Linear Artificial Molecular Muscles

Abstract

Two switchable, palindromically constituted bistable [3]rotaxanes have been designed and synthesized with a pair of mechanically mobile rings encircling a single dumbbell. These designs are reminiscent of a "molecular muscle" for the purposes of amplifying and harnessing molecular mechanical motions. The location of the two cyclobis(paraquat-p-phenylene) (CBPQT 4+ ) rings can be controlled to be on either tetrathiafulvalene (TTF) or naphthalene (NP) stations, either chemically ( 1 H NMR spectroscopy) or electrochemically (cyclic voltammetry), such that switching of inter-ring distances from 4.2 to 1.4 nm mimics the contraction and extension of skeletal muscle, albeit on a shorter length scale. Fast scan-rate cyclic voltammetry at low temperatures reveals stepwise oxidations and movements of one-half of the [3]rotaxane and then of the other, a process that appears to be concerted at room temperature. The active form of the bistable [3]rotaxane bears disulfide tethers attached covalently to both of the CBPQT 4+ ring components for the purpose of its self-assembly onto a gold surface. An array of flexible microcantilever beams, each coated on one side with a monolayer of 6 billion of the active bistable [3]rotaxane molecules, undergoes controllable and reversible bending up and down when it is exposed to the synchronous addition of aqueous chemical oxidants and reductants. The beam bending is correlated with flexing of the surfacebound molecular muscles, whereas a monolayer of the dumbbell alone is inactive under the same conditions. This observation supports the hypothesis that the cumulative nanoscale movements within surface-bound "molecular muscles" can be harnessed to perform larger-scale mechanical work.

Molecular switch based on a cucurbit[6]uril containing bistable [3]rotaxane

“…Matrix assisted laser desorption/absorption mass spectrometry (MALDI-tof-MS) also confirmed the formation of [3]rotaxane (Fig. 1).…”

“…However, there are only a few examples of rotaxanes which resemble linear artificial molecular muscles. [3][4][5] In these systems, the expansion and contraction caused by the movements of the hosts from one recognition site to the other can be monitored via spectroscopic methods ( 1 H NMR, UV-Vis and fluorescence), and cyclic voltammetry.Cucurbit[6]uril (CB6) and its homologues are among the hosts, which can be used in the preparation of rotaxanes. Cucurbituril is prepared from the acid condensation of glycoluril and formaldehyde; it has a hydrophobic cavity, along with two matching hydrophilic carbonyl portals.…”

“…However, there are only a few examples of rotaxanes which resemble linear artificial molecular muscles. [3][4][5] In these systems, the expansion and contraction caused by the movements of the hosts from one recognition site to the other can be monitored via spectroscopic methods ( 1 H NMR, UV-Vis and fluorescence), and cyclic voltammetry.…”

“…To date, many elegantly designed rotaxanes and catenanes have been reported. 2 When appropriately designed, bistable [3]rotaxanes in particular have great potential to act as stimuli-responsive artificial molecular muscles. However, there are only a few examples of rotaxanes which resemble linear artificial molecular muscles.…”

“…The small peaks at m/z 1559 and 1637 are due to the dumbbell of [3]rotaxane with one CB6 and the dumbbell with one CB6 and two potassium ions, respectively. The main peak at m/z 2556 corresponds to the molecular ion of [3]rotaxane. Fig.…”