Chiral Molecular Carbon Nanostructures | Accounts of Chemical

Article Cite This: Acc. Chem. Res. 2019, 52, 1565−1574

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Chiral Molecular Carbon Nanostructures Published as part of the Accounts of Chemical Research special issue “Advanced Molecular Nanocarbons”. Jesús M. Fernández-García,† Paul J. Evans,† Salvatore Filippone,† María Á ngeles Herranz,† and Nazario Martín*,†,‡ †

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Departamento de Química Orgánica, Facultad de Química, Universidad Complutense de Madrid, Avda. Complutense s/n, E-28040 Madrid, Spain ‡ IMDEA-Nanoscience, C/Faraday 9, Campus de Cantoblanco, E-28049 Madrid, Spain CONSPECTUS: Chirality is a fascinating property present in naturally occurring and artificial molecules and materials, observable as chiroptical behavior. The emerging area of carbon nanostructures has undergone tremendous development, with a wide variety of carbon nanoforms reported over the last two decades. However, despite interest in merging chirality and nanocarbons, this has been successfully achieved only in empty fullerenes, whereas in other kinds of fullerenes or carbon nanostructures such as carbon nanotubes, graphene, and graphene quantum dots (GQDs), to name the most popular systems, it is almost unknown. Therefore, controlling chirality in carbon nanostructures currently represents a major challenge for the chemical community. In this Account, we show our progress in the synthesis of chiral molecular carbon nanostructures, namely, metallofullerenes, endohedral fullerenes, GQDs, and curved molecular nanographenes, by using asymmetric catalysis and both topdown and bottom-up chemical approaches. Furthermore, we bring in a new family of lesser-known molecular chiral bilayer nanographenes, where chirality is introduced from the starting helicene moiety and a single enantiomer of the nanographene is synthesized. Some important landmarks in the development of chiral molecular carbon nanostructures shown in this Account are the application of synthesis-tailored, enantiomerically pure metallofullerenes as catalysts for hydrogen transfer reactions and the use of endohedral fullerenes to determine the effect of the incarcerated molecule in the carbon cage on the cis−trans stereoisomerization of optically active pendent moieties. Furthermore, the first top-down synthesis of chiral GQDs by functionalization with chiral alcohols is also presented. An emerging alternative to GQDs, when the desire for purity and atomistic control outweighs the cost of multistep synthesis, is the bottom-up approach, in which molecular nanographenes are formed in precise sizes and shapes and enantiomeric control is feasible. In this regard, a singular and amazing example is given by our synthesis of a single enantiomer of the first chiral bilayer nanographene, which formally represents a new family of molecular nanographenes with chirality controlled and maintained throughout their syntheses. The aforementioned synthetic chiral nanostructures represent groundbreaking nanocarbon systems where chirality is a further dimension of structural control, paving the way to a new scenario for carbon nanoforms in which chirality selection determines the properties of these novel carbon-based materials. Fine-tuning of such properties is envisioned to impact biomedical and materials science applications.

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INTRODUCTION The seminal discovery of fullerenes in 1985 represented the first known molecular carbon allotrope.1 In contrast to the well-known classical allotropes graphite and diamond, exhibiting a reticular structure with carbon atoms (sp2 and sp3, respectively) extending along the three directions of space, fullerene C60 was the first discrete molecule constituted by a precise number of carbon atoms (60). Actually, the following member of the series, C70, was simultaneously observed, and since then, the number of empty fullerenes obeying the isolated pentagon rule (IPR) has been systematically increased.2 Thenceforth, a wide variety of carbon nanostructures have been isolated and characterized, and their properties © 2019 American Chemical Society

and chemical reactivity have been studied. Among these carbon nanostructures, endofullerenes, single- and multiwalled carbon nanotubes, carbon nanoonions, graphene, and graphene quantum dots (GQDs) have received most of the attention by the scientific community.3 Despite the interest in these groundbreaking nanoforms of carbon, still there is a lack of control of essential issues such as regioselectivity4 and, in particular, stereoselectivity.5 In this regard, it is important to take into account the fact that the lack of chirality means the loss of an additional dimension of Received: March 18, 2019 Published: June 4, 2019 1565

