Synthesis of Nonplanar Graphene Nanoribbon with Fjord Edges

Thiophene-backbone arcuate graphene nanoribbons: shotgun synthesis and length dependent properties

Efficient synthetic methods are urgently needed to produce graphene nanoribbons (GNRs) with diverse structures and functions. Precise control over the topological edges of GNRs is also crucial for achieving diverse molecular topologies and desirable electro-optical properties. This study demonstrates a highly efficient "shotgun" synthesis of thiophene-backbone arcuate GNRs, offering a significant advantage over tedious iterative synthesis. This method utilizes a one-pot, three component Suzuki-Miyaura coupling for the precursor, followed by a Scholl reaction for cyclization. The resulting arcuate GNRs have sulfur atoms embedded in the carbon backbone with a combined armchair, cove, and fjord edge structure. This multi-edge architecture is further modified by high-yield oxidation of the electron-rich sulfur atoms to electron-deficient sulfones, enabling precise regulation of the GNRs' electronic properties. These arcuate GNRs with diverse edge structures, heteroatom doping and precise lengths open exciting avenues for their application in optoelectronic devices.

Achieving Precise Control Over the Molecular Periphery of Dibenzoixenes Through Modular Synthesis

Nanographenes and polycyclic aromatic hydrocarbons, both finite forms of graphene, are promising organic semiconducting materials because their optoelectronic and magnetic properties can be modulated through precise control of their molecular peripheries. Several atomically precise edge structures have been prepared by bottom-up synthesis; however, no systematic elucidation of these edge topologies at the molecular level has been reported. Herein, we describe rationally designed modular syntheses of isomeric dibenzoixenes with diverse molecular peripheries, including cove, zigzag, bay, fjord, and gulf structured. The single-crystal structures of dibenzo[a,p]ixene and dibenzo[j,y]ixene reveal enantiomeric pairs with helically twisted cove edges and packing structures. The molecular edge structures are identified from the C-H bonds of the dibenzoixenes using Fourier transform infrared spectroscopy with different vibrational modes, which were further explained using density functional theory calculations. Electron spin resonance spectroscopy indicates that the zigzag-edged molecular periphery significantly affects the magnetic properties of the material. Furthermore, the electrochemical characteristics, examined using dibenzoixenes as anode materials in Li-ion batteries, reveal that the dibenzo[a,p]ixene exhibits promising Li intercalation behaviors with a specific capacity of ~120 mAh g-1. The findings of this study could facilitate the synthesis of larger π ${\pi }$ -extended systems with engineered molecular peripheries and potential application in organic electronics.

Synthesis and Self-Assembly of Monodisperse Graphene Nanoribbons: Access to Submicron Architectures with Long-Range Order and Uniform Orientation

Fabricating organic semiconducting materials into large-scale, well-organized architectures is critical for building high-performance molecular electronics. While graphene nanoribbons (GNRs) hold enormous promise for various device applications, their assembly into a well-structured monolayer or multilayer architecture poses a substantial challenge. Here, we report the preparation of length-defined monodisperse GNRs via the integrated iterative binomial synthesis (IIBS) strategy and their self-assembly into submicrometer architectures with long-range order, uniform orientation, as well as regular layers. The use of short alkyl side chains benefits forming stable multilayers through interlocking structures. By changing the length and backbone shapes of these monodisperse GNRs, various three-dimensional assemblies, including multilayer stripes, monolayer stripes, and nanowires, can be achieved, leading to different photophysical properties and band gaps. The discovery of these intriguing self-assembly behaviors of length-defined GNRs is expected to enable various future applications.

