While the synthesis of nanographenes has advanced greatly in the past few years, development of their atomically precise functionalization strategies remains rare. The ability to modify the carbon scaffold translates to controlling, adjusting, and adapting molecular properties. Towards this end, here, we show that mechanochemistry is capable of transforming graphitization precursors directly into chlorinated curved nanographenes through a Scholl reaction. The halogenation occurs in a regioselective, high-yielding, and general manner. Density Functional Theory (DFT) calculations suggest that graphitization activates specific edge-positions for chlorination. The chlorine atoms allow for precise chemical modification of the nanographenes through a Suzuki or a nucleophilic aromatic substitution reaction. The edge modification enables modulation of material properties. Among the molecules prepared, corannulene-coronene hybrids and laterally fully π-extended helicenes, heptabenzosuperhelicenes, are particularly noteworthy.
Nanographenes are a fascinating class of molecules with potential in materials science1. Thus, in recent years, great progress has been made in developing rational synthesis of such macromolecules2,3,4. A majority of work, however, relates to planar structures. In comparison, the development of general synthetic methods towards curved structures still remains challenging5,6,7,8,9,10,11,12. Even more demanding is the task of addressing these structures in a precise manner for further scaffold functionalization. Such functionalization opportunity could be crucial in adapting material properties such as redox potentials or solubility to meet a certain requirement for a desired application. Typically, nanographene precursors are modified in a multi-step synthetic procedure to gain entry to a certain functionalized structure. Recently, post-synthesis modification of the nanographene edge has emerged as a valuable alternative. In this approach, the nanographene is first synthesized and then activated for further modification. For instance, substitution of nanographene edge with borylation13,14,15 or perchlorination16,17 allows for installation of functional groups through transition-metal-catalyzed coupling reactions. Despite these advances, selective edge functionalization approaches that can occur in a general and efficient manner remain unknown.
In this work, we report a general and high-yielding synthetic route to non-planar nanographenes with precisely functionalizable scaffolds. In our approach, the edge-chlorination occurs within the same step as the nanographene formation. Therefore, nanographenes do not need to be subjected to the activation step separately and can be directly subjected to the functionalization step after their preparation. Furthermore, the edge-chlorination is selective and thus functionalization occurs in a regioselective manner. Finally, unlike the aforementioned solution-phase reactions, the current strategy relies on an environmentally-friendly mechanochemical (solvent-less) reaction18,19,20,21,22,23,24. In this strategy, alkyne-carbonyl metathesis is first employed to give a key corannulene-based indanone molecule. This species is capable of being transformed into a cyclopentadienone and combined with a variety of aryl alkynes and arynes to form graphitization precursors through a Diels-Alder reaction. In these precursors, corannulene represents an electron-deficient curved fragment whereas the Diels-Alder adduct represents an electron-rich fragment. A mechanochemical Scholl reaction then planarizes the adduct and simultaneously annulates it to the curved fragment. This process is referred to as ‘graphitization’, inspired by the earlier use of this term by Borchardt and co-workers in the context of coronene synthesis25. The graphitization creates a large overall curved molecule. A solution-phase Scholl reaction, on the other hand, leads to an intractable mixture of compounds. Remarkably, the mechanochemical process of graphitization activates the curved fragment for chlorination in such a manner that only the adjacent carbon atoms to the annulated structure become chlorinated. Thus, nanographenes can be chemically modified in a regioselective manner. The modifications allow for tuning the electronic properties. Overall, the developed synthesis is general, efficient, and reproducible. Among the various structures prepared, two types of structures are particularly noteworthy. One in which the smallest non-planar and planar graphene motifs, corannulene26,27 and coronene28, are combined with a maximum of π-overlap and share one benzene ring between them. If the planar fragment is viewed as hexabenzocoronene29, then the two structures share phenanthrene-like three aromatic rings between them. Despite efforts to prepare hybrids of these archetypical motifs in nanographene chemistry30, such an extent of structural overlap is exceptional and allows for documenting the properties of such molecular hybrids. The other is an example of a helicene structure that is completely embedded in a π-envelope. This enhances the repertoire of the laterally fully π-extended superhelicenes31. The members of this material class are rare32,33,34. Overall, therefore, this work harnesses graphitization-induced selective edge chlorination in a mechanochemical Scholl reaction to establish an efficient synthesis of curved nanographenes with unique structures and a precisely functionalizable scaffold which allows for tuning of the molecular properties.
