Electrochemical Characteristics of a Self-Propagating Molecular-Based Assembly

Compositionally Controlled Electron Transfer in Metallo-Organics

We demonstrate here the assembly of a nanolayer of electrochromic iron complexes on the top of composite layers of cobalt and ruthenium complexes. Depending on the ratio of the latter two complexes, we can tailor materials that show different electron transport pathways, redox activities, and color transitions. No redox activity of the top layer, consisting of iron complexes, is observable when the relative amount of the ruthenium complexes is low in the underlying composite layer because of the insulating properties of the isostructural cobalt complexes. Increasing the amount of ruthenium complexes opens an electron transport channel, resulting in charge storage in both the cobalt and iron complexes. The trapped charges can be chemically released by redox-active ferrocyanide complexes at the film-water interface.

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.

Covalently Assembled Monolayers of Homo- and Heteroleptic FeII -Terpyridyl Complexes on SiOx and ITO-Coated Glass Substrates: An Experimental and Theoretical Study

Well-defined Fe^II -terpyridyl monolayers were fabricated on SiO_x and conductive ITO-coated glass substrates through covalent-bond formation between the metallo-organic complexes and a preassembled coupling layer. Three different homo- and heteroleptic complexes with terminal pyridyl, amine, and phenyl groups were tested. All the films were found to be densely packed and homogeneous, and consist of molecules standing upright. They exhibited high thermal (up to ≈220 °C) and temporal (up to 5 h at 100 °C) stability. The UV/Vis spectra of the monolayers showed pronounced metal-to-ligand charge-transfer bands with a significant redshift compared with the solution spectra of the metallo-ligands with a pendant pyridyl group quaternized with the coupling layer, whereas the shift was significantly smaller when the coupling layer was bonded to the primary amine (-NH₂ ) group of the complex. Cyclic voltammograms of the monolayers showed reversible, one-electron redox behavior and suggested strong electronic coupling between the confined molecules and the underlying substrate. Analysis of the electrochemistry data allowed us to estimate the charge-transfer rate constant between the metal center and the substrate. Additionally, detailed quantum-chemical calculations were performed to support and rationalize the experimentally observed photophysical properties of the Fe^II -terpyridyl complexes both in the solution state and when bound to a SiO_x -based substrate.

Comparison of DC and AC Transport in 1.5-7.5 nm Oligophenylene Imine Molecular Wires across Two Junction Platforms: Eutectic Ga-In versus Conducting Probe Atomic Force Microscope Junctions

We have utilized DC and AC transport measurements to measure the resistance and capacitance of thin films of conjugated oligophenyleneimine (OPI) molecules ranging from 1.5 to 7.5 nm in length. These films were synthesized on Au surfaces utilizing the imine condensation chemistry between terephthalaldehyde and 1,4-benzenediamine. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy yielded molecular tilt angles of 33-43°. To probe DC and AC transport, we employed Au-S-OPI//GaOx/EGaIn junctions having contact areas of 9.6 × 10(2) μm(2) (10(9) nm(2)) and compared to previously reported DC results on the same OPI system obtained using Au-S-OPI//Au conducting probe atomic force microscopy (CP-AFM) junctions with 50 nm(2) areas. We found that intensive observables agreed very well across the two junction platforms. Specifically, the EGaIn-based junctions showed: (i) a crossover from tunneling to hopping transport at molecular lengths near 4 nm; (ii) activated transport for wires >4 nm in length with an activation energy of 0.245 ± 0.008 eV for OPI-7; (iii) exponential dependence of conductance with molecular length with a decay constant β = 2.84 ± 0.18 nm(-1) (DC) and 2.92 ± 0.13 nm(-1) (AC) in the tunneling regime, and an apparent β = 1.01 ± 0.08 nm(-1) (DC) and 0.99 ± 0.11 nm(-1) (AC) in the hopping regime; (iv) previously unreported dielectric constant of 4.3 ± 0.2 along the OPI wires. However, the absolute resistances of Au-S-OPI//GaOx/EGaIn junctions were approximately 100 times higher than the corresponding CP-AFM junctions due to differences in metal-molecule contact resistances between the two platforms.

