Single Dynamic Covalent Bond Tailored Responsive Molecular Junctions

Quadruple Stimuli-Responsive Behaviour of Acylhydrazone Crystals: Mechanical, Photomechanical, Thermomechanical, and Optical Waveguide Properties

As society advances, the demand for high-performance crystal materials with versatile stimuli-responsive capabilities has increased. However, there remains a notable shortage of simple molecular crystals that exhibit stable and swift responses to various stimuli. In this study, we synthesized a novel acylhydrazone crystal by integrating benzene-fused heterocycles and halogens, achieving a crystal material with multiple stimuli-responsive behaviours. The crystal demonstrates good mechanical flexibility, capable of with standing a strain of 2.3 % under applying mechanical force and recovering its original shape after force removal. Crystal structure analysis reveals this property is due to the strong and flexible intermolecular interactions. It also exhibits rapid photomechanical bending away from light source under UV irradiation, which could be attributed to the facile photoisomerization. Moreover, the crystal undergoes significant thermomechanical transitions, including cracking and jumping, upon heating. Additionally, the prepared crystal exhibits passive optical waveguide of red light at 640 nm, which, combining the photomechanical bending, can achieve controllable light output direction. These multi-stimuli-responsive behaviors make the acylhydrazone crystal a promising candidate for applications in multifunctional sensors, optoelectronic devices, and thermal switches. This work provides a helpful reference for the study of potential crystal material design and functionality of advanced responsive systems.

Boosting the Photoresponse of Azobenzene Single-Molecule Junctions via Mechanical Interlock and Dynamic Anchor

As the most classic photoisomerization system, azobenzene has been widely utilized as a building unit in various photoswitching applications. However, attempts to build azobenzene-based single-molecule photoswitches have met with limited success, giving low on/off ratios. Herein, we demonstrate two designs of azobenzene-based photoresponsive single-molecule junctions, based on mechanically interlocked diazocine and azobenzene-based dynamic anchors, respectively. Molecular conductance measurements using the scanning tunneling microscope breaking junction (STMBJ) technique revealed dramatic conductance changes upon photoillumination, achieving a high on/off ratio of ∼3.7. Using density functional theory (DFT), we revealed peculiar quantum interference (QI) effects in the diazocine molecular switch, indicating that diazocine is an excellent candidate for molecular photoswitches. The asymmetric azobenzene devices with a dynamic anchor exhibit switching behavior between a fully off state and a highly conductive state associated with the trans/cis conformation transition. The findings of this work not only present the design and development of functional molecular devices based on azobenzene units but also provide insight into the fundamental properties of light-induced quantum interference in azobenzene-based molecular devices.

Two-Dimensional Nanoarchitectonics of End-on Oligomers with Versatile Structures and Tunable Negative Differential Resistance

Maximizing the molecular information density requires simultaneously functionalizing distinct monomers and their coupling connections. However, current synthesis generally focuses on distinct monomers rather than coupling reactions because the multistep reactions significantly escalate the synthetic complexity in an exponential increase. Here, we report the two-dimensional nanoarchitectures (2DNs) of end-on oligomers, with versatile molecular structures and negative differential resistance (NDR), synthesized by programmed and surface-initiated step electrosynthesis based on the simultaneous utilization of six reactions including cross- and homocouplings. The resulting vertically end-on oligomers, with similar values in thickness and molecular length, as crystalline 2DNs, exhibit subnanometer uniformity, ultrahigh compressive modulus of 40 GPa, and low-bias NDR at 0.13 V with an ultralow power consumption of down to 0.05 nW/μm2. This highly controlled electrosynthesis provides a unique dimension to enhance the structural diversity of molecular 2D nanomaterials for high-density and low-power consumption electronics.

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.

