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

Solvent-Dependent Stabilization of Metastable Monolayer Polymorphs at the Liquid–Solid Interface

Self-assembly of 1,3,5-tris(4′-biphenyl-4″-carbonitrile)benzene monolayers was studied at the liquid–solid interface by scanning tunneling microscopy. Application of different fatty acid homologues as solvents revealed a solvent-induced polymorphism. Yet, tempering triggered irreversible phase transitions of the initially self-assembled monolayers, thereby indicating their metastability. Interestingly, in either case, the same thermodynamically more stable and more densely packed monolayer polymorph was obtained after thermal treatment, irrespective of the initial structure. Again, the same densely packed structure was obtained in complementary solvent-free experiments conducted under ultrahigh vacuum conditions. Thus, self-assembly of metastable polymorphs at room temperature is explained by adsorption of partially solvated species under kinetic control. The irreversible phase transitions are induced by thermal desolvation, that is, desorption of coadsorbed solvent molecules

Solution Preparation of Two-Dimensional Covalently Linked Networks by Polymerization of 1,3,5-Tri(4-iodophenyl)benzene on Au(111)

The polymerization of 1,3,5-tri(4-iodophenyl)benzene (TIPB) on Au(111) through covalent aryl–aryl coupling is accomplished using a solution-based approach and investigated by scanning tunneling microscopy. Drop-casting of the TIPB monomer onto Au(111) at room temperature results in poorly ordered noncovalent arrangements of molecules and partial dehalogenation. However, drop-casting on a preheated Au(111) substrate yields various topologically distinct covalent aggregates and networks. Interestingly, some of these covalent nanostructures do not adsorb directly on the Au(111) surface, but are loosely bound to a disordered layer of a mixture of chemisorbed iodine and molecules, a conclusion that is drawn from STM data and supported by X-ray photoelectron spectroscopy. We argue that the gold surface becomes covered by a strongly chemisorbed iodine monolayer which eventually inhibits further polymerization

Isoreticular Two-Dimensional Covalent Organic Frameworks Synthesized by On-Surface Condensation of Diboronic Acids

On-surface self-condensation of 1,4-benzenediboronic acid was previously shown to yield extended surface-supported, long-range-ordered two-dimensional covalent organic frameworks (2D COFs). The most important prerequisite for obtaining high structural quality is that the polycondensation (dehydration) reaction is carried out under slightly reversible reaction conditions, <i>i</i>.<i>e</i>., in the presence of water. Only then can the subtle balance between kinetic and thermodynamic control of the polycondensation be favorably influenced, and defects that are unavoidable during growth can be corrected. In the present study we extend the previously developed straightforward preparation protocol to a variety of para-diboronic acid building blocks with the aim to tune lattice parameters and pore sizes of 2D COFs. Scanning tunneling microscopy is employed for structural characterization of the covalent networks and of noncovalently self-assembled structures that form on the surface prior to the thermally activated polycondensation reaction

From Benzenetrithiolate Self-Assembly to Copper Sulfide Adlayers on Cu(111): Temperature-Induced Irreversible and Reversible Phase Transitions

Self-assembly and thermally activated surface chemistry of 1,3,5-benzenetrithiol (BTT) on Cu(111) are studied under ultrahigh vacuum (UHV) conditions by different complementary surface sensitive techniques. Low-energy electron diffraction (LEED) patterns acquired at room temperature and during subsequent heating reveal irreversible phase transitions between in total four different long-range-ordered phases termed α-phase to δ-phase. X-ray photoelectron spectroscopy (XPS) of the different phases facilitates the identification of major chemical changes for the first phase transition from α- to β-phase, whereas in the succeeding phase transitions, no significant chemical shifts are observed anymore. The structural characterization of each phase is carried out by high-resolution scanning tunneling microscopy (STM), and adsorption geometries of the phenyl rings are derived from C 1s near-edge X-ray absorption fine structure (NEXAFS). The combination of the results from this array of experimental techniques leads to a consistent picture of the various phases and underlying processes. Upon room-temperature deposition, BTT fully deprotonates and planar-adsorbed molecules self-assemble into an ordered monolayer. With a temperature onset of 300 K, the carbon–sulfur bonds start dissociating. Sulfur forms a copper sulfide superstructure, whereas the organic remainders form disordered structures. Further heating converts an initial metastable and rarely observed (√3 × √3)<i>R</i> ± 30° copper sulfide superstructure into the more stable and well-known (√7 × √7)<i>R</i> ± 19.1° polymorph

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

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

From Au–Thiolate Chains to Thioether Sierpiński Triangles: The Versatile Surface Chemistry of 1,3,5-Tris(4-mercaptophenyl)benzene on Au(111)

Self-assembly of 1,3,5-tris­(4-mercaptophenyl)­benzene (TMB), a 3-fold symmetric, thiol-functionalized aromatic molecule, was studied on Au(111) with the aim of realizing extended Au–thiolate-linked molecular architectures. The focus lay on resolving thermally activated structural and chemical changes by a combination of microscopy and spectroscopy. Thus, scanning tunneling microscopy (STM) provided submolecularly resolved structural information, while the chemical state of sulfur was assessed by X-ray photoelectron spectroscopy (XPS). Directly after room-temperature deposition, only less well ordered structures were observed. Mild annealing promoted the first structural transition into ordered molecular chains, partly organized in homochiral molecular braids. Further annealing led to self-similar Sierpiński triangles, while annealing at even higher temperatures again resulted in mostly disordered structures. Both the irregular aggregates observed at room temperature and the chains were identified as metal–organic assemblies, whereby two out of the three intermolecular binding motifs are energetically equivalent according to density functional theory (DFT) simulations. The emergence of Sierpiński triangles is driven by a chemical transformation, <i>i.e.</i>, the conversion of coordinative Au–thiolate to covalent thioether linkages, and can be further understood by Monte Carlo simulations. The great structural variance of TMB on Au(111) can on one hand be explained by the energetic equivalence of two binding motifs. On the other hand, the unexpected chemical transition even enhances the structural variance and results in thiol-derived covalent molecular architectures