Switching Effects in Molecular Electronic Devices

Single-Molecule Conductance of Neutral Closed-Shell and Open-Shell Diradical Indenofluorenes

Organic diradicals are highly promising candidates as future components in molecular electronic and spintronic devices because of their low spin-orbit coupling. To advance toward final circuit realizations, a thorough knowledge of the behavior of diradicals within a single-molecule junction framework is imperative. In this work, we have measured for the first time the single-molecule conductance of a neutral open-shell diradical compound, a [2,1-b] isomer of indenofluorene (IF). Our results reveal that the conductance of the [2,1-b] isomer is about 1 order of magnitude higher than that of the corresponding closed-shell regioisomer [1,2-b] IF. This is significant, as it fundamentally demonstrates the possibility of forming stable single-molecule junctions using neutral diradical compounds which are also highly conducting. This opens up a new approach to the development of externally addressable spintronic devices operable at room temperature.

Toward Practical Single-Molecule/Atom Switches

Electronic switches have been considered to be one of the most important components of contemporary electronic circuits for processing and storing digital information. Fabricating functional devices with building blocks of atomic/molecular switches can greatly promote the minimization of the devices and meet the requirement of high integration. This review highlights key developments in the fabrication and application of molecular switching devices. This overview offers valuable insights into the switching mechanisms under various stimuli, emphasizing structural and energy state changes in the core molecules. Beyond the molecular switches, typical individual metal atomic switches are further introduced. A critical discussion of the main challenges for realizing and developing practical molecular/atomic switches is provided. These analyses and summaries will contribute to a comprehensive understanding of the switch mechanisms, providing guidance for the rational design of functional nanoswitch devices toward practical applications.

Exploring Arylazo-3,5-Bis(trifluoromethyl)pyrazole Switches

Arylazopyrazoles stand out among the azoheteroarene photoswitches due to their excellent properties in terms of stability of the least stable isomer and conversion between isomers, leading to their use in several interesting applications. We report herein the synthesis of arylazo-trifluoromethyl-substituted pyrazoles and their switching behavior under light irradiation. UV-vis and NMR experiments showed that arylazo-1H-3,5-bis(trifluoromethyl)pyrazoles displayed very long half-lives in DMSO (days), along with reasonable values of other parameters that characterize a photoswitch. Inclusion of naphthyl moieties as aryl counterparts of the arylazopyrazoles is beneficial only in combination with trifluoromethyl groups, while extending the conjugation by grafting the pyrazole moiety with electron-donating or -withdrawing substituents positively affects the photoswitching behavior, in terms of isomerization yield and half-lives of the least stable isomer. The experimental values were correlated with theoretical calculations indicating the valuable influence of the trifluoromethyl groups onto the photoswitching behavior.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

Theoretical inspection of the spin-crossover [Fe(tzpy)2(NCS)2] complex on Au(100) surface

We explore the deposition of the spin-crossover [Fe(tzpy)₂(NCS)₂] complex on the Au(100) surface by means of density functional theory (DFT) based calculations. Two different routes have been employed: low-cost finite cluster-based calculations, where both the Fe complex and the surface are maintained fixed while the molecule approaches the surface; and periodic DFT plane-wave calculations, where the surface is represented by a four-layer slab and both the molecule and surface are relaxed. Our results show that the bridge adsorption site is preferred over the on-top and fourfold hollow ones for both spin states, although they are energetically close. The LS molecule is stabilized by the surface, and the HS-LS energy difference is enhanced by about 15%-25% once deposited. The different Fe ligand field for LS and HS molecules manifests on the composition and energy of the low-lying bands. Our simulated STM images indicate that it is possible to distinguish the spin state of the deposited molecules by tuning the bias voltage of the STM tip. Finally, it should be noted that the use of a reduced size cluster to simulate the Au(100) surface proves to be a low-cost and reliable strategy, providing results in good agreement with those resulting from state-of-the-art periodic calculations for this system.

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.

Voltage-Controlled Binary Conductance Switching in Gold-4,4'-Bipyridine-Gold Single-Molecule Nanowires

We investigate gold-4,4'-bipyridine-gold single-molecule junctions with the mechanically controllable break junction technique at cryogenic temperature (T = 4.2 K). We observe bistable probabilistic conductance switching between the two molecular binding configurations, influenced both by the mechanical actuation and by the applied voltage. We demonstrate that the relative dominance of the two conductance states is tunable by the electrode displacement, whereas the voltage manipulation induces an exponential speedup of both switching times. The detailed investigation of the voltage-tunable switching rates provides an insight into the possible switching mechanisms.