DOI: 10.1021/acs.accounts.9b00144 Acc. Chem. Res. 2019, 52, 1565−1574

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Accounts of Chemical Research control in these nanocarbon materials. Furthermore, this structural control allows the modification of related properties on demand and therefore their usefulness in potential applications. Chirality is a fascinating concept in chemistry that is not equally present in the different carbon nanoforms known to date. With a few exceptions, most carbon nanostructures exhibit inherent chirality in their structures, as has been found in fullerenes, endofullerenes, and carbon nanotubes. This is not the case, however, for systems like graphene and GQDs, which because of their planarity lack inherent chirality. These structural concepts were studied by different groups at the times of discovery of these new chemical structures based on carbon.6 Therefore, here we focus on chirality as a property to be introduced into the respective achiral carbon nanostructure by chemical protocols. As a consequence of the different chemical structures and reactivities of the known carbon nanostructures, the preparation of chiral derivatives has not been equally developed. Thus, whereas molecular fullerenes have been extensively studied in our group using, in a pioneering manner, asymmetric catalysis to afford a variety of chiral fullerenes with total control of the enantioselectivity,7 this is not the case for other systems, such as carbon nanotubes and graphene. In this Account, we present studies, carried out mostly in our group, directed to the preparation of chiral molecular carbon nanostructures, including fullerenes (both empty and endofullerenes) and nanographenes, formed by both top-down and bottom-up approaches. These studies should pave the way for a future in which the introduction of chirality into graphene and carbon nanotubes can be carried out with certainty.

Figure 1. (a) Enantioselective synthesis of half-sandwich metallo[60]fullerene complexes from enantiomerically pure (top) cis- and (bottom) trans-pyrrolidino[3,4:1,2][60]fullerenes by reaction with the appropriate metal complex. (b) Half-sandwich metallo[60]fullerene complex enantioselectively synthesized from enantiomerically pure pyrrolino[3,4:1,2][60]fullerene together with its X-ray crystal structure.

2. ADVANCES IN CHIRAL FULLERENES (Figure 1a, bottom). After removal of the tert-butyl group, each of the four possible stereoisomeric pyrrolidino[60]fullerenecarboxylic acids is used as a ligand to obtain halfsandwich complexes of iridium, rhodium, and ruthenium by reaction with pentamethylcyclopentadienyliridium(II) dichloride dimer, pentamethylcyclopentadienylrhodium(II) dichloride dimer, and (p-cymene)ruthenium(II) dichloride dimer, respectively. We have prepared a set of optically active fullerene hybrids with metal-centered chirality that have great potential in enantioselective catalysis. Indeed, the addition of the metals occurs diastereospecifically, giving rise to metallofullerenes with two new chiral centers: a chiral nitrogen atom and, more importantly, a stable stereogenic metal atom that adopts a fixed pseudotetrahedral geometry without incurring any process of epimerization. Both new stereocenters feature a configuration that is determined by the chirality of the C2 stereocenter of the pyrrolidine ring, and therefore, the stereogenic metal center can be controlled by the suitable choice of the catalytic system (SM- or RM-selective) (Figure 1a).9 Similar metallo[60]fullerene hybrids based on pyrrolino[60]fullerenes have also been prepared enantioselectively using both metal-mediated processes and organocatalysis (Figure 1b). These new half-sandwich metallo[60]fullerene complexes have been successfully used in photoelectrochemical cells, where they proved to be both good electron acceptor materials and excellent catalysts for photoinduced oxygen reduction reactions.10 Interestingly, metallo[60]fullerene complexes 2 based on pyrrolidino[3,4:1,2][60]fullerenes have turned out to be

