Mechanochemical Scholl Reaction on Phenylated Cyclopentadiene Core: One-Step Synthesis of Fluoreno[5]helicenes.

In this study, we explore feasibility of the mechanochemical approach in the synthesis of tetrabenzofluorenes (fluoreno[5]helicenes). For this, commercially available phenylated cyclopentadiene precursors are subjected to the Scholl reaction in the solid state using FeCl3 as an oxidant and sodium chloride as the solid reaction medium. This ball milling process gave access to the 5-membered ring containing-helicenes in one synthetic step in high (95-96 %) isolated yields. The solution-phase reactions, however, were found to be moderate to low yielding in this regard (10-40 %).

Mechanochemical Synthesis of Corannulene: Scalable and Efficient Preparation of A Curved Polycyclic Aromatic Hydrocarbon under Ball Milling Conditions.

Corannulene, a curved polycyclic aromatic hydrocarbon, is prepared in a multigram scale through mechanochemical synthesis. Initially, a mixer mill approach is examined and found to be suitable for a gram scale synthesis. For larger scales, planetary mills are used. For instance, 15 g of corannulene could be obtained in a single milling cycle with an isolated yield of 90 %. The yields are lower when the jar rotation rate is lower or higher than 400 revolutions per minute (rpm). Cumulatively, 98 g of corannulene is produced through the ball milling-based grinding techniques. These results indicate the future potential of mechanochemistry in the rational chemical synthesis of highly curved nanocarbons such as fullerenes and carbon nanotubes.

Main Group Molecular Switches with Swivel Bifurcated to Trifurcated Hydrogen Bond Mode of Action.

Artificial molecular machines have captured the full attention of the scientific community since Jean-Pierre Sauvage, Fraser Stoddart, and Ben Feringa were awarded the 2016 Nobel Prize in Chemistry. The past and current developments in molecular machinery (rotaxanes, rotors, and switches) primarily rely on organic-based compounds as molecular building blocks for their assembly and future development. In contrast, the main group chemical space has not been traditionally part of the molecular machine domain. The oxidation states and valency ranges within the p-block provide a tremendous wealth of structures with various chemical properties. Such chemical diversity─when implemented in molecular machines─could become a transformative force in the field. Within this context, we have rationally designed a series of NH-bridged acyclic dimeric cyclodiphosphazane species, [(µ-NH){PE(µ-NtBu)2PE(NHtBu)}2] (E = O and S), bis-PV2N2, displaying bimodal bifurcated R21(8) and trifurcated R31(8,8) hydrogen bonding motifs. The reported species reversibly switch their topological arrangement in the presence and absence of anions. Our results underscore these species as versatile building blocks for molecular machines and switches, as well as supramolecular chemistry and crystal engineering based on cyclophosphazane frameworks.

Synthesis of precisely functionalizable curved nanographenes via graphitization-induced regioselective chlorination in a mechanochemical Scholl Reaction.

While the synthesis of nanographenes has advanced greatly in the past few years, development of their atomically precise functionalization strategies remains rare. The ability to modify the carbon scaffold translates to controlling, adjusting, and adapting molecular properties. Towards this end, here, we show that mechanochemistry is capable of transforming graphitization precursors directly into chlorinated curved nanographenes through a Scholl reaction. The halogenation occurs in a regioselective, high-yielding, and general manner. Density Functional Theory (DFT) calculations suggest that graphitization activates specific edge-positions for chlorination. The chlorine atoms allow for precise chemical modification of the nanographenes through a Suzuki or a nucleophilic aromatic substitution reaction. The edge modification enables modulation of material properties. Among the molecules prepared, corannulene-coronene hybrids and laterally fully π-extended helicenes, heptabenzo[5]superhelicenes, are particularly noteworthy.

Non-Fullerene Electron Acceptors Based on Hybridisation of Corannulene and Thiophene-S,S-Dioxide Motifs.

Herein we show that hybridisation of buckybowl corannulene and thiophene-S,S-dioxide motifs is a general approach for the preparation of high electron affinity molecular materials. The devised synthesis is modular and relies on thienannulation of corannnulene-based phenylacetylene scaffolds. The final compounds are highly soluble in common organic solvents. These compounds also exhibit interesting optical properties such as absorption and emission in the blue/green regions of the electromagnetic spectrum. Importantly, a bis-S,S-dioxide derivative exhibits three reversible reductions similar in their strength to the prevalent fullerene-based electron acceptor phenyl-C61 -butyric acid methyl ester (PC61 BM).

