Abstract
The exciting field of polymer mechanochemistry has made great empirical progress in
discovering reactions in which a stretching force accelerates scission of strained
bonds using single molecule force spectroscopy and ultrasonication experiments. Understanding
why these reactions happen, i.e., the fundamental physical processes that govern coupling
of macroscopic motion to chemical reactions, as well as discovering other patterns
of mechanochemical reactivity require complementary techniques, which permit a much
more detailed characterization of reaction mechanisms and the distribution of force
in reacting molecules than are achievable in SMFS or ultrasonication. A molecular
force probe allows the specific pattern of molecular strain that is responsible for
localized reactions in stretched polymers to be reproduced accurately in non-polymeric
substrates using molecular design rather than atomistically intractable collective
motions of millions of atoms comprising macroscopic motion. In this review, we highlight
the necessary features of a useful molecular force probe and describe their realization
in stiff stilbene macrocycles. We describe how studying these macrocycles using classical
tools of physical organic chemistry has allowed detailed characterizations of mechanochemical
reactivity, explain some of the most unexpected insights enabled by these probes,
and speculate how they may guide the next stage of mechanochemistry.