Reversible Spatial Control in Aqueous Media by Visible Light: A Thioindigo Photoswitch that is Soluble and Operates Efficiently in Water.

Aiming to extend the scope of indigoid photoswitches to polar protic environments, we have synthesized a sulfonated thioindigo derivative highly soluble in water. Studies by UV/Vis absorption, fluorescence, and NMR spectroscopy indicate that, despite aggregation effects at micromolar concentrations, the novel dye offers satisfactory performance in aqueous solution in the absence of solvation aides. Enrichment of the metastable cis isomer by irradiation may exceed 65 % and its half-life at room temperature may exceed several hours. Degradation after 20 yellow and blue light irradiation cycles within 2 hours is less than 2 %. Performance can be expected to further improve in future molecular designs if the tendency towards self-association is further reduced. Crucially, photoisomerization of indigoids is not necessarily inhibited by water and, thus, the superior spatial control offered by this class of molecular switches may be of great benefit also in biological systems.