Molecular Self-Assembly from Building Blocks Synthesized on a Surface in Ultrahigh Vacuum: Kinetic Control and Topo-Chemical Reactions

Abstract

Self-assembly of organic molecules on solid surfaces under ultrahigh vacuum conditions has been the focus of intense study, in particular utilizing the technique of scanning tunneling microscopy. The size and complexity of the organic compounds used in such studies are in general limited by thermal decomposition in the necessary vacuum sublimation step. An interesting alternative approach is to deposit smaller molecular precursors, which react with each other on the surface and form the building blocks for the subsequent self-assembly. This has however hitherto not been explored to any significant extent. Here, we perform a condensation reaction between aldehyde and amine precursors codeposited on a Au(111) surface. The reaction product consists of a three-spoke oligo-phenylene-ethynylene backbone with alkyl chains attached through imine coupling. We characterize the self-assembled structures and molecular conformations of the complex reaction product and find that the combined reaction and self-assembly process exhibits pronounced kinetic effects leading to formation of qualitatively different molecular structures depending on the reaction/assembly conditions. At high amine flux/low substrate temperature, compact triimine structures of high conformational order are formed, which inherit organizational motifs from structures formed from one of the reactants. This suggests a topochemical reaction. At low amine flux/high substrate temperature, open porous networks with a high degree of conformational disorder are formed. Both structures are entirely different from that obtained when the triimine product synthesized ex-situ is deposited onto the surface. This demonstrates that the approach of combined self-assembly and on-surface synthesis may allow formation of unique structures that are not obtainable through self-assembly from conventionally deposited building blocks.

Self-Assembly of Nanoporous Chiral Networks with Varying Symmetry from Sexiphenyl-dicarbonitrile on Ag(111)

“…3 Thus, a series of bottom-up fabrication schemes 4 has been developed to realize two-dimensional (2D) layers presenting well-defined open spaces, using hydrogen bonding, [5][6][7][8][9] metal-directed assembly, [10][11][12][13][14] the organization of flexible species, [15][16][17][18] or covalent chemical reactions. 19,20 Nanoporous networks featuring 2D chirality represent a chemically and topologically interesting subclass in this area. [21][22][23][24][25][26] In earlier work, we employed simple ditopic dicarbonitrilepolyphenyl molecular building blocks and studied their selfassembly on the Ag(111) surface.…”

Recent Progress in the Fabrication of Low Dimensional Nanostructures via Surface-Assisted Transforming and Coupling

“…Another important investigation demonstrates that the polymerization of Schiff-base reaction can be delicately controlled using the pulse width, avoiding uncontrolled polymerization [33]. Combining with the alkyl group introduced to adjust the lattice constant and size of low dimensional nanostructure (Figure 4(d)) [11,37], Schiff-base coupling reaction occupied a decisive position in the methodology of surface-enhanced polymerization of nanostructures.…”

Synthesis of One‐Dimensional Schiff Base Polymers that Contain an Oligothiophene Building Block on the Graphite Surface

“…[26] Thermally treated metallic substrates were also reported to catalyze the Ullmann coupling reaction and assist organic molecules in forming 1D and 2D polymers. [15,16,[27][28][29] In addition, intermolecular condensation reactions on surfacesfor example, the self-condensation [19] of boronic acids and their co-condensation [20] with polyols, and condensation of carbonyls [21,22] or aldehydes [23,24] with amines-provide alternative options to form covalent linkages. Studies revealed that the number and position of reactive groups, and the symmetry and rigidity of backbones of the precursors play important roles in the resultant structures and morphologies.…”

“…For instance, selecting proper reactive components can fabricate 1D or branched polymers, [25,[30][31][32][33][34] 2D chiral architectures, [35] and networks with different periods and pore sizes. [21,24] Highly or-dered large-scale 2D surface covalent organic frameworks (COFs) on highly oriented pyrolytic graphite (HOPG) have also been prepared by means of a self-limiting solid-vapor interface reaction method, mild heating under open or closed environments, [36,37] or even by reaction at the solid-liquid interface at room temperature. [38] In these studies, rigid component molecules were usually employed to build polymers of different dimensions, whereas increasing attention has been given to the adsorption and on-surface synthesis of much larger and more flexible species.…”