Regioisomeric π-Extended Nanographene with Long-Lived Phosphorescence Afterglow

The cutouts of graphene sheets, particularly those with a nonplanar topology, present vast opportunities for advancement. Even a slight deviation from the planar structure can lead to intriguing (chiro)optical features for helically twisted nanographenes. In this context, we introduce two regioisomeric π-extended nanographenes that exhibit distinct excited-state characteristics. The helicene structure and the photophysical features can be easily tuned by changing the connecting position of the nanographene to the carbazole core (2,7- and 3,6-). Single-crystal X-ray diffraction analysis confirmed the formation of nanographenes with bent and helical conformations. Both derivatives exhibited thermally activated delayed fluorescence at room temperature and phosphorescence at low temperatures. Notably, the nanographene with the bent structure displayed an impressive red afterglow lasting over 30 seconds, in contrast to the very weak afterglow observed in the helical structure. DFT calculations revealed the existence of an isoenergetic higher triplet state (T8) and comparatively weak spin-orbit coupling (T1-S0), thereby enabling the bent nanographene to exhibit a long-lived component and strong afterglow. Our findings highlight the significance of regioisomeric nanographenes with exceptional optical properties and offer a deeper understanding of the structure-property relationship in nonplanar nanographenes.

Substituent Effects in Scholl-Type Reactions of 1,2-Terphenyls to Triphenylenes

A series of 3,3''- and 4,4''-dimethoxy terphenyls with different second substituents on their ortho-positions have been synthesized and investigated upon the possibility to be oxidatively cyclodehydrogenated to the corresponding triphenylenes under Scholl-type conditions. The experimentally obtained selectivities were supported and explained by quantum chemical calculations and conclusions on the involved mechanisms (acid catalyzed arenium-ion mechanism (AIM) vs radical cation mechanism) were drawn.

Tether-entangled conjugated helices

A new design concept, tether-entangled conjugated helices (TECHs), is introduced for helical polyaromatic molecules. TECHs consist of a linear polyaromatic ladder backbone and periodically entangling tethers with the same planar chirality. By limiting the length of tether, all tethers synchronously bend and twist the backbone with the same manner, and change it into a helical ribbon with a determinate helical chirality. The 3D helical features are customizable via modular synthesis by using two types of synthons, the planar chiral tethering unit (C 2 symmetry) and the docking unit (C 2h symmetry), and no post chiral resolution is needed. Moreover, TECHs possess persistent chiral properties due to the covalent locking of helical configuration by tethers. Concave-type and convex-type oligomeric TECHs are prepared as a proof-of-concept. Unconventional double-helix π-dimers are observed in the single crystals of concave-type TECHs. Theoretical studies indicate the smaller binding energies in double-helix π-dimers than conventional planar π-dimers. A concentration-depend emission is found for concave-type TECHs, probably due to the formation of double-helix π-dimers in the excited state. All TECHs show strong circularly polarized luminescence (CPL) with dissymmetric factors (|g lum|) generally over 10-3. Among them, the (P)-T4-tBu shows the highest |g lum| of 1.0 × 10-2 and a high CPL brightness of 316 M-1 cm-1.

Porphyrin-fused graphene nanoribbons

Graphene nanoribbons (GNRs), nanometre-wide strips of graphene, are promising materials for fabricating electronic devices. Many GNRs have been reported, yet no scalable strategies are known for synthesizing GNRs with metal atoms and heteroaromatic units at precisely defined positions in the conjugated backbone, which would be valuable for tuning their optical, electronic and magnetic properties. Here we report the solution-phase synthesis of a porphyrin-fused graphene nanoribbon (PGNR). This PGNR has metalloporphyrins fused into a twisted fjord-edged GNR backbone; it consists of long chains (>100 nm), with a narrow optical bandgap (~1.0 eV) and high local charge mobility (>400 cm2 V-1 s-1 by terahertz spectroscopy). We use this PGNR to fabricate ambipolar field-effect transistors with appealing switching behaviour, and single-electron transistors displaying multiple Coulomb diamonds. These results open an avenue to π-extended nanostructures with engineerable electrical and magnetic properties by transposing the coordination chemistry of porphyrins into graphene nanoribbons.

Precise synthesis of graphene by chemical vapor deposition

Graphene, a typical representative of the family of two-dimensional (2D) materials, possesses a series of phenomenal physical properties. To date, numerous inspiring discoveries have been made on its structures, properties, characterization, synthesis, transfer and applications. The real practical applications of this magic material indeed require large-scale synthesis and precise control over its structures, such as size, crystallinity, layer number, stacking order, edge type and contamination levels. Nonetheless, studies on the precise synthesis of graphene are far from satisfactory currently. Our review aims to deal with the precise synthesis of four critical graphene structures, including single-crystal graphene (SCG), AB-stacked bilayer graphene (AB-BLG), etched graphene and clean graphene. Meanwhile, existing problems and future directions in the precise synthesis of graphene are also briefly discussed.

