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 ¹H 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.