Post-synthetic breaking of inversion symmetry in metal complex-based host–guest systems

Breaking of inversion symmetry is a significant structural transition that gives rise to high-electrical and optical functionalities in solid-state materials. In general, it is difficult to intentionally break the inversion symmetry of structures by material synthesis. However, recent advances in host–guest chemistry dealing with molecular-based materials have demonstrated several methods for controlling the inversion symmetry of hosts by guests. This post-synthetic dynamic manipulation produces not only polarity switching but also further multifunctionalities of host–guest composites. This study reviews studies on guest-responsive polar materials, which are categorized into five groups: (1) guest adsorption, (2) desorption, (3) exchange control of inversion symmetry, (4) temperature-driven polar transition of composites, and (5) enhancement of polarity by guests. By summarizing these symmetry breaking mechanisms, we provide insights into the rational design of materials with switchable polarity.