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.