The Role of Torsion on the Force-Coupled Reactivity of a Fluorenyl Naphthopyran Mechanophore

The unique reactivity
of molecules under force commands
an understanding
of structure–mechanochemical activity relationships. While
conceptual frameworks for understanding force transduction in many
systems are established, systematic investigations into force-coupled
molecular torsions are limited. Here, we describe a novel fluorenyl
naphthopyran mechanophore for which mechanical force is uniquely coupled
to the torsional motions associated with the overall chemical transformation
as a result of the conformational rigidity imposed by the fluorene
group. Using a combined experimental and theoretical approach, we
demonstrate that variation in the pulling geometry on the fluorene
subunit results in significant differences in mechanochemical activity
due to pronounced changes in how force is coupled to distinct torsional
motions and their coherence with the nuclear motions that accompany
the force-free ring-opening reaction. Notably, subtle changes in polymer
attachment position lead to a >50% difference in the rate of mechanochemical
activation in ultrasonication experiments. Our results offer new insights
into the structural and geometric factors that influence mechanochemical
reactivity by describing how mechanical force is coupled to a reaction
that principally involves torsional motions.