Ferrocene, one of the most attractive organometallic complexes, has received considerable attention since its discovery. On account of their unique sandwich structures, where the internal rotations of the cyclopentadienyl (Cp) rings along the Cp–Fe–Cp axis result in the staggered and eclipsed conformations, ferrocene complexes have been widely used as building blocks for rotary molecular switches showing responsiveness to external stimuli such as pH value, light, or temperature. Whereas most ferrocene-based switches reported to date are operated reversibly in solution, in the solid state an irreversible eclipsed-disorder transition upon heating was observed.<br/>In this work, we aim to improve the flexibility of ferrocene units within supramolecular systems in condensed phase. In order to create larger free volume and weaken their surrounding non-covalent interactions, a series of inclusion complexes of ferrocene-tethered ammonium threads with various lengths and dibenzo-crown ether rings of varied sizes were co-crystallized. Reversible eclipsed-staggered conversion of ferrocenes in the solid state was characterized successfully by single-crystal X-ray diffraction (SCXRD) for the first time.<sup> </sup>Furthermore, crystal structures of these ferrocene complexes were collected by SCXRD at various temperatures with irradiations of different laser power to investigate the relationship between molecular alignment and stimuli-induced motions in crystals. This design, featuring well-oriented large molecular assemblies in crystals, not only enhances the mobilities of ferrocene complexes on the microscopic level, but also leads to macroscopic changes, such as thermal-induced crystal-to-crystal phase transition and photo-induced deformation.

Thermal- and photo-induced reversible rotation of ferrocene complexes in crystals

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