Interrogating the Electronic Structure and Photochemistry of Molecules with Transient X-ray Absorption Spectroscopy

Thesis (Ph.D.)--University of Washington, 2014A detailed understanding of photochemical processes in molecules and materials is necessary for understanding natural phenomena and engineering new technologies. In this thesis, light-induced changes in spin states, rearrangements of electron densities, and proton transfer events are considered. X-ray absorption (XA) spectroscopy is used to investigate the electronic structure of molecules in electronic ground and excited states to reveal fundamental insights on the nature of photochemical events. The photoinduced FeII spin crossover (SCO) reaction is investigated theoretically. Following photoexcitation, an FeII SCO molecule undergoes a conversion from a singlet ground state to a quintet excited state. Transition-potential density functional theory and time-dependent density functional theory (TDDFT) are used to simulate the transient Fe K-edge XA spectra. The spectral signature each spin state is identified in the near-edge and pre-edge regions of the XA spectra. Ruthenium L-edge spectroscopy is used to investigate the electronic structure and photochemistry of solar cell dye molecules and transition metal mixed-valence complexes. The metal-to-ligand charge transfer (MLCT) chemistry is studied in the widely used "N3" dye ([Ru(dcbpy)2(NCS)2¬]) molecule. Ru L-edge XA spectroscopy shows that the formation of the MLCT state corresponds to a light-induced oxidation of the Ru atom from RuII to RuIII. Moreover, charge transfer features in the show the important role of the NCS- ligands in the photochemistry of N3. TDDFT is used to simulate Ru L3-edge spectra of N3 and other Ru complexes. This validates its use as a predictive tool for simulating Ru L3-edge spectroscopy. Finally, the Ru L3-edge spectra of the mixed-valence metal complexes [(NC)5FeIICNRuIII(NH3)¬5]- and [(NC)5RuIICNRuIII(NH3)¬5]-. Quantum chemical simulation using explicit water molecules are required to reproduce experimental spectra highlighting the role of the hydrogen bonding solvent in determining the properties of these complexes. Finally, the photochemistry of 2-thiopyridine (2TP) is investigated. 2TP exists in solution in equilibrium with its tautomer 2-mercaptopyridine (2MP). The possibility of excited state proton transfer from 2TP to 2MP is considered. The transient S K-edge XA spectra are measured for 2TP in acetonitrile. The time-dependence of the transient absorption signal shows that multiple photoproducts are formed. This establishes transient S K-edge XA spectroscopy as a power for tool for studying photochemistry in organic and biological systems