Molecular Wires Research Articles

Estimation of Electron Distribution over Dinuclear Organometallic Molecular Wires by "IR Tag" Analysis of Ancillary Acyl-Cp Ligands.

Understanding the details of electronic properties of mixed-valence (MV) states of organometallic molecular wires is essential to gain insights into electron transfer phenomena. Although the field of MV chemistry is matured, there remain issues to be solved, which cannot be accessed by the conventional analytical methods. Here, we describe the synthesis and properties of diruthenium bridging (diethynylbenzene)diyl wires, (μ-p and m-C≡C-C6H4-C≡C){RuCpR(dppe)}2 1, with the acyl-substituted cyclopentadienyl rings [CpR: Cpe; η5-C5H4COOMe (a-series: ester derivatives), Cpa; η5-C5H4COMe (b-series: acetyl derivatives)], which are installed as IR-tags to estimate electron densities at the metal centers in the MV species. The electrochemical and IR/near IR-spectroelectrochemical studies reveal that the two metal centers in the para-isomers p-1a,b + interact with each other more strongly than those in the meta-isomers m-1a,b + . Electron-spin resonance study also supports the radicals being delocalized over the Ru-(p-C≡C-C6H4-C≡C)-Ru moieties in p-1a,b + . The spectroelectrochemical IR study shows significant higher-energy shifts of the ν(C=O) vibrations brought about upon 1e-oxidation. Spectral simulation on the basis of the Bloch equations allows us to determine the electron transfer rate constants (k et) between the two metal centers being in the orders of 1012 s-1 ( p-1 + ) and 109 s-1 ( m-1 + ). The shifts of the ν(C=O) bands reveal that the charge densities on the para-isomers p-1a,b + are widely delocalized over the Ru-(p-C≡C-C6H4-C≡C)-Ru linkages in contrast to the meta-isomers m-1a,b + , where the electron densities are mainly localized on the metal fragments, as supported by the density functional theory and time-dependent density functional theory studies as well as comparison with the reference mononuclear acetylide complexes, C6H5-C≡C-RuCpR(dppe) 2. We have successfully demonstrated that the carbonyl groups (>C=O) in the ancillary Cp ligands also work as IR-tags to report detailed information on the electron densities at the metal centers and the electron distribution over MV complexes as well.

Small angle symmetry splitting of helicene-based molecular wires on pyrolytic graphite

Synthesis of inherently chiral materials and investigation of their self-assembly into molecular wires or 2D crystals belongs to hot topics in nanotechnology. Here we report an inherent splitting of orientational symmetry of 10–500 nm long molecular wires assembled from 3 nm chiral helicene-based macrocycles on atomically flat HOPG surface. The symmetry splitting of molecular wire orientation by a small angle of (6.5 ± 0.5)° is attributed to interaction of individual helicene-based macrocycle with HOPG. There is a good correlation between experimental AFM data and theoretical simulations using ReaxFF force field and Lennard-Jones (L-J) potential. In particular the new 1000 times faster computational method based on L-J potential and sequential addition of molecules is able to simulate formation of molecular wires and emergence of their symmetry splitting, which was not possible to do by conventional molecular dynamics. The method is also sensitive to chirality of the studied molecules; enantiomers and racemates placed on HOPG form different arrangements. This opens new possibilities for simulations of large molecular systems.

Amine terminated polyynes as candidates for molecular wire applications: A DFT study

The structural and conducting properties of four amine terminated polyynes CAN[n],PHN[n],NCH3[n] and NH2[n], with n = 2,4,6,8 are studied at density functional theory (DFT) level, and compared with two non-amine terminated cases(BT[n] and PY[n]) whose high single molecule conductance was reported experimentally. Helical molecular orbitals delocalized over the whole molecule are observed for amine terminated polyynes which will improve the conductance. The amine terminated polyynes possess high HOMO energy, hence lesser transport barrier, along with reduced bond length alternation (BLA) values. The reduction in BLA of amine terminated polyynes is also confirmed by the shift of the collective CC stretching frequency to lower values. A significant reduction in BLA is noted followed by a transfer of negative charge to the molecule as shown by the BLA of the anions. Also electron reorganization energies are much reduced for CAN [2], CAN [4] and PHN [2] terminated polyynes. In this way the conducting properties of the polyynes are enhanced for amine terminated polyynes.

