The self-assembly of supramolecular monolayers at liquid-solid interfaces has been extensively studied over the last three decades, predominantly by Scanning Tunneling Microscopy. Early on, evidence accumulated that polymorphism-the formation of different monolayer structures from the same molecular building block-is relatively common. A plethora of studies have demonstrated that the specific polymorph expressed can depend systematically and reliably on the type of solvent, solute concentration, temperature, and substrate used. By contrast, spontaneous polymorphism was also observed, whereby different polymorphs emerged under seemingly similar conditions. Although this phenomenon has been known for a long time also in the context of molecular bulk crystals, it is often poorly understood. Here, the self-assembly of hydrogen-bonded trimesic acid monolayers on graphite from heptanoic acid solution yielded either the chickenwire or the flower polymorph, depending on the batch and supplier of the solvent. In a previous study, however, we found the chickenwire polymorph to be most thermodynamically stable in this solvent. This unexpected spontaneous polymorphism was eventually attributed to water impurities in the heptanoic acid solvent, and could be controlled by adding or removing small amounts of water. We anticipate that a fuller and quantitative understanding of the water influence on polymorph selection in hydrogen-bonded monolayers could become a powerful lever for crystal engineering.

Inert supports are essential for harnessing the full potential of on-surface synthesized nanostructures, particularly with regard to their electronic properties. This important milestone can be achieved in two principal ways, either conventional synthesis on metal surfaces followed by subsequent transfer, or direct synthesis on inert surfaces. 1 The latter method not only eliminates a laborious processing step, but also prevents degradation of the nanostructures during transfer. In addition, direct synthesis on inert supports may offer benefits for structure quality by enhancing monomer mobility. Graphitic surfaces are chosen not only because of their compatibility with Scanning Tunneling Microscopy, but also because they are fully inert under ambient conditions. Furthermore, a direct comparison of graphite and graphene sheds light on how the subsurface structure of the topmost graphene layer influences both the synthesis and the properties of nanostructures. Two approaches to on-surface synthesis on graphitic surfaces are discussed: (1) Thermal coupling, where the heating step is carried out in an inert gas atmosphere to prevent premature desorption of the reactants; 2 and (2) photochemical coupling. 3 Two different implementations are presented here: (a) the light-induced formation of intermolecular cyclobutane linkages via thermally forbidden [4+4] cycloadditions, 4 and (b) the photodissociation of iodine substituents resulting in covalent carbon-carbon bonds formed by subsequent radical addition. Conversely, a defined supramolecular self-assembly is essential for the former approach, but detrimental for the latter. (1) Dalton Trans. 50, 10020-10027 (2021). (2) Angew. Chem. Int. Ed. 64, e202422521 (2025). (3) Trends Chem. 4, 471-474 (2022). (4) Nat. Chem. 13, 730–736 (2021).

A key milestone in the field of On-Surface Synthesis (OSS) is the development of generic protocols for covalent coupling on inert surfaces. Typically, metal surfaces are used in OSS, but the strong interactions distort the intrinsic properties of the synthesized nanostructures and therefore compromise their applicability. Inert surfaces, on the other hand, preserve the nanostructures' unique properties, but present challenges during synthesis: Activation energies are generally higher than on metal surfaces, hence thermally activated coupling on inert surfaces is severely hampered by competing premature desorption of the reactants. Using the coupling of 1,3,5-tris(4-mercaptophenyl)benzene (TMB) via carbon-sulfur-carbon thioether bonds, we demonstrate that annealing in a noble gas atmosphere instead of in a vacuum kinetically inhibits desorption. This allows covalent coupling on both inert graphite and even more weakly interacting graphene surfaces. We anticipate generic potential for this approach, with future experiments exploring the expandability of ambient pressure annealing to a portfolio of other coupling protocols.

On-surface synthesis (OSS) facilitates the coupling of larger molecules on solid surfaces into extended covalent nanostructures that are difficult or impossible to achieve by wet chemistry. Its primary analytical tool is scanning probe microscopy (SPM), which provides submolecular views of reactants, products and sometimes intermediates. However, relevant aspects such as subtle chemical changes and structural details remain inaccessible. In addition, direct monitoring of reaction progress in real time by SPM is challenging. This analytical gap is increasingly being filled by complementary analytics: mass spectrometry can be used not only to detect volatile by-products that are released during the reaction, but also to monitor intermediates and higher oligomers. Surface sensitive vibrational spectroscopy, either with electrons or photons, is advantageous for the identification of perceived reaction products, even in cases where the routine approach based on X-ray photoelectron spectroscopy (XPS) is not very promising. X-ray standing wave (XSW) analysis is a less common technique in OSS but well established in surface science, providing experimental access to adsorption heights with picometre accuracy. Its value for detailed comparison and validation of prevailing density functional theory (DFT) based structure calculations cannot be overstated. Recent examples also show the benefits of XSW for less regular structures, such as those often obtained in OSS. Finally, the assessment of reaction kinetics has considerable potential to provide fundamental insights into elementary processes and hidden reaction partners for the unique coupling of larger molecules on surfaces into extended structures. Real-time XPS has sufficient chemical and temporal resolution to monitor reaction kinetics for coupling on surfaces. Ideally, mechanistic insights can be gained by modelling. However, the typically applied linear temperature profiles have limitations that can be overcome by exploring new temperature profiles. Again, the accurate determination of kinetic reaction parameters, such as activation energies, is of paramount importance for benchmarking DFT calculations. Although spectroscopy is already applied for OSS its broader and more systematic implementation appears highly promising for the advancement of the fundamental understanding of OSS, hence eventually also for optimizing the reaction protocols and outcomes.

