BODIPY-Based Hydroxyaryl Derivatives as Fluorescent pH Probes

Seven new 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dyes with phenolic or naphtholic subunits on position 8 and with substituents having different electron driving forces on positions 3 and 5 were synthesized. Their absorption and steady-state fluorescence properties were investigated as a function of solvent. The novel compounds, with the exception of 4,4-difluoro-8-(4-hydroxyphenyl)-3,5-bis-(4-methoxyphenyl)-4-bora-3a,4a-diaza-s-indacene, are characterized by absorption maxima in the range 493−515 nm and small (400−600 cm-1) Stokes shifts. The exceptional dye has absorption maxima in the 570−580 nm region and fluorescence emission maxima around 610−620 nm, depending on the solvent. In aqueous solution, the dyes show a large fluorescent enhancement upon increasing the acidity of the solution. They can be used in aqueous solution as fluorescent pH probes excitable with visible light, with pK a values ranging from 7.5 to 9.3, depending on the substitution pattern on positions 3, 5, and 8.