Impact of device design on the electronic and optoelectronic properties of integrated Ru-terpyridine complexes.

The performance of nanoelectronic and molecular electronic devices relies strongly on the employed functional units and their addressability, which is often a matter of appropriate interfaces and device design. Here, we compare two promising designs to build solid-state electronic devices utilizing the same functional unit. Optically addressable Ru-terpyridine complexes were incorporated in supramolecular wires or employed as ligands of gold nanoparticles and contacted by nanoelectrodes. The resulting small-area nanodevices were thoroughly electrically characterized as a function of temperature and light exposure. Differences in the resulting device conductance could be attributed to the device design and the respective transport mechanism, that is, thermally activated hopping conduction in the case of Ru-terpyridine wire devices or sequential tunneling in nanoparticle-based devices. Furthermore, the conductance switching of nanoparticle-based devices upon 530 nm irradiation was attributed to plasmon-induced metal-to-ligand charge transfer in the Ru-terpyridine complexes used as switching ligands. Finally, our results reveal a superior device performance of nanoparticle-based devices compared to molecular wire devices based on Ru-terpyridine complexes as functional units.

Coordination polymers for emerging molecular devices

Conductive coordination polymers are hybrid materials with the potential to be implemented in the next generation of electronic devices, owing to several desirable properties. A decade ago, only a few scattered examples exhibiting conductivity existed within this class of materials, yet today groups of coordination polymers possess electrical conductivities and mobilities that rival those of inorganic semiconductors. Many currently emerging energy harvesting and storage technologies are limited by the use of inefficient, unstable, and unsustainable charge transport materials with little tunability. Coordination polymers, on the other hand, offer great electrical properties and fine-tunability through their assembly from molecular building blocks. Herein, the structure–function relationship of these building blocks and how to characterize the resulting materials are examined. Solution processability allows devices to step away drastically from conventional fabrication methods and enables cheap production from earth abundant materials. The ability to tune the electrical and structural properties through modifications at the molecular level during the material synthesis stages allows for a large design space, opening the door to a wide spectrum of applications in environmentally friendly technologies, such as molecular wires, photovoltaics, batteries, and sensors. Sustainable, high-performing charge transport materials are crucial for the continued advance of emerging molecular technologies. This review aims to provide examples of how the promising properties of coordination polymers have been exploited to accelerate the development of molecular devices.

Twisted molecular wires polarize spin currents at room temperature

A critical spintronics challenge is to develop molecular wires that render efficiently spin-polarized currents. Interplanar torsional twisting, driven by chiral binucleating ligands in highly conjugated molecular wires, gives rise to large near-infrared rotational strengths. The large scalar product of the electric and magnetic dipole transition moments ([Formula: see text]), which are evident in the low-energy absorptive manifolds of these wires, makes possible enhanced chirality-induced spin selectivity-derived spin polarization. Magnetic-conductive atomic force microscopy experiments and spin-Hall devices demonstrate that these designs point the way to achieve high spin selectivity and large-magnitude spin currents in chiral materials.

