Photo- vs Mechano-Induced Polymorphism and Single Crystal to Single Crystal [2 + 2] Photoreactivity in a Bromide Salt of 4-Amino-Cinnamic Acid

The new molecular salt [1H]Br (1 = 4-amino-cinnamic acid) was obtained in two polymorphic modifications, forms A and B. While mechanical stress induces transformation of form A into form B, an unexpected effect of UV irradiation is observed on form A, i.e., a single crystal to single crystal (SCSC) transformation to a third polymorph, form C. Form B, on the contrary, responds to UV irradiation by undergoing a stepwise SCSC [2 + 2] photodimerization to α-[12H2]Br2, via formation of a solid state solutions in the whole compositional range. All transformations were followed by a combination of solid-state techniques, i.e., single crystal and powder X-ray diffraction, FTIR, and FT-Raman, and by solution 1H NMR. All structural changes also resulted in variations in crystal morphology/texture, and these were monitored via SEM analysis.