The thermal phase transition properties of macrocyclic molecules 1 and 2 consisting of aromatic 1,4-bis(phenylethynyl)benzene (BPEB) components and tetraethylene glycol (TEG) chains were investigated. 1 and 2 are structural isomers with a small difference at the connecting points of the BPEB units with TEG chains, where 1 has a lower structural symmetry than 2. 1 forms a crystal Cr1 upon cooling from the isotropic liquid. 1 at Cr1 shows a crystal-to-crystal polymorphic transition upon heating affording Cr2. Since the Cr1-to-Cr2 phase transition of 1 is an exothermic process, Cr2 is considered to be thermodynamically more stable than Cr1. Indeed, cooling from Cr2 shows no phase transition, and this hysteretic polymorphic transition allows the preparation of 1 in different crystalline states, namely Cr1 and Cr2, at room temperature. In contrast, 2 shows a reversible transition between a crystal and a nematic phase upon temperature changes. Based on the polymorphism of 1, switching of an optical property as a memory function is also demonstrated. Thus, the molecular structure of 1 with a lower structural symmetry than that of 2 is likely an important factor for the crystallization and its thermoresponsive polymorphic phase transition property.
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