Size Matters: [2 + 2] Photoreactivity In Macro- and Microcrystalline Salts of 4-Aminocinnamic Acid

The solid-state photoreactivity of 4-aminocinnamic acid (1) and molecular salts [1H]ClO4 and [1H]HSO4 has been investigated, and structural changes have been monitored by X-ray diffraction techniques and FTIR spectroscopy. While the parent compound 1 and its perchlorate salt [1H]ClO4 are photoinert, irradiation of [1H]HSO4 single crystals results in partial single-crystal to single-crystal (SCSC) [2 + 2] photodimerization, with formation of a {[1H]HSO4}0.36·{[12H2][HSO4]2}0.64 solid solution. If crystalline powders of [1H]HSO4 are irradiated, however, the formation of a monomer/dimer solid solution is followed by “extrusion” and recrystallization of both the phodimerization product [12H2]HSO4 and unreacted [1H]HSO4 as nonisomorphous crystal phases. The outcome of the UV irradiation of [1H]HSO4, therefore, is influenced by the crystal size of the reacting monomer and changes dramatically if the irradiated sample is in the form of single macrocrystals or microcrystalline powder.