Possibilities and Limitations of Kinetic Studies in On-Surface Synthesis by Real Time X-ray Photoelectron Spectroscopy.

The kinetics of coupling reactions on surfaces can be quantitatively studied in real time by X-ray Photoelectron Spectroscopy (XPS). From fitting experimental data, kinetic reaction parameters such as the rate constant's pre-exponential and activation energy can be deduced and compared to quantum chemical simulations. To elucidate the possibilities and limitations of this approach, we propose studies in which experimental data are first simulated and subsequently fitted. Knowing the exact kinetic parameters used in the simulation allows one to evaluate the accuracy of the fit result. Here, several experimental influences, such as the data point density and the addition of noise, are explored for a model reaction with first-order kinetics. The proposed procedure sheds light on the accuracy with which kinetic parameters can be derived and may also help in the design of future experiments.

Steering Self-Assembly of Three-Dimensional Iptycenes on Au(111) by Tuning Molecule-Surface Interactions.

Self-assembly of three-dimensional molecules is scarcely studied on surfaces. Their modes of adsorption can exhibit far greater variability compared to (nearly) planar molecules that adsorb mostly flat on surfaces. This additional degree of freedom can have decisive consequences for the expression of intermolecular binding motifs, hence the formation of supramolecular structures. The determining molecule-surface interactions can be widely tuned, thereby providing a new powerful lever for crystal engineering in two dimensions. Here, we study the self-assembly of triptycene derivatives with anthracene blades on Au(111) by Scanning Tunneling Microscopy, Near Edge X-ray Absorption Fine Structure and Density Functional Theory. The impact of molecule-surface interactions was experimentally tested by comparing pristine with iodine-passivated Au(111) surfaces. Thereby, we observed a fundamental change of the adsorption mode that triggered self-assembly of an entirely different structure.

Carbon-Carbon Coupling on Inert Surfaces by Deposition of En†Route Generated Aryl Radicals.

To facilitate C-C coupling in on-surface synthesis on inert surfaces, we devised a radical deposition source (RDS) for the direct deposition of aryl radicals onto arbitrary substrates. Its core piece is a heated reactive drift tube through which halogenated precursors are deposited and en route converted into radicals. For the proof of concept we study 4,4''-diiodo-p-terphenyl (DITP) precursors on iodine-passivated metal surfaces. Deposition with the RDS at room temperature results in highly regular structures comprised of mostly monomeric (terphenyl) or dimeric (sexiphenyl) biradicals. Mild heating activates progressive C-C coupling into more extended molecular wires. These structures are distinctly different from the self-assemblies observed upon conventional deposition of intact DITP. Direct deposition of radicals renders substrate reactivity unnecessary, thereby paving the road for synthesis on application-relevant inert surfaces.

The Role of Kinetics versus Thermodynamics in Surface-Assisted Ullmann Coupling on Gold and Silver Surfaces.

Surface-assisted Ullmann coupling is the workhorse of on-surface synthesis. Despite its obvious relevance, many fundamental and mechanistic aspects remain elusive. To shed light on individual reaction steps and their progression with temperature, temperature-programmed X-ray photoelectron spectroscopy (TP-XPS) experiments are performed for a prototypical model system. The activation of the coupling by initial dehalogenation is tracked by monitoring Br 3d core levels, whereas the C 1s signature is used to follow the emergence of metastable organometallic intermediates and their conversion to the final covalent products upon heating in real time. The employed 1,3,5-tris(4-bromophenyl)benzene precursor is comparatively studied on Ag(111) versus Au(111), whereby intermolecular bonds and network topologies are additionally characterized by scanning tunneling microscopy (STM). Besides the well-comprehended differences in activation temperatures for debromination, the thermal progression shows marked differences between the two surfaces. Debromination proceeds rapidly on Ag(111), but is relatively gradual on Au(111). While on Ag(111) debromination is well explained by first-order reaction kinetics, thermodynamics prevail on Au(111), underpinned by a close agreement between experimentally deduced and density functional theory (DFT) calculated reaction enthalpies. Thermodynamically controlled debromination on Au(111) over a large temperature range implies an unexpectedly long lifetime of surface-stabilized radicals prior to covalent coupling, as corroborated by TP-XPS of C 1s core levels. These insights are anticipated to play an important role regarding our ability to rationally synthesize atomically precise low-dimensional covalent nanostructures on surfaces.