Results and discussion
In the presence of catalytic Lewis or Brønsted acids, the alkyne-carbonyl metathesis35 provides an efficient access to α,β-unsaturated carbonyl compounds. In the present context, corannulene-based phenylene ethynylene 1 and aldehyde 2 were chosen as the reactants (Fig. 1a). The reaction was carried out with a catalytic amount of SbF5 with ethanol as an additive to efficiently convert the reactants into 2,3-disubstituted indanone 3 in a one-pot procedure. Subsequently, radical benzylic bromination and elimination of HBr in one step leads to the formation of cyclopentadienone 4 (Supplementary Fig. 1). A Diels−Alder cycloaddition between 4 and alkyne 5 yields graphitization precursor 6 under neat conditions (Supplementary Fig. 2).
Recently, mechanochemistry has emerged as a potent synthetic tool18,19,20,21,22,23,24. In this approach, mechanical energy through grinding or milling action is used to promote a chemical reaction36. It is sustainable37 with potential for altering chemical reactivity and product selectivity21,38. The Scholl reaction has been particularly successful under mechanochemical conditions39,40,41,42. Thus, precursor 6 was subjected to an iron(III) chloride-mediated mechanochemical Scholl reaction to furnish nanographene 743. The solvent-less reaction was carried out in a mixer mill equipped with a ZrO2 grinder jar and a ZrO2 ball. Sodium chloride was used as a milling auxiliary. The reaction was milled for 1.5 h at a frequency of 30 Hz. A MALDI-TOF mass spectrometry characterization indicated successful graphitization albeit with bis-chlorination (see Supplementary Information for characterization details). The concrete structural evidence came from the single crystal X-ray diffraction analysis which confirmed the number and position of the chlorine atoms on the nanographene scaffold (Fig. 1b). The isolated yield of 7 was consistently found to be ~90% over several reproductions.
In the Scholl reaction, typically, chlorination of the aromatic scaffold is an undesired side reaction leading to minor products which are removed during purifications. It is also uncontrolled. For instance, in a recent work by Borchardt and co-workers, coronene is chlorinated in a mechanochemical Scholl reaction to give a mixture of products containing 1–18 chlorine atoms44. On the other hand, if chlorination is desired, chlorine atoms are introduced after the synthesis of nanographenes (in a post-synthesis modification step) under harsh and toxic conditions with solvents such as CCl4, elevated temperatures, and long reaction times16. Such procedures culminate into total chlorination (perchlorination) of the molecules. Therefore, the observation of a regioselective bis-chlorination adjacent to the positions where the planar and electron-rich fragment annulates to the non-planar and electron-poor fragment was remarkable. Hence, a new series of graphitization precursors were planned to further probe this aspect (Fig. 2a). Through the use of a terminal phenyl acetylene 8, we obtained a 1:2 mixture of two isomers 9 and 10 (Supplementary Fig. 3) which could be separated easily by column chromatography. These isomers differ in the position of the phenyl rings. Upon a Scholl reaction, 9 would lead to only one connection between the planar and the non-planar fragments while 10 would give two annulations exactly as in the case of nanographene 7. If the regioselectivity originates from activation of the corannulene fragment by the planar fragment, 9 should lead only to mono-chlorination and 10 should give the previously observed bis-chlorination. Indeed, 9 and 10 form mono- and bis-chlorinated compounds 11 and 12, respectively, after the Scholl reaction.
To test modularity of the synthesis as well as the general nature of regioselective chlorination, alkyne precursors were replaced by two aryne precursors in the Diels-Alder reaction (Fig. 2b). In these cases, the aryne, due to its high reactivity, was generated in-situ in a microwave-assisted fluoride-induced 1,2-elimination of ortho-silylaryltriflates reaction45. In one case, benzyne 13 was employed whereas corannulyne 14 was used in the other46. 13 led to the synthesis of 15 (Supplementary Fig. 4) with a naphthalene nucleus while 14 led to the formation of a bis-corannulene system 17. A mechanochemical Scholl reaction then produced 16 and 18, respectively from precursors 15 and 17. Consistent with the previous results, mono-chlorination occurred in 16 (Supplementary Fig. 5) and bis-chlorination in 18. Once again, despite the presence of the naphthalene unit, the chlorination occurred only adjacent to the aromatic fusion place on the corannulene nucleus. These results confirm the regioselective nature of chlorination and the generality of the synthesis.