Cove-Edged Chiral Graphene Nanoribbons with Chirality-Dependent Bandgap and Carrier Mobility

Graphene nanoribbons (GNRs) have garnered significant interest due to their highly customizable physicochemical properties and potential utility in nanoelectronics. Besides controlling widths and edge structures, the inclusion of chirality in GNRs brings another dimension for fine-tuning their optoelectronic properties, but related studies remain elusive owing to the absence of feasible synthetic strategies. Here, we demonstrate a novel class of cove-edged chiral GNRs (CcGNRs) with a tunable chiral vector (n,m). Notably, the bandgap and effective mass of (n,2)-CcGNR show a distinct positive correlation with the increasing value of n, as indicated by theory. Within this GNR family, two representative members, namely, (4,2)-CcGNR and (6,2)-CcGNR, are successfully synthesized. Both CcGNRs exhibit prominently curved geometries arising from the incorporated [4]helicene motifs along their peripheries, as also evidenced by the single-crystal structures of the two respective model compounds (1 and 2). The chemical identities and optoelectronic properties of (4,2)- and (6,2)-CcGNRs are comprehensively investigated via a combination of IR, Raman, solid-state NMR, UV-vis, and THz spectroscopies as well as theoretical calculations. In line with theoretical expectation, the obtained (6,2)-CcGNR possesses a low optical bandgap of 1.37 eV along with charge carrier mobility of ∼8 cm2 V-1 s-1, whereas (4,2)-CcGNR exhibits a narrower bandgap of 1.26 eV with increased mobility of ∼14 cm2 V-1 s-1. This work opens up a new avenue to precisely engineer the bandgap and carrier mobility of GNRs by manipulating their chiral vector.

N=8 Armchair Graphene Nanoribbons: Solution Synthesis and High Charge Carrier Mobility

Structurally defined graphene nanoribbons (GNRs) have emerged as promising candidates for nanoelectronic devices. Low band gap (<1 eV) GNRs are particularly important when considering the Schottky barrier in device performance. Here, we demonstrate the first solution synthesis of 8-AGNRs through a carefully designed arylated polynaphthalene precursor. The efficiency of the oxidative cyclodehydrogenation of the tailor-made polymer precursor into 8-AGNRs was validated by FT-IR, Raman, and UV/Vis-near-infrared (NIR) absorption spectroscopy, and further supported by the synthesis of naphtho[1,2,3,4-ghi]perylene derivatives (1 and 2) as subunits of 8-AGNR, with a width of 0.86 nm as suggested by the X-ray single crystal analysis. Low-temperature scanning tunneling microscopy (STM) and solid-state NMR analyses provided further structural support for 8-AGNR. The resulting 8-AGNR exhibited a remarkable NIR absorption extending up to ∼2400 nm, corresponding to an optical band gap as low as ∼0.52 eV. Moreover, optical-pump TeraHertz-probe spectroscopy revealed charge-carrier mobility in the dc limit of ∼270 cm2  V-1  s-1 for the 8-AGNR.

Curved graphene nanoribbons derived from tetrahydropyrene-based polyphenylenes via one-pot K-region oxidation and Scholl cyclization

