Small molecule motion within and through organic nanomaterials: an anthology

Abstract

This thesis chronicles three distinct projects united by the common theme of small molecule motion in organic materials. Chapter 1 provides a holistic introduction to key background for the main body chapters, Chapters 2-4. The work outlined in the Chapters 2 and 3 was completed under the supervision of Prof. Julia Ortony and pertains to self-assembled small-molecule aramid amphiphile (AA) nanostructures. AAs are noteworthy within the class of self-assembled amphiphile materials because of their unusual mechanical stability borne of strong intermolecular interactions between aramid units. In Chapter 2, I evaluate local conformational dynamics in different chemical domains of an AA nanoribbon through Electron Paramagnetic Resonance (EPR) spectroscopy. These experiments were enabled by co-assembly of AAs with stable nitroxide radical spin labels into the nanoribbon ensemble. Distinct conformational behavior is resolved between domains, and variable temperature studies enable description of each spin label environment through phase transition characterization and activation energy analysis. Chapter 3 explores AA nanostructure morphology in response to pH changes, i.e. ionization-modulated molecular rearrangements. This chapter is divided into Chapters 3A and 3B. In Chapter 3A, the pH dependency of diammonium headgroup AA nanostructures is correlated with aramid backbone flexibility and intermolecular interactions. These diammonium headgroups are also found to exhibit an aggregation-induced pKa drop, which we leverage in Chapter 3 to induce pH responsiveness in a guanidinium headgroup moiety over a physiologically relevant pH range. Finally, Chapter 4 (completed under the supervision of Prof. Zachary Smith) explores molecular transport of CO₂ gas mixtures through a novel guanidinium-functionalized polymer of intrinsic microporosity (PIM-G) membrane. PIM-G shows high permselectivity towards CO₂ over CH₄, N₂, and O₂, and selectivity was further improved by exchanging the polymer’s default Cl− counterion with larger halides. This halide exchange-driven selectivity enhancement occurred without a commensurate drop in CO₂ permeability. This thesis work investigates small molecule motion within novel organic nanomaterials, outlining analytical approaches and structure-property relationships that may be applicable to broad categories of functional materials.