An Integrated Theoretical and Spectroscopic Analysis of the pH-Triggered Ring-Opening Mechanism in a Prototype Rhodamine-Based Fluorescent Dye

A novel ratiometric fluorescent platform, composed of a rodamine derivative and dansyl moiety, was designed and synthesised as a prototype sensor capable of responding to proton concentration. It is well known that, under neutral or basic conditions, rhodamine derivatives in their spirolactam form do not absorb or emit in the visible range. However, metal or proton ions can induce spirolactam ring opening, resulting in visible absorption and strong fluorescence emission. Although many rhodamine derivatives have been developed to detect metal ions or pH changes, the sensing mechanism related to spirolactam ring opening remains not fully understood. To address this, the hybrid platform described in this work was investigated across a wide pH range, particularly under high proton concentration, to study and clarify the proton-mediated ring opening mechanism of the rhodamine spirolactam. This investigation combined spectrophotometric and potentiometric measurements, supported by DFT calculations.