Dissertation/Thesis Abstract

Interpreting Time-Resolved Spectra and Excited State Reactivity with Computational Methods
by Lamb, Robert, Ph.D., Mississippi State University, 2020, 281; 27838297
Abstract (Summary)

Light-harvesting compounds are developed for a variety of purposes pertaining to areas such as energy-capture, chemical transformations, and lighting. There is a need to better understand the reactivity and excited state properties of these compounds. Many experiments focus on gleaning information about reactivity by observing spectral changes over time intervals ranging from femtoseconds to minutes (IR, UV-VIS, and UV-VIS pump-probe spectroscopy). This dissertation focuses on the interpretation of the experimental data from a computational perspective and methodological studies to determine reasonable levels of theory for each system. Three vignettes of this approach will be discussed.

First, a mononuclear tungsten complex was found to be capable of self-sensitized catalytic H2 production. Experimental mechanistic studies employed time-resolved IR spectroscopy to capture spectral signatures of potential catalytic intermediates. DFT computational methods were utilized to predict geometries, energies, and harmonic stretching frequencies of a variety of catalytic intermediates that correlate rather well with experiment.

For studying the excited state, the prototypical system that is both well-known and well-behaved is the [Ru(bpy)3]2+ ion. This complex undergoes a MLCT excitation and ultimately forms a long-lived 3MLCT state with a lifetime on the order of ┬Ás. While several computational studies exist, a systematic study on what dictates an appropriate level of theory for correctly describing this system is absent from the literature. We conduct a systematic study of a series of DFT functionals and basis set combinations to evaluate the relative energies of the MLCT and MC states, as well as correctly predicting the character of excitations observed in the transient UV-VIS spectra of the MLCT excited state.

Finally, the absorption and emission spectra of a series of polycyclic aromatic azaborines were simulated and compared to their experimental values. Experimentally, some compounds exhibit large, solvent-dependent Stokes shifts consistent with CT excitations. Unfortunately, the excited state chemistry is not so straightforward and some of these compounds may become deprotonated in the ES, thus resulting in a charge-separated state. As a by-product of this project, the results from each method suggest that, contrary to literature precedent, typical hybrid functionals appear to overestimate the CT character of the computed excitations.

Indexing (document details)
Advisor: Webster, Charles E.
Commitee: Gwaltney, Steven R., Cui, Xin, Creutz, Sidney, Montiel, Virginia
School: Mississippi State University
Department: Chemistry
School Location: United States -- Mississippi
Source: DAI-B 81/11(E), Dissertation Abstracts International
Subjects: Chemistry, Inorganic chemistry, Computational chemistry
Keywords: Absorption, Emission, Excited state, Ru(bpy)3, TD-DFT, Transition character
Publication Number: 27838297
ISBN: 9798643199199
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