Scanning Tunneling Microscopy and Spectroscopy of Heterotriangulene-based 2D Polymers

Bottom-up synthesized covalent organic frameworks (COFs) provide a means to customize the properties of a 2D polymer from its monomer precursors. The 2D-crystalline DTPA (dimethylmethylene-bridged triphenylamine) COF is synthesized on Au(111) or Ag(111) through Ullman-type coupling¹ . Theory predicts that the as-grown closed-shell electronic structure is semiconducting. Heating in vacuum selectively cleaves methyl groups from the monomer bridge sites, but leaves the framework intact, creating a COF resembling an ultra-flat covalent network of triangulene molecules. Enticing electronic properties are predicted, depending on the specific termination of the bridge sites. Calculations for a H-terminated case indicate that the “radical” COF will be a half-metal (fully spin-polarized density of states at the Fermi energy)². Using an LT-STM, we present new information on the bridge configuration of the demethylated structure, on the electronic structure of the DTPA COF in its closed-shell (methylated) and “radical” (demethylated) forms, and on the interaction between the COF and substrate.

[1] Bieri et al., Chem. Commun. 47 (37) 10239 (2011).

[2] Kan et al., J. Am. Chem. Soc.134 (13) 5718 (2012).