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Enantiomerically Pure Helical Bilayer Nanographenes: A Straightforward Chemical Approach
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Journal of the American Chemical Society

Cite this: J. Am. Chem. Soc. 2024, 146, 50, 34943–34949
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https://doi.org/10.1021/jacs.4c14544
Published December 6, 2024

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Abstract

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The semiconductor properties of nanosized graphene fragments, known as molecular nanographenes, position them as exceptional candidates for next-generation optoelectronics. In addition to their remarkable optical and electronic features, chiral nanographenes exhibit high dissymmetry factors in circular dichroism and circularly polarized luminescence measurements. However, the synthesis of enantiomerically pure nanographenes remains a significant challenge. Typically, these materials are synthesized in their racemic form, followed by separation of the enantiomers using high-performance liquid chromatography (HPLC). While effective, this method often requires expensive instrumentation, extensive optimization of separation conditions, and typically yields analytical quantities of the desired samples. An alternative approach is the enantioselective synthesis of chiral molecular nanographenes; however, to date, only two examples have been documented in the literature. In this work, we present a straightforward chemical method for the chiral resolution of helical bilayer nanographenes. This approach enables the effective and scalable preparation of enantiomerically pure nanographenes while avoiding the need for HPLC. The incorporation of a BINOL core into the polyarene precursor facilitates the separation of diastereomers through esterification with enantiomerically pure camphorsulfonyl chloride. Following the separation of the diastereomers by standard chromatographic column, the hydrolysis of the camphorsulfonyl group yields enantiomerically pure nanographene precursors. The subsequent graphitization, achieved through the Scholl reaction, occurs in an enantiospecific manner and with the concomitant formation of a furan ring and a heterohelicene moiety. The absolute configurations of the final enantiomers, P-oxa[9]HBNG and M-oxa[9]HBNG, have been determined using X-ray diffraction. Additionally, electrochemical, photophysical, and chiroptical properties have been thoroughly evaluated.

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Introduction

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Molecular nanographenes (NGs) have emerged as promising candidates for the advancement of innovative optoelectronic devices, (1) primarily attributable to their semiconductor features stemming from the quantum confinement of electrons at the nanoscale. (2) By using the arsenal of modern organic reactions, the bottom-up synthesis (3) of these carbon-based materials enables the tailored production of molecular structures in customizable sizes, (4) shapes, (5) and consequently, properties at will.
Chiral NGs have recently attracted significant attention due to the interplay between their inherent chirality and the optical and electronic properties they exhibit. (6) In this regard, the π-extension of these molecules not only enhances their properties as excellent fluorophores but also improves the dissymmetry factors controlling the chiroptical properties, namely circular dichroism (CD) and circularly polarized luminescence (CPL). (7) In recent years, numerous research groups have synthesized a variety of NGs that incorporate helical motifs, such as helicenes, (8) resulting in molecules with propeller-like shapes (9) or forming helical bilayers. (10)
Similar to the observation that graphene bilayers modify their electronic structure, (11) exhibiting superconductivity by twisting a specific “magic angle”, (12) helical bilayer NGs (HBNGs) have shown to alter their electronic properties depending on the overlapping degree of both layers. (13)
This intriguing behavior highlights the potential of helical bilayer NGs (HBNGs) for innovative applications in electronics and photonics, exploiting also their chirality, (14) thus paving the way to a new and less-explored scenario. However, the main limitation to address this challenge stems from the difficulty in obtaining enantiomerically pure NGs. Typically, this has mostly been accomplished through high-performance liquid chromatography (HPLC) separation of racemates, a process that is both costly and time-consuming, often yielding only analytical quantities of the desired enantiomerically pure samples, thus preventing their use for practical purposes.
One approach to address this issue is to carry out an enantioselective synthesis of chiral NGs. To the best of our knowledge, only two examples have very recently been described in the literature thus far. Last year, our research group made significant advances by pioneering the enantioselective synthesis of triindane-based molecular NGs (TING, Chart 1). (15) A few months later, Tanaka et al. published a second example detailing the enantiomeric synthesis of π-extended carbohelicenes. (16) These methodologies present the opportunity to produce substantial quantities of enantiomerically pure NGs, which are essential for their further application in devices.

Chart 1

Chart 1. Approaches to Enantiomerically Pure Chiral Bilayer NGs
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In this work, we introduce an alternative and straightforward approach that employs a chemical resolution to synthesize enantiomerically pure helical bilayer NGs through a methodology characterized by its scalability and efficiency, avoiding the use of the HPLC technique. Thus, starting from a commercially available naphthol halide to synthesize a π-extended BINOL, and taking advantage of the high stability of this derivative against racemization, (17) we employ an esterification reaction with an enantiomerically pure camphorsulfonyl chloride. This approach enables the chemical resolution of the resulting diastereomers through straightforward standard chromatographic column techniques. Subsequent hydrolysis and graphitization steps yield two enantiomerically pure helical bilayer NGs M-oxa[9]HBNG and P-oxa[9]HBNG. Single crystal X-ray diffraction determines the absolute configuration of the final products and the enantiospecificity of the final graphitization step. Furthermore, cyclic voltammetry experiments confirm the mixed valence effect of these novel bilayers. Additionally, the study of photophysical and chiroptical properties reveal significant dissymmetry factors, highlighting the singular features of these materials.

Results and Discussion

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Racemic Synthetic Procedure

The synthetic methodology starts from commercially available 7-bromo-2-naphthol (1). Thus, as detailed in Scheme 1, the first step is an oxidative homocoupling of 1, mediated by a Cu-TMDA catalyst, affording 7,7′-dibromo-1,1′-binaphthyl-2,2′-diol (2, 83%). Then, a 2-fold Sonogashira coupling between 2 and two equivalents of 4-(tert-butyl)phenylacetylene (3) is performed, providing 7,7′-bis[(4-(tert-butyl)phenyl)ethynyl]-1,1′-binaphthalyl-2,2′-diol (4, 96%). The resulting alkynyl containing BINOL 4 was reacted with cyclopentadienone 5 to perform a double Diels–Alder cycloaddition, after in situ carbon monoxide extrusion, the BINOL-based polyarene endowed with two penta(4-tert-butylphenyl)phenyl groups 6 is obtained with a remarkably good yield of 93%. The incorporation of a BINOL core into the structure, provides great versatility considering the configurational stability of the isomers (racemization barrier 38 kcal·mol–1). (17a) Finally, a Scholl cyclodehydrogenation using DDQ as oxidant and triflic acid as Brønsted acid is performed, affording a furan-embedded helicene backbone, and thus, the helical bilayer NG oxa[9]HBNG in 61% yield. During the last step, 12 carbon–carbon bonds are formed providing the graphitization of 6. In addition, the cyclodehydration of the phenol groups results in the formation of a carbon–oxygen bond leading to the closure of the furan ring. (18) This reactivity to form furan embedded oxa-helicenes (19) provides the possibility of modulating, and improving the optoelectronic and chiroptical properties, as demonstrated by different studies of heterohelicenes. (20)

Scheme 1

Scheme 1. Synthesis of Racemic Oxa[9]HBNG from 7-Bromo-2-naphthol
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The final product and all nondescribed intermediates, were completely characterized by NMR, HRMS and FT-IR. The 1H NMR of oxa[9]HBNG exhibits 11 singlets and two doublets at low field corresponding to the aromatic protons, and five singlets at high field corresponding to the tert-butyl groups (Figure S7, see Supporting Information). The number of signals reveals the symmetry of the structure due to a C2 axis. As can be seen, the expected phenol signal between 3.50 and 4.00 ppm does not appear in the spectrum, which is evidence of closure to form the furan cycle. In addition, mass spectrometry experiments show the loss of 42 units from 6 (exact mass, 1759.1040) to oxa[9]HBNG (exact mass, 1716.8948), which agrees to the formation of 12 C–C bonds (24 units), and the loss of 18 units due to cyclodehydration to form a new C–O bond.

Chemical Resolution and Enantiospecific Scholl Reaction

The enantiomerically pure preparation of oxa[9]HBNGs starts with the monoesterification of the polyarene 6 with 1R-10-camphorsulfonyl chloride (Scheme 2), affording two diastereomers, R,M-7a (29%) and R,P-7b (28%) easily isolable by silica gel column chromatography. The purity of the diastereomers was determined by 500 MHz 1H NMR, revealing an excellent isolation (purity >95% for both diastereomers, Figures S4 and S5, see Supporting Information). Polyarene diastereomeric derivatives R,M-7a and R,P-7b show clear differences by NMR, as expected from diastereomers (Figure S6, see Supporting Information). The substitution with only one camphorsulfonyl group entails a great asymmetry in the spectra, each diastereomer presents ten singlets, between 1.11 and 1.00 ppm, corresponding to the tert-butyl groups, whereas the starting polyarene 6 exhibits only five singlets (Figure S3, see Supporting Information), revealing the symmetry loss after the functionalization.

Scheme 2

Scheme 2. Chemical Resolution of Racemate 6 and Enantiospecific Synthesis of M-oxa[9]HBNG and P-oxa[9]HBNG
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Enantiopure crystals of R,P-7b, were obtained from a chloroform solution, allowing its structure to be solved by single crystal X-ray diffraction (Figure 1 and Table S1, see Supporting Information). The molecules crystallize in the P212121 space group, and its absolute configuration was unequivocally assigned based on the value of Flack and Hooft parameters. The absence of relevant π–π or C–H···π intramolecular interactions entails a high degree of disorder in several fragments of the molecule, especially relevant for some of the peripheral tert-butyl groups. The packing in the crystal is achieved by weak van der Waals forces, leaving large interstitial spaces that contain disordered solvent molecules.

Figure 1

Figure 1. X-ray structure of R,P-7b showing the chiral auxiliar, 1R-10-camphorsulphonil, and the P configuration of the binaphthol core.

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Once separated, the camphorsulfonyl group of each diasteromer is removed by basic hydrolysis reactions, affording enantiomerically pure BINOL-based polyarenes M-6 (92%)and P-6 (99%). The hydrolysis conditions to afford M-6 and P-6 are mild enough to prevent racemization. Finally, the Scholl reaction providing the helical bilayer NG oxa[9]HBNG occurs enantiospecifically. The configurationally stable atropisomers M-6 and P-6 are transformed into helically full-conjugated NGs with axial-to-helical chirality transfer, leading to enantiomerically pure M-oxa[9]HBNG (51%) and P-oxa[9]HBNG (40%), respectively.
The enantiomeric excess of the M- and P-oxa[9]HBNG were evaluated by chiral HPLC (R,R-Whelk O2 5 μm column, 70% hexane 30% chloroform, recording the absorption by UV–vis and CD at 375 nm, Section 6, see Supporting Information), showing an e.e. >99% for M-oxa[9]HBNG and e.e. >99% for P-oxa[9]HBNG. Considering the total enantiospecificity of the Scholl reaction, the enantiomeric excesses depend only on the enantiopurity of the starting materials M-6 and P-6, determined by the proper separation of the diastereomers by silica gel chromatographic column.
The structures of P-oxa[9]HBNG and M-oxa[9]HBNG were solved by single crystal X-ray diffraction (Figure 2 and Table S2, see Supporting Information) from enantiopure crystals. Each of the compounds crystallize in the C2221 Sohncke space group, with one-half of the molecule in the asymmetric unit. Their absolute configuration was unambiguously assigned by the Bayesian statistical analysis on Bijvoet pairs differences.

Figure 2

Figure 2. X-ray crystal structures of M-oxa[9]HBNG and P-oxa[9]HBNG showing the inherent configuration of the helicene moieties.

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Both molecules of oxa[9]HBNGs display a bent U-shape where the two layers are placed almost parallel to each other (4.34° angle for P-oxa[9]HBNG and 4.95° for M-oxa[9]HBNG) maximizing the overlap of the two NG fragments (Figure S12, see Supporting Information). As many as 12 rings in each layer retain an overall flat disposition (Table S3, see Supporting Information), and 10 of them are involved in strong π–π interactions, with an average distance of 3.61 Å (P-oxa[9]HBNG) and 3.63 Å (M-oxa[9]HBNG) between the centroids of a layer and the plane defined by the ones in the opposite layer (Table S4, see Supporting Information). Regarding the supramolecular arrangement, the molecules are located in sheets parallel to the (110) plane and separated by a distance of c/2, with the oxa[9]HBNG molecules facing alternatingly the [010] and [0–10] directions (see Figure S11, see Supporting Information). The huge spaces left between the layers contain highly disordered solvent molecules.

Electrochemical Properties

The electrochemical properties of oxa[9]HBNG were evaluated by cyclic voltammetry in a 0.1 M solution of tetrabutylammonium hexafluorophosphate in toluene/acetonitrile 4:1 using glassy carbon as working electrode, a reference electrode of Ag/AgNO3, and a platinum wire as counter electrode. The oxidation and reduction potentials vs Fc/Fc+ are shown in Figure 3, oxa[9]HBNG presents two quasi-reversible oxidation waves at E1/2ox1 = 0.37 V and E1/2ox2 = 0.66 V, and two quasi-reversible reduction waves E1/2red1 = −2.27 V and E1/2red2 = −2.52 V. As commented in the introduction, graphene-like bilayers show exotic electronic properties depending on the relative disposition between the graphitized layers. In this way, HBNGs show mixed valence effects strongly related to the overlapping degree between the layers. (13) In the case of oxa[9]HBNG, the mixed valence band effect stabilizes the radical cation formed in the first oxidation between the layers, eventually decreasing the oxidation potential to 0.37 V. This value is comparable with the first oxidation potential of [9]HBNG E1/2ox1 = 0.35 V (Chart 1), (13) a highly overlapped helical bilayer NG with high contribution of mixed valence effects. Furthermore, comparing the first oxidation wave with that of an oxa[7]helical NG reported by Jux et al., in which the HBC layers do not show intramolecular π–π interactions (E1/2ox1 = 0.9 V), (21) oxa[9]HBNG shows a significantly stronger donor character. Therefore, it can be concluded that the contribution of heteroatom in the helicene has a low impact over the oxidation potential, being the strong donor character of oxa[9]HBNG attributed to the mixed valence effect derived from the overlapping between the layers (20 rings involved in the π–π interactions, Table S4, see Supporting Information). DFT calculations indicate that the HOMO orbital is primarily localized on the helicene moiety as expected, due to the electron-donating character of the furan ring. However, the HOMO–1 orbital, which is close in energy, is localized on the HBC layers, thus contributing to the observed mixed-valence effects (see Figure S17). (13)

Figure 3

Figure 3. Cyclic voltammogram of oxa[9]HBNG showing its half-wave redox potentials vs Fc/Fc+ in a 1 M solution of tetrabutylammonium hexafluorophosphate in toluene/acetonitrile 4:1.