Anion-induced palladium nanoparticle formation during the on-surface growth of molecular assemblies

We demonstrate a process that results in the formation of palladium nanoparticles during the assembly of molecular thin films. These nanoparticles are embedded in the films and are generated by a chemical reaction of the counter anions of the molecular components with the metal salt that is used for cross-linking these components.

Multichromophoric electrochromic polymers: colour tuning of conjugated polymers through the side chain functionalization approach

Organic electrochromic materials have gained constantly increasing interest over the years with respect to their inorganic counterpart due to essentially two distinctive characteristics: their processability through solution based low cost processes and their wide colour palette. Such characteristic features enabled their application in displays, smart windows, electronic paper and ophthalmic lenses. Alongside the established concept of donor-acceptor polymers, side chain functionalized multichromophoric polymers are gaining attention as a highly performing and synthetically feasible alternative, particularly relevant to applications requiring a complete colourlessness in one of the accessible redox states of the material. The primary aim of the present article is to review all the results involving the tuning of the native electrochromic properties of simple conjugated polymers through the introduction of a discrete electrochromic molecule as a side chain substituent.

Optical and electrochemical properties of covalent assembled bis(4′-carboxylic phenyl terpyridyl) Ru(ii)-monolayer

Monomolecular layers of Ru(ii)-4′carboxylicphenyl-terpyridyl complex are prepared on glass and ITO-coated glass substrates. The optical, electrochemical and stability studies indicate that the present system is highly robust.

Pyridine coordination chemistry for molecular assemblies on surfaces

CONSPECTUS: Since the first description of coordination complexes, many types of metal-ligand interactions have creatively been used in the chemical sciences. The rich coordination chemistry of pyridine-type ligands has contributed significantly to the incorporation of diverse metal ions into functional materials. Here we discuss molecular assemblies (MAs) formed with a variety of pyridine-type compounds and a metal containing cross-linker (e.g., PdCl2(PhCN2)). These MAs are formed using Layer-by-Layer (LbL) deposition from solution that allows for precise fitting of the assembly properties through molecular programming. The position of each component can be controlled by altering the assembly sequence, while the degree of intermolecular interactions can be varied by the level of π-conjugation and the availability of metal coordination sites. By setting the structural parameters (e.g., bond angles, number of coordination sites, geometry) of the ligand, control over MA structure was achieved, resulting in surface-confined metal-organic networks and oligomers. Unlike MAs that are constructed with organic ligands, MAs with polypyridyl complexes of ruthenium, osmium, and cobalt are active participants in their own formation and amplify the growth of the incoming molecular layer. Such a self-propagating behavior for molecular systems is rare, and the mechanism of their formation will be discussed. These exponentially growing MAs are capable of storing metal salts that can be used during the buildup of additional molecular layers. Various parameters influencing the film growth mechanism will be presented, including (i) the number of binding sites and geometry of the organic ligands, (ii) the metal and the structure of the polypyridyl complexes, (iii) the influence of the metal cross-linker (e.g., second or third row transition metals), and (iv) the deposition conditions. By systematic variation of these parameters, switching between linear and exponential growth could be demonstrated for MAs containing structurally well-defined polypyridyl complexes. The porosity of the MAs has been estimated by using electrochemically active probes. Incorporating multiple polypyridyl complexes of osmium and ruthenium into a single assembly give rise to composite materials that exhibit interesting electrochemical and electrochromic properties. These functional composites are especially attractive as they exhibit properties that neither of each metal complex possesses individually. Some of our MAs have very high coloration efficiencies, redox stability, fast responsive times and operate at voltages < 1.5 V. Moreover, their electrochemical properties are dependent on the deposition sequence of the polypyridyl complexes, resulting in MAs that possesses distinctive electron transfer pathways. Finally, some of these MAs are described in terms of their practical applications in electrochromic materials, storage-release chemistry, solar cells, and electron transport properties.

Interfacial mass transfer by controlled multilayer disassembly

We demonstrated the one-pot disassembly of self-propagating molecular assemblies (SPMAs) by ligand exchange and the subsequent covalent binding of the molecular components to other surfaces. These functionalized surfaces are suitable for regenerating the SPMAs.