Precise synthesis of graphene nanoribbons (GNRs) is of great interest to chemists and materials scientists because of their unique opto-electronic properties and potential applications in carbon-based nanoelectronics and spintronics. In addition to the tunable edge structure and width, introducing curvature in GNRs is a powerful structural feature for their chemi-physical property modification. Here, we report an efficient solution synthesis of the first pyrene-based GNR (PyGNR) with curved geometry via one-pot K-region oxidation and Scholl cyclization of its corresponding well-soluble tetrahydropyrene-based polyphenylene precursor. The efficient A2B2-type Suzuki polymerization and subsequent Scholl reaction furnishes up to ∼35 nm long curved GNRs bearing cove- and armchair-edges. The construction of model compound 1, as a cutout of PyGNR, from a tetrahydropyrene-based oligophenylene precursor proves the concept and efficiency of the one-pot K-region oxidation and Scholl cyclization, which is clearly revealed by single crystal X-ray diffraction analysis. The structure and optical properties of PyGNR are investigated by Raman, FT-IR, solid-state NMR, STM and UV-Vis analysis with the support of DFT calculations. PyGNR exhibits a narrow optical bandgap of ∼1.4 eV derived from a Tauc plot, qualifying as a low-bandgap GNR. Moreover, THz spectroscopy on PyGNR estimates its macroscopic charge mobility μ as ∼3.6 cm2 V-1 s-1, outperforming several other curved GNRs reported via conventional Scholl reaction.

Interplay of structure and photophysics of individualized rod-shaped graphene quantum dots with up to 132 sp² carbon atoms

Nanographene materials are promising building blocks for the growing field of low-dimensional materials for optics, electronics and biophotonics applications. In particular, bottom-up synthesized 0D graphene quantum dots show great potential as single quantum emitters. To fully exploit their exciting properties, the graphene quantum dots must be of high purity; the key parameter for efficient purification being the solubility of the starting materials. Here, we report the synthesis of a family of highly soluble and easily processable rod-shaped graphene quantum dots with fluorescence quantum yields up to 94%. This is uncommon for a red emission. The high solubility is directly related to the design of the structure, allowing for an accurate description of the photophysical properties of the graphene quantum dots both in solution and at the single molecule level. These photophysical properties were fully predicted by quantum-chemical calculations.

Helical fused 1,2:8,9-dibenzozethrene oligomers with up to 201° end-to-end twist: "one-pot" synthesis and chiral resolution

Twisted polyarenes with persistent chirality are desirable but their synthesis has remained a challenge. In this study, we present a "one-pot" synthesis of 1,2:8,9-dibenzozethrene (DBZ) and its vertically fused dimers and trimers using nickel-catalyzed cyclo-oligomerization reactions. X-ray crystallographic analysis confirmed highly twisted helical structures that consist of equal parts left- and right-handed enantiomers. Notably, the end-to-end twist between the terminal anthracene units measured 66°, 130°, and 201° for the DBZ monomer, dimer, and trimer, respectively, setting a new record among twisted polyarenes. Furthermore, the chiral resolution by HPLC yielded two enantiomers for the fused DBZ dimer and trimer, both of which maintained stable configurations and showed absorption dissymmetry factors of around 0.008-0.009. Additionally, their optical and electrochemical properties were investigated, which exhibited a chain-length dependence.

Electron-phonon interaction toward engineering carrier mobility of periodic edge structured graphene nanoribbons

Graphene nanoribbons have many extraordinary electrical properties and are the candidates for semiconductor industry. In this research, we propose a design of Coved GNRs with periodic structure ranged from 4 to 8 nm or more, of which the size is within practical feature sizes by advanced lithography tools. The carrier transport properties of Coved GNRs with the periodic coved shape are designed to break the localized electronic state and reducing electron-phonon scattering. In this way, the mobility of Coved GNRs can be enhanced by orders compared with the zigzag GNRs in same width. Moreover, in contrast to occasional zero bandgap transition of armchair and zigzag GNRs without precision control in atomic level, the Coved GNRs with periodic edge structures can exclude the zero bandgap conditions, which makes practical the mass production process. The designed Coved-GNRs is fabricated over the Germanium (110) substrate where the graphene can be prepared in the single-crystalline and single-oriented formants and the edge of GNRs is later repaired under "balanced condition growth" and we demonstrate that the propose coved structures are compatible to current fabrication facility.

