Planarization of Twisted Push-Pull Probes by Stretching Rather than by Compression: Core-Substituted Fluorescent Flippers as Materials Mechanosensors.

Fluorescent flippers are twisted push-pull mechanophores that report planarization with red-shifted absorption and an increase in fluorescence intensity and lifetime. Until today, their planarization by physical forces has focused on compression to visualize physical forces in biology. Here, we show that planarization can also be achieved by stretching of flipper probes that are equipped with tethers in their core and to visualize mechanical stress in polymeric materials. The synthesis of dithieno-[3,2-:2',3'-]-thiophene dimers with alcohols extending from their twisted core is accomplished in 17 steps. Covalently integrated into polyurethanes, these core-substituted flippers exhibit an excitation-wavelength-dependent fluorescence enhancement upon polymer stretching, validating their mode of action. Noncovalently interfaced flipper controls are much less responsive. In light of the importance their fluorogenic compression has reached in biology, synthetic access to flippers that can be stretched rather than compressed and use in materials rather than life sciences opens up significant, fundamentally new perspectives for flipper research.