On-Surface Ullmann Coupling: The Influence of Kinetic Reaction Parameters on the Morphology and Quality of Covalent Networks

Abstract

On-surface Ullmann coupling is a versatile and appropriate approach for the bottom-up fabrication of covalent organic nanostructures. In two-dimensional networks, however, the kinetically controlled and irreversible coupling leads to high defect densities and a lack of long-range order. To derive general guidelines for optimizing reaction parameters, the structural quality of 2D porous covalent networks was evaluated for different preparation protocols. For this purpose, polymerization of an iodine- and bromine-functionalized precursor was studied on Au(111) by scanning tunneling microscopy under ultrahigh vacuum conditions. By taking advantage of the vastly different temperature thresholds for C-Br and C-I cleavage, two different polymerization routes were compared - hierarchical and direct polymerization. The structural quality of the covalent networks was evaluated for different reaction parameters, such as surface temperatures, heating rates, and deposition rates by statistical analysis of STM data. Experimental results are compared to Monte Carlo simulations.

Recent advancement of Ullmann-type coupling reactions in the formation of C–C bond

“…In this method, the halogenated precursor molecules are deposited onto metal surfaces and depending on the type of metal surface, coupling proceeds either directly [25] or via a metastable organic intermediate [26].…”

Interchain-linked Graphene Nanoribbons from Dibenzo[g,p]chrysene via Two-zone Chemical Vapor Deposition

“…15 Self-assembly of molecules on a metal surface enables to form the 2D structures by interactions with the substrate as well as those with the neighboring molecules. 68 Molecular 2D structures like covalent organic frameworks (COFs) are of great interest because the covalent bonding structures are preferable for various applications due to their thermal and chemical stability.…”

“…68 Molecular 2D structures like covalent organic frameworks (COFs) are of great interest because the covalent bonding structures are preferable for various applications due to their thermal and chemical stability. 913 Various chemical reactions to form covalent bonds have been reported, such as Ullman coupling, 14,15 Glaser coupling, 16 decarboxylative coupling, 17 acylation reaction, 18 imidization reaction, 19 and condensation reaction. 20 Most of these materials reported have been based on sp 3 -hybridized carbons, which lack π-conjugation in 2D system.…”

A Shape-Persistent Polyphenylene Spoked Wheel

“…5 With the combination of sp-, sp 2 -, and sp 3 -hybridized carbon atoms, a number of 2D carbon materials with different architectures have also been synthesized and/or theoretically predicted. 2,[6][7][8][9][10][11][12][13][14][15][16][17][18] In particular, graphene has attracted major attention during the last decade, which also stimulated the studies of other 2D carbon materials such as γ-graphyne and graphdiyne ( Figure 1). 10,11,14,19,20 Atomically precise syntheses of γ-graphyne and graphdiyne remain elusive, but widely studied phenylene-ethynylene-based macrocycles can be regarded as their molecular subunits.…”

“…Nevertheless, the number of reported examples of 2D carbon materials is still limited. 2,[6][7][8][9][10][11][12][13][14][15][16][17][18] By replacing ethynylene groups of γ-graphyne and graphdiyne with p-phenylene units, two purely sp 2 -hybridized 2D carbon structures with hexagonal symmetry can be conceived, which we name graphenylene-1 and graphenylene-2, respectively ( Figure 1). Notably, graphenylene-1 corresponds to a single-layer substructure of the hypothetical carbon allotrope "cubic graphite", 26 the synthesis of which still remains elusive.…”