Ultrafast Aqueous Solvation Dynamics around Photo-active Organic and Inorganic Complexes

Most chemical reactions occur in solution, yet the solvent is often considered a mere spectator rather than an active participant. Understanding the molecular interactions between reactants and their surrounding solvent can advance the development of new technologies such as solar-panel efficiency, and optoelectronics. This thesis employs both computational and experimental methods to explore the solvent’s role in light-induced processes and how measuring the change in solvent heat can be utilized to estimate electronic state energies.

The first part of this thesis investigates the solvent shell response to an electron transfer to the solvent from the photoionized hallide, one of the structurally simplest solute systems for studying aqueous solvation. The structural dynamics of the solvent is observed with Time-Resolved X-ray Solution Scattering (TR-XSS). The findings reveal that the solvation structure around iodine (I0) involves a solvent shell expansion of approximately 0.5 Å for the nearest solvent oxygen atoms compared to iodide (I⁻), with the best-fit Lennard-Jones parameters of ϵ = 0.6 kcal/mol and σ = 3.75 Å. Additionally, the study shows that water molecule rearrangement around iodine occurs within 0.5–1 ps, with an initial delay due to the breaking of the hydrogenbond network. The average solvated electron ejection radius is 5±3 Å, with primary geminate recombination occurring around 20 ps.

The second part of this thesis investigates the solvent shell response to intramolecular charge transfer. The research utilizes surface hopping dynamics to simulate the excited state dynamics of an aqueous-solvated organic solute molecule (CBQ, 6-cyanobenzquinuclidine). A total of 10000 ground state trajectories are simulated over 10 ps, where 3728 of them are excited and simulated further over a period of 10 ps. The excited state simulations show strong solvent coordination to the donor atom, which disappears after photoexcitation. Around 1% of the excited trajectories populate the triplet state after 10 ps, while the rest remain in the lowest excited singlet state. Grouping the trajectories into two sets based on the electronic state at 10 ps reveals different solvation structures between the two trajectory sets. The trajectories ending in the singlet state show changes in solvation structure after photoexcitation, while those ending in the triplet state do not. Coordination number analysis indicates a correlation between the solvation structure and the populated electronic state. Normal mode analysis shows no differences between the two trajectory sets in the ground state but identifies four normal modes that differ between the two trajectory sets in the excited state. These four normal modes all show significant movement of the donor atom.

The final part of this thesis explores how changes in solvent heat, measured by TR-XSS, can be used for calorimetry to determine the energy difference between electronic states. Based on TR-XSS measurements of seven modified molecular spin switch systems ([Fe(4’-R-terpy)₂]²⁺, terpy = 2,2’:6’,2”-terpyridine, R = substitution group) in aqueous solution, the estimated lifetimes and excitation fractions align with results from other experimental techniques, and fall within the expected range from theoretical calculations. However, the estimated solvent temperatures are overestimated in comparison to the expected energies involved, resulting in an overestimation of the quintet energies. Literature review indicates a consistent overestimation of solvent temperatures for similar TR-XSS studies and various causes for this are discussed. Due to the insufficient TR-XSS data quality, the quintet energies remain inconclusive. The measurements are ultimately utilized for ligand-dependent structural characterization of the excited-state quintet structure of the seven modified systems. The results show that the structural change from ground to quintet state is unaffected by the substitution group in the 4’ position of the terpy ligands. However, the quintet lifetimes exhibit significant variation, ranging from 1 to 16 ns. Specifically, the most electron-withdrawing group (SO₂−Me) corresponds to the shortest lifetime, while the most electron-donating group (OH) is associated with the longest lifetime.