Cocrystal of 1,1-Dicyano-2-(4-hydroxyphenyl)-ethene with l-Proline and Induced Conformational Polymorphism of 1,1-Dicyano-2-(4-hydroxy- 3-methoxyphenyl)-ethene

Crystallization of 1,1-dicyano-2-(4-hydroxyphenyl)-ethene (I) and 1,1-dicyano-2-(4-hydroxy-3-methoxyphenyl)-ethene (II) with l-proline (III) and l-tartaric acid (IV) was carried out. Compound I formed cocrystals with l-proline I·III. However, cocrystallization of II was unsuccessful; instead in the presence of III or IV formation of two conformational polymorphs IIa and IIb was observed. In cocrystals I·III, hydrogen bonded chains of proline molecules with molecules of I attached to them were found. Similar synthons were found in the literature in other proline-containing crystals, and we believe that it is possible to use these acentric supramolecular associates in crystal engineering of nonlinear optical materials. Possible reasons for the formation of two conformational polymorphs of compound II are discussed based on the results of X-ray analysis, molecular and crystal structure energy calculations, and thermal behavior of compound II.