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However, the oxygen embedded in the helicene seems to be more important regarding the electron acceptor character of oxa[9]HBNG. The LUMO orbital of the carbon-based NG, [9]HBNG, is mainly located in the helicene backbones. (22) In the case of oxa[9]HBNG, the electron-donor character of both the helicene (due to the electron-donor character of the furan ring), and the hexa-peri-hexabenzocoronene (HBC) layers, entails the destabilization of the LUMO orbital (see Figure S17). Thus, the potential of the first reduction wave increases (in absolute value) to E1/2red1 = −2.27 V, revealing a weaker electron acceptor character vs [9]HBNG (E1/2red1 = −2.18 V).

Photophysical Properties of the Racemic Mixture

The photophysical properties of oxa[9]HBNG were evaluated by UV–vis absorption and emission measurements in chloroform (Figure 4). The absorption spectrum of oxa[9]HBNG displays nonstructured broad bands in the ultraviolet region of the absorption spectrum (344, 371, 398 nm) very similar to those of the previously described [9]HBNG, (13) and more structured weak bands in the visible region (450, 476, 498 nm).
The emission spectrum of oxa[9]HBNG shows well-structured bands with maxima at 500, 537, and 580 nm, which contrasts with the excimer-like broad bands displayed by [9]HBNG. (13) As illustrated in Figure 4, the emission bands of oxa[9]HBNG (orange shaded curve) are blue-shifted with respect to the previously described carbon-based [9]HBNG (purple shaded curve), and are more comparable to those of the hexa-tert-butyl-substituted HBC (tBu-HBC, blue shaded curve). These observations agree with the cyclic voltammetry data previously described. The incorporation of the oxygen atom in the helicene modifies the structure and entails the modification of the LUMO orbital, which is not farther localized in the helicene moiety, as for carbon-based [9]HBNG. This modification involves the variation of the photophysical properties, and the emission bands are no longer excimer-like. The fluorescence quantum yield of oxa[9]HBNG resulted of Φem = 0.30, determined by comparison with the emission spectra of fluoresceine in ethanol (Φem = 0.97) at 432 nm. The optical band gap calculated in the intersection of the absorption and emission spectra E0–0 = 2.55 eV, clearly fits with the electrochemical band gap E = 2.51 eV, resulting from the difference between the electron affinity and the ionization potential (Figure S14, see Supporting Information). This value is also comparable to the HOMO–LUMO gap obtained from DFT calculations, which is 2.93 eV (Figure S17).

Figure 4

Figure 4. Absorption (solid lines) and emission (shaded curves) spectra of oxa[9]HBNG and [9]HBNG in chloroform.

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Chiroptical Properties of Enantiomerically Pure HBNGs

The electronic circular dichroism (CD) has been recorded in chloroform for both enantiomerically pure NGs M-oxa[9]HBNG and P-oxa[9]HBNG. As illustrated in Figure 5, the spectra of the enantiomers (P-oxa[9]HBNG, brown solid line, and M-oxa[9]HBNG, orange solid line) show specular image signals and two main regions with prominent Cotton effects. One intense broad band from the ultraviolet region to the visible region (300 to 470 nm), and one of lower intensity at lower energy in the visible region (480 nm). The absorption dissymmetry factors represented with dashed lines in Figure 5, are gabs = −2.7 × 10–3 at 452 nm for M-oxa[9]HBNG, and gabs = +2.6 × 10–3 at 452 nm for P-oxa[9]HBNG.

Figure 5

Figure 5. CD spectra and gabs values of M-oxa[9]HBNG and P-oxa[9]HBNG.

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The CPL has also been recorded in chloroform for both enantiomerically pure NGs M-oxa[9]HBNG and P-oxa[9]HBNG (Figure 6). Both spectra are symmetrical and show three main regions with different Cotton effects. The emission dissymmetry factors (dashed lines, Figure 6) are, glum = +2.6 × 10–3 at 500 nm for M-oxa[9]HBNG, and glum = −2.4 × 10–3 at 500 nm for P-oxa[9]HBNG. The low variation between the gasb and the glum values indicates that the structures present similar geometries in the fundamental and the first excited state. Furthermore, these dissymmetry factors are in the typical range of values presented by helical NGs.

Figure 6

Figure 6. CPL spectra and glum values of M-oxa[9]HBNG and P-oxa[9]HBNG.

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Conclusions

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The synthesis of enantiomerically pure helical bilayer NGs M-oxa[9]HBNG and P-oxa[9]HBNG by an easily scalable methodology, is described. The synthesis starts from commercially available 7-bromo-2-naphthol that, in the presence of copper(II), yields axially chiral racemic dibromoBINOL 2 which has been π-extended to the BINOL-based polyarene 6. Taking advantage of the stable axial chirality and reactivity of the obtained BINOL-based structures, a chiral resolution was carried out by functionalizing 6 with enantiomerically pure camphorsulfonyl chloride. The separation of the obtained diastereomers R,M-7a and R,P-7b by conventional silica gel chromatographic column afforded an isomeric excess >95% (by NMR). After a hydrolysis step under mild conditions, enantiomerically pure BINOL-based polyarenes P- and M-6 were obtained which, under Scholl reaction conditions, lead to the corresponding enantiomerically pure P- and M-oxa[9]HBNG. During the Scholl oxidation two different types of reactions take place, the expected cyclodehydrogenation (providing the graphene-like structure), and the enantiospecific cyclodehydration of the phenol groups yielding the furan embedded helicene (axial-to-helical chirality transfer). The performance of the silica column separation determines the enantiomeric excess of the final NGs (e.e. >99% by chiral HPLC for each enantiomer). All structures were characterized by spectroscopic techniques (NMR, FT-IR, HRMS) and single-crystal X-ray diffraction confirmed the configuration of the BINOL core in diastereomeric polyarene R,P-7b and the corresponding enantiomer obtained after hydrolysis and Scholl reaction, P-oxa[9]HBNG. Furthermore, the crystal structure of the other enantiomeric NG M-oxa[9]HBNG has also been obtained. These studies showed the great overlapping between the layers, being 20 rings involved in the intramolecular π–π interactions.
The large overlap of the structure explains the variation of the donor character. The influence of the electron-donor character of oxygen in the helicene backbone has a lower impact on the oxidation potentials (E1/2ox1 = 0.35 V and E1/2ox1 = 0.66 V) than the influence of the overlapping degree between the layers leading to a mixed valence band effect. The influence of the electron-donor character of the oxygen-containing helicene is more reflected in the acceptor character. Thus, the first reduction potential for oxa[9]HBNG (E1/2red1 = −2.27 V) is closer to that of tBu-HBC (E1/2red1 = −2.24 V). In addition, the emission spectra show structured bands suggesting the lower excimer-like contribution. Finally, the chiroptical properties of M- and P-oxa[9]HBNG have been studied by means of electronic circular dichroism and circularly polarized luminescence, from which absorption and emission dissymmetry factors in the typical range of helical NGs, with gabs values of −2.7 × 10–3 at 452 nm for M-oxa[9]HBNG and +2.6 × 10–3 at 452 nm for P-oxa[9]HBNG, and glum values of +2.6 × 10–3 for M-oxa[9]HBNG and −2.4 × 10–3 for P-oxa[9]HBNG.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c14544.

  • Synthetic procedures, additional figures/schemes of physical properties and characterization data. Data for R,P-7b (CCDC: 2388461), M-oxa[9]HBNG (CCDC: 2388463) and P-oxa[9]HBNG (CCDC: 2388462) (PDF)

Accession Codes

Deposition numbers 2388461–2388463 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via the joint Cambridge Crystallographic Data Centre (CCDC) and Fachinformationszentrum Karlsruhe Access Structures service.

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Author Information

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  • Corresponding Author
    • Nazario Martín - Departamento de Química Orgánica I, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, 28040 Madrid, Spain;  IMDEA-Nanociencia, C/Faraday, 9, Campus de Cantoblanco, 28049 Madrid, Spain;  Orcidhttps://orcid.org/0000-0002-5355-1477;  Email: [email protected]
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  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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P.I.-G., J.M.F.-G. and N.M. acknowledge financial support from the Spanish MICIN (project PID2020-114653RB I00), they also acknowledge financial support from the ERC (SyG TOMATTO ERC-2020-951224) and from the “(MAD2D-CM)-UCM” project funded by Comunidad de Madrid, by the Recovery, Transformation and Resilience Plan, and by NextGenerationEU from the European Union.

