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.