Light‐ and Redox‐Gated Molecular Brakes Consisting of a Pentiptycene Rotor and an Indole Pad

Two photochemically and electrochemically active alkenes 3Me and 3An containing pentiptycene and indole groups have been synthesized and investigated as light and/or redox‐gated molecular brakes. The pentiptycene group functions as the four‐bladed rotor, the indole group as the brake pad, and the vinylene group as the switch module. The E configuration corresponds to the brake‐off state, in which the rotation of the rotor is free with a rotation rate of 10⁸‐10⁹ at ambient temperature according to DFT calculations. The Z configuration corresponds to the brake‐on state, in which the rotation rate is decreased to 10¹‐10², depending on the N‐substituent of indole, according to line‐shape analysis of variable temperature ¹³C NMR spectra. The overall braking effect reaches a factor of 10⁶‐10⁸. While the combined photochemical E → Z and electrochemical Z → E switching has a higher capacity than the two‐way photochemical switching in the case of 3Me , the switching capacity are comparable for the two methods in 3An . The results also show that photochemical E‐Z isomerization is much more reliable than the electrochemical counterpart, as the stability of the redox intermediates plays a critical role in determining the robustness of the molecular brakes via electrochemical switching.     

Improved and new syntheses of potential molecular electronics devices

New syntheses of ethyl and nitro substituted oligo(phenylene ethynylene)s (OPEs) have been developed. To further explore whether the presence of nitro functionality in OPEs leads to switching and memory capabilities, new nitro substituted OPEs have been designed and synthesized. An isatogen-based system, a structure that is isomeric to the nitro OPE, has been synthesized. Additionally, pyridine-based and chromium-based compounds have been synthesized. We surmise that redox reactions of these candidates may impart switching capabilities and electrochemical studies are shown. U-shaped OPEs were synthesized to inhibit leakage of metals deposited during formation of top contacts on self-assembled monolayers (SAMs). The OPEs contain either thiol-based moieties or isonitrile groups to enable formation of SAMs on metal substrates.     

Electrochemical investigations on stability and protonation behavior of pyridine-terminated aromatic self-assembled monolayers

Based on electrochemical methods such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), Au(111) electrodes modified by self-assembled monolayers (SAMs) of a homologous series of pyridine-terminated thiols with aromatic backbones have been investigated. An important correlation between the chain structure and film integrity in electrolytic media was found. Monolayers with odd numbers of methylene spacers in the molecular chain showed superior barrier properties compared to even numbered counterparts. A positive influence of an increase in the number of attached phenyl rings on the integrity of SAMs was observed. Furthermore, cathodic desorption of the investigated SAMs is characterized by multiwave desorption peaks and extraordinarily large cathodic charges indicating an unusual desorption process. Moreover, protonation behavior of the SAMs has been investigated by X-ray photoelectron spectroscopy (XPS) and electrochemical methods. Protonation has been found to be reversible and surface pK(a) values have been determined to be around 5 for all investigated monolayers.     

A pH-controllable electrochemical molecule switch employing a new electrochemical measurement system as switching transducer

A new design concept of electrochemical pH-controllable molecular switch is presented by utilizing a new electrochemical measurement system as switching transducer. A pH sensor is connected in series between the terminal points of the working and counters electrodes of a potentisostat, and immersed in the solution together with a reference electrode, establishing a novel electrochemical measurement system. In this system, the variation of pH-controllable interface potential at the pH-sensing film/solution interface can be converted to current response when amperometry technique is employed. Based on this unique current–potential relationship, a pH-controllable switch is designed to monitor the protonation and deprotonation reaction of pH-sensing molecule. The current direction interchanges between positive and negative via pH control, illustrating a reversible conformation transition between protonated state and deprotionated state of molecule. The magnitude of current value represents the degree of protonation and deprotonation reaction of molecule. The strategy is successfully demonstrated with a remarkably reversible polyaniline-based pH-controllable switch, which confirms the feasibility of the novel electrochemical measurement system as switching transducer for designing electrochemical pH-controllable switches. This study may open up a potential avenue to construct the electrochemical pH-controllable switches.     

