Similar polymorphism of derivatives of salicylideneanilines with varying halo-substitution†

Four halo-substituted derivatives of salicylideneanilines (4BB, 4BC, 4CB, and 4CC) were prepared and recrystallized in order to investigate the impact of the substitution on the polymorphism of derivatives. Two similar forms, each with a quadrilateral or needle morphology and a yellow or orange color, were observed in all derivatives. The polymorphs' crystal structure and stability were examined. It was discovered that crystals of the same form adopted comparable molecular arrangements in terms of packing style and conformation. According to the DSC pattern and microscopic observations, 4BB, 4CB, and 4CC showed monotropic solid-to-solid transition during the heating process. The contact of seed crystals may also cause a martensitic transformation, in which the transition process is continuous, whereas, the two forms of 4BC crystals can stably exist. In the solution state, comparable transition behavior between polymorphs was observed.