Mechanistic Implications of the Assembly of Organic Thiocyanates on Precious Metals

Thiocyanate assembly is shown to be an effective method for assembling thiolate structures on platinum, silver, and gold. The assemblies were studied by infrared reflection spectroscopy and X-ray photoelectron spectroscopy (XPS). Two cyanide species were identified on the surfaces:  the first corresponding to adsorbed cyanide and the second to a form commonly seen as an intermediate during cyanide etching of metals. The presence of the second species supports the theory that cyanide is leaving the surface as M(CN)x, resulting in a thiolate monolayer. Comparison of thiocyanate assemblies on evaporated gold and silver to those on template-stripped gold demonstrates the integral role of surface morphology in the expulsion of (CN)ads from the surface of the metals

Substituent Parameters Impacting Isomer Composition and Optical Properties of Dihydroindolizine Molecular Switches

In an attempt to understand which factors influence constitutional isomer control of 6′- and 8′-substituted dihydroindolizines (DHIs), a series of asymmetric pyridines was condensed with dimethyl spiro­[cycloprop[2]­ene-1,9′-fluorene]-2,3-dicarboxylate. The substituents on the pyridial derivatives ranged from donating to withdrawing and demonstrated control over the isomeric ratios for all DHIs. Substituent control proved to be selective for the highly donating amino, which exclusively formed the 8′ isomer. The same ratios were reproduced via photolytic experiments, which suggested that the condensation reaction is dominated by the product’s thermodynamic stability. The electronic influences of the substituents extends beyond isomer control, as it impacts the DHIs’ optical properties and electrocyclization (switching) rates to the spiro conformers. Our results allow us to predict the syntheses and properties of future 6′- or 8′-substituted DHIs, molecules that will be applied in understanding the role of the dipole vector orientation to work function switching

Spectroscopic Evidence of Work Function Alterations Due to Photoswitchable Monolayers on Gold Surfaces

Taking advantage of surfaces’ response to interfacial dipoles, a class of photochromophores (dihydroindolizine) is demonstrated to alter the work function of the underlying substrate (∼170 meV). This same molecule also provides spectroscopic signatures for correlating the change in molecular structure to the induced change in the surfaces’ electronic properties. Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) allows analysis of the characteristic dihydroindolizine CC (1559 cm^–1) and pyridinium (1643 cm^–1) stretch as a function of photoexcitation. Structural assignments of this photochromophore are corroborated to density function theory calculations. Conformational changes in the monolayers appear in parallel with work function changes and are consistent with both its rate and magnitude

Spontaneous Assembly of Organic Thiocyanates on Gold Sufaces. Alternative Precursors for Gold Thiolate Assemblies

Thiolate self-assembly on gold has proven to be a valuable technique for assembling monolayers on a wide variety of substrates. However, the oxidative instability of the thiols, especially aromatic thiols and α,ω-dithiols, presents several difficulties. Shown here is that thiocyanates, easily synthesized stable thiol derivatives, can be directly assembled on gold surfaces with no auxiliary reagents required. Assembly is complete in 24 h and leaves a similar gold thiolate structure as seen in typical thiol self-assembled monolayers

Effects of Hindered Internal Rotation on Packing and Conductance of Self-Assembled Monolayers

Self-assembled monolayers of 4-thiobiphenyl and 2-thiophenanthrene molecules on Au{111} were probed using scanning tunneling microscopy (STM) to investigate their assembled order, to measure the conductance through them, and to determine what molecular features are necessary for molecules to exhibit conductance switching. The 4-thiobiphenyl molecules assemble into two distinct packing structures, whereas no order is observed for 2-thiophenanthrene. Both molecules show rectifying behavior, but although 4-thiobiphenyl is more conductive at positive sample biases, 2-thiophenanthrene is more conductive at negative sample bias. Individual molecules of each type were also inserted into alkanethiol SAMs and tracked over several hours to investigate the conductance states of each molecule. Both molecules exhibit two conductance states, indicating that internal ring rotation is not required for conductance switching