Regioselectivity and aromaticity
Intrigued by the chlorination results, a computational study at the dispersion corrected B3LYP-D3/def2-SVP level was carried out to gain more insight into the regioselectivity of the process. To this end, we computed the preferred chlorination sites in the representative model precursors 7′ and 18′ (leading to the observed chlorinated compounds 7 and 18, respectively, where the t-Bu groups were replaced by methyl groups) by means of the corresponding condensed Fukui functions (fk-)47. These reactivity indicators were chosen because they have been successfully used in the past to rationalize the regioselectivity of related electrophilic substitutions including chlorination and bromination reactions48,49. As depicted in Fig. 3a, the positions adjacent to the newly formed bonds between the planar and the curved corannulene fragments exhibit the highest fk− values and therefore are the most susceptible positions to be chlorinated, which is consistent with the experimental findings.
To further gain understanding of these results, the aromatic nature of the newly prepared systems and their associated ring currents were studied50,51,52. For this, we first computed the variation of the Nuclear Independent Chemical Shift (NICS)53 values of the different rings in going from the parent cyclopentadienone 4 to model nanographenes 7′ and 18′. As graphically shown in Fig. 3b and Supplementary Fig. 6, the aromaticity of the rings belonging to the corannulene moiety in 4 remains essentially unaltered upon the fusion in the corresponding nanographenes. Interestingly, in all cases, whereas the central six-membered ring fused to the corannulene fragment (i.e. that formed during the Diels-Alder reaction) is aromatic, the six-membered rings fused to the edge corannulene rings (i.e. those formed during the Scholl reaction) become clearly antiaromatic. This is consistent with the Anisotropy of the Induced Current Density (AICD)54,55 method, which nicely confirms the occurrence of a paratropic (i.e. antiaromatic) ring current in these fused six-membered rings (Fig. 3b). Strikingly, these antiaromatic rings are fused to the six-membered rings of the corannulene where the chlorination takes place, therefore suggesting a strong correlation between aromaticity and reactivity.
Solution-phase Scholl reaction
At this point, we became interested in a comparison of the mechanochemical reaction with the traditional solution-phase reaction. For this, the graphitization reaction of the precursor 6 was carried out under typical Scholl conditions using DDQ/TfOH, DDQ/CH3SO3H, and FeCl3 in dichloromethane as a solvent. Unfortunately, the solution-phase reaction reproducibly produced an intractable reaction mixture. In 1H NMR spectroscopy, the crude spectrum showed a number of signals in the area of 6–10.5 ppm indicating a diverse reaction mixture (Supplementary Fig. 7).
The regioselective chlorination offered an opportunity to functionalize the nanographene scaffold. For this, initially, a palladium-catalyzed Suzuki reaction with boronic acid 19 was carried out and afforded the product 20 in good isolated yield (41%) (Fig. 4a)56. X-ray crystallography confirmed the structure of the bis-coupled product (Fig. 5a). Encouraged by these results, nucleophilic substitution reaction on 7 was targeted. Scott and co-workers have established the utility of such reactions on the corannulene scaffold to achieve the synthesis of thioethers57. By using thiocresol as the nucleophile, the synthesis of bis-thioether derivative 21 was achieved in an isolated yield of 72%. In this way, the regioselective chlorination allows for further chemical modification of the nanographene structure.
