Liquid-free single-crystal to single-crystal transformations in coordination polymers

The design and synthesis of functional materials with desired properties is the ultimate goal for chemical pursuers. Single-crystal to single-crystal (SCSC) transformations are an important component of solid-state reactions. These transformations not only create new materials but also provide an opportunity to explore the process of forming a chemical bond, which is conducive to making function-oriented crystal synthesis a reality. In this review, we provide a broad overview of SCSC transformations that take place in polymers or metal-organic frameworks, and classify them into several groups based on their various inducing stimuli, such as UV light, loss/uptake of solvent vapor, temperature change, mechanical force and their synergic impact. Unlike other reviews, the SCSC transformations discussed in this paper are confined to transformations via an absolutely liquid-free mode (both the reactant and product are in the solid phase), while those involving liquids, in which the transformation occurs in a recrystallization manner, are strictly excluded. These dynamic processes all involve the breakage and formation of covalent or non-covalent bonds, accompanied by drastic structural rearrangement in the crystalline phase. Changes in the physical properties of these complexes resulting from SCSC transformations are also illustrated. Thanks to the regular characteristics of crystal structures before and after the reactions, the transformation processes may provide deep insights into the rational design of crystalline materials.