Weak intermolecular interactions as binding probes and analytical tools

Abstract

Over the past several decades, polyelectrolytes have received a great deal of attention in supramolecular chemistry. Poly(amidoamine), or PAMAM, dendrimers are hyperbranched polyelectrolytes that can bind to small charged molecules through non-covalent interactions. These supramolecular structures are often involved in cooperative binding events to guest molecules in which they display enhanced affinity. This dissertation describes a series of studies aimed at gaining a deeper understanding of the fundamental intermolecular interactions involved in these binding events, and their relative importance. In order to accomplish this, this work introduces a series of small-molecule PAMAM analogues and focuses on using NMR spectroscopy to study their non-covalent interactions with carboxylates in deuterated methanol. In Chapter 2, the design and synthesis of these model molecules are discussed. Chapter 3 and Chapter 4 detail the NMR spectral changes that accompany proton transfer in deuterated methanol by the PAMAM analogues and carboxylates, respectively. Chapter 5 is focused on the observable non-covalent interactions between the protonated PAMAM analogues and carboxylates. The “intact” analogue formed the strongest binding interactions and induced the greatest change in carboxylate proton chemical shifts. The primary ammonium in the protonated PAMAM analogues was found to be the main driver of these interactions. These studies have provided a more complete understanding of the fundamental intermolecular interactions involved. Chapter 6 details a separate project based on a different family of fluorene-based conjugated polymers with analytical applications to the sensing of polycyclic aromatic hydrocarbons (PAHs). Through extensive benchtop titration studies, the fluorescence of these polymers was found to respond differentially to the presence of these hydrocarbons in organic solution. The decrease in fluorescence intensity was found to originate from a combination of static and dynamic quenching. An analytical array was constructed based on these polymers which was able to generate differential responses for each PAH analyte. Linear discriminant analysis (LDA) was successfully used to discriminate 16 PAHs (the infamous “EPA sixteen”) in solution. These studies provided insight into the interactions between these conjugated polymers and PAHs, and the polymer design features which lead to observable interactions with PAHs.