Abstract

Light-induced chemical transformations are practical across many chemical and biological systems. From intramolecular hydrogen-bonding coordinates to reactions which alter the carbon framework of a molecule, the application of light can induce conformational changes that can subsequently be harnessed for a variety of tasks. However, molecular-level mechanistic insight about photo-induced chemical reactions remains difficult to acquire using current methodologies. In this dissertation, several intramolecular reactions are investigated and characterized, ranging from ground- and excited-state intramolecular proton transfers to the photochemical synthesis of a complex organic cyclization structure. The reaction coordinates are directly interrogated by utilizing ultrafast infrared spectroscopic techniques, such as time-resolved transient absorption and two-dimensional spectroscopy, which monitor vibrational motions within molecules that are highly sensitive to structural changes over the course of a reaction. First, a study on the intramolecular H-bond dynamics in a set of β-diketones based on the framework of acetylacetone was performed. For the compounds with shorter H atom donor-acceptor distances, which correspond to stronger H-bonding interactions, the calculated H atom potentials are quite soft and result in a red-shifted OH stretch vibrational mode frequency and H atom dislocation after vibrational excitation of this mode. Coupling between the OH stretch to many low-frequency normal mode coordinates was shown to contribute to the broad experimental OH stretch vibrational signature. Polarization-dependent transient absorption studies suggest significant isotopic-dependent differences on the orientational relaxation timescale, occurring around ~600 fs and ~2 ps for the OH and OD stretching regions, respectively. These timescales were interpreted as pertaining to H/D atom transfer events influenced by the presence of intramolecular structural rearrangements. Next, the proton transfer dynamics upon electronic excitation of 3-hydroxyflavone (3HF) and 3-hydroxy-2-(thiophen-2-yl)chromen-4-one (3HTC) were studied using transient infrared (TRIR) spectroscopy in a relatively nonpolar solvent (CDCl3). Rapid proton transfer dynamics (<100 fs) were determined to occur in both systems, shortly followed by vibrational relaxation of the tautomeric conformation of the electronic excited state. The faster vibrational relaxation timescale of 3HF (~1 ps) compared to that of 3HTC (~3 ps) was attributed to the greater number of vibrational modes, with coupling to low-frequency vibrational modes invoking motion along the H-atom transfer coordinate observed for the OH bend in both the ground and electronic excited state. Finally, transient infrared spectroscopy was applied to a more complex chemical reaction, tracking the kinetics and dynamics of the cyclization of an intramolecular [2+2] cycloaddition prompted by the application of visible light. Within ~300 ps, vibrational signatures corresponding to formation of the cyclized product were observed. Application of a four-step sequential reaction model via global lifetime analysis returned evolution-associated spectral profiles, the first two of which are hypothesized to derive from fast vibrational relaxation from the initial excited electronic state (~160 fs) and internal conversion (~3 ps). Following these steps, a pivotal intermediate state or species appears to form on a ~30 ps timescale, possibly caused by an intersystem crossing into the triplet manifold. This intermediate then undergoes a rate-limiting transformation, ultimately resulting in product formation on a timescale of ~260 ps. With this information regarding the occurrence of rapid product formation and possible mechanistic insights, practical measures can be taken in macroscopic synthetic procedures to optimize the efficiency of product formation for this and similar chemical reactions.

Committee Chair

Joseph Fournier

Committee Members

Courtney Reichhardt; Joseph Fournier; Kade Head-Marsden; Richard Loomis; Richard Mabbs

Degree

Doctor of Philosophy (PhD)

Author's Department

Chemistry

Author's School

Graduate School of Arts and Sciences

Document Type

Dissertation

Date of Award

12-8-2025

Language

English (en)

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