Zwitterionic N₂O₂‑Type Protonated Dipyrrin Bearing a Phosphate Anionic Moiety as a pH-Responsive Fluorescence Indicator

Zwitterionic protonated-dipyrrin 1 bearing a phosphate unit was synthesized from the N₂O₂-type tetradentate dipyrrin ligand. Compound 1 is in equilibrium with the deprotonated form 1′ with a pK_a value of 5.8. Compound 1 exhibited a pH-responsive fluorescence under physiological conditions; the fluorescence intensity increased in aqueous media as the pH increased. In living cells, 1 also exhibited emission responsive to pH. Thus, 1 should be applicable as a pH probe for detecting tumor cells.