Based Diarylethene

Proton-responsive photochromic molecules are attractive for their ability to react on non-invasive rapid optical stimuli and the importance of protonation/deprotonation processes in various fields. Conventionally, their acidic/basic sites are on hetero-atoms, which are orthogonal to the photo-active π-center. Here, we incorporate azulene, an acid-sensitive pure hydrocarbon, into the skeleton of a diarylethene-type photoswitch. The latter exhibits a novel proton-gated negative photochromic ring-closure and its optical response upon protonation in both open and closed forms is much more pronounced than those of diarylethene photoswitches with hetero-atom based acidic/basic moieties. The unique behavior of the new photoswitch can be attributed to direct protonation on its π-system, supported by 1H NMR and theoretical calculations. Our results demonstrate the great potential of integrating non-alternant hydrocarbons into photochromic systems for the development of multi-responsive molecular switches.

Solvent-molecule interaction induced gating of charge transport through single-molecule junctions

To explore solvent gating of single-molecule electrical conductance due to solvent-molecule interactions, charge transport through single-molecule junctions with different anchoring groups in various solvent environments was measured by using the mechanically controllable break junction technique. We found that the conductance of single-molecule junctions can be tuned by nearly an order of magnitude by varying the polarity of solvent. Furthermore, gating efficiency due to solvent-molecule interactions was found to be dependent on the choice of the anchor group. Theoretical calculations revealed that the polar solvent shifted the molecular-orbital energies, based on the coupling strength of the anchor groups. For weakly coupled molecular junctions, the polar solvent-molecule interaction was observed to reduce the energy gap between the molecular orbital and the Fermi level of the electrode and shifted the molecular orbitals. This resulted in a more significant gating effect than that of the strongly coupled molecules. This study suggested that solvent-molecule interaction can significantly affect the charge transport through single-molecule junctions.

Photo-induced carbocation-enhanced charge transport in single-molecule junctions

We report the first example of photo-induced carbocation-enhanced charge transport in triphenylmethane junctions using the scanning tunneling microscopy break junction (STM-BJ) technique. The electrical conductance of the carbocation state is enhanced by up to 1.5 orders of magnitude compared to the initial state, with stability lasting for at least 7 days. Moreover, we can achieve light-induced reversible conductance switching with a high ON-OFF ratio in carbocation-based single-molecule junctions. Theoretical calculations reveal that the conductance increase is due to a significant decrease of the HOMO-LUMO gap and also the enhanced transmission close to the Fermi levels when the carbocation forms. Our findings encourage continued research toward developing optoelectronics and carbocation-based devices at the single-molecule level.

Automatic classification of single-molecule charge transport data with an unsupervised machine-learning algorithm

Single-molecule electrical characterization reveals the events occurring at the nanoscale, which provides guidelines for molecular materials and devices. However, data analysis to extract valuable information from the nanoscale measurement data remained as a major challenge. Herein, an unsupervised deep leaning method, a deep auto-encoder K-means (DAK) algorithm, is developed to distinguish different events from single-molecule charge transport measurements. As validated by three single-molecule junction systems, the method applies to the recognition for multiple compounds with various events and offers an effective data analysis method to track reaction kinetics at the single-molecule scale. This work opens the possibility of using deep unsupervised approaches to studying the physical and chemical processes at the single-molecule level.

Azulene-Derived Fluorescent Probe for Bioimaging: Detection of Reactive Oxygen and Nitrogen Species by Two-Photon Microscopy

Two-photon fluorescence microscopy has become an indispensable technique for cellular imaging. Whereas most two-photon fluorescent probes rely on well-known fluorophores, here we report a new fluorophore for bioimaging, namely azulene. A chemodosimeter, comprising a boronate ester receptor motif conjugated to an appropriately substituted azulene, is shown to be an effective two-photon fluorescent probe for reactive oxygen species, showing good cell penetration, high selectivity for peroxynitrite, no cytotoxicity, and excellent photostability.