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    ACS Applied Materials & Interfaces (2020), 12 (25), 28426-28434CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Self-heating in light-emitting electrochem. cells (LECs) has been long overlooked, while it has a significant impact on (i) device chromaticity by changing the electroluminescent band shape, (ii) device efficiency because of thermal quenching and exciton dissocn. reducing the external quantum efficiency (EQE), and (iii) device stability because of thermal degrdn. of excitons and eliminate doped species, phase sepn., and collapse of the intrinsic emitting zone. Herein, we reveal, for the first time, a direct relationship between self-heating and the early changes in the device chromaticity as well as the magnitude of the error comparing theor./exptl. EQEs-i.e., an overestimation error of ca. 35% at usual pixel working temps. of around 50 °C. This has been realized in LECs using a benchmark nanographene-i.e., a substituted hexa-peri-hexabenzocoronene-as an emerging class of emitters with outstanding device performance compared to the prior art of small-mol. LECs-for example, luminances of 345 cd/m2 and EQEs of 0.35%. As such, this work is a fundamental contribution highlighting how self-heating is a crit. limitation toward the optimization and wide use of LECs.
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    (d) Das, S.; Pandey, D.; Thomas, J.; Roy, T. The Role of Graphene and Other 2D Materials in Solar Photovoltaics. Adv. Mater. 2019, 31, e1802722  DOI: 10.1002/adma.201802722
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  2. 2
    (a) Ritter, K. A.; Lyding, J. W. The Influence of Edge Structure on the Electronic Properties of Graphene Quantum Dots and Nanoribbons. Nat. Mater. 2009, 8 (3), 235,  DOI: 10.1038/nmat2378
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    2a
    The influence of edge structure on the electronic properties of graphene quantum dots and nanoribbons
    Ritter, Kyle A.; Lyding, Joseph W.
    Nature Materials (2009), 8 (3), 235-242CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Graphene shows promise as a future material for nanoelectronics owing to its compatibility with industry-std. lithog. processing, electron mobilities up to 150 times greater than Si and a thermal cond. twice that of diamond. The electronic structure of graphene nanoribbons (GNRs) and quantum dots (GQDs) was predicted to depend sensitively on the crystallog. orientation of their edges; however, the influence of edge structure was not verified exptl. Here, we use tunneling spectroscopy to show that the electronic structure of GNRs and GQDs with 2-20 nm lateral dimensions varies on the basis of the graphene edge lattice symmetry. Predominantly zigzag-edge GQDs with 7-8 nm av. dimensions are metallic owing to the presence of zigzag edge states. GNRs with a higher fraction of zigzag edges exhibit a smaller energy gap than a predominantly armchair-edge ribbon of similar width, and the magnitudes of the measured GNR energy gaps agree with recent theor. calcns.
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    (b) Sun, Z.; Ye, Q.; Chi, C.; Wu, J. Low Band Gap Polycyclic Hydrocarbons: from Closed-shell Near Infrared Dyes and Semiconductors to Open-Shell Radicals. Chem. Soc. Rev. 2012, 41, 7857,  DOI: 10.1039/c2cs35211g
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    2b
    Low band gap polycyclic hydrocarbons: from closed-shell near infrared dyes and semiconductors to open-shell radicals
    Sun, Zhe; Ye, Qun; Chi, Chunyan; Wu, Jishan
    Chemical Society Reviews (2012), 41 (23), 7857-7889CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. Low band gap (Eg < 1.5 eV) polycyclic hydrocarbons have become one of the most important types of materials for many applications, for example, as semiconductors in org. field effect transistors (OFETs), as light-harvesting dyes in org. solar cells and photodetectors, as near IR (NIR) fluorescent probes in high resoln. bio-imaging and bio-sensing, and as chromophores in non-linear optics. The benzenoid polycyclic hydrocarbons as nano-sized graphene fragments also serve as perfect model compds. to understand the fundamental structure-property relationship of graphene. The ground state of these mols. can be described as either a closed-shell or an open-shell structure on the basis of their mol. size and edge structure. In this review, a summary will be given on a series of low band gap polycyclic hydrocarbons about their synthesis, phys. properties and material applications.
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    (c) Li, S.-Y.; He, L. Recent Progresses of Quantum Confinement in Graphene Quantum Dots. Front. Phys. 2021, 17, 33201,  DOI: 10.1007/s11467-021-1125-2
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    (d) Arimura, S.; Matsumoto, I.; Sekiya, R.; Haino, T. Intermediate Color Emission via Nanographenes with Organic Fluorophores. Angew. Chem., Int. Ed. 2024, 63, e202315508  DOI: 10.1002/anie.202315508
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    (e) Sekiya, R.; Haino, T. Edge-Functionalized Nanographenes. Chem.─Eur. J. 2021, 27, 187– 199,  DOI: 10.1002/chem.202003370
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  3. 3
    (a) Grzybowski, M.; Sadowski, B.; Butenschon, H.; Gryko, D. T. Synthetic Applications of Oxidative Aromatic Coupling-From Biphenols to Nanographenes. Angew. Chem., Int. Ed. 2020, 59, 2998,  DOI: 10.1002/anie.201904934
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    3a
    Synthetic Applications of Oxidative Aromatic Coupling-From Biphenols to Nanographenes
    Grzybowski, Marek; Sadowski, Bartlomiej; Butenschoen, Holger; Gryko, Daniel T.
    Angewandte Chemie, International Edition (2020), 59 (8), 2998-3027CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Oxidative arom. coupling occupies a fundamental place in the modern chem. of arom. compds. It is a method of choice for the assembly of large and bewildering architectures. Considerable effort was also devoted to applications of the Scholl reaction for the synthesis of chiral biphenols and natural products. The ability to form biaryl linkages without any prefunctionalization provides an efficient pathway to many complex structures. Although the chem. of this process is only now becoming fully understood, this reaction continues to both fascinate and challenge researchers. This is esp. true for heterocoupling, i.e., oxidative arom. coupling with the chemoselective formation of a C-C bond between two different arenes. Anal. of the progress achieved in this field since 2013 reveals that many groups have contributed by pushing the boundary of structural possibilities, expanding into surface-assisted (cyclo)dehydrogenation, and developing new reagents.
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    (b) Grzybowski, M.; Skonieczny, K.; Butenschon, H.; Gryko, D. T. Comparison of Oxidative Aromatic Coupling and the Scholl Reaction. Angew. Chem., Int. Ed. 2013, 52, 9900,  DOI: 10.1002/anie.201210238
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    3b
    Comparison of Oxidative Aromatic Coupling and the Scholl Reaction
    Grzybowski, Marek; Skonieczny, Kamil; Butenschoen, Holger; Gryko, Daniel T.
    Angewandte Chemie, International Edition (2013), 52 (38), 9900-9930CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Does the dehydrogenative coupling of arom. compds. mediated by AlCl3 at high temps. and also by FeCl3, MoCl5, PIFA, or K3[Fe(CN)6] at room temp. proceed by the same mechanism in all cases. With the growing importance of the synthesis of arom. compds. by double C-H activation to give various biaryl structures, this question becomes pressing. Since some of these reactions proceed only in the presence of non-oxidizing Lewis acids and some only in the presence of certain oxidants, the authors venture the hypothesis that, depending on the electronic structure of the substrates and the nature of the "catalyst", two different mechanisms can operate. One involves the intermediacy of a radical cation and the other the formation of a sigma complex between the acid and the substrate. The goal of this Review is to encourage further mechanistic studies hopefully leading to an in-depth understanding of this phenomenon.
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    (c) Jolly, A.; Miao, D.; Daigle, M.; Morin, J. F. Emerging Bottom-Up Strategies for the Synthesis of Graphene Nanoribbons and Related Structures. Angew. Chem., Int. Ed. 2020, 59, 4624,  DOI: 10.1002/anie.201906379
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    3c
    Emerging Bottom-Up Strategies for the Synthesis of Graphene Nanoribbons and Related Structures
    Jolly, Anthony; Miao, Dandan; Daigle, Maxime; Morin, Jean-Francois
    Angewandte Chemie, International Edition (2020), 59 (12), 4624-4633CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review discusses the synthesis of graphene nanoribbons and polycyclic arom. hydrocarbons approximating them by soln.-phase synthesis. The topochem. polymn. and benzannulation reactions of aryl alkynes, photochem. oxidative cyclocondensation (Mallory) and cyclodehydrohalogenation reactions, and palladium-catalyzed reactions are discussed.
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    (d) Zhu, C.; Wang, D.; Wang, D.; Zhao, Y.; Sun, W. Y.; Shi, Z. Bottom-up Construction of pi-Extended Arenes by a Palladium-Catalyzed Annulative Dimerization of o-Iodobiaryl Compounds. Angew. Chem., Int. Ed. 2018, 57, 8848,  DOI: 10.1002/anie.201803603
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    (e) Lu, D.; Zhuang, G.; Wu, H.; Wang, S.; Yang, S.; Du, P. A Large pi-Extended Carbon Nanoring Based on Nanographene Units: Bottom-Up Synthesis, Photophysical Properties, and Selective Complexation with Fullerene C(70). Angew. Chem., Int. Ed. 2017, 56, 158,  DOI: 10.1002/anie.201608963
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  4. 4
    (a) Liu, Z.; Fu, S.; Liu, X.; Narita, A.; Samori, P.; Bonn, M.; Wang, H. I. Small Size, Big Impact: Recent Progress in Bottom-Up Synthesized Nanographenes for Optoelectronic and Energy Applications. Adv. Sci. 2022, 9, e2106055  DOI: 10.1002/advs.202106055
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    (b) Baier, D. M.; Gratz, S.; Jahromi, B. F.; Hellmann, S.; Bergheim, K.; Pickhardt, W.; Schmid, R.; Borchardt, L. Beyond the Scholl Reaction - One-step Planarization and Edge Chlorination of Nanographenes by Mechanochemistry. RSC Adv. 2021, 11, 38026,  DOI: 10.1039/D1RA07679E
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    (c) Lungerich, D.; Papaianina, O.; Feofanov, M.; Liu, J.; Devarajulu, M.; Troyanov, S. I.; Maier, S.; Amsharov, K. Dehydrative pi-Extension to Nanographenes with Zig-zag Edges. Nat. Commun. 2018, 9, 4756,  DOI: 10.1038/s41467-018-07095-z
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    4c
    Dehydrative π-extension to nanographenes with zig-zag edges
    Lungerich Dominik; Papaianina Olena; Feofanov Mikhail; Amsharov Konstantin; Lungerich Dominik; Liu Jia; Devarajulu Mirunalini; Maier Sabine; Troyanov Sergey I
    Nature communications (2018), 9 (1), 4756 ISSN:.
    Zig-zag nanographenes are promising candidates for the applications in organic electronics due to the electronic properties induced by their periphery. However, the synthetic access to these compounds remains virtually unexplored. There is a lack in efficient and mild strategies origins in the reduced stability, increased reactivity, and low solubility of these compounds. Herein we report a facile access to pristine zig-zag nanographenes, utilizing an acid-promoted intramolecular reductive cyclization of arylaldehydes, and demonstrate a three-step route to nanographenes constituted of angularly fused tetracenes or pentacenes. The mild conditions are scalable to gram quantities and give insoluble nanostructures in close to quantitative yields. The strategy allows the synthesis of elusive low bandgap nanographenes, with values as low as 1.62 eV. Compared to their linear homologues, the structures have an increased stability in the solid-state, even though computational analyses show distinct diradical character. The structures were confirmed by X-ray diffraction or scanning tunneling microscopy.
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    (d) Oró, A.; Romeo-Gella, F.; Perles, J.; Fernández-García, J. M.; Corral, I.; Martín, N. Tetrahedraphene: A Csp(3)-Centered 3D Molecular Nanographene Showing Aggregation-Induced Emission. Angew. Chem., Int. Ed. 2023, 62, e202312314  DOI: 10.1002/anie.202312314
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  5. 5
    (a) Lombardi, F.; Lodi, A.; Ma, J.; Liu, J.; Slota, M.; Narita, A.; Myers, W. K.; Müllen, K.; Feng, X.; Bogani, L. Quantum Units from the Topological Engineering of Molecular Graphenoids. Science 2019, 366, 1107,  DOI: 10.1126/science.aay7203
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    5a
    Quantum units from the topological engineering of molecular graphenoids
    Lombardi, Federico; Lodi, Alessandro; Ma, Ji; Liu, Junzhi; Slota, Michael; Narita, Akimitsu; Myers, William K.; Mullen, Klaus; Feng, Xinliang; Bogani, Lapo
    Science (Washington, DC, United States) (2019), 366 (6469), 1107-1110CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
    A review. Robustly coherent spin centers that can be integrated into devices are a key ingredient of quantum technologies. Vacancies in semiconductors are excellent candidates, and theory predicts that defects in conjugated carbon materials should also display long coherence times. However, the quantum performance of carbon nanostructures has remained stunted by an inability to alter the sp2-carbon lattice with at. precision. Here, we demonstrate that topol. tailoring leads to superior quantum performance in mol. graphene nanostructures. We unravel the decoherence mechanisms, quantify nuclear and environmental effects, and observe spin-coherence times that outclass most nanomaterials. These results validate long-standing assumptions on the coherent behavior of topol. defects in graphene and open up the possibility of introducing controlled quantum-coherent centers in the upcoming generation of carbon-based optoelectronic, electronic, and bioactive systems.
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    (b) Majewski, M. A.; Stępień, M. Bowls, Hoops Saddles: Synthetic Approaches to Curved Aromatic Molecules. Angew. Chem., Int. Ed. 2019, 58, 86,  DOI: 10.1002/anie.201807004
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    (c) Rickhaus, M.; Mayor, M.; Juríček, M. Chirality in Curved Polyaromatic Systems. Chem. Soc. Rev. 2017, 46, 1643,  DOI: 10.1039/C6CS00623J
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    5c
    Chirality in curved polyaromatic systems
    Rickhaus, Michel; Mayor, Marcel; Juricek, Michal
    Chemical Society Reviews (2017), 46 (6), 1643-1660CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. Carbon allotropes constituted of sp2-hybridized carbon atoms display a variety of properties that arise from their delocalised π-conjugated electronic structure. Apart from carbon's planar allotropic form graphene, bent or curved structures, such as carbon nanotubes or fullerenes, resp., have been discovered. In this Tutorial Review, we analyze and conceptually categorise chiral synthetic mol. fragments of non-planar sp2-carbon allotropes, including hypothetical forms of carbon that have been proposed to exist as stable entities. Two types of mol. systems composed of equally or differently sized rings are examd.: bent with zero Gaussian curvature and curved with pos. or neg. Gaussian curvature. To affirm that a system is chiral, two conditions must be fulfilled: (1) both reflective symmetry elements, an inversion center and a mirror plane, must be absent and (2) the system must be stereochem. rigid. It is therefore crucial to not only consider the symmetry of a given system as if it was a rigid object but also its structural dynamics. These principles serve as guidelines for the design of mol. fragments that encode and transcribe chirality into larger systems.
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    (d) Zhang, Y.; Pun, S. H.; Miao, Q. The Scholl Reaction as a Powerful Tool for Synthesis of Curved Polycyclic Aromatics. Chem. Rev. 2022, 122, 14554,  DOI: 10.1021/acs.chemrev.2c00186
