Crystal engineering has recently emerged as a method of choice for the design and the construction of functional materials. Solid-state synthesis, of the most commonly studied aspects of crystal engineering, has been shown to provide access to molecular targets that are hardly obtainable using principles of conventional (i.e. solution-based) organic synthesis. Reactions in the solid state are, however, not routinely used in organic synthetic chemistry. The scarce use of solid-state reactions can be attributed to the difficulty of predicting molecular arrangements in the solid state, as well as to the lack of methodologies to control crystal packing. Template-directed solid-state synthesis is a recently developed modus operandi that enables control over reactivity within multi-component crystals. The thesis is focused on the application of template-directed solid-state approach to [2+2] photocycloaddition reactions in the solid state, as well as on the understanding of intermolecular interactions in crystals. Synthetic templates have been utilized to construct cocrystals that enable a class of hitherto underdeveloped organic solid-state reactions, namely [2+2] cross-photoaddition reactions. In addition, products derived form templated solid state reactions, namely tetrapyridylcyclobutanes, have been utilized to generate exceptional materials, such as thixothropic hydrogels based on nano-dimensional metal-organic particles. The utility of crystal engineering has also been expanded to the nanoscience and the development of nanomaterials. A crystallization method for the preparation of nano-dimensional cocrystals has been developed. The method has been shown to enable single-crystal-to-single-crystal [2+2] photodimerizations of olefins. In addition, nano-dimensional cocrystals have been shown to exhibit distinctive mechanical properties upon single-crystal-to-single-crystal transformations.
In addition to solid-state reactions and materials derived therefrom, we systematically studied hierarchies of supramolecular synthons in pharmaceutical cocrystals comprised of multi-functional molecules. Pharmaceutical cocrystals have been recently shown to exhibit physical properties superior to those of parent drugs. Our studies involved xanthine alkaloids as pharmaceutical agents and a series of hydroxylated benzoic acids as cocrystal formers. Synthon hierarchies have been established for three xanthine alkaloids. We also discovered pharmaceutical salts that formed where cocrystallization was expected to occur. Reasons contributing to such unexpected salt formation were investigated using X-ray crystallography and computational methods. The established synthon hierarchies are expected to contribute to a better understanding of self-assembly processes in cocrystals that is crucial for the development of state-of-art drugs, and the design of organic reactions in the solid state.