A Direct Dynamics model was developed to represent the
processes exhibited in molecular complexes of significant
size and intricacy.
Light-induced chemical processes are accompanied by molecular motion at the femtosecond time scale. Uncovering this dynamical motion is central to understanding chemical reactions on a fundamental level. The thesis focuses on the excess excitation energy dissipation via dynamic changes in molecular structure, vibrations and solvation. The studies are necessary for understanding data from X-ray Free Electron Laser (XFEL) experiments, which have become possible at new international large-scale facilities.
Working towards discovering the nature of the chemical reaction entails scrutinizing molecular systems of increasing size and complexity, which again motivates the need for development of new theoretical tools.
The main part of the work concerns the development of a Direct Dynamics model which combines the quantum mechanical description of Density Functional Theory (DFT) with the classical Molecular Mechanical method to represent the processes exhibited in molecular complexes of significant size and intricacy. The term Direct Dynamics arises from the notion that the atomic motion is simulated by directly calculating the classical forces on the nuclei, influenced by the explicitly calculated electronic density of the Quantum Mechanical part of the system. The resulting atomic motion is collected into trajectories which can be analyzed to reveal information about the transient changes.
The method was employed to simulations of two transition metal complexes in solution, to uncover their energy dissipation channels, and how they are affected by the solvent. The main complex studied was [Ir2(dimen)4]2+. In this bi-metallic Iridium complex, excited state bond formation results in a large Ir-Ir contraction with oscillatory behavior. Implementation of the modeling tool reproduced the experimentally observed metal pinching oscillation. Further, the experimentally obtained model was improved by analyzing the pinch, twist, and breathing modes in relation to the solvent. The system proved interesting, since the coherence lifetime was actually increased by solvation, since the solvent (acetonitrile) can block the Intramolecular Vibrational energy Redistribution (IVR), which would cause de-coherence.
Also studied was the effect of solvation on a bi-centred Ru-Co complex. Contrasted to [Ir2(dimen)4]2+ it does not have direct metalmetal interactions, and is believed to have a narrower Ground State (GS) thermal distribution of geometries. However, the structural changes in the complex due to excitation are more subtle. Both specific and non-specific solvation dynamics of the solvent shell around the complex as a consequence of the excited state electron transfer was observed. Use of the developed modeling tool proved capable of recovering several processes which could not be described by purely classical force-field methods.
The project also included participating in experiments on these systems carried out at the LCLS and SACLA XFEL facilities in USA and Japan.
In conclusion, the current limitations of the developed Direct Dynamics method were overshadowed by its efficacy. While many such studies are still confined to interpreting results from single trajectories of few picoseconds, this implementation has shown its efficiency, accuracy and power in the continuing exploration of the world of femtochemistry.
Caption: Illustration of solvation dynamics following electronic excitation of a molecule, which changes its dipole moment.