Isopyrazole‐Masked Tetraketone: Tautomerism and Functionalization for Fluorescent Metal Ligands.

An isopyrazole (4H‐pyrazole)‐embedded nonane‐2,4,6,8‐tetraone derivative was synthesized as a masked‐tetraketone for stepwise and multiple functionalization. The masked‐tetraketone has six possible tautomers, but exists predominantly as a 12π‐electron conjugated keto‐enamine‐imine‐enol form, which was also observed in the solid state by single crystal X‐ray analysis. Stepwise functionalization to introduce metal‐coordination and fluorescence‐enhancement sites into the masked tetraketone furnished a fluorescent chromophore that indicates metal coordination on a microgram scale by considerably changing the solid‐state emission. [ABSTRACT FROM AUTHOR]