Mechanically Activated Luminescence in Polyurethanes Incorporating Calixarene Mechanophores

The conformational properties of cone calix[4]arenes are exploited to develop the first example of a calixarene-based mechanoluminophore. By functionalizing the distal positions at the upper rim of a calix[4]arene with pyrene moieties, we produce a macrocycle incorporating two 1,6-bis(phenylethynyl)pyrene fluorophores. The flexibility of the calix[4]arene scaffold facilitates the formation of intramolecular ground-state excimers involving the pyrene units, as evidenced by ¹H NMR and fluorescence spectroscopy. Incorporating the calixarene mechanoluminophore into linear polyurethanes yields uniform films that exhibit the characteristic pyrene excimer emission. When these films are stretched, either manually or using a universal tensile tester, a fluorescence shift from green-yellow to blue is observed under irradiation with UV light. Such fluorescence change is reversible and repeatable over numerous stress and release cycles. We attribute this behavior to conformational changes in the calix[4]arene triggered by mechanical force, and in particular to the temporary dissociation of the intramolecular pyrene excimers into pyrene monomers. Thus, the flexible calix[4]arene scaffold enables the reversible separation of the two fluorophores while preserving their spatial proximity. We propose that this design concept can provide general guidelines for creating supramolecular mechanophores free of intermolecular aggregation effects.