Mechanical Taming of Hardy-Cope Rearrangements.

Bullvalene is a fluxional molecule that rapidly rearranges via low-barrier [3,3] sigmatropic rearrangements, allowing each of its 10 carbon atoms to exchange rapidly at room temperature, accessing all possible configurational permutations with submillisecond half-lives. We previously demonstrated that the incorporation of this fluxional core into polymeric systems imbues materials with force-responsive properties, as bullvalene's fluxionality can reversibly "respond" to applied mechanical stress. However, direct observation of such rearrangements within the bullvalene core in response to applied force has remained elusive in soft materials because of the complexity of the resulting isomer distributions and the rapid thermal reversion of perturbed isomers. In this work, we utilize a polymer-chain-centered fluorinated bullvalene core to provide direct spectroscopic evidence of changes to the bullvalene isomer distribution in response to mechanical force. These findings position bullvalene as the first example in a unique class of mechanophores capable of undergoing multiple mechanochemical reactions without requiring an external stimulus for subsequent mechanochemical activity. We anticipate these results to provide key insights into the development of force-responsive materials with enhanced mechanical properties and macromolecular sensing capabilities.