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    5d
    The Scholl Reaction as a Powerful Tool for Synthesis of Curved Polycyclic Aromatics
    Zhang, Yiqun; Pun, Sai Ho; Miao, Qian
    Chemical Reviews (Washington, DC, United States) (2022), 122 (18), 14554-14593CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. These curved polycyclic aroms. are not only unique objects of structural org. chem. in relation to the nature of aromaticity, but also play an important role in bottom-up approaches to precise synthesis of nanocarbons of unique topol. Moreover, they have received considerable attention in the fields of supramol. chem. and org. functional materials because of their interesting properties and promising applications. Despite the great success of Scholl reactions in synthesis of curved polycyclic aroms., 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 aroms. with interesting structures and properties, and aims to shed light on the key factors that affect Scholl reactions in synthesizing sterically strained polycyclic aroms.
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    (e) Fernández-García, J. M.; Evans, P. J.; Medina Rivero, S.; Fernández, I.; García-Fresnadillo, D.; Perles, J.; Casado, J.; Martín, N. pi-Extended Corannulene-Based Nanographenes: Selective Formation of Negative Curvature. J. Am. Chem. Soc. 2018, 140, 17188,  DOI: 10.1021/jacs.8b09992
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  6. 6
    (a) Fernández-García, J. M.; Izquierdo-García, P.; Buendía, M.; Filippone, S.; Martín, N. Synthetic Chiral Molecular Nanographenes: The Key Figure of the Racemization Barrier. Chem. Commun. 2022, 58, 2634,  DOI: 10.1039/D1CC06561K
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    6a
    Synthetic chiral molecular nanographenes: the key figure of the racemization barrier
    Fernandez-Garcia, Jesus M.; Izquierdo-Garcia, Patricia; Buendia, Manuel; Filippone, Salvatore; Martin, Nazario
    Chemical Communications (Cambridge, United Kingdom) (2022), 58 (16), 2634-2645CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    A review. Chirality is one of the most intriguing concepts of chem., involving living systems and, more recently, materials science. In particular, the bottom-up synthesis of mol. nanographenes endowed with one or several chiral elements is a current challenge for the chem. community. The wilful introduction of defects in the sp2 honeycomb lattice of mol. nanographenes allows the prepn. of chiral mols. with tuned band-gaps and chiroptical properties. There are two requirements that a system must fulfill to be chiral: (i) lack of inversion elements (planes or inversion centers) and (ii) to be configurationally stable. The first condition is inherently established by the symmetry group of the structure, however, the limit between conformational and configurational isomers is not totally clear. In this feature article, the chirality and dynamics of synthetic mol. nanographenes, with special emphasis on their racemization barriers and, therefore, the stability of their chiroptical properties are discussed. The general features of nanographenes and their bottom-up synthesis, including the main defects inducing chirality in mol. nanographenes are firstly discussed. In this regard, the most common topol. defects of mol. NGs as well as the main techniques used for detg. their energy barriers are presented. Then, the manuscript is structured according to the dynamics of mol. nanographenes, classifying them in four main groups, depending on their resp. isomerization barriers, as flexible, detectable, isolable and rigid nanographenes. In these sections, the different strategies used to increase the isomerization barrier of chiral mol. nanographenes that lead to configurationally stable nanographenes with defined chiroptical properties are discussed.
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    (b) Fernández-García, J. M.; Evans, P. J.; Filippone, S.; Herranz, M. A.; Martín, N. Chiral Molecular Carbon Nanostructures. Acc. Chem. Res. 2019, 52, 1565,  DOI: 10.1021/acs.accounts.9b00144
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    6b
    Chiral Molecular Carbon Nanostructures
    Fernandez-Garcia, Jesus M.; Evans, Paul J.; Filippone, Salvatore; Herranz, Maria Angeles; Martin, Nazario
    Accounts of Chemical Research (2019), 52 (6), 1565-1574CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review. In this Account, we show our progress in the synthesis of chiral mol. carbon nanostructures, namely, metallofullerenes, endohedral fullerenes, GQDs, and curved mol. nanographenes, by using asym. catalysis and both top-down and bottom-up chem. approaches. Furthermore, we bring in a new family of lesser-known mol. 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 mol. 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 det. the effect of the incarcerated mol. 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 alcs. 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 mol. 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 mol. 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 dets. 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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    (c) Anderson, H. V.; Gois, N. D.; Chalifoux, W. A. New Advances in Chiral Nanographene Chemistry. Org. Chem. Front. 2023, 10, 4167,  DOI: 10.1039/D3QO00517H
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  7. 7
    (a) Schnitzlein, M.; Shoyama, K.; Würthner, F. A Highly Fluorescent Bora[6]helicene Exhibiting Circularly Polarized Light Emission. Chem. Sci. 2024, 15, 2984– 2989,  DOI: 10.1039/d3sc05171d
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    (b) Niu, W.; Fu, Y.; Deng, Q.; Qiu, Z. L.; Liu, F.; Popov, A. A.; Komber, H.; Ma, J.; Feng, X. Enhancing Chiroptical Responses in Helical Nanographenes via Geometric Engineering of Double [7]Helicenes. Angew. Chem., Int. Ed. 2024, 63, e202319874  DOI: 10.1002/anie.202319874
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    (c) Qiu, Z.; Ju, C. W.; Frederic, L.; Hu, Y.; Schollmeyer, D.; Pieters, G.; Müllen, K.; Narita, A. Amplification of Dissymmetry Factors in pi-Extended [7]- and [9]Helicenes. J. Am. Chem. Soc. 2021, 143, 4661,  DOI: 10.1021/jacs.0c13197
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    7c
    Amplification of Dissymmetry Factors in π-Extended [7]- and [9]Helicenes
    Qiu, Zijie; Ju, Cheng-Wei; Frederic, Lucas; Hu, Yunbin; Schollmeyer, Dieter; Pieters, Gregory; Muellen, Klaus; Narita, Akimitsu
    Journal of the American Chemical Society (2021), 143 (12), 4661-4667CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    π-Extended helicenes constitute an important class of polycyclic arom. hydrocarbons with intrinsic chirality. Herein, we report the syntheses of π-extended [7]helicene 4 and π-extended [9]helicene 6 through regioselective cyclodehydrogenation in high yields, where a "prefusion" strategy plays a key role in preventing undesirable aryl rearrangements. The unique helical structures are unambiguously confirmed by X-ray crystal structure anal. Compared to the parent pristine [7]helicene and [9]helicene, these novel π-extended helicenes display significantly improved photophys. properties, with a quantum yield of 0.41 for 6. After optical resoln. by chiral high-performance liq. chromatog., the chiroptical properties of enantiomers 4-P/M and 6-P/M are investigated, revealing that the small variation in helical length from [7] to [9] can cause an approx. 10-fold increase in the dissymmetry factors. The circularly polarized luminescence brightness of 6 reaches 12.6 M-1 cm-1 as one of the highest among carbohelicenes.
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  8. 8
    (a) Liu, X.; Jin, Z.; Qiu, F.; Guo, Y.; Chen, Y.; Sun, Z.; Zhang, L. Hexabenzoheptacene: A Longitudinally Multihelicene Nanocarbon with Local Aromaticity and Enhanced Stability. Angew. Chem., Int. Ed. 2024, 63, e202407547  DOI: 10.1002/anie.202407547
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    (b) Zhu, Y.; Wang, J. Helical Synthetic Nanographenes with Atomic Precision. Acc. Chem. Res. 2023, 56, 363,  DOI: 10.1021/acs.accounts.2c00767
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    8b
    Helical Synthetic Nanographenes with Atomic Precision
    Zhu, Yanpeng; Wang, Jiaobing
    Accounts of Chemical Research (2023), 56 (3), 363-373CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review. Understanding and harnessing the properties of nanoscale mol. entities are considered as new frontiers in basic chem. In this regard, synthetic nanographene with at. precision has attracted much attention recently. For instance, taking advantage of the marvelous bonding capability of C, flat, curved, ribbon-type, or cone-shaped nanographenes were prepd. in highly controllable and elegant manner, allowing exploration of fascinating mol. architectures with intriguing optical, electrochem., and magnetic characteristics. This stands in stark contrast to other C-rich nanomaterials, such as graphite oxides or C quantum dots, which preclude thorough studies because of complicate structural defects. Undoubtedly, synthetic nanographene contributes strongly to modern arom. chem. and represents a vibrant field that may deliver transforming functional materials crucial for optoelectronics, nanotechnologies, and biomedicine. In many cases, synthesis and characterization of nanographene compds. are highly demanding. Low soly., high mol. strain, undesired selectivity, as well as incomplete or excessive C-C bond formation are common impediments, that require formidable efforts to control the mol. 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 relations, and lay down foundation for exploring synthetic nanographenes at a high level of sophistications. The contributions to this field are summarized by presenting helical synthetic nanographenes, such as hexapole [7]helicene (H7H), N-doped H7H, hexapole [9]helicene (H9H), superhelicene, and supertwistacene. This kind of giant synthetic nanographene reaches the size domain of C quantum dots, albeit has precise at. structure. It provides a unique platform to study arom. chem. and chirality at the nanoscale. Synthetic methods and point out, in particular, the strengths and pitfalls of Scholl oxidn., which are expected to be valuable for making synthetic nanographenes in general are discussed. The authors illustrate their exciting electrochem. and photophys. performance, which include, but are not limited to, reversible multielectron redox chem., record high panchromatic absorption, impressive photothermal behavior, and extremely strong Cotton effect. These unusual characteristics are convincingly traced back to their 3-dimensional conjugated architectures, highlighting the crit. roles of π-electron delocalization, heteroatom-doping, substitution, and mol. symmetry in detg. nanographenes' properties and functions. Lastly, the authors put forward the 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.
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    (c) Peng, L. J.; Wang, X. Y.; Li, Z. A.; Gong, H. Y. All Carbon Helicenes and π-Extended Helicene Derivatives. Asian J. Org. Chem. 2023, 12, e202300543  DOI: 10.1002/ajoc.202300543
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    (d) Izquierdo-García, P.; Fernández-García, J. M.; Perles, J.; Fernández, I.; Martín, N. Electronic Control of the Scholl Reaction: Selective Synthesis of Spiro vs Helical Nanographenes. Angew. Chem., Int. Ed. 2023, 62, e202215655  DOI: 10.1002/anie.202215655
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    8d
    Electronic Control of the Scholl Reaction: Selective Synthesis of Spiro vs Helical Nanographenes
    Izquierdo-Garcia, Patricia; Fernandez-Garcia, Jesus M.; Perles, Josefina; Fernandez, Israel; Martin, Nazario
    Angewandte Chemie, International Edition (2023), 62 (7), e202215655CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A Scholl reaction controlled by the electronic effects on the starting substrates was described. Anthracene-based polyphenylenes led to spironanographenes under Scholl conditions. In contrast, an electron-deficient anthracene substrate afforded a helically arranged mol. nanographene formed by two orthogonal dibenzo[fg,ij]phenanthro-[9,10,1,2,3-pqrst]pentaphene (DBPP) moieties linked through an octafluoroanthracene core. D. Functional Theory (DFT) calcns. predicted that electronic effects control either the first formation of spirocycles and subsequent Scholl reaction to form spironanographene , or the expected dehydrogenation reaction leading solely to the helical nanographene, I. The crystal structures of four of the new spiro compds. were solved by single crystal X-ray diffraction. The photophys. properties of the new mol. nanographene I revealed a remarkable dual fluorescent emission.
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  9. 9
    (a) Navakouski, M.; Zhylitskaya, H.; Chmielewski, P. J.; Lis, T.; Cybińska, J.; Stępień, M. Stereocontrolled Synthesis of Chiral Heteroaromatic Propellers with Small Optical Bandgaps. Angew. Chem., Int. Ed. 2019, 58, 4929,  DOI: 10.1002/anie.201900175
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    9a
    Stereocontrolled synthesis of chiral heteroaromatic propellers with small optical bandgaps
    Navakouski, Maksim; Zhylitskaya, Halina; Chmielewski, Piotr J.; Lis, Tadeusz; Cybinska, Joanna; Stepien, Marcin
    Angewandte Chemie, International Edition (2019), 58 (15), 4929-4933CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Chiral heteroarom. propellers based on radially π-extended hexapyrrolohexaazacoronenes were obtained in a concise synthesis from suitably functionalized donor-acceptor monopyrroles. To overcome steric hindrance, a new cyclodehydrogenation method was developed, and it uses bromine electrophiles as oxidative coupling agents instead of the commonly employed high-potential oxidants. The new reaction offers high yields of propeller-shaped targets, even for electron-deficient precursors, and shows electrophile-dependent stereoselectivity, with N-bromosuccinimide and dibromine yielding, resp. D6- and C2-sym. products. The propeller azacoronenes are chiral and can be sepd. into configurationally stable enantiomers. In addn. to providing steric bulk, peripheral functionalization considerably affects the electronic properties of the propellers, which exhibit reduced optical and electrochem. band gaps, and a more clearly defined electroredn. behavior.
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    (b) Zhu, Y.; Guo, X.; Li, Y.; Wang, J. Fusing of Seven HBCs toward a Green Nanographene Propeller. J. Am. Chem. Soc. 2019, 141, 5511,  DOI: 10.1021/jacs.9b01266
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    9b
    Fusing of Seven HBCs toward a Green Nanographene Propeller
    Zhu, Yanpeng; Guo, Xiaoyu; Li, Yang; Wang, Jiaobing
    Journal of the American Chemical Society (2019), 141 (13), 5511-5517CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    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 arom. hydrocarbons ever prepd. using scalable soln. chem. Despite its unusual mol. size, single-crystal X-ray structural anal. (resoln. 0.9 Å) and baseline chiral resoln. are achieved. NP is sol. in various org. solvents and can be fully characterized by common spectroscopic and voltammetric techniques. It has a strong panchromatic absorption from the UV to the near-IR (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 coeff. larger than 100 000 M-1 cm-1. Moreover, a record-high Cotton effect in the visible spectrum is obsd. for enantiopure NP, with |Δε| values of 1182 and 1090 M-1 cm-1 at 374 and 405 nm, resp. These photophys. properties evolve significantly compared to those of the propeller-shaped hexapole [7]helicene.
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    (c) Izquierdo-García, P.; Fernández-García, J. M.; Fernández, I.; Perles, J.; Martín, N. Helically Arranged Chiral Molecular Nanographenes. J. Am. Chem. Soc. 2021, 143, 11864,  DOI: 10.1021/jacs.1c05977
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    9c
    Helically Arranged Chiral Molecular Nanographenes
