Fine‐Tuning the Spring‐Like Motion of an Anion‐Based Triple Helicate by Tetraalkylammonium Guests

Supramolecular springs are a class of molecular devices that may provide implications to the macroscopic spring behavior from the molecular level. Helical structures are suitable molecular springs because the specific twisting of the helical strands can cause spring‐like (extension‐contraction) movement along the axis upon external stimuli. Herein we report an anion‐based triple‐helicate spring, which can undergo reversible contraction–extension motion through introduction and removal of tetraalkylammonium cations, including TMA+ and analogous irregular tetrahedral cations bearing different alkyl chains, while the relative orientation of the two phosphate ions changes to facilitate guest inclusion. Notably, the degree of contraction (shortening of the helical pitch) can be fine‐tuned by varying the shape and size of guest cation. However, with the larger cations (TEA+, TPA+ and TBA+), the meso‐helicate configuration is obtained, which interconverts with the helicate by addition/removal of TMA+ ion. An anion‐coordination‐driven triple helicate molecular spring undergoes reversible extension‐contraction motion by addition/removal of tetraalkylammonium guest cations, which is accompanied by an “orientational” isomerization (i.e. rotation of an anion) of the helicate to facilitate guest binding. The spring‐like motion is modulated by both the size and shape of the cationic guest.