Unusual In Situ Transformation of Metastable Co-Crystal Forms into a Stable Form, in Solution and Solid State, and the [2 + 2] Photochemical Reactivity in the Crystalline Forms

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Isolation of multiple composition co-crystals from a one-pot crystallization flask and in situ transformations of the crystals are not common phenomena. We describe, herein, thoroughly characterized three unusual co-crystals of citric acid (CA) and 1,2-bis(4-pyridyl)ethene ( bpyee ) with different molecular compositions (1:1, α; 2:2, β; and 2:3, γ) that were obtained from a one-pot crystallization flask of a CH 3 OH solution with equimolar co-formers. The three co-crystals can be distinguished by their morphology and appearance at various stages of the crystallization process. In the first instance, needle-shaped (β) crystals are formed on the edge of the flask, followed by plates (γ) in the bulk of the solution, and finally, the entire flask with only rod-shaped crystals (α), indicating the in situ conversion of β- and γ-forms into α-form. Interestingly, in the solid state, isolated crystals are observed to undergo conversion, but in a different way than solution growth, as characterized by in situ variable-temperature powder X-ray diffraction, with β- and α-forms converting into γ-form. Furthermore, all of the forms undergo [2 + 2] photochemical reactivity upon UV irradiation due to the favorable Schmidt’s photoreactive distance between the adjacent bpyee molecules in α- and γ-forms, with the former following a single-crystal-to-single-crystal transformation process also, while β-form, despite having bpyee molecules separated by 7.68 Å, through conversion into γ-form.