Fluorescent pH-Responsive Probes Based on Water-Soluble Boron-Dipyrromethene (BODIPY) Derivatives, Featuring Long-Wavelength Emission

We describe here the synthesis of water-soluble red/NIR-emissive, boron-dipyrromethene (BODIPY) derivatives displaying optical (absorption and emission) responses in pH range of 4-8. Substitution close to the tertiary aniline or the phenol subunits selected as the proton-sensitive sites allowed us to finely tune the pH ranges. Furthermore, the introduction of sulfobetaine functions at the boron centre of these pH-responsive BODIPYs afforded valuable fluorescent dyes in the red/NIR region in aqueous media, for which the steric hindrance and electrostatic repulsions prevent their non-emissive aggregation. All the absorption and emission studies, as well as the protonation properties were investigated in aqueous, ethanolic and saline solutions (mimicking physiological conditions). Interestingly, the systems present a fluorescent ratiometric protonation response in EtOH, but the non-protonated form is almost a non-fluorescent species under quasi-physiological conditions (saline aqueous solutions) due to the fading of the emissive character of the low-lying charge-transfer transition in the presence of a supporting electrolyte.