Solvatochromic and Proton-Responsive characteristics of Bi-1,3,4-Oxadiazole derivatives with symmetric dimethylamino substitution

D-A molecules find extensive use in intelligent stimulus-response systems due to their exceptional attributes, including high sensitivity, rapid response, wide compatibility, and structural adaptability. The strength of Intramolecular Charge Transfer (ICT) plays a pivotal role in determining the performance of these devices. To enhance the ICT strength and explore new applications for D-A molecules, we meticulously designed a pair of symmetric dimethylamino-substituted bi-1,3,4-oxadiazole derivatives (DMAOXD and DMAOXDBEN). These symmetric D-A-A-D molecules, with strong electron donor terminals, displayed a modest redshift of less than 25 nm in the UV-vis absorption spectra. However, there was a significant redshift in the emission spectra (140 nm for DMAOXD and 170 nm for DMAOXDBEN) when transitioning from cyclohexane to dimethyl sulfoxide, indicating a pronounced ICT characteristic. Theoretical calculations support the idea that the dimethylaminophenyl unit serves as an electron donor in both DMAOXD and DMAOXDBEN, while the 1,3,4-oxadiazole and central benzene ring act as acceptors. The pronounced ICT characteristic observed in DMAOXD and DMAOXDBEN can be attributed to long-distance electron transfer. Additionally, it's noteworthy that the emission of DMAOXD and DMAOXDBEN solution samples can be quenched by adding trifluoroacetic acid (TFA) and restored by the addition of triethylamine (TEA). Inspired by this, a pattern created with ink samples containing DMAOXD and DMAOXDBEN can be concealed through fumigation with TFA and subsequently revealed by treating them with TEA, suggesting their potential use in data encryption.

Fully Reversible and Super-Fast Photo-Induced Morphological Transformation of Nanofilms for High-Performance UV Detection and Light-Driven Actuators

Flexible and highly ultraviolet (UV) sensitive materials garner considerable attention in wearable devices, adaptive sensors, and light-driven actuators. Herein, a type of nanofilms with unprecedented fully reversible UV responsiveness are successfully constructed. Building upon this discovery, a new system for ultra-fast, sensitive, and reliable UV detection is developed. The system operates by monitoring the displacement of photoinduced macroscopic motions of the nanofilms based composite membranes. The system exhibits exceptional responsiveness to UV light at 375 nm, achieving remarkable response and recovery times of < 0.3 s. Furthermore, it boasts a wide detection range from 2.85 µW cm-2 to 8.30 mW cm-2, along with robust durability. Qualitative UV sensing is accomplished by observing the shape changes of the composite membranes. Moreover, the composite membrane can serve as sunlight-responsive actuators for artificial flowers and smart switches in practical scenarios. The photo-induced motion is ascribed to the cis-trans isomerization of the acylhydrazone bonds, and the rapid and fully reversible shape transformation is supposed to be a synergistic result of the instability of the cis-isomers acylhydrazone bonds and the rebounding property of the networked nanofilms. These findings present a novel strategy for both quantitative and qualitative UV detection.

Highly Reversible Molecular Photoswitches with Transition Metal Dichalcogenides Electrodes

The molecule-electrode coupling plays an essential role in photoresponsive devices with photochromic molecules, and the strong coupling between the molecule and the conventional electrodes leads to/ the quenching effect and limits the reversibility of molecular photoswitches. In this work, we developed a strategy of using transition metal dichalcogenides (TMDCs) electrodes to fabricate the thiol azobenzene (TAB) self-assembled monolayers (SAMs) junctions with the eutectic gallium-indium (EGaIn) technique. The current-voltage characteristics of the EGaIn/GaOx //TAB/TMDCs photoswitches showed an almost 100% reversible photoswitching behavior, which increased by ∼28% compared to EGaIn/GaOx //TAB/AuTS photoswitches. Density functional theory (DFT) calculations showed the coupling strength of the TAB-TMDCs electrode decreased by 42% compared to that of the TAB-AuTS electrode, giving rise to improved reversibility. our work demonstrated the feasibility of 2D TMDCs for fabricating SAMs-based photoswitches with unprecedentedly high reversibility.

Dual reactivity based dynamic covalent chemistry: mechanisms and applications

Dynamic covalent chemistry (DCC) focuses on the reversible formation, breakage, and exchange of covalent bonds and assemblies, setting a bridge between irreversible organic synthesis and supramolecular chemistry and finding wide utility. In order to enhance structural and functional diversity and complexity, different types of dynamic covalent reactions (DCRs) are placed in one vessel, encompassing orthogonal DCC without crosstalk and communicating DCC with a shared reactive functional group. As a means of adding tautomers, widespread in chemistry, to interconnected DCRs and combining the features of orthogonal and communicating DCRs, a concept of dual reactivity based DCC and underlying structural and mechanistic insights are summarized. The manipulation of the distinct reactivity of structurally diverse ring-chain tautomers allows selective activation and switching of reaction pathways and corresponding DCRs (C-N, C-O, and C-S) and assemblies. The coupling with photoswitches further enables light-mediated formation and scission of multiple types of reversible covalent bonds. To showcase the capability of dual reactivity based DCC, the versatile applications in dynamic polymers and luminescent materials are presented, paving the way for future functionalization studies.

