Photon catalysis and microdroplet chemistry : applications of electric fields to influence reaction dynamics
Keywords: Chemistry.

A large part of chemistry is driven by localized electric fields between charged species, for example when an electron is exchanged in between a redox reaction pair, or a nucleophile attacks a positively charged part of a molecule. Ideally, scientists try to not only observe and understand these processes, but also gain control over them in order to steer reactions in a certain direction. In this thesis, the use of electric fields to influence reactions was studied in several reactive systems, including various chemical environments and two different ways to apply electric fields. The first part of this thesis work investigates the impact of an electric field generated by a focused laser pulse of nonresonant radiation, which is referred to as photon catalysis. The second part explores other ways to apply electric fields to chemical reactions: on the surface of microdroplets, and by voltage applied to a semiconductor on the nanoscale. Chapter 1 introduces the concept of photon catalysis. It outlines how a focused laser pulse of nonresonant radiation can act as a catalyst through its strong electric field: no photons are used up or changed in the interaction, but upon the application of the electric field laser, particular reaction pathways can be favored over others, resulting in a net change in the reaction dynamics. Furthermore, Chapter 1 lays the groundwork for the following chapters by characterizing the electric field generated by a nanosecond laser pulse, and by describing the experimental setup for photon catalysis on gas-phase reactions. It also presents the first studies on photon catalysis carried out in this lab, and defines which properties of a reactive system render it a good candidate to showcase the photon catalytic effect. Chapter 2 shows how the application of a nonresonant, focused laser pulse of infrared radiation changes the outcome of the dissociation of deuterium iodide. Depending on the excitation wavelength, deuterium iodide can dissociate via different reaction pathways after the excitation, yielding a product mix of two distinct deuterium (D) species. The relative product ratio at which these two species are formed is changed in the presence of the electric field supplied by the infrared laser pulse, indicating a change in the reaction dynamics. The magnitude and the direction of change are dependent on the excitation wavelength. The underlying mechanism for the change is explored both experimentally and theoretically, and there is an agreement that the observed effect is rather caused by an AC-Stark shift of the potential energy surfaces, than by molecular alignment of the reactants. Chapter 3 continues to explore photon catalysis by extending the studies to a more complex molecule, phenol. The photodissociation of phenol along the OH bond involves two well-characterized dissociation pathways, which are populated to different degrees depending on the excitation wavelength. The reaction products are phenoxy radicals and hydrogen atoms of characteristic speeds. In this study, two features in the potential energy landscape are probed: a conical intersection, and the minimum energy threshold that it requires to dissociate the molecule. Similarly to lowering an activation barrier, the conical intersection is lowered by the electric-field induced Stark-shift, generated by the focused, nonresonant laser pulse. Therefore, the pathway that lies higher in energy is opened up wider than under field-free conditions. The dissociation origin experiences a smaller Stark-shift, yet allows for phenol dissociation at a wavelength that is not sufficient to yield any dissociation under field-free conditions. The postulated mechanism is supported by theoretical calculations. Chapter 4 transfers the concept of photon catalysis from gas-phase reactions to solution-phase systems. It outlines the changes and challenges of the chemical environment that reactants and the laser beam face, and proposes potential experimental setups. Following successful setup development, the impact of the electric field on the photoisomerization of stilbene is investigated: When a solution of cis-stilbene (CS) in cyclohexane is irradiated with ultraviolet photons, photoisomerization to trans-stilbene (TS) is promoted, and an irreversible ring-closure reaction to form phenanthrene (PH) is observed. At wavelengths around the red absorption onset, the TS formation is increased by the application of the electric field laser, and at excitation wavelengths in the center of the absorption range, the CS is increasingly converted to both TS and PH. This change is partially due to local heating in the reaction solution, which can be subtracted as background at the edge wavelengths, but is overwhelming at the absorption center. Multiphoton processes are not observed in measurable amounts. The end of this chapter highlights the promising perspectives for further use and development of photon catalysis in condensed-phase systems. Chapter 5 approaches a different application of electric fields in chemistry: the conversion of low-value polycyclic aromatic hydrocarbons to compounds with higher petrochemical utility. A new method to obtain higher conversion rates is proposed, which involves two ways of how electric fields can be applied in chemical reactions. First, the reactant solution is sprayed with a sheath gas from a small nozzle, generating micron-sized liquid droplets that exhibit strong electric fields on the surface, enhanced by an electric double layer if the solution contains water. Second, these droplets subsequently hit immobilized anatase nanoparticles, which are charged with 2 kV, and continuously wetted. The applied voltage results in an electron-hole separation, where the oxidative hole converts the water to highly reactive hydroxy radicals. The combination of the electric field on the microdroplet surface with the hydroxy radicals is required to obtain high degradation yields of the sample molecule rubrene. The method is validated on selected other molecules as well.