    Izquierdo-Garcia, Patricia; Fernandez-Garcia, Jesus M.; Fernandez, Israel; Perles, Josefina; Martin, Nazario
    Journal of the American Chemical Society (2021), 143 (30), 11864-11870CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A benchtop soln.-phase synthesis of mol. nanographenes composed of two orthogonal dibenzo[fg,ij]phenanthro[9,10,1,2,3-pqrst]pentaphene (DBPP) moieties covalently connected through a tetrafluorobenzene ring is described. The helical arrangement of these three covalently linked mol. fragments leads to the existence of a chiral axis which gives rise to a racemic mixt., even with the mol. moieties being sym. substituted. X-ray diffraction studies show that both enantiomers cocrystallize in a single crystal, and the racemic mixt. can be resolved by chiral HPLC. Asym. substitution in DBPP moieties affords a pair of diastereoisomers whose rotational isomerization has been studied by 1H NMR. Addnl., the electrochem. and photophys. properties derived from these new mol. nanographenes reveal an electroactive character and a significant fluorescent behavior.
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  10. 10
    (a) Yang, L.; Ju, Y. Y.; Medel, M. A.; Fu, Y.; Komber, H.; Dmitrieva, E.; Zhang, J. J.; Obermann, S.; Campaña, A. G.; Ma, J.; Feng, X. Helical Bilayer Nonbenzenoid Nanographene Bearing a [10]Helicene with Two Embedded Heptagons. Angew. Chem., Int. Ed. 2023, 62, e202216193  DOI: 10.1002/anie.202216193
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    10a
    Helical Bilayer Nonbenzenoid Nanographene Bearing a [10]Helicene with Two Embedded Heptagons
    Yang, Lin; Ju, Yang-Yang; Medel, Miguel A.; Fu, Yubin; Komber, Hartmut; Dmitrieva, Evgenia; Zhang, Jin-Jiang; Obermann, Sebastian; Campana, Araceli G.; Ma, Ji; Feng, Xinliang
    Angewandte Chemie, International Edition (2023), 62 (4), e202216193CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The precision synthesis of helical bilayer nanographenes (NGs) with new topol. is of substantial interest because of their exotic physicochem. properties. However, helical bilayer NGs bearing non-hexagonal rings remain synthetically challenging. Here we present the efficient synthesis of the first helical bilayer nonbenzenoid nanographene (HBNG1) from a tailor-made azulene-embedded precursor, which contains a novel [10]helicene backbone with two embedded heptagons. Single-crystal X-ray anal. reveals its highly twisted bilayer geometry with a record small interlayer distance of 3.2Å among the reported helical bilayer NGs. Notably, the close interlayer distance between the two layers offers intramol. through-space conjugation as revealed by in situ spectroelectrochem. studies together with DFT simulations. Furthermore, the chiroptical properties of the P/M enantiomers of HBNG1 are also evaluated by CD and circularly polarized luminescence.
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    (b) Niu, W.; Fu, Y.; Qiu, Z. L.; Schurmann, C. J.; Obermann, S.; Liu, F.; Popov, A. A.; Komber, H.; Ma, J.; Feng, X. pi-Extended Helical Multilayer Nanographenes with Layer-Dependent Chiroptical Properties. J. Am. Chem. Soc. 2023, 145, 26824,  DOI: 10.1021/jacs.3c09350
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    (c) Ju, Y. Y.; Chai, L.; Li, K.; Xing, J. F.; Ma, X. H.; Qiu, Z. L.; Zhao, X. J.; Zhu, J.; Tan, Y. Z. Helical Trilayer Nanographenes with Tunable Interlayer Overlaps. J. Am. Chem. Soc. 2023, 145, 2815,  DOI: 10.1021/jacs.2c08746
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    10c
    Helical Trilayer Nanographenes with Tunable Interlayer Overlaps
    Ju, Yang-Yang; Chai, Ling; Li, Kang; Xing, Jiang-Feng; Ma, Xiao-Hui; Qiu, Zhen-Lin; Zhao, Xin-Jing; Zhu, Jun; Tan, Yuan-Zhi
    Journal of the American Chemical Society (2023), 145 (5), 2815-2821CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Herein, two trilayer nanographenes were synthesized by covalently linking nanographene layers through helicene bridges. The structural characterization of the trilayer nanographenes revealed a compact trilayer-stacked architecture. The introduction of a furan ring into the helicene linker modulates the interlayer overlap and π-conjugation of the trilayer nanographenes, enabling the tuning of the interlayer coupling, as demonstrated by optical, electrochem., and theor. analyses. Both synthesized trilayer nanographenes were rigid chiral nanocarbons and showed a chirality transfer from the helicene moiety to the stacked nanographene layers. These helical trilayer nanographenes reported here represent the covalently linked multilayer nanographenes rather than bilayer ones, showing the tunable multilayer stacking structure.
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    (d) Evans, P. J.; Ouyang, J.; Favereau, L.; Crassous, J.; Fernández, I.; Perles, J.; Martín, N. Synthesis of a Helical Bilayer Nanographene. Angew. Chem., Int. Ed. 2018, 57, 6774,  DOI: 10.1002/anie.201800798
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    10d
    Synthesis of a Helical Bilayer Nanographene
    Evans, Paul J.; Ouyang, Jiangkun; Favereau, Ludovic; Crassous, Jeanne; Fernandez, Israel; Perles, Josefina; Martin, Nazario
    Angewandte Chemie, International Edition (2018), 57 (23), 6774-6779CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A rigid, inherently chiral bilayer nanographene has been synthesized as both the racemate and enantioenriched M isomer (with 93 % ee) in three steps from established helicenes. This folded nanographene is composed of two hexa-peri-hexabenzocoronene layers fused to a [10]helicene, with an interlayer distance of 3.6 Å as detd. by X-ray crystallog. The rigidity of the helicene linker forces the layers to adopt a nearly aligned AA-stacked conformation, rarely obsd. in few-layer graphene. By combining the advantages of nanographenes and helicenes, we have constructed a bilayer system of 30 fused benzene rings that is also chiral, rigid, and remains sol. in common org. solvents. We present this as a mol. model system of bilayer graphene, with properties of interest in a variety of potential applications.
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  11. 11
    Li, G.; Luican, A.; Lopes dos Santos, J. M. B.; Castro Neto, A. H.; Reina, A.; Kong, J.; Andrei, E. Y. Observation of Van Hove Singularities in Twisted Graphene Layers. Nat. Phys. 2010, 6, 109,  DOI: 10.1038/nphys1463
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    11
    Observation of Van Hove singularities in twisted graphene layers
    Li, Guohong; Luican, A.; Lopes dos Santos, J. M. B.; Castro Neto, A. H.; Reina, A.; Kong, J.; Andrei, E. Y.
    Nature Physics (2010), 6 (2), 109-113CODEN: NPAHAX; ISSN:1745-2473. (Nature Publishing Group)
    Electronic instabilities at the crossing of the Fermi energy with a Van Hove singularity in the d. of states often lead to new phases of matter such as supercond., magnetism or d. waves. However, in most materials this condition is difficult to control. In the case of single-layer graphene, the singularity is too far from the Fermi energy and hence difficult to reach with std. doping and gating techniques. Here we report the observation of low-energy Van Hove singularities in twisted graphene layers seen as two pronounced peaks in the d. of states measured by scanning tunneling spectroscopy. We demonstrate that a rotation between stacked graphene layers can generate Van Hove singularities, which can be brought arbitrarily close to the Fermi energy by varying the angle of rotation. This opens intriguing prospects for Van Hove singularity engineering of electronic phases.
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  12. 12
    Cao, Y.; Fatemi, V.; Fang, S.; Watanabe, K.; Taniguchi, T.; Kaxiras, E.; Jarillo-Herrero, P. Unconventional Superconductivity in Magic-angle Graphene Superlattices. Nature 2018, 556, 43,  DOI: 10.1038/nature26160
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    12
    Unconventional superconductivity in magic-angle graphene superlattices
    Cao, Yuan; Fatemi, Valla; Fang, Shiang; Watanabe, Kenji; Taniguchi, Takashi; Kaxiras, Efthimios; Jarillo-Herrero, Pablo
    Nature (London, United Kingdom) (2018), 556 (7699), 43-50CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    The behavior of strongly correlated materials, and in particular unconventional superconductors, has been studied extensively for decades, but is still not well understood. This lack of theor. understanding has motivated the development of exptl. techniques for studying such behavior, such as using ultracold atom lattices to simulate quantum materials. Here we report the realization of intrinsic unconventional supercond.-which cannot be explained by weak electron-phonon interactions-in a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle. For twist angles of about 1.1°-the first 'magic' angle-the electronic band structure of this 'twisted bilayer graphene' exhibits flat bands near zero Fermi energy, resulting in correlated insulating states at half-filling. Upon electrostatic doping of the material away from these correlated insulating states, we observe tunable zero-resistance states with a crit. temp. of up to 1.7 K. The temp.-carrier-d. phase diagram of twisted bilayer graphene is similar to that of copper oxides (or cuprates), and includes dome-shaped regions that correspond to supercond. Moreover, quantum oscillations in the longitudinal resistance of the material indicate the presence of small Fermi surfaces near the correlated insulating states, in analogy with underdoped cuprates. The relatively high superconducting crit. temp. of twisted bilayer graphene, given such a small Fermi surface (which corresponds to a carrier d. of about 1011 per square centimeter), puts it among the superconductors with the strongest pairing strength between electrons. Twisted bilayer graphene is a precisely tunable, purely carbon-based, two-dimensional superconductor. It is therefore an ideal material for investigations of strongly correlated phenomena, which could lead to insights into the physics of high-crit.-temp. superconductors and quantum spin liqs.
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  13. 13
    Izquierdo-García, P.; Fernández-García, J. M.; Medina Rivero, S.; Šámal, M.; Rybáček, J.; Bednárová, L.; Ramírez-Barroso, S.; Ramírez, F. J.; Rodríguez, R.; Perles, J.; García-Fresnadillo, D.; Crassous, J.; Casado, J.; Stará, I. G.; Martín, N. Helical Bilayer Nanographenes: Impact of the Helicene Length on the Structural, Electrochemical, Photophysical, and Chiroptical Properties. J. Am. Chem. Soc. 2023, 145, 11599,  DOI: 10.1021/jacs.3c01088
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    13
    Helical Bilayer Nanographenes: Impact of the Helicene Length on the Structural, Electrochemical, Photophysical, and Chiroptical Properties
    Izquierdo-Garcia, Patricia; Fernandez-Garcia, Jesus M.; Medina Rivero, Samara; Samal, Michal; Rybacek, Jiri; Bednarova, Lucie; Ramirez-Barroso, Sergio; Ramirez, Francisco J.; Rodriguez, Rafael; Perles, Josefina; Garcia-Fresnadillo, David; Crassous, Jeanne; Casado, Juan; Stara, Irena G.; Martin, Nazario
    Journal of the American Chemical Society (2023), 145 (21), 11599-11610CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Helical bilayer nanographenes (HBNGs) are chiral π-extended arom. compds. consisting of two π-π stacked hexabenzocoronenes (HBCs) joined by a helicene, thus resembling van der Waals layered 2D materials. Herein, we compare [9]HBNG, [10]HBNG, and [11]HBNG helical bilayers endowed with [9], [10], and [11]helicenes embedded in their structure, resp. Interestingly, the helicene length defines the overlapping degree between the two HBCs (no. of benzene rings involved in π-π interactions between the two layers), being 26, 14, and 10 benzene rings, resp., according to the X-ray anal. Unexpectedly, the electrochem. study shows that the lesser π-extended system [9]HBNG shows the strongest electron donor character, in part by interlayer exchange resonance, and more red-shifted values of emission. Furthermore, [9]HBNG also shows exceptional chiroptical properties with the biggest values of gabs and glum (3.6 x 10-2) when compared to [10]HBNG and [11]HBNG owing to the fine alignment in the configuration of [9]HBNG between its elec. and magnetic dipole transition moments. Furthermore, spectroelectrochem. studies as well as the fluorescence spectroscopy support the aforementioned exptl. findings, thus confirming the strong impact of the helicene length on the properties of this new family of bilayer nanographenes.
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  14. 14
    (a) Chiesa, A.; Privitera, A.; Macaluso, E.; Mannini, M.; Bittl, R.; Naaman, R.; Wasielewski, M. R.; Sessoli, R.; Carretta, S. Chirality-Induced Spin Selectivity: An Enabling Technology for Quantum Applications. Adv. Mater. 2023, 35, e2300472  DOI: 10.1002/adma.202300472
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    (b) Bloom, B. P.; Paltiel, Y.; Naaman, R.; Waldeck, D. H. Chiral Induced Spin Selectivity. Chem. Rev. 2024, 124, 1950,  DOI: 10.1021/acs.chemrev.3c00661
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  15. 15
    Buendía, M.; Fernández-García, J. M.; Perles, J.; Filippone, S.; Martín, N. Enantioselective Synthesis of a Two-fold Inherently Chiral Molecular Nanographene. Nat. Synth. 2024, 3, 545,  DOI: 10.1038/s44160-024-00484-x
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    There is no corresponding record for this reference.
  16. 16
    Morita, F.; Kishida, Y.; Sato, Y.; Sugiyama, H.; Abekura, M.; Nogami, J.; Toriumi, N.; Nagashima, Y.; Kinoshita, T.; Fukuhara, G.; Uchiyama, M.; Uekusa, H.; Tanaka, K. Design and Enantioselective Synthesis of 3D π-Extended Carbohelicenes for Circularly Polarized Luminescence. Nat. Synth. 2024, 3, 774,  DOI: 10.1038/s44160-024-00527-3
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    There is no corresponding record for this reference.
  17. 17
    (a) Tkachenko, N. V.; Scheiner, S. Optical Stability of 1,1′-Binaphthyl Derivatives. ACS Omega 2019, 4, 6044,  DOI: 10.1021/acsomega.9b00619
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    17a
    Optical Stability of 1,1'-Binaphthyl Derivatives
    Tkachenko, Nikolay V.; Scheiner, Steve
    ACS Omega (2019), 4 (3), 6044-6049CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
    The racemization process of various 1,1'-binaphthyl derivs. is studied by quantum calcns. The preferred racemization pathway passes through a transition state belonging to the Ci symmetry group. The energy barrier for this process is independent of solvation, the electron-withdrawing/releasing power of substituents, or their ability to engage in H-bonds within the mol. The primary factor is instead the substituent size. The barrier is thus reduced when the -OH groups of BINOL are replaced by H. There is a drop in barrier also when the substituents are moved from the 2,2' positions to 6,6' where they will not come close to one another in the transition state. Upon removal of the peripheral arom. rings of the binaphthyl system, the biphenyl system undergoes a facile racemization. It is concluded that the optimal means of improving optical stability of 1,1'-binaphthyl systems is the substitution of large bulky groups in the 2,2' positions.
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    (b) Li, S.; Li, R.; Zhang, Y. K.; Wang, S.; Ma, B.; Zhang, B.; An, P. BINOL-like Atropisomeric Chiral Nanographene. Chem. Sci. 2023, 14, 3286,  DOI: 10.1039/D2SC06244E
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  18. 18
    Nakanishi, K.; Fukatsu, D.; Takaishi, K.; Tsuji, T.; Uenaka, K.; Kuramochi, K.; Kawabata, T.; Tsubaki, K. Oligonaphthofurans: Fan-Shaped and Three-Dimensional π-Compounds. J. Am. Chem. Soc. 2014, 136, 7101,  DOI: 10.1021/ja502209w
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    18
    Oligonaphthofurans: Fan-Shaped and Three-Dimensional π-Compounds
    Nakanishi, Kentaro; Fukatsu, Daisuke; Takaishi, Kazuto; Tsuji, Taiki; Uenaka, Keita; Kuramochi, Kouji; Kawabata, Takeo; Tsubaki, Kazunori
    Journal of the American Chemical Society (2014), 136 (19), 7101-7109CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
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  • Abstract

