Self-Assembly of Nanographenes

Nonplanar Nanographene with a Large Conjugated π-Surface

Conjugated π-surfaces are ubiquitous in molecules and materials. However, large π-surfaces up to a few nanometers in size are difficult to construct in an atomically precise manner. They tend to aggregate because of strong π-π interactions, resulting in notorious problems for both purification and spectroscopic investigations. Here, by contrast, we report the design, synthesis, and full characterizations of a nonplanar nanographene 1, which has a large, precise, and nonstacked π-surface. It is soluble in common organic solvents and allows for thorough investigations. The structure of 1, comprising 85 fused rings with an extended π-surface of 3 nm in size, is unambiguously confirmed by single-crystal X-ray diffraction. Unusual electronic structures, record-high near-infrared absorption, pronounced magnetic shielding, and ultrastrong heteromolecular van der Waals complexations are demonstrated, enabling us to establish a clear structure-property relationship, which has been elusive for decades. These results have broad implications for studying and understanding various phenomena and processes relevant to both discrete and interacting π-surfaces.

Triterpenoids-templated self-assembly nanosystem for biomimetic delivery of CRISPR/Cas9 based on the synergy of TLR-2 and ICB to enhance HCC immunotherapy

Combination immunotherapy has shown promising potential for enhancing the objective response rate compared to immune checkpoint blockade (ICB) monotherapy. However, combination therapy with multi-drugs is limited by the different properties of the agents and inconsistent synergistic targeted delivery. Herein, based on a universal triterpene template and the anticancer active agent ursolic acid (UA), a cytomembrane-coated biomimetic delivery nanoplatform (UR@M) prepared by the self-assembly of a PD-L1 targeted CRISPR/Cas9 system and UA was designed for hepatocellular carcinoma (HCC) treatment. UR@M showed enhanced tumor accumulation in vivo with homologous tumor targeting, and CRISPR in the nanosystem exhibited potent gene-editing efficiency of 76.53% in vitro and 62.42% in vivo with no off-target effects. UA activated the natural immune system through the TLR-2-MyD88-TRAF6 pathway, which synergistically enhanced the proliferation of natural killer cells and dendritic cells and realized excellent immune cytotoxic T cell infiltration by combining with the ICB of PD-L1. The strategy of work along both lines based on innate immune and adaptive immunity displayed a significant effect in tumor regression. Overall, the UA-templated strategy "killed three birds with one stone" by establishing a self-assembly nanosystem, inducing tumor cell death, and promoting synergistic immunostimulation for HCC treatment.

Intermediate Color Emission via Nanographenes with Organic Fluorophores

Photoluminescence (PL) color can be tuned by mixing fluorophores emitting the three primary colors in an appropriate ratio. When color tuning is achieved on a single substrate, we can simplify device structures. We demonstrated that nanographenes (NGs), which are graphene fragments with a size of tens of nanometers, could be utilized as carriers of fluorophores. The addition of red- and blue-light-emitting fluorophores on the edge successfully reproduced the purple light. The relative PL intensities of the fluorophores could be regulated by the excitation wavelength, enabling multicolor emission between blue and red light. Owing to the triphenylamine units of the fluorophores, the NGs showed PL enhancement due to aggregation. This characteristic was valuable for the fabrication of solid polymer materials. Specifically, the functionalized NGs can be dispersed into polyvinylidene difluoride. The resultant polymer films emitted red, blue, and purple color. Our study demonstrated the potential applicability of NGs for fluorophore carriers capable of reproducing intermediate colors of light.