Fabrication of an electrochemical sensor based on gold nanoparticle-functionalized nanocarbon black hybrid nanocomposite for sensitive detection of anti-cancer drug formestane in biological and pharmaceutical samples

• A novel Au@f-NCB hybrid nanocomposite was successfully synthesized and characterized. • The Au@f-NCB modified MW-GCPE exhibited superior performance toward electrosensing of FMST with high sensitivity. • The new sensor was used successively for analysis of FMST in biological fluids, Lentaron depot ampoules and plasma of patient. • Electrosensing of FMST in the presence of anticancer drug DOX was explored. A gold nanoparticle decorated functionalized nanocarbon black (Au@f-NCB) hybrid nanocomposite–modified molecular wire glassy carbon paste electrode (MW-GCPE) was constructed for sensitive determination of anticancer drug formestane (FMST) by square-wave adsorptive anodic stripping voltammetry (SW-AASV). The synthesized Au@f-NCB hybrid nanocomposite was characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and dispersive X-ray spectroscopy (EDX). The fabricated Au@f-NCB/MW-GCPE was found to have higher electrocatalytic activity towards the oxidation of FMST than bare GCPE. The developed electrochemical sensor allowed the ultrasensitive and selective detection of FMST with a linear range (0.02–5.2 µM), low limit of detection (7.14 nM), and high sensitivity (90.0 μA μ M −1 cm −2 ). The constructed Au@f-NCB nanocomposite modified MW-GCPE revealed good repeatability, reproducibility, and higher stability. Furthermore, the developed sensor has been shown to be highly sensitive for detecting FMST in human fluids, Lentaron depot ampoules, and patient plasma. In addition, electrosensing of FMST in the presence of the anti-cancer chemotherapy drug doxorubicin (DOX) was explored, and simultaneous detection of the two drugs was achieved.

Current spin polarization of a platform molecule with compression effect

Using the first-principles method, the spin-dependent transport properties of a novel platform molecule containing a freestanding molecular wire is investigated by simulating the spin-polarized scanning tunneling microscope experiment with Ni tip and Au substrate electrodes. Transport calculations show that the total current increases as the tip gradually approaches to the substrate, which is consistent with the conductance obtained from previous experiment. More interestingly, the spin polarization (SP) of current modulated by compression effect has the completely opposite trend to the total current. Transmission analyses reveal that the reduction of SP of current with compression process originates from the promotion of spin-down electron channel, which is controlled by deforming the molecule wire. In addition, the density of states shows that the SP of current is directly affected by the organic–ferromagnetic spinterface. The weak orbital hybridization between the Ni tip and propynyl of molecule results in high interfacial SP, whereas the breaking of the C≡C triple of propynyl in favor of the Ni–C–C bond induces the strong orbital hybridization and restrains the interfacial SP. This work proposes a new way to control and design the SP of current through organic–ferromagnetic spinterface using functional molecular platform.

Molecular electronics sensors on a scalable semiconductor chip: A platform for single-molecule measurement of binding kinetics and enzyme activity

For nearly 50 years, the vision of using single molecules in circuits has been seen as providing the ultimate miniaturization of electronic chips. An advanced example of such a molecular electronics chip is presented here, with the important distinction that the molecular circuit elements play the role of general-purpose single-molecule sensors. The device consists of a semiconductor chip with a scalable array architecture. Each array element contains a synthetic molecular wire assembled to span nanoelectrodes in a current monitoring circuit. A central conjugation site is used to attach a single probe molecule that defines the target of the sensor. The chip digitizes the resulting picoamp-scale current-versus-time readout from each sensor element of the array at a rate of 1,000 frames per second. This provides detailed electrical signatures of the single-molecule interactions between the probe and targets present in a solution-phase test sample. This platform is used to measure the interaction kinetics of single molecules, without the use of labels, in a massively parallel fashion. To demonstrate broad applicability, examples are shown for probe molecule binding, including DNA oligos, aptamers, antibodies, and antigens, and the activity of enzymes relevant to diagnostics and sequencing, including a CRISPR/Cas enzyme binding a target DNA, and a DNA polymerase enzyme incorporating nucleotides as it copies a DNA template. All of these applications are accomplished with high sensitivity and resolution, on a manufacturable, scalable, all-electronic semiconductor chip device, thereby bringing the power of modern chips to these diverse areas of biosensing.

Polyyne [3]Rotaxanes: Synthesis via Dicobalt Carbonyl Complexes and Enhanced Stability.

