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.
Recent studies have shown that "crystal fluidity" in the form of fast conformational motions is critical for largeamplitude rotational motion in crystals. To explore this concept, we designed a crystalline assembly featuring two diethynylbenzene (DEB) molecular rotators linked to tetraphenylethylene (TPE), a fluorophore known to emit with intensities that depend on the rigidity of the medium. We envisioned that an increase in crystal fluidity as a function of increasing temperature would facilitate rotational motion of the DEB while diminishing the fluorescence intensity of the TPE. The aggregation-induced emission of the TPE moiety was confirmed when its fluorescence intensity increased by the addition of water to a THF solution. While bulk solids showed a relatively strong TPE emission with a lifetime of 4 ± 1 ns, no significant changes were observed between measurements carried out from 77 to 298 K, indicating that the crystal environment has limited motion within the excited-state lifetime. This conclusion was confirmed by the quadrupolar echo 2 H NMR line-shape analysis of a deuterium-labeled sample between 198 and 298 K, which revealed rotational correlation times in the microsecond regime, suggesting that rotational fluidity is 3 orders of magnitude too slow to affect fluorescence emission.
Herein, we report the use of fluorescence anisotropy decay for measuring the rotation of six shape-persistent molecular rotors with central naphthalene (2), anthracene (3a, 3b, and 3c), tetracene (4), and pentacene (5) rotators axially linked by triple bonds to bulky trialkylsilyl groups of different sizes. Steady-state and time-resolved polarization measurements carried out in mineral oil confirmed that the vibrationally resolved lowest-energy absorption bands are characterized by a transition dipole moment oriented along the short acene axes, in the direction of the alkyne linkers. Fluorescence lifetimes increased significantly with increasing acene size and moderately with a decrease in the size of the trialkylsilyl group. The fluorescence anisotropy decay for all compounds in mineral oil with a viscosity of ca. 21.6 cP at 40 °C was completed within the fluorescence lifetime, so that the rotational time constants could be obtained via their rotational correlation times, which increased with silyl protecting group size rather than acene size, indicating that polarization decay is determined by tumbling of the molecular rotor about the long acene axis. These results suggest that monitoring the rotational motion of bis(silylethynyl)acenes in restricted media should be possible for media with viscosity values on the order of 21.6 cP or greater.
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