Photophysical properties of an on/off fluorescent pH indicator excitable with visible light based on a borondipyrromethene-linked phenol

A borondipyrromethene-derived pH indicator (available as methyl ester (1) and sodium salt (2)) for the near-neutral pH range with ultra bright fluorescence in the red spectral region has been synthesized by linking o-chlorophenol to the 3-position of a difluoroboradiazaindacene derivative. Absorption and steady-state and time-resolved fluorescence measurements have been used to study the photophysical properties of the BODIPY-based pH probe. The fluorescence lifetime (3.8 ± 0.2 ns) and the very high (nearly 1.0) fluorescence quantum yield of dye 1 are not dependent on the solvent. In aqueous solution, the water-soluble compound 2 undergoes a reversible protonation–deprotonation reaction in the near-neutral pH range with a pKa of 7.60, which is practically insensitive to low ionic strength. Fluorimetric titrations as a function of pH produce fluorescence emission enhancements at lower pH.