Synthesis of Bioctacene-Incorporated Nanographene with Near-Infrared Chiroptical Properties

We report the synthesis of a hexabenzoperihexacene (HBPH) with two incorporated octacene substructures, which was unambiguously characterized by single-crystal X-ray analysis. The theoretical isomerization barrier of the (P,P)-/(P,M)-forms was estimated to be 38.4 kcal mol^-1 , and resolution was achieved by chiral HPLC. Notably, the enantiomers exhibited opposite circular dichroism responses up to the near-infrared (NIR) region (830 nm) with a high g_abs value of 0.017 at 616 nm. Moreover, HBPH demonstrated NIR emission with a maximum at 798 nm and an absolute PLQY of 41 %. The excited-state photophysical properties of HBPH were investigated by ultrafast transient absorption spectroscopy, revealing an intriguing feature that was attributed to the rotational and/or conformational dynamics of HBPH after excitation. These results provide new insight into the design of chiral nanographene with NIR optical properties for potential chiroptical applications.

Solution-Synthesized Extended Graphene Nanoribbons Deposited by High-Vacuum Electrospray Deposition

Solution-synthesized graphene nanoribbons (GNRs) facilitate various interesting structures and functionalities, like nonplanarity and thermolabile functional groups, that are not or not easily accessible by on-surface synthesis. Here, we show the successful high-vacuum electrospray deposition (HVESD) of well-elongated solution-synthesized GNRs on surfaces maintained in ultrahigh vacuum. We compare three distinct GNRs, a twisted nonplanar fjord-edged GNR, a methoxy-functionalized "cove"-type (or also called gulf) GNR, and a longer "cove"-type GNR both equipped with alkyl chains on Au(111). Nc-AFM measurements at room temperature with submolecular imaging combined with Raman spectroscopy allow us to characterize individual GNRs and confirm their chemical integrity. The fjord-GNR and methoxy-GNR are additionally deposited on nonmetallic HOPG and SiO2, and fjord-GNR is deposited on a KBr(001) surface, facilitating the study of GNRs on substrates, as of now not accessible by on-surface synthesis.

Precise Structural Regulation and Band-Gap Engineering of Curved Graphene Nanoribbons

Graphene nanoribbons (GNRs)─quasi-one-dimensional graphene cutouts─have drawn growing attention as promising candidates for next-generation electronic and spintronic materials. Theoretical and experimental studies have demonstrated that the electronic and magnetic properties of GNRs critically depend on their widths and edge topologies. Thus, the preparation of structurally defined GNRs is highly desirable not only for their fundamental physicochemical studies but also for their future technological development in carbon-based nanoelectronics. In the past decade, significant efforts have been made to construct a wide variety of GNRs with well-defined widths and edge structures via bottom-up synthesis. In addition to extensively studied planar GNRs consisting of armchair, zigzag, or gulf edges, curved GNRs (cGNRs) bearing cove ([4]helicene unit) or fjord ([5]helicene unit) regions along the ribbon edges have received increasing interest after we presented the first attempt to synthesize the fully cove-edged GNRs in 2015. Profiting from their novel edge topologies, cGNRs usually exhibit an unprecedented narrow band gap and high carrier transport mobility in comparison to the planar GNRs with similar widths. Moreover, cGNRs with particular out-of-plane-distorted structures are expected to provide further opportunities in nonlinear optics and asymmetric catalysis. However, the synthesis of cGNRs bearing cove or fjord edges remains underdeveloped due to the absence of efficient synthetic strategies/methods and suitable molecular precursor design.In this Account, we present the recent advances in the bottom-up synthesis and characterization of structurally defined cGNRs containing cove or fjord edges, mainly from our research group. First, the synthetic strategies toward cGNRs bearing cove edges are described, including the design of molecular monomers and polymer precursors as well as the corresponding polymerization methods, such as Ullmann coupling, Yamamoto coupling, A₂B₂-type Diels-Alder polymerization, followed by Scholl-type cyclodehydrogenation. The synthesis of typical model compounds is also described to support the understanding of the related cGNRs. In addition, the synthesis of cGNRs containing fjord edges from other research groups via the regioselective Scholl reaction, Hopf cyclization or regioselective photochemical cyclodehydrochlorination approach is presented. Second, we discuss the optoelectronic properties of the as-synthesized cGNRs and reveal the design principle to obtain cGNRs with high charge carrier mobilities. Finally, the challenges and prospects in the design and synthesis of cGNRs are offered. We anticipate that this Account will further stimulate the development of cGNRs through a collaborative effort between different disciplines.

