Fluorescent push–pull pH-responsive probes for ratiometric detection of intracellular pH

A family of fluorescent push–pull pH-responsive probes based on 2-dicyanomethylidene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran as a strong electron acceptor group is described. Small structural variations allow obtaining pKa ranging from 4.8 to 8.6, underlining the role of the substituent in modulating the acidic properties. Remarkable changes in the optical properties (in particular the fluorescence intensity ratios) were observed as a function of pH. The most interesting probes with pKa close to neutrality were used for ratiometric imaging of intracellular pH.