Combined Experimental and Theoretical Investigations of a Photoswitching Dimorphic Chlorinated Schiff Base

Polymorphism in organic compounds is a fascinating phenomenon with diverse applications. Rational development of polymorphic forms is challenging and demands more investigations. Subtle variations in functional groups can lead to variation in intra- as well as intermolecular interactions and promote polymorphism. We report chlorinated sulfonated Anil 1 and its crystallization in two polymorphic forms, 1a and 1b, which are conformational polymorphs. Structural studies reveal conformational polymorphism arises due to rotation about the C═N–C═C bond and subsequent prototropism, as planar form 1a exists in keto while twisted form 1b exists as an enol tautomer. Color variation is attributed to the difference in extent of molecular conjugation, while emission switching arises due to variation in molecular packing. Density functional theory (DFT) studies have been carried out to understand molecular packing and explain optical properties. To further understand the effect of the nature of halogen atoms on electronic and optical properties, bromo and iodo derivatives of 1 have been reported and studied. Aggregation-induced emission (AIE) studies and sensitivity of acids, nitroaromatics like picric acid, and heavy metal ions like Hg(II) have been reported for the polymorphs.