Synthesis of two-dimensional covalent organic frameworks at surfaces

Promovendus/a: Niklas Herrmann

Promotor(en): Prof. dr. Steven De Feyter, De heer Kunal Mali

Two-dimensional materials (2DMs), such as graphene, have properties which render them interesting for many applications, for example in energy storage, catalysis, optics or medicine. To harvest their full potential, designing and synthesising tailored 2DMs is an important goal. 2D covalent organic frameworks (2D COFs) are one class of 2DMs, produced by chemically connecting molecules, forming an ordered, "crystalline" 2D sheet. In this work, the synthesis of different 2D COFs is investigated. Unlike most of the so far published 2D COFs, which are synthesised under harsh reaction conditions (high temperatures and pressures, long reaction times), mild conditions at the interface between a solid substrate and an organic solvent are used. This approach does not only allow the synthesis of single layers of 2D COFs, both also the use of scanning tunnelling microscopy (STM), a technique using the quantum-mechanical tunnel effect to directly image very small structures such as molecules with very high resolution.

Using STM and other analytical techniques, the formation mechanism of different 2D COFs was investigated. For a boroxine-based 2D COF, the potential influence of the STM imaging onto the growth of the COF was probed and found to be negligible. For another type of 2D COF, synthesised from a so called "two-in-one" monomer, a very fast reaction under mild conditions was found. Additionally, this COF is not forming and growing on the used solid substrate, but in the solution above, contrasting to most of the so far reported COFs synthesised at the liquid/solid interface.

The self-assembly of a porphyrin-based molecule and a so far unreported novel molecule, pyrene-1,6-diboronic acid was also probed by interfacial STM. The last chapter of the presented work presents the design and synthesis of two new boroxine 2D COF monomer molecules, which have the ability to bind to different metals. While the envisioned metal-containing 2D COF was not formed, complex chemical reaction pathways were found and analysed by using various analytical techniques