Examining and Controlling the Effects of Hydration on Nanoconfined H+ and e- Transfer Processes.

With the support of the Macromolecular, Supramolecular and Nanochemistry Program in the Division of Chemistry, Professor Mark C. Lipke of Rutgers University will study how acid-base chemistry and electron transfer processes are altered inside nanoscale molecular containers, thereby providing knowledge that may aid the design of catalysts, sensors, and other chemically functional materials. Graduate students, undergraduate students, and high school interns engaged in this research will receive research training in fundamental organic, inorganic, and physical chemistry, as well as advanced topics of nanoscience, electrochemistry, and energy science. The Lipke group will also engage in educational outreach with local high schools to promote careers in science by discussing modern research topics and career paths to becoming scientists. It is well known that nanoconfinement can alter the rates and thermodynamics of proton and electron transfer processes. However, past studies of this behavior have not fully addressed the complex interplay of factors—especially the hydration of the confined environment—that tune proton and electron transfer. In ordinary solutions, acid-base chemistry and electron transfer chemistry are strongly influenced by the interactions of charged chemical entities with solvent molecules, such as the transfer of protons (H+) from an acidic molecule to water molecules to generate solvated protons known as hydronium ions. Such chemistry can be altered significantly in molecular containers that only have enough space to hold a small number of solvent molecules since these tightly confined spaces prevent solvent molecules from moving freely. The Lipke group will study these effects in well-defined molecular containers to shed light on how such structures can be used to deliberately control proton and electron transfer events that underpin important chemical functions, such as electrochemical synthetic methods. The Lipke group has developed robust redox-active porphyrin nanocages that enable clear measurements of how hydration and other variables influence acidity and redox chemistry. The nanocages can host a variety of acid-functionalized guests and display redox activity that can be characterized clearly by electrochemical methods, thereby providing systems for measuring H+ and H+/e- transfer processes in a confined environment. Furthermore, the uptake of water by the host-guest complexes can be characterized by 1H NMR spectroscopy in organic solvents, providing a way to reliably test the effects of hydration on H+ and H+/e- transfer. These nanocages will be used to examine how different structural and chemical parameters, such as size, hydrogen-bonding ability, hydrophobicity, and counterions, affect H+ and e- transfer under conditions in which the pore is hydrated. Complementing experimental studies, Professor Lipke will collaborate with a computational chemistry expert, Professor Richard C. Remsing, who will employ molecular dynamics and density functional theory calculations to elucidate atomic level explanations of experimental observations. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.