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 [ 1 H]ClO 4 and [ 1 H]HSO 4 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 [ 1 H]ClO 4 are photoinert, irradiation of [ 1 H]HSO 4 single crystals results in partial single-crystal to single-crystal (SCSC) [2 + 2] photodimerization, with formation of a {[ 1 H]HSO 4 } 0.36 ·{[ 1 2 H 2 ][HSO 4 ] 2 } 0.64 solid solution. If crystalline powders of [ 1 H]HSO 4 are irradiated, however, the formation of a monomer/dimer solid solution is followed by “extrusion” and recrystallization of both the phodimerization product [ 1 2 H 2 ]HSO 4 and unreacted [ 1 H]HSO 4 as nonisomorphous crystal phases. The outcome of the UV irradiation of [ 1 H]HSO 4, therefore, is influenced by the crystal size of the reacting monomer and changes dramatically if the irradiated sample is in the form of single macro crystals or micro crystalline powder.