Synthetic Strategies for the Selective Functionalization of Carbon Nanodots Allow Optically Communicating Suprastructures

The surface of Carbon Nanodots (CNDs) stands as a rich chemical platform, able to regulate the interactions between particles and external species. Performing selective functionalization of these nanoscale entities is of practical importance, however, it still represents a considerable challenge. In this work, we exploited the organic chemistry toolbox to install target functionalities on the CND surface, while monitoring the chemical changes on the material's outer shell through nuclear magnetic resonance spectroscopy. Following this, we investigated the use of click chemistry to covalently connect CNDs of different nature en-route towards covalent suprastructures with unprecedent molecular control. The different photophysical properties of the connected particles allowed their optical communication in the excited state. This work paves the way for the development of selective and addressable CND building blocks which can act as modular nanoscale synthons that mirror the long-established reactivity of molecular organic synthesis.

Supramolecular Chemistry of Carbon-Based Dots Offers Widespread Opportunities

Carbon dots are an emerging class of nanomaterials that has recently attracted considerable attention for applications that span from biomedicine to energy. These photoluminescent carbon nanoparticles are defined by characteristic sizes of <10 nm, a carbon-based core and various functional groups at their surface. Although the surface groups are widely used to establish non-covalent bonds (through electrostatic interactions, coordinative bonds, and hydrogen bonds) with various other (bio)molecules and polymers, the carbonaceous core could also establish non-covalent bonds (ππ stacking or hydrophobic interactions) with π-extended or apolar compounds. The surface functional groups, in addition, can be modified by various post-synthetic chemical procedures to fine-tune the supramolecular interactions. Our contribution categorizes and analyzes the interactions that are commonly used to engineer carbon dots-based materials and discusses how they have allowed preparation of functional assemblies and architectures used for sensing, (bio)imaging, therapeutic applications, catalysis, and devices. Using non-covalent interactions as a bottom-up approach to prepare carbon dots-based assemblies and composites can exploit the unique features of supramolecular chemistry, which include adaptability, tunability, and stimuli-responsiveness due to the dynamic nature of the non-covalent interactions. It is expected that focusing on the various supramolecular possibilities will influence the future development of this class of nanomaterials.

Synthetic chiral molecular nanographenes: the key figure of the racemization barrier

Chirality is one of the most intriguing concepts of chemistry, involving living systems and, more recently, materials science. In particular, the bottom-up synthesis of molecular nanographenes endowed with one or several chiral elements is a current challenge for the chemical community. The wilful introduction of defects in the sp² honeycomb lattice of molecular nanographenes allows the preparation of chiral molecules with tuned band-gaps and chiroptical properties. There are two requirements that a system must fulfill to be chiral: (i) lack of inversion elements (planes or inversion centres) and (ii) to be configurationally stable. The first condition is inherently established by the symmetry group of the structure, however, the limit between conformational and configurational isomers is not totally clear. In this feature article, the chirality and dynamics of synthetic molecular nanographenes, with special emphasis on their racemization barriers and, therefore, the stability of their chiroptical properties are discussed. The general features of nanographenes and their bottom-up synthesis, including the main defects inducing chirality in molecular nanographenes are firstly discussed. In this regard, the most common topological defects of molecular NGs as well as the main techniques used for determining their energy barriers are presented. Then, the manuscript is structured according to the dynamics of molecular nanographenes, classifying them in four main groups, depending on their respective isomerization barriers, as flexible, detectable, isolable and rigid nanographenes. In these sections, the different strategies used to increase the isomerization barrier of chiral molecular nanographenes that lead to configurationally stable nanographenes with defined chiroptical properties are discussed.

