Hydrogen‐Bonded Crystalline Molecular Machines with Ultrafast Rotation and Displacive Phase Transitions

Two new crystalline rotors 1 and 2 assembled through N−H⋅⋅⋅N hydrogen bonds by using halogenated carbazole as stators and 1,4‐diaza[2.2.2]bicyclooctane (DABCO) as the rotator, are described. The dynamic characterization through 1H T1 relaxometry experiments indicate very low rotational activation barriers (Ea) of 0.67 kcal mol−1 for 1 and 0.26 kcal mol−1 for 2, indicating that DABCO can reach a THz frequency at room temperature in the latter. These Ea values are supported by solid‐state density functional theory computations. Interestingly, both supramolecular rotors show a phase transition between 298 and 250 K, revealed by differential scanning calorimetry and single‐crystal X‐ray diffraction. The subtle changes in the crystalline environment of these rotors that can alter the motion of an almost barrierless DABCO are discussed here. Moving molecules: Two crystalline rotors assembled through N−H⋅⋅⋅N hydrogen bonds by using halogenated carbazole as stators and 1,4‐diaza[2.2.2]bicyclooctane (DABCO) as rotator, are described. The dynamic characterization through 1H T1 relaxometry experiments indicate very low rotational activation barriers of 0.67 and 0.26 kcal mol−1, indicating that DABCO can reach a THz frequency at room temperature.