Symmetry and spacing controls in periodic covalent functionalization of graphite surfaces templated by self-assembled molecular networks

We herein present the periodic covalent functionalization of graphite surfaces, creating a range of patterns of different symmetries and pitches at the nanoscale. Self-assembled molecular networks (SAMNs) of rhombic-shaped bis(dehydrobenzo[12]annulene) (bisDBA) derivatives having alkyl chain substituents of different lengths were used as templates for covalent grafting of electrochemically generated aryl radicals. Scanning tunneling microscopy (STM) observations at the 1,2,4-trichlorobenzene/graphite interface revealed that these molecules form a variety of networks that contain pores of different shapes and sizes. The covalently functionalized surfaces show hexagonal, oblique, and quasi-rectangular periodicities. This is attributed to the favorable aryl radical addition at the pore(s). We also confirmed the successful transmission of chirality information from the SAMNs to the alignment of the grafted aryls. In one case, the addition of a guest molecule was used to switch the SAMN symmetry and periodicity, leading to a change in the functionalized surface periodicity from oblique to hexagonal in the presence of the guest molecule. This contribution highlights the potential of SAMNs as templates for the controlled formation of nanopatterned carbon materials.

Rational design of two-dimensional covalent tilings using a C6-symmetric building block via on-surface Schiff base reaction

Three types of well-ordered covalent two-dimensional tilings including triangular, rhombille and semi-regular Archimedean tilings were successfully constructed via on-surface Schiff base reaction. Among them, the covalent organic framework (COF) constructed from a C6 symmetry monomer and C3 symmetry monomer is the first reported COF with kgd (rhombille tiling) topology.

Donor-acceptor interactions between cyclic trinuclear pyridinate gold(i)-complexes and electron-poor guests: nature and energetics of guest-binding and templating on graphite

Aromatic stacking interactions of π-basic Au(i) complexes with π-acids were analyzed experimentally, theoretically and at the solid/liquid interface using STM.

Donor–acceptor-type interactions between π-electron systems are of high relevance in the design of chemical sensors. Due to their electron-rich nature, cyclic trinuclear complexes (CTCs) of gold(i) are ideal receptor sites for electron-deficient aromatic analytes. Scanning tunneling microscopy provided insight into the structures of two-dimensional crystals of pyridinate gold CTCs that form on a graphite template at the solid/liquid interface. One polymorph thereof – in turn – templated the on-top co-adsorption of π-acidic pyrazolate CTCs as electron-poor guests up to a certain threshold. From NMR titration experiments, we quantified free energies of –6.1 to –7.5 kcal mol^–1 for the binding between pyridinate gold(i) CTCs and π-acidic pyrazolate CTCs. Quantum chemical calculations revealed that these interactions are largely dominated by London dispersion. These results give a more detailed insight into a rational design of sensitive CNT- or graphene-based sensors for π-acidic analytes, such as electron-deficient aromatics.

Discrete polygonal supramolecular architectures of isocytosine-based Pt(ii) complexes at the solution/graphite interface

Polygonal supramolecular architectures of a Pt(ii) complex including trimers, tetramers, pentamers and hexamers were self-assembled via hydrogen bonding between isocytosine moieties; their structure at the solid/liquid interface was unravelled by in situ scanning tunneling microscopy imaging. Density functional theory calculations provided in-depth insight into the thermodynamics of their formation by exploring the different energy contributions attributed to the molecular self-assembly and adsorption processes.

Hydrogen-Bonded Macrocyclic Supramolecular Systems in Solution and on Surfaces

Cyclization into closed assemblies is the most recurrent approach to realize the noncovalent synthesis of discrete, well-defined nanostructures. This review article particularly focuses on the noncovalent synthesis of monocyclic hydrogen-bonded systems that are self-assembled from a single molecule with two binding-sites. Taking advantage of intramolecular binding events, which are favored with respect to intermolecular binding in solution, can afford quantitative amounts of a given supramolecular species under thermodynamic control. The size of the assembly depends on geometric issues such as the monomer structure and the directionality of the binding interaction, whereas the fidelity achieved relies largely on structural preorganization, low degrees of conformational flexibility, and templating effects. Here, we discuss several examples described in the literature in which cycles of different sizes, from dimers to hexamers, are studied by diverse solution or surface characterization techniques.

Self-assembly of Natural and Unnatural Nucleobases at Surfaces and Interfaces

The self-assembly of small organic molecules interacting via non-covalent forces is a viable approach towards the construction of highly ordered nanostructured materials. Among various molecular components, natural and unnatural nucleobases can undergo non-covalent self-association to form supramolecular architectures with ad hoc structural motifs. Such structures, when decorated with appropriate electrically/optically active units, can be used as scaffolds to locate such units in pre-determined positions in 2D on a surface, thereby paving the way towards a wide range of applications, e.g., in optoelectronics. This review discusses some of the basic concepts of the supramolecular engineering of natural and unnatural nucleobases and derivatives thereof as well as self-assembly processes on conductive solid substrates, as investigated by scanning tunnelling microscopy in ultra-high vacuum and at the solid/liquid interface. By unravelling the structure and dynamics of these self-assembled architectures with a sub-nanometer resolution, a greater control over the formation of increasingly sophisticated functional systems is achieved. The ability to understand and predict how nucleobases interact, both among themselves as well as with other molecules, is extremely important, since it provides access to ever more complex DNA- and RNA-based nanostructures and nanomaterials as key components in nanomechanical devices.

Study of the Edge-on Self-Assembly of Axially Substituted Silicon(IV) Phthalocyanine Derivatives in a Template on the HOPG Surface

Molecular conformation is an important issue related to the self-assembly architecture and property. The self-assembly of silicon(IV) phthalocyanines covalently linked to the 5-N-cytidine or 4-carboxyphenoxy moiety at the axial positions, namely, SiPc(NC)2 and SiPc(CP)2, respectively, has been studied by means of scanning tunneling microscopy (STM) at the solid-liquid interface. The intermolecular axial hydrogen bonding in combination with the stabilizing role of the TCDB template brings about supramolecular self-assembled structures of silicon(IV) phthalocyanines in an edge-on orientation. Two pyridine compounds, 4,4'-bipyridine (BPY) and 1,2-di(4-pyridyl)ethylene (DPE), can tune the supramolecular structure, leading to interestingly axial self-assemblies of SiPc(CP)2 with BPY and DPE in an edge-on manner by hydrogen bonding. The results indicate that the axial substituents and the axial ligands can regulate and precisely control the conformation and arrangement of the phthalocyanine moiety on the graphite surface.

Self-assembly of N3-substituted xanthines in the solid state and at the solid-liquid interface

The self-assembly of small molecular modules interacting through noncovalent forces is increasingly being used to generate functional structures and materials for electronic, catalytic, and biomedical applications. The greatest control over the geometry in H-bond supramolecular architectures, especially in H-bonded supramolecular polymers, can be achieved by exploiting the rich programmability of artificial nucleobases undergoing self-assembly through strong H bonds. Here N(3)-functionalized xanthine modules are described, which are capable of self-associating through self-complementary H-bonding patterns to form H-bonded supramolecular ribbons. The self-association of xanthines through directional H bonding between neighboring molecules allows the controlled generation of highly compact 1D supramolecular polymeric ribbons on graphite. These architectures have been characterized by scanning tunneling microscopy at the solid-liquid interface, corroborated by dispersion-corrected density functional theory (DFT) studies and X-ray diffraction.