Atomic‐Level Elucidation of the Initial Stages of Self‐Assembled Monolayer Metallization and Nanoparticle Formation

The development of high‐performance molecular electronics and nanotech applications requires deep understanding of atomic level structural, electronic, and magnetic properties of electrode/molecular interfaces. Recent electrochemical experiments on self‐assembled monolayers (SAMs) have identified highly practical means to generate nanoparticles and metal monolayers suspended above substrate surfaces through SAM metallizations. A rational basis why this process is even possible is not yet well‐understood. To clarify the initial stages of interface formation during SAM metallization, we used first‐principles spin‐polarized density functional theory (DFT) calculations to study Pd diffusion on top of 4‐mercaptopyridine (4MP) SAMs on Au(111). After distinguishing potential‐energy surfaces (PESs) for different spin configurations for transition metal atoms on the SAM, we find adatom diffusion is not possible over the clean 4MP–SAM surface. Pre‐adsorption of transition‐metal atoms, however, facilitates atomic diffusion that appears to explain multiple reports on experimentally observed island and monolayer formation on top of SAMs. Furthermore, these diffusions most likely occur by moving across low‐lying and intersecting PESs of different spin states, opening the possibility of magnetic control over these systems. Vertical diffusion processes were also investigated, and the electrolyte was found to play a key role in preventing metal permeation through the SAM to the substrate.     

Self-assembled monolayers of electroactive polychlorotriphenylmethyl radicals on Au(111)

Two new polychlorotriphenylmethyl (PTM) derivatives bearing a thioacetate and a disulfide group have been synthesized to anchor on gold substrate. On the basis of these molecules, three strategies were followed to prepare self-assembled monolayers (SAMs) of electroactive PTMs. The resulting SAMs were fully characterized by contact angle, atomic force microscopy (AFM), and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). The high coverage surface and stability of the SAMs were demonstrated by cyclic voltammetry. In addition, the electrochemical experiments proved that these SAMs are bistable since it is possible to reversibly switch between the PTM radical state to the corresponding anion. The magnetic response was investigated by electron paramagnetic resonance. We observed that when the PTM SAMs are in their radical form they confer magnetic functionality to the surface, whereas when they are in the anionic state, the surface is diamagnetic. Thus, the PTM-modified substrates are multifunctional surfaces since they combine magnetic and electroactive properties. The reported results show the high potential of these materials for the fabrication of surface molecular devices.     

Diastereoselective construction of azetidin-2-ones by electrochemical intramolecular C-C bond forming reaction

A convenient method for synthesis of optically active azetidin-2-ones using electrochemical oxidation has been exploited. The method consists of a diastereoselective intramolecular C-C bond forming reaction between active methylene and methyne groups through an electrochemical system in which positive iodine species acted as mediators under mild conditions.     

Proton- and redox-controlled switching of photo- and electrochemiluminescence in thiophenyl-substituted boron-dipyrromethene dyes

A luminescent molecular switch in which the active thiol/disulfide switching element is attached to a meso-phenyl-substituted boron-dipyrromethene (BDP) chromophore as the signalling unit is presented. The combination of these two functional units offers great versatility for multimodal switching of luminescence: 1) deprotonation/protonation of the thiol/thiolate moiety allows the highly fluorescent meso-p-thiophenol-BDP and its nonfluorescent thiolate analogue to be chemically and reversibly interconverted, 2) electrochemical oxidation of the monomeric dyes yields the fluorescent disulfide-bridged bichromophoric dimer, also in a fully reversible process, and 3) besides conventional photoexcitation, the well separated redox potentials of the BDP also allow the excited BDP state to be generated electrochemically (i.e., processes 1) and 2) can be employed to control both photo- and electrochemiluminescence (ECL) of the BDP). The paper introduces and characterizes the various states of the switch and discusses the underlying mechanisms. Investigation of the ortho analogue of the dimer provided insight into potential chromophore-chromophore interactions in such bichromophoric architectures in both the ground and the excited state. Comparison of the optical and redox properties of the two disulfide dimers further revealed structural requirements both for redox switches and for ECL-active molecular ensembles. By employing thiol/disulfide switching chemistry and BDP luminescence features, it was possible to create a prototype molecular ensemble that shows both fully reversible proton- and redox-gated electrochemiluminescence.     

