On-Surface Synthesis of Functionalized Honeycombene Macrocycles and Related Two-Dimensional Polymer Structures

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On-surface synthesis is a promising approach for the modification and functionalization of interfaces and for generating new functional materials. While this approach has found some attention for the synthesis of one- and two-dimensional polymers during the past five years, the on-surface synthesis of large organic molecules is still its infancy. Likewise, suitable reactions for covalent bond formation on surfaces are scarce. Encouraged by our joint preliminary studies in our next-door laboratories, we will combine the expertise in organic synthesis from the Hilt group with the surface science competence in the Gottfried group. Specifically, we will use transition-metal catalyzed methods for the flexible synthesis of (poly)functionalized oligophenylene precursors in combination with ultrahigh-vacuum (UHV) based methods for the synthesis, analysis and characterization of complex macrocycles and polymeric frameworks on metal or metal oxide surfaces. Product analysis will be performed with scanning tunneling microscopy (STM), photoelectron spectroscopy (XPS/UPS) and other methods for full structural and compositional analysis. The following aims will be pursued: (a) On-surface synthesis of large conjugated macrocycles (honeycombenes), which undergo self-assembly to form long-range ordered structures on single-crystal surfaces of metals and metal oxides. (b) Functionalization at the periphery of the honeycombene building blocks will be used for further covalent/coordination linkage as a novel approach to well-defined 2D polymers. (c) The inward-functionalization of the honeycombenes will enable follow-up molecular recognition in the central cavity or secondary assembly processes, e.g., the size control of catalytically active metal nanoparticles. (d) We will attempt to generate various honeycombenes on a milligram scale utilizing unprecedented on-surface synthesis on metal nanoparticles. (e) We intend to identify novel bond formation reactions towards poly-functionalized honeycombenes and related structures, expanding the presently very limited arsenal of suitable on-surface reactions. In particular, catalysis by absorbed adatoms will be explored to generate novel macrocyclic structures, such as molecular dodecagrams, via metal-catalyzed [2+2]-cycloaddition reactions. (f) Finally, the reaction mechanisms of the surface-mediated reactions will be investigated using a combination of surface spectroscopic and microscopic techniques, to establish a basis for a systematic development and optimization of these reactions.