A fluorescent sensor for pH based on rhodamine fluorophore

In this study, an ‘Off–On’-type acidic pH fluorescent chemosensor, 3′,6′-Bis (ethylamino)-2′,7′-dimethyl-2-(2-oxoethylideneamino)spiro[isoindoline-1,9′-xanthen]-3-one (RG1) has been designed, synthesized, and characterized by high-resolution mass spectrometry (HRMS), X-ray crystallography, infrared spectroscopy (IR), and 1H NMR and 13C NMR spectroscopy. The design tactics for the sensor was based on the change in structure between spirocyclic (non-fluorescent) and ring-open (fluorescent) forms of rhodamine dyes. Fluorescence “off–on” behaviors of RG1 were investigated on the basis of variable acid concentrations. The pH titrations showed a ca. 581-fold increase in fluorescence intensity within the pH range of 7.5 to 1.1 with a pKa value of 2.32 in acetonitrile–water (1:1, v/v) solution when excited at 510 nm. The fluorescence change of RG1 was fully reversible and took place mainly within the pH range from 1.0 to 4.0, which was valuable for pH researches in acidic environments.

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► A pH fluorescent chemosensor RG1 based on rhodamine dye was synthesized.
► RG1 showed a 581-fold fluorescence enhancement in aqueous solution.
► The fluorescence change of RG1 is fully reversible.
► RG1 is valuable for pH researches in acidic environments with a pKa of 2.32.