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.