Force-activated ratiometric fluorescence switching of tensile mechano-fluorophoric polyurethane elastomers with enhanced toughnesses improved by mechanically interlocked [c2] daisy chain rotaxanes

The extended and contracted conformations of [c2] daisy chain rotaxanes were introduced in mechano-fluorophoric polyurethane (PU) scaffolds via step-growth polymerizations to obtain distinct mechanical and optical features upon external forces. Particularly, the inherent ductile and stretchable capabilities along with notable toughnesses of PU films containing very low contents of [c2] daisy chain moieties with long-range sliding motions were obviously improved. Furthermore, appealing Förster resonance energy transfer (FRET) behaviors with the optimum energy transfer efficiency ca. 26.02% between blue-emitting tetraphenylethylene (TPE) and yellow-orange-emitting rhodamine derivatives could be acquired in the PU film under tensile conditions, which could be assigned to the incorporation of bi-fluorophoric daisy chain rotaxane into PU frameworks to gain reversible dual fluorescence switching behaviors upon stretching and relaxation processes. Additionally, the stretching deformations of PU films were inspected by X-ray diffraction (XRD) and FTIR techniques for the verification of correlated morphological properties of stretching states in the oriented daisy chain rotaxane-based PU films. Interestingly, impressive shape recovery and reversible ratiometric mechano-fluorophoric fluorescence switching properties of [c2] daisy chain rotaxane-based PU films could be attained upon heating, suggesting potential applications of PU films comprising artificial molecular muscles with glorious mechanical and optical behaviors in advanced polymer networks.