Alkyl substituent-dependent systematic change in cold crystallization of azo molecules.

The thermal behavior of alkyl-derivatized 1-(2,4-dimethylphenylazo)-4-naphthol and 1-(2,4-dimethylphenylazo)-2-naphthol (2,4-DM-4-C and 2,4-DM-2-C, respectively) was investigated. The change in the position of the alkyl substituent led to a variation in the thermal behavior, including the cold crystallization, which is a heat-storing phenomenon. In addition, a comprehensive study of the alkyl chain length revealed that 2,4-DM-4-C had better crystallinity and exhibited cold crystallization with short alkyl chains. The π-π, C-H⋯N, and C-H⋯π interactions stabilized the crystal structure of 2,4-DM-4-C. On the other hand, the polymorphism of 2,4-DM-2-C inhibited the formation of a uniform crystalline phase during cooling, which led to poor crystallinity. The only difference between the compounds, the position of the substituent, resulted in a clear variation in the cold crystallization and heat storage properties.