Helical Bilayer Nonbenzenoid Nanographene Bearing a [10]Helicene with Two Embedded Heptagons

Abstract The precision synthesis of helical bilayer nanographenes (NGs) with new topology is of substantial interest because of their exotic physicochemical 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 analysis reveals its highly twisted bilayer geometry with a record small interlayer distance of 3.2 Å among the reported helical bilayer NGs. Notably, the close interlayer distance between the two layers offers intramolecular through‐space conjugation as revealed by in situ spectroelectrochemistry studies together with DFT simulations. Furthermore, the chiroptical properties of the P/M enantiomers of HBNG1 are also evaluated by circular dichroism and circularly polarized luminescence.

For instance, Martín's group reported the helical bilayer nanographene (NG) with pristine [10]helicene backbone bearing two hexa-peri-hexabenzocoronene as end-units (III, Figure 1b). The rigidity of the helicene linker forces the two NG layers to adopt a bilayer geometry, [9] which not only maintains the π-conjugation through the molecule but also bestows the resultant π-system with unique chirality. Larger NG helicoids bearing central π-extended helicenes or an increasing number of NG blades leads to enhanced πdelocalization in novel aromatics. [8b, 10] Despite the very few examples of helicene-bridged bilayer NGs that have been reported, [9,11] the helical bilayer NG containing nonbenzenoid substructure is essentially unknown so far mostly due to the lack of a proper synthetic strategy to incorporate nonhexagonal rings into the higher helicene backbone.
Herein, we report the synthesis of the first example of a helical bilayer nonbenzenoid nanographene (HBNG1) containing a [10]helicene with two embedded heptagons as a novel chiral moiety (Figure 1c). The second layer in HBNG1 is well constructed based on the predesigned precursor 5 with defined conformation by an efficient Scholl reaction. HBNG1 is composed of 35 fused rings with one pentagon and two heptagons. The introduction of the second heptagon largely overcomes the steric hindrance within the inner helical rim to form this highly congested bilayer structure. Single-crystal X-ray diffraction (SCXRD) analysis of HBNG1 clearly elucidates its highly twisted bilayer structure with a close interlayer distance of 3.2 Å. The extensive intramolecular interaction between the two layers is demonstrated by 2D NMR spectroscopy and density functional theory (DFT) simulation. The embedded heptagons exhibit large nonplanarity values and strong antiaromatic characters. Cyclic voltammetry (CV) analysis indicates its amphoteric redox properties and its oxidized states are fully investigated by in situ spectroelectrochemistry (SEC). More interestingly, the small interlayer distance in HBNG1 induces the unique intramolecular through-space interaction between the two layers, which is clearly revealed by the SEC measurements together with the calculations. Furthermore, the P/M enantiomers of HBNG1 are resolved by chiral highperformance liquid chromatography (HPLC), and their chiroptical properties are studied by electronic circular dichroism (ECD) and circularly polarized luminescence (CPL). This study will stimulate the design and synthesis of other novel helical bilayer or multilayer NGs incorporated with nonbenzenoid substructures.
The synthesis of HBNG1 starts from heptagon-embedded super [6]helicene 2 that has been described in our previous report (Scheme 1). [12] First, the iodine-substituted compound 2 was coupled with 4-tert-butylphenylacetylene through a Sonogashira reaction to give the intermediate 3 in 82 % yield. Subsequent Diels-Alder reaction between 3 and 2,3,4,5-tetrakis(4-tert-butyl-phenyl)cyclopentadienone (4) afforded the key precursor 5 in 52 % yield. After that, the Scholl reaction of 5 using DDQ/TfOH at 0°C for 30 minutes provided the HBNG1, together with partially cyclized and oxidized byproducts, resulting in difficult purification (Figure S1). Based on this result, we reduced the reaction time to 15 minutes, yielding a mixture of HBNG1 and partially cyclized compounds without the presence of unwanted oxidized byproducts. After a simple work-up, the obtained crude residue was further treated under the same reaction conditions. To our delight, the partially cyclized byproducts were fully converted and the target HBNG1 was isolated as a red solid with a yield of 31 % over two steps.
