Correlating the orbital overlap area and vibrational frequency shift of an isocyanide moiety adsorbed on Pt and Pd covered Au(111) surfaces

Orbital interactions between adsorbed molecules and the underlying metal surfaces play critical roles in a wide range of surface and interfacial processes. Establishing a correlation between an experimental observable (e.g., vibrational frequency shift of the adsorbed molecule) and the orbital interactions is of vital importance. Herein, theoretical calculations are used to investigate the vibrational frequency shift of phenyl isocyanide molecules as a probe molecule adsorbed on mono- and bi-layer Pt and Pd covered Au(111) surfaces and Pd₂Au₄ and Pt₂Au₄ clusters. By analyzing the density of states (DOS) of the adsorption system, we show that the orbital overlap area of d electronic DOS with a molecular σ or π* orbital, particularly their ratio (R_d-σ/d-π*), can be a meaningful descriptor to explain the frequency shift of the CN moiety. This hypothesis has been verified by simulations for phenyl isocyanide with electron donating NH₂- and withdrawing CF₃- substituent groups, formonitrile and carbon monoxide. Quasi-linear dependence of the frequency shift on R_d-σ/d-π* is observed for both the red and blue shift regions. Our findings build up on previous notions of electronic interactions, which will provide a more quantitative and solid footing to understand and analyze the frequency shift of adsorbed molecules on metal surfaces and the related electronic interactions and catalytic properties.

Sub-Nanometer Mapping of the Interfacial Electric Field Profile Using a Vibrational Stark Shift Ruler

The characterization of electrical double layers is important since the interfacial electric field and electrolyte environment directly affect the reaction mechanisms and catalytic rates of electrochemical processes. In this work, we introduce a spectroscopic method based on a Stark shift ruler that enables mapping the electric field strength across the electric double layer of electrode/electrolyte interfaces. We use the tungsten-pentacarbonyl(1,4-phenelenediisocyanide) complex attached to the gold surface as a molecular ruler. The carbonyl (CO) and isocyanide (NC) groups of the self-assembled monolayer (SAM) provide multiple vibrational reporters situated at different distances from the electrode. Measurements of Stark shifts under operando electrochemical conditions and direct comparisons to density functional theory (DFT) simulations reveal distance-dependent electric field strength from the electrode surface. This electric field profile can be described by the Gouy-Chapman-Stern model with Stern layer thickness of ∼4.5 Å, indicating substantial solvent and electrolyte penetration within the SAM. Significant electro-induction effect is observed on the W center that is ∼1.2 nm away from the surface despite rapid decay of the electric field (∼90%) within 1 nm. The applied methodology and reported findings should be particularly valuable for the characterization of a wide range of microenvironments surrounding molecular electrocatalysts at electrode interfaces and the positioning of electrocatalysts at specific distances from the electrode surface for optimal functionality.

Probing Molecular Mechanisms during the Oscillatory Adsorption of Propyl Chain Functionalized Organosilane Films with Sum Frequency Generation Spectroscopy

The selectivity rules of sum frequency generation spectroscopy were exploited to determine propyl chain order during the time-dependent oscillatory adsorption of propyltrimethoxysilane (PTMS) and Langmuir-type growth of propyldimethylmethoxysilane (PDMMS). During the early stages of film growth, molecular packing density determines the extent of propyl chain defects within both films with high surface coverage resulting in a film with fewer defects. Following this, an ordered monolayer-like film stabilizes on the Al₂O₃ substrate for both silanes. Although this result is intuitive for the Langmuir-type growth of PDMMS, the stabilization of molecular ordering despite the continuing oscillation in PTMS surface coverage indicates the presence of a stable monolayer, while it is the oligomerized PTMS dendrimers which continue to desorb and readsorb to the substrate. We also reveal for the first time, the formation of a physisorbed bilayer during the self-assembly process of PTMS. The presence of this ordered, physisorbed bilayer on top of the covalently bound PTMS film plays a key role in the process of the molecular self-assembly mechanism and is proposed to enable further condensation of the covalently bound film.

A quantum chemical study of substituent effects on CN bonds in aryl isocyanide molecules adsorbed on the Pt surface

A periodicity implemented scheme of natural bond orbital (NBO) theory and normal mode analysis has been employed to investigate the tendency of the chemical bond strength of aryl isocyanide molecules with different para-substituted groups adsorbed on the Pt(111) surface. The NC bond order shows a clear correspondence with the NC stretching frequency; both of them exhibit a "volcano-like" profile as a function of the Hammett constant of the para-substituted groups for isolated molecules. When a molecule is adsorbed on the Pt(111) surface, the NC stretching frequency variations are determined by the resultant effect of σ donation and π back-donation between the molecule and the surface. The present comprehensive and systematic computations clarify the electron donating and withdrawing effects of the substituted groups on the interaction between the aryl isocyanide molecule and the transition metal substrate.

Chemical self-assembly strategies for designing molecular electronic circuits

Design principles are demonstrated for fabricating molecular electronic circuits using the inherently self-limiting growth of molecular wires between gold nanoparticles from the oligomerization of 1,4-phenylene diisocyanide.

