Thiourea as a template for photodimerization of azastilbenes.

In this study we have explored the potential of thiourea (TU) as a template to preorient stilbazoles and bispyridylethylenes (azastilbenes) in the crystalline state. TU is able to preorient eleven azastilbenes toward dimerization in the crystalline state. While cocrystals of these eleven olefins photodimerized to a single dimer expected based on crystal packing, pure crystals of these olefins either were nonreactive or gave a mixture of dimers. The differential photobehavior of the pure crystals and cocrystals highlights the importance of TU in templating the olefins in a photoreactive orientation in the crystalline state. X-ray crystallographic and photochemical studies have identified a few azastilbenes that photodimerize in spite of not being arranged in an ideal orientation in the crystalline state. These as well as a few examples already reported in the literature suggest that it is important to recognize that molecules could experience large amplitude motions in the crystalline state, especially when energized by light. Short-term lattice instability caused by photoexcitation can be effective in driving a photochemical reaction. Thus one should view the crystalline arrangement of molecules upon light exposure as "dynamic" rather than "static" as determined from X-ray structure analysis.