Large Converse Piezoelectric Effect Measured on a Single Molecule on a Metallic Surface

A Helicene-Based Single-Molecule Inductor and Capacitor with Frequency-Dependent Charge-Transport Pathways

Despite extensive studies has been explored on single-molecule switches and rectifiers, the design of single-molecule inductors has not been explored due to the experimental challenges in the investigation of frequency-dependent charge transport at the single-molecule scale. In this study, we synthesized a helicene-based helical molecular wire and carried out meticulous single-molecule conductance measurements, combined with current-voltage (IV) studies with varying frequencies using the scanning tunneling microscope break junction (STM-BJ) technique. Our results reveal the formation of a single-molecule junction and highlight the unique behavior of the molecular wire in response to different alternating current (AC) varying frequencies. The transport of charges occurs selectively either through the coiled backbone of the conjugated helical structure or vertically via π-π stacking, depending on the frequency of the applied AC. Notably, our investigation demonstrates the functionality of the wire as an inductor at low frequencies, and a capacitor at high frequencies. This work lays the foundation for a systematic approach to designing, fabricating, and implementing single-molecule logic devices such as inductors and wave filters.

Adding multiple electrons to helicenes: how they respond?

Helicenes of increasing dimensions and complexity have recently burst into the scene due to their unique structures coupled with interesting chiral, optical, and conducting properties. The helicene-related research has quickly progressed from fundamental curiosity to a diverse range of applications in organic catalysis, optoelectronic devices, chiroptical switches, sensors, and energy storage. The in-depth understanding of electron accepting properties of helicenes should further advance their materials chemistry applications, however, previous reports only relied on spectrocopic and electrochemical studies, while their structural changes weren't extensively discussed. Therefore, we initiated a broad investigation of chemical reduction behaviour of helicenes ranging in size and properties coupled with X-ray diffraction characterization of the reduced products. The responses of helicenes with different structures to the stepwise electron addition were investigated using a combination of X-ray crystallography, spectroscopic methods, and calculations. This study revealed topology- and charge-dependent consequences of chemical reduction ranging from reversible geometry perturbation to irreversible core transformation and site-specific reactivity of helicenes in addition to original alkali metal coordination patterns. This overview is focused on the crystallographically confirmed examples stemming from chemical reduction reactions of different helicenes with alkali metals. The opened discussion should stimulate further exploration of reactivity and complexation of novel π-expanded and heteroatom-doped helicenes based on the revealed structure-property correlations, thus advancing their applications as intriguing new materials.

Hybrid-DFT study of halide perovskites, an energy-efficient material under compressive pressure for piezoelectric applications

Recent studies have reported that lead-halide perovskites are the most efficient energy-harvesting materials. Regardless of their high-output energy and structural stability, lead-based products have risk factors due to their toxicity. Therefore, lead-free perovskites that offer green energy are the expected alternatives. We have taken CsGeX3(X = Cl, Br, and I) as lead-free halide perovskites despite knowing the low power conversion rate. Herein, we have tried to study the mechanisms of enhancement of energy-harvesting capabilities involving an interplay between structure and electronic properties. A density functional theory simulation of these materials shows a decrease in the band gaps, lattice parameters, and volumes with increasing applied pressure. We report the high piezoelectric responses and high electro-mechanical conversion rates, which are intriguing for generating electricity through mechanical stress.

Tetrahydrobenzo[h]quinoline derivatives as a novel chemotype for dual antileishmanial-antimalarial activity graced with antitubercular activity: Design, synthesis and biological evaluation

