The Roles of Intrinsic Barriers and Crystal Fluidity in Determining the Dynamics of Crystalline Molecular Rotors and Molecular Machines

Crystalline solids are a promising platform for the development of molecular machines. They have the potential of combining the molecular-level control of physical properties caused by isomerizations, conformational motions, or chemical reactions with the emergent properties that arise from long-range order and multiscale phenomena. However, the construction of crystalline molecular machinery has been challenging due to the difficulties associated with the design of structures capable of supporting high order and controlled molecular motion in the solid state, a platform that we term amphidynamic crystals. With ultrafast rotation as the target, previous work on amphidynamic crystals has explored the creation of free space around the rotator, the advantages of volume-conserving rotational motions, and the challenges associated with correlated rotations, or gearing motions. In this Perspective we report the results of a systematic analysis of a large number of examples from our work and that of others, where we demonstrate that the creation of free space alone does not always result in ultrafast dynamics. In a limit that applies to porous crystals with large empty volumes such as MOFs and other extended solids, internal motions fall in the regime of activation control, with dynamics determined by the intrinsic (gas-phase) electronic barriers for rotation around the bond that connects the rotator and the stator. By contrast, internal rotation in close-packed molecular crystals falls in the regime of diffusion-controlled dynamics and depends on the ability of the rotator surroundings to distort and create transient cavities. We refer to this property as crystal fluidity and suggest that it may be used as an additional guiding principle for the design of crystalline molecular machines. We describe here the general principles behind the promising field of crystalline molecular machinery, the analytical methods to analyze rotational dynamics of crystalline solids, and the key structural concepts that may help their future development.