Advances in Mechanistic Photochemistry: Dienes and Trienes
Redwood, Christopher Evan (author)
Saltiel, Jack (professor directing dissertation)
Cogan, Nicholas G. (university representative)
Alabugin, Igor V. (committee member)
Hilinski, Edwin F. (committee member)
Stagg, Scott (committee member)
Florida State University (degree granting institution)
College of Arts and Sciences (degree granting college)
Department of Chemistry and Biochemistry (degree granting department)
Found throughout nature, science, industry, and medicine, conjugated dienes and trienes are ubiquitous. While their behavior changes upon substitution, they are all built from the same fundamental units of either 1,3-butadiene or the 1,3,5-hexatrienes. These basic units undergo fascinatingly complex photochemistry, which is mimicked or changed upon substitution. Developing a proper understanding the photochemistry of these fundamental units, and the reasons for divergence from their basic behavior upon substitution, enables their use in optical applications and provides evidence which can be used to advance modern molecular quantum mechanics. The compounds studied are the 1,4-diphenyl-1,3-butadienes, the isomers of Vitamin D3, and the 1,3,5-hexatrienes. These compounds undergo reaction upon excitation with ultraviolet light that causes bond breakage, reordering, and reorganization. The primary process studied herein is the cis-trans photoisomerization of double bonds. This process is described primarily through the application of the one bond twist, hula-twist, and bicycle pedal photoisomerization mechanisms. Significant debate focused on finding a unifying explanation to excited state cis-trans photoisomerization currently surrounds these mechanisms. The reader is encouraged to access the abstract of the electronic dissertation to view the movies modeled for these cis-trans photoisomerizations, which are described later on page 63 of the dissertation. This dissertation is split between three parts. First, the photoisomerization of the 1,4-diphenyl-1,3-butadienes was studied in ethanol to search for evidence of conical intersections, which are considered equivalent to transition states in photochemical reactions. Second, since the validity of the hula-twist mechanism has been questioned on many occasions in the literature, the prima facie evidence used for its justification, the photoisomerizations of Previtamin D3 in volume confining media, were reinvestigated using more powerful methods. To conclude this work, the photoisomerizations of the 1,3,5-hexatrienes were reexamined, as no experimental evidence currently accounts for the majority of their photochemical decay. Surprisingly, results provided prior to this work account for less than 10% of the hexatrienes excited state decay. Extracting the information contained in this dissertation required the development and application of unique and sophisticated spectral decomposition techniques for UV-Vis, fluorescence, fluorescence excitation, and 1H-NMR spectroscopies. The results of this work suggest that conical intersections exist close to the twisted excited singlet geometries of the 1,4-diphenyl-1,3-butadienes, show that the original experimental evidence for the hula-twist mechanism was based upon a misinterpretation of the photoisomerizations of Previtamin D3, and reveal that cis-trans photoisomerization accounts for nearly all of the excited state decay of the 1,3,5-hexatrienes in solution. The impact of the first study reconciles previous reports for the photoaddition of alcohol to the 1,4-diphenyl-1,3-butadienes under a general path of addition to a short lived phantom intermediate. This mechanism is likely general and extends to similar molecules which undergo photoaddition of alcohol in the singlet state. It additionally confirms that the twisted singlet intermediate of the 1,4-diphenyl-1,3-butadienes is zwitterionic in nature, and that the bicycle pedal photoisomerization mechanism occurs in a step-wise and not concerted manner, in certain circumstances. The second study raises concern regarding the validity of the conclusions from numerous studies which reported hula-twist products, and perhaps should be reconsidered as photoisomerizations from one bond twist or bicycle pedal precursors. The concluding study signifies need for renewed interest in the photochemistry of the 1,3,5-hexatrienes by theory. The latest theoretical studies on the matter are not in harmony with the recently obtained experimental evidence.
1,4-diphenyl-1,3-butadiene, Chemometrics and Self Modeling, cis-1,3,5-hexatriene, cis-trans Photoisomerization, trans-1,3,5-hexatriene, Vitamin D3
April 19, 2016.
A Dissertation submitted to the Department of Chemistry and Biochemistry in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
Includes bibliographical references.
Jack Saltiel, Professor Directing Dissertation; Nick Cogan, University Representative; Igor Alabugin, Committee Member; Edwin F. Hilinski, Committee Member; Scott Stagg, Committee Member.
Florida State University
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