Oxidative cyclo-rearrangement of helicenes into chiral nanographenes

Nanographenes are emerging as a distinctive class of functional materials for electronic and optical devices. It is of remarkable significance to enrich the precise synthetic chemistry for these molecules. Herein, we develop a facile strategy to recompose helicenes into chiral nanographenes through a unique oxidative cyclo-rearrangement reaction. Helicenes with 7~9 ortho-fused aromatic rings are firstly oxidized and cyclized, and subsequently rearranged into nanographenes with an unsymmetrical helicoid shape through sequential 1,2-migrations. Such skeletal reconstruction is virtually driven by the gradual release of the strain of the highly distorted helicene skeleton. Importantly, the chirality of the helicene precursor can be integrally inherited by the resulting nanographene. Thus, a series of chiral nanographenes are prepared from a variety of carbohelicenes and heterohelicenes. Moreover, such cyclo-rearrangement reaction can be sequentially or simultaneously associated with conventional oxidative cyclization reactions to ulteriorly enrich the geometry diversity of nanographenes, aiming at innovative properties.

F ollowing the impressive development of graphene materials 1,2 , nanographenes with size limited in the nanometer scale are currently emerging as prominent semiconducting substances due to their open bandgap 3 , which is highly useful for electronic devices 4 , energy storage 5 , and sensors 6 . Conventionally, nanographenes are prepared by fragmentation of relatively large-size carbon derivatives 7,8 . Although the fabrication procedures are normally simple and ready to scale up, it is of enormous difficulty to make atomistically precise products. To this end, "bottom-up" synthetic strategies are being developed to construct nanographenes with accurate and welltunable molecular structures 3 . Amongst, intramolecular cyclization of tailor-made dendritic oligophenylenes plays an indispensable role as a result of its high conversion and broad scope (Fig. 1a, left). This generally involves graphitization through Scholl-type oxidative cyclization 9 , photocyclization 10 , and HF zipping reactions 11 . On the other hand, annulative π-extension (APEX) 12,13 , alkyne benzannulation 14,15 , and aryne cyclotrimerization 16 reactions, either by enlarging a polycyclic aromatic core or fitting together aromatic fragments, are typical alternative pathways to complex nanographenes (Fig. 1a, right). These advances have provided an access to a vast number of nanographenes with relatively regular planar, curved, and helical geometries. However, the synthesis of more sophisticated and distinctive nanographenes remains a remarkable challenge as a consequence of the fact that most current reactions only allow the fusion of aromatic fragments into a larger system within a constant geometry regime.
Helicenes are a type of chiral polycyclic aromatic molecules constituted by ortho-fused benzenoid cycles [17][18][19] . The distorted sp 2 -carbon skeleton renders helicenes with relatively higher chemical activities. Thus, helicenes undergo intramolecular cyclization through Diels-Alder reaction into 3D bridge-ring frameworks 20 or deform into nearly planar structures through oxidative cyclization 21 . Recent studies by scanning tunnel microscopy showed that the skeleton of helicene could transform on metal surfaces 22,23 . This provides a unique portal to manipulate the aromatic skeletons and to construct nanographenes.
Herein, we aim to precisely harness the skeletal reconstruction of helicenes under a high-energy state generated by controlled oxidation in solution (Fig. 1b). We report the formation of chiral nanographenes through enantio-persisting oxidative cyclo-rearrangement of a variety of helicenes and study the skeleton transformation of the helicene precursors and the properties of the resulting chiral nanographenes with the assistance of theoretical calculations. This is followed by the extension of the aromatic structures through sequential or simultaneous combination with conventional oxidative cyclization reactions.