2.1. Chiral Fullerenes for Catalysis

A first direct approach toward chiral carbon nanostructures is based on the preparation of optically active molecular materials by enantioselective functionalization of fullerenes. This is not a trivial process since the fullerene cage consists of noncoordinating sp2 carbon atoms, which allow neither activation of the substrates nor the consequent chiral induction. In a previous review, we reported on the use of organo- and transition-metal catalysis to overcome this obstacle, allowing the preparation of diverse chiral [60]-, [70]-, and endohedral fullerenes.7 While the importance of fullerene hybrids has been recognized by the preparation of a great number of them,8 the lack of methodologies for their stereoselective synthesis hampers the availability of chiral metallofullerenes. In order to address this issue, we have undertaken the preparation of brand-new chiral fullerene hybrids with the aim of achieving a new class of enantioselective catalysts and to explore the potential of synergy between fullerenes and transition metals. As a first step, we pointed our attention to active transition metals known as catalysts in different chemical transformations, namely, iridium, rhodium, and ruthenium. Thus, the addition of a tert-butyl iminoester onto C60 catalyzed by AgOAc/(R,R)-BPE or Cu(OAc)2/(R)-FeSulPhos affords the two cis-pyrrolidino[60]fullerene stereoisomers (1cis) with R,R and S,S configurations, respectively, at C2 and C5 of the pyrrolidine ring (Figure 1a, top). Similarly, Cu(OTf)2/(R)- and (S)-DTBM-Segphos direct the formation of the two enantiomers of the trans cycloadduct, 1trans 1566


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of endohedral fullerenes, namely, the respective pyrrolidino[3,4:1,2][60]fullerenes of endohedrals H2@C60,18 H2O@ C60,17 and [email protected] These compounds had previously been prepared by encapsulation of a molecule of hydrogen,20 water,21 and hydrogen fluoride,22 respectively, and further reacted in a enantioselective N-metalated azomethine ylide cycloaddition.17−19 We carried out a kinetic study of the cis−trans isomerization of 2,5-disubstituted pyrrolidinofullerenes with two endohedrals bearing H2O and HF as incarcerated molecules in the C60 sphere, together with pristine C60. Theoretical calculations and experimental data clearly revealed a stepwise mechanism that leads to a zwitterionic intermediate as a result of the breaking of the C4−C5 pyrrolidine bond, while the absolute configuration of C2 of the pyrrolidine ring is maintained. Interestingly, the stabilization of the zwitterionic intermediate is based on the fullerene anion and the formed benzylic cation but also is assisted by a hydrogen bond from the inner species through the fullerene shell (Figure 3). Experimental evidence revealed that the cis−trans isomerization is stereospecific since the optically pure (2S,5S)-cis stereoisomer gave rise to an optically pure (2S,5R)-transpyrrolidino[60]fullerene without any loss of optical purity. According to the aforementioned results, the comparative kinetic study showed first-order kinetics, with the isomer-

excellent reusable homogeneous/heterogeneous catalysts in hydrogen transfer reactions (Figure 2). Thus, these systems,

Figure 2. Metallo[60]fullerene complexes based on pyrrolidino[3,4:1,2][60]fullerenes are efficient homogeneous/heterogeneous reusable catalysts in hydrogen transfer reactions.

particularly those bearing the iridium atom, proved to be highly efficient in the reduction of ketones with isopropanol as the hydrogen donor solvent as well as in the N-alkylation of amines with alcohols.11 2.2. Stereoselective Synthesis on Endohedral Fullerenes

Retro-cycloaddition reactions represent a beautiful example of less-explored reversible covalent chemistry that is well-known in fullerene chemistry.12 These reactions have found utility, for instance, in protection−deprotection protocols13 and, more recently, in the first example of fullerenes used for chiral resolution of helicene racemates.14 However, despite interest in the aforementioned chemical transformations, studies with control of the stereoselectivity have not been properly addressed to date and still remain a significant scientific challenge. This is particularly true in endohedral fullerenes. In this regard, endohedral fullereneshaving a chemical species (atom, molecule, or cluster) in the inner cavity of the fullerene carbon cagerepresent a valuable and a singular scenario where the incarcerated species can play a noninnocent role in the outcome of the reaction occurring on the fullerene surface.15 The first asymmetric synthesis of the chemically functionalized endohedral fullerene derivative La@C72(C6H3Cl2) by the 1,3-cycloaddition reaction of an N-metalated azomethine ylide under very mild experimental conditions was reported in 2011.16 Eight different compounds were obtained and were characterized by spectroscopic techniques and circular dichroism (CD) measurements. Interestingly, selective cycloaddition at the C13−C14 and C27−C28 double bonds of La@ C72(C6H3Cl2) was proposed according to the spectroscopic data and density functional theory (DFT) calculations (B3LYP and M06-2X functionals). We recently reported the first cis−trans isomerization reaction from enantiomerically enriched [60]fullerene derivatives. The experimental findings and DFT calculations showed that this reaction occurs in a completely stereospecific manner.17 In order to gain a better understanding of this process and determine the role of the inner species, we have also carried out a cis−trans isomerization reaction on a series