Electrical Matching at Metal/Molecule Contacts for Efficient Heterogeneous Charge Transfer

In a metal/molecule hybrid system, unavoidable electrical mismatch exists between metal continuum states and frontier molecular orbitals. This causes energy loss in the electron conduction across the metal/molecule interface. For efficient use of energy in a metal/molecule hybrid system, it is necessary to control interfacial electronic structures. Here we demonstrate that electrical matching between a gold substrate and π-conjugated molecular wires can be obtained by using monatomic foreign metal interlayers, which can change the degree of d-π* back-donation at metal/anchor contacts. This interfacial control leads to energy level alignment between the Fermi level of the metal electrode and conduction molecular orbitals, resulting in resonant electron conduction in the metal/molecule hybrid system. When this method is applied to molecule-modified electrocatalysts, the heterogeneous electrochemical reaction rate is considerably improved with significant suppression of energy loss at the internal electron conduction.

Nanostructuring of Au(111) during the Adsorption of an Aromatic Isocyanide from Solution

We present a combined vibrational and morphological characterization of the self-assembly of 1,4-phenylene-diisocyanide (PDI) on Au(111) from methanol solution. Vibrational sum frequency generation (vSFG) and scanning tunneling microscopy (STM) have been applied to determine the adsorption geometry of the PDI-Au adatom complexes as well as the morphological transformations of the Au(111) substrate upon SAM formation from solutions with PDI concentrations in the μM to mM range. At low concentration/coverage, PDI adsorbs in flat adsorption geometry, with both isocyanide groups attached to Au adatoms on the Au(111) surface. Transformation to a standing-up phase is observed with increasing concentration/coverage. In contrast to findings for PDI adsorbed in ultrahigh vacuum, PDI does not form a long-range-ordered monolayer phase when adsorbed from solution. In addition, the Au(111) surface is subjected to structural modifications. Au vacancy islands and ad-islands, which are typical substrate defects formed during the self-assembly of aromatic thiols on Au(111), are also created during PDI adsorption from solution. At low PDI concentration, the Au vacancy islands and ad-islands are found at specific sites mediated by the herringbone reconstruction of the Au(111) surface, giving rise to long-range-ordered structures. These structures do not form during UHV adsorption of PDI on Au(111) nor has a similar ordering effect been observed for any related thiol-SAM system investigated so far.

Molecular orientation of organic thin films on dielectric solid substrates: a phase-sensitive vibrational SFG study

Phase-sensitive SFG spectroscopy is employed to determine the absolute molecular orientation on the solid/air and solid/liquid interfaces.

Determination of Adsorbate Structures from 1,4-Phenylene Diisocyanide on Gold

The structure of the 1-D oligomer chains that form on a Au(111) surface following adsorption of 1,4-phenylene diisocyanide (PDI) is explored using reflection-absorption infrared spectroscopy and scanning tunneling microscopy (STM). The experimental work is complemented by first-principles density functional theory calculations, which indicate that the previously proposed gold-PDI oligomer chains in which the PDI molecule bridged gold adatoms are thermodynamically stable. In addition, the calculated vibrational modes for this structure are in excellent agreement with the experimental infrared data. The linkage of the PDI units by gold adatoms is confirmed by comparing STM images collected as a function of tip bias with images for the calculated structure by the Bardeen method.

High-density monolayers of metal complexes: preparation and catalysis

Catalysts are one of the key materials for realizing a sustainable society. However, we may encounter problematic cases where conventional catalyst systems cannot provide effective solutions. We thus believe that the establishment of novel methods of catalyst preparation is currently necessary. Utilization of high-density monolayers of molecular metal complexes is our strategy, and we expect that this methodology will enable facile and systematic screening of unique and efficient catalysts. This Personal Account describes our challenges to establish such an immature method in catalyst preparation as well as the related background and perspective. Preparation and catalysis by high-density monolayers of Rh complexes with N-heterocyclic carbene, structurally compact phosphine and diisocyanide ligands on gold surfaces are presented. The catalytic application of a high-density Pd-bisoxazoline complex prepared on a single-crystal silicon surface is also shown. Uniquely high catalyst turnover numbers and high chemoselectivities were observed with these catalyst systems.

Characterizing the photoinduced switching process of a nitrospiropyran self-assembled monolayer using in situ sum frequency generation spectroscopy

Sum frequency generation (SFG) vibrational spectroscopy is employed to investigate the reversible, photoinduced spiro→merocyanine isomerization of a self-assembled monolayer, the result of attachment of nitrospiropyran to a gold surface using a dithiolane anchoring group. The attachment of these molecular "alligator clips" to spiropyran molecules provide an easily accessible method to self-assemble a robust monolayer of spiropyran on a gold surface, which allows photoswitching of the spiropyran units. Probing the symmetric and antisymmetric stretching modes of the nitro group allows the determination of the structural orientation of the charged moiety with respect to the surface normal as well as the isomerization rates under photoinduced switching conditions. The photoisomerization of the spiropyran SAM on the gold surface is much faster than the rates of switching spiropyrans in a solid crystalline form, and the rate of thermal relaxation of the opened to closed form in this study is found to be on the same time scale as the relaxation of spiropyran when present in solutions with polar solvents.