Anomalous Optical Properties of Citrazinic Acid under Extreme pH Conditions

Abstract

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.

Polymerization‐Driven Photoluminescence in Alkanolamine‐Based C‐Dots

“…The fluorescence lifetime of the samples after different reaction times and the corresponding QY are shown in Figure 12. The decay profile of photoluminescence is analyzed with a two exponential law and the average lifetime has been calculated by using Equation [36] : …”

Reversible Aggregation of Molecular-Like Fluorophores Driven by Extreme pH in Carbon Dots

“…Figure 6 a shows the UV–Vis spectra of CU25 dots at the increase of pH values. At high pH, the deprotonation of the fluorophores [ 25 , 29 ] and the subsequent formation of carbonyl groups determine the rise of the absorption band at around 328 nm, assigned to the n–π* electron transitions of C=O bonds [ 30 ]. This process affects the emission yield with a gradual quenching of the green component and the appearance of the blue one ( Figure 6 b–d).…”

“…The results suggest the presence of two fluorophores, which have specific optical responses to a change of the chemical environment. In a recent work [25] it has been shown that the citrazinic acid can form aggregates with a weak green emission, if dissolved in acidic solutions ( Figure S2b). This phenomenon is explained with the protonation of the citrazinic acid molecules, associated with a blueshift and a weakening of the band at 275 nm.…”

“…This process affects the emission yield with a gradual quenching of the green component and the appearance of the blue one (Figure 6b-d). At pH 14, the molecular-like fluorophores are in a de-protonated form [25,28] which inhibits the formation of hydrogen bonds between fluorophores and avoids aggregation. This is in agreement with the disappearance of the absorption bands at 425 nm for CU2 ( Figure 5a) and 410 nm for CU25 ( Figure 5b) attributed to the aggregates [2].…”

“…How these fluorophores are sensitive to the external environment is, however, not clear. It has been reported that the variation of pH may induce fluorescence tuning and quenching [12,25].…”