Fluorescent pH probes based on boron dipyrromethene dyes

Two novel fluorescent pH probes based on 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene dyes were configured as a “fluorophore-spacer-receptor” system. Their absorption and fluorescence properties were investigated in various solvents; in aqueous solution, based on their photo-induced electron transfer, the probes exhibited 3-fold (pH 8.70–4.93) and 14-fold (pH 9.02–3.29) fluorescence enhancement upon increase in the acidity of the solution, respectively. The probes can be used as fluorescent pH probes which are excitable with visible light; the pKa values of the probes were determined from the fluorescence changes.