Figure 3. (a) Kinetic study of the cis−trans isomerization of 2,5disubstituted pyrrolidinofullerenes of endohedrals bearing H2, H2O, and HF as incarcerated molecules in the C60 sphere. (b) Zwitterionic intermediate showing the stabilizing hydrogen bonds from the incarcerated species. 1567


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Accounts of Chemical Research ization of 2,5-disubstituted pyrrolidinofullerenes in H2O@C60 occurring faster than in HF@C60, whereas the slowest kinetics was observed for empty C60 because of the lack of hydrogenbonding assistance.

3. TOP-DOWN SYNTHESIS OF CHIRAL GRAPHENE QUANTUM DOTS A further degree of complexity emerges in chiral molecular systems when the collective behavior of multiple chiral units is considered in the bulk, and important changes in morphology and material properties can be found.23 In this regard, novel approaches to carbon chiral nanomaterials are currently going beyond chiral molecular compositions, and other chiral materials such as polymers,24 nanoparticles,25 self-assembled systems,26 nanotubes,27 and metal−organic frameworks are being considered.28 Among the variety of carbon nanomaterials, graphene has received much attention because its extraordinary properties are promising for numerous applications.29 However, despite this interest, a band gap between the conduction and valence bands of graphene needs to be introduced for applications in electronics. Recent strategies to generate a band gap in graphene by chemical cutting have resulted in innovative materials such as graphene quantum dots.30 GQDs can be obtained from different easily affordable carbon material sources (graphite, carbon fiber, graphene, carbon nanotubes) via top-down chemical (acidic oxidation, hydrothermal or solvothermal synthesis, microwave- or sonication-assisted methods), electrochemical, or physical approaches. These low-cost production methods, together with the good photostability of GQDs and their low toxicity and good solubility in water, have provoked considerable attention over the past decade. Furthermore, quantum-confinement effects and defect engineering in GDQs permit the tuning of their remarkable electronic and optical properties.30 To the best of our knowledge, the first synthesis of chiral GQDs was recently reported by our group31 and also by Violi and Kotov.32 In quite similar approaches, the GQDs obtained under acidic conditions were chemically modified with enantiomerically pure alcohols or amino acids, respectively. In our case, commercial graphite was exfoliated and oxidatively fragmented using a mixture of concentrated H2SO4 and HNO3, followed by neutralization of the excess acid and dialysis. In a subsequent step, the carboxylic acid-terminated GQDs were treated with thionyl chloride to create acyl chloride groups on the GQD surface. Thionyl chloride was used as the solvent in large excess, enabling the reaction to proceed quantitatively. After removal of the excess thionyl chloride with argon as the carrier gas, the GQDs endowed with acid chlorides were reacted in situ with (R)- or (S)-2-phenyl-1-propanol. Following filtration and extensive washing with different solvents, GQDs bearing enantiomerically pure esters as organic addends were obtained. Initially, the morphology of the chiral GQDs (CGQDs) was investigated by microscopy techniques such as transmission electron microscopy and atomic force microscopy. A heterogeneous distribution was observed, with a mixture of isolated and few-layer GQDs with sizes in the range from 3 to 20 nm and heights of 3−5 nm.31 Analysis of the optoelectronic properties of both CGQDs nanomaterials revealed UV−vis absorption spectra with the typical absorption of an aromatic π system and a strong photoluminescence response upon excitation at 350 nm (Figure 4).

Figure 4. Absorption (solid) and emission (dashed, excitation at 350 nm) of a solution of S-CGQDs in H2O. Inset: photograph taken under 365 nm illumination.

