Mechanical Peeling of Anthracene Dimers.

In polymer mechanochemistry, mechanophores are activated by the pulling action of polymer arms. This activation can be achieved from various pulling geometries, the most common being shearing, where tensile stress is distributed across the entire structure, and peeling, which concentrates tension on one scissile bond at a time. The latter usually leads to a better activation and is often associated to pulling points with a cis arrangement in cyclic mechanophores. Here we show that such a cis arrangement is not always conducive of an efficient peeling activation. The investigation of four isomers of butterfly-shaped anthracene dimers show that only one of the cis isomers undergoes a formal retro-4+4 cycloaddition to regenerate the anthracene cores. Combining with experimental and theoretical studies, we have revealed the reaction pathway and shown that the asymmetric force transmission is crucial for the mechanochemical activation. Furthermore, altering the rigidity of the anchoring handle from a flexible alkyl chain to a rigid phenyl handle accelerates the reaction by sevenfold. Our findings establish new structural and geometric design principles for mechanochemical transformations.