Acid‐ and Nucleophile‐Gated Photoisomerization of Phosphaindirubin

Abstract 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.