The presence of the ester groups on the CGQDs surface was confirmed by 13C NMR and FTIR spectroscopy: the characteristic resonance signals of the phenyl substituents (120−150 ppm) and the newly formed esters (168 ppm) were observed in the 13C NMR spectrum, and in the FTIR spectrum the CO stretching vibrations at 1727−1731 cm−1 and the C−O−C asymmetric (1300 cm−1) and symmetric (1200 cm−1) stretching modes were clearly observed. Finally, the chiroptical properties of the brand-new CGQDs were investigated by CD measurements in N-methylpyrrolidone (NMP) solutions. The chiral moieties at the GQD surface did not have any obvious absorption over 300 nm (see Figure 4), and for that reason, pyrene was added to the CGQDs solution to aid in the formation of supramolecular assemblies, which could trigger chiral transfer from the CGQDs to the supramolecular assemblies by means of strong π−π stacking forces. Interestingly, the pyrene/CGQDs NMP solution displayed a CD signal (Figure 5). In particular, the CGQDs containing the S enantiomerically pure units (SCGQDs) exhibited a positive Cotton effect over the absorption range of 330−340 nm, while those containing the R enantiomerically pure units (R-CGQDs) showed the opposite negative Cotton effect over the same absorption range. The obtained S- and R-CGQDs/pyrene compositions are not proper enantiomer pairs, and the differences observed in the other CD signals of the R- and S-CGQDs/pyrene aggregates could be related to the nonidentical surface functionalization, contributions of different CGQDs sizes and agglomerates, and their assembly with the pyrene molecules. This elemental example proves the transmission of chirality through the assembly of nanomaterials, a property worthy of exploration in nanomaterials for further technological applications.

4. BOTTOM-UP SYNTHESIS OF CHIRAL GRAPHENE QUANTUM DOTS Having taken advantage of the large-scale and ready availability of carbon materials prepared from top-down methods by functionalization of fullerenes and GQDs, we sought to expand our synthetic toolbox with bottom-up, benchtop approaches. This synthetic philosophy relies on commercially available small molecules as opposed to bulk materials like graphite and fullerenes. While they typically require more complex synthetic 1568


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extension (APEX) of an arene template to construct one or more fused aromatic rings.34 Taking into account the high availability of corannulene due to its kilogram-scale production35we decided to expand the π system of this curved polycyclic aromatic hydrocarbon (PAH) using a stepwise methodology consisting of a Diels− Alder cycloaddition followed by a Scholl cyclodehydrogenation reaction (Scheme 1).36 This bottom-up approach yielded new curved molecular nanographenes with interesting optoelectronic properties. Scheme 1. Syntheses of Curved Nanographenes by Stepwise π Extension of Corannulene Involving Diels−Alder Cycloaddition and Scholl Cyclodehydrogenation Reactions

Figure 5. (a) Cartoon depicting a possible assembly formed by SCGQDs and pyrene. (b) Top: Circular dichroism spectra of the SCGQDs/pyrene (red) and R-CGQDs/pyrene (black) aggregates in NMP. Bottom: UV−vis spectrum of pyrene in NMP.

The synthesis started with the bromination of corannulene using IBr with the objective of performing a subsequent Sonogashira cross-coupling reaction on bromocorannulene (3). The alkynylcorannulene produced, 4, was a suitable compound to carry out a further stepwise π extension. The first step was a Diels−Alder cycloaddition reaction, using cyclopentadienone 5 as the diene, where tert-butyl groups were introduced to improve the solubility of the final nanographene. The resulting compound 6 is a hexaphenylbenzene derivative in which one of the phenyl groups is substituted by a corannulene fragment. From intermediate 6, subsequent Scholl oxidation afforded different products (7−9) depending on the new bonds formed. Following the four rings forming the dibenzo[fg,ij]phenanthro[9,10,1,2,3-pqrst]pentaphene (DBBP) moiety (black arrows), a seven-membered ring and a sixmembered ring could be further closed between corannulene and DBBP (blue and green arrows), yielding nanographenes with different types of Gaussian curvature (Scheme 1). In carbon nanostructures, curvature is formed when non-sixmembered rings are present.37 In this way, structures having

design and multistep syntheses, the vast knowledge base of chemical reactivity allows for the atom-specific creation of structures unavailable by highly energetic (arc discharge synthesis of fullerenes, chemical vapor deposition synthesis of graphene) or highly reactive (exfoliation and etching of graphite to produce GQDs) methods. The addition of methods to make nanographenes by conventional synthesis to the expertise of our research thereby completes a full battery of synthetic approaches to carbon nanomaterials, allowing us to apply the strategy most relevant to our synthetic goals in any given situation. The most common synthetic method consists of stepwise π extension from small aromatic components involving Diels− Alder, cross-coupling, or alkyne cyclotrimerization reactions followed by cyclodehydrogenation (Scholl reaction).33 More recently, Itami et al. have described a single-step annulative π 1569


