Synthesis and Acid-Sensing Behavior of Rhodamine-Thiazolothiazole Conjugates

This study reports the synthesis and characterization of two novel fluorescent conjugates, RhodTzTzCHO and RhodTzTzRhod, which integrate rhodamine B hydrazide units with a thiazolo[5,4-d]thiazole (TzTz) core via phenyl bridges. While rhodamine derivatives typically undergo ring-opening in acidic media, these conjugates exhibit a particularly unusual pH-dependent behavior: they remain in the nonfluorescent, closed spirolactam form across a wide range of acidic environments, including hydrochloric, sulfuric, and acetic acids. Significant ring-opening, evidenced by the emergence of intense colour and fluorescence, occurs exclusively in the presence of trifluoroacetic acid. This unique selectivity was thoroughly investigated using nuclear magnetic resonance, mass spectrometry, and density functional theory calculations to identify possible protonated structures in solution. Our results demonstrate that the ring opening in both RhodTzTzCHO and RhodTzTzRhod conjugates is strongly influenced by the electronic properties of the bonded groups and the solvent, leading to an extraordinary stabilization of the protonated form with the spirocyclic ring closed, particularly in dimethylformamide. These findings establish rhodamine-TzTz conjugates as robust sensors for specific acidic conditions, suggesting possible utility in contexts where selective acid detection is required.