Tailoring coordination environments of single-atom electrocatalysts for hydrogen evolution by topological heteroatom transfer

The rational design of carbon-supported transition-metal single-atom catalysts requires the precise arrangement of heteroatoms within the single-atom catalysts. However, achieving this design is challenging due to the collapse of the structure during the pyrolysis. Here, we introduce a topological heteroatom-transfer strategy to prevent the collapse and accurately control the P coordination in carbon-supported single-atom catalysts. As an illustration, we have prepared self-assembled helical fibers with encapsulated cavities. Within these cavities, adjustable functional groups can chelate metal ions (Nx···Mn+···Oy), facilitating the preservation of the structure during the pyrolysis based phosphidation. This process allows for the transfer of heteroatoms from the assembly into single-atom catalysts, resulting in the precise coordination tailoring. Notably, the Co-P2N2-C catalyst exhibits electrocatalytic performance as a non-noble metal single-atom catalyst for alkaline hydrogen evolution, attaining a current density of 100 mA cm-2 with an overpotential of only 131 mV.

Application of Exciton Coupling for Characterization of Nanographene Edge

The structural characterization of nonstoichiometric nanographene (NG)-organic hybrid materials is usually difficult. The number of substituents on the edge and their arrangements are frequently questioned but are difficult to answer. Since the number of functional groups is closely related to the distance between the nearest neighbors (dISD ), the extraction of dISD from spectroscopic data could provide important information on their structural characterization. We show that exciton coupling, which is a theoretical prediction of the absolute structures of discrete molecules, is a possible candidate to address this issue. The comparison of the calculated CD spectra of the chiral chromophores extracted from the model NG edge with the observed edge spectra indicated a dISD of ca. 8 Å; this corresponded to substitution on every other armchair edge. Furthermore, an up-up-down-down alternate orientation was found to be a possible edge structure. Although the procedure was limited to NGs carrying chiral substituents, our method could facilitate the detailed structural characterization of NG-organic hybrid materials.

Facile band gap tuning in graphene-brucite heterojunctions

The zero band gap of pure graphene is a well-known issue that limits some specific applications of graphene in opto- and microelectronics. This led to several research studies in the so-called van der Waals composites (known as heterostructures, or heterojunctions), where two monolayers of different materials are stacked and held together by dispersive interactions. In this paper, we introduced and considered a single layer of brucite Mg(OH)2, an overlooked 2D material that can be easily produced by exfoliation (like graphene from graphite), for the creation of the heterojunction. First principles simulations showed that brucite/graphene composites can modify the electronic properties (position of the Dirac cone with respect to the Fermi level and band gap) according to the crystallographic stacking and the presence of point defects. The present work represents then an important step forward in understanding and finding new ways to design two-dimensional materials with tailored electronic and physical properties.

Light-driven self-assembly of spiropyran-functionalized covalent organic framework

Controlling the number of molecular switches and their relative positioning within porous materials is critical to their functionality and properties. The proximity of many molecular switches to one another can hinder or completely suppress their response. Herein, a synthetic strategy involving mixed linkers is used to control the distribution of spiropyran-functionalized linkers in a covalent organic framework (COF). The COF contains a spiropyran in each pore which exhibits excellent reversible photoswitching behavior to its merocyanine form in the solid state in response to UV/Vis light. The spiro-COF possesses an urchin-shaped morphology and exhibits a morphological transition to 2D nanosheets and vesicles in solution upon UV light irradiation. The merocyanine-equipped COFs are extremely stable and possess a more ordered structure with enhanced photoluminescence. This approach to modulating structural isomerization in the solid state is used to develop inkless printing media, while the photomediated polarity change is used for water harvesting applications.

Esterase-Activated, pH-Responsive, and Genetically Targetable Nano-Prodrug for Cancer Cell Photo-Ablation

