Synthesis and Characterisation of Asymmetric Perylene-based Supramolecular Polymers

Abstract

Supramolecular polymers (SMPs) are a unique class of materials where highly directional and reversible non-covalent interactions link the monomers. Owing to the reversibility of non-covalent interactions, SMPs exhibit significantly dynamic behaviour, enabling them to display various properties such as self-healing, processability, and stimuli-responsiveness. The ability to control the self-assembly behaviour of these functional molecules offers pathways for optimising their properties and structures for targeted applications. Perylene diimides (PDIs) have attracted considerable interest for their increasing prominence as building blocks in the construction of SMPs through solution-based self-assembly. PDIs feature a variety of appealing properties, including optoelectronic properties, that enable the investigation of their self-assembly, thermal and photochemical stability, N-type semiconductivity, and facile chemical modification. Asymmetric PDIs, which bear unique substituents at each imide position, are considered an essential subgroup within PDI derivatives owing to their substantial synthetic versatility. Despite the promising properties of asymmetric PDI derivatives, there have been limited studies on tuning their self-assembly behaviour and morphology, especially in terms of variation of the hydrophobic-hydrophilic balance by introducing asymmetric hydrophilic and hydrophobic chains. The work described in this thesis explores the formation of SMPs based on asymmetric PDIs and investigates the effects of variation of asymmetric hydrophilic and hydrophobic substitution on the properties of the resulting SMPs. A series of asymmetric PDI derivatives were synthesised with modifications of the amide linker, long alkyl and oligo (ethylene glycol) (OEG) chains at the imide position. Tetrahydrofuran/water (THF/H2O) mixtures were found to be an optimal system to induce the self-assembly behaviour of these asymmetric PDI derivatives; the behaviour of these new systems was monitored by recording ultraviolet-visible (UV/Vis) spectra and analysing changes in absorbance intensity. Temperature-dependent UV/Vis studies yielded thermodynamic parameters for the self-assembly process and revealed a transition in the supramolecular polymerisation mechanism due to variations in asymmetric hydrophilic and hydrophobic substitutions. Detailed transmission electron microscopy (TEM) and atomic force microscopy (AFM) investigations revealed the formation of different nanofiber-based supramolecular structures, demonstrating a potential strategy for controlling the supramolecular architecture of asymmetric PDIs. Overall, this thesis provides insights into the influence of asymmetric hydrophilic and hydrophobic substitutions on the self-assembly and supramolecular polymerisation of PDIs. The strategies developed in this work may offer potential pathways for controlling self-assembly behaviour and tailoring material properties, thereby contributing to the broader design of functional supramolecular polymers.