X-ray Tracking of Electronic and Structural Dynamics During Chemical Reactions Using Free Electron Lasers

Mads Laursen

Research output: Book/ReportPh.D. thesis

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Many transition metal complexes have optical properties which can be utilized in the study of fundamental and applied photochemistry. In some cases these complexes can be used in solar cell applications and for photo-catalysis. How these complexes function in practical applications is connected to their excited state molecular structure and their excited state energy landscape. The photophysical nature of the exited state of the complex entails that the complex will inevitably relax back into the ground state, sometimes via one or more intermediate states. The energy flow associated with this relaxation process is related to the degrees of freedom and the vibrational modes of the molecule and is of significant interest in relation to the design of new compounds with tailored properties. This thesis describe the ultrafast photochemistry of several different transition metal complexes studied using Time-Resolved Wide Angle X-ray Scattering (TR-WAXS) at X-ray Free Electron Lasers (XFEL’s). First, the thesis describes the theoretical background of how X-rays can be utilized to reveal the structure of matter and the role X-ray scattering for the study of the photochemistry of transition metal complexes in solution. The technique of TR-WAXS is outlined and the associated data reduction and treatment is described. Thefirstsystemstudied,usingTR-WAXS,istherutheniumbasedcomplex,[Ru3(CO)12]. Following photo-excitation, this complex undergoes a series of structural changes, includingbondbreakageandreformation. Ourexperimentsandanalysisaddresshowthis reaction process occur on the <50 ps timescale. The experimental data is modelled using density functional theory (DFT) and molecular modelling. From our analysis several intermediate structures are suggested including a previously not reported intermediate. All intermediates suggested by our analysis, save the new intermediate, are reported by complementary optical techniques. Thus, combining the new XFEL based results with existing literature generate a suggestion for a complete and consistent picture of the photo-fragmentation cycle of [Ru3(CO)12]. The second system studied is di-platinum complex, [Pt2(P2O5H2)4]−4 (abbreviated PtPOP) coordinated to thallium (Tl-PtPOP). The ultrafast picosecond (<5 ps) dynamics of the system was studied with optical Transient Absorption and TR-WAXS. Time-domain Fourier transform of the experimental data of the two techniques show two distinct temporal modes, which was assigned the vibrational modes of Tl-Pt and Pt-Pt. The TR-WAXS data was modelled using DFT structural optimization and previously unknown ground state structure of Tl-PtPOP was suggested. Based on this structure the ultrafast structural dynamics (t<5ps) of Tl-PtPOP was interpreted from the XFEL data through a structural optimization of the excited state structure. The final part of the thesis regards the role of the X-ray energy on a TR-WAXS structuralanalysis. With the access to larger X-ray energies a typical TR-WAXS experiment has access to larger scattering vectors. How this effects a TR-WAXS structural analysis was studied through the modelling of simulated TR-WAXS experimental data. Modelling the simulated data using an incomplete model shows that the prediction power of an incomplete model increases with an increase in X-ray energy.
Original languageEnglish
Place of PublicationLyngby, Denmark
PublisherTechnical University of Denmark
Number of pages70
Publication statusPublished - 2018


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