Acid‐ and Nucleophile‐Gated Photoisomerization of Phosphaindirubin

Nature uses protonation and microenvironmental effects to modulate photoisomerization, as seen in rhodopsins and GFP. Inspired by this, we report phosphaindirubin (PI), a visible-light responsive photoswitch bearing a stereogenic phosphorus center that exhibits reversible Z/E isomerization controlled by light, acid, and nucleophiles. While structurally related to photoinert isoindigo, phosphaindirubin undergoes efficient Z→E isomerization in low-polarity solvents but remains inert in polar media unless protonated. Acid gating alters the excited-state landscape, enabling switching under light irradiation in acetonitrile. Strikingly, the thermal back-isomerization of PI is accelerated by nucleophiles, including pyridine and iodide, offering an underexplored mechanism for catalyzing double bond rotation. This triple-responsiveness to light, acid, and nucleophile enables reversible, fatigue-resistant cycling between Z- and E-forms. These findings introduce a new design principle for photoswitches based on dynamic, multi-stimuli gating of excited-state and ground-state reactivity.