Electrochemical Studies of Chemically Modified Nanometer‐Sized Electrodes

Self‐assembled monolayers (SAMs) of 4‐aminothiophenol (4‐ATP) has been successfully deposited onto nanometer‐sized gold (Au) electrodes. The cyclic voltammograms obtained on a 4‐ATP SAMs modified electrode show peaks and the peak height is proportional to the scan rate, which is similar to that on an electroactive SAMs modified macro electrode. The electrochemical behavior and mechanism of outer‐sphere electron transfer reaction on the 4‐ATP SAMs modified nanometer‐sized electrode has also been studied. The 4‐ATP SAMs modified electrode is further modified with platinum (Pt) nanoparticles. Electrochemical behaviors show that there is electrical communication between Pt nanoparticles and Au metal on the Pt nanoparticles/4‐ATP SAMs/Au electrode. However, scanning electron microscopic image shows that the Pt nanoparticles are not evenly covered the electrode.     

Ion recognition properties of self-assembled monolayers (SAMs)

In the search for new sensors, self-assembled monolayers (SAMs) have gained intensive interest due to their nanometre size, highly-ordered structures, and molecular recognition properties. This article presents an overview of ion recognition at SAM-modified surface/solution interfaces, and brings up to date the most notable examples for the sensing of cations and anions. Sensing is achieved with SAMs containing redox active and inactive receptors using techniques such as fluorescence spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy.     

An evanescent-field-driven self-assembled molecular photoswitch for macrocycle coordination and release

Two self-assembled monolayer (SAM) films containing the photoswitchable 4-pyridylazophenoxy chromophore have been deposited onto a gold-coated glass substrate. One film contains the chromophore as a single component, 1 SAM, and the other is doped with a nonphotoactive component as a 1:1 mixture, 2 SAM. The reversible photoswitching performances of 1 SAM and 2 SAM via the evanescence field using light of appropriate wavelengths have been investigated by UV spectroscopic and electrochemical monitoring. In principle, the trans-form SAMs present a coordinating surface, the "on" state, that can be switched "off" in the cis form. This has been illustrated by immersing both the as-deposited (trans form) SAMs and the photoswitched (predominantly cis form) SAMs into solutions of cobalt and zinc tetraphenylporphyrin (CoTPP and ZnTPP, respectively) and an octaoctyl-substituted cobalt phthalocyanine. In a further phase of this study, the remote control of binding events at the surface of the SAMs has been demonstrated through evanescent-field-driven photoswitching of trans-form SAMs coordinated at the surfaces with examples of these metallomacrocycles. This photoswitching was undertaken with the constructs immersed in neat toluene, and the macrocycles were released from the surface into the solvent. The release was measured by spectroscopic monitoring of the material remaining on the constructs. The study was extended to develop an in situ release/coordination cycle. Thus, irradiation of a construct of ZnTPP bound to the surface of trans-form 2 SAM using waveguided light at 365 nm releases the macrocycle into a toluene solution of ZnTPP. Further irradiation of the SAM, now in its cis form, with waveguided 439 nm light regenerates the trans form, which recoordinates ZnTPP from the solution. The results demonstrate the potential for using waveguided light to control molecular events within and at the surfaces of SAM constructs.     

基于电致酸/碱理论的手性光学切换体系

手性分子光学开关在光学储存、 光学通信以及三维立体显示等领域有着重要的应用价值. 但是目前的手性分子光学开关存在材料种类少、 光学可调性差、 稳定性差等缺点, 如何构筑出具有高光学可调性以及稳定性的手性光学切换体系依旧是一项严峻的挑战. 本工作基于电致酸/碱理论, 通过将手性联萘官能团引入到酸响应的罗丹明主体结构中, 设计并合成了一种新型的酸响应手性光学开关分子, 并将其与电致酸材料相结合, 成功实现了电场驱动的手性光学开关过程, 开发了一种新型的手性光学切换体系. 发现在合适的电场控制下, 其颜色、 荧光以及圆二色谱信号均能发生可逆的变化. 这种方法为构筑新型手性光学开关体系提供了一种新思路, 对手性光学开关材料的应用拓展具有重要的参考价值.     

