Solvent-Free Photoresponsive Artificial Muscles Rapidly Driven by Molecular Machines

We prepared photoresponsive actuators as both hydrogels and dry gels consisting of 4-arm poly(ethylene glycol) (PEG) cross-linked by a [c2]daisy chain, which is a double-threaded [2]rotaxane dimer with alpha-cyclodextrin (alpha CD) and stilbene. The obtained gels showed fast and large deformation triggered by UV irradiation in both wet and dry states. The UV/vis spectroscopy results, NMR measurements and tensile tests on the gels revealed that the actuation is driven by photoisomerization of the stilbene unit in the [c2]daisy chain. The responsiveness of these gels depends on the molecular weight of the 4-arm PEG. These results suggest that alpha CD recognizes trans-stilbene prior to UV irradiation to maintain the length of the PEG chain in the polymer network and that photoisomerization allows alpha CD to leave the cis-stilbene moiety and move onto the PEG chain because the association constant of alpha CD with cis-stilbene is quite low. Thus, the sliding motion of the alpha CD unit shrinks the [c2]daisy chain, leading to the contraction of the gels. In both wet and dry states, these actuations are repeatable through reversible photoisomerization of the stilbene moiety using different wavelengths of UV-light irradiation and can be used to perform bending and lifting actions (for 15 times heavier weight compared to the dry gel).