New strategies for synthesizing polyyne polyrotaxanes are being developed as an approach to stable carbyne “insulated molecular wires”. Here we report an active metal template route to polyyne [3]rotaxanes, using dicobalt carbonyl masked alkyne equivalents. We synthesized two [3]rotaxanes, both with the same C28 polyyne dumbbell component, one with a phenanthroline‐based macrocycle and one using a 2,6‐pyridyl cycloparaphenylene nanohoop. The thermal stabilities of the two rotaxanes were compared with that of the naked polyyne dumbbell in decalin at 80 °C, and the nanohoop rotaxane was found to be 4.5 times more stable.

Polyyne [3]Rotaxanes: Synthesis via Dicobalt Carbonyl Complexes and Enhanced Stability

AbstractNew strategies for synthesizing polyyne polyrotaxanes are being developed as an approach to stable carbyne “insulated molecular wires”. Here we report an active metal template route to polyyne [3]rotaxanes, using dicobalt carbonyl masked alkyne equivalents. We synthesized two [3]rotaxanes, both with the same C28 polyyne dumbbell component, one with a phenanthroline‐based macrocycle and one using a 2,6‐pyridyl cycloparaphenylene nanohoop. The thermal stabilities of the two rotaxanes were compared with that of the naked polyyne dumbbell in decalin at 80 °C, and the nanohoop rotaxane was found to be 4.5 times more stable.

Electronic Transport in Molecular Wires of Precisely Controlled Length Built from Modular Proteins.

DNA molecular wires have been studied extensively because of the ease with which molecules of controlled length and composition can be synthesized. The same has not been true for proteins. Here, we have synthesized and studied a series of consensus tetratricopeptide repeat (CTPR) proteins, spanning 4 to 20 nm in length, in increments of 4 nm. For lengths in excess of 6 nm, their conductance exceeds that of the canonical molecular wire, oligo(phenylene-ethylenene), because of the more gradual decay of conductance with length in the protein. We show that, while the conductance decay fits an exponential (characteristic of quantum tunneling) and not a linear increase of resistance with length (characteristic of hopping transport), it is also accounted for by a square-law dependence on length (characteristic of weakly driven hopping). Measurements of the energy dependence of the decay length rule out the quantum tunneling case. A resonance in the carrier injection energy shows that allowed states in the protein align with the Fermi energy of the electrodes. Both the energy of these states and the long-range of hopping suggest that the reorganization induced by hole formation is greatly reduced inside the protein. We outline a model for calculating the molecular-electronic properties of proteins.

Empirical Parameter to Compare Molecule-Electrode Interfaces in Large-Area Molecular Junctions.

This paper describes a simple model for comparing the degree of electronic coupling between molecules and electrodes across different large-area molecular junctions. The resulting coupling parameter can be obtained directly from current–voltage data or extracted from published data without fitting. We demonstrate the generalizability of this model by comparing over 40 different junctions comprising different molecules and measured by different laboratories. The results agree with existing models, reflect differences in mechanisms of charge transport and rectification, and are predictive in cases where experimental limitations preclude more sophisticated modeling. We also synthesized a series of conjugated molecular wires, in which embedded dipoles are varied systematically and at both molecule–electrode interfaces. The resulting current–voltage characteristics vary in nonintuitive ways that are not captured by existing models, but which produce trends using our simple model, providing insights that are otherwise difficult or impossible to explain. The utility of our model is its demonstrative generalizability, which is why simple observables like tunneling decay coefficients remain so widely used in molecular electronics despite the existence of much more sophisticated models. Our model is complementary, giving insights into molecule–electrode coupling across series of molecules that can guide synthetic chemists in the design of new molecular motifs, particularly in the context of devices comprising large-area molecular junctions.

