We described a new approach for the solid-state photodimerization of alkenes in which the cation-pi interaction between a pyridinium and an aromatic ring serves as a powerful tool to control the alignment of alkenes in a crystal. The irradiation of 4-azachalcone (1a) gave the synHT dimer 2a in 49% yield, whereas the photolysis of 4'-methoxy-4-azachalcone (1b) afforded the synHH dimer 3b in 67% yield. On the other hand, the photodimerization of 1a.HCl and 1b.HCl exclusively afforded the synHT dimers 2a and 2b, respectively, in quantitative yields. The structures of the produced dimers were clarified by X-ray crystallographic analyses. A comparison of the X-ray packing structures of 1a and 1b with 1a.HCl and 1b.HCl clarified the significant differences between them. The separations of the two reactive double bonds of the HCl salts are much shorter than those of 1a and 1b, strongly suggesting the existence of an intermolecular cation-pi interaction in the pyridinium salts. These results are evidence of the significant effectiveness in the cation-pi controlled solid-state photodimerization on the stereoselective formation of cyclobutane dimers.

Exposure of 4-azachalcones to HCl gas produced the corresponding HCl salts with a head-to-tail stacked alignment, irradiation of which produced the corresponding syn-HT dimers with high regio- and stereoselectivities, thus showing the effectiveness of the cascade process in crystals.

Exposure of yellow anhydrate organic crystals to water vapor gave red hydrate crystals with significant changes in the molecular arrangement.