Carbon Dot-like Molecular Nanoparticles, Their Photophysical Properties, and Implications for LEDs

Abstract

Carbon dot-like molecular nanoparticles (CMPs) exhibit strong photoluminescence (PL) emissions that can be tuned throughout the visible spectrum. Recently, CMPs gained attention due to excitation-independent PL emission and higher PL quantum yield (QY) compared to conventional carbon dots (CDs). In this study, CMPs composed of molecular fluorophores are synthesized, under mild conditions, by a solvothermal method and their structure is investigated. The CMPs have an average size of 19.5 nm with a single-digit PL QY from 3.5 to 9.4% depending upon the solvents they are dispersed in. Two distinct excitationwavelength-independent PL emission bands are observed from the CMPs: one is a deep-violet emission band centered at ∼400 nm and another is a green emission band at ∼520 nm. Structural and optical characterizations reveal that the CMPs are composed of two different types of molecular fluorophores. The molecular fluorophores are separated by dissolving the CMPs in low-polarity solvents and separating the molecules by solvent extraction. The PL QY of the green-emitting molecules after separation increases to 53.6%. Nuclear magnetic resonance characterization reveals that the violet-emitting molecules mainly have an aliphatic structure with an abundance of oxygen, while the green-emitting molecules have an aromatic structure and are identified as N 4 ,N 11 -dimethyldibenzo[a,h]phenazine-4,11-diamine (BPD). Finally, timedependent density functional theory (TD-DFT) calculations are carried out to further study the excited states in the BPD molecules and to confirm that the green emission arises from the same molecules. Furthermore, BPD shows strong PL QY when dispersed in polymers, which is relevant for applications in UV-pumped light-emitting devices.

Synthesis and Characterization of Amorphous Lawsone Polymer Dots for Fluorescent Applications

“…This fluorophore is effectively shielded from solvent induced excited state deactivation pathways, leading to enhanced emission intensity and fluorescence quantum yield (37%), which is significantly higher than the quantum yield of pure lawsone (<0.5%). This suggests that certain electronic transitions, inherently present in lawsone, can be stabilized through some type covalent/supramolecular cross-linking or aggregation during the synthesis process. , The stabilization of these electronic states and enhanced emission are consistent with a CEE mechanism. Further analysis of the mirror symmetry shown in the excitation and emission spectra provide further evidence of a molecular-like fluorophore in the LPDs due to the observation of vibronic progression as found in other CD systems. ,, Vibronic progression occurs when a certain electronic transition couples with a vibrational mode to produce multiple excitation and emission peaks that are evenly spaced.…”

“…This suggests that certain electronic transitions, inherently present in lawsone, can be stabilized through some type covalent/supramolecular cross-linking or aggregation during the synthesis process. , The stabilization of these electronic states and enhanced emission are consistent with a CEE mechanism. Further analysis of the mirror symmetry shown in the excitation and emission spectra provide further evidence of a molecular-like fluorophore in the LPDs due to the observation of vibronic progression as found in other CD systems. ,, Vibronic progression occurs when a certain electronic transition couples with a vibrational mode to produce multiple excitation and emission peaks that are evenly spaced. The spacing in energy between the subsequent excitation and emission peaks can be correlated with the vibrational mode of a functional group.…”

“…This suggests that certain electronic transitions, inherently present in lawsone, can be stabilized through some type covalent/supramolecular cross-linking or aggregation during the synthesis process. 46,47 The stabilization of these electronic states and enhanced emission are consistent with a CEE mechanism. Further analysis of the mirror symmetry shown in the excitation and emission spectra provide further evidence of a molecular-like fluorophore in the LPDs due to the observation of vibronic progression as found in other CD systems.…”

“…Further analysis of the mirror symmetry shown in the excitation and emission spectra provide further evidence of a molecular-like fluorophore in the LPDs due to the observation of vibronic progression as found in other CD systems. 6,47,48 Vibronic progression occurs when a certain electronic transition couples with a vibrational mode to produce multiple excitation and emission peaks that are evenly spaced. The spacing in energy between the subsequent excitation and emission peaks can be correlated with the vibrational mode of a functional group.…”

Polypropylene-Derived Luminescent Carbon Dots

“…Carbon dots (CDs), as a quasi-0D material class, have been actively investigated since the first report by Xu et al in 2004. , The attractive optical properties of CDs, including tunable photoluminescence , and high quantum yield, as well as their distinctive low cost and toxicity, , are relevant for bioimaging sensors, − ion detectors, − and optoelectronic devices. ,− Apart from applications based on luminescence, CDs are promising in electrodes and for conductive inks. , Early discoveries of CDs involved energy-intensive top-down methods, , where large-area carbon sources such as graphite and graphene are used as starting materials. More recently, less energy-intensive synthesis approaches have been explored in which organic molecules, or carbon-rich polymers are utilized as precursors for the reaction.…”