![]() ![]() Density functional theory (DFT) for the ground state and time-dependent DFT calculations for excited states have been performed for the gas phase and for aqueous solution using a polarized continuum model. Using a combination of computational methods, we analyzed the UV absorption and fluorescence properties of the IPCA monomer and stacked IPCA dimers as basic models for the fluorescent centers in CDs. 5-oxo-1,2,3,5-tetrahydroimidazo-pyridine-7-carboxylic acid (IPCA), a molecular fluorophore, which is being created during the synthesis of CDs from citric acid and ethylenediamin, has been identified as an origin for the fluorescence of CDs. The explanation of the origin of the fluorescence properties of carbon dots (CDs) represents an important task because of the great interest in the promising capabilities of these nanomaterials. 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. The computed vibrational fingerprint of the formation of aggregates is confirmed by surface-enhanced Raman spectroscopy. 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. 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. 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. 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. 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. ![]()
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