Comparison of templating ablities of urea and thioruea during photodimerization of bipyridylethyelene and stilbazole crystals

Abstract Photodimerization of cocrystals of four bispyridylethylenes and two stilbazoles with urea as a template in the solid state has been investigated following our success with thiourea. Four investigated olefins photodimerized quantitatively to a single dimer in the crystalline state only. The reactivity of urea-olefin crystals is understood on the basis of their packing arrangements in the crystalline state. In reactive crystals the adjacent reactive molecules are within 4.2 Å and parallel, whereas the unreactive ones have their adjacent molecules are farther than 4.6Å and nonparallel. Thus, with the knowledge of crystal packing the reactivity of urea-olefin crystals is predictable on the basis of Schmidt's topochemical postulates. The templating property of urea, similar to thiourea, derives from its ability to form hydrogen bonds with itself and the guest olefins. Despite the similarities in molecular structures of urea and thiourea their subtle electronic properties, yet to be fully understood, affect the crystal packing and consequently their reactivity in the crystalline state. Further work is needed to fully exploit the templating properties of urea. Templating properties of urea in solid-state photodimerization of stilbazoles and bispyridylethylenes have been established through a study that combined photochemistry and X-ray crystallography. The templating ability of urea derives from its ability to form hydrogen bond with itself and with coguests stilbazoles and bispyridylethylenes. At this stage, it is not easy to predict when urea will and when will not function as a template.