No abstract

The changes in the surface morphology of a salt crystal of 4-(2,5-diisopropylbenzoyl)benzoic acid with (S)-phenylethylamine upon photoirradiation were determined under atomic force microscopy. The crystal underwent enantiospecific photocyclization of the benzophenone derivative to give (S)-cyclobutenol as the sole product via single-crystal-to-single-crystal transformation. Upon UV irradiation, numbers of hemispheric unevennesses appeared on the (001) and (010) single-crystal surfaces with heights of tens of nanometers. Prolonged irradiation caused the hemispheres to merge, restoring essentially flat surfaces at the completion of the reaction. In contrast, the roughness of the (100) surface resulting from cutting with a razor blade decreased on photoirradiation, and a smooth surface was obtained. The morphological changes likely resulted from the changes in the molecular structure of the benzophenone derivative; the movement of the salt bond chains near the crystal surface then became easier as the melting point decreased on photoirradiation.

Changes in the surface morphology of salt crystals composed of 4-(2,5-diisopropylbenzoyl)benzoic acid with (S)-phenylethylamine and 2,4-dichlorobenzylamine were observed upon photoirradiation. The reaction processes were monitored using Fourier transform infrared (FT-IR) spectroscopy and surface morphologies determined using atomic force microscopy. The crystals underwent enantiospecific and enantioselective photocyclization of the benzophenone derivative via a single-crystal-to-single-crystal transformation. UV irradiation of the salt microcrystals at wavelengths greater than 290 nm induced the formation of hemispheric features on the initially platelike (001) face, which gradually grew to heights of tens of nanometers. The maximum relative surface roughness was several percent of the 1 µm crystal thickness. Prolonged irradiation after the reactions had reached completion decreased the average feature height, and the surface eventually returned to its initial smooth state. On the basis of X-ray crystallographic data obtained before and after photocyclization, these morphological changes likely resulted from changes in the molecular structure of the benzophenone chromophore upon photoirradiation, resulting in molecular motion at the crystal surface.