Anomalous Optical Properties of Citrazinic Acid under Extreme pH Conditions

Citrazinic acid (CZA) is a weakly fluorescent molecular compound whose optical properties are dependent on aggregation states and chemical environment. This molecule and its derivatives have been recently identified as the source of the intense blue emission of carbon dots obtained from citric acid with a nitrogen source, such as ammonia or urea. Citrazinic acid has a strong tendency to aggregate and form tautomers whose optical properties are largely unexplored. At extreme acidic and basic pH values, we have observed an "anomalous" optical response of citrazinic acid, attributed to the formation of aggregates from the tautomers. We have characterized the molecule, both at pH = 1 and 14, using UV-vis, NMR, steady-state, and time-resolved fluorescence spectroscopy. At extremely low pH values, the protonation causes luminescence quenching and the appearance of new emissions. On the contrary, high pH values are responsible for deprotonation and splitting of the excitation spectra.