Cold Crystallization of Ferrocene-Hinged π-Conjugated Molecule Induced by the Limited Conformational Freedom of Ferrocene.

Crystallization that proceeds above the glass transition temperature upon heating the glassy or amorphous state is referred to as "cold crystallization", which has often been observed in supercooled phases of molecular materials followed by the transition to the thermodynamically stable crystalline phase. Although this behavior is common for the macromolecules with high structural flexibility among segments preserving the wide temperature range of corresponding glassy phases, cold crystallization of small molecules is relatively rare and there is still less knowledge on the design guideline of such molecules. Here we report a ferrocene-hinged molecule DC12 carrying two units of didodecyl-substituted pentathiophenes at the 1,1'-positions. Due to the rotational freedom of the ferrocene unit, DC12 forms amorphous solid on cooling from its isotropic melt. The amorphous state was realized even by slow cooling such as 0.1 °C min-1. The possible reason for the easy formation of the amorphous phase is the coexistence of various conformations of DC12 originating from the open conformers of the ferrocene. On heating from the amorphous phase, DC12 shows cold crystallization with the estimated activation energy of 61 kJ mol-1. The crystalline phase is composed of the closed form of DC12 molecules packed in a lamellar fashion, and the degree of crystallinity is remarkably high compared with the case of macromolecular materials. The crystallization is triggered by the intermolecular interactions among the dodecyl chains and rotational flexibility of the ferrocene unit. This work provides an unprecedented example that ferrocenes act as heat-controllable rotational hinges in condensed phases. Considering that the cold crystallization phenomenon is related to heat-storage materials, the design strategy presented in this work will be novel to realize both easily formed amorphous phases and highly crystalline phases formed through cold crystallization.