Abstract
<jats:p>Mechanofluorescent mechanophores enable the direct visualization of mechanical stress and damage in polymeric materials. The sensitivity of these systems depends on both efficient force transduction to the reactive unit and the brightness of the fluorophore generated upon activation. Here, we report coumarin–styrene cyclobutane mechanophores designed to overcome the limitations of our previously reported coumarin–acrylate system, using a phenylcontaining linkage to promote more efficient force transduction and an ether-substituted coumarin to increase the fluorescence quantum yield of the activated product. The mechanophores were prepared in two or three metal-free synthetic steps through a catalyst-free, visible-light-driven [2+2] photocycloaddition and were readily incorporated into linear poly(methyl acrylate) (PMA), linear poly(methyl methacrylate) (PMMA), and cross-linked PMA networks. CoGEF calculations and geometric analysis predicted a lower rupture force and more favorable alignment of the applied force with the scissile cyclobutane bond than for the previously reported coumarin–acrylate analogue. Upon ultrasonication in solution, centrally incorporated mechanophores in linear PMA underwent force-induced cyclobutane scission to form the corresponding coumarin and styrene products, as confirmed by size-exclusion chromatography, UV-vis absorption and fluorescence spectroscopy, and 1H NMR spectroscopy. Cross-linked PMA networks containing the mechanophore also exhibited a pronounced fluorescence response during tensile deformation, which was monitored in situ by fluorescence spectroscopy. Scratching mechanophore-functionalized films of linear PMMA generated localized fluorescence that could be directly visualized by fluorescence microscopy. These results establish coumarin–styrene cyclobutane mechanophores as a modular and synthetically accessible platform in which force transduction and fluorescence output can be tuned to enable sensitive detection and imaging of mechanical activation in polymeric materials.</jats:p>