摘要： We have synthesized and characterized a thioindigo photoswitch featuring phenolic residues forming an intramolecular H bond exclusively in the cis isomer. In acetone water mixtures, wavelength dependent photostationary states their extremes of up to 95 % and down to 11 % cis isomers at ca. 580 and 480 nm, respectively. Within a few hours, room temperature thermal isomerization is negligible in neutral and mildly acidic solutions. Protic media decrease trans→cis but increase cis→trans quantum yields. In CD3OD solution, in which light of any wavelength absorbed triggers the latter process almost exclusively, 19F NMR titration provides pKa values of 7.2 (trans) and 5.0 (cis; both rescaled to water) corresponding to a change in dissociation constant of 2.2 orders of magnitude. Future derivatives featuring solubility and reversible photochromism in plain water may provide pH modulation of similar amplitude in a range adjustable by the choice of phenolic substituents.