During the purification of 20, trace side-products 22 and 23 were obtained which are likely to be formed through dehalogenation reaction (Fig. 4b). The isolation of these side-products was useful as it provided the fundamental corannulene-coronene hybrid and the intermediate mono-phenyl structure for comparing the structure-property relationship in the series 23, 22, and 20 (Supplementary Fig. 8 and Table 1). Finally, since the Suzuki reaction has a broader scope, all the chloride substrates (11, 12, 16, 18) were subjected to coupling with the boronic acid partner 19 to give expected products 24, 25, 26, and 27 (Figs. 4b and 5b–e, and Supplementary Fig. 9). X-ray crystallographic analyses indicated the bowl depth to be relatively deeper for the nanographenes (e.g. 1.00 Å for 23) than the parental bowl motif corannulene (0.87 Å) (Fig. 5 and Supplementary Table 1)58. Compounds 18 and 27 are unique as they are fully π-extended—super – helicene structures mentioned before (Fig. 5f).
In UV-Vis spectroscopy, corannulene and coronene display broad absorption bands below 400 nm59,60. In contrast, the nanographenes display several moderate-intensity absorption bands in the 400–550 nm region (Fig. 6a and Supplementary Figs. 10–22). This 150 nm bathochromic shift is a reflection of the extended π-conjugation in the nanographenes. The most red-shifted band is observed at 510 nm for the thioether-functionalized nanographene 21. The electron-donating nature of the sulfur atoms in 21 can rationalize the bathochromic shift and indicates an opportunity for varying the band-gap through post-synthesis functionalization approach. In 20 and 27, the absorption bands are red-shifted when compared to 23 and 18, respectively. Furthermore, both bis-functionalized nanographenes, 20 and 21, display tailing of absorption until 650 nm (Supplementary Figs. 8 and 15–16). These results indicate that the (thio)phenyl rings can electronically communicate with the main aromatic scaffold and underline the importance of functionalization in tuning the optical properties. The use of phenyl groups carrying electron donors (such as N,N-dimethylaniline) is expected to lead to a stronger bathochromic shift. In fluorescence emission spectroscopy, the nanographenes are emissive in the blue-green region of the electromagnetic spectrum (Fig. 6b and Supplementary Figs. 10–22).
Time-dependent (TD) Density Functional Theory (DFT) calculations were carried out on the model compounds 7′ and 18′ (where the t-Bu groups were replaced by methyl groups) to determine the nature of the electronic transitions associated with the observed UV/Vis absorptions located at 500–550 nm. Our TD-DFT calculations (TD-B3LYP-D3/def2-SVP level) reproduce the occurrence of an electronic transition at ca. 500 nm (λcalc = 481 and 467 nm, for 7′ and 18′ respectively). This transition is the result of the one-electron promotion from HOMO to LUMO, which can be viewed as π-molecular orbitals delocalized along the entire system (Supplementary Fig. 23), therefore confirming the extension of the π-conjugation in the newly prepared nanographenes. Moreover, the HOMO→LUMO transition in 21 is significantly red-shifted (lcalc = 540 nm) with respect to the analogous transition involving 7′ or 18′, which is consistent with the experimental findings and a direct consequence of the involvement of the lone-pair of the sulfur atoms in the corresponding frontier molecular orbitals (Supplementary Fig. 23).
Würthner’s curved imides61,62,63,64 and Scott and Itami’s warped nanographenes65 represent elegant examples of electron-poor nanographenes. In this context, the electrochemical behavior of the synthesized nanographenes was studied with the help of square-wave voltammetry (SWV) and cyclic voltammetry (CV) in dichloromethane. These results indicated that all nanographenes are electron deficient molecules likely due to molecular curvature as is exemplified by the parental motif corannulene66 (Fig. 6c and Supplementary Figs. 24–27). Overall, the first one-electron reduction occurs in the region of –1.8 to –2.0 V vs Fc/Fc+ (Fc = ferrocene), 300–500 mV more easily than parental corannulene66. The chlorinated structures show limited chemical reversibility of the first reduction process, in the sense that the oxidative (ipox) to reductive (ipred) peak current ratios (ipox/ipred) were «1 (at a scan rate of 0.1 V s−1) indicating chemical instability of the reduced compounds. However, as seen in modified cases for compounds 20–27 (Fig. 6c) whose CVs have ipox/ipred-ratios that approach unity, functionalization is a means to improve the lifetime of the reduced compounds.
