Polymorphism in a <i>trans</i>-Cinnamic Acid Derivative Exhibiting Two Distinct β‑type Phases: Structural Properties, [2 + 2] Photodimerization Reactions, and Polymorphic Phase Transition Behavior

We report the discovery of a rare case of polymorphism among trans-cinnamic acid derivatives in which both polymorphs are classified as β-type structures based on their solid-state structural properties and their solid-state photoreactivity. Specifically, for 3-fluoro-trans-cinnamic acid, crystallization from many solvent systems leads to formation of one polymorph (denoted β1), although, in a few cases, concomitant crystallization of another polymorph (denoted β2) is also observed. On heating the β1 polymorph, a solid-state phase transition occurs at ca. 119 °C to produce the β2 polymorph. This polymorphic phase transition is irreversible, and the β2 polymorph remains stable on subsequent cooling to ambient temperature. Both the β1 and the β2 polymorphs undergo a topochemical [2 + 2] photodimerization reaction upon UV irradiation to produce 3,3′-difluoro-β-truxinic acid as the photoproduct in almost 100% yield. However, these reactions are associated with complete loss of crystallinity, preventing the structural properties of the directly produced solid photoproduct from being determined by single-crystal or powder X-ray diffraction techniques.