Structural Transformations in Crystals Induced by Radiation and Pressure. Part 6. The Reactivity of Difluorocinnamic Acids under Ambient and High PressuresComparative Studies

The influence of the molecular environment and the intramolecular features on the [2 + 2] photodimerization of 2,5-difluorocinnamic acid (1) and 3,5-difluorocinnamic acid (2) in crystals were analyzed. The crystal structures of (1) were determined at pressures of 0.1 MPa and 0.3 and 0.9 GPa before the [2 + 2] photodimerization as well as for several steps of the reaction. The determined structures revealed the influence of pressure on the reaction rate: the reaction was faster at 0.3 GPa than at 0.1 MPa, but there was not a significant difference between the rates at 0.3 and 0.9 GPa. For (2), the crystal structure was determined before the photochemical reaction, and the unit cell parameters were monitored along with the reaction progress at 0.1 MPa and 0.4 GPa, which showed higher reactivity of (2) at 0.4 GPa. Despite the similar orientations, distances between two neighboring monomers, and volumes of free space in the studied compounds, (2) reacted noticeably worse than (1). The differences in reactivity were rationalized in terms of the substituent effect of fluorine. The results were compared with those of our previous studies carried out for 2,6-difluorocinnamic acid (3).