Influence of a Coordinated Metal Center on Charge Transport through a Series of Porphyrin Molecular Junctions

Porphyrin-based molecular wires as promising candidates for nanoelectronic devices have attracted much attention. Therefore, it is fundamentally important to investigate structure–electrical properties involving such molecules. Herein, a series of 5-ethynyl-2,3-dihydrobenzo[b]thiophene-substituted free-metal porphyrins and metalloporphyrins have been synthesized. Rigid, structurally well-defined, and highly conjugated porphyrin-based molecular wires offer a good platform to explore the impact of coordinated metal ions (Cu, Fe, and Zn) on charge transport at a single-molecule scale. Using the scanning tunneling microscope break-junction (STM-BJ) technique, it is found that single-molecule conductance can change by nearly 500% when the central coordinated metal ions are changed. The theoretically simulated energy-dependent transmission spectra reveal that the spin state of FeTPP can tune charge transport at E = EF, and transmission coefficient T(E) is well correlated with the experimental observation. Our work proves a pronounced metal ion effect on charge transport through a porphyrin plane, providing guidance to design high-performance porphyrin-based nanodevices.

A one-pot synthesis of oligo(arylene–ethynylene)-molecular wires and their use in the further verification of molecular circuit laws

A convenient two-step, one-pot synthesis of oligo(arylene–ethynylene) (OAE) type molecular wires in yields of up to 70% via in situ desilylation of protected bis(alkynes) Me3SiC≡CArC≡CSiMe3 (Ar = 2,5-thienyl, 1,4-naphthylene, 9,10-anthrylene) and subsequent Sonogashira cross-coupling with S-(4-iodophenyl) ethanethiolate, 4-iodothioanisole, or 5-bromo-3,3-dimethyl-2,3-dihydrobenzo[b]thiophene is described. The in situ desilylation avoids the manipulation of the sensitive terminal dialkynes (HC≡CArC≡CH), whilst the general approach presented has some advantages over alternative synthetic strategies based on coupling of aryl dihalides (XArX) by avoiding the multi-step preparation and purification of the terminal alkynes S-(4-ethynylphenyl) ethanethiolate, 4-ethynylthioanisole and 5-ethynyl 3,3-dimethyl-2,3-dihydrobenzo[b]thiophene. The molecular conductance of the resulting thiolate or thioether functionalised OAE molecular wires has been determined using scanning tunneling microscope break junction (STM-BJ) methods. The trends in molecular conductance do not track simply with the degree of aromaticity of the molecular core despite the rather similar molecular lengths. Rather, the STM-BJ data are better correlated with the nature of the anchor group, highlighting the important role of electrode–molecule coupling on electron transport in a molecular junction. The experimental conductance data are in good agreement with recently described quantum circuit rules, further highlighting the potential for these relationships to be used as predictive tools in molecular electronics research.

Study of electronic structure, electrical characteristics and transport properties of heteroatoms (N, O & S) based Anthracene molecular Nanowires-A DFT Study

For designing the electronic devices using molecular wires, it is important to understand the electronic properties at the molecular level. In this paper, the simulation methodology for analyzing the electrical conductivity of anthracene conjugated molecule sandwich between Au atoms via linker Sulfur atom, is proposed. This theoretical analysis is carried out for different applied electric field ranges from 0 to 0.26 VÅ-1 to metal electrode Au atom using density functional theory (DFT). The effect of external applied electric field towards the structural variation of metal attached anthracene molecule [WH] is well explored and compared the results with that of hetero atoms [N, S, and O] substituted heterocyclic systems [HAA, HTA and HOA]. Further, the ability of electron flow through these systems were explored from the analysis frontier molecular orbitals and density of states (DOS). The bonding nature of the studied molecules were well described from the QTAIM analysis and finally the reactive sites for electrophilic and nucleophilic attack was identified from the simulation of molecular electrostatic potential surface. Non-covalent interaction (NCI-RDG) analysis is addressed to characterize the interactions in heteroatom substituted anthracene based molecular nanowires. In our overall study, the outcomes are exposing that the significates feature and importance of electrical conductivity nature and transport characteristic of hetero-atoms (N, S, O) substituted anthracene based molecular nanowires.

Self-assembled molecular nanowires on prepatterned Ge(001) surfaces.

