(37) Of peculiar interest, in the dry state, the corresponding xerogel is able to perform a fast pseudo-reversible deformation induced by an alternation of UV and visible lights. (34) Another important advance was made by the group of Harada who synthesized a covalent 3D polymer network integrating daisy chains in which cyclodextrins (rings) bind to photoswitchable azobenzene groups (axles). In these examples, high degrees of polymerization were demonstrated, leading for instance to ( i) microscopic extension/contraction of single chain polymers (33) ( ii) supramolecular polymerization/depolymerization process (36) ( iii) changes of mesophase morphologies in bundles of supramolecular polymers (35) and ( iv) macroscopic sol–gel transitions in supramolecular polymer networks. (32) Our group recently introduced coordination (33) and hydrogen-bonded (34−36) main chain supramolecular polymers involving pH-responsive bistable daisy chain rotaxanes ( Figure 1b).
(27−30) The first covalent oligomers based on daisy chain rotaxanes have been described independently by the groups of Stoddart (31) and Grubbs. By introducing different binding stations for the macrocycles on the axles, the controlled gliding of their subunits can lead to internal contraction and extension based on the dynamic of their mechanical bond. (26) Their complex molecular structure involves two rings linked to two axles that are double threaded ( Figure 1a). (21−25)Ī particularly interesting class of artificial nanomachines to be further coupled in space and time are bistable daisy chain rotaxanes (so-called “molecular muscles”). (17−20) Following these lines, a few recent works have demonstrated the possible integration of large numbers of artificial molecular machines in order to connect them with the outside world. (12−16) However, to make use of the work produced by these nanomachines, one should link them to other elements which can amplify and/or transduce their mechanical motion in something useful. (7−9) Their individual functioning can be actuated at thermodynamic equilibrium (molecular switches) (10,11) or out-of-equilibrium (molecular motors). (3,4) Partly inspired by biomolecular machines, (5,6) chemists have synthesized a number of artificial molecules capable of relative internal translations or rotations. (1,2) These important biological responses can be modeled as a mechanical continuum going from subcellular components to tissues and organs. In living systems, the integration in space and time of protein machineries with the dynamic structuring of the cytoskeleton is central to many functions such as growth, motility, contraction, and mechanotransduction.