Design and Investigation of a Series of Rhodamine-Based Fluorescent Probes for Optical Measurements of pH

A series of structurally similar fluorescent probes (1-4), synthesized from rhodamine B, were designed to optically measure pH. Each probe had a unique off-on response as the solution went from basic to acidic. Probes 1-3 exhibited a spirocyclic quenching of the pyronin B fluorophore, whereas probe 4 was quenched by PET from the amine moiety.