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    Chart 1

    Chart 1. Approaches to Enantiomerically Pure Chiral Bilayer NGs
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    Scheme 1

    Scheme 1. Synthesis of Racemic Oxa[9]HBNG from 7-Bromo-2-naphthol
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    Scheme 2

    Scheme 2. Chemical Resolution of Racemate 6 and Enantiospecific Synthesis of M-oxa[9]HBNG and P-oxa[9]HBNG
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    Figure 1

    Figure 1. X-ray structure of R,P-7b showing the chiral auxiliar, 1R-10-camphorsulphonil, and the P configuration of the binaphthol core.

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    Figure 2

    Figure 2. X-ray crystal structures of M-oxa[9]HBNG and P-oxa[9]HBNG showing the inherent configuration of the helicene moieties.

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    Figure 3

    Figure 3. Cyclic voltammogram of oxa[9]HBNG showing its half-wave redox potentials vs Fc/Fc+ in a 1 M solution of tetrabutylammonium hexafluorophosphate in toluene/acetonitrile 4:1.

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    Figure 4

    Figure 4. Absorption (solid lines) and emission (shaded curves) spectra of oxa[9]HBNG and [9]HBNG in chloroform.

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    Figure 5

    Figure 5. CD spectra and gabs values of M-oxa[9]HBNG and P-oxa[9]HBNG.

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    Figure 6

    Figure 6. CPL spectra and glum values of M-oxa[9]HBNG and P-oxa[9]HBNG.

