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

Abstract

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 solidstate structural properties and their solid-state photoreactivity. Specifically, for 3-fluoro-transcinnamic acid, crystallization from many solvent systems leads to formation of one polymorph (denoted β 1 ), although, in a few cases, concomitant crystallization of another polymorph (denoted β 2 ) is also observed. On heating the β 1 polymorph, a solid-state phase transition occurs at ca. 119 °C to produce the β 2 polymorph. This polymorphic phase transition is irreversible, and the β 2 polymorph remains stable on subsequent cooling to ambient temperature. Both the β 1 and the β 2 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 singlecrystal or powder X-ray diffraction techniques.

Multi-modal mechanophores based on cinnamate dimers

“…19 ), suggesting mechanochemically generated cinnamates in the polymer backbones underwent photochemical [2 + 2] cycloaddition to form cyclobutane cross-links. This conclusion is supported by the appearance upon sonication of P2 of an IR resonance at 1640 cm −1 , which was previously associated with the cinnamate moiety 29 , 62 , 63 , and its disappearance upon irradiation of the sonicated sample at 365 nm, suggestive of photochemical dimerization of mechanochemically generated cinnamates (Supplementary Fig. 20 ).…”

Single-crystal-to-single-crystal phase transition of 18β-glycyrrhetinic acid isopropyl ester

“…Phase transitions in organic compounds are not an uncommon phenomenon (Zakharov et al, 2018;Li et al, 2019;Khoj et al, 2013;Palmer et al, 2012;Hirai et al, 2020;Colmenero, 2020;de Moura Oliveira et al, 2021;Rok et al, 2017). However, very often, during phase transformation, profound changes in the crystal structures cause the loss of crystal integrity.…”

Polymorphic phase transformations of 3-chloro-trans-cinnamic acid and its solid solution with 3-bromo-trans-cinnamic acid

“…A previous study reported (Ahn et al, 2001) that the polymorph of 3-BrCA undergoes a polymorphic transformation to the polymorph on heating above 373 K. In the present article, we investigate the possibility that a phase transformation may also occur for the polymorph of 3-ClCA. Polymorphic phase transformations (Taylor et al, 2014;Khoj et al, 2013;Yates & Sparkes, 2013;Palmer et al, 2012;Pete et al, 2015;Rubin-Preminger et al, 2004;Sørensen & Simonsen, 1989;Das et al, 2010;Hu & Englert, 2005;Zhang et al, 2005;Ahn et al, 2006;Williams et al, 2012) represent a 'tipping point' at which a gradual change in conditions (e.g. temperature) leads to the sudden conversion of one crystalline material into another (Dunitz, 1991;Meijer et al, 2003).…”