A sub 20 nm metal-conjugated molecule junction acting as a nitrogen dioxide sensor

The interaction of a gas molecule with a sensing material causes the highest change in the electronic structure of the latter, when this material consists of only a few atoms. If the sensing material consists of a short, conductive molecule, the sensing action can be furthermore probed by connecting such molecules to nanoelectrodes. Here, we report that NO2 molecules that adhere to 4,4'-biphenyldithiol (BPDT) bound to Au surfaces lead to a change of the electrical transmission of the BPDT. The related device shows reproducible, stable measurements and is so far the smallest (<20 nm) gas sensor. It demonstrates modulation of charge transport through molecules upon exposure to nitrogen dioxide down to concentrations of 55 ppb. We have evaluated several devices and exposure conditions and obtained a close to linear dependence of the sensor response on the gas concentration.

Chemical Locking in Molecular Tunneling Junctions Enables Nonvolatile Memory with Large On-Off Ratios

This paper describes the reversible chemical locking of sypiropyran switches bound to metallic surfaces to enable the encoding of nonvolatile information. Data are encoded spatially by selectively locking the spiropyran moieties in their merocyanine form using a combination of exposure to acid and UV light. Without exposure to acid, the merocyanine form spontaneously converts back to the spiropyran form. Bits are resolved by defining the regions of the monolayer that are exposed to acid, using a "soft punchcard" fabricated from a silicone elastomer. Information is read by measuring the tunneling charge-transport through the monolayer using eutectic Ga-In top-contacts. The merocyanine form is more than three orders of magnitude more conductive than the spiropyran form, allowing the differentiation of bits. Photoelectron spectroscopy shows that the monolayers are undamaged by exposure to light, acid, base, and applied bias, enabling proof-of-concept devices in which an 8-bit ASCII encoded six-character string is written, erased, and rewritten.

Side-group chemical gating via reversible optical and electric control in a single molecule transistor

By taking advantage of large changes in geometric and electronic structure during the reversible trans–cis isomerisation, azobenzene derivatives have been widely studied for potential applications in information processing and digital storage devices. Here we report an unusual discovery of unambiguous conductance switching upon light and electric field-induced isomerisation of azobenzene in a robust single-molecule electronic device for the first time. Both experimental and theoretical data consistently demonstrate that the azobenzene sidegroup serves as a viable chemical gate controlled by electric field, which efficiently modulates the energy difference of trans and cis forms as well as the energy barrier of isomerisation. In conjunction with photoinduced switching at low biases, these results afford a chemically-gateable, fully-reversible, two-mode, single-molecule transistor, offering a fresh perspective for creating future multifunctional single-molecule optoelectronic devices in a practical way.

It remains a challenge to fully control molecular electronics. Here, Meng et al. show a reversible two-mode single-molecule switch, where the conductance through the molecular backbone is controlled by an in situ chemical gating via bias-dependent trans–cis isomerisation on an azobenzene sidegroup.

Two-Terminal Molecular Memory through Reversible Switching of Quantum Interference Features in Tunneling Junctions

Large-area molecular tunneling junctions comprising self-assembled monolayers of redox-active molecules are described that exhibit two-terminal bias switching. The as-prepared monolayers undergo partial charge transfer to the underlying metal substrate (Au, Pt, or Ag), which converts their cores from a quinoid to a hydroquinoid form. The resulting rearomatization converts the bond topology from a cross-conjugated to a linearly conjugated π system. The cross-conjugated form correlates to the appearance of an interference feature in the transmission spectrum that vanishes for the linearly conjugated form. Owing to the presence of electron-withdrawing nitrile groups, the reduction potential and the interference feature lie close to the work function and Fermi level of the metallic substrate. We exploited the relationship between conjugation patterns and quantum interference to create nonvolatile memory in proto-devices using eutectic Ga-In as the top contact.

Molecular Photoswitching Aided by Excited-State Aromaticity

Central to the development of optoelectronic devices is the availability of efficient synthetic molecular photoswitches, the design of which is an arena where the evolving concept of excited-state aromaticity (ESA) is yet to make a big impact. The aim of this minireview is to illustrate the potential of this concept to become a key tool for the future design of photoswitches. The paper starts with a discussion of challenges facing the use of photoswitches for applications and continues with an account of how the ESA concept has progressed since its inception. Then, following some brief remarks on computational modeling of photoswitches and ESA, the paper describes two different approaches to improve the quantum yields and response times of switches driven by E/Z photoisomerization or photoinduced H-atom/proton transfer reactions through simple ESA considerations. It is our hope that these approaches, verified by quantum chemical calculations and molecular dynamics simulations, will help stimulate the application of the ESA concept as a general tool for designing more efficient photoswitches and other functional molecules used in optoelectronic devices.