Nanoscale electrical investigation of layer-by-layer grown molecular wires

Nanoscopic metal-molecule-metal junctions consisting of Fe-bis(terpyridine)-based ordered nanostructures are grown in layer-by-layer fashion on a solid support. Hopping is demonstrated as the main charge-transport mechanism both experimentally and theoretically.

Sequence-dependent assembly to control molecular interface properties

Variation's what you need: variation of the assembly sequence in which layers of two isostructural metal complexes are built up leads to molecular materials with electrochemical properties that depend on the assembly sequence. These properties vary from reversible electron transfer to unidirectional current flows and even charge trapping. The sequence-dependent assembly strategy has implications for various disciplines that involve self-assembly.

Long-range electron transport of ruthenium-centered multilayer films via a stepping-stone mechanism

We studied electron transport of Ru complex multilayer films, whose structure resembles redox-active complex films known in the literature to have long-range electron transport abilities. Hydrogen bond formation in terms of pH control was used to induce spontaneous growth of a Ru complex multilayer. We made a cross-check between electrochemical measurements and I-V measurements using PEDOT:PSS to eliminate the risk of pinhole contributions to the mechanism and have found small β values of 0.012-0.021 Å(-1). Our Ru complex layers exhibit long-range electron transport but with low conductance. On the basis of the results of our theoretical-experimental collaboration, we propose a modified tunneling mechanism named the "stepping-stone mechanism", where the alignment of site potentials forms a narrow band around E(F), making resonant tunneling possible. Our observations may support Tuccito et al.'s proposed mechanism.

Structural analysis of porphyrin multilayer films on ITO assembled using copper(I)-catalyzed azide-alkyne cycloaddition by ATR IR

We report the use of grazing-angle attenuated total reflectance (GATR) IR and polarized UV-vis to determine the molecular structure of porphyrin based molecular multilayer films grown in a layer-by-layer (LbL) fashion using copper-catalyzed azide-alkyne cycloaddition (CuAAC). The molecular orientation and bonding motif present in multilayer films of this type could impact their photophysical and electrochemical properties as well as potential applications. Multilayer films of M(II) 5,10,15,20-tetra(4-ethynylphenyl)porphyrin (1 M = Zn, 2 M = Cu) and azido based linkers 3-5 were used to fabricate the films on ITO substrates. Electrochemically determined coverage of films containing 2 match the trends observed in the absorbance. GATR-IR spectral analysis of the films indicate that CuAAC reactivity is leading to 1,4-triazole linked multilayers with increasing porphyrin and linker IR characteristic peaks. Films grown using all azido-linkers (3-5) display an oscillating trend in azide IR intensity suggesting that the surface bound azido group reacts with 1 and that further layering can occur through additional reaction with linkers, regenerating the azide surface. Films containing linker 5 in particular show an overall increase in azide content suggesting that only two of the three available groups react during multilayer fabrication, causing an overall buildup of azide content in the film. Films of 1 with linker 3 and 5 showed an average porphyrin plane angle of 46.4° with respect to the substrate as determined by GATR FT-IR. Polarized UV-vis absorbance measurements correlate well with the growth angle calculated by IR. The orientation of the porphyrin plane within the multilayer structures suggests that the CuAAC-LbL process results in a film with a trans bonding motif.

Synergism in multicomponent self-propagating molecular assemblies

Multicomponent self-propagating molecular assemblies (SPMAs) have been generated from an organic chromophore, a redox-active polypyridyl complex, and PdCl(2). The structure of the multicomponent SPMA is not a linear combination of two assemblies generated with a single molecular constituent. Surface-confined assemblies formed from only the organic chromophore and PdCl(2) are known to follow linear growth, whereas the combination of polypyridyl complexes and PdCl(2) results in exponential growth. The present study demonstrates that an iterative deposition of both molecular building blocks with PdCl(2) results in an exponentially growing assembly. The nature of the assembly mechanism is dictated by the polypyridyl complex and overrides the linear growth process of the organic component. Relatively smooth, multicomponent SPMAs have been obtained with a thickness of ∼20 nm on silicon, glass, and indium-tin oxide (ITO) coated glass. Detailed information of the structure and of the surface-assembly chemistry were obtained using transmission optical (UV/Vis) spectroscopy, ellipsometry, atomic force microscopy (AFM), synchrotron X-ray reflectivity (XRR), and electrochemistry.