Derivatives with tetrahydrobenzo[h]quinoline chemotype were synthesized via one-pot reactions and evaluated for their antileishmanial, antimalarial and antitubercular activities. Based on a structure-guided approach, they were designed to possess antileishmanial activity through antifolate mechanism, via targeting Leishmania major pteridine reductase 1 (Lm-PTR1). The in vitro antipromastigote and antiamastigote activity are promising for all candidates and superior to the reference miltefosine, in a low or sub micromolar range of activity. Their antifolate mechanism was confirmed via the ability of folic and folinic acids to reverse the antileishmanial activity of these compounds, comparably to Lm-PTR1 inhibitor trimethoprim. Molecular dynamics simulations confirmed a stable and high potential binding of the most active candidates against leishmanial PTR1. For the antimalarial activity, most of the compounds exhibited promising antiplasmodial effect against P. berghei with suppression percentage of up to 97.78%. The most active compounds were further screened in vitro against the chloroquine resistant strain P. falciparum, (RKL9) and showed IC50 value range of 0.0198-0.096 μM, compared to IC50 value of 0.19420 μM for chloroquine sulphate. Molecular docking of the most active compounds against the wild-type and quadruple mutant pf DHFR-TS structures rationalized the in vitro antimalarial activity. Some candidates showed good antitubercular activity against sensitive Mycobacterium tuberculosis in a low micromolar range of MIC, compared to 0.875 μM of isoniazid. The top active ones were further tested against a multidrug-resistant strain (MDR) and extensively drug-resistant strain (XDR) of Mycobacterium tuberculosis. Interestingly, the in vitro cytotoxicity test of the best candidates displayed high selectivity indices emphasizing their safety on mammalian cells. Generally, this work introduces a fruitful matrix for new dual acting antileishmanial-antimalarial chemotype graced with antitubercular activity. This would help in tackling drug-resistance issues in treating some Neglected Tropical Diseases.

Helical Bilayer Nanographenes: Impact of the Helicene Length on the Structural, Electrochemical, Photophysical, and Chiroptical Properties

Helical bilayer nanographenes (HBNGs) are chiral π-extended aromatic compounds consisting of two π-π stacked hexabenzocoronenes (HBCs) joined by a helicene, thus resembling van der Waals layered 2D materials. Herein, we compare [9]HBNG, [10]HBNG, and [11]HBNG helical bilayers endowed with [9], [10], and [11]helicenes embedded in their structure, respectively. Interestingly, the helicene length defines the overlapping degree between the two HBCs (number of benzene rings involved in π-π interactions between the two layers), being 26, 14, and 10 benzene rings, respectively, according to the X-ray analysis. Unexpectedly, the electrochemical study shows that the lesser π-extended system [9]HBNG shows the strongest electron donor character, in part by interlayer exchange resonance, and more red-shifted values of emission. Furthermore, [9]HBNG also shows exceptional chiroptical properties with the biggest values of g_abs and g_lum (3.6 × 10^-2) when compared to [10]HBNG and [11]HBNG owing to the fine alignment in the configuration of [9]HBNG between its electric and magnetic dipole transition moments. Furthermore, spectroelectrochemical studies as well as the fluorescence spectroscopy support the aforementioned experimental findings, thus confirming the strong impact of the helicene length on the properties of this new family of bilayer nanographenes.

Chemo- and Regioselective Multiple C(sp2 )-H Insertions of Malonate Metal Carbenes for Late-Stage Functionalizations of Azahelicenes

Chiral quinacridines react up to four times, step-by-step, with α-diazomalonates under RuII and RhII catalysis. By selecting the catalyst, [CpRu(CH3 CN)3 ][PF6 ] (Cp=cyclopentadienyl) or Rh2 (oct)4 , chemo and regioselective insertions of derived metal carbenes are achieved in favor of mono- or bis-functionalized malonate derivatives, respectively, (r.r.>49 : 1, up to 77 % yield, 12 examples). This multi-introduction of malonate groups is particularly useful to tune optical and chemical properties such as absorption, emission or Brønsted acidity but also cellular bioimaging. Density-functional theory further elucidates the origin of the carbene insertion selectivity and also showcases the importance of conformations in the optical response.