Carbon Nanodots for All-in-One Photocatalytic Hydrogen Generation

Carbon nanodots (CNDs) were photochemically altered to produce dihydrogen under light irradiation. Within the complex structure of CNDs, photo-oxidation takes place at citrazinic acid molecular fluorophore sites. Important is the fact that the resulting CND materials have a dual function. On one hand, they absorb light, and on the other hand, they photo- and electrocatalytically produce dihydrogen from water and seawater, without any external photosensitizer or cocatalyst. Record HER activities of 15.15 and 19.70 mmol(H₂) g(catalyst)^-1 h^-1 were obtained after 1 h of 75 mW/cm² Xe lamp illumination, from water and seawater, respectively. This impressive performance outweighs the remaining structural uncertainties. A full-fledged physicochemical investigation based on an arsenal of steady-state and time-resolved spectroscopic characterizations together with microscopy enabled a comprehensive look into the reaction mechanism. For an efficient dihydrogen formation, a precatalytic activation by means of reduction with a sacrificial electron donor is imperative.

Interfacing Carbon Dots for Charge-Transfer Processes

Carbon dots (CDs) are a booming material and the most recent incomer in the big family of carbon nanostructures. Specifically, CDs are nanosized fluorescent core-shell nanoparticles with tunable absorption and emission spectra, with high solubility in aqueous media and common organic solvents. Herein, the origins and the development of these unique nanoscale structures are discussed, key synthetic routes are briefly described, and the utilization of CDs in light-induced charge-transfer schemes is mainly focused upon. Beyond the impact of the CD's surface on the photoluminescence properties, functionalization, by covalent or supramolecular means, permits controllable incorporation of new functionalities with novel photophysical properties. Furthermore, the dual nature of CDs as electron donating or electron accepting species, unveiled upon interfacing them with organic chromophores, highlights their potentiality in managing diverse charge-transfer processes. Novel mechanisms, such as symmetry-breaking photoinduced charge-transfer can be activated upon covalent functionalization of CDs with organic dyes. Without a doubt, participation of CDs in energy conversion schemes opens up a wide avenue that may lead to the development of novel prototype devices suitable for technological applications and related to photonics and optoelectronics.

Symmetry-Breaking Charge-Transfer Chromophore Interactions Supported by Carbon Nanodots

Carbon dots (CDs) and their derivatives are useful platforms for studying electron-donor/acceptor interactions and dynamics therein. Herein, we couple amorphous CDs with phthalocyanines (Pcs) that act as electron donors with a large extended π-surface and intense absorption across the visible range of the solar spectrum. Investigations of the intercomponent interactions by means of steady-state and pump-probe transient absorption spectroscopy reveal symmetry-breaking charge transfer/separation and recombination dynamics within pairs of phthalocyanines. The CDs facilitate the electronic interactions between the phthalocyanines. Thus, our findings suggest that CDs could be used to support electronic couplings in multichromophoric systems and further increase their applicability in organic electronics, photonics, and artificial photosynthesis.

Orthogonal Adsorption of Carbon Dots and DNA on Nanoceria

Carbon dots (CDs) are highly fluorescent nanomaterials with surface carboxyl and amino groups. However, their exact structure remains under debate. In this work, we probed the surface properties of CDs by physically adsorbing them onto various nanomaterials. Three types of nanomaterials, including CeO₂ nanoparticles (nanoceria), gold nanoparticles, and graphene oxide were tested. Among them, nanoceria strongly adsorbed the CDs and quenched their fluorescence. For the tested anions to compete with the CDs for adsorption, only phosphate and F^- induced desorption of the CDs from nanoceria, and the phosphate-induced desorption was less compared to that by F^-. This was opposite to the desorption of DNA from nanoceria, where phosphate induced more DNA desorption. Furthermore, using calcein and fluorescein as representative dyes for comparison, we conclude that the CDs might use their carboxyl groups to adsorb on nanoceria, while DNA uses its phosphate backbone for adsorption. This difference may explain their occupying different surface sites on nanoceria and different displacement by phosphate and F^-. Using nanomaterials as probes to understand the surface properties of CDs is effective, and such understanding might in turn be used for building hybrid materials for applications.

Ping-Pong Energy Transfer in Covalently Linked Porphyrin-MoS2 Architectures

Molybdenum disulfide nanosheets covalently modified with porphyrin were prepared and fully characterized. Neither the porphyrin absorption nor its fluorescence was notably affected by covalent linkage to MoS2 . The use of transient absorption spectroscopy showed that a complex ping-pong energy-transfer mechanism, namely from the porphyrin to MoS2 and back to the porphyrin, operated. This study reveals the potential of transition-metal dichalcogenides in photosensitization processes.

