Helical Synthetic Nanographenes with Atomic Precision.

ConspectusUnderstanding and harnessing the properties of nanoscale molecular entities are considered as new frontiers in basic chemistry. In this regard, synthetic nanographene with atomic precision has attracted much attention recently. For instance, taking advantage of the marvelous bonding capability of carbon, flat, curved, ribbon-type, or cone-shaped nanographenes have been prepared in highly controllable and elegant manner, allowing one to explore fascinating molecular architectures with intriguing optical, electrochemical, and magnetic characteristics. This stands in stark contrast to other carbon-rich nanomaterials, such as graphite oxides or carbon quantum dots, which preclude thorough investigations because of complicate structural defects. Undoubtedly, synthetic nanographene contributes strongly to modern aromatic chemistry and represents a vibrant field that may deliver transforming functional materials crucial for optoelectronics, nanotechnologies, and biomedicine.Nonetheless, in many cases, synthesis and characterization of nanographene compounds are highly demanding. Low solubility, high molecular strain, undesired selectivity, as well as incomplete or excessive C-C bond formation are common impediments, that require formidable efforts to control the molecular geometry, to modulate the edge structure, to achieve accurate doping, or to push the upper size boundary. These endeavors are indispensable for establishing structure-property relationships, and lay down foundation for exploring synthetic nanographenes at a high level of sophistications.In this Account, we summarize our contributions to this field by presenting a series of helical synthetic nanographenes, such as hexapole [7]helicene (H7H), nitrogen-doped H7H, hexapole [9]helicene (H9H), superhelicene, and supertwistacene. This kind of giant synthetic nanographene reaches the size domain of carbon quantum dots, albeit has precise atomic structure. It provides a unique platform to study aromatic chemistry and chirality at the nanoscale. We discuss synthetic methods and point out, in particular, the strengths and pitfalls of Scholl oxidation, which are expected to be valuable for making synthetic nanographenes in general. In addition, we illustrate their exciting electrochemical and photophysical performance, which include, but are not limited to, reversible multielectron redox chemistry, record high panchromatic absorption, impressive photothermal behavior, and extremely strong Cotton effect. These unusual characteristics are convincingly traced back to their three-dimensional conjugated architectures, highlighting the critical roles of π-electron delocalization, heteroatom-doping, substitution, and molecular symmetry in determining nanographenes' properties and functions. Lastly, we put forward our understanding on the challenges and opportunities that lies ahead and hope this Account will inspire ever more ambitious achievements from this attractive area of research.

Double π-Extended Undecabenzo[7]helicene.

This work reports the first double π-extended undecabenzo[7]helicene 1, which is a large chiral nanographene, composed of 65 fused rings and 186 conjugated carbon atoms. The molecular identity of 1 has been confirmed by single crystal X-ray diffraction. A wine coloured solution of 1 in dichloromethane absorbs light from ultraviolet to the near infrared, featuring an extremely large molar absorption coefficient of 844 000 M-1 cm-1 at 573 nm. Optically pure 1 shows a record high electronic circular dichroism intensity in the visible spectral range (|Δϵ|=1375 M-1 cm-1 at 430 nm) known for any discrete polycyclic aromatic hydrocarbon. These unusual photophysical properties of 1 contrast sharply with those of a mono-undecabenzo[7]helicene derivative 2.

Supertwistacene: A Helical Graphene Nanoribbon.

This work describes a synthetic chiral graphene nanoribbon, named supertwistacene 1. It has four superbenzene (HBC) units linearly fused in a helical manner. The structure of 1, 4.3 nm in length, with an end-to-end twist of 117°, was confirmed by single-crystal X-ray diffraction. In contrast to various twistacene compounds and their analogues, 1 has a very stable configuration. It resists thermal isomerization even when being heated at 200 °C for 16 h. Enantiopure 1 obtained by chiral HPLC shows distinct CD signals in a broad spectral range until 600 nm. In addition, two smaller congeners of 1, the trimer 2 and the dimer 3, were also prepared and systematically investigated. Combining theoretical and experimental studies on 1-3 presents a big picture on their (chir)optical and electronic characteristics.

