Analysis of Solvent Polarity Effects on the Mechanochemistry of Spiropyran Ring Opening

Abstract

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 have not been quantified. Here, we analyze single-molecule force spectroscopy studies of SP ring opening in five solvents that span normalized Reichardt solvent polarity factors (ETN) of 0.1 to 0.59. Individual multi-mechanophore 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 results are rationalized as follows. First, the trend in mechanochemical reactivity is consistent with the developing zwitterionic character on going from SP to the ring-opened merocyanine product. Second, the force dependence of the rate constant (∆x‡) was calculated for all solvent cases and found to increase with ETN, which is interpreted to reflect a shift in the transition state to a later and more product-like position. Finally, the inferred shift in 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.