Mechanosensitive Fluorescent Probes, Changing Color Like Lobsters during Cooking: Cascade Switching Variations

Fluorescent flipper probes have been introduced recently to image physical forces in biology. Their design is inspired by the combination of planarization and polarization that makes the color of astaxanthin, a carotenoid, turn blue in living lobsters or shrimps. Flipper probes are constructed around twisted dithienothiophene dimers. Upon planarization, donors and acceptors placed on both sides are coupled to generate push-pull systems that shift excitation maxima to the red, while the emission wavelength is mechanoinsensitive. To assure chemical stability, these donors and acceptors have to turn on only upon planarization. In living lobster, this is achieved most beautifully with non-covalent hydrogen bonds to and from the surrounding, planarizing protein. With flipper probes, the unorthodox chalcogen bonds prove best to produce turn-on donors and acceptors. The specific objective of this study was to explore different turn-on donors for the resulting chalcogen-bonding cascade switches. The focus is on substitution of the original triazoles with ethylenedioxythiophene (EDOT) and ortho-hydroxyphenyl (HOP) donors. Design, synthesis and evaluation of the respective flipper probes are described.