Surface-confined 2D polymerization of a brominated copper-tetraphenylporphyrin on Au(111)

TL;DR

This study demonstrates a surface-confined synthesis of a 2D covalent organic network from a brominated copper-tetraphenylporphyrin on Au(111), involving controlled debromination and polymerization steps monitored by spectroscopy and microscopy.

Contribution

It introduces a coupling-limited on-surface synthesis method for 2D covalent networks starting from a halogenated metallo-porphyrin on Au(111), combining experimental and theoretical analysis.

Findings

Successful formation of a covalently bonded 2D network.

Identification of defect types and their effects.

Insights into HOMO-LUMO gap changes due to polymerization.

Abstract

A coupling-limited approach for the Ullmann reaction-like on-surface synthesis of a two-dimensional covalent organic network starting from a halogenated metallo-porphyrin is demonstrated. Copper-octabromo-tetraphenylporphyrin molecules can diffuse and self-assemble when adsorbed on the inert Au(111) surface. Splitting-off of bromine atoms bonded at the macrocyclic core of the porphyrin starts at room temperature after the deposition and is monitored by X-ray photoelectron spectroscopy for different annealing steps. Direct coupling between the reactive carbon sites of the molecules is, however, hindered by the molecular shape. This leads initially to an ordered non-covalently interconnected supramolecular structure. Further heating to 300{\deg}C and an additional hydrogen dissociation step is required to link the molecular macrocycles via a phenyl group and form large ordered polymeric networks. This approach leads to a close-packed covalently bonded network of overall good quality. The structures are characterized using scanning tunneling microscopy. Different kinds of lattice defects and, furthermore, the impact of polymerization on the HOMO-LUMO gap are discussed. Density functional theory calculations corroborate the interpretations and give further insight into the adsorption of the debrominated molecule on the surface and the geometry and coupling reaction of the polymeric structure.