A Latent Mechanoacid for Time-Stamped Mechanochromism and Chemical Signaling in Polymeric Materials

Mechanically coupled proton transduction offers potential for stress-responsive polymeric materials whose properties can be switched via acid-triggered coloration, polymerization/cross-linking, or degradation. The utility of currently available mechanoacids, however, is limited by modest force-free stability or a scissile response that caps mechanoacid generation at one proton per strained polymer chain. Here, we report a new mechanoacid based on 2-methoxy-substituted gem-dichlorocyclopropane (MeO-gDCC). Pulsed ultrasonication leads to the mechanochemical ring opening of the MeO-gDCC and the subsequent elimination of either HCl or MeCl, with ∼0.58 equiv of HCl released per mechanophore activation and ∼67 protons per chain scission event. Single-molecule force spectroscopy reveals that the methoxy substituent lowers the force required for rapid (kopen ∼102 s–1) ring opening to ca. 900 pN, vs 1300 pN required for the parent gDCC. The utility of the mechanoacid is demonstrated in silicone elastomers, where its mechanical activation leads to a strain-triggered color change prior to fracture of the elastomer. The postactivation kinetics of coloration are used to demonstrate a new concept in mechanochromism, namely, a spectroscopic indicator of not only whether and where a mechanical event has occurred but when it occurred.