Competitive Metal Coordination of Hexaaminotriphenylene on Cu(111) by Intrinsic Copper Versus Extrinsic Nickel Adatoms.

The interplay between the self-assembly and surface chemistry of 2,3,6,7,10,11-hexaaminotriphenylene (HATP) on Cu(111) was complementarily studied by high-resolution scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) under ultra-high vacuum conditions. To shed light on the competitive metal coordination, comparative experiments were carried out on pristine and nickel-covered Cu(111). Directly after room-temperature deposition of HATP onto pristine Cu(111), self-assembled aggregates were observed by STM, and XPS results indicated still protonated amino groups. Annealing up to 200 °C activated the progressive single deprotonation of all amino groups as indicated by chemical shifts of both the N 1s and C 1s core levels in the XP spectra. This enabled the formation of topologically diverse π-d conjugated coordination networks with intrinsic copper adatoms. The basic motif of these networks was a metal-organic trimer, in which three HATP molecules were coordinated by Cu3 clusters, as corroborated by the accompanying density functional theory (DFT) simulations. Additional deposition of more reactive nickel atoms resulted in both chemical and structural changes with deprotonation and formation of bis(diimino)-Ni bonded networks already at room temperature. Even though fused hexagonal metal-coordinated pores were observed, extended honeycomb networks remained elusive, as tentatively explained by the restricted reversibility of these metal-organic bonds.

What can be inferred from moiré patterns? A case study of trimesic acid monolayers on graphite.

Self-assembly of benzene-1,3,5-tricarboxylic acid (trimesic acid - TMA) monolayers at the alkanoic acid-graphite interface is revisited. Even though this archetypal model system for hydrogen bonded porous networks is particularly well studied, the analysis of routinely observed superperiodic contrast modulations known as moiré patterns lags significantly behind. Fundamental questions remain unanswered such as, are moiré periodicity and orientation always the same, i.e. is exclusively only one specific moiré pattern observed? What are the geometric relationships (superstructure matrices) between moiré, TMA, and graphite lattices? What affects the moiré pattern formation? Is there any influence from solvent, concentration, or thermal treatment? These basic questions are addressed via scanning tunneling microscopy experiments at the liquid-solid interface, revealing a variety of different moiré patterns. Interestingly, TMA and graphite lattices were always found to be ∼5° rotated with respect to each other. Consequently, the observed variation in the moiré patterns is attributed to minute deviations (<2°) from this preferred orientation. Quantitative analysis of moiré periods and orientations facilitates the determination of the TMA lattice parameter with picometer precision.

Post-Synthetic Decoupling of On-Surface-Synthesized Covalent Nanostructures from Ag(111).

The on-surface synthesis of covalent organic nanosheets driven by reactive metal surfaces leads to strongly adsorbed organic nanostructures, which conceals their intrinsic properties. Hence, reducing the electronic coupling between the organic networks and commonly used metal surfaces is an important step towards characterization of the true material. We demonstrate that post-synthetic exposure to iodine vapor leads to the intercalation of an iodine monolayer between covalent polyphenylene networks and Ag(111) surfaces. The experimentally observed changes from surface-bound to detached nanosheets are reproduced by DFT simulations. These findings suggest that the intercalation of iodine provides a material that shows geometric and electronic properties substantially closer to those of the freestanding network.

Large area synthesis of a nanoporous two-dimensional polymer at the air/water interface.

We present the synthesis of a two-dimensional polymer at the air/water interface and its nm-resolution imaging. Trigonal star, amphiphilic monomers bearing three anthraceno groups on a central triptycene core are confined at the air/water interface. Compression followed by photopolymerization on the interface provides the two-dimensional polymer. Analysis by scanning tunneling microscopy suggests that the polymer is periodic with ultrahigh pore density.