Fine-Tuning the Molecular Design for High-Performance Molecular Diodes Based on Pyridyl Isomers

Better control of molecule-electrode coupling (Γ) to minimize leakage current is an effective method to optimize the functionality of molecular diodes. Herein we embedded 5 isomers of phenypyridyl derivatives, each with an N atom placed at a different position, in two electrodes to fine-tune Γ between self-assembled monolayers (SAMs) and the top electrode of EGaIn (eutectic Ga-In terminating in Ga2 O3 ). Combined with electrical tunnelling results, characterizations of electronic structures, single-level model fittings, and DFT calculations, we found that the values of Γ of SAMs formed by these isomers could be regulated by nearly 10 times, thereby contributing to the leakage current changing over about two orders of magnitude and switching the isomers from resistors to diodes with a rectification ratio (r+ =|J(+1.5 V)/J(-1.5 V)|) exceeding 200. We demonstrated that the N atom placement can be chemically engineered to tune the resistive and rectifying properties of the molecular junctions, making it possible to convert molecular resistors into rectifiers. Our study provides fundamental insights into the role of isomerism in molecular electronics and offers a new avenue for designing functional molecular devices.

Resonant transport in a highly conducting single molecular junction via metal-metal covalent bond

Achieving highly transmitting molecular junctions through resonant transport at low bias is key to the next-generation low-power molecular devices. Although resonant transport in molecular junctions was observed by connecting a molecule between the metal electrodes via chemical anchors by applying a high source-drain bias (>1 V), the conductance was limited to <0.1G0, G0 being the quantum of conductance. Herein, we report electronic transport measurements by directly connecting a ferrocene molecule between Au electrodes under ambient conditions in a mechanically controllable break junction setup (MCBJ), revealing a conductance peak at ∼0.2G0 in the conductance histogram. A similar experiment was repeated for ferrocene terminated with amine (-NH2) and cyano (-CN) anchors, where conductance histograms exhibit an extended low conductance feature, including the sharp high conductance peak, similar to pristine ferrocene. The statistical analysis of the data and density functional theory-based transport calculation suggest a possible molecular conformation with a strong hybridization between the Au electrodes, and that the Fe atom of ferrocene is responsible for a near-perfect transmission in the vicinity of the Fermi energy, leading to the resonant transport at a small applied bias (<0.5 V). Moreover, calculations including van der Waals/dispersion corrections reveal a covalent-like organometallic bonding between Au and the central Fe atom of ferrocene, having bond energies of ∼660 meV. Overall, our study not only demonstrates the realization of an air-stable highly transmitting molecular junction, but also provides important insights about the nature of chemical bonding at the metal/organo-metallic interface.

Single-molecule photoelectron tunnelling spectroscopy

Experimental mapping of transmission is essential for understanding and controlling charge transport through molecular devices and materials. Here we developed a single-molecule photoelectron tunnelling spectroscopy approach for mapping transmission beyond the HOMO-LUMO gap of the single diketopyrrolopyrrole molecule junction using an ultrafast-laser combined scanning tunnelling microscope-based break junction set-up at room temperature. Two resonant transport channels of ultrafast photocurrent are found by our photoelectron tunnelling spectroscopy, ranging from 1.31 eV to 1.77 eV, consistent with the LUMO + 1 and LUMO + 2 in the transmission spectrum obtained by density functional theory calculations. Moreover, we observed the modulation of resonant peaks by varying bias voltages, which demonstrates the ability to quantitatively characterize the effect of the electric field on frontier molecular orbitals. Our single-molecule photoelectron tunnelling spectroscopy offers an avenue that allows us to explore the nature of energy-dependent charge transport through single-molecule junctions.

