Ultrafast electronic and coupled electronic-nuclear dynamics of solvated metal complexes

The work presented in this thesis concerns the topic of ultrafast electronic and structural dynamics of solvated transition metal complexes induced by absorption of visible light. Methods of investigations include time-resolved X-ray scattering experiments conducted at an X-ray Free Electron Laser (XFEL), along with dynamics simulations of solvated metal complexes, at several levels of theory.

In particular, the results of structural dynamics related to excited states of metal-to-ligand charge transfer (MLCT) character and the interaction of the metal complexes with solvent are presented. The first part of the thesis describes the basics of the experimental work employing time-resolved wide angle X-ray scattering (TR-WAXS) measurements conducted at XFELs.

The experimental part describes the background theory of X-ray scattering as a useful tool to determine changes in molecular structure, and how we apply the technique to study the structural dynamics of transition metal complexes in solution. The second part covers the relevant aspects of the theoretical concepts of the computational methods for the studied transition metal complexes.

It covers the different levels of theory involved in the study, including different ways to simulate the solvent, and how we employ simulations to study ultrafast structural dynamics. The third part of the thesis is focused on the investigations of the structural dynamics of the two main systems under study, namely the [Ru(bpy)3]2+ and [Fe(bpy)(CN)4]2- complexes (bpy = 2,2’-bipyridine) inaqueous solution.

The studies of [Ru(bpy)3]2+ concerns the analysis of the early (<3.5 ps) times after photoexcitation from ultrafast TR-WAXS measurements. The analysis uses the methods described in part I, and several of the computational tools presented in part II, to create a model and compare to the experimental data.

The results show only small structural changes (0-0.01 Å) of the bond lengths of the [Ru(bpy)3]2+ system in the MLCT excited states, following absorption of visible light. However, despite the small structural changes in the system, the structural dynamics give rise to a quite significant change in the measured scattering signals upon photoexcitation.

The observed features in the signals are suggested to arise from structural changes in both the solute and solvent, with indications of the solvent response arising faster than the solute structural response. The studies of [Fe(bpy)(CN)4]2- in solution mainly employ ultrafast excited state dynamics simulations of the earliest 700 fs after excitation.

Results include the simulated absorption spectrum, which shows good agreement with experiment, along with the time-dependant electronic populations, charge transfer character of the excited states, radial distribution functions (RDF), hydrogen bond analysis and calculated time-resolved difference scattering signals.

Excitation into the lowest energy band of the absorption spectrum leads to populations of excited states of predominantly 1MLCT character, which relaxes to triplet states of mainly 3MLCTand 3MC character in a branched decay mechanism. The study finds that 3MC states are populated with a time constant of 0:53 ± 0:09 ps and the overall intersystem crossing time is 0:21 ± 0:01 ps.

The RDFs shows a strong interaction between the solute and solvent through hydrogen bonding to the cyanides, which weakens following photoexcitation. The structural changes of the solute within the first 100 fs are small (∼ 0:05 Å), in accordance with the observation of low structural changes associated with population of MLCT states.

For both systems, the nature of the solvent effects the photoinduced dynamics and thus the local environment plays a more important role than previously anticipated. For future investigations of structural dynamics related to MLCT excited states of transition metal complexes, the solvent effects should also be considered.