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Accounts of Chemical Research only six-membered rings present zero Gaussian curvature, as in graphene or nanotubes. When rings with five or fewer atoms are inserted, the structure has positive Gaussian curvature with spherical shape, as in fullerenes or buckybowls. On the other hand, when the structure possess seven-membered or larger rings, it has a negative Gaussian curvature with a saddle shape. Furthermore, if the resulting structure is stereochemically rigid and lacks an inversion center and mirror plane, the introduction of curvature in carbon nanostructures also leads to chirality.38 In our case, depending on the cyclodehydrogenation conditions employed, it was possible to direct the synthesis to the formation of nanographenes with positive or negative curvature. Employing FeCl3 as the oxidant and chilling the reaction mixture to −50 °C afforded chiral [6]helicene nanographene 7 with positive Gaussian curvature due to the presence of corannulene with a five-membered ring in the structure. Interestingly, this compound is the result of the formation of five C−C bonds, four in DBPP and the other one between the corannulene and DBPP units. Changing the oxidant to DDQ at 0 °C enabled the creation of an additional C−C bond, forming a new seven-membered ring between corannulene and DBBP, to afford nanographene 8 with negative Gaussian curvature. Finally, when the reaction was performed at 80 °C with FeCl3, the introduction of a chlorine atom in the nanographene 9 with negative curvature was possible. These molecular nanographenes differ only in the formation, or not, of the C−C bond leading to the formation of the sevenmembered ring and negative curvature. This minimal detail induces important changes in the structure of the nanographenes. The corannulene fragment is flatter on the nanographene with negative curvature, as shown by the Xray data. This study revealed average p-orbital axis vector (POAV) angles of the hub carbons of the corannulene of 7.91° for 7 and 7.87° for 9. The observed geometrical change also influences the packing of the molecules in the crystal. While 7 experiences π−π interactions between the flat areas of DBPP, 8 and 9 do not because of the negative curvature, which extends into the DBPP fragment, resulting in a porous packing structure (Figure 6). As a consequence of the π-extended system in these molecular nanographenes, they are better electron acceptors and electron donors than the starting material, corannulene. However, the most remarkable property of these new structures is their fluorescence, resulting from the confinement of the electrons in the π system. Interestingly, comparison between 7 and 8 resulted in several significant differences (Figure 7). The negatively curved nanographene 8 showed a red-shifted and broader emission band at 523 nm than [6]helicene nanographene 7, which emits with a maximum at 494 nm. This experimental finding could be a consequence of the greater flexibility of the molecule due to the double curvature. Furthermore, the quantum yield of the more rigid [6]helicene nanographene 7 (50%) was higher than that of 8 (25%) and much higher than that of pristine and symmetric hexabenzocoronene (HBC), one of the simplest nanographenes (5.8%). Finally, CD spectroscopy was used to measure the chiroptical properties of the racemic compound 7 previously resolved by chiral HPLC. The two CD spectra perfectly fit as specular images, as expected for enantiomers (Figure 8).

Figure 6. Solid-state packing of curved nanographenes 7, 8, and 9. Red dashes indicate π−π interactions in 7. The lack of these interactions in 8 and 9 results in porous packing of these nanographenes.

Figure 7. Normalized absorption (solid) and emission (dashed) spectra of 7 (black) and 8 (red). Inset: fluorescence of 7 irradiated at 365 nm.

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indicates that the steric bulk at the ends of the helicene during the cycloaddition reaction is sufficient to prevent thermal racemization, which has a half-life of 13.4 min at 221.7 °C for the parent [6]helicene.50 Starting from a racemic mixture of helicenes provided the racemic helical bilayer nanographene (Scheme 2). Scheme 2. Enantioretentive Synthesis of the M Isomer of a Helical Bilayer Nanographene and Crystal Structures of Key Intermediatesa

Figure 8. Circular dichroism spectra of (P)-7 and (M)-7 in CH2Cl2. (HPLC experimental conditions: stationary phase, COSMOSIL Cholester column; mobile phase, 3:2 THF/IPA; flow rate, 1 mL/ min).

