Two-dimensional (2D) functional molecular networks

This thesis aims to investigate the formation of complex two-dimensional (2D) on solid surfaces. Investigations have been carried out to synthesise and characterise novel 2D functional molecular networks with controllable morphologies and properties on highly oriented pyrolytic graphite (HOPG) surfaces. Two molecular systems that undergo non-covalent 2D self-assembly and one molecular system that demonstrates the formation of 2D covalent-organic frameworks (2D-COFs) have been studied. Following an introduction in chapter 1, chapter 2 provides a background to the doctoral research. Chapter 3 presents the experimental and theoretical methodology used during the thesis. Chapter 4 details scanning tunnelling microscopy (STM) investigations of the interfacial self-assembly of a tetra-carboxylic acid derivative, 5, 5'-(anthracene-2,6-diyl) diisophthalic acid (ATC). This prochiral molecule forms chiral domains when it self-assembles into 2D molecular network at a liquid-solid interface. The morphology of the ATC networks is tuneable by changing the concentration of ATC in solution. Three distinct structural arrangements of ATC have been observed within the same 2D self-assembled network. Chapter 5 describes STM investigations of self-assembled porphyrin networks organised by selective hydrogen bonding between DNA nucleobases at a liquid-solid interface. Driven by homo-molecular thymine-thymine and hetero-molecular thymine-adenine interactions, both mono-component and bi-component self-assembled structures have been formed using freebase and metal containing thymine functionalised porphyrins. Finally, in chapter 6, a Schiff-base condensation reaction has been utilised to grow highly ordered 2D-COFs using tetra-amine functionalised porphyrin and benzene-1,4-dicarboxaldehyde (BDA) at a vapour-solid interface. These structures have been investigated experimentally using STM, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The chemical composition of the 2D-COFs was found to be closely related to the environmental conditions during growth. Our results demonstrate that complementary experimental techniques that probe both the structure and the chemical composition of 2D materials are necessary when investigating 2D-COF materials