The formation of HBNG1 from precursor 5 was first confirmed by matrix-assisted laser desorption/ionizationtime-of-flight mass spectrometry (MALDI-TOF MS). The experimental mass spectrum shows the desired ion [M] + with an error of + 1.32 ppm to the theoretical value and matches the simulated isotopic distribution pattern perfectly ( Figure 2a). Notably, HBNG1 exhibits good solubility in common organic solvents, such as n-hexane, DCM, THF and toluene, owing to its highly twisted geometry. The structure of HBNG1 was then fully characterized by means of 1 H, 13 C and 2D NMR experiments ( Figure S3-11). The proton sequence was determined by evaluating long-range correlations (lr-COSY) and through-space correlations observed by rotating-frame nuclear Overhauser effect spectroscopy (ROESY). Interestingly, ROESY correlations between both layers were observed ( Figure 2b and Figure S7). For example, the phenyl-hydrogens H 11 (8.31 ppm), H 12 (8.22 ppm) and H 13 (8.61 ppm) located on the plane of centroid rings C and D (Figure 3c) show ROESY correlations to the hydrogens of the tert-butyl group t Bu 28/29 (1.95 ppm) on the plane of centroid ring I, indicating the close distance between the two layers (Scheme 1 and Figure 2b).
A suitable crystal was grown by slow vapor diffusion of methanol into a solution of HBNG1 in chlorobenzene. [13] SCXRD analysis revealed its unique helical bilayer geometry bearing two heptagons in the [10]helicene of the inner rim ( Figure 3a and Figure S12). Three distinct NG blades were laterally fused to a [10]helicene, forming a twisted bilayer nonbenzenoid structure ( Figure 3b). The average perpendicular distance between the centroid rings C and H in HBNG1 equals 3.24 Å (Figure 3a and Figure S13), [14] which is a record small distance among those of the reported helical bilayer NGs, as a result of the embedded heptagons. The angle between the terminal rings A and J in the inner [10]helicene (θ = 21.64°) indicates a more parallel arrangement compared with the reported III (θ = 42.76°) (Figure 3c), [9] suggesting the highly distorted geometry of HBNG1. In addition, the angle between the two planes of the centroid rings C and H is determined as 9.9° (Figure S12), much larger than the value in molecule III (2.04°). [9] Remarkably, an average torsion angle of 29.6°is found in the inner rim of HBNG1, and the highest torsion angle is 49.5°near the heptagonal ring G (C14À C25À C31À C44, marked in blue in Figure 3c), which is one of the largest angles among the reported heptagonembedded helical nanographene. [3,12,15] The deformation of CÀ CÀ C bond angles away from the natural value of 120°a lso illustrates the highly twisted geometry, ranging from 106.2°to 132.4°in the azulene and heptagon units (Figure S14). The nonplanarity value [16] of the embedded heptagonal ring G equals 0.243 Å (Figure 3d), revealing a highly distorted ring conformation. Furthermore, the CÀ C bond lengths in the heptagonal ring G lie within the range of 1.41-1.50 Å, which are significantly longer than that of typical C(sp 2 )À C(sp 2 ) bond in benzene (1.39 Å). In addition, HBNG1 crystallizes in the I 4 1 /a space group with a pair of M-and P-helicenes. In the packing model, the two adjacent molecules are almost located at a perpendicular angle to each other due to the CÀ HÀ π interactions (the surrounding tert-butyl groups block the interactions between π-faces, Figure 3e).
To determine the attractive interlayer interaction in HBNG1, reduced density gradient (RDG) simulations were performed. As shown in Figure 3f, there are strong noncovalent van der Waals interactions (green surface) between the two layers (see details in Figure S22). In addition, the strong steric hindrance in the helical inner rim (red color in Figure 3f) is also revealed by RDG analysis. The nucleusindependent chemical shift (NICS) and anisotropy of the induced current density (ACID) analyses are used to get further insight into the aromaticity of HBNG1. NICS(1) zz calculations indicate that the pentagon exhibits a slight antiaromatic feature (ring K: 3.69 ppm) and the heptagons show strong anti-aromatic character (ring E: 17.15 ppm; ring G: 10.40 ppm) (Figure 3d and Figure S23). Furthermore, the  ACID simulations also exhibit counter-clockwise ring current for the embedded heptagon and azulene subunit, demonstrating their antiaromatic behavior ( Figure S24).