Results and discussion
Oxidative cyclo-rearrangement of carbohelicenes. It has been reported previously that [5]helicene and [6]helicene derivatives undergo Scholl cyclization upon oxidation and form planar or negatively curved structures (Fig. 2a) 24,25 . Initially, we explored the oxidative reactions of primitive carbo [6]helicene 6H in the presence of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and various acids (e.g., CF 3 COOH, CH 3 SO 3 H, CF 3 SO 3 H) ( Fig. 2b and Supplementary Table 1). However, all the attempts ended with unreacted 6H or a mess of unidentifiable products, indicating an uncontrollable oxidation process. Then, we shifted our focus to carbo [8]helicene 8H, a longer helicene with the two ends fully overlapping with each other. Although the reactions conducted under less acidic conditions (in the presence of CF 3 COOH or CH 3 SO 3 H) led to similar undesired products, the mixture containing triflic acid (CF 3 SO 3 H) predominantly gave rise to a highly fluorescent compound (O8H, oxidized product for 8H) with a yield of 49% ( Fig. 2b  ). X-ray diffraction of a single crystal further confirmed the formation of an unsymmetric chiral π-conjugated system in which a benzo[ghi]perylene moiety was fused with a [6]helicene skeleton (Fig. 2b). Further studies on the oxidation of enantiopure 8H precursors showed that the helical chirality of 8H was integrally transferred to O8H, where P-8H formed exclusively P-O8H and M-8H solely gave rise to M-O8H ( Supplementary Fig. 96).
Density functional theory (DFT) calculations on the possibly involved intermediates and transition states indicated that the above oxidation reaction may involve four major steps (Fig. 2d). First, 8H undergoes an intramolecular oxidative cyclization (between C 1 and C 20c ) with the assistance of DDQ and CF 3 SO 3 H and transforms into a cationic intermediate 8H-IM1 with a newly-formed hexagonal cycle in the center through either a radical cation or an arenium cation pathway 26,27 . Then, the cationic 8H-IM1 reforms into a spiro intermediate 8H- distorted structure 28 . Subsequently, 8H-IM2 transforms into a helical cationic intermediate 8H-IM3 through a second 1,2migration via a homoallylic transition state 8H-TS2 29,30 . Notably, the migration might undergo through a homobenzylic pathway to afford a less crowded outward product but this is probably disfavored by relatively higher activation energy ( Supplementary  Fig. 104). Importantly, the helical chirality of 8H is integrally recorded by the spiro moiety in the first 1,2-migration and intactly recovered in the second 1,2-migration, thus keeping the memory of its chirality. Finally, deprotonation of cationic 8H-IM3 leads to the formation of a relatively stable neutral product O8H. It appears that the electron-withdrawing substituents destabilizes the initial cation and hence favor the formation of β structures, while the electron-donating substituents stabilized the cationic moiety and thus promote the formation of α structures.  Fig. 2 Oxidative cyclo-rearrangement of carbo [8]helicenes. a Cyclization of [5] and [6]helicene derivatives. b Oxidation of primitive 6H and 8H by 2,3dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in various acidic conditions. The crystal structure of O8H is shown in thermal ellipsoid with a 30% probability. c Oxidative cyclo-rearrangement of substituted carbo [8]helicenes. d Proposed mechanism for the oxidative cyclo-rearrangement of 8H. Free energies are given in kcal mol −1 and the free energy of 8H is set as 0.
The fusion of the benzo[ghi]perylene and helicene moieties in an absolute chiral fashion provides a distinctive platform to tune the properties of nanographenes. Most intuitively, these molecules showed a combined aromaticity nature of benzo [ghi] perylene and helicene according to the harmonic oscillator model of aromaticity (HOMA) 34 (Fig. 4a) and the nucleus-independent chemical shifts (NICS) 35 (Fig. 4b). Typically, O7H, O8H, and O9H showed high aromaticity in rings B, C, and F as a benzo [ghi] perylene fashion, and also in the terminal ring (H, I, and J, respectively) as a helicene feature. Besides, the newly formed heptagonal cycle of OO8H presented a typical anti-aromaticity nature (HOMA = − 0.35, and NICS(0) = 12.2). Regarding the optical properties, O7H, O8H, and O9H displayed intense benzo [ghi]perylene-type absorption bands with several vibronic progression in the region of 400-500 nm (Fig. 4c). These bands can be majorly attributed to the HOMO → LUMO transitions with the optical bandgaps of 2.59, 2.54, and 2.40 eV, respectively, which are comparable with the electrochemical bandgaps (2.54, 2.49, and 2.41 V, respectively, Fig. 4f). The enantiopure O8H and O9H molecules revealed intense mirror-image electronic circular dichroism (ECD) signals with typical helicene characteristics, where for the P enantiomers, positive Cotton effects were formed in a longer wavelength region (350-500 nm), and negative ones were formed in shorter wavelengths (250-350 nm) (Fig. 4c). These molecules displayed blue to yellow emissions (Fig. 4e), and the enantiopure O8H and O9H products showed intense circularly polarized luminescence (CPL) with prominent dissymmetric factors g lum values 36,37 of ±0.0055 and ±0.012, respectively (Fig. 4d).
