Dynamic molecular assemblies toward a new frontier in materials chemistry

The crystal lattice energy of pi-molecules is dominated by weak and anisotropic intermolecular interactions ranging in energy from 1 to similar to 20 kJ mol(-1), which can be tuned by the introduction of specific interaction sites to the parent pi-molecular core. In various types of pi-molecular crystals, supramolecular approaches using directional hydrogen-bonding, halogen bonding, pi-stacking, host-guest, and hydrophobic interactions have been effectively used to achieve lattice dynamics including proton transfer, ionic transport, and molecular rotations in the closest-packing molecular assemblies. Such approaches enable the formation of dynamic multi-functional intrinsic pi-electronic materials with electrical conductivity, magnetism, and unique optical responses. Short-range collective proton transfer occurs along intermolecular hydrogen-bonding networks and is correlated with the dipole inversion and ferroelectricity, whereas long-range proton transport is directly associated with bulk protonic conduction. Dynamic proton and ionic transport in the molecular assemblies can be designed for protons via hydrogen bonding and for Li+ and/or Na+ via ionic channels, where the freedom of motion is coupled with the intrinsic pi-electronic properties. When the dynamic molecular rotation of the polar structural unit is controllable by an outer electric field, the dipole inversion and dielectric responses of the molecular assemblies are associated with ferroelectricity. Careful design of the polar rotary unit has the potential to create artificial molecular motors or futuristic molecular assembly machines. These dynamic molecular assemblies can be coupled with intrinsic pi-electronic functions, offering a new direction in the future of materials chemistry.