Triple Regioselective Functionalization of Cationic [4]Helicenes via Iridium-Catalyzed Borylation and Suzuki Cross-Coupling Reactivity

In essentially one-pot, using Ir- and Pd-catalysis, tris(arene)-functionalized cationic [4]helicenes are synthesized with full regioselectivity and enantiospecificity starting from a trivial precursor (17 examples). This poly-addition of aryl groups improves key optical properties, that is, fluorescence quantum yields and lifetimes. Electronic circular dichroism and circularly polarized luminescence signatures are observed up to the far-red domain, in particular with additional arenes prone to aggregation.

A Strategy for Polar Crystals with Dipolar Heterohelicenes

Polar crystals have attracted interest for the applications to polar materials with piezo- and pyroelectricity, and second harmonic generation. Despite their potential utility for flexible polar materials, a strategy for ordering polar helicenes have remained elusive. Here, we demonstrate creation of polar crystal with unsymmetrically substituted aza[5]helicenes tuned by substituents. The usymmetric aza[5]helicenes have been prepared through regioselective monoprotiodesilylations. We disclosed triisopropylsilyl-substituted derivatives show 1D chain columnar packings. In particular, enantiopure crystals showed spontaneous polarization. Optical and single-crystal X-ray diffraction experiments with other derivatives, as well as theoretical calculations, revealed that the presence of triisopropylsilyl or electron-withdrawing aryl substituents is essential for forming the 1D chain columnar structure. Hirshfeld surface analyses further showed that CH-π interactions between 1D chain columns regulate the polar assembly. Finally, we determined the polarizability of the nitro derivative by ab initio calculation to be 4.53 µC/cm 2 . This value corroborates the first example of a spontaneously polar crystal of helicenes. We believe that this study will contribute to the development of polar materials from organic molecules.

Fabrication of β-Phase-Enriched PVDF Sheets for Self-Powered Piezoelectric Sensing

The fabrication of self-powered pressure sensors based on piezoelectric materials requires flexible piezoelectric generators produced with a continuous, large-scale, and environmentally friendly approach. In this study, continuous poly(vinylidene fluoride) (PVDF) sheets with a higher β-phase content were facilely fabricated by the melt-extrusion-calendering technique and a PVDF-based piezoelectric generator (PEG) was further assembled. Such a PEG exhibits a remarkable piezoelectric output performance. Moreover, it possesses prominent stability even after working for a long time, exhibiting potential applications for real-time monitoring of various human movements (i.e., hopping, running, and walking) and gait. This work not only provides the possibility of continuous and environmentally friendly fabrication of PVDF sheets with remarkable piezoelectric properties but also paves a new promising pathway for powering portable microelectronic applications without any external power supply.

Towards dielectric relaxation at a single molecule scale

Dielectric relaxation lies at the heart of well-established techniques of dielectric spectroscopy essential to diverse fields of research and technology. We report an experimental route for increasing the sensitivity of dielectric spectroscopy ultimately towards the scale of a single molecule. We use the method of radio frequency scanning tunneling microscopy to excite a single molecule junction based on a polar substituted helicene molecule by an electric field oscillating at 2-5 GHz. We detect the dielectric relaxation of the single molecule junction indirectly via its effect of power dissipation, which causes lateral displacement. From our data we determine a corresponding relaxation time of about 300 ps-consistent with literature values of similar helicene derivatives obtained by conventional methods of dielectric spectroscopy.

Mechanically induced single-molecule helicity switching of graphene-nanoribbon-fused helicene on Au(111)

Helicene is a functional material with chirality caused by its characteristic helical geometry. The inversion of its helicity by external stimuli is a challenging task in the advanced control of the molecular chirality. This study fabricated a novel helical molecule, specifically a pentahelicene-analogue twisted aromatic hydrocarbon fused with a graphene nanoribbon, via on-surface synthesis using multiple precursors. Noncontact atomic force microscopy imaging with high spatial resolution confirmed the helicity of the reaction products. The helicity was geometrically converted by pushing a CO-terminated tip into the twisted framework, which is the first demonstration of helicity switching at the single-molecule scale.

