The First Ferrocene-Based Molecular Ferroelectric Accompanied by Spatial Symmetry Operation Breaking.

Ferroelectric materials with switchable spontaneous polarization have attracted significant interest over the past decades. Molecular ferroelectrics particularly offer unique advantages such as structural tunability, inducible intrinsic homochirality, and ease of processing, features often unattainable in inorganic ceramics. Ferrocene derivatives have been widely used in pharmaceuticals, sensing, and solar energy conversion. However, developing a single-component ferrocene-based ferroelectric remains a major challenge. In this work, we report, for the first time, a chiral, single-component ferrocene-based ferroelectric, Fe(C₅H₅)(C₅H₄COO-CHOL), via the homochirality strategy. Notably, this compound undergoes a reversible structural phase transition at 193 K, switching from the polar space group C2 to another polar space group, P2_1, while maintaining the same point group. This transition only involves a symmetry operation change from a 2-fold rotation and screw rotation (C2) to a single 2-fold screw axis (P2₁). The ferroelectric nature was confirmed through piezoresponse force microscopy and polarization-electric field hysteresis loop measurements. Its non-centrosymmetric structure was validated by a polarized second-harmonic generation measurement. To our knowledge, this is the first report of a homochiral, single-component ferrocene-based ferroelectric undergoing phase transitions involving only spatial symmetry operation breaking. This work broadens the collection of chiral and ferrocene-based ferroelectrics and offers valuable insights for designing new ferroelectric materials.