Spectroscopic Characterization of Electronic and Magnetic Relaxation Phenomena in Molecular Systems

Citation

Ribson, Ryan Dillon (2022) Spectroscopic Characterization of Electronic and Magnetic Relaxation Phenomena in Molecular Systems. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/e2mj-qs04. https://resolver.caltech.edu/CaltechTHESIS:04192022-171213261

Abstract

The thesis herein describes the application of time-resolved spectroscopic techniques to the understanding of a variety of electronic and magnetic relaxation phenomena in molecular systems. Chapter I presents the techniques and theory behind transient absorption spectroscopy and electron paramagnetic resonance spectroscopy, which are two tools that are used throughout the thesis. Chapter II recounts the study of singlet fission in a series of bipentacene dipyridyl pyrrolides, including HDPP-Pent, Li₂(DPP-Pent)₂, and KDPP-Pent. Using transient absorption and kinetic modeling, we found that deprotonation and metal coordination induced a change in the rate of singlet fission (~7 fold increase going from HDPP-Pent to Li₂(DPP-Pent)₂) and ultimate triplet yield. Chapter III details the study of the temperature-dependent magnetic relaxation studies of S = ½ spin systems copper (II) phthalocyanine (CuPc) and vanadyl phthalocyanine (VOPc). Although the spin-lattice relaxation time (T₁) of CuPc is greater than that of VOPc at low temperatures (<30 K), the CuPc T₁’s decline more substantially with temperature than those of VOPc, which we attribute to the increased spin-orbit coupling constant of Cu over V. Ultimately, the phase memory times (T₂) are T₁-limited in CuPc by 150 K, whereas room temperature coherence is observed in VOPc. In Chapter IV, 2,9-dialkyl substituted 1,10-phenanthroline complexes of Cu(I) are studied computationally to assign entatic energies to the steric contributions attributed to the ligand that dictate the electrochemical and photophysical properties of the complexes. We performed experimental validation of reduction potential, low-temperature emission bandwidth and excited state relaxation energies, and ³MLCT lifetimes to support the computational work. In Chapter V, we present ongoing work toward the characterization of triplet and triplet pair states generated via singlet fission in HDPP-Pent, Li₂(DPP-Pent)₂, and KDPP-Pent by time-resolved electron paramagnetic resonance spectroscopy in collaboration with Drs. Jens Niklas and Oleg Poluektov. Finally, in Chapter VI, we present data collected toward the photophysical characterization of a series of Ni(II) 2,2’-bipyridine aryl halide complexes synthesized by David Cagan, which are relevant for photochemical transformations. We provide supporting materials for Chapters II, III, and V in Appendices A, B, and C, respectively.

Thesis Files