Fusing of Seven HBCs toward a Green Nanographene Propeller.

This work presents a green chiral nanographene propeller (NP), which is built by fusing seven hexabenzocoronenes in a helical arrangement. It contains 258 conjugated carbon atoms and represents the largest three-dimensional conjugated polycyclic aromatic hydrocarbons ever prepared using scalable solution chemistry. Despite its unusual molecular size, single-crystal X-ray structural analysis (resolution 0.9 Å) and baseline chiral resolution are achieved. NP is soluble in various organic solvents and can be fully characterized by common spectroscopic and voltammetric techniques. It has a strong panchromatic absorption from the ultraviolet to the near-infrared (λmax = 659 nm, ε = 179 000 M-1 cm-1). For instance, more than half of the spectral range between 300 and 800 nm witnesses an extinction coefficient larger than 100 000 M-1 cm-1. Moreover, a record-high Cotton effect in the visible spectrum is observed for enantiopure NP, with |Δε| values of 1182 and 1090 M-1 cm-1 at 374 and 405 nm, respectively. These photophysical properties evolve significantly compared to those of the propeller-shaped hexapole [7]helicene.

Hexapole [9]Helicene.

Herein we present the first hexapole [9]helicene (H9H). Co-catalyzed [2+2+2] cyclotrimerization of a dinaphthopyrene (DNP) functionalized alkyne provides the hexaaryl benzene precursor 2, which is transformed into H9H via a dehydrocyclization reaction. Formation of each embedded [9]helicene involves forging of a new C-C bond, which stitches together two [4]helicene subunits of the neighboring DNP blades, reminiscent of the initial method Martin developed for the preparation of [9]helicene in the 1960s. Single-crystal X-ray analysis of both 2 and H9H discloses their extremely distorted and crowded structural features. Chiral resolution, optical and electronic properties of H9H are also presented.

A Nitrogen-Doped Hexapole [7]Helicene versus Its All-Carbon Analogue.

Two synthetic nanographenes (NGs), N-H7H and C-H7H, were prepared. N-H7H is doped with nitrogen, and C-H7H is the all-carbon analogue. Both are hexapole [7]helicenes (H7Hs), and their structures were identified by single-crystal X-ray diffraction. Sharp contrasts in absorption (abs λmax , 683 vs. 593 nm), emission (em λmax , 894 vs. 777 nm), and electrochemical behavior (ox E1 , 0.28 vs. 0.53 V) were observed between N-H7H and C-H7H, and the origin of these differences was rationalized by theoretical calculations. Studies on N-H7H and C-H7H set a clear example to elucidate the remarkable effects of N-doping on the physical properties of NGs.

Synthesis and Characterization of Hexapole [7]Helicene, A Circularly Twisted Chiral Nanographene.

We report the synthesis and characterization of two hexapole [7]helicenes (H7Hs). Single crystal X-ray diffraction unambiguously confirms the molecular structure. H7H absorbs light, with distinct Cotton effect, from ultraviolet to the near-infrared (λmax = 618 nm). Cyclic voltammetry reveals nine reversible redox states, consecutively from -2 to +6. These chiroptical and electronic properties of H7H are inaccessible from helicene's small homologues.

Synthesis and Characterization of Ferrocene Based Hemicages.

We present a series of tripodal ligands L1-3, which fold into hemicages C1-3 by using coordination-driven dynamic combinational chemistry. The identities of these hemicages were characterized using 1H NMR, 1H-1H COSY, DOSY, and ESI-TWIM-MS. Free rotation of the ferrocene structural units in the ligands affords an adaptable directionality, which is essential for the construction of these hemicages. Encapsulation of adamantane by C2 indicates the presence of a well-defined inner cavity as the binding pocket.

Janusarene: A Homoditopic Molecular Host.

