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.

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.

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.

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.

Evolution of adsorption heights in the on-surface synthesis and decoupling of covalent organic networks on Ag(111) by normal-incidence X-ray standing wave.

Structural characterization in on-surface synthesis is primarily carried out by Scanning Probe Microscopy (SPM) which provides high lateral resolution. Yet, important fresh perspectives on surface interactions and molecular conformations are gained from adsorption heights that remain largely inaccessible to SPM, but can be precisely measured with both elemental and chemical sensitivity by Normal-Incidence X-ray Standing Wave (NIXSW) analysis. Here, we study the evolution of adsorption heights in the on-surface synthesis and post-synthetic decoupling of porous covalent triazine-phenylene networks obtained from 2,4,6-tris(4-bromophenyl)-1,3,5-triazine (TBPT) precursors on Ag(111). Room temperature deposition of TBPT and mild annealing to ∼150 °C result in full debromination and formation of organometallic intermediates, where the monomers are linked into reticulated networks by C-Ag-C bonds. Topologically identical covalent networks comprised of triazine vertices that are interconnected by biphenyl units are obtained by a thermally activated chemical transformation of the organometallic intermediates. Exposure to iodine vapor facilitates decoupling by intercalation of an iodine monolayer between the covalent networks and the Ag(111) surface. Accordingly, Scanning Tunneling Microscopy (STM), X-ray Photoelectron Spectroscopy (XPS) and NIXSW experiments are carried out for three successive sample stages: organometallic intermediates, covalent networks directly on Ag(111) and after decoupling. NIXSW analysis facilitates the determination of adsorption heights of chemically distinct carbon species, i.e. in the phenyl and triazine rings, and also for the organometallic carbon atoms. Thereby, molecular conformations are assessed for each sample stage. The interpretation of experimental results is informed by Density Functional Theory (DFT) calculations, providing a consistent picture of adsorption heights and molecular deformations in the networks that result from the interplay between steric hindrance and surface interactions. Quantitative adsorption heights, i.e. vertical distances between adsorbates and surface, provide detailed insight into surface interactions, but are underexplored in on-surface synthesis. In particular, the direct comparison with an in situ prepared decoupled state unveils the surface influence on the network structure, and shows that iodine intercalation is a powerful decoupling strategy.

Synthesis on inert surfaces.

Extraordinarily robust extended covalent organic nanostructures with unprecedented structures and intriguing chemical and electronic properties are currently synthesized on metal surfaces. Envisaged electronic applications, for instance in field effect transistors or sensors, however, demand insulating supports. To obviate the need for a cumbersome post-synthetic transfer from the metal growth surface to the target substrate, synthesis directly on inert surfaces is highly desirable. Albeit reversible polycondensations are broadly established on inert graphite surfaces, carbon-carbon (C-C) coupling remains mostly elusive. Thermally activated coupling on weakly interacting supports suffers from the "desorption problem", that is the premature desorption of reactants due to increased reaction barriers, which becomes even worse on inert surfaces due to diminished desorption barriers. Consequently, C-C coupling on inert surfaces requires new paradigms. We propose either photochemical coupling or activation of monomers prior to deposition as possible alternatives, discuss the current state-of-the-art and identify future challenges.

On-surface photopolymerization of two-dimensional polymers ordered on the mesoscale.

The use of solid supports and ultra-high vacuum conditions for the synthesis of two-dimensional polymers is attractive, as it can enable thorough characterization, often with submolecular resolution, and prevent contamination. However, most on-surface polymerizations are thermally activated, which often leads to high defect densities and relatively small domain sizes. Here, we have obtained a porous two-dimensional polymer that is ordered on the mesoscale by the two-staged topochemical photopolymerization of fluorinated anthracene triptycene (fantrip) monomers on alkane-passivated graphite surfaces under ultra-high vacuum. First, the fantrip monomers self-assemble into highly ordered monolayer structures, where all anthracene moieties adopt a suitable arrangement for photopolymerization. Irradiation with violet light then induces complete covalent crosslinking by [4+4] photocycloaddition to form a two-dimensional polymer, while fully preserving the long-range order of the self-assembled structure. The extent of the polymerization is confirmed by local infrared spectroscopy and scanning tunnelling microscopy characterization, in agreement with density functional theory calculations, which also gives mechanistic insights.

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.

Quantifying the Ultraslow Desorption Kinetics of 2,6-Naphthalenedicarboxylic Acid Monolayers at Liquid-Solid Interfaces.

