Organic Molecular Wires Research Articles

Formation and Evolution of Metallocene Single-Molecule Circuits with Direct Gold-π Links.

Single-molecule circuits with group 8 metallocenes are formed without additional linker groups in scanning tunneling microscope-based break junction (STMBJ) measurements at cryogenic and room-temperature conditions with gold (Au) electrodes. We investigate the nature of this direct gold-π binding motif and its effect on molecular conductance and persistence characteristics during junction evolution. The measurement technique under cryogenic conditions tracks molecular plateaus through the full cycle of extension and compression. Analysis reveals that junction persistence when the metal electrodes are pushed together correlates with whether electrodes are locally sharp or blunt, suggesting distinct scenarios for metallocene junction formation and evolution. The top and bottom surfaces of the "barrel"-shaped metallocenes present the electron-rich π system of cyclopentadienyl rings, which interacts with the gold electrodes in two distinct ways. An undercoordinated gold atom on a sharp tip forms a donor-acceptor bond to a specific carbon atom in the ring. However, a small, flat patch on a dull tip can bind more strongly to the ring as a whole through van der Waals interactions. Density functional theory (DFT)-based calculations of model electrode structures provide an atomic-scale picture of these scenarios, demonstrating the role of these bonding motifs during junction evolution and showing that the conductance is relatively independent of tip atomic-scale structure. The nonspecific interaction of the cyclopentadienyl rings with the electrodes enables extended conductance plateaus, a mechanism distinct from that identified for the more commonly studied, rod-shaped organic molecular wires.

Controlling and Optimizing Photoinduced Charge Transfer across Ultrathin Silica Separation Membrane with Embedded Molecular Wires for Artificial Photosynthesis.

Ultrathin amorphous silica membranes with embedded organic molecular wires (oligo(p-phenylenevinylene), three aryl units) provide chemical separation of incompatible catalytic environments of CO2 reduction and H2O oxidation while maintaining electronic and protonic coupling between them. For an efficient nanoscale artificial photosystem, important performance criteria are high rate and directionality of charge flow. Here, the visible-light-induced charge flow from an anchored Ru bipyridyl light absorber across the silica nanomembrane to Co3O4 water oxidation catalyst is quantitatively evaluated by photocurrent measurements. Charge transfer rates increase linearly with wire density, with 5 nm-2 identified as an optimal target. Accurate measurement of wire and light absorber densities is accomplished by the polarized FT-IRRAS method. Guided by density functional theory (DFT) calculations, four wire derivatives featuring electron-donating (methoxy) and -withdrawing groups (sulfonate, perfluorophenyl) with highest occupied molecular orbital (HOMO) potentials ranging from 1.48 to 0.64 V vs NHE were synthesized and photocurrents evaluated. Charge transfer rates increase sharply with increasing driving force for hole transfer from the excited light absorber to the embedded wire, followed by a decrease as the HOMO potential of the wire moves beyond the Co3O4 valence band level toward more negative values, pointing to an optimal wire HOMO potential around 1.3 V vs NHE. Comparison with photocurrents of samples without nanomembrane indicates that silica layers with optimized wires are able to approach undiminished electron flux at typical solar intensities. Combined with the established high proton conductivity and small-molecule blocking property, the charge transfer measurements demonstrate that oxidation and reduction catalysis can be efficiently integrated on the nanoscale under separation by an ultrathin silica membrane.

Single-Molecule Junction of a Cationic Rh(III) Polyyne Molecular Wire.