Oxidative cyclo-rearrangement of heterohelicenes. Heteroatomdoping is an efficient and practical method to tailor the physical and chemical properties of polycyclic aromatic molecules 38,39 . To this end, we prepared sulfur-doped or/and nitrogen-doped methylthio [8]helicene 8H mt , aza [8]helicene 8H a , and thioaza [8] helicene 8H ta . In a typical oxidative cyclo-rearrangement condition with one equivalent of DDQ, we observed that 8H mt transformed not only into an oxidized product O8H mt but also into a further oxidized product OO8H mt (Fig. 5a). O8H mt showed an O8H-like structure with the thiophene cycle located at the singlestranded helicoid end, while OO8H mt displayed a negatively curved feature caused by the heptagonal cycle. Due to the doping of sulfur, OO8H mt revealed considerably bathochromic-shifted absorption and emission bands compared to OO8H (Fig. 5b, c). 8H a and 8H ta also transformed into the rearranged products O8H a and O8H ta , respectively (Fig. 5d, e). It should be noted that protonation may occur to the nitrogen atoms, but obviously, this did not hinder the following oxidative cyclo-rearrangement, and the products can be neutralized by NaOH at the end. This protonation chemistry was further utilized to switch the chiroptical properties of enantiopure O8H a (Supplementary Fig. 98, Fig. 5f, g), and remarkably the protonated product O8H a H + showed a  Fig. 3 Stepwise oxidations of 7H, 8H, and 9H. a Two-step oxidation of 7H, 8H, and 9H. b Possible cyclization processes following a radical cation mechanism and their activation energies (free energies of the corresponding transition states, TS, in kcal mol −1 ). The free energies of the radical cation intermediates O7H ·+ , O8H ·+ , O9H ·+ are set as 0.
significantly bathochromic-shifted CPL compared to the neutral product O8H a (orange vs. green).
Fabrication of more sophisticated nanographenes. The oxidative cyclo-rearrangement reactions were ultimately associated with other reactions to fabricate more sophisticated nanographenes with distinctive geometries and properties. We first prepared an oligophenylene-like helicene derivative 6H DPT with a diphenyltriphenylene (DPT) group substituted on position 2 of 6H. Upon treatment with an excessive amount of DDQ in the presence of CF 3 SO 3 H, 6H DPT directly transformed to O6H DPT which showed a [6]helicene moiety fused to a slightly arched π-extended structure with a length of ca. 1.5 nm (Fig. 6a, b) 40 . This was probably generated by the formation of a [8]helicene moiety at first via Scholl cyclization followed by an oxidative cyclo-rearrangement at the more πextended side. The highly extended π-system rendered O6H DPT with a strong absorption band around 490 nm and an intense greenish-yellow fluorescence around 510 nm (Fig. 6c). We also synthesized another bromo-substituted product O8H' Br by treating 2-bromo [8]helicene (8H' Br ) with DDQ and CF 3 SO 3 H and subsequently fused it with a perylene diimide (PDI) moiety through boration and Suzuki coupling. Oxidative photocyclization of the resulting product O8H' PDI finally afforded a dark brown product CO8H' PDI (Fig. 6d).
Single-crystal analysis revealed that CO8H' PDI possessed a bilayer conjugated structure [40][41][42] with the benzo[ghi]perylene and PDI moieties fused by a [8]helicene bridge (Fig. 6e). The two layers showed a typical AB stacking pattern with a short interlayer distance of 3.4 Å. The unsymmetric bilayer structure presented a strong intramolecular charge transfer complexation with the HOMO located at the benzo[ghi]perylene layer and the LUMO at the PDI layer (Fig. 6g), leading to a broad absorption band from ca. 570 to 710 nm (Fig. 6f). Besides, the CO8H' PDI molecules were found to be tightly piled up in the crystals with the PDI moiety of one molecule and the benzo[ghi]perylene fragment of the neighboring molecule packed in an AB stacking manner with a close distance of ca. 3.6 Å (Fig. 6e).
