Seed-triggered solid-to-solid transformation between color polymorphs: Striking differences between quasi-isomorphous crystals of dichloro-substituted salicylideneaniline regioisomers

Two regioisomers, N-(4′-chlorosalicylidene)-4-chloroaniline (4CC) and N-(5′-chlorosalicylidene)-4-chloroaniline (5CC), were prepared and were found to exhibit color polymorphism with yellow and orange forms. Microscopy observations revealed that only 4CC exhibited a solid-to-solid transition from the yellow to orange form during heating, and that contact with orange seed crystals could trigger the transformation. The transformation was tracked by thermal analysis and variable-temperature infrared spectroscopy. When polymorphs of the same color were compared, the 4CC and 5CC crystal structures were similar, whereas the molecular arrangement of 4CC crystals was significantly disordered with respect to the molecular orientation. Assuming that the disorder results from stacking faults, crystal models were constructed free of disorder and their molecular environment was analyzed in detail. Finally, it was proposed that certain types of stacking faults could have promoted the solid-to-solid transition.