Revealing highly unbalanced energy barriers in the extension and contraction of the muscle-like motion of a [c2]daisy chain.

Nanoscale muscle-like materials have aroused great interest as they may provide controllable mechanical operations by artificial actuations. Molecular designs to achieve the desired motion at the macroscopic scale in experiments require atomic level understanding. By systematic quantum chemical and molecular dynamics calculations we reveal that the length change is not only due to the linear telescoping from the dibenzo[24]crown-8 recognition at two docking stations but also the folding/unfolding of two bulky stoppers. The extension and contraction processes of a [c2]daisy chain under acidic vs. basic conditions are exothermic but need to cross very different energy barriers, being at least double the height under acidic compared to basic conditions, hindering balanced cyclic motions at moderate excitation. Our result suggests that to realize the desired muscle-like motion one should adopt sufficiently high external excitation, using for example reasonably high temperature and further optimizing the solution used.