The nanographenes also display a second reduction process at potentials ~200–400 mV more negative, with a similar current magnitude to the first reduction process, suggesting a second one-electron reduction to form the dianions. However, only compound 27 displays an ipox/ipred-ratio close to unity for the second one-electron transfer process, indicating that only this nanographene forms a dianion that has a lifetime in solution of at least several seconds. Nanographene 27 is symmetrical and conjugated, thus the difference in potential between the first two one-electron reduction processes can be used to estimate the comproportionation equilibrium constant (Kc) for the internal electron transfer (Eq. 1)67, where E01 and E02 are the formal potentials (that approximate the standard electrode potentials) of the first and second reduction processes, respectively, F is the Faraday constant (96485 C mol−1), R is the gas constant (8.3143 J K−1 mol−1) and T is the temperature (in K). Using a value of 0.2 V for the separation between the first two reduction processes for nanographene 27 leads to a Kc value of approximation 2.4 × 103, indicating intermediate internal electronic interactions between the two symmetrical halves of the molecule.
Overall, it is anticipated that electron-withdrawing substituents such as cyano or trifluoromethyl groups on the phenyl ring can further enhance the electron affinity of the nanographenes. Furthermore, large nanographenes such as 27 present some interesting materials for future electrochemical studies.
In summary, alkyne-carbonyl metathesis provides a corannulene-based indanone molecule which paves the way to a cyclopentadienone moiety useful in a Diels-Alder reaction with a variety of alkynes and arynes to give graphitization precursors. An iron(III) chloride-mediated mechanochemical Scholl reaction then annulates the Diels-Alder adducts to the corannulene nucleus. Interestingly the graphitization process activates the adjacent positions for a chlorination reaction. Thus, chlorinated curved nanographenes are obtained after the graphitization step. The isolated chemical yields for all the involved chemical processes are >60%. Each reaction is carried out multiple times and provides isolated yields within a similar range. Therefore, overall, the developed synthetic route can be described as general, efficient, and reproducible. Our DFT calculations indicate that the regioselectivity of the chlorination reaction can be rationalized by means of the condensed Fukui functions, which indicate that the edge corannulene rings bear the most reactive positions. In addition, it is found that aromaticity also plays a role in controlling regioselectivity. The regioselective chlorination offers an opportunity to functionalize the nanographene scaffold in an atomically precise manner. This could be achieved with the help of a boronic acid or a thiol molecule. Such functionalization strategy which efficiently alters the chemical structure and material properties can be potentially harnessed to prepare much stronger electron acceptors by placing electron-withdrawing groups on nanographenes. Alternatively, the placement of electron donating groups is expected to lead to red-absorbing/emitting nanographenes. A future goal would also be to render the nanographenes soluble in water for biological applications by placing polyethylene glycol (PEG) chains through the use of commercially available thiol-end-functionalized PEGs. It is anticipated that such constructs would be of interest as bio-relevant radical scavengers and antioxidants due to the electron-deficient nature of the curved aromatic scaffold.
All data supporting the findings of this study are available within the article and Supplementary information files, and also are available from the corresponding author upon request. The X-ray crystallographic coordinates for structures reported in this study have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition numbers CCDC 2202286 (compound 4), CCDC 2202106 (compound 6), CCDC 2234615 (compound 7), CCDC 2202097 (compound 10), CCDC 2202098 (compound 15), CCDC 2202099 (compound 17), CCDC 2202100 (compound 20), CCDC 2202101 (compound 22), CCDC 2202102 (compound 24), CCDC 2202103 (compound 25), CCDC 2202104 (compound 26), and CCDC 2202105 (compound 27). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary Data 1 contains the cartesian coordinates of the structures.
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Financial support from the Ministry of Education Singapore under the AcRF Tier 1 (MOE T1 RG11/21) and AcRF Tier 2 (MOE-T2EP10221-0002), M.C.S., and the Spanish MCIN/AEI/10.13039/501100011033 (Grants PID2019-106184GB-I00 and RED2018-102387-T), I.F., are gratefully acknowledged.
The authors declare no competing interests.
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Stanojkovic, J., William, R., Zhang, Z. et al. Synthesis of precisely functionalizable curved nanographenes via graphitization-induced regioselective chlorination in a mechanochemical Scholl Reaction. Nat Commun 14, 803 (2023). https://doi.org/10.1038/s41467-023-36470-8
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