The synthesis and applications of pillar[n]arene in selective guest binding and surface modification

Lewis, Alexander ORCID: 0000-0002-7735-9914 (2023). The synthesis and applications of pillar[n]arene in selective guest binding and surface modification. University of Birmingham. Ph.D.

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Abstract

Macrocyclic chemistry has garnered interest in a plethora of scientific fields. Whether in the exploration of new antibiotics, or in the adsorption of greenhouse gases, the breadth of macrocyclic research is constantly extending. Underpinning most of this research, however, is a key theme; to bind and encapsulate a particular species. Natural processes provided the foundations of such research, from ion capture in hemoglobin, to the protein binding antimicrobial qualities of erythromycin and clarithromycin. Consequently, the subsequent biomimetic approach to the synthetic design of macrocycles was the dominant force in synthetic macrocyclic research. That was until the works from the likes of Pederson, Lehn and Cram, who introduced not only new chemical motifs, but a new approach to the design of macrocycles.

Pillar[n]arenes were introduced to the family of synthetic macrocycles by Tomoki Ogoshi in 2008. They were heralded for their simplicity, rigidity, and their promising host-guest properties. These para-bridged symmetrical macrocycles have since become a guiding force in macrocyclic research, aided by their ease of synthesis, and the almost endless possibilities for functionalization. Akin to other macrocycles, research into the host-guest properties of pillar[5]arene has been demonstrated as paramount to the macrocycle’s success in pursuing real-world applications.

Although pillar[5]arenes selectivity towards cationic and electron poor molecules has been well established, this research has rarely expanded into specific applications. An example of such an application is presented herein, that is the selective binding of common pesticides and their metabolites. Pesticides perform a vital role in providing means to produce the ever-increasing supply of agrarian products needed for our expanding population. Parallel to their need, however, is the growing concern surrounding the environmental and anthropomorphic damage that these agrochemicals provide. The safe management and control of these biocides are therefore paramount to ensuring a sustainable future. Herein, the host-guest properties of pillar[n]arenes are utilized to pursue a method of creating a selective ‘kill-switch’ to prevent the more harmful metabolites of nicotinoids and neonicotinoids. Through proton NMR titrations, the binding constants of the 5 and 6 membered pillar[n]arenes towards a series of insecticides and their respective metabolites are measured. Whereas the cationic nicotine metabolite, N-ethylnicotinium is successfully encapsulated, imidacloprid and its metabolite desnitroimidacloprid show contrary results. The results therefore demonstrate that our understanding of the host-guest chemistry of pillar[5]arene must propel solely beyond electrostatic interactions.

Surface chemistry plays a vital role in transferring the properties of molecules to the macromolecular scale. The adoption of macrocycles onto surfaces, therefore provides a unique opportunity to transplant the host-guest properties of a given molecule to a stable macromolecular platform. This work details a method of adsorbing pillar[5]arenes onto a gold surface using thiol chemistry. Through scanning tunneling microscopy, the topographic nature of the pillar[5]arene/ metal surface is examined.

An expansion to this surface modification is an attempt to affix the dynamic rotaxane to a porous material. Herein, a ‘semi-modular’ approach to rotaxane design where the asymmetric components of the rotaxane can be tailored to a specific surface is employed. Paired with these rotaxanes are novel metal organic frameworks containing moieties capable of covalent post-synthetic surface modification.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):