The integration of mechanophores, motifs that transduce mechanical forces into chemical reactions, allows creating materials with stress-dependent properties. Typical mechanophores are activated by cleaving weak covalent bonds, but these reactions can also be triggered by other stimuli, and this renders the behavior unspecific. Here we show that this problem can be overcome by extending the molecular-shuttle function of rotaxanes to mechanical activation. A mechanically interlocked mechanophore composed of a fluorophore-carrying macrocycle and a dumbbell-shaped molecule containing a matching quencher was integrated into a polyurethane elastomer. Deformation of this polymer causes a fluorescence turn-on, due to the spatial separation of fluorophore and quencher. This process is specific, efficient, instantly reversible, and elicits an easily detectable optical signal that correlates with the applied force.

Three mechanoresponsive polyurethane elastomers whose blue, green, and orange photoluminescence can be reversibly turned on by mechanical force were prepared and combined to create a blend that exhibits deformation-induced white photoluminescence. The three polyurethanes contain rotaxane-based supramolecular mechanoluminophores based on π-extended pyrene, anthracene, or 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4 H -pyran (DCM) luminophores, respectively, and 1,4,5,8-naphthalenetetracarboxylic diimide as an electronically matched quencher. Each polymer shows instantly reversible, strain-dependent switching of its photoluminescence intensity when stretched and relaxed, as deformation leads to a spatial separation of the luminophore and quencher. The present study shows that the photoluminescence color can easily be tailored by variation of the luminophore and also by combining several mechanophores in one material and demonstrates that adaptability is a key advantage of supramolecular approaches to create mechanoresponsive polymers.

Polymers that display useful mechanochemical responses, such as changes of their fluorescence characteristics, are attracting great interest. Here, we introduce the fluorescent dithiomaleimide (DTM) motif as a mechanofluorophore and report the mechanoresponse of two polymer types containing this motif. Poly(methyl acrylate) (PMA) and poly(ε-caprolactone)s (PCL) featuring one DTM moiety in the center of each chain (PMA-DTM and PCL-DTM) were synthesized by controlled radical and coordination-insertion ring-opening polymerizations using bifunctional DTM-containing initiators. Upon ultrasonic treatment of PMA-DTM or PCL-DTM of sufficiently high initial molecular weight, both the molecular weight and the fluorescence intensity decreased with similar kinetics, while no significant fluorescence changes were observed for DTM-free reference polymers. The results show that the DTM motif can serve as a mechanophore that displays a mechanically induced fluorescence turn-off.