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Spectroscopic Studies of Ground and Excited State Mixed Valence


Mixed valence systems have two charge bearing units (M) that share a radical electron. Coupling between the sites is mediated by a covalent bridge (B). These systems have been extensively studied since the 1960's on account of their interesting electronic structure and as an ideal system for understanding intermolecular electron transfer. One of the characteristic signatures of mixed valence is the intervalence charge transfer (IVCT): a low energy electronic absorption where the radical electron is formally transferred from one site to the other. However, this phenomenon is not unique to the ground state and has also been shown to occur in the excited state. In the electronic absorption spectrum excited state mixed valence manifests as a set of two transitions. The first three studies presented in this dissertation will focus on understanding the excited state geometric distortions upon photoexcitation of ground state and excited state mixed valence systems.

The first chapter of this dissertation is an introduction to the underlying principles and history of excited state mixed valence. Important tools to analyze and interpret mixed valence systems which are used throughout the remainder of this dissertation are presented including the neighboring orbital model and the time-dependent theory of spectroscopy. Several aspects of these tools as they apply to the coupling and transition dipole moments are presented through three historical models and additional pedagogical examples.

The following three chapters are fundamental studies of mixed valence systems. The first study investigates the relationship between ground and excited state mixed valence in the radical anion of 9,9-dimethyl-2,7-dinitrofluorene. Apparent similarities in the absorption spectrum are analyzed using time-dependent theory, the neighboring orbital model, and resonance Raman spectroscopy. The second study reviews the excited state mixed valence of diisopropyl ditolyl radical cation. This study presents an essential expansion of the transition dipole moment analysis that is typically applied to excited state mixed valence systems. The third mixed valence study examines the importance of coordinate dependent coupling in a dialkylaniline ether. The ground state geometry of this compound should forbid any coupling between the aniline moieties, however, movement along a low frequency twisting coordinate facilitates coupling giving rise to mixed valence transitions.

The final two chapters of this thesis are applied spectroscopic studies. The first applied study characterizes the weak electronic interaction between iron and ruthenium in a ferrocene complex. This is achieved by combining electronic absorption spectroscopy with Raman spectroscopy in resonance with a near IR iron-ruthenium charge transfer. The profile of the iron-ruthenium Raman stretch aids in the assignment of the electronic transition. The second applied study the photoisomerization of an azobenzene based linker in metal organic framework. Photophysical studies are present to understand the dynamics of the linker and subsequently demonstrate the ability of the metal organic framework to store molecular cargo and release it on demand.

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