Conformational Behavior and Optical Properties of a Fluorophore Dimer as a Model of Luminescent Centers in Carbon Dots

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 Detect Water-to-Ice Phase Transition and Act as Alcohol Sensors via Fluorescence Turn-Off/On Mechanism

“…Vertical ionization energies and electron affinities of model structures representing carbon core, hydrophilic CD surface, and 5-oxo-1,2,3,5-tetrahydroimidazo­[1,2-α]­pyridine-7-carboxylic acid (IPCA) (see structures in Figure S21) were computed at the CAM-B3LYP/ aug -cc-pVTZ − /CPCM , level of theory using the geometries optimized at the CAM-B3LYP/6-31G­(d)/CPCM level (coronene) or taken from references. , All calculations were performed with the Gaussian16 program…”

Formation of Citrazinic Acid Ions and Their Contribution to Optical and Magnetic Features of Carbon Nanodots: A Combined Experimental and Computational Approach

“…This approach focuses on CND molecular state which was largely confirmed in previous works [ 10 , 11 , 12 , 13 ]. The molecule-like model was also supported by quantum-chemistry calculations, where different N-doped aromatic moieties were considered, together with their aggregated forms, as the origin of the optical properties of bottom-up synthesized CNDs [ 14 , 15 , 16 , 17 , 18 ].…”

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

“…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 features experimentally recorded. Indeed, if we consider the case of IPCA, a reaction of cyclization between CA and ethylene-diamine was hypothesized to produce the molecule that acts as a seed for the formation of the CDs and cause, with its aggregates, the excitation dependent emission features recorded in those systems [28,67,68]. In a similar way, we recently studied the formation of CZA in the synthesis of CA and urea (the reference system in the present study) and we demonstrated that the formation of various ionic species and different aggregates can explain the optical features of citric acid derived CDs [65,66,69,70].…”

“…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.…”