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      There is no corresponding record for this reference.
    4. 4
      (a) Liu, Z.; Fu, S.; Liu, X.; Narita, A.; Samori, P.; Bonn, M.; Wang, H. I. Small Size, Big Impact: Recent Progress in Bottom-Up Synthesized Nanographenes for Optoelectronic and Energy Applications. Adv. Sci. 2022, 9, e2106055  DOI: 10.1002/advs.202106055
      There is no corresponding record for this reference.
      (b) Baier, D. M.; Gratz, S.; Jahromi, B. F.; Hellmann, S.; Bergheim, K.; Pickhardt, W.; Schmid, R.; Borchardt, L. Beyond the Scholl Reaction - One-step Planarization and Edge Chlorination of Nanographenes by Mechanochemistry. RSC Adv. 2021, 11, 38026,  DOI: 10.1039/D1RA07679E
      There is no corresponding record for this reference.
      (c) Lungerich, D.; Papaianina, O.; Feofanov, M.; Liu, J.; Devarajulu, M.; Troyanov, S. I.; Maier, S.; Amsharov, K. Dehydrative pi-Extension to Nanographenes with Zig-zag Edges. Nat. Commun. 2018, 9, 4756,  DOI: 10.1038/s41467-018-07095-z
      4c
      Dehydrative π-extension to nanographenes with zig-zag edges
      Lungerich Dominik; Papaianina Olena; Feofanov Mikhail; Amsharov Konstantin; Lungerich Dominik; Liu Jia; Devarajulu Mirunalini; Maier Sabine; Troyanov Sergey I
      Nature communications (2018), 9 (1), 4756 ISSN:.
      Zig-zag nanographenes are promising candidates for the applications in organic electronics due to the electronic properties induced by their periphery. However, the synthetic access to these compounds remains virtually unexplored. There is a lack in efficient and mild strategies origins in the reduced stability, increased reactivity, and low solubility of these compounds. Herein we report a facile access to pristine zig-zag nanographenes, utilizing an acid-promoted intramolecular reductive cyclization of arylaldehydes, and demonstrate a three-step route to nanographenes constituted of angularly fused tetracenes or pentacenes. The mild conditions are scalable to gram quantities and give insoluble nanostructures in close to quantitative yields. The strategy allows the synthesis of elusive low bandgap nanographenes, with values as low as 1.62 eV. Compared to their linear homologues, the structures have an increased stability in the solid-state, even though computational analyses show distinct diradical character. The structures were confirmed by X-ray diffraction or scanning tunneling microscopy.
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      (d) Oró, A.; Romeo-Gella, F.; Perles, J.; Fernández-García, J. M.; Corral, I.; Martín, N. Tetrahedraphene: A Csp(3)-Centered 3D Molecular Nanographene Showing Aggregation-Induced Emission. Angew. Chem., Int. Ed. 2023, 62, e202312314  DOI: 10.1002/anie.202312314
      There is no corresponding record for this reference.
    5. 5
      (a) Lombardi, F.; Lodi, A.; Ma, J.; Liu, J.; Slota, M.; Narita, A.; Myers, W. K.; Müllen, K.; Feng, X.; Bogani, L. Quantum Units from the Topological Engineering of Molecular Graphenoids. Science 2019, 366, 1107,  DOI: 10.1126/science.aay7203
      5a
      Quantum units from the topological engineering of molecular graphenoids
      Lombardi, Federico; Lodi, Alessandro; Ma, Ji; Liu, Junzhi; Slota, Michael; Narita, Akimitsu; Myers, William K.; Mullen, Klaus; Feng, Xinliang; Bogani, Lapo
      Science (Washington, DC, United States) (2019), 366 (6469), 1107-1110CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
      A review. Robustly coherent spin centers that can be integrated into devices are a key ingredient of quantum technologies. Vacancies in semiconductors are excellent candidates, and theory predicts that defects in conjugated carbon materials should also display long coherence times. However, the quantum performance of carbon nanostructures has remained stunted by an inability to alter the sp2-carbon lattice with at. precision. Here, we demonstrate that topol. tailoring leads to superior quantum performance in mol. graphene nanostructures. We unravel the decoherence mechanisms, quantify nuclear and environmental effects, and observe spin-coherence times that outclass most nanomaterials. These results validate long-standing assumptions on the coherent behavior of topol. defects in graphene and open up the possibility of introducing controlled quantum-coherent centers in the upcoming generation of carbon-based optoelectronic, electronic, and bioactive systems.
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      (b) Majewski, M. A.; Stępień, M. Bowls, Hoops Saddles: Synthetic Approaches to Curved Aromatic Molecules. Angew. Chem., Int. Ed. 2019, 58, 86,  DOI: 10.1002/anie.201807004
      There is no corresponding record for this reference.
      (c) Rickhaus, M.; Mayor, M.; Juríček, M. Chirality in Curved Polyaromatic Systems. Chem. Soc. Rev. 2017, 46, 1643,  DOI: 10.1039/C6CS00623J
      5c
      Chirality in curved polyaromatic systems
      Rickhaus, Michel; Mayor, Marcel; Juricek, Michal
      Chemical Society Reviews (2017), 46 (6), 1643-1660CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. Carbon allotropes constituted of sp2-hybridized carbon atoms display a variety of properties that arise from their delocalised π-conjugated electronic structure. Apart from carbon's planar allotropic form graphene, bent or curved structures, such as carbon nanotubes or fullerenes, resp., have been discovered. In this Tutorial Review, we analyze and conceptually categorise chiral synthetic mol. fragments of non-planar sp2-carbon allotropes, including hypothetical forms of carbon that have been proposed to exist as stable entities. Two types of mol. systems composed of equally or differently sized rings are examd.: bent with zero Gaussian curvature and curved with pos. or neg. Gaussian curvature. To affirm that a system is chiral, two conditions must be fulfilled: (1) both reflective symmetry elements, an inversion center and a mirror plane, must be absent and (2) the system must be stereochem. rigid. It is therefore crucial to not only consider the symmetry of a given system as if it was a rigid object but also its structural dynamics. These principles serve as guidelines for the design of mol. fragments that encode and transcribe chirality into larger systems.
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      (d) Zhang, Y.; Pun, S. H.; Miao, Q. The Scholl Reaction as a Powerful Tool for Synthesis of Curved Polycyclic Aromatics. Chem. Rev. 2022, 122, 14554,  DOI: 10.1021/acs.chemrev.2c00186
      5d
      The Scholl Reaction as a Powerful Tool for Synthesis of Curved Polycyclic Aromatics
      Zhang, Yiqun; Pun, Sai Ho; Miao, Qian
      Chemical Reviews (Washington, DC, United States) (2022), 122 (18), 14554-14593CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. These curved polycyclic aroms. are not only unique objects of structural org. chem. in relation to the nature of aromaticity, but also play an important role in bottom-up approaches to precise synthesis of nanocarbons of unique topol. Moreover, they have received considerable attention in the fields of supramol. chem. and org. functional materials because of their interesting properties and promising applications. Despite the great success of Scholl reactions in synthesis of curved polycyclic aroms., 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 aroms. with interesting structures and properties, and aims to shed light on the key factors that affect Scholl reactions in synthesizing sterically strained polycyclic aroms.
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      (e) Fernández-García, J. M.; Evans, P. J.; Medina Rivero, S.; Fernández, I.; García-Fresnadillo, D.; Perles, J.; Casado, J.; Martín, N. pi-Extended Corannulene-Based Nanographenes: Selective Formation of Negative Curvature. J. Am. Chem. Soc. 2018, 140, 17188,  DOI: 10.1021/jacs.8b09992
      There is no corresponding record for this reference.
    6. 6
      (a) Fernández-García, J. M.; Izquierdo-García, P.; Buendía, M.; Filippone, S.; Martín, N. Synthetic Chiral Molecular Nanographenes: The Key Figure of the Racemization Barrier. Chem. Commun. 2022, 58, 2634,  DOI: 10.1039/D1CC06561K
      6a
      Synthetic chiral molecular nanographenes: the key figure of the racemization barrier
      Fernandez-Garcia, Jesus M.; Izquierdo-Garcia, Patricia; Buendia, Manuel; Filippone, Salvatore; Martin, Nazario
      Chemical Communications (Cambridge, United Kingdom) (2022), 58 (16), 2634-2645CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      A review. Chirality is one of the most intriguing concepts of chem., involving living systems and, more recently, materials science. In particular, the bottom-up synthesis of mol. nanographenes endowed with one or several chiral elements is a current challenge for the chem. community. The wilful introduction of defects in the sp2 honeycomb lattice of mol. nanographenes allows the prepn. of chiral mols. with tuned band-gaps and chiroptical properties. There are two requirements that a system must fulfill to be chiral: (i) lack of inversion elements (planes or inversion centers) and (ii) to be configurationally stable. The first condition is inherently established by the symmetry group of the structure, however, the limit between conformational and configurational isomers is not totally clear. In this feature article, the chirality and dynamics of synthetic mol. nanographenes, with special emphasis on their racemization barriers and, therefore, the stability of their chiroptical properties are discussed. The general features of nanographenes and their bottom-up synthesis, including the main defects inducing chirality in mol. nanographenes are firstly discussed. In this regard, the most common topol. defects of mol. NGs as well as the main techniques used for detg. their energy barriers are presented. Then, the manuscript is structured according to the dynamics of mol. nanographenes, classifying them in four main groups, depending on their resp. isomerization barriers, as flexible, detectable, isolable and rigid nanographenes. In these sections, the different strategies used to increase the isomerization barrier of chiral mol. nanographenes that lead to configurationally stable nanographenes with defined chiroptical properties are discussed.
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      (b) Fernández-García, J. M.; Evans, P. J.; Filippone, S.; Herranz, M. A.; Martín, N. Chiral Molecular Carbon Nanostructures. Acc. Chem. Res. 2019, 52, 1565,  DOI: 10.1021/acs.accounts.9b00144
      6b
      Chiral Molecular Carbon Nanostructures
      Fernandez-Garcia, Jesus M.; Evans, Paul J.; Filippone, Salvatore; Herranz, Maria Angeles; Martin, Nazario
      Accounts of Chemical Research (2019), 52 (6), 1565-1574CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review. In this Account, we show our progress in the synthesis of chiral mol. carbon nanostructures, namely, metallofullerenes, endohedral fullerenes, GQDs, and curved mol. nanographenes, by using asym. catalysis and both top-down and bottom-up chem. approaches. Furthermore, we bring in a new family of lesser-known mol. 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 mol. 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 det. the effect of the incarcerated mol. 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 alcs. 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 mol. 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 mol. 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 dets. 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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      (c) Anderson, H. V.; Gois, N. D.; Chalifoux, W. A. New Advances in Chiral Nanographene Chemistry. Org. Chem. Front. 2023, 10, 4167,  DOI: 10.1039/D3QO00517H
      There is no corresponding record for this reference.
    7. 7
      (a) Schnitzlein, M.; Shoyama, K.; Würthner, F. A Highly Fluorescent Bora[6]helicene Exhibiting Circularly Polarized Light Emission. Chem. Sci. 2024, 15, 2984– 2989,  DOI: 10.1039/d3sc05171d
      There is no corresponding record for this reference.
      (b) Niu, W.; Fu, Y.; Deng, Q.; Qiu, Z. L.; Liu, F.; Popov, A. A.; Komber, H.; Ma, J.; Feng, X. Enhancing Chiroptical Responses in Helical Nanographenes via Geometric Engineering of Double [7]Helicenes. Angew. Chem., Int. Ed. 2024, 63, e202319874  DOI: 10.1002/anie.202319874
      There is no corresponding record for this reference.
      (c) Qiu, Z.; Ju, C. W.; Frederic, L.; Hu, Y.; Schollmeyer, D.; Pieters, G.; Müllen, K.; Narita, A. Amplification of Dissymmetry Factors in pi-Extended [7]- and [9]Helicenes. J. Am. Chem. Soc. 2021, 143, 4661,  DOI: 10.1021/jacs.0c13197
      7c
      Amplification of Dissymmetry Factors in π-Extended [7]- and [9]Helicenes
      Qiu, Zijie; Ju, Cheng-Wei; Frederic, Lucas; Hu, Yunbin; Schollmeyer, Dieter; Pieters, Gregory; Muellen, Klaus; Narita, Akimitsu
      Journal of the American Chemical Society (2021), 143 (12), 4661-4667CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      π-Extended helicenes constitute an important class of polycyclic arom. hydrocarbons with intrinsic chirality. Herein, we report the syntheses of π-extended [7]helicene 4 and π-extended [9]helicene 6 through regioselective cyclodehydrogenation in high yields, where a "prefusion" strategy plays a key role in preventing undesirable aryl rearrangements. The unique helical structures are unambiguously confirmed by X-ray crystal structure anal. Compared to the parent pristine [7]helicene and [9]helicene, these novel π-extended helicenes display significantly improved photophys. properties, with a quantum yield of 0.41 for 6. After optical resoln. by chiral high-performance liq. chromatog., the chiroptical properties of enantiomers 4-P/M and 6-P/M are investigated, revealing that the small variation in helical length from [7] to [9] can cause an approx. 10-fold increase in the dissymmetry factors. The circularly polarized luminescence brightness of 6 reaches 12.6 M-1 cm-1 as one of the highest among carbohelicenes.
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    8. 8
      (a) Liu, X.; Jin, Z.; Qiu, F.; Guo, Y.; Chen, Y.; Sun, Z.; Zhang, L. Hexabenzoheptacene: A Longitudinally Multihelicene Nanocarbon with Local Aromaticity and Enhanced Stability. Angew. Chem., Int. Ed. 2024, 63, e202407547  DOI: 10.1002/anie.202407547
      There is no corresponding record for this reference.
      (b) Zhu, Y.; Wang, J. Helical Synthetic Nanographenes with Atomic Precision. Acc. Chem. Res. 2023, 56, 363,  DOI: 10.1021/acs.accounts.2c00767
      8b
      Helical Synthetic Nanographenes with Atomic Precision
      Zhu, Yanpeng; Wang, Jiaobing
      Accounts of Chemical Research (2023), 56 (3), 363-373CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review. Understanding and harnessing the properties of nanoscale mol. entities are considered as new frontiers in basic chem. In this regard, synthetic nanographene with at. precision has attracted much attention recently. For instance, taking advantage of the marvelous bonding capability of C, flat, curved, ribbon-type, or cone-shaped nanographenes were prepd. in highly controllable and elegant manner, allowing exploration of fascinating mol. architectures with intriguing optical, electrochem., and magnetic characteristics. This stands in stark contrast to other C-rich nanomaterials, such as graphite oxides or C quantum dots, which preclude thorough studies because of complicate structural defects. Undoubtedly, synthetic nanographene contributes strongly to modern arom. chem. and represents a vibrant field that may deliver transforming functional materials crucial for optoelectronics, nanotechnologies, and biomedicine. In many cases, synthesis and characterization of nanographene compds. are highly demanding. Low soly., high mol. strain, undesired selectivity, as well as incomplete or excessive C-C bond formation are common impediments, that require formidable efforts to control the mol. 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 relations, and lay down foundation for exploring synthetic nanographenes at a high level of sophistications. The contributions to this field are summarized by presenting helical synthetic nanographenes, such as hexapole [7]helicene (H7H), N-doped H7H, hexapole [9]helicene (H9H), superhelicene, and supertwistacene. This kind of giant synthetic nanographene reaches the size domain of C quantum dots, albeit has precise at. structure. It provides a unique platform to study arom. chem. and chirality at the nanoscale. Synthetic methods and point out, in particular, the strengths and pitfalls of Scholl oxidn., which are expected to be valuable for making synthetic nanographenes in general are discussed. The authors illustrate their exciting electrochem. and photophys. performance, which include, but are not limited to, reversible multielectron redox chem., record high panchromatic absorption, impressive photothermal behavior, and extremely strong Cotton effect. These unusual characteristics are convincingly traced back to their 3-dimensional conjugated architectures, highlighting the crit. roles of π-electron delocalization, heteroatom-doping, substitution, and mol. symmetry in detg. nanographenes' properties and functions. Lastly, the authors put forward the 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.
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      (c) Peng, L. J.; Wang, X. Y.; Li, Z. A.; Gong, H. Y. All Carbon Helicenes and π-Extended Helicene Derivatives. Asian J. Org. Chem. 2023, 12, e202300543  DOI: 10.1002/ajoc.202300543
      There is no corresponding record for this reference.
      (d) Izquierdo-García, P.; Fernández-García, J. M.; Perles, J.; Fernández, I.; Martín, N. Electronic Control of the Scholl Reaction: Selective Synthesis of Spiro vs Helical Nanographenes. Angew. Chem., Int. Ed. 2023, 62, e202215655  DOI: 10.1002/anie.202215655
      8d
      Electronic Control of the Scholl Reaction: Selective Synthesis of Spiro vs Helical Nanographenes
      Izquierdo-Garcia, Patricia; Fernandez-Garcia, Jesus M.; Perles, Josefina; Fernandez, Israel; Martin, Nazario
      Angewandte Chemie, International Edition (2023), 62 (7), e202215655CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A Scholl reaction controlled by the electronic effects on the starting substrates was described. Anthracene-based polyphenylenes led to spironanographenes under Scholl conditions. In contrast, an electron-deficient anthracene substrate afforded a helically arranged mol. nanographene formed by two orthogonal dibenzo[fg,ij]phenanthro-[9,10,1,2,3-pqrst]pentaphene (DBPP) moieties linked through an octafluoroanthracene core. D. Functional Theory (DFT) calcns. predicted that electronic effects control either the first formation of spirocycles and subsequent Scholl reaction to form spironanographene , or the expected dehydrogenation reaction leading solely to the helical nanographene, I. The crystal structures of four of the new spiro compds. were solved by single crystal X-ray diffraction. The photophys. properties of the new mol. nanographene I revealed a remarkable dual fluorescent emission.
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    9. 9
      (a) Navakouski, M.; Zhylitskaya, H.; Chmielewski, P. J.; Lis, T.; Cybińska, J.; Stępień, M. Stereocontrolled Synthesis of Chiral Heteroaromatic Propellers with Small Optical Bandgaps. Angew. Chem., Int. Ed. 2019, 58, 4929,  DOI: 10.1002/anie.201900175
      9a
      Stereocontrolled synthesis of chiral heteroaromatic propellers with small optical bandgaps
      Navakouski, Maksim; Zhylitskaya, Halina; Chmielewski, Piotr J.; Lis, Tadeusz; Cybinska, Joanna; Stepien, Marcin
      Angewandte Chemie, International Edition (2019), 58 (15), 4929-4933CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Chiral heteroarom. propellers based on radially π-extended hexapyrrolohexaazacoronenes were obtained in a concise synthesis from suitably functionalized donor-acceptor monopyrroles. To overcome steric hindrance, a new cyclodehydrogenation method was developed, and it uses bromine electrophiles as oxidative coupling agents instead of the commonly employed high-potential oxidants. The new reaction offers high yields of propeller-shaped targets, even for electron-deficient precursors, and shows electrophile-dependent stereoselectivity, with N-bromosuccinimide and dibromine yielding, resp. D6- and C2-sym. products. The propeller azacoronenes are chiral and can be sepd. into configurationally stable enantiomers. In addn. to providing steric bulk, peripheral functionalization considerably affects the electronic properties of the propellers, which exhibit reduced optical and electrochem. band gaps, and a more clearly defined electroredn. behavior.
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      (b) Zhu, Y.; Guo, X.; Li, Y.; Wang, J. Fusing of Seven HBCs toward a Green Nanographene Propeller. J. Am. Chem. Soc. 2019, 141, 5511,  DOI: 10.1021/jacs.9b01266
      9b
      Fusing of Seven HBCs toward a Green Nanographene Propeller
      Zhu, Yanpeng; Guo, Xiaoyu; Li, Yang; Wang, Jiaobing
      Journal of the American Chemical Society (2019), 141 (13), 5511-5517CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      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 arom. hydrocarbons ever prepd. using scalable soln. chem. Despite its unusual mol. size, single-crystal X-ray structural anal. (resoln. 0.9 Å) and baseline chiral resoln. are achieved. NP is sol. in various org. solvents and can be fully characterized by common spectroscopic and voltammetric techniques. It has a strong panchromatic absorption from the UV to the near-IR (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 coeff. larger than 100 000 M-1 cm-1. Moreover, a record-high Cotton effect in the visible spectrum is obsd. for enantiopure NP, with |Δε| values of 1182 and 1090 M-1 cm-1 at 374 and 405 nm, resp. These photophys. properties evolve significantly compared to those of the propeller-shaped hexapole [7]helicene.
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      (c) Izquierdo-García, P.; Fernández-García, J. M.; Fernández, I.; Perles, J.; Martín, N. Helically Arranged Chiral Molecular Nanographenes. J. Am. Chem. Soc. 2021, 143, 11864,  DOI: 10.1021/jacs.1c05977
      9c
      Helically Arranged Chiral Molecular Nanographenes
      Izquierdo-Garcia, Patricia; Fernandez-Garcia, Jesus M.; Fernandez, Israel; Perles, Josefina; Martin, Nazario
      Journal of the American Chemical Society (2021), 143 (30), 11864-11870CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A benchtop soln.-phase synthesis of mol. nanographenes composed of two orthogonal dibenzo[fg,ij]phenanthro[9,10,1,2,3-pqrst]pentaphene (DBPP) moieties covalently connected through a tetrafluorobenzene ring is described. The helical arrangement of these three covalently linked mol. fragments leads to the existence of a chiral axis which gives rise to a racemic mixt., even with the mol. moieties being sym. substituted. X-ray diffraction studies show that both enantiomers cocrystallize in a single crystal, and the racemic mixt. can be resolved by chiral HPLC. Asym. substitution in DBPP moieties affords a pair of diastereoisomers whose rotational isomerization has been studied by 1H NMR. Addnl., the electrochem. and photophys. properties derived from these new mol. nanographenes reveal an electroactive character and a significant fluorescent behavior.
