Mehdi Esmaeili PhD Defence

Thesis Title: Photoreactivity of Crystalline Unsaturated Compounds: Reaction Pathways and Kinetics

Abstract:

     This research explored how the photoreactivity of unsaturated molecules in the solid-state can be manipulated and studied. In this thesis, we explored the photoreactivity of selected unsaturated compounds in different crystalline forms, establishing protocols and methodologies for quantitative kinetic measurements.
     To carry out this research, we first screened the cocrystallization and photoreactivity of selected reactants in both pure crystalline and cocrystalline forms using the PXRD method. Subsequently, we conducted a series of qualitative and quantitative experiments utilizing 1H NMR and qHNMR spectroscopy to identify photoproducts and determine reaction kinetics. The obtained results were then utilized for modelling to analyze the solid-state photoreaction mechanism and to extract kinetic parameters.
We conducted a detailed investigation into the photoreaction kinetics of seven different systems, including four crystalline and three cocrystalline solids. The progression of reactions was documented both qualitatively and quantitatively by analyzing the compositions of the reaction mixture over time. The constructed kinetic curves revealed multiple concurrent reaction pathways in most of the systems studied.
     Four unsaturated carboxylic acids from the initially selected compounds exhibited photoreactivity in both crystalline and cocrystalline forms. Incorporating these four reactants into five different
dipeptides resulted in the formation of cocrystals in all the 20 mixtures while maintaining their photoreactivity. The statistics obtained demonstrate a high success rate in cocrystal formation while preserving photoreactivity. At the same time, we observed significant differences in reaction pathways and kinetics between reactant crystals and their cocrystals with dipeptides. These results revealed that the cocrystallization of reactants can serve as a tool to control their photochemical reactivity.
     Analysis of the measured absorption spectra of the selected compounds in both solution and the solid-state revealed substantial differences in the absorptivity and, thereby, the influence of molecular arrangements on the UV absorption of molecules. In all cases, the energy required to excite the reactant molecule was lower in the solid-state than in solution.
The novelty of this research is its thorough exploration of solid-state photoreactivity, utilizing dipeptide-based crystals as reaction matrices for the photoreactant molecules, comprehensive kinetic studies of concurrent photoreactions in the solid-state, and employing modelling to analyze quantitative data from systems showing complex or multiple reaction pathways.