Kinetic effects in monolayer self-assembly at liquid-solid interfaces are not well explored but can provide unique insights. We use variable-temperature scanning tunneling microscopy (STM) to quantify the desorption kinetics of 2,6-naphthalenedicarboxylic acid (NDA) monolayers at nonanoic acid-graphite interfaces. Quantitative tracking of the decline of molecular coverages by STM between 57.5 and 65.0 °C unveiled single-exponential decays over the course of days. An Arrhenius plot of rate constants derived from fits results in a surprisingly high energy barrier of 208 kJ mol-1 that strongly contrasts with the desorption energy of 16.4 kJ mol-1 with respect to solution as determined from a Born-Haber cycle. This vast discrepancy indicates a high-energy transition state. Expanding these studies to further systems is the key to pinpointing the molecular origin of the remarkably large NDA desorption barrier.

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.

The influence of ortho-methyl substitution in organometallic self-assembly - a comparative study on Cu(111) vs. Ag(111).

Metal surface-induced dehalogenation of precursors is known to initiate self-assembly of organometallic networks, where tectons are connected via carbon-metal-carbon (C-M-C) bonds. Even though reversibility of the C-M-C bonds facilitates structural equilibration, defects associated with highly bent organometallic linkages are still commonly observed. By introducing a steric hindrance to reduce the C-M-C bond angle flexibility, we find well ordered organometallic networks of an ortho-methyl substituted 1,3,5-tris(p-bromophenyl)benzene analogue on Cu(111) after room-temperature (RT) deposition and on Ag(111) after annealing.

Remote functionalization in surface-assisted dehalogenation by conformational mechanics: organometallic self-assembly of 3,3',5,5'-tetrabromo-2,2',4,4',6,6'-hexafluorobiphenyl on Ag(111).

Even though the surface-assisted dehalogenative coupling constitutes the most abundant protocol in on-surface synthesis, its full potential will only become visible if selectivity issues with polybrominated precursors are comprehensively understood, opening new venues for both organometallic self-assembly and on-surface polymerization. Using the 3,3',5,5'-tetrabromo-2,2',4,4',6,6'-hexafluorobiphenyl (Br4F6BP) at Ag(111), we demonstrate a remote site-selective functionalization at room temperature and a marked temperature difference in double- vs. quadruple activation, both phenomena caused by conformational mechanical effects of the precursor-surface ensemble. The submolecularly resolved structural characterization was achieved by Scanning Tunneling Microscopy, the chemical state was quantitatively assessed by X-ray Photoelectron Spectroscopy, and the analysis of the experimental signatures was supported through first-principles Density-Functional Theory calculations. The non-planarity of the various structures at the surface was specifically probed by additional Near Edge X-ray Absorption Fine Structure experiments. Upon progressive heating, Br4F6BP on Ag(111) shows the following unprecedented phenomena: (1) formation of regular organometallic 1D chains via remote site-selective 3,5'-didebromination; (2) a marked temperature difference in double- vs. quadruple activation; (3) an organometallic self-assembly based on reversibility of C-Ag-C linkages with a thus far unknown polymorphism affording both hexagonal and rectangular 2D networks; (4) extraordinary thermal stability of the organometallic networks. Controlled covalent coupling at the previously Br-functionalized sites was not achieved for the Br4F6BP precursor, in contrast to the comparatively studied non-fluorinated analogue.

Immersion-scanning-tunneling-microscope for long-term variable-temperature experiments at liquid-solid interfaces.

Fundamental insights into the kinetics and thermodynamics of supramolecular self-assembly on surfaces are uniquely gained by variable-temperature high-resolution Scanning-Tunneling-Microscopy (STM). Conventionally, these experiments are performed with standard ambient microscopes extended with heatable sample stages for local heating. However, unavoidable solvent evaporation sets a technical limit on the duration of these experiments, hence prohibiting long-term experiments. These, however, would be highly desirable to provide enough time for temperature stabilization and settling of drift but also to study processes with inherently slow kinetics. To overcome this dilemma, we propose a STM that can operate fully immersed in solution. The instrument is mounted onto the lid of a hermetically sealed heatable container that is filled with the respective solution. By closing the container, both the sample and microscope are immersed in solution. Thereby solvent evaporation is eliminated and an environment for long-term experiments with utmost stable and controllable temperatures between room-temperature and 100 °C is provided. Important experimental requirements for the immersion-STM and resulting design criteria are discussed, the strategy for protection against corrosive media is described, the temperature stability and drift behavior are thoroughly characterized, and first long-term high resolution experiments at liquid-solid interfaces are presented.