Reversible polymorphic crystalline transition of a push–pull-type molecule: {4-[4,5-bis(methylsulfanyl)-1,3-dithiol-2-ylidene]cyclohexa-2,5-dien-1-ylidene}malononitrile (BMDCM)

A push–pull-type molecule {4-[4,5-bis(methylsulfanyl)-1,3-dithiol-2-ylidene]cyclohexa-2,5-dien-1-ylidene}malononitrile (BMDCM) turned out to show two different polymorphs depending on the phase in which the crystal growth was made. BMDCM molecules stack in the head-to-tail fashion in both polymorphs to cancel out their large dipole moments with each other. On the other hand, the relative arrangements of the molecule stacking columns are clearly different between the two polymorphs. A solution-grown crystal makes molecular long axes in all the columns align in the same direction. In contrast, a vapor-grown crystal exhibits alternate orientation of the molecular long axes among neighboring columns being almost perpendicular to each other. While there is such a large structural difference between the two polymorphs, we have found the polymorphic transition between them occurs essentially in a reversible manner.