Structure-Imposed Electronic Topology in Cove-Edged Graphene Nanoribbons

In cove-edged zigzag graphene nanoribbons (ZGNR-Cs), one terminal CH group per length unit is removed on each zigzag edge, forming a regular pattern of coves that controls their electronic structure. Based on three structural parameters that unambiguously characterize the atomistic structure of ZGNR-Cs, we present a scheme that classifies their electronic state (i.e., if they are metallic, topological insulators, or trivial semiconductors) for all possible widths N, unit lengths a, and cove position offsets at both edges b, thus showing the direct structure-electronic structure relation. We further present an empirical formula to estimate the band gap of the semiconducting ribbons from N, a, and b. Finally, we identify all geometrically possible ribbon terminations and provide rules to construct ZGNR-Cs with a well-defined electronic structure.

Synthesis of zigzag- and fjord-edged nanographene with dual amplified spontaneous emission

We report the synthesis of a dibenzodinaphthocoronene (DBDNC) derivative as a novel nanographene with armchair, zigzag, and fjord edges, which was characterized by NMR and X-ray crystallography as well as infrared (IR) and Raman spectroscopies. Ultrafast transient absorption (TA) spectroscopy revealed the presence of stimulated emission signals at 655 nm and 710 nm with a relatively long lifetime, which resulted in dual amplified spontaneous emission (ASE) bands under ns-pulsed excitation, indicating the promise of DBNDC as a near-infrared (NIR) fluorophore for photonics. Our results provide new insight into the design of nanographene with intriguing optical properties by incorporating fjord edges.

The Scholl Reaction as a Powerful Tool for Synthesis of Curved Polycyclic Aromatics

The past decade has witnessed remarkable success in the synthesis of curved polycyclic aromatics through Scholl reactions which enable oxidative aryl-aryl coupling even in company with the introduction of significant steric strain. These curved polycyclic aromatics are not only unique objects of structural organic chemistry in relation to the nature of aromaticity but also play an important role in bottom-up approaches to precise synthesis of nanocarbons of unique topology. Moreover, they have received considerable attention in the fields of supramolecular chemistry and organic functional materials because of their interesting properties and promising applications. Despite the great success of Scholl reactions in synthesis of curved polycyclic aromatics, the outcome of a newly designed substrate in the Scholl reaction still cannot be predicted in a generic and precise manner largely due to limited understanding on the reaction mechanism and possible rearrangement processes. This review provides an overview of Scholl reactions with a focus on their applications in synthesis of curved polycyclic aromatics with interesting structures and properties and aims to shed light on the key factors that affect Scholl reactions in synthesizing sterically strained polycyclic aromatics.

Electronic, transport, magnetic and optical properties of graphene nanoribbons review

Low dimensional materials have attracted great research interest from both theoretical and experimental point of view. These materials exhibit novel physical and chemical properties due to the confinement effect in low dimensions. The experimental observations of graphene open a new platform to study the physical properties of materials restricted to two dimensions. This featured article provides a review on the novel properties of quasi one-dimensional (1D) material known as graphene nanoribbon. Graphene nanoribbons can be obtained by unzipping carbon nanotubes (CNTs) or cutting the graphene sheet. Alternatively, it is also called the finite termination of graphene edges. It gives rise different edge geometries namely zigzag and armchair among others. There are various physical and chemical techniques to realize these materials. Depending on the edge type termination, these are called the zigzag and armchair graphene nanoribbons (ZGNR and AGNR). These edges play an important role in controlling the properties of graphene nanoribbons. The present review article provides an overview of the electronic, transport, optical and magnetic properties of graphene nanoribbons. However, there are different ways to tune these properties for device applications. Here, some of them are highlighted such as external perturbations and chemical modifications. Few applications of graphene nanoribbon have and chemical modifications. Few applications of graphene nanoribbon have also been briefly discussed.