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    10. 10
      (a) Yang, L.; Ju, Y. Y.; Medel, M. A.; Fu, Y.; Komber, H.; Dmitrieva, E.; Zhang, J. J.; Obermann, S.; Campaña, A. G.; Ma, J.; Feng, X. Helical Bilayer Nonbenzenoid Nanographene Bearing a [10]Helicene with Two Embedded Heptagons. Angew. Chem., Int. Ed. 2023, 62, e202216193  DOI: 10.1002/anie.202216193
      10a
      Helical Bilayer Nonbenzenoid Nanographene Bearing a [10]Helicene with Two Embedded Heptagons
      Yang, Lin; Ju, Yang-Yang; Medel, Miguel A.; Fu, Yubin; Komber, Hartmut; Dmitrieva, Evgenia; Zhang, Jin-Jiang; Obermann, Sebastian; Campana, Araceli G.; Ma, Ji; Feng, Xinliang
      Angewandte Chemie, International Edition (2023), 62 (4), e202216193CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The precision synthesis of helical bilayer nanographenes (NGs) with new topol. is of substantial interest because of their exotic physicochem. properties. However, helical bilayer NGs bearing non-hexagonal rings remain synthetically challenging. Here we present the efficient synthesis of the first helical bilayer nonbenzenoid nanographene (HBNG1) from a tailor-made azulene-embedded precursor, which contains a novel [10]helicene backbone with two embedded heptagons. Single-crystal X-ray anal. reveals its highly twisted bilayer geometry with a record small interlayer distance of 3.2Å among the reported helical bilayer NGs. Notably, the close interlayer distance between the two layers offers intramol. through-space conjugation as revealed by in situ spectroelectrochem. studies together with DFT simulations. Furthermore, the chiroptical properties of the P/M enantiomers of HBNG1 are also evaluated by CD and circularly polarized luminescence.
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      (b) Niu, W.; Fu, Y.; Qiu, Z. L.; Schurmann, C. J.; Obermann, S.; Liu, F.; Popov, A. A.; Komber, H.; Ma, J.; Feng, X. pi-Extended Helical Multilayer Nanographenes with Layer-Dependent Chiroptical Properties. J. Am. Chem. Soc. 2023, 145, 26824,  DOI: 10.1021/jacs.3c09350
      There is no corresponding record for this reference.
      (c) Ju, Y. Y.; Chai, L.; Li, K.; Xing, J. F.; Ma, X. H.; Qiu, Z. L.; Zhao, X. J.; Zhu, J.; Tan, Y. Z. Helical Trilayer Nanographenes with Tunable Interlayer Overlaps. J. Am. Chem. Soc. 2023, 145, 2815,  DOI: 10.1021/jacs.2c08746
      10c
      Helical Trilayer Nanographenes with Tunable Interlayer Overlaps
      Ju, Yang-Yang; Chai, Ling; Li, Kang; Xing, Jiang-Feng; Ma, Xiao-Hui; Qiu, Zhen-Lin; Zhao, Xin-Jing; Zhu, Jun; Tan, Yuan-Zhi
      Journal of the American Chemical Society (2023), 145 (5), 2815-2821CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Herein, two trilayer nanographenes were synthesized by covalently linking nanographene layers through helicene bridges. The structural characterization of the trilayer nanographenes revealed a compact trilayer-stacked architecture. The introduction of a furan ring into the helicene linker modulates the interlayer overlap and π-conjugation of the trilayer nanographenes, enabling the tuning of the interlayer coupling, as demonstrated by optical, electrochem., and theor. analyses. Both synthesized trilayer nanographenes were rigid chiral nanocarbons and showed a chirality transfer from the helicene moiety to the stacked nanographene layers. These helical trilayer nanographenes reported here represent the covalently linked multilayer nanographenes rather than bilayer ones, showing the tunable multilayer stacking structure.
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      (d) Evans, P. J.; Ouyang, J.; Favereau, L.; Crassous, J.; Fernández, I.; Perles, J.; Martín, N. Synthesis of a Helical Bilayer Nanographene. Angew. Chem., Int. Ed. 2018, 57, 6774,  DOI: 10.1002/anie.201800798
      10d
      Synthesis of a Helical Bilayer Nanographene
      Evans, Paul J.; Ouyang, Jiangkun; Favereau, Ludovic; Crassous, Jeanne; Fernandez, Israel; Perles, Josefina; Martin, Nazario
      Angewandte Chemie, International Edition (2018), 57 (23), 6774-6779CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A rigid, inherently chiral bilayer nanographene has been synthesized as both the racemate and enantioenriched M isomer (with 93 % ee) in three steps from established helicenes. This folded nanographene is composed of two hexa-peri-hexabenzocoronene layers fused to a [10]helicene, with an interlayer distance of 3.6 Å as detd. by X-ray crystallog. The rigidity of the helicene linker forces the layers to adopt a nearly aligned AA-stacked conformation, rarely obsd. in few-layer graphene. By combining the advantages of nanographenes and helicenes, we have constructed a bilayer system of 30 fused benzene rings that is also chiral, rigid, and remains sol. in common org. solvents. We present this as a mol. model system of bilayer graphene, with properties of interest in a variety of potential applications.
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    11. 11
      Li, G.; Luican, A.; Lopes dos Santos, J. M. B.; Castro Neto, A. H.; Reina, A.; Kong, J.; Andrei, E. Y. Observation of Van Hove Singularities in Twisted Graphene Layers. Nat. Phys. 2010, 6, 109,  DOI: 10.1038/nphys1463
      11
      Observation of Van Hove singularities in twisted graphene layers
      Li, Guohong; Luican, A.; Lopes dos Santos, J. M. B.; Castro Neto, A. H.; Reina, A.; Kong, J.; Andrei, E. Y.
      Nature Physics (2010), 6 (2), 109-113CODEN: NPAHAX; ISSN:1745-2473. (Nature Publishing Group)
      Electronic instabilities at the crossing of the Fermi energy with a Van Hove singularity in the d. of states often lead to new phases of matter such as supercond., magnetism or d. waves. However, in most materials this condition is difficult to control. In the case of single-layer graphene, the singularity is too far from the Fermi energy and hence difficult to reach with std. doping and gating techniques. Here we report the observation of low-energy Van Hove singularities in twisted graphene layers seen as two pronounced peaks in the d. of states measured by scanning tunneling spectroscopy. We demonstrate that a rotation between stacked graphene layers can generate Van Hove singularities, which can be brought arbitrarily close to the Fermi energy by varying the angle of rotation. This opens intriguing prospects for Van Hove singularity engineering of electronic phases.
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    12. 12
      Cao, Y.; Fatemi, V.; Fang, S.; Watanabe, K.; Taniguchi, T.; Kaxiras, E.; Jarillo-Herrero, P. Unconventional Superconductivity in Magic-angle Graphene Superlattices. Nature 2018, 556, 43,  DOI: 10.1038/nature26160
      12
      Unconventional superconductivity in magic-angle graphene superlattices
      Cao, Yuan; Fatemi, Valla; Fang, Shiang; Watanabe, Kenji; Taniguchi, Takashi; Kaxiras, Efthimios; Jarillo-Herrero, Pablo
      Nature (London, United Kingdom) (2018), 556 (7699), 43-50CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      The behavior of strongly correlated materials, and in particular unconventional superconductors, has been studied extensively for decades, but is still not well understood. This lack of theor. understanding has motivated the development of exptl. techniques for studying such behavior, such as using ultracold atom lattices to simulate quantum materials. Here we report the realization of intrinsic unconventional supercond.-which cannot be explained by weak electron-phonon interactions-in a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle. For twist angles of about 1.1°-the first 'magic' angle-the electronic band structure of this 'twisted bilayer graphene' exhibits flat bands near zero Fermi energy, resulting in correlated insulating states at half-filling. Upon electrostatic doping of the material away from these correlated insulating states, we observe tunable zero-resistance states with a crit. temp. of up to 1.7 K. The temp.-carrier-d. phase diagram of twisted bilayer graphene is similar to that of copper oxides (or cuprates), and includes dome-shaped regions that correspond to supercond. Moreover, quantum oscillations in the longitudinal resistance of the material indicate the presence of small Fermi surfaces near the correlated insulating states, in analogy with underdoped cuprates. The relatively high superconducting crit. temp. of twisted bilayer graphene, given such a small Fermi surface (which corresponds to a carrier d. of about 1011 per square centimeter), puts it among the superconductors with the strongest pairing strength between electrons. Twisted bilayer graphene is a precisely tunable, purely carbon-based, two-dimensional superconductor. It is therefore an ideal material for investigations of strongly correlated phenomena, which could lead to insights into the physics of high-crit.-temp. superconductors and quantum spin liqs.
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    13. 13
      Izquierdo-García, P.; Fernández-García, J. M.; Medina Rivero, S.; Šámal, M.; Rybáček, J.; Bednárová, L.; Ramírez-Barroso, S.; Ramírez, F. J.; Rodríguez, R.; Perles, J.; García-Fresnadillo, D.; Crassous, J.; Casado, J.; Stará, I. G.; Martín, N. Helical Bilayer Nanographenes: Impact of the Helicene Length on the Structural, Electrochemical, Photophysical, and Chiroptical Properties. J. Am. Chem. Soc. 2023, 145, 11599,  DOI: 10.1021/jacs.3c01088
      13
      Helical Bilayer Nanographenes: Impact of the Helicene Length on the Structural, Electrochemical, Photophysical, and Chiroptical Properties
      Izquierdo-Garcia, Patricia; Fernandez-Garcia, Jesus M.; Medina Rivero, Samara; Samal, Michal; Rybacek, Jiri; Bednarova, Lucie; Ramirez-Barroso, Sergio; Ramirez, Francisco J.; Rodriguez, Rafael; Perles, Josefina; Garcia-Fresnadillo, David; Crassous, Jeanne; Casado, Juan; Stara, Irena G.; Martin, Nazario
      Journal of the American Chemical Society (2023), 145 (21), 11599-11610CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Helical bilayer nanographenes (HBNGs) are chiral π-extended arom. compds. consisting of two π-π stacked hexabenzocoronenes (HBCs) joined by a helicene, thus resembling van der Waals layered 2D materials. Herein, we compare [9]HBNG, [10]HBNG, and [11]HBNG helical bilayers endowed with [9], [10], and [11]helicenes embedded in their structure, resp. Interestingly, the helicene length defines the overlapping degree between the two HBCs (no. of benzene rings involved in π-π interactions between the two layers), being 26, 14, and 10 benzene rings, resp., according to the X-ray anal. Unexpectedly, the electrochem. study shows that the lesser π-extended system [9]HBNG shows the strongest electron donor character, in part by interlayer exchange resonance, and more red-shifted values of emission. Furthermore, [9]HBNG also shows exceptional chiroptical properties with the biggest values of gabs and glum (3.6 x 10-2) when compared to [10]HBNG and [11]HBNG owing to the fine alignment in the configuration of [9]HBNG between its elec. and magnetic dipole transition moments. Furthermore, spectroelectrochem. studies as well as the fluorescence spectroscopy support the aforementioned exptl. findings, thus confirming the strong impact of the helicene length on the properties of this new family of bilayer nanographenes.
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    14. 14
      (a) Chiesa, A.; Privitera, A.; Macaluso, E.; Mannini, M.; Bittl, R.; Naaman, R.; Wasielewski, M. R.; Sessoli, R.; Carretta, S. Chirality-Induced Spin Selectivity: An Enabling Technology for Quantum Applications. Adv. Mater. 2023, 35, e2300472  DOI: 10.1002/adma.202300472
      There is no corresponding record for this reference.
      (b) Bloom, B. P.; Paltiel, Y.; Naaman, R.; Waldeck, D. H. Chiral Induced Spin Selectivity. Chem. Rev. 2024, 124, 1950,  DOI: 10.1021/acs.chemrev.3c00661
      There is no corresponding record for this reference.
    15. 15
      Buendía, M.; Fernández-García, J. M.; Perles, J.; Filippone, S.; Martín, N. Enantioselective Synthesis of a Two-fold Inherently Chiral Molecular Nanographene. Nat. Synth. 2024, 3, 545,  DOI: 10.1038/s44160-024-00484-x
      There is no corresponding record for this reference.
    16. 16
      Morita, F.; Kishida, Y.; Sato, Y.; Sugiyama, H.; Abekura, M.; Nogami, J.; Toriumi, N.; Nagashima, Y.; Kinoshita, T.; Fukuhara, G.; Uchiyama, M.; Uekusa, H.; Tanaka, K. Design and Enantioselective Synthesis of 3D π-Extended Carbohelicenes for Circularly Polarized Luminescence. Nat. Synth. 2024, 3, 774,  DOI: 10.1038/s44160-024-00527-3
      There is no corresponding record for this reference.
    17. 17
      (a) Tkachenko, N. V.; Scheiner, S. Optical Stability of 1,1′-Binaphthyl Derivatives. ACS Omega 2019, 4, 6044,  DOI: 10.1021/acsomega.9b00619
      17a
      Optical Stability of 1,1'-Binaphthyl Derivatives
      Tkachenko, Nikolay V.; Scheiner, Steve
      ACS Omega (2019), 4 (3), 6044-6049CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
      The racemization process of various 1,1'-binaphthyl derivs. is studied by quantum calcns. The preferred racemization pathway passes through a transition state belonging to the Ci symmetry group. The energy barrier for this process is independent of solvation, the electron-withdrawing/releasing power of substituents, or their ability to engage in H-bonds within the mol. The primary factor is instead the substituent size. The barrier is thus reduced when the -OH groups of BINOL are replaced by H. There is a drop in barrier also when the substituents are moved from the 2,2' positions to 6,6' where they will not come close to one another in the transition state. Upon removal of the peripheral arom. rings of the binaphthyl system, the biphenyl system undergoes a facile racemization. It is concluded that the optimal means of improving optical stability of 1,1'-binaphthyl systems is the substitution of large bulky groups in the 2,2' positions.
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      (b) Li, S.; Li, R.; Zhang, Y. K.; Wang, S.; Ma, B.; Zhang, B.; An, P. BINOL-like Atropisomeric Chiral Nanographene. Chem. Sci. 2023, 14, 3286,  DOI: 10.1039/D2SC06244E
      There is no corresponding record for this reference.
    18. 18
      Nakanishi, K.; Fukatsu, D.; Takaishi, K.; Tsuji, T.; Uenaka, K.; Kuramochi, K.; Kawabata, T.; Tsubaki, K. Oligonaphthofurans: Fan-Shaped and Three-Dimensional π-Compounds. J. Am. Chem. Soc. 2014, 136, 7101,  DOI: 10.1021/ja502209w
      18
      Oligonaphthofurans: Fan-Shaped and Three-Dimensional π-Compounds
      Nakanishi, Kentaro; Fukatsu, Daisuke; Takaishi, Kazuto; Tsuji, Taiki; Uenaka, Keita; Kuramochi, Kouji; Kawabata, Takeo; Tsubaki, Kazunori
      Journal of the American Chemical Society (2014), 136 (19), 7101-7109CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Using a bottom-up method, we prepd. a series of oligonaphthofurans composed of alternating naphthalene rings and furan rings. The largest compd. (compd. 25) contained 8 naphthalene units and 7 furan units. DFT calcns. revealed that these compds. were fan-shaped mols. and each naphthalene ring was oriented in an alternate mountain-valley fold conformation because of steric repulsion by the hydrogens at the peri-positions. We investigated the optical properties that derived from their fan-shaped and mountain-valley sequences. As the no. of arom. rings of the oligonaphthofurans increased, the peaks of the longest wavelength absorptions in the UV-vis spectra (HOMO-LUMO energy gap) of these compds. steadily red-shifted, although the shapes of spectra were not sustained because of the decreasing molar absorption coeffs. (ε's) of their λmax. We compared our results with those reported for other types of oligoarom. compds. such as acenes 1, ethene-bridged p-phenylenes 2, rylenes 3, oligofurans 4, and oligonaphthalenes 5. The slopes of the plots between the transition energies (HOMO-LUMO energy gap) of the oligoarom. compds. and the reciprocal of the no. of arom. rings indicated that the efficiency of π conjugation of the oligonaphthofurans was comparable with that of linear and rigid acenes and rylenes. The higher-order compds. 22 and 25 aggregated even under high diln. conditions (∼10-6 M).
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    19. 19
      Areephong, J.; Ruangsupapichart, N.; Thongpanchang, T. A Concise Synthesis of Functionalized 7-oxa-[5]Helicenes. Tetrahedron Lett. 2004, 45, 3067,  DOI: 10.1016/j.tetlet.2004.02.105
      There is no corresponding record for this reference.
    20. 20
      (a) Guido, C. A.; Zinna, F.; Pescitelli, G. CPL Calculations of [7]Helicenes with Alleged Exceptional Emission Dissymmetry Values. J. Mater. Chem. C 2023, 11, 10474,  DOI: 10.1039/D3TC01532G
      There is no corresponding record for this reference.
      (b) Zhang, L.; Song, I.; Ahn, J.; Han, M.; Linares, M.; Surin, M.; Zhang, H.-J.; Oh, J. H.; Lin, J. π-Extended Perylene Diimide Double-heterohelicenes as Ambipolar Organic Semiconductors for Broadband Circularly Polarized Light Detection. Nat. Commun. 2021, 12, 142,  DOI: 10.1038/s41467-020-20390-y
      20b
      Pi-Extended perylene diimide double-heterohelicenes as ambipolar organic semiconductors for broadband circularly polarized light detection
      Zhang, Li; Song, Inho; Ahn, Jaeyong; Han, Myeonggeun; Linares, Mathieu; Surin, Mathieu; Zhang, Hui-Jun; Oh, Joon Hak; Lin, Jianbin
      Nature Communications (2021), 12 (1), 142CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
      Despite great challenges, the development of new mol. structures with multiple and even conflicting characteristics are eagerly pursued for exploring advanced applications. To develop high-performance chiral org. semiconducting mols., a distorted π-system is required for strong coupling with circularly polarized light (CPL), whereas planar π-stacking systems are necessary for high charge-carrier mobility. To address this dilemma, in this work, we introduce a skeleton merging approach through distortion of a perylene diimide (PDI) core with four fused heteroaroms. to form an ortho-π-extended PDI double-[7]heterohelicene. PDI double helicene inherits a high dissymmetry factor from the helicene skeleton, and the extended π-planar system concurrently maintains a high level of charge transport properties. In addn., ortho-π-extension of the PDI skeleton brings about near-IR (NIR) light absorption and ambipolar charge transport abilities, endowing the corresponding org. phototransistors with high photoresponsivity of 450 and 120 mA W-1 in p- and n-type modes resp., along with a high external quantum efficiency (89%) under NIR light irradiations. Remarkably, these multiple characteristics enable high-performance broadband CPL detections up to NIR spectral region with chiral org. semiconductors.
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      (c) Cei, M.; Di Bari, L.; Zinna, F. Circularly Polarized Luminescence of Helicenes: A Data-informed Insight. Chirality 2023, 35, 192,  DOI: 10.1002/chir.23535
      20c
      Circularly polarized luminescence of helicenes: A data-informed insight
      Cei, Matteo; Di Bari, Lorenzo; Zinna, Francesco
      Chirality (2023), 35 (4), 192-210CODEN: CHRLEP; ISSN:0899-0042. (Wiley-Liss, Inc.)
      A review. Helicenes are an interesting scaffold for chiroptical properties and in particular circularly polarized luminescence (CPL). In this short review, we collect the luminescence (glum) and absorption (gabs) dissymmetry factors assocd. to the first Cotton effect of the electronic CD (ECD) spectrum. Considering the data for 170 [n]-helicenes (n = 4-11), overall we found reasonable correlations between glum and gabs. Despite a few notable exceptions, this would confirm a similarity in the stereochem. of the ground and emitting excited states for most helicenes. These results may be useful in rationalizing chiroptical data and help chemists in designing new helicene structures with the desired CPL properties.
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    21. 21
      Reger, D.; Haines, P.; Amsharov, K. Y.; Schmidt, J. A.; Ullrich, T.; Bonisch, S.; Hampel, F.; Gorling, A.; Nelson, J.; Jelfs, K. E.; Guldi, D. M.; Jux, N. A Family of Superhelicenes: Easily Tunable, Chiral Nanographenes by Merging Helicity with Planar pi Systems. Angew. Chem., Int. Ed. 2021, 60, 18073,  DOI: 10.1002/anie.202103253
      There is no corresponding record for this reference.
    22. 22
      Zhou, Z.; Fernández-García, J. M.; Zhu, Y.; Evans, P. J.; Rodríguez, R.; Crassous, J.; Wei, Z.; Fernández, I.; Petrukhina, M. A.; Martín, N. Site-Specific Reduction-Induced Hydrogenation of a Helical Bilayer Nanographene with K and Rb Metals: Electron Multiaddition and Selective Rb(+) Complexation. Angew. Chem., Int. Ed. 2022, 61, e202115747  DOI: 10.1002/anie.202115747
      22
      Site-Specific Reduction-Induced Hydrogenation of a Helical Bilayer Nanographene with K and Rb Metals: Electron Multiaddition and Selective Rb+ Complexation
      Zhou, Zheng; Fernandez-Garcia, Jesus M.; Zhu, Yikun; Evans, Paul J.; Rodriguez, Rafael; Crassous, Jeanne; Wei, Zheng; Fernandez, Israel; Petrukhina, Marina A.; Martin, Nazario
      Angewandte Chemie, International Edition (2022), 61 (10), e202115747CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The chem. redn. of π-conjugated bilayer nanographene 1 (C138H120) with K and Rb in the presence of 18-crown-6 affords [K+(18-crown-6)(THF)2][{K+(18-crown-6)}2(THF)0.5][C138H1223-] (2) and [Rb+(18-crown-6)2][{Rb+(18-crown-6)}2(C138H1223-)] (3). Whereas K+ cations are fully solvent-sepd. from the trianionic core thus affording a "naked" 1.3- anion, Rb+ cations are coordinated to the neg. charged layers of 1.3-. According to DFT calcns., the localization of the first two electrons in the helicene moiety leads to an unprecedented site-specific hydrogenation process at the carbon atoms located on the edge of the helicene backbone. This uncommon redn.-induced site-specific hydrogenation provokes dramatic changes in the (electronic) structure of 1 as the helicene backbone becomes more compressed and twisted upon chem. redn., which results in a clear slippage of the bilayers.
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c14544.

    • Synthetic procedures, additional figures/schemes of physical properties and characterization data. Data for R,P-7b (CCDC: 2388461), M-oxa[9]HBNG (CCDC: 2388463) and P-oxa[9]HBNG (CCDC: 2388462) (PDF)

    Accession Codes

    Deposition numbers 2388461–2388463 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via the joint Cambridge Crystallographic Data Centre (CCDC) and Fachinformationszentrum Karlsruhe Access Structures service.


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