Mechanical (De)Activation of Rotaxane Mechanophores: Axle Length Matters

We recently introduced rotaxane-based supramolecular mechanophores that are composed of a cyclic molecule featuring a luminophore, which is threaded onto an axle molecule that contains a matching quencher. Elastic polymers containing such motifs exhibit instantly reversible photoluminescence changes upon application and removal of mechanical force due to the mechanically induced separation of emitter-quencher assemblies. Here, we report on the importance of the length of the rotaxane axle. We investigated thermoplastic polyurethane elastomers, which contain similar rotaxane mechanophores that differ only in axle length. While the increase of the fluorescence intensity observed when the materials are stretched follows the applied stress and is similar for all materials, the fluorescence intensity decay that occurs when the force is gradually reduced depends on the axle length, becoming slower when the axle length is increased. Materials containing rotaxanes with longer axles also show a slower reduction of the fluorescence intensity in stress relaxation experiments. The different relaxation behaviors appear to be related to kinetic effects, which are related to the increasing difficulty to re-establish the original assemblies of ring and quencher molecules as the axle length and therewith the translational degree of freedom are increased.