Influence of Defects on the Reactivity of Organic Surfaces

Molecular orientation within organic solids limits the range of applicable surface reactions, with reactive functionalities often recessed and inaccessible to adsorbates. To induce reactivity in heretofore inert orientations of acenes, a defect-mediated mechanism is utilized to functionalize thin-film phase pentacene. This mechanism was demonstrated via correlation of reaction data to defect density, determined via polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and atomic force microscopy (AFM). By controllably varying the amount of grain boundaries in the acene films, the reaction can be varied from near zero to coverage exceeding a monolayer. The extensive coverage suggests that the reaction propagates from the defects throughout the grains, a prediction borne out via direct observation of reaction progression along the surface from a single dislocation (via scanning electron microscopy). The results support a mechanism whereby the reaction is initialized at the defect sites, especially boundaries of crystal domains, which opens the unfavorable molecular orientation of the (001) pentacene to incoming adsorbates. This exact film configuration and its successful reaction is especially relevant to organic thin-film transistor (OTFT) devices

Electrochemical Origin of Voltage-Controlled Molecular Conductance Switching

We have studied electron transport in bipyridyl-dinitro oligophenylene-ethynelene dithiol (BPDN) molecules both in an inert environment and in aqueous electrolyte under potential control, using scanning tunneling microscopy. Current−voltage (IV) data obtained in an inert environment were similar to previously reported results showing conductance switching near 1.6 V. Similar measurements taken in electrolyte under potential control showed a linear dependence of the bias for switching on the electrochemical potential. Extrapolation of the potentials to zero switching bias coincided with the potentials of redox processes on these molecules. Thus switching is caused by a change in the oxidation state of the molecules

Actuation of Self-Assembled Two-Component Rodlike Nanostructures

A model for predicting the effects of stimuli-induced contraction of the polypyrrole ends of two-segment gold-polypyrrole nanorods on their assembly into curved superstructures is presented. The model and experimental data presented here show that small changes (ca. 3%) in the diameter of the polypyrrole segment of each rod will induce dramatic changes (up to 20%) in the radii of the resulting superstructures, providing a convenient means for actuating their opening and closing. We show experimentally that this actuation can be affected via humidity, temperature, and light

Monolayer-Induced Changes in Metal Penetration and Wetting for Metal-on-Organic Interfaces

Metal deposition onto organic materials results in a myriad of issues at the metal-on-organic interface, necessitating a highly adaptable interlayer. A single cysteamine-based monolayer on evaporated tetracene thin films is demonstrated to be highly multifunctional, inhibiting metal penetration and increasing contact wetting for the silver-on-organic interface. Cross sections of the monolayer-coated sample, imaged via transmission electron microscopy, show that silver penetration decreased by up to 40% compared to untreated tetracene. Substantial morphology differences are observed between treated and untreated samples; metal poorly wets untreated samples, forming nanoparticle clusters, while monolayer-coated samples are uniform. Scanning electron microscopy indicates that the monolayers prevent contact discontinuities (hundreds of nanometers in size) that were observed on untreated samples with thin (20 nm) silver contacts. Monolayers do not add any significant barrier to charge transport compared to untreated samples, with conductivity values measured to be near identical. When these are combined with earlier reports of cysteamine improving contact adhesion and durability, monolayers can effectively address most of the issues plaguing metal-on-organic devices

Neutral Complexes of First Row Transition Metals Bearing Unbound Thiocyanates and Their Assembly on Metallic Surfaces

A series of transition metal coordination complexes designed to assemble on gold surfaces was synthesized, their electronic structure and transitions analyzed, and their magnetic properties studied. By taking advantage of recently developed thiocyanate assembly protocols, these molecules were then assembled onto a gold surface, without the need for an inert atmosphere, to give a loosely packed monolayer. The assembled molecules exhibit properties similar to that of the bulk molecules, indicating little change in molecular structure outside of chemisorption