Solvent Polarity Effects on the Mechanochemistry of Spiropyran Ring Opening.

The spiropyran mechanophore (SP) is employed as a reporter of molecular tension in a wide range of polymer matrices, but the influence of surrounding environment on the force-coupled kinetics of its ring opening has not been quantified. Here, we report single-molecule force spectroscopy studies of SP ring opening in five solvents that span normalized Reichardt solvent polarity factors ( E _T ^N ) of 0.1-0.59. Individual multimechanophore polymers were activated under increasing tension at constant 300 nm s ^-1 displacement in an atomic force microscope. The extension results in a plateau in the force-extension curve, whose midpoint occurs at a transition force f * that corresponds to the force required to increase the rate constant of SP activation to approximately 30 s ^-1 . More polar solvents lead to mechanochemical reactions that are easier to trigger; f * decreases across the series of solvents, from a high of 415 ± 13 pN in toluene to a low of 234 ± 9 pN in n -butanol. The trend in mechanochemical reactivity is consistent with the developing zwitterionic character on going from SP to the ring-opened merocyanine product. The force dependence of the rate constant (Δ x ^‡ ) was calculated for all solvent cases and found to increase with E _T ^N , which is interpreted to reflect a shift in the transition state to a later and more productlike position. The inferred shift in the transition state position is consistent with a double-well (two-step) reaction potential energy surface, in which the second step is rate determining, and the intermediate is more polar than the product.