AbstractEin wichtiger Meilenstein auf dem Gebiet der Oberflächensynthese (On‐Surface‐Synthesis – OSS) ist die Entwicklung generischer Protokolle für die kovalente Kupplung auf inerten Oberflächen. Normalerweise werden für die OSS Metalloberflächen verwendet. Jedoch beeinflussen ihre starken Wechselwirkungen die intrinsischen Eigenschaften der synthetisierten Nanostrukturen und beeinträchtigen daher ihre Anwendbarkeit. Inerte Oberflächen hingegen bewahren die einzigartigen Eigenschaften der Nanostrukturen, stellen aber eine Herausforderung bei der Synthese dar: Die Aktivierungsenergien sind in der Regel höher als auf Metalloberflächen, wodurch die thermisch aktivierte Kupplung auf inerten Oberflächen durch konkurrierende vorzeitige Desorption der Reaktanden erheblich eingeschränkt wird. Anhand der Kupplung von 1,3,5‐Tris(4‐Mercaptophenyl)Benzol (TMB) über Kohlenstoff‐Schwefel‐Kohlenstoff Thioether‐Bindungen demonstrieren wir, dass beim Heizen in einer Edelgasatmosphäre statt im Vakuum die Desorption kinetisch gehemmt ist. Dies ermöglicht die kovalente Vernetzung sowohl auf inerten Graphitoberflächen als auch auf noch schwächer wechselwirkenden Graphenoberflächen. Wir antizipieren generisches Potenzial für diese Methode. In zukünftigen Experimenten wird untersucht, ob das Heizen bei Umgebungsdruck auf andere Kopplungsprotokolle ausgeweitet werden kann.

AbstractAt elevated temperatures, the prototypical organic solvents used to study the self‐assembly of supramolecular monolayers at liquid–solid interfaces alter a graphite substrate by intercalation. As a consequence, less strongly bound supramolecular monolayers become thermodynamically unstable, as probed by scanning tunneling microscopy. Complementary characterization by atomic force microscopy, confocal Raman spectroscopy and low energy electron microscopy consistently points to subsurface changes in the top few layers of the graphite substrate due to solvent intercalation. High‐temperature annealing at 900 °C in the vacuum restores the adsorption properties of the graphite substrates, indicating a high activation energy for deintercalation. However, strongly adsorbing hydrogen‐bonded monolayers of trimesic acid inhibit solvent intercalation and thus protect the graphite substrate. Mildly solvent‐intercalated graphite may prove useful as an easily prepared graphitic material with further weakened adsorption properties.

We attained the synthesis of mesoscale-ordered 2D polymers by the topochemical on-surface photopolymerization of fluorinated anthracene triptycene (fantrip) monomers. The underlying protocol is two-staged: (1) Self-assembly of the monomers into a photopolymerizable monolayer structure, where the photoactive anthracene moieties are face-to-face stacked; (2) cross-linking of the self-assembled monolayer into a covalent 2D polymer by photochemically excited [4+4] cycloadditions between the antiparallel aligned anthracene blades. Thereby, the long-range order attained in step (1) is transferred into the covalent state. Yet, the topochemical approach depends crucially on achieving the reactive monomer packing with the appropriate mutual alignment of the anthracene blades. For this initial self-assembly, the underlying surface plays a decisive role, which also extends to the photopolymerization. We show that the photoactive monolayer packing of fantrip monomers can be achieved on both alkane-passivated graphite and iodine-passivated Au(111) or Ag(111). Although the fantrip self-assembly appears essentially similar on these substrates, there are distinct differences in the subsequent photopolymerization. These can be rationalized by the photoexcitation of the substrate and the subsequent transfer of hot carriers to the molecular networks. Nat. Chem. 13, 730-736 (2021) Eur. J. Org. Chem. 2021, 5478–5490 (2021) Trends Chem. 4, 471-474 (2022) Angew. Chem. Int. Ed. 61, e202201044 (2022)

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.

