Solvent Effects: A Signature of J- and H-Aggregate of Carbon Nanodots in Polar Solvents

Abstract

The secret behind excitation-dependent/-independent photoluminescence of carbon nanodots (CDs) is not yet revealed completely. To address this issue, a detailed investigation on solvent polarity-dependent optical properties of citric acid–urea co-derived nitrogen-doped carbon nanodots (NCDs) was carried out. The interpretation on UV–visible spectral data reveals the presence of H-aggregates formed through hydrogen bonding. In addition, dipole–dipole interaction-mediated J-aggregates are clearly evident. The broad and intense excitation band of NCDs is mostly contributed by highly emissive J-like self-assembly of H-aggregates in polar solvents. Time-resolved fluorescence spectra of NCDs show triexponential decay kinetics. The three lifetime components correspond to long-lived H-aggregates, short-lived J-aggregates, and JH-aggregates of intermediate lifetime. Moreover, fluorescence of NCD is influenced by concentration and storage time. Accordingly, mismatch in spectral shapes of excitation and absorption spectra of NCD can be successfully correlated to aggregate species of NCDs that exist even in very dilute solutions.

Role of the Anilinium Ion on the Selective Polymerization of Anilinium 2-Acrylamide-2-methyl-1-propanesulfonate

“…The π-stacking effect has been investigated in several molecules, analyzing solvent type, concentration, and time, among other variables [ 62 , 63 , 64 ]. Additionally, UV-Vis spectroscopy is one of the characterization techniques for identifying the presence of π-stacking [ 65 ].…”

“…This effect in Poly(Ani-AMPS) is produced by the inter- or intra-chain or both interactions among the anilinium cations, AMPS anion, and solvent. The equilibrium forces between the AMPS anion and anilinium cation (van der Waals forces, electrostatic repulsion, steric hindrance) allow the interaction and the final conformation, too [ 62 , 66 ].…”

Citric Acid-Based Carbon Dots and Their Application in Energy Conversion

“…It was shown that the size of these aggregates strongly depends on the concentration and that the maximum emission wavelength changes as the effect of the interaction with different solvents. A systematic investigation of the effect of nine polar solvents on the CA/U-CD absorption, emission, and lifetimes was performed by Anjali Devi et al The complex trends in the CA/U-CD optical features were explained with the self-assembly of the fluorophores into different coexisting structures, i.e., H-aggregates, J-aggregates, and combinations of the two, whose presence was also supported by TEM characterization. , Moreover, in the work of Bhuyan et al, it is shown how the reversible formation of aggregates from water to acetone goes in parallel with the blue emission suppression in favor of the green emission enhancement, reflecting the fluorescence resonance energy transfer (FRET)-mediated energy migration from the blue fluorophores to the green fluorophores inside the aggregate . Finally, Yao et al performed the microwave-assisted irradiation of citric acid and urea, achieving isolation by means of reversed phase C18 column chromatography of the fluorophore 5-oxopyrrolidine-3-carboxylic acid (OPCA) .…”

“…For example, many molecules identified among the unpurified CA-CD mixtures form aggregates, slowing down the purification process. On the other hand, these aggregates could correspond to the CA-CDs themselves, as pointed out in several studies; thus, dialysis would not be the best option for their purification. ,− Beside dialysis, chromatography techniques provide a useful approach for understanding the composition of the raw CDs. Silica column chromatography, − C18 reverse phase chromatography, , gel permeation chromatography, , size-exclusion chromatography, and ion exchange chromatography were exploited for the purification and/or characterization of the fluorophores for various CA-CD systems.…”

Solvent Effect on Structural Elucidation of Photoluminescent Graphitic Carbon Nanodots

“…On the other hand, a study on the effect of surrounding media, namely, solvent effect, revealed additional photophysical characteristics of the CNDs . Recent reports on the solvatochromic response of CNDs revealed important emission parameters. − …”