Unexpected Acid-Triggered Formation of Reversibly Photoswitchable Stenhouse Salts from Donor-Acceptor Stenhouse Adducts.

Donor-acceptor Stenhouse adducts (DASAs) are reversibly photoswitchable dyes, which are able to interconvert between a red/NIR absorbing triene-like state and a colorless cyclic state. Although optically attractive for multiple applications, their low solubility and lack of photoswitching in water impede their use in aqueous environments. We developed water-soluble DASAs based on indoline as donor and methyl, or trifluoromethyl, pyrazolone-based acceptors. In acetonitrile, photophysical analysis and photochemical studies, accounted with a three-state kinetic model, confirmed the reversible photoswitching mechanism previously proposed. In water, the colorless cyclic state is a thermodynamic sink at neutral pH values. In contrast, in acidic conditions, we observed a fast scrambling of DASAs' end-group resulting in the in situ formation of Stenhouse salts (StS), which are in turn capable of reversible photoswitching. We believe that this unexpected result is of interest not only for the future design of DASAs with improved stability, but also for further development and applications of StS as photoswitchable probes.