Reversible Control of Crystalline Rotors by Squeezing Their Hydrogen Bond Cloud Across a Halogen Bond-Mediated Phase Transition

We report on a crystalline rotor
that undergoes a reversible phase transition at 145 K. Variable-temperature
X-ray and 1H spin–lattice relaxation experiments,
and calculations of rotational barriers, provide a description (i)
of the way in which the rotators’ dynamics changes back and
forth at the onset of the phase transition and (ii) of the mechanism
responsible for the abrupt switching of the crystalline rotors from
a very low-energy 4-fold degenerate equilibrium state, in which the
rotation is ultrafast (9.6 GHz at 145 K), to a single higher-energy
state associated with a slower motion (2.3 GHz at 145 K). Our results
provide evidence that the reversible change observed in the rotational
barriers at the transition is due to a cooperative modulation of the
C–Hrotator···Istator hydrogen
bond cloud across a C–Istator···Istator–C halogen bond-mediated phase transition. In
addition, we report evidence for second-harmonic generation from this
material, thereby confirming with a second example the benefit of
using polarized light to probe the torsional degree of freedom of
chiral helix blades, as well as symmetry and dimensionality of large
collections of chiral rotors in the solid state.