Insight into the Molecular Model in Carbon Dots through Experimental and Theoretical Analysis of Citrazinic Acid in Aqueous Solution

Abstract

The molecular emission model is the most accredited one to explain the emission properties of carbon dots (CDs) in a low-temperature bottom-up synthesis approach. In the case of citric acid and urea, the formation of a citrazinic acid (CZA) single monomer and oligomers is expected to affect the optical properties of the CDs. It is therefore mandatory to elucidate the possible role of weak bonding interactions in determining the UV absorption spectrum of some molecular aggregates of CZA. Although this carboxylic acid is largely exploited in the synthesis of luminescent CDs, a full understanding of its role in determining the final emission spectra of the produced CDs is still very far to be achieved. To this aim, by relying on purely first-principles density functional theory calculations combined with experimental optical characterization, we built and checked the stability of some molecular aggregates, which could possibly arise from the formation of oligomers of CZA, mainly dimers, trimers, and some selected tetramers. The computed vibrational fingerprint of the formation of aggregates is confirmed by surface-enhanced Raman spectroscopy. The comparison of experimental data with calculated UV absorption spectra showed a clear impact of the final morphology of the aggregates on the position of the main peaks in the UV spectra, with particular regard to the 340 nm peak associated with n-π* transition.

Synthesis and Photophysical Properties of Fluorescent 6-Aryl-D-π-A Coumarin Derivatives

“…Ground and excited states were simulated within DFT and TDDFT frameworks, respectively, by exploiting the B3LYP hybrid functional with the 6-31G(d,p) basis set as already reported for similar compounds. 19 The simulations were accomplished under vacuum and with a model solvent, chloroform. In the latter cases, solvation effects arising from the interaction of coumarin derivatives with chloroform were treated with the self-consistent reaction field model by simulating the dielectric solvent through the polarizable continuum model calculation with-in the integral equation formalism.…”

Towards N–N-Doped Carbon Dots: A Combined Computational and Experimental Investigation

“…The whole set of experimentally observed features here reported calls for the formation of CDs: the rounded nanoparticle observed by means of TEM (mean size 5-12 nm), the measured D and G vibrational bands, the absorption band around 350 nm, the excitation dependent emission spectra and the non-single exponential decay times. As already proposed for parent CA derived systems, those features are related to the formation of some specific molecule, as the IPCA and CZA molecules reported in those cases [11,28,65,66]. Starting from this consideration, we hypothesized that the pyridinone molecules produced during the synthesis of CA and hydrazine, their aggregated forms or some polymeric structure derived by further reactions of the monomer units can be incorporated into the structure of the CDs and produce the optical…”

“…None of the precursor vibrational modes were present in both the 1350-1400 and 1550-1600 cm −1 regions, whilst the pyridinone structure shows two bands at 1400 and 1580 cm −1 . To further explore the scenario, we considered for the Hy-CD case the formation of some typical aggregates already proposed for the CZA and IPCA systems [65][66][67][68][69], namely the parallel and anti-parallel stack dimers and the head-to-tail in plane dimer. We calculated the absorption spectra for the aggregate systems at a fixed distance between the monomer units, namely at 0.366 nm for the stack dimers and at 0.288 for the heat-to-tail dimers, assuming the same geometries of the parent CZA system.…”

Advances in the Methods for the Synthesis of Carbon Dots and Their Emerging Applications

“…In particular, CDs were considered as nanoparticles with sp 3 -hybridized amorphous carbon cores that contain partially sp 2 -hybridized carbon domains, and it was predicted that the emission of CDs can be red shifted by increase of hybridization factor of those domains within CDs [ 159 ]. Another theoretical investigation showed that covalent-bonded dimers of polyaromatic hydrocarbons at the CDs surface, in contrast to non-interacting monomers, resulted in a red shift and broadening of the PL band, with a decrease in its intensity [ 160 ]. CQDs are widely used in dye-sensitized solar cells, LEDS, photovoltaic cells, electroluminescent CD-based LEDs and CDs in Perovskite solar cells.…”