Coupled Rocking Motion in a Light-Driven Rotary Molecular Motor

Coupled motion is
ubiquitous in Nature as it forms the base for
the direction, amplification, propagation, and synchronization of
movement. Herein, we present experimental proof for the coupling of
the rocking motion of a dihydroanthracene stator moiety with the light-induced
rotational movement of an overcrowded alkene-based molecular motor.
The motor was desymmetrized, introducing two different alkyl substituents
to the stator part of the molecular scaffold, resulting in the formation
of two diastereomers with opposite axial chirality. The structure
of the two isomers is determined with nuclear Overhauser effect spectroscopy
NMR and single-crystal X-ray analysis. The desymmetrization enables
the study of the coupled motion, that is, rotation and oscillation,
by 1H NMR, findings that are further supported by density
functional theory calculations. A new handle to regulate the rotational
speed of the motor through functionalization in the bottom half was
also introduced, as the thermal barrier for thermal helix inversion
is found to be largely dependent on the alkyl substituents and its
orientation toward the upper half of the motor scaffold. In addition
to the commonly observed successive photochemical and thermal steps
driving the rotation of the motor, we find that the motor undergoes
photochemically driven rotation in three of the four steps of the
rotation cycle. Hence, this result extends the scope of molecular
motors capable of photon-only rotary behavior.