Unbalanced 2D Chiral Crystallization of Pentahelicene Propellers and Their Planarization into Nanographenes

The chiral self-assembly of trispentahelicene propellers on a gold surface has been investigated in ultrahigh vacuum by means of scanning tunneling microscopy and time-of-flight secondary ion mass spectrometry. The trispentahelicene propellers aggregate into mirror domains with an enantiomeric ratio of 2 : 1. Thermally induced cyclodehydrogenation leads to planarization into nanographenes, which self-assemble into closed-packed layers with two different azimuths. Further treatment induces in part dimerization and trimerization by intermolecular cyclodehydrogenation.

Intrinsically Polar Piezoelectric Self-Assembled Oligopeptide Monolayers

Flexible, biocompatible piezoelectric materials are of considerable research interest for a variety of applications, but many suffer from low response or high cost to manufacture. Herein, novel piezoelectric force and touch sensors based on self-assembled monolayers of oligopeptides are presented, which produce large piezoelectric voltage response and are easily manufactured without the need for electrical poling. While the devices generate modest piezoelectric charge constants (d33 ) of up to 9.8 pC N-1 , they exhibit immense piezoelectric voltage constants (g33 ) up to 2 V m N-1 . Furthermore, a flexible device prototype is demonstrated that produces open-circuit voltages of nearly 6 V under gentle bending motion. Improvements in peptide selection and device construction promise to further improve the already outstanding voltage response and open the door to numerous practical applications.

Transition from Homochiral Clusters to Racemate Monolayers during 2D Crystallization of Trioxa[11]helicene on Ag(100)

The phenomenon of chiral crystallization into homochiral crystals is known for more than 170 years, yet it is still poorly understood. Studying crystallization on surfaces under well-defined condition seems a promising approach towards better understanding the intermolecular chiral recognition mechanisms during nucleation and growth. The two-dimensional aggregation of racemic trioxaundecahelicene on the single crystalline silver(100) surface has been investigated with scanning tunneling microscopy and with non-contact atomic force microscopy, as well as molecular modeling simulations. A transition from homochiral cluster motifs to heterochiral assembly into large islands with increasing coverage is observed. Force field modelling confirms higher stability of heterochiral arrangements from twelve molecules on. Results are discussed with respect to previous findings for the all-carbon heptahelicene on the same surface.

Influence of Electronic Structure Modeling and Junction Structure on First-Principles Chiral Induced Spin Selectivity

We have carried out a comprehensive study of the influence of electronic structure modeling and junction structure description on the first-principles calculation of the spin polarization in molecular junctions caused by the chiral induced spin selectivity (CISS) effect. We explore the limits and the sensitivity to modeling decisions of a Landauer/Green's function/two-component density functional theory approach to CISS. We find that although the CISS effect is entirely attributed in the literature to molecular spin filtering, spin-orbit coupling being partially inherited from the metal electrodes plays an important role in our calculations on ideal carbon helices, even though this effect cannot explain the experimental conductance results. Its magnitude depends considerably on the shape, size, and material of the metal clusters modeling the electrodes. Also, a pronounced dependence on the specific description of exchange interaction and spin-orbit coupling is manifest in our approach. This is important because the interplay between exchange effects and spin-orbit coupling may play an important role in the description of the junction magnetic response. Our calculations are relevant for the whole field of spin-polarized electron transport and electron transfer, because there is still an open discussion in the literature about the detailed underlying mechanism and the magnitude of physical parameters that need to be included to achieve a consistent description of the CISS effect: seemingly good quantitative agreement between simulation and the experiment can be caused by error compensation, because spin polarization as contained in a Landauer/Green's function/two-component density functional theory approach depends strongly on computational and structural parameters.

