Structural transformations in crystals induced by radiation and pressure. Part 8. Photochemical behaviour of 4-chlorocinnamide

Abstract

Molecules of trans-4-chlorocinnamide were arranged in a head-to-tail fashion and under UV–vis irradiation underwent [2 + 2] photodimerization. The photochemical reaction was conducted at ambient pressure (0.1 MPa) and at high pressure of 0.5 GPa, 1.5 GPa and 2.0 GPa in a step-by-step manner. This enabled us to monitor the photo-induced changes in the unit cell parameters, intramolecular geometry, free space between molecules and intermolecular contacts. The crystals at high pressure were cracking faster than in ambient conditions, which was explained by the lack of sufficient free space in the crystals. The high pressure also had an impact on the size of changes during the [2 + 2] photoreaction: the higher the pressure, the smaller the changes occurred. A slight rotation of reactant molecules during the photoreaction was observed and the angle between the molecular plane and the (100), (010) and (001) planes was monitored for the studied pressures. The dependence between the initial distance for reactant molecules and the size of the changes that took place during the [2 + 2] photoreaction for the head-to-tail molecular architecture was described. We also showed that the [2 + 2] photodimerization, and not pressure, can have a bigger impact on intermolecular interactions.