Molecular transport, reactivity and structure in organic crystals

The work presented in this thesis is based on studies concerning structural properties of molecular solids, and is composed of three different parts. First, structural aspects of solid state dimerization reactions have been studied using 1H NMR and powder X-ray diffraction. The experiments are based on the photoreactions of cinnamic acids in the solid state. These materials show photochemical behaviour upon irradiation by UV light. The /-phase (photoreactive polymorph) of 4-cMoro-/ra s-cinnamic acid was chosen as the primary material for study, and a combination of powder X-ray diffraction and solution state 1H NMR spectroscopy techniques have been used to follow the evolution of the solid-state photoreaction, and to determine the structural evolution of the photoproduct. In addition, the structure of a photoreactive material (the /-phase of 4-fluoro-/raws-cinnammic acid) that cannot be prepared as a single crystal suitable for single crystal X-ray diffraction has been determined directly from powder X-ray diffraction data. This research exploits modern developments in the application of powder X-ray diffraction techniques in order to obtain new levels of understanding of a classical solid state reaction. The second part of this thesis is based on the investigation of transport of molecules through linear tunnels in solid organic inclusion compounds (host-guest systems). This project is focused on incommensurate inclusion compounds in which guest molecules are contained within a system of one-dimensional tunnels (with a diameter of about 5.5 A) in a crystalline urea host structure. It was shown previously that net transport of guest molecules in one direction along the crystal can be achieved by insertion of new guest molecules at one end of the crystal (by dipping it into the liquid phase of another potential guest), with the original guest molecules expelled from the other end of the crystal. Such phenomena have considerable potential as the basis of selective micro-scale separation techniques, based on discrimination of molecular size, shape and chirality. In the present work, the spatial distribution of the guest molecules in the crystal has been studied together with the changes in the spatial distribution of the guest molecules as a function of time during the transport process. This process has been investigated directly by using confocal Raman microspectrometry as an in situ probe of the molecular transport process. The data obtained from such experiments have provided access to information on the kinetics of the transport process and the time-dependence of the spatial distribution of guest molecules within the channel system. A model has been developed to understand the kinetics of the transport process, leading to detailed mechanistic insights. The last part of the thesis is focused on the design of hydrogen-bonded organic complexes. Underlying all structural aspects of organic molecular crystals is the fundamental question of what factors control the molecular packing arrangement, and an important related issue is how to apply an understanding of these factors in the design of crystals with desired structural properties. The work has focused on the study of components with hydrogen bonding capability (urea/diols and /-phenylenediamine/diols), with a view to the formation of hydrogen-bonded complexes with predictable structural features. Optical microscopy, single crystal X-ray diffraction, powder X-ray diffraction and confocal Raman microspectroscopy have been applied in this work