Aggregated Molecular Fluorophores in the Ammonothermal Synthesis of Carbon Dots

Abstract

Recently, molecular fluorophores were shown to be formed in the bottom-up chemical synthesis, contributing to the emission of carbon dots (CDs), derived from a citric acid precursor. We applied an ammonothermal synthesis toward CDs, employing two reactants citric acid and supercritical ammonia functioning as both solvent and precursor. The resulting nanoparticles are identified as amorphous aggregates of molecular fluorophores based on citrazinic acid derivatives, which resemble many of the emission features typically reported to be characteristic for CDs. The aggregates absorb and emit at short and long wavelengths of the spectrum, a feature prior ascribed to intrinsic CD core and surface states, respectively. We identify three emission states: a high energy and a low energy aggregate state as well as an energy transfer state between both. Energy transfer is triggered only upon excitation within a narrow high energy spectral range, resulting in a characteristic bluegreen double emission. The high energy aggregate state exhibits a trapping mechanism elongating emission lifetime. To further analyze aggregated molecular fluorophores, we studied aqueous solutions and films of citrazinic acid and polyvinylpyrrolidone and demonstrated their concentration dependent optical behavior. Since fluorophore aggregates reproduce the emissive features of CDs, the contribution of sp 2 /sp 3 carbonized products and graphitic domains to the emission features of CDs must be carefully evaluated in future studies.

Excitons in Carbonic Nanostructures

“…In a number of brilliant studies, it was shown that, in C-dots formed by thermal treatment of citric acid + amine, the aromatic fluorophores and their associates are observed well before the carbonization occurs [18,[88][89][90][91]. Moreover, the fluorescent C-dots may totally consist of organic dyes [92][93][94]. As we seen below, these facts may suggest the principle of formation of ordered H-aggregated structures.…”

“…Whereas the "molecular view" is dominant in the discussions of fluorescence mechanisms of carbon nanoparticles [18,93,144], suggesting molecular emitters located in the particle core [19], being extended to the surface [20] or forming the whole particle [94], there are spectroscopic properties that do not fit. In addition to the discussed above (Section 3.3) systematically observed very strong Stokes shifts, carbonic nanoparticles may possess the fluorescence lifetimes, τ F , that are unexpectedly much longer than that of typical organic dyes (e.g., fluorescein and rhodamines [261]).…”

Customizing the Electrochemical Properties of Carbon Nanodots by Using Quinones in Bottom‐Up Synthesis

“…[49] Angewandte Chemie Zuschriften 5158 www.angewandte.de excitation wavelength-dependence is observed, leading to multi-color fluorescence under different excitation wavelengths.T herefore,their typical emission spectrum may arise not only from particles of different size but also from ad istribution of emissive domains and fluorophores on each dot. [57][58][59][60][61][62] All CNDs showed the typical wavelength-dependent emission in the blue region of the UV/Vis spectrum (Figure 2dand the Supporting Information, Figures S10 and S11).…”

Customizing the Electrochemical Properties of Carbon Nanodots by Using Quinones in Bottom‐Up Synthesis

“…The emission is attributed to core and surface state related emission, in particular when excitation wavelength‐dependence is observed, leading to multi‐color fluorescence under different excitation wavelengths. Therefore, their typical emission spectrum may arise not only from particles of different size but also from a distribution of emissive domains and fluorophores on each dot . All CNDs showed the typical wavelength‐dependent emission in the blue region of the UV/Vis spectrum (Figure d and the Supporting Information, Figures S10 and S11).…”