A Steric-Repulsion-Driven Clutch Stack of Triaryltriazines: Correlated Molecular Rotations and a Thermoresponsive Gearshift in the Crystalline Solid

We
report that a newly developed type of triaryltriazine rotor,
which bears bulky silyl moieties on the para position
of its peripheral phenylene groups, forms a columnar stacked clutch
structure in the crystalline phase. The phenylene units of the crystalline
rotors display two different and interconvertible correlated molecular
motions. It is possible to switch between these intermolecular geared
rotational motions via a thermally induced crystal-to-crystal phase
transition. Variable-temperature solid-state 2H NMR measurements
and X-ray diffraction studies revealed that the crystalline rotor
is characterized by a vertically stacked columnar structure upon introducing
a bulky Si moiety with bent geometry as the stator. The structure
exhibits correlated flapping motions via a combination of 85°
and ca. 95° rotations between 295 and 348 K, concurrent with
a negative entropy change (ΔS‡ = −23 ± 0.3 cal mol–1 K–1). Interestingly, heating the crystal beyond 348 K induces an anisotropic
expansion of the column and lowers the steric congestion between the
adjacent rotators, thus altering the correlated motions from a flapping
motion to a correlated 2-fold 180° rotation with a lower entropic
penalty (ΔS‡ = −14
± 0.5 cal mol–1 K–1). The
obtained results of our study suggest that the intermolecular stacking
of the C3-symmetric rotator driven by
the steric repulsion of the bulky stator represents a promising strategy
for producing various correlated molecular motions in the crystalline
phase. Moreover, direct and reversible modulation of the intermolecularly
correlated rotation is achieved via a thermally induced crystal-to-crystal
phase transition, which operates as a gearshift function at the molecular
level.