Dynamic Characterization of Crystalline Supramolecular Rotors Assembled through Halogen Bonding

A modular molecular kit for the preparation
of crystalline molecular
rotors was devised from a set of stators and rotators to gain simple
access to a large number of structures with different dynamic performance
and physical properties. In this work, we have accomplished this with
crystalline molecular rotors self-assembled by halogen bonding of
diaza­bicyclo­[2.2.2]­octane, acting as a rotator,
and a set of five fluorine-substituted iodo­benzenes that take
the role of the stator. Using variable-temperature 1H T1 spin–lattice relaxation measurements,
we have shown that all structures display ultrafast Brownian rotation
with activation energies of 2.4–4.9 kcal/mol and pre-exponential
factors of the order of (1–9) × 1012 s–1. Line shape analysis of quadrupolar echo 2H NMR measurements in selected examples indicated rotational trajectories
consistent with the 3-fold or 6-fold symmetric potential of the rotator.