Tailoring the cavities of hydrogen-bonded amphidynamic crystals using weak contacts: towards faster molecular machines

This work describes the use of C-HMIDLINE HORIZONTAL ELLIPSISF-C contacts in the solid-state from the stator towards the rotator to fine-tune their internal motion, by constructing a set of interactions that generate close-fitting cavities in three supramolecular rotors 1-3I. The crystal structures of these rotors, determined by synchrotron radiation experiments at different temperatures, show the presence of such C-HMIDLINE HORIZONTAL ELLIPSISF-C contacts between extended carbazole stators featuring fluorinated phenyl rings and the 1,4-diazabicyclo[2.2.2]octane (DABCO) rotator. According to the H-2 NMR results, using deuterated samples, and periodic density functional theory computations, the rotators experience fast angular displacements (preferentially 120 degrees jumps) due to their low rotational activation energies (E-a = 0.8-2.0 kcal mol(-1)). The higher rotational barrier for 1 (2.0 kcal mol(-1)) is associated with a larger number of weak C-HMIDLINE HORIZONTAL ELLIPSISF-C contacts generated by the stators. This strategy offers the possibility to explore the correlation among weak intermolecular forces, cavity shape, and internal dynamics, which has strong implications in the design of future fine-tuned amphidynamic crystals.