Single-molecule conductance studies on metal-containing inorganic and organometallic molecular wires are relatively less explored compared to those on organic molecular wires. Furthermore, conductance and transmission profiles of the metal-containing wires insensitive to the metal centers often hinder rational design for high performance wires. Here, synthesis and single-molecule conductance measurements of the bis(butadiynyl)rhodium wires with tetracarbene ligands 1H and 1Au are reported as rare examples for Rh(III) diacetylide molecular wires. The rhodium wires derived from the terminal acetylene and gold-functionalized precursors show comparable, high single-molecule conductance ((6-7) × 10-3 G0) as determined by the STM break-junction measurements, suggesting formation of virtually the same covalently linked metal electrode-molecule-metal electrode junctions. The values for the metallapolyynes are larger than those of the organic polyyne wires having the similar molecular lengths. The hybrid DFT-NEGF calculations of the model systems suggest that profiles of transmission spectra are highly sensitive to the presence and species of the metal fragments doped into the polyyne molecular wire because the conductance orbitals of the metallapolyynes molecular junctions carry significant metal fragment characters. Thus, the metallapolyyne junctions turn out to be suitable platforms for rationally designed molecular wires.

Carbon-Bridged Oligo(phenylene vinylene)s: A de Novo Designed, Flat, Rigid, and Stable π-Conjugated System.

The modern history of conducting organic systems started with a fortuitous error in 1967 on acetylene polymerization, followed by a rational discovery in 1976 on the effects of doping that generates a polaron and, hence, dramatically increases conductivity. Not unexpectedly, however, the prototypical polyacetylene suffers many problems, including C-C single bond rotation, short effective conjugation length, radiationless deactivation, and instability of the polarons. Several strategies have been put in place to solve these problems. An early approach relied on partial rigidification of the polyene structure by conversion into polymers with thiophene, pyrrole, and benzene linkages. An oligo(phenylene vinylene) (OPV) is an all-carbon analogue of polyacetylene, where every other diene unit in the polyene chain is converted to a benzene unit, still leaving many C-C single bonds freely rotating in the molecule. We considered adding additional carbon bridges to rigidify the OPV skeleton entirely to create a carbon-bridged OPV (COPV). Making such a compound was an obvious challenge. This Account describes the authors' efforts to design and synthesize a series of COPV molecules, where the benzene rings in OPV are bridged by sp3 carbon atoms to form a bicyclo[3.3.0]octatriene framework bearing a tetrasubstituted olefin at the ring fusion. This olefinic bond is so strained that it resists further deformation or conversion to sp3 centers, and hence, it is chemically stable despite the strain. The sp3 carbon bridges can bear organic side chains that hinder intermolecular interactions, rendering the excited states stable and long-lived even in the solid state. They also increase solubility, a common problem among rigid molecular systems. With these structural features, the COPV molecules were found to be well behaved both at a single-molecule level and as a bulk material. We reported in 2009 a method for the synthesis of COPVs and have, since then, reported their structures and physicochemical properties, including basic photophysical properties of neutral and charged derivatives, thermal and photostability, and fast electron transfer. These properties have rendered the COPV molecules useful for electronic and photonic research, for example, lasers, solar cells, and molecular wire applications. Noteworthy discoveries in the area connecting chemistry and physics include inelastic tunneling and long-range resonance tunneling at ambient temperature, which were previously observed only for organic molecular wires placed under cryogenic conditions. Given the ready availability of the COPV skeleton bearing a wide variety of substituents, this class of molecules will serve as versatile building blocks for fundamental and applied research on physicochemical and materials chemistry.

Heterobinuclear Light Absorber Coupled to Molecular Wire for Charge Transport across Ultrathin Silica Membrane for Artificial Photosynthesis.

Coupling of robust, all-inorganic heterobinuclear light absorbers to metal oxide catalysts for water oxidation across an ultrathin product-separating silica membrane requires charge transfer through organic molecular wires embedded in the silica. A synthetic approach for assembling the bimetallic units on the silica surface is introduced that is compatible with the presence of encapsulated organic molecules. Accurate selection and fine tuning of the concentration of embedded conducting wires are enabled by a two-step method consisting of surface attachment of a tripodal anchor, trimethoxysilyl aniline, followed by attachment of p-oligo(phenylene vinylene) through amide linkage. Each step of the assembly process was monitored and characterized by a combination of Fourier transform infrared, Fourier transform-Raman, and UV-vis spectroscopy techniques. Hole transfer was observed from transient CoIII, formed by TiIVOCoII → TiIIIOCoIII charge transfer excitation of the chromophore, to p-oligo(phenylene vinylene) molecule within the 8 ns width of the photolysis laser pulse by transient optical absorption spectroscopy of the wire radical cation. The rectifying property of the light absorber-wire assembly enabled by appropriate selection of redox potentials of metals and embedded wire obviates the need for a molecularly defined linkage between the components. Combined with the previously observed ultrafast hole injection from the embedded wires to Co oxide catalyst, the result implies visible-light-induced hole transfer from visible-light-excited binuclear light absorber to water oxidation catalyst across the silica separation membrane in a few nanoseconds or faster. Demonstration and understanding of this interfacial charge-transfer step is critical for developing nanoscale core-shell architectures for complete photosynthetic cycles.

