Modulating the Optical Properties of Citrazinic Acid through the Monomer-to-Dimer Transformation

Abstract

Understanding the luminescence of carbon dots is a highly challenging task because of the complex reactions involved in the synthesis process. Several by-products form at different reaction stages and become possible sources of emission. Citrazinic acid and its derivatives, in particular, have been identified as intermediates that give rise to blue fluorescence. Full comprehension of the optical properties of citrazinic acid itself is, however, still lacking. In particular, citrazinic acid has the property of forming different tautomers and aggregates such as dimers. However, the nature of these chemical species and the correlation with their relative optical properties have been only partially explored. In the present work, we have used a combination of spectroscopic techniques, UV−visible and fluorescence spectroscopy, time-resolved photoluminescence and computational simulation, to study the different species which citrazinic acid forms in water as a function of CZA molarity. A monomer-todimer transformation and a hypsochromic shift are observed with concentration. The monomer is in the keto structure and does not form other tautomers while the dimers are fluorescent J-type aggregates. The formation of aggregates strongly modulates the optical properties of citrazinic acid.

Polymerization‐Driven Photoluminescence in Alkanolamine‐Based C‐Dots

“…The spectra are characterized by two main absorption bands peaking at 338 nm and at 230 nm. They are assigned to n–π* and π–π* transitions, respectively [24] …”

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

“…The CU2 C-dots are characterized by a blue fluorescence ( Figure 1 b,c) peaking at 450 nm, which corresponds to the characteristic citrazinic acid emission in water [ 27 ]. The C-dots show also a weaker green component assigned to the formation of a fluorophore identified with HPPT.…”

“…Citric acid and urea derived C-dots are known to exhibit these bands [12]. Nevertheless, they are also detected in the absorption spectrum of citrazinic acid dissolved in water [27]. Correspondingly, the CU2 C-dots absorption spectrum is mainly identified by molecular origin, as a result of the formation of citrazinic acid molecule by citric acid and urea reaction at high temperature (190 • C).…”

Oil-Dispersible Green-Emitting Carbon Dots: New Insights on a Facile and Efficient Synthesis

“…Indeed, 2-pyridones are a prominent class of fluorophores, well known, inter alia, as dyes, pigments or fluorescent bio-markers [52,53], which have been reported to form upon oxidation or disproportionation reactions, resulting from the treatment of pyridinium salts with NaOH under mild conditions [54]. In general, the formation, alongside the CDs, of blue-emitting molecular fluorophores has been largely demonstrated in popular CDs synthetic approaches based on the carbonization of citric acid in the presence of di-or triamines [26][27][28][29][30][31]. In particular, blue-emitting 2-pyridone fluorophores, such as citrazinic acid or imidazo[1,2-α]pyridine-7-carboxylic acid, 1,2,3,5-tetrahydro-5-oxo-(IPCA), have been identified and isolated [26,27].…”

“…In fact, a fundamental understanding of the synthetic processes is essential in order to direct the synthesis to the achievement of CDs with well-defined properties that may, finally, address technological applications. Furthermore, it has been widely shown, in the hydrothermal or microwave-assisted syntheses, that fluorescent molecules can be originated alongside the CDs in the reactions, and it is commonly accepted that such species significantly contribute, becoming even predominant, to the overall emission of the CDs [26][27][28][29][30][31]. Therefore, the identification of such emitting species and their comprehensive spectroscopic characterization is fundamental in order to distinguish the CDs' emission contribution.…”