Activatable prodrugs have drawn considerable attention for cancer cell ablation owing to their high specificity in drug delivery systems. However, phototheranostic prodrugs with dual organelle-targeting and synergistic effects are still rare due to low intelligence of their structures. Besides, the cell membrane, exocytosis, and diffusional hindrance by the extracellular matrix reduce drug uptake. Moreover, the up-regulation of heat shock protein and short singlet-oxygen lifetime in cancer cells hamper photo-ablation efficacy, especially in the mono-therapeutic model. To overcome those obstacles, we prepare an esterase-activated DM nano-prodrug, which is conjugated by diiodine-substituted fluorogenic malachite green derivative (MG-2I) and phototherapeutic agent DPP-OH via hydrolyzable ester linkage, having pH-responsiveness and genetically targetable activity for dual organelles-targeting to optimize photo-ablation efficacy. The DM nanoparticles (NPs) present improved pH-responsive photothermal/photodynamic property by the protonation of diethylaminophenyl units in acidic environment. More importantly, the MG-2I and DPP-OH moieties can be released from DM nano-prodrug through overexpressed esterase; then specifically target lysosomes and mitochondria in CT-26 Mito-FAP cells. Hence, near-infrared DM NPs can trigger parallel damage in dual-organelles with strong fluorescence and effective phototoxicity, thus inducing serious mitochondrial dysfunction and apoptotic death, showing excellent photo-ablation effect based on esterase-activated, pH-responsive, and genetically targetable activities.

Substituent-Induced Supramolecular Aggregates of Edge Functionalized Nanographenes

Precisely controlled molecular assemblies often display intriguing morphologies and/or functions arising from their structures. The application of the concept of the self-assembly for controlling the aggregation of nanographenes (NGs) is challenging. The title NGs are those carrying both long alkyl chains and tris(phenylisoxazolyl)benzene (TPIB) on the edge. The former group secures the affinity of NGs for organic solvents, and the latter group drives the 1D arrangement of NGs through the interactions between the TPIB units. The concentration-dependent and temperature variable ¹ H NMR, UV-vis, and PL spectra demonstrate the aggregation of NGs in 1,2-dichloroethane, and the aggregation is controllable by the regulation of the solvent polarity. AFM images give the stacked structures of the NGs, and these aggregates turn out to be network polymeric structures at a high concentration. These observations demonstrate that the synergy of the face-to-face interactions between the surfaces and the interactions between the TPIB units are effective for controlling the self-assembly of the NGs.

Computational Studies on the Structures of Nanographenes with Various Edge Functionalities

Computational studies have often been carried out on hydrogen-terminated nanographenes (NGs). These structures are, however, far from those deduced from experimental observations, which have suggested armchair edges with two carboxy groups on the edges as dominant. We conducted computational studies on NGs consisting of C₄₂ , C₆₀ , C₇₈ , C₉₆ , C₁₄₂ , and C₁₇₄ carbon atoms with hydrogen, carboxy, and N-methyl imide-terminated armchair edges. DFT calculations inform distorted basal planes and similar HOMO-LUMO gaps, indicating that the edge oxidation and functionalization do not very influence the electronic structure. Comparison of observed UV-vis spectra of carboxy- and N-octadecyl chain terminated NGs with calculated spectra of model NGs informs the contribution of π-π* transitions on the basal plane to the absorptions in the visible region. A dimeric structure of NG and octadecyl-installed NG demonstrate that both the distorted basal planes and the steric contacts among the functional groups widen the surface-to-surface distance thereby allowing the invasion of solvent molecules between the surfaces. This picture is consistent with the improved solubility of edge-modified NGs.

Supramolecular Assembly of Edge Functionalized Top-Down Chiral Graphene Quantum Dots

The construction of supramolecular assemblies of heterogeneous materials at the nanoscale is an open challenge in science. Herein, new chiral graphene quantum dots (GQDs) prepared by amidation reaction introducing chiral amide groups and pyrene moieties into the periphery of GQDs are described. The analytical and spectroscopic data show an efficient chemical functionalization and the morphological study of the supramolecular ensembles using SEM and AFM microscopies reveals the presence of highly ordered fibers of several micrometers length. Fluorescence studies, using emission spectroscopy and confocal microscopy, reveal that the fibers stem from the π-π stacking of both pyrenes and GQDs, together with the hydrogen bonding interactions of the amide groups. Circular dichroism analysis supports the chiral nature of the supramolecular aggregates.