Mimicking the electrophysiological microenvironment of bone tissue using electroactive materials to promote its regeneration

The process of bone tissue repair and regeneration is complex and requires a variety of physiological signals, including biochemical, electrical and mechanical signals, which collaborate to ensure functional recovery. The inherent piezoelectric properties of bone tissues can convert mechanical stimulation into electrical effects, which play significant roles in bone maturation, remodeling and reconstruction. Electroactive materials, including conductive materials, piezoelectric materials and electret materials, can simulate the physiological and electrical microenvironment of bone tissue, thereby promoting bone regeneration and reconstruction. In this paper, the structures and performances of different types of electroactive materials and their applications in the field of bone repair and regeneration are reviewed, particularly by providing the results from in vivo evaluations using various animal models. Their advantages and disadvantages as bone repair materials are discussed, and the methods for tuning their performances are also described, with the aim of providing an up-to-date account of the proposed topics.

Double layer crystallization of heptahelicene on noble metal surfaces

Resolution of enantiomers of chiral compounds via crystallization is the dominant method in chemical industry, but chiral recognition at the molecular level during this process is still poorly understood. Using single metal surfaces in ultrahigh vacuum as model system, the enantio-related transition from the monolayer structure into a double layer of the racemic mixture of heptahelicene has been studied with scanning tunneling microscopy. Submolecular resolution reveals enantiopure second layers on Ag(111) and almost enantiopure second layers on Au(111). In analogy to previous results on Cu(111), it is concluded that transition from the 2D first layer racemate into a layered racemate occurs.

Fiber-Based Energy Conversion Devices for Human-Body Energy Harvesting

Following the rapid development of lightweight and flexible smart electronic products, providing energy for these electronics has become a hot research topic. The human body produces considerable mechanical and thermal energy during daily activities, which could be used to power most wearable electronics. In this context, fiber-based energy conversion devices (FBECD) are proposed as candidates for effective conversion of human-body energy into electricity for powering wearable electronics. Herein, functional materials, fiber fabrication techniques, and device design strategies for different classes of FBECD based on piezoelectricity, triboelectricity, electrostaticity, and thermoelectricity are comprehensively reviewed. An overview of fiber-based self-powered systems and sensors according to their superior flexibility and cost-effectiveness is also presented. Finally, the challenges and opportunities in the field of fiber-based energy conversion are discussed.

Helically Chiral Aromatics: The Synthesis of Helicenes by [2 + 2 + 2] Cycloisomerization of π-Electron Systems

Advanced molecular nanocarbons are now in the spotlight reflecting the basic discoveries of fullerenes, carbon nanotubes, and graphene. This research area includes also the chemistry, physics, and nanoscience of nonplanar polycyclic hydrocarbons, many of which exhibit helical chirality, such as iconic helicenes and their congeners. The combination of unique π-electron systems with the chirality phenomenon makes them highly attractive in various fields of science. Helicenes are polyaromatic compounds that are composed of all-angularly annulated benzene units, but other (hetero)cycles can also be embedded into their backbone. Even though they do not contain any stereogenic center, they are inherently chiral owing to the helical shape they adopt. Hexahelicene and higher homologues are conformationally stable within a reasonable range of temperatures and, therefore, can be obtained in an enantiopure form through a racemate resolution or asymmetric synthesis. An amazing array of synthetic methods for their preparation has been developed, but only a few of them have passed the tough scrutiny to be general, robust and practical methods such as traditional photocyclodehydrogenation of diaryl olefins and recently developed transition-metal-catalyzed [2 + 2 + 2] cycloisomerization of π-electron systems, which is discussed in this Account. Alkyne [2 + 2 + 2] cycloisomerization is a highly exergonic process and is therefore suitable for forming the strained helicene backbone, three (or more) cycles of which are closed in a single operation. The typical starting materials are aromatic triynes (optionally cyanodiynes or ynedinitriles) or tetraynes with diynes that undergo intramolecular or intermolecular cyclization, respectively, catalyzed by various complexes mainly of Ni⁰, Co^I, or Rh^I. Utilizing this synthetic methodology, various [5]-, [6]-, [7]-, [9]-, [11]-, [13]-, [16]-, [17]-, and [19]helicenes or their congeners, including functionalized derivatives, can be effectively prepared. Moreover, asymmetric synthesis (both catalytic and stoichiometric) of nonracemic helicenes has already been demonstrated. It relies on [2 + 2 + 2] cycloisomerization of centrally chiral triynes followed by an asymmetric transformation of the first order (controlled by the 1,3-allylic-type strain) or on enantioselective [2 + 2 + 2] cycloisomerization of alkynes catalyzed by chiral complexes mainly of Ni⁰ or Rh^I. Intriguingly, advanced helical architectures were formed such as the longest helicenes (up to oxa[19]helicene by closing 12 rings in a single synthetic operation) or laterally extended helicenes (e.g., pyreno[7]helicenes). Utilizing the aforementioned synthetic methodology, the tailor-made helical molecular nanocarbons are now better accessible to be applied in enantioselective catalysis, chirality sensing, spintronics (based on chirality induced spin selectivity), chiroptics (to produce circularly polarized light emission), organic/molecular electronics, or chiral single molecule devices.