Explosives sensing by using electron-rich supramolecular polymers: role of intermolecular hydrogen bonding in significant enhancement of sensitivity.

We demonstrate here that supramolecular interactions enhance the sensitivity towards detection of electron-deficient nitro-aromatic compounds (NACs) over discrete analogues. NACs are the most commonly used explosive ingredients and are common constituents of many unexploded landmines used during World War II. In this study, we have synthesised a series of pyrene-based polycarboxylic acids along with their corresponding discrete esters. Due to the electron richness and the fluorescent behaviour of the pyrene moiety, all the compounds act as sensors for electron-deficient NACs through a fluorescence quenching mechanism. A Stern-Volmer quenching constant determination revealed that the carboxylic acids are more sensitive than the corresponding esters towards NACs in solution. The high sensitivity of the acids was attributed to supramolecular polymer formation through hydrogen bonding in the case of the acids, and the enhancement mechanism is based on an exciton energy migration upon excitation along the hydrogen-bond backbone. The presence of intermolecular hydrogen bonding in the acids in solution was established by solvent-dependent fluorescence studies and dynamic light scattering (DLS) experiments. In addition, the importance of intermolecular hydrogen bonds in solid-state sensing was further explored by scanning tunnelling microscopy (STM) experiments at the liquid-solid interface, in which structures of self-assembled monolayer of the acids and the corresponding esters were compared. The sensitivity tests revealed that these supramolecular sensors can even detect picric acid and trinitrotoluene in solution at levels as low as parts per trillion (ppt), which is much below the recommended permissible level of these constituents in drinking water.

Iodine passivation facilitates on-surface synthesis of robust regular conjugated two-dimensional organogold networks on Au(111).

Two-dimensional conjugated organogold networks with anthra-tetrathiophene repeat units are synthesized by thermally activated debrominative coupling of 2,5,9,12-tetrabromoanthra[1,2-b:4,3-b':5,6-b'':8,7-b''']tetrathiophene (TBATT) precursor molecules on Au(111) surfaces under ultra-high vacuum (UHV) conditions. Performing the reaction on iodine-passivated Au(111) surfaces promotes formation of highly regular structures, as revealed by scanning tunneling microscopy (STM). In contrast, coupling on bare Au(111) surfaces results in less regular networks due to the simultaneous expression of competing intermolecular binding motifs in the absence of error correction. The carbon-Au-carbon bonds confer remarkable robustness to the organogold networks, as evidenced by their high thermal stability. In addition, as suggested by density functional theory (DFT) calculations and underscored by scanning tunneling spectroscopy (STS), the organogold networks exhibit a small electronic band gap in the order of 1.0 eV due to their high π-conjugation.

Mechanistic insights into on-surface reactions from isothermal temperature-programmed X-ray photoelectron spectroscopy.

On-surface synthesis often proceeds under kinetic control due to the irreversibility of key reaction steps, rendering kinetic studies pivotal. The accurate quantification of reaction rates also bears potential for unveiling reaction mechanisms. Temperature-Programmed X-ray Photoelectron Spectroscopy (TP-XPS) has emerged as an analytical tool for kinetic studies with splendid chemical and sufficient temporal resolution. Here, we demonstrate that the common linear temperature ramps lead to fitting ambiguities. Moreover, pinpointing the reaction order remains intricate, although this key parameter entails information on atomistic mechanisms. Yet, TP-XPS experiments with a stepped temperature profile comprised of isothermal segments facilitate the direct quantification of rate constants from fitting time courses. Thereby, rate constants are obtained for a series of temperatures, which allows independent extraction of both activation energies and pre-exponentials from Arrhenius plots. By using two analogous doubly versus triply brominated aromatic model compounds, we found that their debromination on Ag(111) is best modeled by second-order kinetics and thus proceeds via the involvement of a second, non-obvious reactant. Accordingly, we propose that debromination is activated by surface supplied Ag adatoms. This hypothesis is supported by Density Functional Theory (DFT) calculations. We foresee auspicious prospects for this TP-XPS variant for further exploring the kinetics and mechanisms of on-surface reactions.

