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The organic solid state is becoming more widely utilized to synthesize molecules that are inaccessible or difficult to realize from the liquid phase. A solid-state synthesis provides strict control of both geometry and stereochemistry. While essential for efficient and sustainable organic syntheses, catalysis in the organic solid state is expected to be difficult to achieve due to low diffusion rates of molecules in the crystals. However, recently a ditopic receptor was used as a small-molecule supramolecular catalyst to direct a topochemical [2+2] photodimerization of an olefin. To achieve catalytic turnover, mechanochemistry, in the form of a manual mortar-and-pestle dry grinding, had to be employed. A main obstacle in the manual grinding method is the fact that the mechanochemistry and photoreaction are conducted separately. Thus, an automated method that enables simultaneous grinding and irradiation would be preferred. However, such technology is not commercially available. We have developed a simple, readily accessible, and automated method to achieve the mechanochemical preparation of supramolecular materials. Importantly, method enables simultaneous grinding and irradiation. In effect, the vortex grinding serves as a ball mill that is UV irradiation transparent. The vortex method has been applied to the preparation of cocrystal and metal organic frameworks, as well as supramolecular catalysis. When conducted using vortex method, supramolecular catalysis proceeds four time faster than when conducted using manual grinding. Accelerated rate of catalysis is attributed to combination of external stress exerted by grinding and internal stress arising from photoreaction.
The scope of supramolecular catalysis is expanded to reaction of 2,2'-bpe, catalyzed by a similar ditopic receptor res. Importantly, in this system catalytic turnover proceeds spontaneously. The mechanism of catalysis was investigated using X-ray diffraction studies and gas-phase DFT calculations. The studies uncovered that 2,2'-bpe and 2,2'-tpcb undergo rotational motion to release accumulated stress akin to action of a supramolecular torsional spring.
We report an integration of aromatic stacks into discrete assemblies based on hydrogen bonds in the solid state. We used indolo[2,3-a]carbazole to organize aromatics into double, triple, and quadruple stacks within cocrystals. We also showed that aromatics within quadruple stack undergo topochemical [2+2] photodimerization to give a single photoproduct stereoselectively in up to maximal yield.
We reveal the first example of a product of a templated solid-state that acts as a template in subsequent reaction. Both reactions proceed in 100% yield and stereoselectively. We also report the unique case of polymorphism where both polymorphs are photoreactive and within one polymorph photoreaction proceeds in SCSC manner.
Due to effects of crystal packing, homology in solid state is difficult to achieve. We present the application of the template screening approach to solid-state photoreactivity among all olefins of a homologous series. To assess electronic effects of substituents on stability and hydrogen bonds of assemblies, we preformed gas-phase DFT calculations on a series of substituted assemblies. We found, that the strength of hydrogen-bonding varies and correlates with electronic nature of the substituents.
Lastly, we incorporated principles of supramolecular chemistry, solid-state reactivity, green chemistry and sustainability in undergraduate curriculum. In particular, students employ supramolecular chemistry to control photoreactivity in the organic solid state using the template approach. To form supramolecular assemblies, mechanochemistry, in form of mortar-and-pestle grinding, is used. To study reactivity in crystals, X-ray diffraction is used. Students are involved in all steps necessary to obtain crystal structure: growing suitable crystals, selection of the best crystals, data collection, solution, refinement and analysis of the crystal structures. Overall, students have hands-down experience in all aspects of crystallography. Although essential in chemical, biochemical, pharmaceutical and materials science, crystallography is rarely taught in undergraduates laboratories.