Chemical Design and Physical Properties of Dynamic Molecular Assemblies

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 (Col(h)) and lamellar (L-a) liquid crystal phases enables the dipole inversion of polar N-H center dot center dot center dot 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.