A homoditopic molecular host, janusarene, is presented that has two back-to-back compactly arranged nanocavities for guest complexation. The unique two-face structural feature of janusarene allows it to bind and align various guest compounds concurrently, which include spherical pristine fullerene C60 and planar polycyclic aromatic hydrocarbons (PAHs), such as pyrene, perylene, and 9,10-dimethylanthracene. The host-guest interactions were characterized by single-crystal X-ray diffraction. A pairwise encapsulation of the PAH guests by janusarene enables PAH dimers to be obtained that deliver spectroscopic properties distinct from those of PAHs dissolved in solution, or in the bulk state. A monotopic control host was also synthesized and used to characterize the host-guest complexing behavior in solution.

Photoswitchable intramolecular through-space magnetic interaction.

The interaction between two TEMPO spin centers connected to a photoswitchable overcrowded alkene changes from noncoupled (three-line EPR spectrum) in the trans state, where the two spin centers are separated by ∼22 Å, to strongly coupled (five-line EPR spectrum) in the cis state, where the separation is ∼7 Å, upon photoswitching. Importantly, the performance of the alkene switching unit is essentially unaffected by the spin centers.

Photoswitchable intramolecular H-stacking of perylenebisimide.

Dynamic control over the formation of H- or J-type aggregates of chromophores is of fundamental importance for developing responsive organic optoelectronic materials. In this study, the first example of photoswitching between a nonstacked and an intramolecularly H-stacked arrangement of perylenebisimides (PBI) is demonstrated. The system is composed of a central switching unit (overcrowded alkene) tethered to two PBI chromophores. cis-trans isomerization of the switching unit, induced by alternate irradiation at 312 and 365 nm, can drive two PBI chromophores reversibly between an intramolecularly "aggregated" and "nonaggregated" state. Distinct changes in UV-vis absorption and fluorescence spectra are observed following photoisomerization. This approach allows for efficient control of intramolecular H-stack formation with no significant intermolecular interactions spanning over at least 4 orders of magnitude of concentration (from 10(-8) to 10(-4) M) and a range of solvents and temperatures.

On-surface synthesis of gold-coronene molecular wires.

Perchlorocoronene undergoes random dehalogenation at elevated temperatures, resulting in the formation of a disordered gold-coronene complex on Au(111) surfaces. The dehalogenation can be guided by introducing intermolecular constraint from accompanying molecules coexisting on the surface. Owing to the intermolecular electrostatic interactions, gold-coronene wires are intercalated between the polyphenylene chains, which hinders the transverse dehalogenation and results in an improved tendency toward linear gold-coronene molecular wires.

Supramolecular templates for nanoflake-metal surfaces.

A sustainable method for the fabrication of metallic surfaces with flower-like fractal morphology was developed by using a three-dimensional supramolecular assembly as a template. Modifying Au nanoflakes with self-assembled monolayers or polymers allows the surface wettability to be adjusted from superhydrophobic to superhydrophilic (see figure). Furthermore, Au nanoflakes present excellent substrates for surface-enhanced Raman spectroscopy (SERS).

Supramolecular shape shifter: polymorphs of self-organized fullerene assemblies.

Studies of hierarchical supramolecular assemblies of a fullerene derivative bearing three hexadecyloxy chains (1) have been carried out in various conditions, such as different organic solvents, temperatures, and with ultrasonication. The dimensional control of the hierarchical supramolecular architectures from the fullerene derivative (1), such as spherical vesicles, fibers, nano-disks, and uncommon conical and flower-shaped assemblies, was previously achieved. Here, further morphologies such as microspheres, windmill-like sheets, baton-, maracas-, and jellyfish-like assemblies were discovered. Shape shifting of supramolecular assemblies was seen only in the case of the fullerene derivative (1). Reference derivatives bearing three eicosyloxy (2) and three dodecyloxy chains (3) never showed such polymorphs, however, these derivatives self-organized into microparticles with nano-flake outer surfaces and ultra-thin disks, respectively. The fullerene derivatives studied here formed interdigitated bilayer structures with different interspacing length. There are two essential intermolecular forces present due to fullerene or aliphatic chains. Fullerene moieties exhibit strong pi-pi interactions, while van der Waals interactions between aliphatic chains can be altered by variation of their length. Three hexadecyloxy chains at the fullerene moiety (1) were the most effective substitution pattern for stimulating supramolecular shape shifting.

