Abstract
In contrast to ground state (thermal) chemistry where internal energy is statistically distributed among the molecular degrees of freedom, photo-activation of a reaction entails initial localization of the internal energy in a highly non-statistical manner. The result is often reaction rates faster than those predicted by statistical models, but also a more critical dependence between molecular structure and (photo)-reactivity. This thesis presents a collection of studies of various types of photoinduced processes in neutral, anionic, or cationic molecular species as introduced below. The investigations have primarily employed femtosecond time-resolved photoionization or photodetachment techniques.
Intersystem crossing (ISC) in neutral organic species is conventionally assumed to be slow due to the spin-forbidden nature of the process; this assumption has been challenged during the past decade by several observations of ultrafast ISC in organic molecules. In this thesis nitro-aromatic compounds, methylated benzene-derivatives, and small esters comprise the three classes of molecules whose photoinduced properties collectively establish the foundation for discussing whether the current description of ISC between excited states of organic molecules is adequate. The investigations presented here indicate that early activation of specific molecular modes in some cases may provide a means for ISC to become ultrafast and capable of competing with processes such as (spin-allowed) internal conversion.
Nucleobase anions are posited to be involved in DNA damage, where the molecular dipole moment of a nucleobase acts as electron-antenna and gateway for low-energy electrons to access the valence system of DNA. The investigations presented herein explore the abilities of adenine, thymine, and uracil to capture electrons and subsequently transition from dipole to valence-bound anions in different energy regimes. The investigations indicate that dipole-bound anion formation depends strongly on the magnitude of the molecular dipole moment, while the formation and stability of valence-bound anions depends on ring-puckering of the nucleobases. In certain cases specific ring-puckering can stabilize the valence-anions to prolong their lifetimes significantly, even when formed in the scattering continuum.
The study of cation dynamics aims at understanding peculiar isotope effects in the decomposition of ionized dihaloalkanes. Br-C-Br bending is activated by pump-induced ionization, which shows up as oscillating ion transitions arising from probe-induced fragmentation. The source of the isotopic preference is analyzed by tracking temporal changes in the contribution from each isotopomer to the total fragment-ion signal. Preliminary studies indicate that this approach can provide new insight into the origin of the isotope effect; the investigation thereby constitutes a novel way of investigating isotope effects.
Common for all reactions discussed in this thesis is that the early-time dynamics initiated by photon-absorption have large impact on the evolution of the ensuing reaction. The diversity of the processes studied illustrates the versatility of the experimental method as a means to increase knowledge on the complex interplay between structural dynamics and (photo)-reactivity.
Intersystem crossing (ISC) in neutral organic species is conventionally assumed to be slow due to the spin-forbidden nature of the process; this assumption has been challenged during the past decade by several observations of ultrafast ISC in organic molecules. In this thesis nitro-aromatic compounds, methylated benzene-derivatives, and small esters comprise the three classes of molecules whose photoinduced properties collectively establish the foundation for discussing whether the current description of ISC between excited states of organic molecules is adequate. The investigations presented here indicate that early activation of specific molecular modes in some cases may provide a means for ISC to become ultrafast and capable of competing with processes such as (spin-allowed) internal conversion.
Nucleobase anions are posited to be involved in DNA damage, where the molecular dipole moment of a nucleobase acts as electron-antenna and gateway for low-energy electrons to access the valence system of DNA. The investigations presented herein explore the abilities of adenine, thymine, and uracil to capture electrons and subsequently transition from dipole to valence-bound anions in different energy regimes. The investigations indicate that dipole-bound anion formation depends strongly on the magnitude of the molecular dipole moment, while the formation and stability of valence-bound anions depends on ring-puckering of the nucleobases. In certain cases specific ring-puckering can stabilize the valence-anions to prolong their lifetimes significantly, even when formed in the scattering continuum.
The study of cation dynamics aims at understanding peculiar isotope effects in the decomposition of ionized dihaloalkanes. Br-C-Br bending is activated by pump-induced ionization, which shows up as oscillating ion transitions arising from probe-induced fragmentation. The source of the isotopic preference is analyzed by tracking temporal changes in the contribution from each isotopomer to the total fragment-ion signal. Preliminary studies indicate that this approach can provide new insight into the origin of the isotope effect; the investigation thereby constitutes a novel way of investigating isotope effects.
Common for all reactions discussed in this thesis is that the early-time dynamics initiated by photon-absorption have large impact on the evolution of the ensuing reaction. The diversity of the processes studied illustrates the versatility of the experimental method as a means to increase knowledge on the complex interplay between structural dynamics and (photo)-reactivity.
| Original language | English |
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| Publisher | Department of Chemistry, Faculty of Science, University of Copenhagen |
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| Number of pages | 214 |
| Publication status | Published - 2017 |