An Autonomously Reciprocating Transmembrane Nanoactuator

Biological molecular machines operate far from equilibrium by coupling chemical potential to repeated cycles of dissipative nanomechanical motion. This principle has been exploited in supramolecular systems that exhibit true machine behavior in solution and on surfaces. However, designed membrane‐spanning assemblies developed to date have been limited to simple switches or stochastic shuttles, and true machine behavior has remained elusive. Herein, we present a transmembrane nanoactuator that turns over chemical fuel to drive autonomous reciprocating (back‐and‐forth) nanomechanical motion. Ratcheted reciprocating motion of a DNA/PEG copolymer threaded through a single α‐hemolysin pore was induced by a combination of DNA strand displacement processes and enzyme‐catalyzed reactions. Ion‐current recordings revealed saw‐tooth patterns, indicating that the assemblies operated in autonomous, asymmetric cycles of conformational change at rates of up to one cycle per minute.     

Three‐State Single‐Molecule Naphthalenediimide Switch: Integration of a Pendant Redox Unit for Conductance Tuning

We studied charge transport through core‐substituted naphthalenediimide (NDI) single‐molecule junctions using the electrochemical STM‐based break‐junction technique in combination with DFT calculations. Conductance switching among three well‐defined states was demonstrated by electrochemically controlling the redox state of the pendent diimide unit of the molecule in an ionic liquid. The electrical conductances of the dianion and neutral states differ by more than one order of magnitude. The potential‐dependence of the charge‐transport characteristics of the NDI molecules was confirmed by DFT calculations, which account for electrochemical double‐layer effects on the conductance of the NDI junctions. This study suggests that integration of a pendant redox unit with strong coupling to a molecular backbone enables the tuning of charge transport through single‐molecule devices by controlling their redox states.     

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.     

How to Switch Spin‐Crossover Metal Complexes at Constant Room Temperature

Spin‐crossover metal complexes represent a highly promising class of molecular switches, the diverse physicochemical properties of which can be reversibly changed by different physical and chemical stimuli. One of the most interesting and examined features of these materials is the change of magnetic properties by changing the temperature or by irradiation with light at low temperatures. However, most prospective applications of such complexes require functioning at room temperature. This Concept article provides an overview about how the switching of spin‐crossover metal complexes can be achieved at constant room temperature. The principles of switching by different physical and chemical methods in solution and in the solid state are presented in an easy‐to‐read form for nonspecialists. These are further supported and clarified by examples from the literature. The overview might also be interesting for experts that target spin‐crossover systems functioning at ambient conditions.     

Tuning Aqueous Supramolecular Polymerization by an Acid-Responsive Conformational Switch

Besides their widespread use in coordination chemistry, 2,2’-bipyridines are known for their ability to undergo cis–trans conformational changes in response to metal ions and acids, which has been primarily investigated at the molecular level. However, the exploitation of such conformational switching in self-assembly has remained unexplored. In this work, the use of 2,2’-bipyridines as acid-responsive conformational switches to tune supramolecular polymerization processes has been demonstrated. To achieve this goal, we have designed a bipyridine-based linear bolaamphiphile, 1 , that forms ordered supramolecular polymers in aqueous media through cooperative aromatic and hydrophobic interactions. Interestingly, addition of acid (TFA) induces the monoprotonation of the 2,2’-bipyridine moiety, leading to a switch in the molecular conformation from a linear (trans) to a V-shaped (cis) state. This increase in molecular distortion along with electrostatic repulsions of the positively charged bipyridine-H⁺ units attenuate the aggregation tendency and induce a transformation from long fibers to shorter thinner fibers. Our findings may contribute to opening up new directions in molecular switches and stimuli-responsive supramolecular materials.     