Coherent Resonant Electron Tunneling at 9 and 300 K through a 4.5 nm Long, Rigid, Planar Organic Molecular Wire.

Organic molecular wires that operate stably at ambient temperatures are a necessary first step toward practical and useful molecular-scale electronic devices, which have thus far been hampered by many factors, including the structural and electron configurational instability of organic molecules. We report here that a single disulfanyl carbon-bridged oligo(phenylenevinylene) (COPV6) molecule embedded between thermally stable electroless Au-plated electrodes of a 4 nm nanogap undergoes coherent resonant tunneling at both 9 and 300 K and functions even after storage in air at room temperature. Such enormous stability is ascribed to the unique structural characteristics of COPV6, that is, rigidity, planarity, thermal stability, resistivity against oxidation and reduction, and an organic insulating sheath that protects the π-system. When sandwiched between the gaps without pinning, this molecule behaves as a Coulomb island with sequential single-electron tunneling at 9 K that disappears at 300 K while maintaining a stable electron flow.

Lattice-Directed Construction of Metal–Organic Molecular Wires of Pentacene on the Au(110) Surface

The construction of metal–organic molecular wires is important for the design of specific functional devices but has been a great challenge for experimental technology. Here we report the formation of one-dimensional metal–organic structures by direct deposition of pentacene molecules on the Au(110) surface with subsequent thermal annealing. These metal–organic molecular wires were systematically explored by scanning tunneling microscopy (STM) and density functional theory calculations. At submonolayer coverage, during annealing at ∼470 K, the adsorbed molecules induce both Au(110)-(1 × 3) surface reconstruction, where two atomic rows are missing every three rows on the Au(110) surface, with the end-to-end pentacene configuration and Au(110)-(1 × 6) surface reconstruction, where five rows are missing every six rows on the surface, with the side-by-side configuration. Further annealing at ∼520 K results in Au-adatom-coordinated metal–organic molecular wires with a new side-by-side configuration of pentacen...

Inorganic and Organometallic Molecular Wires for Single-Molecule Devices.

Recent developments of single-molecule conductance measurements allow us to understand fundamental conducting properties of molecular wires. While a wide variety of organic molecular wires have been studied so far, inorganic and organometallic molecular wires have received much less attention. However, molecular wires with transition-metal atoms show interesting features and functions distinct from those of organic wires. These properties originate mainly from metal-ligand dπ-pπ interactions and metal-metal d-d interactions. Thanks to the rich combination of metal atoms and supporting ligands, frontier orbital energies of the molecular wires can be finely tuned to lead to highly conducting molecular wires. Moreover, the unique electronic structures of metal complexes are susceptible to subtle environmental changes, leading to potential functional molecular devices. This article reviews recent advances in the single-molecule conductance study of inorganic and organometallic molecular wires.

Electron transfer through organic molecular wires: A theoretical study

Density Functional Theory (DFT) and Time Dependent DFT (TDDFT) calculations have been carried out on a series of Donor-Bridge-Acceptor (DBA) systems, where the bridges are π-conjugated molecular wires. In these systems a lack of strong dependence of the charge-transfer (CT) rate on large variations in the D-A distance has been observed. The results show that a suitably defined length, Reff, shows significantly smaller variation over the different bridges which have widely varying geometrical lengths. The distance Reff is determined on the basis of application of a modified Mulliken’s formula for the CT energies and comparison to those calculated by TDDFT.

