No abstract

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. 5-Oxo-1,2,3,5-tetrahydroimidazo-[1,2-α]-pyridine-7-carboxylic acid (IPCA), a molecular fluorophore, which is being created during the synthesis of CDs from citric acid and ethylenediamine, has been identified as an origin for the fluorescence of CDs. 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. 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. Classical molecular dynamics (MD) simulations of the dimer in the ground state have been carried out as well to investigate spontaneous association processes of IPCA and to analyze the ground state dynamics. Due to the complex charge distribution of the monomer and various possibilities of forming hydrogen bonds, in total, seven dimer structures have been identified at the DFT level as ground state minima with similar energies. Stabilities have been confirmed by domain-based local pair natural orbital (DLPNO) coupled cluster with singles and doubles and perturbative triples CCSD­(T) calculations. The MD simulations confirm this picture, showing rotational flexibility processes of the two monomers with respect to each other. The lowest excited states have been characterized in terms of their orbital excitations and excitonic splitting. These calculations demonstrate the dominance of monomer π → π* transitions for the explanation of the observed UV spectra. Optimization of the dimer in the excited state did not lead to well-defined single structures but shows the picture of either intersection pathways to S1/S0 crossings which would quench fluorescence or stacked dimers with red-shifted fluorescence in comparison to the UV absorption. In the gas phase, both types of processes have been observed, whereas in solution only the stacked structures were found without any non-adiabatic radiationless deactivation processes.

Carbon dots (CDs), one of the youngest members of the carbon nanostructure family, are now widely experimentally studied for their tunable fluorescence properties, bleaching resistance, and biocompatibility. Their interaction with biomolecular systems has also been explored experimentally. However, many atomistic details still remain unresolved. Molecular dynamics (MD) simulations enabling atomistic and femtosecond resolutions simultaneously are a well-established tool of computational chemistry which can provide useful insights into investigated systems. Here we present a full procedure for performing MD simulations of CDs. We developed a builder for generating CDs of a desired size and with various oxygen-containing surface functional groups. Further, we analyzed the behavior of various CDs differing in size, surface functional groups, and degrees of functionalization by MD simulations. These simulations showed that surface functionalized CDs are stable in a water environment through the formation of an extensive hydrogen bonding network. We also analyzed the internal dynamics of individual layers of CDs and evaluated the role of surface functional groups on CD stability. We observed that carboxyl groups interconnected the neighboring layers and decreased the rate of internal rotations. Further, we monitored changes in the CD shape caused by an excess of charged carboxyl groups or carbonyl groups. In addition to simulations in water, we analyzed the behavior of CDs in the organic solvent DMF, which decreased the stability of pure CDs but increased the level of interlayer hydrogen bonding. We believe that the developed protocol, builder, and parameters will facilitate future studies addressing various aspects of structural features of CDs and nanocomposites containing CDs.