Born-Haber cycle for monolayer self-assembly at the liquid-solid interface: assessing the enthalpic driving force.

The driving force for self-assembly is the associated gain in free energy with decisive contributions from both enthalpy and entropy differences between final and initial state. For monolayer self-assembly at the liquid-solid interface, solute molecules are initially dissolved in the liquid phase and then become incorporated into an adsorbed monolayer. In this work, we present an adapted Born-Haber cycle for obtaining precise enthalpy values for self-assembly at the liquid-solid interface, a key ingredient for a profound thermodynamic understanding of this process. By choosing terephthalic acid as a model system, it is demonstrated that all required enthalpy differences between well-defined reference states can be independently and consistently assessed by both experimental and theoretical methods, giving in the end a reliable value of the overall enthalpy gain for self-assembly of interfacial monolayers. A quantitative comparison of enthalpy gain and entropy cost reveals essential contributions from solvation and dewetting, which lower the entropic cost and render monolayer self-assembly a thermodynamically favored process.

Control of intermolecular bonds by deposition rates at room temperature: hydrogen bonds versus metal coordination in trinitrile monolayers.

Self-assembled monolayers of 1,3,5-tris(4'-biphenyl-4"-carbonitrile)benzene, a large functional trinitrile molecule, on the (111) surfaces of copper and silver under ultrahigh vacuum conditions were studied by scanning tunneling microscopy and low-energy electron diffraction. A densely packed hydrogen-bonded polymorph was equally observed on both surfaces. Additionally, deposition onto Cu(111) yielded a well-ordered metal-coordinated porous polymorph that coexisted with the hydrogen-bonded structure. The required coordination centers were supplied by the adatom gas of the Cu(111) surface. On Ag(111), however, the well-ordered metal-coordinated network was not observed. Differences between the adatom reactivities on copper and silver and the resulting bond strengths of the respective coordination bonds are held responsible for this substrate dependence. By utilizing ultralow deposition rates, we demonstrate that on Cu(111) the adatom kinetics plays a decisive role in the expression of intermolecular bonds and hence structure selection.

Adsorption structure determination of a large polyaromatic trithiolate on Cu(111): combination of LEED-I(V) and DFT-vdW.

The adsorption geometry of 1,3,5-tris(4-mercaptophenyl)benzene (TMB) on Cu(111) is determined with high precision using two independent methods, experimentally by quantitative low energy electron diffraction (LEED-I(V)) and theoretically by dispersion corrected density functional theory (DFT-vdW). Structural refinement using both methods consistently results in similar adsorption sites and geometries. Thereby a level of confidence is reached that allows deduction of subtle structural details such as molecular deformations or relaxations of copper substrate atoms.

Kinetic versus thermodynamic polymorph stabilization of a tri-carboxylic acid derivative at the solid-liquid interface.

The carboxylic acid moiety gives rise to structural variability in surface-supported self-assembly due to the common expression of various H-bonding motifs. Self-assembly of 3-fold symmetric tricarboxylic acid derivatives on surfaces typically results in monolayer structures that feature the common 2-fold cyclic R22(8) H-bond motif for at least one of the carboxylic acid groups. Polymorphs that are exclusively based on 3-fold cyclic R33(12) H-bonds were predicted but remained elusive. Here, we show the emergence of such a superflower (SF) structure purely based on R33(12) H-bonds for L-benzene-1,3,5-tricarbonyl phenylalanine (L-BTA), a molecule derived from the well-studied trimesic acid (TMA). In contrast to TMA, L-BTA is not completely planar and is also equipped with additional functional groups for the formation of secondary intermolecular bonds. At the heptanoic acid-graphite interface we transiently observe a SF structure, which is dynamically converted into a chicken-wire structure that only exhibits R22(8) H-bonds. Interestingly, when using nonanoic acid as a solvent the initially formed SF structure remained stable. This unexpected behaviour is rationalized by accompanying force field simulations and experimental determination of solvent-dependent L-BTA solubility.