Charge Transport through Organic Molecular Wires Embedded in Ultrathin Insulating Inorganic Membrane

A hybrid organic-inorganic membrane for facilitating artificial photosynthesis is made by embedding surface bound molecular wires in an inorganic ~2 nm thick silica layer by low temperature atomic layer deposition. This membrane is meant to facilitate electron and proton transport between water oxidation and carbon dioxide reaction sites while blocking products and intermediate reactions by compartmentalizing the reaction microenvironments. Here the membrane is studied in a flat form as an intermediate step towards a working artificial photosynthesis system. The membrane is characterized using FT-IR, XPS and EDS-STEM, confirming the wires are intact and embedded in silica. Charge transport through the wires is confirmed by directly measuring the short circuit photocurrent generated by hole injection from a Ru based dye solution while at the same time blocking charge transport from Sn based dye solution which does not have the proper energy level alignment to inject charges to the wires. The incorporation of a molecular wire in an inorganic matrix may facilitate the incorporation of other molecular electronic building block and enable their use in other applications and fields of study.

Charge Transport through Organic Molecular Wires Embedded in Ultrathin Insulating Inorganic Layer

Dense phase silica layers with thickness of a few nanometers featuring embedded organic molecular wires of type p-oligo(phenylenevinylene) are shown by visible light sensitized electrochemical measurements to transport charges across the insulating membrane. We find that such hybrid materials combination allows electronic charge transport only through the wires, while blocking molecular transport. Embodiment of the wire molecules in the silica was accomplished by atomic layer deposition under mild temperature conditions. Grown on Co oxide films for water oxidation, with the wire molecules covalently anchored on the oxide surface, the layer functions as a proton conducting separation membrane. Characterization by XPS, FT-IR and STEM/EDX confirms the integrity of the silica-encapsulated organic wires. Cyclic voltammetry with redox couple of selected potential relative to the energy levels of the wire molecules shows that the membrane is free of pinholes. The new type of membrane allows separation of incompa...

Electron transfer through rigid organic molecular wires enhanced by electronic and electron-vibration coupling.

Electron transfer (ET) is a fundamental process in a wide range of biological systems, photovoltaics and molecular electronics. Therefore to understand the relationship between molecular structure and ET properties is of prime importance. For this purpose, photoinduced ET has been studied extensively using donor-bridge-acceptor molecules, in which π-conjugated molecular wires are employed as bridges. Here, we demonstrate that carbon-bridged oligo-p-phenylenevinylene (COPV), which is both rigid and flat, shows an 840-fold increase in the ET rate compared with the equivalent flexible molecular bridges. A 120-fold rate enhancement is explained in terms of enhanced electronic coupling between the electron donor and the electron acceptor because of effective conjugation through the COPVs. The remainder of the rate enhancement is explained by inelastic electron tunnelling through COPV caused by electron-vibration coupling, unprecedented for organic molecular wires in solution at room temperature. This type of nonlinear effect demonstrates the versatility and potential practical utility of COPVs in molecular device applications.

Inorganic core-shell assemblies for closing the artificial photosynthetic cycle.

Co oxide (Co(3)O(4)) nanotubes are shown to act as an efficient water oxidation catalyst when driven with a visible light sensitizer (pH 7). The nanotubes form the core of a Co(3)O(4)-SiO(2) core-shell nanotube design for separating the carbon dioxide photoreduction from the oxygen evolution reaction. Amorphous dense phase silica of a few nanometers depth is shown to conduct protons while blocking molecular oxygen. Organic molecular wires embedded in the silica shell provide controlled charge transport between the light absorber on one side and the Co(3)O(4) catalyst on the other side. Hence, the silica shell is suitable as a membrane of an assembly for closing the photosynthetic cycle on the nanometer scale under product separation.

