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 molecular precursors. The 2D-crystalline DTPA (dimethylmethylene-bridged triphenylamine) COF is synthesized on metal surfaces through Ullman-type coupling¹. Theory predicts that the as-grown COF electronic structure is semiconducting. Heating in vacuum selectively cleaves methyl groups from the monomer bridge sites, creating a new COF. Enticing electronic properties are predicted, depending on the termination of the bridge sites including a half-metal (fully spin-polarized density of states at the Fermi energy) for the H-terminated case². Using cryogenic STM, we present new information on the bridge configuration of the demethylated structure, on the electronic structure of the DTPA COF in its methylated and demethylated forms, and on the COF/substrate interaction. Furthermore, by studying a range of DTPA self-assemblies, we have gained a deeper understanding of the electronic bands of the fully grown COF.

[1] Bieri et al., Chem. Commun. 47 (37) 10239 (2011).
[2] Kan et al., J. Am. Chem. Soc.134 (13) 5718 (2012).