Celebrating Women in Chemistry.

This special collection of Chemistry - An Asian Journal features contributions made by female scientists working in the field of chemistry. In her editorial, Mihaiela C. Stuparu briefly describes the background that led to the conception of this special collection. The table of contents graphic was created by Lim Pei Xi.

Mechanochemical transformation of planar polyarenes to curved fused-ring systems.

The transformation of planar aromatic molecules into π-extended non-planar structures is a challenging task and has not been realized by mechanochemistry before. Here we report that mechanochemical forces can successfully transform a planar polyarene into a curved geometry by creating new C-C bonds along the rim of the molecular structure. In doing so, mechanochemistry does not require inert conditions or organic solvents and provide better yields within shorter reaction times. This is illustrated in a 20-minute synthesis of corannulene, a fragment of fullerene C60, in 66% yield through ball milling of planar tetrabromomethylfluoranthene precursor under ambient conditions. Traditional solution and gas-phase synthetic pathways do not compete with the practicality and efficiency offered by the mechanochemical synthesis, which now opens up a new reaction space for inducing curvature at a molecular level.

Synthesis of azahelicenes through Mallory reaction of imine precursors: corannulene substrates provide an exception to the rule in oxidative photocyclizations of diarylethenes.

Typically, the synthesis of phenanthrene-based polycyclic aromatic hydrocarbons relies on the Mallory reaction. In this approach, stilbene (PhCH[double bond, length as m-dash]CHPh)-based precursors undergo an oxidative photocyclization reaction to join the two adjacent aromatic rings into an extended aromatic structure. However, if one C[double bond, length as m-dash]C carbon atom is replaced by a nitrogen atom (C[double bond, length as m-dash]N), the synthesis becomes practically infeasible. Here, we show the very first examples of a successful Mallory reaction on stilbene-like imine precursors involving the molecularly curved corannulene nucleus. The isolated yields exceed 90% and the resulting single and double aza[4]helicenes exhibit adjustable high affinity for electrons.

Aggregation-free and high stability core-shell polymer nanoparticles with high fullerene loading capacity, variable fullerene type, and compatibility towards biological conditions.

Fullerenes have unique structural and electronic properties that make them attractive candidates for diagnostic, therapeutic, and theranostic applications. However, their poor water solubility remains a limiting factor in realizing their full biomedical potential. Here, we present an approach based on a combination of supramolecular and covalent chemistry to access well-defined fullerene-containing polymer nanoparticles with a core-shell structure. In this approach, solvophobic forces and aromatic interactions first come into play to afford a micellar structure with a poly(ethylene glycol) shell and a corannulene-based fullerene-rich core. Covalent stabilization of the supramolecular assembly then affords core-crosslinked polymer nanoparticles. The shell makes these nanoparticles biocompatible and allows them to be dried to a solid and redispersed in water without inducing interparticle aggregation. The core allows a high content of different fullerene types to be encapsulated. Finally, covalent stabilization endows nanostructures with stability against changing environmental conditions.

Corannulene: A Curved Polyarene Building Block for the Construction of Functional Materials.