Understanding the charge transport properties of redox active metal-organic conjugated wires

For Rh₂-organic molecular wires, we found that weaker coupling systems built using longer bridging ligands exhibit better electrical conductance.

Layer-by-layer assembly of the dirhodium complex [Rh₂(O₂CCH₃)₄] (Rh₂) with linear N,N′-bidentate ligands pyrazine (L_S) or 1,2-bis(4-pyridyl)ethene (L_L) on a gold substrate has developed two series of redox active molecular wires, (Rh₂L_S)_n@Au and (Rh₂L_L)_n@Au (n = 1–6). By controlling the number of assembling cycles, the molecular wires in the two series vary systematically in length, as characterized by UV-vis spectroscopy, cyclic voltammetry and atomic force microscopy. The current–voltage characteristics recorded by conductive probe atomic force microscopy indicate a mechanistic transition for charge transport from voltage-driven to electrical field-driven in wires with n = 4, irrespective of the nature and length of the wires. Whilst weak length dependence of electrical resistance is observed for both series, (Rh₂L_L)_n@Au wires exhibit smaller distance attenuation factors (β) in both the tunneling (β = 0.044 Å^–1) and hopping (β = 0.003 Å^–1) regimes, although in (Rh₂L_S)_n@Au the electronic coupling between the adjacent Rh₂ centers is stronger. DFT calculations reveal that these wires have a π-conjugated molecular backbone established through π(Rh₂)–π(L) orbital interactions, and (Rh₂L_L)_n@Au has a smaller energy gap between the filled π*(Rh₂) and the empty π*(L) orbitals. Thus, for (Rh₂L_L)_n@Au, electron hopping across the bridge is facilitated by the decreased metal to ligand charge transfer gap, while in (Rh₂L_S)_n@Au the hopping pathway is disfavored likely due to the increased Coulomb repulsion. On this basis, we propose that the super-exchange tunneling and the underlying incoherent hopping are the dominant charge transport mechanisms for shorter (n ≤ 4) and longer (n > 4) wires, respectively, and the Rh₂L subunits in mixed-valence states alternately arranged along the wire serve as the hopping sites.

Conformation-induced light emission switching of N-acylhydrazone systems

Bis-N-acylhydrazones bearing different substituents were found to display different colour emissions, through ESIPT or AIE, as a result of conformation switching, triggered by physical stimuli.

Photoconductance from Exciton Binding in Molecular Junctions

We report on a theoretical analysis and experimental verification of a mechanism for photoconductance, the change in conductance upon illumination, in symmetric single-molecule junctions. We demonstrate that photoconductance at resonant illumination arises due to the Coulomb interaction between the electrons and holes in the molecular bridge, so-called exciton-binding. Using a scanning tunneling microscopy break junction technique, we measure the conductance histograms of perylene tetracarboxylic diimide (PTCDI) molecules attached to Au-electrodes, in the dark and under illumination, and show a significant and reversible change in conductance, as expected from the theory. Finally, we show how our description of the photoconductance leads to a simple design principle for enhancing the performance of molecular switches.

Immobilizing Organic-Based Molecular Switches into Metal-Organic Frameworks: A Promising Strategy for Switching in Solid State

Organic-based molecular switches (OMS) are essential components for the ultimate miniaturization of nanoscale electronics and devices. For practical applications, it is often necessary for OMS to be incorporated into functional solid-state materials. However, the switching characteristics of OMS in solution are usually not transferrable to the solid state, presumably because of spatial confinement or inefficient conversion in densely packed solid phase. A promising way to circumvent this issue is harboring the functional OMS within the robust and porous environment of metal-organic frameworks (MOFs) as their organic components. In this feature article, recent research progress of OMS-based MOFs is briefly summarized. The switching behaviors of OMS under different stimuli (e.g., light, redox, pH, etc.) in the MOF state are first introduced. After that, the technological applications of these OMS-based MOFs in different areas, including CO₂ adsorption, gas separation, drug delivery, photodynamic therapy, and sensing, are outlined. Finally, perspectives and future challenges are discussed in the conclusion.