First-principles study of one-dimensional sandwich wires [(P)5TM]∞ (TM = Ti, V, Cr, Mn, Fe, Co)

Since the discovery of ferrocene, many one-dimensional metallic sandwich molecular wires have been identified. However, most of the known systems are assembled from organic molecules. Suffering from many drawbacks has, however, hampered their widespread applications. With the goal of breaking this logjam, we provide a blueprint for the designing of a variety of novel sandwich molecular wires ([(P)5TM]∞, TM = Ti, V, Cr, Mn, Fe, and Co) assembled from ferrocene-like inorganic molecules (P)5TM, offering evidence of the existence of inorganic molecular wires in this class. We present first-principles calculations to investigate systematically the electronic and magnetic properties of such novel inorganic sandwich molecular wires. Compared with the organic molecular wires, all the inorganic [(P)5TM]∞ wires are of large magnetic moment. Among them, we find that [(P)5V]∞, [(P)5Cr]∞ and [(P)5Mn]∞ display ferromagnetic character, while for [(P)5Ti]∞, [(P)5Fe]∞ and [(P)5Co]∞, the magnetic coupling is antiferromagnetic. More remarkably, the TM atoms distributed in these wires show regular docking and lead to structures with ordered spin signals, which is a long-term dream of spintronics. We propose that the difference in magnetic coupling for the studied systems is related to the competition between two exchange interactions of TM atoms. Specifically, we propound that the general mechanism for the formation of stable 1D [(P)5TM]∞ involves the transfer of one electron from the TM atom to the P5 ligand forming and TM+ alternating structure.

Cyanopolyynes as Organic Molecular Wires in the Interstellar Medium

Cyanopolyynes (H[C≡C]n-CN or HC2n+1N, where n = 1, 2, 3, …, n) are commonly observed in the interstellar medium (ISM) as well as in the envelopes of carbon-rich stars. These linear molecular structures can be well described with a one-dimensional conduction model, which considers the scattering processes of electrons through the charge transfer conduction bridge of the H[C≡C]n-molecular wire containing the CN group as an electron-acceptor terminal unit. Therefore, our results using this model enable a better understanding of the longest molecules observed in interstellar space and provide new insight into why these particular cyanopolyynes reach a maximum length, such as is observed from astronomical experimental spectral data and cosmological chemical models. Dipole moments and geometrical parameters of these cyanopolyynes were obtained from ab initio molecular orbital calculations using the restricted Hartree-Fock approach and 6-311G* basis set, in order to obtain the inner resistance as a new parameter of chemical reaction feasibility for this molecular series. Using this last molecular parameter, we have been able to analyze the possibility of identifying long molecular species that can be found under local thermodynamic equilibrium in some ISM such us HC25H, HC27H, and HC29N, which have not been observed at present.

Magnetic Manipulation and Half-Metal Prediction of One-Dimensional Bimetallic Organic Sandwich Molecular Wires [CpTM1CpTM2]∞ (TM1 = Ti, Cr, Fe; TM2 = Sc−Co)

We systematically investigate the stability and electronic and magnetic properties of one-dimensional (1D) bimetallic organic sandwich molecular wires (BOSMWs), [CpTiCpTM]∞ (TM = Sc−Co, Cp = C5H5), [CpCrCpTM]∞ (TM = V, Mn, Co), and [CpFeCpTM]∞ (TM = Cr, Co), using ab initio methods. All the BOSMWs are highly stable due to mixed ionic−covalent bonding. With the exceptions of [CpTiCpV]∞, [CpTiCpMn]∞, and [CpCrCpV]∞ exhibiting antiferromagnetic behavior, all the other BOSMWs are ferromagnetic with tunable magnetic moments. In particular, magnetic moments of [CpTiCpCo]∞ and [CpCrCpMn]∞ can be as high as 5 μB per unit cell. Our calculations further show that [CpTiCpTM]∞ (TM = Cr, Fe), [CpCrCpTM]∞ (TM = Fe, Co), and [CpFeCpCo]∞ are robust half-metals (HMs) with large HM gaps. Most importantly, we identify an empirical valence electron filling rule for these BOSMWs, and a BOSMW is found to be a half-metallic ferromagnet whenever N − 5(10) = 5(7) (N is the sum of the valence electrons of two metal atoms). This electron filling rule, together with the HM equations formulized in this study, can be extended to predict new HM BOSMWs.