Use of Nitrogen-Doped Carbon Nanodots for the Photocatalytic Fluoroalkylation of Organic Compounds

The use of amine-rich N-doped carbon nanodots (NCNDs) for the photochemical radical perfluoroalkylation of organic compounds is reported. This operationally simple approach occurs under mild conditions producing valuable new C-C bonds. The chemistry is driven by the ability of NCNDs to directly reach an electronically excited state upon light absorption, thereby successively triggering the formation of reactive radical species from simple perfluoroalkyl iodides. Preliminary mechanistic studies are also reported.

Carbon Nanodots for Charge-Transfer Processes

In recent years, carbon nanodots (CNDs) have emerged as an environmentally friendly, biocompatible, and inexpensive class of material, whose features sparked interest for a wide range of applications. Most notable is their photoactivity, as exemplified by their strong luminescence. Consequently, CNDs are currently being investigated as active components in photocatalysis, sensing, and optoelectronics. Charge-transfer interactions are common to all these areas. It is therefore essential to be able to fine-tune both the electronic structure of CNDs and the electronic communication in CND-based functional materials. The complex, but not completely deciphered, structure of CNDs necessitates, however, a multifaceted strategy to investigate their fundamental electronic structure and to establish structure-property relationships. Such investigations require a combination of spectroscopic methods, such as ultrafast transient absorption and fluorescence up-conversion techniques, electrochemistry, and modeling of CNDs, both in the absence and presence of other photoactive materials. Only a sound understanding of the dynamics of charge transfer, charge shift, charge transport, etc., with and without light makes much-needed improvements in, for example, photocatalytic processes, in which CNDs are used as either photosensitizers or catalytic centers, possible. This Account addresses the structural, photophysical, and electrochemical properties of CNDs, in general, and the charge-transfer chemistry of CNDs, in particular. Pressure-synthesized CNDs (pCNDs), for which citric acid and urea are used as inexpensive and biobased precursor materials, lie at the center of attention. A simple microwave-assisted thermolytic reaction, performed in sealed vessels, yields pCNDs with a fairly homogeneous size distribution of ∼1-2 nm. The narrow and excitation-independent photoluminescence of pCNDs contrasts with that seen in CNDs synthesized by other techniques, making pCNDs optimal for in-depth physicochemical analyses. The atomistic and electronic structures of CNDs were also analyzed by quantum chemical modeling approaches that led to a range of possible structures, ranging from heavily functionalized, graphene-like structures to disordered amorphous particles containing small sp² domains. Both the electron-accepting and -donating performances of CNDs make the charge-transfer chemistry of CNDs rather versatile. Both covalent and noncovalent synthetic approaches have been explored, resulting in architectures of various sizes. CNDs, for example, have been combined with molecular materials ranging from electron-donating porphyrins and extended tetrathiafulvalenes to electron-accepting perylendiimides, or nanocarbon materials such as polymer-wrapped single-walled carbon nanotubes. In every case, charge-separated states formed as part of the reaction cascades initiated by photoexcitation. Charge-transfer assemblies including CNDs have also played a role in technological applications: for example, a proof-of-concept dye-sensitized solar cell was designed and tested, in which CNDs were adsorbed on the surface of mesoporous anatase TiO₂. The wide range of reported electron-donor-acceptor systems documents the versatility of CNDs as molecular building blocks, whose electronic properties are tunable for the needs of emerging technologies.

Charge transfer in graphene quantum dots coupled with tetrathiafulvalenes

Water-soluble fluorescent graphene quantum dots have been successfully prepared through a top-down approach, that is, starting with graphite, and covalently functionalizing it with π-extended tetrathiafulvalene. Charge transfer investigations reveal noticeably slower charge recombination when compared with exTTF nanoconjugates featuring carbon nanodots, for which a larger presence of trap states is observed.