Photo initiated molecular processes elucidated by quantum chemistry and theoretical spectroscopy

Rott, Florian (2022): Photo initiated molecular processes elucidated by quantum chemistry and theoretical spectroscopy. Dissertation, LMU München: Fakultät für Chemie und Pharmazie

Abstract

Processes initiated by the interaction between light and matter are a fundamental step in various chemical, physical and biological phenomena. The present work investigates the photoinduced processes in artificial molecular machines and small molecules with the help of quantum chemical calculations. The research was performed in close collaboration with experimentalists, allowing an in-depth look at the underlying mechanisms of these ultrafast processes. The first part addresses the relaxation pathways after photoexcitation of the photoswitch hemithioindigo (HTI) and the artificial molecular motors, motor-1 and motor-2. The pho- tochromic compound HTI is a novel photoswitch capable of performing efficient isomerization upon irradiation with non-damaging visible light. Based on time-resolved absorption and emis- sion experiments and supported by high level quantum chemical calculations, a comprehensive reaction model for its photoisomerization, including the effects of different solvents as well as substitutions, is established. The structure of both molecular motors, motor-1 and motor-2, is based on the HTI moiety. By clever design, this switch was turned into a molecular motor, capable of unidirectional rotation. These motors are among the first light-powered molecular motors that operate under ambient and non-damaging conditions. The underlying processes for their multistep rotation was elucidated through multiscale broadband transient absorption mea- surements and quantum chemical investigations of their excited state potential energy surfaces. From these findings, pathways to improve the rotational speeds and efficiency of light-driven molecular motors in general could be developed. The second part of this work addresses the theoretical simulation of the ultrafast spec- troscopy technique known as attosecond transient absorption spectroscopy (ATAS). Attosecond pulses in the extreme ultraviolet (XUV) or X-ray region provide a powerful tool for investigating ultrafast nuclear and even electron dynamics in atoms, molecules and solids. Due to their high photon energy, they are able to create electron wave packets extremely well localized in time. This makes them an excellent choice for triggering photochemical reaction in a pump-probe scenario. Further, their broad bandwidth provides element, charge and electronic state sensitive insights by probing the inner-valence and core-level states of the excited molecules. To aid the interpretation of the experimental data and provide further insights into these complex inter- actions between light and matter, a comprehensive framework simulating XUV/X-ray transient absorption spectra is presented. Using ab initio non-adiabatic molecular dynamics (NAMD), the ultrafast processes of excited molecules after laser excitation is simulated, enabling the res- olution of both the changes in the electronic structure and the nuclear motion over time. Based on this information, the time-dependent XUV/X-ray transient absorption spectra are calculated by applying high-level multi-reference methods, namely restricted active space self-consistend field (RASSCF) and restricted active space perturbation theory (RASPT2). This framework is utilized in the two studies on the molecules vinyl bromide and trifluoroiodomethane. For both molecules the ultrafast coupled nuclear-electron dynamics after strong-field ionization could be explained in great detail.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: quantum chemistry, ultrafast chemical reactions, photoswitches, molecular motors, theoretical spectroscopy
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 540 Chemie
• Fakultäten: Fakultät für Chemie und Pharmazie
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 28. April 2022
• 1. Bericht­erstatter:in: Vivie-Riedle, Regina de