In summary, we have developed a distinctive oxidative cyclorearrangement method to recompose the skeleton of helicenes to precisely fabricate chiral nanographenes with unique topologies and absolute configurations integrally inherited from their helical precursors. The facile introduction of a variety of substituents at the periphery and heteroatoms in the aromatic core, along with the sequential oxidative cyclo-rearrangement reactions, have provided diverse access to a rich array of nanographenes. The flexible association with other conventional oxidative cyclization reactions has further allowed the designable fabrication of more sophisticated chiral nanographenes. Overall, this work demonstrates a unique route to activate and reform a highly distorted aromatic system (e.g., helicenes) into flattened but lateral extended derivatives, which is virtually driven by the gradual release of the internal strains. The design of analogous energyreservoir precursors together with controlled skeletal reconstruction reactions may open up an innovative pathway for future nanographene synthesis.

Methods
Typical oxidative cyclo-rearrangement reaction. 21.4 mg of 8H (0.05 mmol) and 13.6 mg of DDQ (0.06 mmol, 1.2 equiv.) were suspended in 20 mL of dried CH 2 Cl 2 under argon at 20°C, and 0.1 mL of CF 3 SO 3 H was added to the suspension dropwise with vigorous stirring. The mixture was stirred for 30 min and then quenched by 5 mL of saturated NaHCO 3 aqueous solution. The mixture was extracted by CH 2 Cl 2 and the organic layer was washed with water twice and dried over Na 2 SO 4 . The solvent was removed at reduced pressure and the residue was purified by silica column chromatography (heptane/CH 2 Cl 2 = 10/1, v/v) to afford O8H as orange solid (10.5 mg, yield 49%).
Theoretical calculations. DFT calculations were carried out using Gaussian 09 program. Geometrical optimization calculations were carried out at the PBE0-D3/ def2-SVP level without any symmetry assumptions unless otherwise stated. The Cartesian coordinates of the optimized geometries are provided in Supplementary Data 1. Harmonic vibration frequency calculations were performed at the same level for obtaining the thermodynamic energies and verifying the resulting geometries as local minima (with all the frequencies real) or saddle points (with only one imaginary frequency). The assignment of the saddle points was performed using the intrinsic reaction coordinate (IRC) calculations. Nucleus-independent chemical shifts (NICS) values were calculated at the GIAO-PBE0/def2-SVP level on the optimized structures. Time-dependent density functional theory (TD-DFT) calculations were performed at the PBE0-D3/def2-SVP level on the optimized geometries for the lowest 200 vertical singlet electronic excitations. No special shift or scaling was applied unless otherwise mentioned. Isosurfaces of molecular orbitals (MOs) were drawn using the IQmol program with the isovalue set to 0.1. All calculations were performed under the SMD continuum solvent model for CH 2 Cl 2 .

Data availability
The authors declare that all the data supporting the findings of this study are available within the paper and Supplementary Information files, and also are available from the corresponding authors upon reasonable request. The X-ray crystallographic coordinates for structures reported in this study have been deposited    Fig. 6 Synthesis of more sophisticated nanographenes. a Synthesis of O6H DPT through the association of oxidative cyclo-rearrangement with Scholl cyclization. b Crystal structure of O6H DPT . c UV-vis absorption (red) and fluorescence (yellow) spectra of O6H DPT (in CH 2 Cl 2 , c = 2.0 × 10 −5 mol L −1 ). d Synthesis of O8H' Br via oxidative cyclo-rearrangement and sequential formation of CO8H' PDI via further fusion with a PDI moiety. e Stacking of CO8H' PDI molecules in a crystal (alkyl side chains and hydrogen atoms are omitted for clarity). f UV-vis absorption spectrum of CO8H' PDI (in CH 2 Cl 2 , c = 2.0 × 10 −5 mol L −1 ). Calculated excitation energies and oscillator strengths of CO8H' PDI-CH3 (the alkyl side chains are replaced with methyl groups for simplicity) are displayed as sticks (the first 10 excitations). g HOMO and LUMO of CO8H' PDI-CH3 .