Cold Crystallization of a Macrocyclic Molecule with Semiflexible Silyl Ether Units

Cold crystallization, an exothermic phase transition upon heating of a glassy state, is of interest in heat storage materials. While such behavior is common in polymers, small-molecule systems have also been investigated. Herein, we report a semiflexible macrocyclic compound composed of four silyl ether units and aromatic linkers that exhibits distinct cold crystallization. A key for the molecular design is semiflexible silyl ether units significantly affecting the macrocyclic shape. Differential scanning calorimetry revealed the formation of a glassy state by melt quenching, followed by exothermic crystallization. Powder X-ray diffraction indicated that the molecular conformation and packing in the crystals after cold crystallization are similar to those of solution-grown crystals. In contrast, a biphenylene-bridged macrocycle and a reference compound with a nonmacrocyclic structure did not show this behavior. These results suggest that a macrocyclic structure with suitable conformational mobility may help in the design of small molecular systems showing cold crystallization as heat storage materials.