Nanographenes and Graphene Nanoribbons as Multitalents of Present and Future Materials Science

As cut-outs from a graphene sheet, nanographenes (NGs) and graphene nanoribbons (GNRs) are ideal cases with which to connect the world of molecules with that of bulk carbon materials. While various top-down approaches have been developed to produce such nanostructures in high yields, in the present perspective, precision structural control is emphasized for the length, width, and edge structures of NGs and GNRs achieved by modern solution and on-surface syntheses. Their structural possibilities have been further extended from "flatland" to the three-dimensional world, where chirality and handedness are the jewels in the crown. In addition to properties exhibited at the molecular level, self-assembly and thin-film structures cannot be neglected, which emphasizes the importance of processing techniques. With the rich toolkit of chemistry in hand, NGs and GNRs can be endowed with versatile properties and functions ranging from stimulated emission to spintronics and from bioimaging to energy storage, thus demonstrating their multitalents in present and future materials science.

Small Size, Big Impact: Recent Progress in Bottom-Up Synthesized Nanographenes for Optoelectronic and Energy Applications

Bottom-up synthesized graphene nanostructures, including 0D graphene quantum dots and 1D graphene nanoribbons, have recently emerged as promising candidates for efficient, green optoelectronic, and energy storage applications. The versatility in their molecular structures offers a large and novel library of nanographenes with excellent and adjustable optical, electronic, and catalytic properties. In this minireview, recent progress on the fundamental understanding of the properties of different graphene nanostructures, and their state-of-the-art applications in optoelectronics and energy storage are summarized. The properties of pristine nanographenes, including high emissivity and intriguing blinking effect in graphene quantum dots, superior charge transport properties in graphene nanoribbons, and edge-specific electrochemistry in various graphene nanostructures, are highlighted. Furthermore, it is shown that emerging nanographene-2D material-based van der Waals heterostructures provide an exciting opportunity for efficient green optoelectronics with tunable characteristics. Finally, challenges and opportunities of the field are highlighted by offering guidelines for future combined efforts in the synthesis, assembly, spectroscopic, and electrical studies as well as (nano)fabrication to boost the progress toward advanced device applications.

Solution Synthesis and Characterization of a Long and Curved Graphene Nanoribbon with Hybrid Cove-Armchair-Gulf Edge Structures

Curved graphene nanoribbons (GNRs) with hybrid edge structures have recently attracted increasing attention due to their unique band structures and electronic properties as a result of their nonplanar conformation. This work reports the solution synthesis of a long and curved multi-edged GNR (cMGNR) with unprecedented cove-armchair-gulf edge structures. The synthesis involves an efficient A₂ B₂ -type Diels-Alder polymerization between a diethynyl-substituted prefused bichrysene monomer (3b) and a dicyclopenta[e,l]pyrene-5,11-dione derivative (6) followed by FeCl₃ -mediated Scholl oxidative cyclodehydrogenation of the obtained polyarylenes (P1). Model compounds 1a and 1b are first synthesized to examine the suitability and efficiency of the corresponding polymers for the Scholl reaction. The successful formation of cMGNR from polymer P1 bearing prefused bichrysene units is confirmed by FTIR, Raman, and solid-state NMR analyses. The cove-edge structure of the cMGNR imparts the ribbon with a unique nonplanar conformation as revealed by density functional theory (DFT) simulation, which effectively enhances its dispersibility in solution. The cMGNR has a narrow optical bandgap of 1.61 eV, as estimated from the UV-vis absorption spectrum, which is among the family of low-bandgap solution-synthesized GNRs. Moreover, the cMGNR exhibits a carrier mobility of ≈2 cm² V^-1 s^-1 inferred from contact-free terahertz spectroscopy.

Cove-Edged Hexa-peri-hexabenzo-bis-peri-octacene: Molecular Conformations and Amplified Spontaneous Emission

The bottom-up synthesis of an unprecedentedly large cove-edged nanographene, hexa-peri-hexabenzo-bis-peri-octacene (HBPO), is reported in this work. Chiral high-performance liquid chromatography and density functional theory (DFT) calculations revealed multiple conformations in solution. Two different molecular conformations, "waggling" and "butterfly", were found in crystals by X-ray crystallography, and the selectivity of conformations could be tuned by solvents. The optoelectronic properties of HBPO were investigated by UV/Vis absorption and fluorescence spectroscopies, cyclic voltammetry, and DFT calculations. The contorted geometry and branched alkyl groups suppress the aggregation of HBPO in solution, leading to a high fluorescence quantum yield of 79 %. The optical-gain properties were explored through transient absorption and amplified spontaneous emission spectroscopies, which enrich the choices of edge structures for potential applications in laser cavities.

