Fluorescent pH sensors based on BODIPY structure sensitive in acidic media

Two BODIPY phenolic esters 1 and 2 were synthesized and their acid-catalyzed hydrolytic and esterase enzymatic stability was demonstrated. Their photophysical properties were investigated in solvents of different polarity. Their spectral properties are the typical of BODIPY dyes with narrow absorption and emission spectra in the visible spectral region and large molar absorption coefficients (≈500 nm, ε = 40000-50000 M−1cm−1). The derivative with methyl substituted BODIPY core 2 shows quantum yields of fluorescence one magnitude higher (Φf = 0.30–0.55) compared to the BODIPY without methyl groups. Solvents affect very weakly spectral properties, with more pronounced effect of the solvent polarizability than the solvent polarity. Both BODIPY dyes show pH responsive fluorescence in aqueous solution (pKa = 2.72 ± 0.08 and 4.18 ± 0.05), with quenching of fluorescence by protonation due to PET from the BODIPY to phenol, rendering them applicable for intracellular pH measurements. The feasibility of the PET in the protonated form of 1 was corroborated by electrochemical measurements and calculation of ΔETGo for PET, and (time-dependent) density functional theory computations. This less frequently encountered quenching mechanism where the BODIPY is an electron donor provides new opportunities in the rational design of new generations of fluorescent sensors. The applicability for intracellular measurements was demonstrated by confocal fluorescence microscopy on H460 human cancer cell line.