Photodimerization of HCl salts of azastilbenes in the solid state.

A simple strategy that is based on conversion of an electron-rich pyridyl to an electron-deficient pyridinium ion is able to orient thirteen of the sixteen olefins investigated towards a single dimer in quantitative yield in the crystalline state. The reagent used is HCl, the method involves grinding the olefin with a drop of HCl and the dimerization is achieved by exposure of the crystalline stilbazolium·HCl salts to light. The weak force involved in modifying the crystal packing is most likely charge-transfer interaction.