Water‐Soluble Pd ₈ L ₄ Barrel for Binding of Versatile Hydrophobic Dyes and Visible‐Light Promoted Catalysis in Aqueous Medium

ABSTRACT Achieving efficient supramolecular recognition of hydrophobic dyes and their use as photosensitizers in water remain a major challenge because of their inherent insolubility and loss of activity in aqueous medium. We address this issue by designing a metal‐organic barrel ( M ) that combines water solubility with a spacious hydrophobic cavity. This barrel functions as a versatile host capable of binding a broad range of polyaromatic guests, from small hydrocarbons to fullerenes. Interestingly, the barrel protects fragile fluorophores such as benzothiadiazole, benzoselenadiazole, quinoxaline, and BODIPY dyes from aggregation‐caused quenching (ACQ), thereby preserving their excited‐state integrity in water. Binding of the hydrophobic benzothiadiazole‐dithiophene (BDT) photosensitizer within the barrel transforms it into an efficient photocatalyst in water, enabling C─H functionalization of indole derivatives with malonate. Overall, this work demonstrates a water‐soluble barrel that serves as a powerful host capable of solubilizing hydrophobic dyes and enabling interesting catalytic reactivity directly in water.