A Reversible Rhodamine B Based pH Probe with Large Pseudo-Stokes Shift

A reversible and sensitive pH probe DPE-Rh operates by Förster resonance energy transfer from 1,2-diphenylethyne (DPE) to Rhodamine B (Rh). In the presence of H⁺ , the spirolactam ring of the Rhodamine B unit was opened and this resulted in ca. 1000-fold enhancement of fluorescence intensity with linear change over the pH range of 2.0 to 5.5. The Förster resonance energy transfer offered this probe an effective excitation-emission wavelength shift of around 240 nm with about 100 % quenching of the donor emission. The response of the sensor is tolerant towards a wide range of metal ions and the sensing mechanism was deduced by ¹ H NMR spectrometry. This FRET-based molecule not only provides a sensitive pH probe, but also suggests an effective strategy to eliminate the interference of excitation light.