Dissertation Defense: Michael Bertocchi
Candidate Name: Michael Bertocchi
Advisor: Richard G. Weiss, Ph.D.
Part 1: Charge- and Electron-Transfer Processes of Sterically Graded Pyrenes, Part 2: Polymer Chain Dynamics Probed by Pyrenyl Excimer Fluorescence
Charge- and electron-transfer processes are among the simplest and most important class of chemical reactions because they involve directly no bond making or bond breaking. They play a key role in photosynthesis, molecular recognition, and photovoltaic applications. They have been invoked to understand fundamental aspects of complex mechanistic processes for many years. Although significant progress has been made to understand their nature, important aspects of the mechanism(s) remain nebulous. Historically, a challenge has been how to quantify the various competing factors related to electron transfer processes. They include kinetic considerations, thermodynamics, and the nature of the molecular environment.
This dissertation examines the mechanistic areas where these factors intersect and explores them in ways that allow their separation. The steric and electronic factors have been assessed quantitatively using fluorescence quenching experiments that employ electronically related pyrenyl molecules with differential steric factors as a primary tool. The quenching molecules include structurally diverse sets of tertiary aliphatic amines and N,N-dialkylanilines in organic media and iodide salts in aqueous media. Steric and electronic properties of the quenchers and the pyrenes (e.g., sizes, shapes, conformational labilities, enthalpies of hydration of ions, excitation energies, and oxidation and reduction potentials) have been correlated with the steady-state and dynamic fluorescence quenching data, supplemented with DFT calculations, to make quantitative assessments of the steric and electronic factors controlling the quenching processes.
Furthermore, the stiffness of polymer chains has been explored by probing excimer formation (N.B., a combination of charge transfer, excitation resonance, and π-electron delocalization) in two types of pyrenyl-labeled polymer backbones, poly(isobutylene-alt-maleicanhydride)s and (3-aminopropyl)methylsiloxane- dimethylsiloxane copolymers. The abilities of the pyrenyl labels to form excimers has been examined by steady-state fluorescence intensity measurements, time-correlated single photon counting experiments, and time-resolved emission spectra. The degree of excimer formation among the labels is influenced by the relative stiffness of the polymer backbone and has provided insights into the dependence of the polymer conformations and their labilities on the degree of pyrenyl substitution, the nature of the appended alkyl chains, temperature, and solvent properties. The data demonstrate the ability of fluorescence decays acquired from labeled polymers to provide quantitative insights into the local molecular environments of the pyrenyl groups.
Thursday, November 9, 2017 at 12:30pm to 2:30pm
Reiss Science Building, 238
37th and O St., N.W., Washington