5. ENANTIORETENTIVE SYNTHESIS OF A CHIRAL BILAYER MOLECULAR NANOGRAPHENE The controllable synthesis of curved nanographenes shown above, in which the formation of a carbon−carbon bond in the final step of the synthesis as dictated by reaction conditions determines the type of curvature (positive and helical, positive, or negative) of the resulting structure, represents one type of structural control available by our chemistry. The helical products in this case were obtained as racemic mixtures. The synthesis of chiral molecular nanographenes is an emerging topic in which the power of bottom-up benchtop synthesis has been well-demonstrated.39−47 However, in all cases the synthesis has not been enantiospecific. Access to the pure enantiomers has to date been possible only by chiral chromatography. To produce an enantioenriched chiral nanographene synthetically, we designed the synthesis of a helical bilayer molecular nanographene in which an enantiopure starting helicene can be carried through three synthetic steps to rapidly elaborate the chiral backbone into a πextended helical bilayer.48 Examples of such bilayer nanographenes are still rare, and choosing a rigid helicene linker further allows the two layers to stack in an AA arrangement with carbon atoms centered over each other in opposite layers. This arrangement does not occur in crystalline graphite and has shown interesting properties;49 therefore, having a molecular model of this interaction is quite relevant. Our π extension strategy was similar to the above synthesis of curved nanographenes. From enantiopure helicene 10, Sonogashira coupling at room temperature offered 11 with 4tert-butylphenyl groups at the termini. Microwave heating of this dialkyne with tetra-4-tert-butylphenyl cyclopentadienone 5 without solvent offered the bis(pentaphenylphenyl)-functionalized helicene 12, a product of Diels−Alder cycloaddition followed by the liberation of carbon monoxide, in a reasonable 30% yield. Scholl cyclodehydrogenation cleanly assembled the two hexa-peri-hexabenzocoronene layers and formally extended the helicene backbone from [6]helicene to [10]helicene, completing the synthesis of the helical bilayer nanographene 13. Chiral HPLC analysis of the final product showed an enantiomeric excess of 93%. This retention of stereochemistry

a

Protons and solvent have been omitted for clarity, and red highlights are used to illustrate key transformations.

Photophysical studies were performed (Figure 9). The broad UV−vis absorption of 13 supports the formation of a large,

Figure 9. Absorption (solid black), emission, (dashed black, excitation at 380 nm), and circular dichroism spectra (red) of a solution of M-13 (1.8 × 10−5 M, dichloromethane). Inset: photograph of M-13 solution under 365 nm irradiation.

extended π system, and the featureless fluorescence upon irradiation at 380 nm suggests that the excited state is delocalized over the entire molecule. Furthermore, the chiral nature of this nanographene was confirmed by CD spectroscopy, showing a change in sign at 380 nm, which coincides with the maximum absorption in the ultraviolet. X-ray crystallography confirmed the sophisticated architecture of the molecule (Scheme 2). Because of the rigid 1571



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hexagonal PAH framework, the lattices of the two layers are almost aligned in an AA stacking pattern radially slip-stacked by 4.06°. Strong nonbonding interactions, as predicted by DFT calculations, pull the layers nearly parallel with an interlayer angle of 2.04°. These interactions, combined with the forced AA arrangement, lead to an interlayer distance of 3.55 Å, the same as in folded AA-bilayer graphene.48 Therefore, we have demonstrated that our in-the-flask bottom-up synthetic strategy allows access to new graphitic structures with controlled curvature and chirality and creative unnatural geometries like bilayers and warped graphene flakes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Paul J. Evans: 0000-0003-3016-579X María Á ngeles Herranz: 0000-0001-9155-134X Nazario Martín: 0000-0002-5355-1477 Notes