Self-assembly made durable: water-repellent materials formed by cross-linking fullerene derivatives.

Fullerene flakes: A diacetylene-functionalized fullerene derivative self-organizes into flakelike microparticles (see picture). Both the diacetylene and C(60) moieties can be effectively cross-linked, which leads to supramolecular materials with remarkable resistivity to solvent, heat, and mechanical stress. Moreover, the surface of the cross-linked flakelike objects is highly durable and water-repellent.

Electron transport and electrochemistry of mesomorphic fullerenes with long-range ordered lamellae.

Fullerenes, C60, modified with long alkyl chains form long-range ordered lamellar mesophases permitting a high C60 content. The mesomorphic fullerenes feature reversible electrochemistry and a comparably high electron carrier mobility making them attractive components for fullerene-based soft materials.

A julolidine-fused anthracene derivative: synthesis, photophysical properties, and oxidative dimerization.

We describe the synthesis and characterization of a julolidine-fused anthracene derivative J-A, which exhibits a maximum absorption of 450 nm and a maximum emission of 518 nm. The fluorescent quantum yield was determined to be 0.55 in toluene. J-A dimerizes in solution via oxidative coupling. Structure of the dimer was characterized using single crystal X-ray diffraction.

De novo endo-functionalized organic cages as cooperative multi-hydrogen-bond-donating catalysts.

Two endo-functionalized organic cages as oxyanion hole mimics were achieved via dynamic covalent chemistry, which exhibit good size selectivity, catalytic activity and broad substrate scopes for Friedel-Crafts reactions. The modular character of the synthesis provides an easy way to modify the size, shape and inner function of the molecular cavity.

Covalent tethering of photo-responsive superficial layers on hydrogel surfaces for photo-controlled release.

The diffusion and transport of substances between a hydrogel and its environment have received tremendous research interest, due to the wide range of applications of hydrogel materials in fields related to drug carriers and drug delivery vehicles. To date, much research has been done to tailor the diffusion and transport of substances through hydrogels, where most efforts were focused on tuning the 3D network properties of the hydrogel including loop size, hydrophobicity of building blocks and the stimuli-responsive properties of backbones. These conventional strategies, however, usually suffer from complicated fabrication procedures and result in a homogeneous increase in hydrophobicity of the hydrogel network, leading to low efficiency control over the diffusion of substances through the hydrogel. Herein, a facile strategy that can functionalize the surfaces of hydrogels, while keeping the interior network unchanged, was reported, and is realized by quaternization reaction confined to the hydrogel/oil interface. Owing to the introduction of the photo-responsive molecule IBSP as a modifier, the surface wettability of the resulting hydrogel can be controlled by light both in air and underwater environments. Consequently, the diffusion rate of a substance through this modified hydrogel can be regulated by light, which brings convenience to the controlled release of hydrogels and other hydrogel-related fields.

Graphene-like nanoribbons periodically embedded with four- and eight-membered rings.

Embedding non-hexagonal rings into sp2-hybridized carbon networks is considered a promising strategy to enrich the family of low-dimensional graphenic structures. However, non-hexagonal rings are energetically unstable compared to the hexagonal counterparts, making it challenging to embed non-hexagonal rings into carbon-based nanostructures in a controllable manner. Here, we report an on-surface synthesis of graphene-like nanoribbons with periodically embedded four- and eight-membered rings. The scanning tunnelling microscopy and atomic force microscopy study revealed that four- and eight-membered rings are formed between adjacent perylene backbones with a planar configuration. The non-hexagonal rings as a topological modification markedly change the electronic properties of the nanoribbons. The highest occupied and lowest unoccupied ribbon states are mainly distributed around the eight- and four-membered rings, respectively. The realization of graphene-like nanoribbons comprising non-hexagonal rings demonstrates a controllable route to fabricate non-hexagonal rings in nanoribbons and makes it possible to unveil their unique properties induced by non-hexagonal rings.