It is a long-standing goal to fabricate conductive molecular nanowires (NWs) on semiconductor surfaces. Anchoring molecules to pre-patterned surface nanostructures is a practical approach to construct molecular NWs on semiconductor surfaces. Previously, well-ordered inorganic Ge NWs were deduced to spontaneously grow onto Pt/Ge(001) surfaces after annealing at an elevated temperature. In this work, we further demonstrate that organic 7,7,8,8-tetracyanoquinodimethane (TCNQ) molecular NWs can self-assemble onto the atomic NWs on Pt/Ge(001) surfaces. The outer nitrogen atoms in TCNQ molecules hybridize with under-coordinated Ge atoms in Ge NWs with an energy release of ∼1.14 eV per molecule, and electrons transfer from Ge NWs to the frontier orbitals of anchored TCNQs resulting in a negatively charged state. This largely tailors the electronic configurations of TCNQs and Pt/Ge(001) surfaces, enhancing the electron transport along the dimer row direction. The TCNQ molecular NWs coupled with the Ge NWs represent an exemplary showcase for the fabrication of molecular NWs on semiconductor surfaces.

A straightforward method to quantify the electron-delocalizing ability of π-conjugated molecules.

Electronic delocalization is essential to the properties of π-conjugated molecules. We introduce the inter-fragment delocalization index (IFDI) as an easy-to-use computational method for quantifying the electronic delocalization in π-conjugated oligomers and molecular wire models. We show that the IFDI is related to the torsion barriers of π-conjugated dimers, and to the single-molecule conductance of several π-conjugated fragments. The IFDI is a useful screening technique for comparing different π-conjugated subunits as components in organic electronics, since it can quantify the influence of substitution position, structure, and (anti)aromaticity on delocalization.

The impact of G-quadruplex dynamics on inter-tetrad electronic couplings: a hybrid computational study.

The G-quadruplex is a fascinating nucleic acid motif with implications in biology, medicine, and nanotechnologies. G-quadruplexes can form in the telomeres at the edges of chromosomes and in other guanine-rich regions of the genome. They can also be engineered for exploitation as biological materials for nanodevices. Their higher stiffness and higher charge transfer rates make them better candidates in nanodevices than duplex DNA. For the development of molecular nanowires, it is important to optimize electron transport along the wire axis. One powerful basis to do so is by manipulating the structure, based on known effects that structural changes have on electron transport. Here, we investigate such effects, by a combination of classical simulations of the structure and dynamics and quantum calculations of electronic couplings. We find that this structure-function relationship is complex. A single helix shape parameter alone does not embody such complexity, but rather a combination of distances and angles between stacked bases influences charge transfer efficiency. By analyzing linear combinations of shape descriptors for different topologies, we identify the structural features that most affect charge transfer efficiency. We discuss the transferability of the proposed model and the limiting effects of inherent flexibility.

Room‐Temperature Nonvolatile Molecular Memory Based on Partially Unzipped Nanotube

AbstractNonvolatile memories have attracted a lot of interest because they retain the data when the power is interrupted. Smaller size and improved performance of nonvolatile memories are pursued both for basic research and applications. In this work, a molecular wire made of seamless junctions between semiconducting single‐walled carbon nanotubes (SWNT) and partially unzipped segments of the SWNTs are reported. This novel nanostructure is demonstrated to be a nonvolatile memory, which works at room temperature under atmospheric conditions. The characteristics of the device are measured with a four‐terminal configuration and a non‐local voltage (Vnon‐local) is used as the storage signal. An electrical hysteresis of Vnon‐local is observed, wherein two states with different Vnon‐local can be switched by the application of an electric field through an insulating gate device structure, exhibiting nonvolatile characteristics. Vnon‐local can be modulated with external magnetic fields and the mechanism of the electrical hysteresis is attributed to the magnetic moments at the partially unzipped SWNT. The smaller size of SWNT and high working temperature may lead to the development of molecular nanomagnets as nonvolatile memory devices for practical applications.