ESIPT-active fluorescent probes for ratiometric analysis of carbonate ions in aqueous media: structural modifications for enhanced selectivity and response

This study focuses on the design and synthesis of two ESIPT-active fluorescent probes that differ in the position of the hydroxyl group. The isomer with the hydroxyl group at the <i>ortho</i> position in compound 1 exhibited a large population of the keto tautomer in equilibrium, forming more compact nanoaggregates in aqueous media. In contrast, the enol form was predominant in compound 2, which has the hydroxyl group at the terminal position, leading to significantly larger aggregate sizes compared to the first probe. Both compounds demonstrated a ratiometric color-changing response with carbonate ions, accompanied by a turn-on fluorescence response. Mechanistic studies revealed that hydrogen bonding (or possible deprotonation) induced by carbonate ions influenced the keto-enol equilibrium and conformational flexibility, which affected both the optical properties and aggregation behavior of the probes. Comparative analysis showed that compound 1 had a superior response towards carbonate ions compared to probe 2. The position of the hydroxyl group altered the binding cleft available for carbonate ions. Furthermore, the naphthyl-substituted analogue of probe 1 formed larger aggregates and exhibited a stronger response to carbonate ions than probe 1 itself. Cross-reactivity studies confirmed the high selectivity of compound 1 for carbonate ions, with minimal interference from other anions. These results motivated the use of the current system for analyzing carbonate ions in natural water samples collected from the laboratory tap and nearby ponds. The high recovery rates and low standard deviation confirmed the suitability of this system for the quantitative analysis of carbonate in real-life samples.