Composition and sequence-controlled conductance of crystalline unimolecular monolayers

Understanding how the charge travels through sequence-controlled molecules has been a formidable challenge because of simultaneous requirements in well-controlled synthesis and well-manipulated orientation. Here, we report electrically driven simultaneous synthesis and crystallization as a general strategy to study the conductance of composition and sequence-controlled unioligomer and unipolymer monolayers. The structural disorder of molecules and conductance variations on random positions can be extremely minimized, by uniform synthesis of monolayers unidirectionally sandwiched between electrodes, as an important prerequisite for the reproducible measurement on the micrometer scale. These monolayers show tunable current density and on/off ratios in four orders of magnitude with controlled multistate and massive negative differential resistance (NDR) effects. The conductance of monolayer mainly depends on the metal species in homo-metal monolayers, while the sequence becomes a matter in hetero-metal monolayers. Our work demonstrates a promising way to release an ultra-rich variety of electrical parameters and optimize the functions and performances of multilevel resistive devices.

Scaling of quantum interference from single molecules to molecular cages and their monolayers

The discovery of quantum interference (QI) is widely considered as an important advance in molecular electronics since it provides unique opportunities for achieving single-molecule devices with unprecedented performance. Although some pioneering studies suggested the presence of spin qubit coherence and QI in collective systems such as thin films, it remains unclear whether the QI can be transferred step-by-step from single molecules to different length scales, which hinders the application of QI in fabricating active molecular devices. Here, we found that QI can be transferred from a single molecule to their assemblies. We synthesized and investigated the charge transport through the molecular cages using 1,3-dipyridylbenzene (DPB) as a ligand block with a destructive quantum interference (DQI) effect and 2,5-dipyridylfuran (DPF) as a control building block with a constructive quantum interference (CQI) effect using both single-molecule break junction and large area junction techniques. Combined experiments and calculations revealed that both DQI and CQI had been transferred from the ligand blocks to the molecular cages and the monolayer thin film of the cages. Our work introduced QI effects from a ligand to the molecular cage comprising 732 atoms and even their monolayers, suggesting that the quantum interference could be scaled up within the phase-coherent distance.

A trefoil knot self-templated through imination in water

The preparation of topologically nontrivial molecules is often assisted by covalent, supramolecular or coordinative templates that provide spatial pre-organization for all components. Herein, we report a trefoil knot that can be self-assembled efficiently in water without involving additional templates. The direct condensation of three equivalents of a tetraformyl precursor and six equivalents of a chiral diamine produces successfully a [3 + 6] trefoil knot whose intrinsic handedness is dictated by the stereochemical configuration of the diamine linkers. Contrary to the conventional wisdom that imine condensation is not amenable to use in water, the multivalent cooperativity between all the imine bonds within the framework makes this trefoil knot robust in the aqueous environment. Furthermore, the presence of water is proven to be essential for the trefoil knot formation. A topologically trivial macrocycle composed of two tetraformyl and four diamino building blocks is obtained when a similar reaction is performed in organic media, indicating that hydrophobic effect is a major driving force behind the scene.

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.

Light-Driven Charge Transport and Optical Sensing in Molecular Junctions

Probing charge and energy transport in molecular junctions (MJs) has not only enabled a fundamental understanding of quantum transport at the atomic and molecular scale, but it also holds significant promise for the development of molecular-scale electronic devices. Recent years have witnessed a rapidly growing interest in understanding light-matter interactions in illuminated MJs. These studies have profoundly deepened our knowledge of the structure–property relations of various molecular materials and paved critical pathways towards utilizing single molecules in future optoelectronics applications. In this article, we survey recent progress in investigating light-driven charge transport in MJs, including junctions composed of a single molecule and self-assembled monolayers (SAMs) of molecules, and new opportunities in optical sensing at the single-molecule level. We focus our attention on describing the experimental design, key phenomena, and the underlying mechanisms. Specifically, topics presented include light-assisted charge transport, photoswitch, and photoemission in MJs. Emerging Raman sensing in MJs is also discussed. Finally, outstanding challenges are explored, and future perspectives in the field are provided.

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.

Multiple heteroatom substitution effect on destructive quantum interference in tripodal single-molecule junctions

Destructive quantum interference manipulating the electron transport in tripodal meta-linked phenyl derivatives can be modulated by adjusting the number and the position of the substituted heteroatom(s) inside the molecular core.