This Account describes a body of research in the design and synthesis of molecular materials prepared from corannulene. Corannulene (C20H10) is a molecular bowl of carbon that can be visualized as the hydrogen-terminated cap of buckminsterfullerene. Due to this structural resemblance, it is often referred to as a buckybowl. The bowl can invert, accept electrons, and form host-guest complexes. Due to these characteristics, corannulene presents a useful building block in materials chemistry.In macromolecular science, for example, assembly of amphiphilic copolymers carrying a hydrophobic corannulene block enables micelle formation in water. Such micellar nanostructures can host large amounts of fullerenes (C60 and C70) in their corannulene-rich core through complementarity of the curved π-surfaces. Covalent stabilization of the assembled structures then leads to the formation of robust water-soluble fullerene nanoparticles. Alternatively, use of corannulene in a polymer backbone allows for the preparation of electronic and redox-active materials. Finally, a corannulene core enables polymer chains to respond to solution temperature changes and form macroscopic fibrillar structures. In this way, the corannulene motif brings a variety of properties to the polymeric materials.In the design of non-fullerene electron acceptors, corannulene is emerging as a promising aromatic scaffold. In this regard, placement of sulfur atoms along the rim can cause an anodic shift in the molecular reduction potential. Oxidation of the sulfur atoms can further enhance this shift. Thus, a variation in the number, placement, and oxidation state of the sulfur atoms can create electron acceptors of tunable and high strengths. An advantage of this molecular design is that material solubility can also be tuned. For example, water-soluble electron acceptors can be created and are shown to improve the moisture resistance of perovskite solar cells.Host-guest complexation between corannulene and γ-cyclodextrin under flow conditions of a microfluidic chamber allows for the preparation of water-soluble nanoparticles. Due to an oligosaccharide-based sugarcoat, the nanoparticles are biocompatible while the corannulene component renders them active toward nonlinear absorption and emission properties. Together, these attributes allow the nanoparticles to be used as two-photon imaging probes in cancer cells.Finally, aromatic extension of the corannulene nucleus is seen as a potential route to nonplanar nanographenes. Typically, such endeavors rely upon gas-phase synthesis or metal-catalyzed coupling protocols. Recently, two new approaches have been established in this regard. Photochemically induced oxidative cyclization, the Mallory reaction, is shown to be a general method to access corannulenes with an extended π-framework. Alternatively, solid-state ball milling can achieve this goal in a highly efficient manner. These new protocols bring practicality and sustainability to the rapidly growing area of corannulene-based nanographenes.In essence, corannulene presents a unique building block in the construction of functional materials. In this Account, we trace our own efforts in the field and point toward the challenges and future prospects of this area of research.

Synthesis and Properties of Bis-corannulenes.

Corannulenecarbaldehyde and corannulenylmethyl triphenylphosphonium bromide are combined through the Wittig olefination reaction to furnish dicorannulenylethene with 70% yield. A subsequent oxidative photocyclization leads to annulation of the corannulene nuclei to produce a C42H18 nanographene structure in 59% yield. Interestingly, only the trans isomer of the dicorannulenylethene forms cocrystals with fullerene C60 through concave-convex and convex-convex π-π stacking interactions. The Mallory photocyclization could be extended to a phenanthrene-based diarylethene precursor to yield a large bicorannulene system.

Corannulene Chalcogenides.

The introduction of chalcogen atoms into a polycyclic aromatic hydrocarbon structure is an established method to tune material properties. In the context of corannulene (C20 H10 ), a fragment of fullerene C60 , such structural adjustments have given rise to an emerging class of functional and responsive molecular materials. In this minireview, our aim is to discuss the synthesis and properties of such chalcogen (sulfur, selenium, and tellurium) derivatives of corannulene.

Novel amphiphilic corannulene additive for moisture-resistant perovskite solar cells.

The addition of amphiphilic triethylene glycol based corannulene molecules provides multiple Lewis basic sites that assist in perovskite grain growth, and improve the charge carrier collection and moisture resistance of perovskite solar cells. This study paves the way for utilization of more molecules from corannulene families in perovskite research.

Selenium and Tellurium Derivatives of Corannulene: Serendipitous Discovery of a One-Dimensional Stereoregular Coordination Polymer Crystal Based on Te-O Backbone and Side-Chain Aromatic Array.

Monobromo-, tetrabromo-, and pentachloro-corannulene are subjected to nucleophilic substitution reactions with tolyl selenide and phenyl telluride-based nucleophiles generated in situ from the corresponding dichalcogenides. In the case of selenium nucleophile, the reaction provides moderate yields (52-77 %) of the targeted corannulene selenoethers. A subsequent oxidation of the selenium atoms proceeds smoothly to furnish corannulene selenones in 81-93 % yield. In the case of tellurides, only monosubstitution of the corannulene scaffold could be achieved albeit with concomitant oxidation of the tellerium atom. Unexpectedly, this monotelluroxide derivative of corannulene (RR'Te=O, R=Ph, R'=corannulene) is observed to form a linear coordination polymer chain in the crystalline state. In this chain, Te-O constitutes the polymer backbone around which the aromatic groups (R and R') arrange as polymer side-chains. The polymer crystal is stabilized through intramolecular π-π stacking interactions of the side-chains and intermolecular hydrogen and halogen bonding interactions with the solvent (chloroform) molecules. Interestingly, each diad of the polymer chain is racemic. Therefore, in terms of stereoregularity, the polymer chain can be described as syndiotactic.

Mechanochemical Synthesis of Corannulene-Based Curved Nanographenes.

