Mechanochemically induced ring opening of spiropyran into merocyanine in various solvent mediates was systematically evaluated using single-molecule force spectroscopy. The force necessary to trigger ring opening of spiropyran in a rate (~10 s<sup>-1</sup>) that accommodates the extension of a single polymer chain (300 nm/s) highly depends on the solvent mediate, with a high force in nonpolar solvent and a low force in polar solvent (415 pN in toluene vs 235 pN in methanol). It was found that there is a linear correlation (slope = -380 pN) between the triggering force and the relative polarity of solvent mediate. This dependence was attributed to a heterolytic, polar transition state in the mechanically induced ring opening process. Further analysis using Bell-Even or Cusp model revealed that the ring opening undergoes dual transition states, and the observed mechanical coupling is an overall distance change along the force vector across all transition states. The results highlight the environment effects of polymer matrices on the mechanochemical reactivities of spiropyran when used as force probes.

Solvent effects on a mechanochemical reaction

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