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 toot to control the alignment of alkenes in a crystal. The irradiation of 4-azachalcone (1 a) 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 center dot HCl and 1b center dot 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 Mth 1a center dot HCl and 1b center dot 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 I a 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.