Crystal Fluidity Reflected by Fast Rotational Motion at the Core, Branches, and Peripheral Aromatic Groups of a Dendrimeric Molecular Rotor

Low
packing densities are key structural features of amphidynamic
crystals built with static and mobile components. Here we report a
loosely packed crystal of dendrimeric rotor 2 and the
fast dynamics of all its aromatic groups, both resulting from the
hyperbranched structure of the molecule. Compound 2 was
synthesized with a convergent strategy to construct a central phenylene
core with stators consisting of two layers of triarylmethyl groups.
Single crystal X-ray diffraction analysis confirmed a low-density
packing structure consisting of one molecule of 2 and
approximately eight solvent molecules per unit cell. Three isotopologues
of 2 were synthesized to study the motion of each segment
of the molecule in the solid state using variable temperature quadrupolar
echo 2H NMR spectroscopy. Line shape analysis of the spectra
reveals that the central phenylene, the six branch phenylenes, and
the 18 periphery phenyls all display megahertz rotational dynamics
in the crystals at ambient temperature. Arrhenius analysis of the
data gives similar activation energies and pre-exponential factors
for different parts of the structure. The observed pre-exponential
factors are 4–6 orders of magnitude greater than those of elementary
site-exchange processes, indicating that the dynamics are not dictated
by static energetic potentials. Instead, the activation energies for
rotations in the crystals of 2 are controlled by temperature
dependent local structural fluctuations and crystal fluidity.