Photoinduced proton exchange between molecular switches

The identification of strategies to establish communication between independent molecules is an essential requirement for the development of operating principles to manipulate information at the molecular level. In this context, we have devised a strategy to exchange signals between pairs of complementary molecular switches. It is based on the photoinduced ring closing of a merocyanine to produce a spiropyran with the concomitant release of a proton. The liberated proton is captured by either one of two pyridine derivatives with the formation of their conjugate acids. This transformation induces a significant increase in chemical shift for the resonances of the pyridyl protons and, in one instance, also a pronounced color change. The overall process is fully reversible and the pair of communicating molecules reverts to the original state in the dark. Relying on this mechanism, an optical input is transduced into a detectable spectroscopic output after the controlled intermolecular exchange of protons. A simple analysis of the signal transduction operated by the communicating molecular switches reveals that a binary digit is passed unaltered from the input to the output even although the nature of the signal carrying the information changes at each step. Furthermore, the different nature of input and output implies that the state of the ensemble of molecules can be probed non-destructively at any point in time. The timescales of the switching steps, however, are seriously limited by the slow reaction kinetics. The photoinduced transformation occurs within minutes, but the thermal reaction reverts the switch state only after several hours.     

Self-assembled monolayers of alkanethiols on gold prepared in a hexagonal lyotropic liquid crystalline phase of Triton X-100/water system

In this paper, we have reported a new method of preparing self-assembled monolayers (SAMs) of decanethiol and hexadecanethiol on gold surface by using a lyotropic liquid crystalline phase as an adsorbing medium. The stability and blocking ability of these SAMs were characterized using grazing angle Fourier transform infrared (FTIR) spectroscopy and electrochemical techniques such as cyclic voltammetry and electrochemical impedance spectroscopy. The lyotropic liquid crystalline medium possesses a hexagonal structure consisting of a nonionic surfactant Triton X-100, water, and the corresponding thiol, which provides a highly hydrophobic environment to solubilize the alkanethiols and later to facilitate their delivery to the gold surface. We find that the SAMs formed from the hexagonal liquid crystalline phase are highly compact and have excellent electrochemical blocking ability towards the redox probes compared to conventional SAMs prepared from commonly used organic solvents such as ethanol. From the impedance studies, we have determined the capacitance of the monolayer-coated electrodes and the surface coverage of the SAM, which has been found to be >99.98% on gold surface. We have also estimated the extent of ionic permeability through the film and measured the rate constants for the redox reactions on the SAM-modified electrodes. Our results show that the rate constants of [Fe(CN)6](3-/4-) and [Ru(NH3)6](2+/3+) redox couples are very much lower in the case of monolayers prepared in liquid crystalline phase compared to the SAM formed in 1 mM thiol in ethanol solution, suggesting a better blocking ability of the SAMs in the former case. From the grazing angle FTIR spectroscopic studies and capacitance measurements, we have ruled out any coadsorption of surfactant molecules on the Au surface. These results suggest that SAMs of very low defect density and extremely low ionic permeability can be obtained when a hexagonal lyotropic liquid crystalline phase is used as an adsorbing medium.     

Self assembled monolayers on silicon for molecular electronics

We present an overview of various aspects of the self-assembly of organic monolayers on silicon substrates for molecular electronics applications. Different chemical strategies employed for grafting the self-assembled monolayers (SAMs) of alkanes having different chain lengths on native oxide of Si or on bare Si have been reviewed. The utility of different characterization techniques in determination of the thickness, molecular ordering and orientation, surface coverage, growth kinetics and chemical composition of the SAMs has been discussed by choosing appropriate examples. The metal counterelectrodes are an integral part of SAMs for measuring their electrical properties as well as using them for molecular electronic devices. A brief discussion on the variety of options available for the deposition of metal counterelectrodes, that is, soft metal contacts, vapor deposition and soft lithography, has been presented. Various theoretical models, namely, tunneling (direct and Fowler-Nordheim), thermionic emission, Poole-Frenkel emission and hopping conduction, used for explaining the electronic transport in dielectric SAMs have been outlined and, some experimental data on alkane SAMs have been analyzed using these models. It has been found that short alkyl chains show excellent agreement with tunneling models; while more experimental data on long alkyl chains are required to understand their transport mechanism(s). Finally, the concepts and realization of various molecular electronic components, that is, diodes, resonant tunnel diodes, memories and transistors, based on appropriate architecture of SAMs comprising of alkyl chains (sigma- molecule) and conjugated molecules (pi-molecule) have been presented.