The authors declare no competing financial interest. Biographies

6. CONCLUSIONS AND PERSPECTIVES The generation and selection of chirality represent a sophisticated degree of control over the chemical structure of molecules and materials exhibiting chiroptical properties. In this regard, although the great variety of carbon nanostructures that have emerged in recent decades have been extensively studied, their chiroptical properties have been significantly less explored. It is important to note, however, that these studies have led to good control in some carbon nanostructures like fullerenes, whereas in other nanocarbons such as graphene or GQDs it is still in its infancy. Over the last 10 years we have undertaken a program in our research group directed at the development of new carbon nanoforms endowed with the property of chirality. As a result of these efforts, the use of asymmetric catalysis in fullerenes allowed the synthesis, for the first time, of chiral fullerenes at will. In this Account, we have presented our first studies on other elaborate chiral carbon nanostructures, namely, metalofullerenes, endofullerenes, GQDs, and curved molecular nanographenes. Furthermore, we have also highlighted our recent results on a new family of almost unknown molecular chiral bilayer nanographenes based on enantiopure helicenes. In the aforementioned nanocarbons, we have used asymmetric catalysis and also both top-down and bottom-up synthetic approaches. Top-down methods allow the production of bulk and cheap nanographenes or GQDs. However, the preparation of graphenic materials showing optoelectronic properties at will requires precise control of the size and morphology of the GQDs. As as result, bottom-up methods have emerged as an efficient alternative for the preparation of molecular GQDs. This approach allows the synthesis of molecular nanographenes by design with total control of their size and topology and therefore their optoelectronic properties. Some of the examples discussed in this Account attest to these findings. However, although much has been achieved, still there is much to be done. In this regard, in addition to the control of chirality in other nanoforms of carbon, such as single-walled carbon nanotubes, which is largely unexplored, the introduction of chiral elements into planar carbon nanostructures is still a great and parallel synthetic goal. Molecular nanographenes bearing inherent helicene moieties are well-known in the literature but only as racemates. We have targeted the enantioselective preparation of synthetic molecular nanographenes with total control of the inherent chirality of the helicene moieties in their chemical structure as a main task for the near future. These studies will pave the way to the breakthrough scenario where specific stereoisomers of chiral nanographenes and other nanoforms of carbon are available at will.

Jesú s M. Fernández-Garcıá is a postdoctoral researcher at Universidad Complutense de Madrid (UCM). He obtained his Ph.D. at the University of Oviedo under the supervision of Prof. Enrique Aguilar and Prof. Manuel A. Fernández-Rodrı ́guez (University of Alcalá de Henares). He joined Prof. Nazario Martı ́n’s group and is working on bottom-up synthesis of molecular nanographenes. Paul J. Evans is a postdoctoral researcher at UCM. He was born and raised in Columbus, Ohio, USA. He received his M.A. from Boston University in 2014 and his Ph.D. from the University of Oregon in 2015, both under the supervision of Prof. Ramesh Jasti. He joined Prof. Nazario Martı ́n’s group in 2016 and has dedicated his work in the lab to the bottom-up synthesis of curved and bilayer molecular nanographenes. Salvatore Filippone is an associate professor at UCM. He obtained his Ph.D. at the University of Pavia under the supervision of Prof. Giuseppe Faita. He then moved to ENS of Paris for a postdoctoral fellowship, working with Prof. André Rassat and Prof. René Bensasson. He joined Prof. Nazario Martı ́n’s group and is working on chiral functionalization of fullerenes. Marı ́a Á ngeles Herranz obtained her Ph.D. from UCM and carried out postdoctoral stays at the University of Miami and Clemson University in the U.S. before joining the group of Prof. Nazario Martı ́n at UCM. Currently she is an associate professor at UCM, and her research interests focus on the chemistry, electronic and chiroptical properties of carbon nanostructures. Nazario Martı ́n has research interests spanning a range of targets, with emphasis on the molecular and supramolecular chemistry of carbon nanostructures and their applications in energy and nanoscience. He is a fellow of the Royal Society of Chemistry and a member of the Academia Europaea and belongs to the Editorial Advisory Board of Accounts of Chemical Research. He is a recipient of an Advanced Grant from the European Research Council.

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ACKNOWLEDGMENTS This work was supported by the European Research Council (ERC-2012-ADG_20120216 (Chirallcarbon)), the Ministerio de Ciencia, Innovación y Universidades (CIENCIA) of Spain (Projects CTQ2017-83531-R and CTQ2017- 84327-P), and the CAM (QUIMTRONIC, Project Y2018/NMT-4783).

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