The temporal evolution of the reactant concentrations as measured by XPS for different temperature profiles reveals that the debromination of organic molecules on Ag(111) is activated by Ag adatoms.

Regular and robust 2D conjugated organogold networks with anthra-tetrathiophene repeat units are synthesized by debrominative coupling on iodine-passivated Au(111). Dynamic error correction becomes feasibly through iodine-induced bond reversibility.

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.

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.

Light is an auspicious stimulus for the synthesis of covalent organic nanostructures on surfaces. By contrast to most thermally induced chemistry, photochemical couplings can proceed both on inert surfaces and at lower temperatures. Prospects are manifold, ranging from exploiting new classes of surfaces and reactions to attaining higher quality structures.

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.

The development of widely applicable methods for the synthesis of C-C-bonded nanostructures on inert and insulating surfaces is a challenging yet rewarding milestone in the field of on-surface synthesis. This would enable studies of nearly unperturbed covalent nanostructures with unique electronic properties as graphene nanoribbons (GNR) and π-conjugated 2D polymers. The prevalent Ullmann-type couplings are almost exclusively carried out on metal surfaces to lower the temperature required for initial dehalogenation well below the desorption threshold. To overcome the necessity for the activation of monomers on the target surface, we employ a recently developed Radical Deposition Source (RaDeS) for the direct deposition of radicals onto inert surfaces for subsequent coupling by addition reactions. The radicals are generated en route by indirect deposition of halogenated precursors through a heated reactive tube, where the dehalogenation reaction proceeds. Here, we use the ditopic 6,11-diiodo-1,2,3,4-tetraphenyltriphenylene (DITTP) precursor that afforded chevron-like GNR on Au(111) via the usual two-staged reaction comprised of monomer-coupling into covalent polymers and subsequent formation of an extended GNR by intramolecular cyclodehydrogenation (CDH). As a model system for inert surfaces, we use Ag(111) passivated with a closed monolayer of chemisorbed iodine that behaves in an inert manner with respect to dehalogenation reactions and facilitates the progressive coupling of radicals into extended covalent structures. We deposit the DITTP-derived biradicals onto both iodine-passivated and pristine Ag(111) surfaces. While on the passivated surface, we directly observe the formation of covalent polymers, on pristine Ag(111) organometallic intermediates emerge instead. This has decisive consequences for the further progression of the reaction: heating the organometallic chain directly on Ag(111) results in complete desorption, whereas the covalent polymer on iodine-passivated Ag(111) can be transformed into the GNR. Yet, the respective CDH proceeds directly on Ag(111) after thermal desorption of the iodine passivation. Accordingly, future work is aimed at the further development of approaches for the complete synthesis of GNR on inert surfaces.

Self-assembly of supramolecular monolayers at liquid-solid interfaces has matured into an established research field. Numerous studies unveiled crucial influences of solvent, solute concentration, and temperature on the kinetics and thermodynamics of monolayer formation and their specific role for structure selection. Yet, almost all experiments are carried out on highly inert graphite surfaces that are straightforward to prepare. However, the strong focus on graphite leaves the crucial impact of the underlying surface severely underexplored. Here, we show that passivation of Au(111) with a chemisorbed monolayer of iodine atoms renders it sufficiently inert for studies at liquid- solid interfaces, even at elevated temperatures. By using aromatic homologues of benzene tricarboxylic acids as a well-explored model system and by a one-to-one comparison to graphite, we unveil that molecule-surface interactions can cause substrate-induced polymorphism, crucially affect the supramolecular monolayer's thermodynamic stability, or even result in the emergence of new polymorphs. These experiments underscore a decisive and specific thermodynamic influence of the underlying surface. We expect our study to stimulate further research on the surface influence on interfacial monolayers by employing this accessible and easy-to prepare surface with the aim to establish a new lever for steering supramolecular self-assembly.

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.

2D polymers are a relatively new class of macromolecules. Therefore, it is not astounding that so far research focused on how to provide access to this intriguing class of organic 2D materials, how to prove their existence, and how to assess their structural quality. Studies concerning the formation mechanism are comparatively scarce. We here collect and compare all the mechanistic information available for 2D polymer synthesis by photochemical means and point towards research directions to be followed in order to advance the fundamental understanding and, thus, fast development of this field. Because the two current starting situations for the photochemical synthesis of 2D polymers are layered single crystals and surface-supported monolayers, the prominent analytical tools are X-ray diffraction (XRD), local vibration spectroscopy, e. g. tip enhanced Raman spectroscopy (TERS), and scanning probe microscopy (SPM), e. g. low temperature scanning tunnelling microscopy (LT STM) in ultra-high vacuum (UHV), but also atomic force microscopy (AFM). With their advantages and shortcomings, they will therefore play an important role throughout this mini review.