Ratiometric Flapping Force Probe That Works in Polymer Gels

A ratiometric flapping force probe that can evaluate the nanoscale stress concentration in the polymer chain network of common organogels has been developed. Stress-dependent dual-fluorescence properties of the chemically doped flapping force probe has been demonstrated even when the probe is solvated in the wet materials (Figure 1). The fluorescence ratiometric analysis is robust against the local concentration change induced by the macroscopic polymer deformation. While the force-responsive FRET dyads, widely used in mechanobiology, are sensitive to the distance and orientation of the two chromophores, the flapping fluorophore works as a single-component flexible force probe regardless of the FRET efficiency. Realtime and reversible spectral response to the mechanical stress is observed with a low threshold on the order of sub-MPa compression due to its conformational flexibility. The previously reported flapping probe only shows a negligible response in the solvated environments because the undesired spontaneous planarization occurs in the S1 excited state, even without mechanical force. The excited-state engineering by changing the flapping wings from the anthraceneimide units to the pyreneimide units endows this molecule with the force probe function in the wet conditions. The structurally modified force probe also has an advantage in terms of a wide dynamic range of the fluorescence response in solvent-free elastomers, which enabled the ratiometric fluorescence imaging of the molecular-level stress concentration during the crack growth in a stretched polyurethane film. The percentage of the stressed force probes has been experimentally estimated to be approximately 30–40% before the fracture of the elastomers. The flapping force probe is useful for elucidating the toughening mechanism of recently focused unique topological gels and elastomers at molecular level.