Integration of Nanographenes and Organic Chemistry - Toward Nanographene-based Two-Dimensional Materials

Graphene and its relatives have received considerable attention from the fields of physics and chemistry since the isolation of pristine graphene sheets. Nanographenes (NGs) are graphene fragments that are a few to tens of nanometers in diameter. Compared to graphene and its relatives, such as graphene oxides, NGs can be handled more easily, and their large π surface and oxygen functional groups on the edge allow postsynthetic modifications. The study of NGs is gradually shifting from the development of synthetic procedures to postsynthetic modification. From the structural point of view, NGs can be regarded as two-dimensional carbon polymers. Their unique structures and affinity for organic molecules make NGs excellent scaffolds for two-dimensional materials, which are now an important topic in organic and polymer chemistry. In this conceptual article, we introduce the position of NGs from the perspective of two-dimensional substances and briefly summarize both the structural features of NGs and the effects of functionalization on their physical properties. These are valuable when producing reasonable strategies for their postsynthetic modifications.

Template-Free Synthesis of Porous Fluorescent Carbon Nanomaterials with Gluten for Intracellular Imaging and Drug Delivery

A series of carbon nanomaterials, including carbon dots, carbon nanorings (CNRs), and porous carbon nanoballs, were facilely prepared by a template-free hydrothermal treatment of gluten as the sole carbon source. Driven by the hydrophobicity interaction, a concentration-dependent self-assembly of gluten was observed in an aqueous solution, leading to the subsequent formation of different morphologies of carbon nanomaterials in a hydrothermal treatment. Among these carbon nanomaterials, the CNRs exhibit bright photoluminescence with a quantum yield of 47.0%. Furthermore, CNRs also have a large surface area and low toxicity, making them an excellent drug carrier for chemotherapeutics. A model drug molecule doxorubicin (DOX) was successfully loaded on the CNRs, and the CNRs-DOX complexes exhibit a pH-dependent DOX release behavior. Compared with free DOX, the CNRs-DOX complexes can induce a higher level of apoptosis and lower level of necrosis, showing promise as anticancer agents.

Fluorescence Enhancement by Supramolecular Sequestration of a C54-Nanographene Trisimide by Hexabenzocoronene

A supramolecular trilayer nanographene complex consisting of a newly synthesized D_3h-symmetric C₅₄-nanographene trisimide (NTI 1) and two hexabenzocoronenes (HBC) has been obtained by self-assembly. This 1:2 complex is structurally well-defined according to UV/vis and single crystal X-ray studies and exhibits high thermodynamic stability even in polar halogenated solvents. Complexation of NTI 1 by two HBC molecules protects the NTI 1 π-surface efficiently from oxygen quenching, thereby leading to a sequestration-induced fluorescence enhancement under ambient conditions.

Nanographene - A Scaffold of Two-Dimensional Materials

Substances can be divided into 0D to 3D species based on the number of repeating units (atom, ion, and molecule) and their arrangements in space (point, linear, layer, and solid). Discrete substances belong to 0D species, polymers are examples of 1D species, and molecular crystals are 3D species. Most of the substances belong to one of these species. On the other hand, those categorized into 2D species wherein the repeating units organize a layer are less explored. 2D species have a surface and edges. The incorporation of these structural features into a molecular design can realize multifunctionalized systems that are difficult to achieve by conventional organic synthesis. The development of 2D species is, therefore, the frontier of organic, inorganic, and polymer chemistry. Nanographenes (NGs) are suitable scaffolds for realizing 2D species due to several factors, such as chemical stability and oxygen-containing functional groups on the surface and on the edge, allowing postsynthetic modifications. Our group has utilized NGs with tens of nanometers in diameters for developing 2D species. Carboxy groups on the edge enable us to install various substituents into NGs, offering NG-based functional materials. These studies demonstrate that the integration of NGs with organic chemistry can widen the scope of their applications other than optical materials that are a main application of NGs. We introduce our recent studies on the development of NG-based functional materials realized by postsynthetic modifications. We hope that this account will contribute to the development of the chemistry of 2D species.