Electrochemical and Photochemical Cyclization and Cycloreversion of Diarylethenes and Diarylethene-Capped Sexithiophene Wires

A combined theoretical and experimental study was performed on diarylethenes and diarylethene-capped sexithiophenes aiming at an improved understanding of the electrochemical and photochemical ring-opening and ring-closing mechanisms. Theoretical calculations, based on DFT and TDDFT, suggested that the spatial distribution and the occupancy of the frontier orbitals determine and control the diarylethenes' ring-opening and ring-closing upon photoirradiation and electrochemical oxidation. Optimized geometries, potential energy surfaces, and activation energies between the open-ring and closed-ring forms were calculated for diarylethenes in the neutral ground state, excited states, and mono- and dicationic states. Analysis of the frontier orbitals was employed to understand the cyclization and cycloreversion of diarylethenes and to predict and explain the switching properties of diarylethene-capped sexithiophene molecular wires. The TDDFT data were verified with experimentally measured UV/vis spectra. The DFT calculations estimated open-shell ground states of diarylethene-capped sexithiophene dications, which were verified with EPR spectroscopy, and the broadening of the peaks in the EPR spectra were explained with the calculated singlet-triplet splitting. The good agreement of experiment and theory allows for the understanding of switching behavior of diarylethenes in solutions, in metal break junctions, in monolayers on metal surfaces, and as a part of complex organic molecular wires.

Single molecule charge transport: from a quantum mechanical to a classical description

This paper explores charge transport at the single molecule level. The conductive properties of both small organic molecules and conjugated polymers (molecular wires) are considered. In particular, the reasons for the transition from fully coherent to incoherent charge transport and the approaches that can be taken to describe this transition are addressed in some detail. The effects of molecular orbital symmetry, quantum interference, static disorder and molecular vibrations on charge transport are discussed. All of these effects must be taken into account (and may be used in a functional way) in the design of molecular electronic devices. An overview of the theoretical models employed when studying charge transport in small organic molecules and molecular wires is presented.

Cofacially‐Arrayed Polybenzenoid Nano Structures

Based on Moore's Law, every 18 months the number of transistors that can be placed per unit area on a microchip will double. However, by the year 2017 this trend will cease to exist due to the size restrictions imposed by silicon, the main component of microchips; this has led to the birth of molecular electronics. An organic molecule that has been extensively tested for its potential in molecular electronics is DNA. However, recent research has shown that DNA cannot function as a molecular wire due to its fragile nature. This is why Raj Rathore's group is developing organic molecular wires based on robust macromolecular structures. The specific assembly designed to study the wire behavior consists of a triad which is made up of polyfluorenes as an electron donor site, a spacer unit (or wire) made of polyphenylenes, and an electron donor‐acceptor complexation site composed of hexamethylbenzene (HMB). A chloranil molecule—an electron acceptor—complexes with the HMB of the triad, and when a laser is shined on HMB/CA complex, a hole is introduced in the molecular wire which will travel 30 Å to the polyfluorene donor, via an electron hopping mechanism. The Marquette University High School SMART Team (Students Modeling A Research Topic) created a physical model of this molecular wire using 3D printing technology. Supported by a grant from NIH‐NCRR‐SEPA.

Computational Modeling of the Electronic Structure of Oligothiophenes with Various Side Chains

Oligothiophenes are promising candidates as organic molecular wires. Density functional theory is employed to investigate eight oligothiophene series with various side chains, end groups, and spacers ranging in length from 3 to 12 monomers. Redox potentials are evaluated for a number of species on the basis of the thermodynamic cycle formalism (J. Phys. Chem. A 2002, 106, 7407) and agree well with the experimental results. It is confirmed that polarons rather than bipolarons play the role of charge carriers for charged oligomers with sufficiently long backbone. Our calculations also shed light on the detailed oxidization processes in cyclic voltammetry experiments and, hence, provide significant implications on the design of novel oligothiophene-based conductive materials.