Mechanoresponsive molecules as force sensors and self-healing materials
February 16, 2016 - 2:00pm to 3:00pm
Molecules that show a defined response to mechanical force (mechanophores) can be used as the building blocks of mechanoreponsive materials. For example, the mechanical activation of a latent catalyst can generate fluorophores that allow the early detection of bond rupture processes. The catalyst might also start a chain reaction that initiates the reformation of bonds in the material, thus introducing self-healing properties. I will give an introduction to mechanochemistry experiments and compare them to force experiments in biology. In a first study, theoretical calculations of the load rate dependence of the mechanical activation of a latent catalyst (N-heterocyclic carbene) reveal kinetic effects that explain the diverging results from AFM and ultrasound experiments. A second application targets the formation of a triazole unit in a cycloaddition reaction between azides and alkynes, which is a widely used tool in materials chemistry, where it allows the functionalization of polymers and surfaces as well as the synthesis of macromolecular structures. We have investigated theoretically the mechanically induced reversion of this reaction (cycloreversion), which would allow for the straightforward design of mechanoresponsive materials. We show that cycloreversion is principally possible and that the pulling geometry is the most important parameter. This geometry effect has also been observed in biological systems e.g. for the mechanical separation of DNA or b-strands in proteins. It appears to be a general principle that also applies to the mechanical rupture of covalent bonds in ring structures.