Thermosalient Amphidynamic Molecular Machines: Motion at the Molecular and Macroscopic Scales

The supramolecular amphidynamic rotor 1, composed of two carbazole molecules acting as the stator and a DABCO rotator, exhibits remarkable thermosalience above 316 K. During this process, the crystals spontaneously transduce collective molecular displacements into macroscopic movement due to a phase transition, which is described by single-crystal X-ray analyses from 100 K to 320 K. The fast rotation in the low-temperature phase (I) occurs with a low activation energy E_a(I) ≈ 2.6 kcal mol⁻¹ and a pre-exponential factor A(I) ≈ 10¹² s⁻¹. Increased symmetry of the cavity in the high-temperature phase (II) resulted in slower dynamics, regardless of a smaller rotational barrier, E_a(II) ≈ 0.5 kcal mol⁻¹, due to the large reduction in the pre-exponential factor to A(II) ≈ 10⁷ s⁻¹. These results demonstrate that a relatively small distortion of lattice framework leads to drastic dynamic effects at both molecular and macroscopic scales, helping us to understand responsive crystalline materials. Amphidynamic crystals are a promising platform for the design of artificial molecular machines that rely on thermal activation of rapidly moving molecular elements in a lattice. The conversion of thermal energy into mechanical work at the macroscopic scale is an emergent property that could enable the design of all-organic artificial muscles in soft robotics. The thermosalient effect is a visually observable motion of crystals that occurs due to a sudden release of strain accumulated in the crystal lattice over a phase transition. The rapid switching of the entire crystal structure occurs on time scales that are several orders of magnitude faster than those of common phase transitions, resulting in self-actuation of the crystals. An amphidynamic thermosalient crystal that is capable of molecular and macroscopic motion is a precedent of being dynamic at two levels of structural hierarchy and provides insights into the relationship between the underlying molecular and lattice dynamics. A thermosalient molecular rotor obtained from the cocrystallization between DABCO and carbazole is reported. The cocrystal shows ultrafast rotation at low temperatures with a low rotational barrier of 2.63 kcal mol⁻¹. A phase transition above 320 K causes the crystals to jump or explode, with a concomitant decrease in the rotational frequency of DABCO. The double dynamic behavior was characterized by X-ray diffraction, solid-state NMR, calorimetry, and relaxometry, and the results established the dynamics at the molecular and macroscopic levels.