Switching and Quantifying the Single-Molecule Mechanochemical Reactivity of Four-Membered Carbocycle Mechanophores...

The reactivity of four-membered carbocycle mechanophores, such as cyclobutane (CB), cyclobutene (CBE), and benzocyclobutene (BCB), has been explored in the context of fundamental reaction mechanisms and the manipulation of polymer network properties. Despite their similar scaffolding, the mechanochemical reaction paths of these carbocycles are quite different due to the configuration or absence of π bonds opposite the sites of applied force. Here, we report a CBE diarylethene (DAE) mechanophore that can be reversibly toggled between open and bridged states, providing access to two reactivity patterns from a single species. The mechanochemical reactivity of both states is observed by single-molecule force spectroscopy (SMFS) and pulsed ultrasonication experiments. The two states give indistinguishable ring-opened products, but their reactivity differs significantly. The greater mechanochemical reactivity of the bridged state is evident in a lower transition force observed in the SMFS experiments (810 pN, vs 1520 pN for the open state) and by the greater conversion of bridged vs open isomers observed in pulsed sonication. The use of external triggers to switch between states of different reactivity offers promise as a quantitative probe of mechanochemical contributions to the performance of materials and devices.