Chemical Design and Physical Properties of Dynamic Molecular Assemblies
Tomoyuki Akutagawa
Abstract
Abstract The thermally activated motional freedom of protons (H+), ions (M+), and molecules can be controlled using supramolecular approaches. In single crystals, motional freedom is enabled because of the small size of H+ and M+ (e.g., Li+ and Na+), and the thermally activated motion of small molecular units can yield molecular rotator structures in electrically conducting and magnetic crystals. The design of hydrogen-bonded networks and rotator–stator structures is a rational method to form functional dynamic molecular assemblies, and the thermally activated motional freedom of alkylamide (–CONHCnH2n+1) chains in discotic hexagonal columnar (Colh) and lamellar (La) liquid crystal phases enables the dipole inversion of polar N–H⋯O= hydrogen-bonded chains, enabling a ferroelectric response to an applied external electric field. The thermally activated rotational freedom of neutral radicals in plastic crystals results in multifunctional dielectric, magnetic, and optical properties at the order–disorder phase transition. In hydrogen-bonded host–guest molecular crystals, dynamic structural transformations are coupled with highly reversibly guest adsorption–desorption in the crystalline state. Further, changes in the fluorescence colour of excited-state intramolecular proton transfer (ESIPT) systems can be exploited for solid-state molecular sensing, in which both dynamic molecular rotation and conformational transformations drastically affect the fluorescent responses.