Investigating Excited States of Photoactive Transition Metal Complexes Using Soft X-ray Spectroscopy

Abstract

This research uses soft X-ray spectroscopy and computational chemistry to investigate transition metal complexes’ electronic structure and photophysics. Specifically, the excited states in ruthenium ([Ru(bpy)2(dppp2)]2+), chromium (Cr(acac)3), and cobalt (Cobalamin) were studied. Chapter 1 provides a general introduction to the research, while Chapter 2 details the experimental and theoretical methods used in this thesis. Chapter 3 investigates the electronic structure of the [Ru(bpy)2(dppp2)]2+ complex using time-dependent density functional theory (TDDFT), focusing on its two low-lying triplet 3MLCT excited states. The spectral features observed in the experiment were reproduced computationally, and the origin of each feature was found using difference density distributions. The results showed that the first spectral features in the excited state correspond to a state with an electron transfer from the metal to a single ligand. Chapter 4 introduces an innovative configuration for conducting femtosecond time-resolved soft X-ray absorption spectroscopy (XAS) in solution. The approach utilizes a beam-splitting off-axis zone plate (BOZ) and is denoted BOZ-XAS. A key achievement of this thesis is the presentation of the analysis and interpretation of the first results of the liquid jet implementation of the BOZ-XAS method. The successful implementation of the setup was confirmed through its application to the ultrafast photoinduced spin crossover in [Fe(bpy)3]2+. Next, the effectiveness of the BOZ-XAS method was demonstrated by measuring time-resolved XAS at the Co L-edge in a 7 mM solution of cyanocobalamin, highlighting its sensitivity and efficiency. The ground state of cyanocobalamin is low-spin Co(III) with a fully occupied t2g set. Excitation causes an electron to move to the π∗ orbital, forming the initial excited state. Subsequent relaxation processes can lead to the formation of the LMCT state. However, the similarities observed at 200 fs and 1 ps might suggest either relaxation processes occurring at the S1 potential energy level or the involvement of two electronic states with comparable spectral signatures. This result points to the need for further investigation into the relaxation mechanisms and electronic states. In Chapter 5, picosecond Cr L-edge X-ray absorption spectroscopy was employed to investigate the electronic dynamics of Cr(acac)3, focusing on metal-centered (MC) spin-flip excited states initiated by LMCT. Experimental and theoretical integration elucidated the multiplet structure at the Cr L3-edge, revealing insights into transient electronic structure and photochemical properties. The analysis highlights significant spectral differences between ground and excited states. The analysis of spin state contributions shows that the quartet nature in the ground state spectrum shifts to a doublet character in the valence excited state. This study underscores the utility of picosecond Cr L-edge XAS for understanding transition metal complex dynamics, with implications for advanced applications like molecular sensing and photocatalysis. Finally, Chapter 6 investigates the electronic structures of cyanocobalamin (CNCbl) and methylcobalamin (MeCbl) as thin film samples using resonant inelastic X-ray scattering (RIXS) and X-ray absorption spectroscopy complemented by CASSCF-NEVPT2 calculations. The static 2p3d RIXS spectra at the Co L3- edge were analyzed to study d-d excitations, revealing the 3T1g first electronic excitation for both complexes. Although there were difficulties due to the thin film damage, the computational analysis successfully pinpointed the initial spectral feature associated with the triplet transition and the subsequent feature linked to the singlet transitions.