X-ray Transient Absorption Spectroscopy: Characterizing the Excited-State Structural Dynamics of Transition Metal Complexes

With the development of 3rd generation synchrotron sources and more advanced laser systems, ultrafast pump-probe experiments implementing an X-ray probe are now feasible, opening the door to new methods for excited-state structural characterization. X-ray transient absorption (XTA) spectroscopy measures the structure around a unique atomic center in a system in any medium while simultaneously measuring the electronic structure of that unique atom. When combined with time-resolved optical spectroscopy, such as optical transient absorption (TA), the excited-state dynamics of any system can be fully characterized, potentially guiding the design of more effective solar energy harvesting devices. In this thesis, the excited-state structural and electronic dynamics of two transition metal complexes are analyzed. The correlation between their atomic rearrangements and the functions of these chemical systems is deciphered, answering long-standing fundamental questions regarding their dynamics and reaction mechanisms.;The first portion of this work focuses on myoglobin and iron protoporphyrin IX and the dissociation/recombination dynamics of carbon monoxide to their iron centers. The properties of these systems differ due to the presence of the protein macrostructure, which is present for myoglobin and absent for porphyrin. The ligand recombination dynamics are measured by a combined optical TA and XTA approach and the heme reorganization associated with the formation of the "domed" structure upon ligand loss are measured. The effect of the protein globin on these structural reorganizations is discussed.;The second portion of this work focuses on the relationship between ground-state structure and excited-state dynamics of copper(I) bisphenanthrolines. Changing the substituents at the 2,9 positions of the phenanthroline ligand causes massive alterations in the ground state structure, optical absorption, and excited-state dynamics. The photophysical properties of two such complexes, the tetrahedral [Cu(I)(dtbp)2]+ (with tBu substituents), and the heavily flattened [Cu(I)(dpp)2]+ (with phenyl substituents), are probed using TA, XTA, and TD-DFT methods. The relationship of these different groups on the Jahn-Teller states are discussed, as well as their effects on the flattening, intersystem crossing, and phosphorescence rates. Carboxylated forms of [Cu(I)(dpp)2]+ are attached to TiO2; ultrafast electron injection into the TiO 2 is observed, and conclusions are drawn regarding the electron injection mechanism.