Synthesis of Racemic, Diastereopure, and Enantiopure Carba- or Oxa[5]-, [6]-, [7]-, and -[19]helicene (Di)thiol Derivatives

A series of carba- or oxa[5]-, [6]-, [7]-, and -[19]helicene (di)thiols was prepared. The Miyazaki-Newman-Kwart rearrangement of (dimethylcarbamothioyl)oxy (oxa)helicenes in a flow reactor or nucleophilic substitution of dichloro (oxa)helicenes with alkanethiolates were used in the sulfanylation step. Despite the high temperatures employed in this key step, no conformational scrambling was observed during the asymmetric synthesis of the diastereo- and enantiopure oxahelicenes. Single-molecule conductivity of the longest oxa[19]helicene dithiol derivative was studied by the scanning tunneling microscopy break-junction method.

Heterochiral recognition among functionalized heptahelicenes on noble metal surfaces

No matter which heptahelicene derivative, all assemble into heterochiral zigzag chains on gold and silver(111) surfaces.

Theory of electron, phonon and spin transport in nanoscale quantum devices

At the level of fundamental science, it was recently demonstrated that molecular wires can mediate long-range phase-coherent tunnelling with remarkably low attenuation over a few nanometre even at room temperature. Furthermore, a large mean free path has been observed in graphene and other graphene-like two-dimensional materials. These create the possibility of using quantum and phonon interference to engineer electron and phonon transport through nanoscale junctions for a wide range of applications such as molecular switches, sensors, piezoelectricity, thermoelectricity and thermal management. To understand transport properties of such devices, it is crucial to calculate their electronic and phononic transmission coefficients. The aim of this tutorial article is to outline the basic theoretical concepts and review the state-of-the-art theoretical and mathematical techniques needed to treat electron, phonon and spin transport in nanoscale molecular junctions. This helps not only to explain new phenomenon observed experimentally but also provides a vital design tool to develop novel nanoscale quantum devices.

Torque-Induced Change in Configuration of a Single NO Molecule on Cu(110)

We demonstrated that a nitric oxide (NO) molecule on Cu(110) acts as an "ON-OFF-ON toggle switch" that can be turned on and off by repulsive force and electron injection, respectively. On the surface, NO molecules exist in three configurations: flat along the [001] direction (ON), upright (OFF), and flat along [001[over ¯]] (ON). An NO-functionalized tip, which was characterized by scanning tunneling microscopy and inelastic electron tunneling spectroscopy, can convert an upright NO adsorbate into a flat-lying NO. Atomic force microscopy and a simulation of the interactions between the NO molecules reveal that a repulsive force not aligned with the N-O bond provides the torque that detrudes the NO toggle; i.e., the upright NO adsorbate is tilted away from the tip. Therefore, the NO adsorbate behaves as a nonvolatile sensor for the detection of locally applied repulsive torque.