It is shown that corannulene-based strained π-surfaces can be obtained through the use of mechanochemical Suzuki and Scholl reactions. Besides being solvent-free, the mechanochemical synthesis is high-yielding, fast, and scalable. Therefore, gram-scale preparation can be carried out in a facile and sustainable manner. The synthesized nanographene structure carries positive (bowl-like) and negative (saddle-like) Gaussian curvatures and adopts an overall quasi-monkey saddle-type of geometry. In terms of properties, the non-planar surface exhibits a high electron affinity that was measured by cyclic voltammetry, with electrolysis and in situ UV/vis spectroscopy experiments indicating that the one-electron reduced state displays a long lifetime in solution. Overall, these results indicate the future potential of mechanochemistry in accessing synthetically challenging and functional curved π-systems.

Corannulene-Based Electron Acceptors: Combining Modular and Practical Synthesis with Electron Affinity and Solubility.

It is shown in this work that high electron affinity can be combined with high solubility and practical accessibility in corannulene-based electron acceptors. The electron affinity originates from the presence of three different types of electron-withdrawing groups (imide, sulfone, and trifluoromethyl) on the aromatic scaffold. The imide substituent further hosts a long alkyl chain (C18 H37 ) to boast solubility in a wide range of organic solvents. The synthesis is modular and consists of three simple steps from a commonly available corannulene derivative with an overall isolated yield of 22-27 %.

Synthesis of thermoresponsive oligo(ethylene glycol) polymers through radical ring-opening polymerization of vinylcyclopropane monomers.

Polyvinylcyclopropanes are an old class of polymers typically known for their low polymerization-induced shrinkage properties. In this work, we show that they are capable of exhibiting a thermally triggered aggregation process in aqueous solutions. The phase transition is sharp, tunable within the temperature range of 25-46 °C, and relatively insensitive to environmental conditions. It is anticipated that this preliminary study will shine new light on polyvinylcyclopropanes and lead to new avenues in their studies and future application.

A general approach to non-fullerene electron acceptors based on the corannulene motif.

Here, we show that oxidation of exo-cyclic sulfur atoms enhances the molecular reduction potential of non-planar polycyclic aromatic hydrocarbons and allows for a systematic bridging of the electron affinity gap between corannulene, a fragment of fullerene C60, and the prevalent fullerene-based electron acceptor phenyl-C61-butyric acid methyl ester (PCBM).

Corannulene: a molecular bowl of carbon with multifaceted properties and diverse applications.

Unlike typical polycyclic aromatic hydrocarbons, such as coronene, which are flat and planar, corannulene is a molecular bowl of carbon. It can be imagined as the cap region of fullerene C60 or an end of a single-walled carbon nanotube. This structural distinction manifests itself in unique properties. For example, corannulene exhibits bowl-flipping dynamics, electron accepting capability, and formation of a ball-in-socket type of interaction with C60. These varied properties allow for application of corannulene in a myriad of disciplines ranging from organic electronics and sensing to energy storage and self-assembly. In this feature article, our goal is to discuss the major synthetic developments in corannulene chemistry which allow the scientific community access to this beautiful molecule in a practical fashion, the unique properties of the corannulene nucleus that sets it apart from the planar polynuclear aromatic hydrocarbons, and lastly its applications in the arena of materials chemistry.

Photochemical Synthesis and Electronic Properties of Extended Corannulenes with Variable Fluorination Pattern.

The first family of extended and fluorinated corannulenes is prepared through a highly efficient and modular synthetic strategy. In this strategy, corannulene aldehyde could be combined with the fluorine-carrying phosphonium ylides to furnish stilbene-like vinylene precursors. A photochemically induced oxidative cyclization process of these precursors gives rise to the fluorinated and curved polycyclic aromatic hydrocarbons. A UV-vis absorption study shows that aromatic extension results in a bathochromic shift of about 12 nm. Fluorination further shifts the absorption spectrum to the red region, and a maximum shift of about 22 nm is detected for a compound carrying two trifluoromethyl groups. A cyclic and square-wave voltammetry investigation reveals that the extension of the corannulene scaffold increases the reduction potential by 0.11 V. Placement of fluorine or trifluoromethyl groups further enhances the electron affinities. In this regard, the presence of one trifluoromethyl group equals the effect of three aromatic fluorine atoms. Molecules with two trifluoromethyl groups, meanwhile, exhibit the highest reduction potentials of -1.93 and -1.83 V. These values are 0.37 and 0.46 V higher than those of the parental corannulene and demonstrate the utility of the present design concept by efficiently accessing effective electron acceptors based on the buckybowl motif.