Molecular Origin of Mechanical Sensitivity of the Reaction Rate in Anthracene Cyclophane Isomerization Reveals Structural Motifs for Rational Design of Mechanophores

The observed pressure sensitivity of the isomerization reaction rate of bis-anthracene cyclophane photoisomer has attracted significant attention in the rational design of mechanically sensitive materials. However, the molecular origin of this sensitivity remains unclear. We developed an ab initio molecular model to quantify the effect of pressure on the reaction rate and to elucidate its molecular origin. Pressure-induced deformations and changes along the reaction free-energy surfaces are estimated from ab initio molecular dynamics trajectories. Our model predicts a barrier reduction of ∼2 kcal/mol at 0.9 GPa (in agreement with experiment). The barrier reduction is linear in the low-pressure regime (up to 2 GPa) but has a nonlinear dependence at higher pressures. We find that pressure alters the reaction path and that the mechanical sensitivity of the reaction rate is caused by an uneven distribution of the free-energy increase along the reaction surface. The uneven distribution primarily results from destabilization of the reactant, which has a lower mechanical rigidity along a particular deformation mode (flattening of anthracene rings as they are pushed toward each other in the cyclophane framework). Exploiting similar structural motifs to maximize rigidity differences along the reaction coordinate represents a promising rational design strategy.