Redox-Active Metaphosphate-Like Terminals Enable High-Capacity MXene Anodes for Ultrafast Na-Ion Storage

2D transition metal carbides and/or nitrides, so-called MXenes, are noted as ideal fast-charging cation-intercalation electrode materials, which nevertheless suffer from limited specific capacities. Herein, it is reported that constructing redox-active phosphorus-oxygen terminals can be an attractive strategy for Nb₄ C₃ MXenes to remarkably boost their specific capacities for ultrafast Na⁺ storage. As revealed, redox-active terminals with a stoichiometric formula of PO₂ - display a metaphosphate-like configuration with each P atom sustaining three PO bonds and one PO dangling bond. Compared with conventional O-terminals, metaphosphate-like terminals empower Nb₄ C₃ (denoted PO₂ -Nb₄ C₃ ) with considerably enriched carrier density (fourfold), improved conductivity (12.3-fold at 300 K), additional redox-active sites, boosted Nb redox depth, nondeclined Na⁺ -diffusion capability, and buffered internal stress during Na⁺ intercalation/de-intercalation. Consequently, compared with O-terminated Nb₄ C₃ , PO₂ -Nb₄ C₃ exhibits a doubled Na⁺ -storage capacity (221.0 mAh g^-1 ), well-retained fast-charging capability (4.9 min at 80% capacity retention), significantly promoted cycle life (nondegraded capacity over 2000 cycles), and justified feasibility for assembling energy-power-balanced Na-ion capacitors. This study unveils that the molecular-level design of MXene terminals provides opportunities for developing simultaneously high-capacity and fast-charging electrodes, alleviating the energy-power tradeoff typical for energy-storage devices.

Inducing Single-Handed Helicity in a Twisted Molecular Nanoribbon

Molecular conformation has an important role in chemistry and materials science. Molecular nanoribbons can adopt chiral twisted helical conformations. However, the synthesis of single-handed helically twisted molecular nanoribbons still represents a considerable challenge. Herein, we describe an asymmetric approach to induce single-handed helicity with an excellent degree of conformational discrimination. The chiral induction is the result of the chiral strain generated by fusing two oversized chiral rings and of the propagation of that strain along the nanoribbon's backbone.

Cove-Edged Graphene Nanoribbons with Incorporation of Periodic Zigzag-Edge Segments

Structurally precision graphene nanoribbons (GNRs) are promising candidates for next-generation nanoelectronics due to their intriguing and tunable electronic structures. GNRs with hybrid edge structures often confer them unique geometries associated with exotic physicochemical properties. Herein, a novel type of cove-edged GNRs with periodic short zigzag-edge segments is demonstrated. The bandgap of this GNR family can be tuned using an interplay between the length of the zigzag segments and the distance of two adjacent cove units along the opposite edges, which can be converted from semiconducting to nearly metallic. A family member with periodic cove-zigzag edges based on N = 6 zigzag-edged GNR, namely 6-CZGNR-(2,1), is successfully synthesized in solution through the Scholl reaction of a unique snakelike polymer precursor (10) that is achieved by the Yamamoto coupling of a structurally flexible S-shaped phenanthrene-based monomer (1). The efficiency of cyclodehydrogenation of polymer 10 toward 6-CZGNR-(2,1) is validated by FT-IR, Raman, and UV-vis spectroscopies, as well as by the study of two representative model compounds (2 and 3). Remarkably, the resultant 6-CZGNR-(2,1) exhibits an extended and broad absorption in the near-infrared region with a record narrow optical bandgap of 0.99 eV among the reported solution-synthesized GNRs. Moreover, 6-CZGNR-(2,1) exhibits a high macroscopic carrier mobility of ∼20 cm² V^-1 s^-1 determined by terahertz spectroscopy, primarily due to the intrinsically small effective mass (m*_e = m*_h = 0.17 m₀), rendering this GNR a promising candidate for nanoelectronics.