The UV/Vis spectrum of HBNG1 shows a broad absorption together with a hump peak from 503 to 561 nm ( Figure 4a). According to the time-dependent density functional theory (TD-DFT) calculations, the broad hump absorption bands are attributed to the combination of HOMO!LUMO + 1, HOMOÀ 1!LUMO and HOMOÀ 2!LUMO transitions ( Figure S25 and Table S3). The longest wavelength absorption band of HBNG1 is centered at 628 nm and absorbs up to 680 nm due to the extended π-conjugation. The optical energy gap of HBNG1 is determined to be 1.86 eV from the onset of its UV/Vis absorption. The emission spectrum of HBNG1 in DCM solution displays a maximum at 602 nm with a shoulder at 665 nm ( Figure S16) and its photoluminescence quantum yield in DCM is estimated to be 0.32 %. [15a, 17] In addition, the electrochemical behavior of HBNG1 was investigated by CV and square wave voltammetry (SWV). HBNG1 exhibits four reversible oxidation with half-wave potentials at 0.20, 0.49, 0.97 and 1.25 V, and one reversible reduction wave at À 1.93 V (vs. Fc + /Fc) was identified in the CV curve. SWV measurements provided an apparent observation of the highest oxidation event (Figure 4b). The CV data at different scan rates reveal the good electrochemical reversibility of the first two-oxidative waves of HBNG1 ( Figure S20). The HOMO/LUMO levels are thus estimated to be À 4.9/ À 3.01 eV based on the onset potentials of the first oxidation/reduction waves. The electrochemical energy gap is thus calculated to be 1.89 eV, which is in good accordance with its optical energy gap.
Considering the lower oxidation potentials of HBNG1, its redox behavior was further studied by in situ electron paramagnetic resonance (EPR)/UV/Vis-NIR SEC. During the first oxidation, the absorption bands at 485, 530, 602, 645 and 845 nm dominate in the UV/Vis-NIR spectra (Figure 4c). Simultaneously, an EPR signal with a g value of 2.0025 was obtained, indicating the formation of radical cation species (Figure 4c). The second oxidation leads to the appearance of the absorption band at 1175 nm, while the bands at 485 and 845 nm are red and blue shifted, respectively. The EPR intensity decreases during the second oxidation indicating that the dication specie has a diamagnetic dication character ( Figure S21). The formation of the radical trication during further oxidation is indicated by the increase in the EPR intensity and appearance of the band around 900 nm (Figure 4c). Interestingly, the observation of an intervalence charge transfer band in the NIR region for the one-oxidized species (Figure 4c) as well as the corresponding TD-DFT calculations reveal the through-space conjugation among the two layers in HBNG1 (Table S2). The calculated electronic coupling (V 12 ) value is larger than half of the reorganization energy (λ/2), indicating that it is a Robin-Day class III mixed-valence compound. [18] In addition, the spin density of HNBG1 * + reflects the electronic communication between these two layers based on the calculation ( Figure 5).
The resolution of the racemic HBNG1 into its two enantiomers was achieved by chiral stationary phase HPLC (CSP-HPLC). A full CSP-HPLC optimization study allowed the identification of the best chromatographic conditions for its enantioresolution ( Figure S15 and Table S1). A cellulosebased stationary phase and a gradient of n-hexane/DCM mixture as mobile phase at room temperature were used for this purpose. After enantioresolution, their chiroptical properties were studied ( Figure 6). The ECD response was measured for the two collected fractions. Both enantiomers displayed mirror images with several opposite Cotton effects in the UV/Vis region. The first eluted fraction showed two bands of major intensity with negative Cotton effect; the first one at 360 nm (j Δɛ j = 95.9 M À 1 cm À 1 , g abs = j Δɛ j /ɛ = 1.9 × 10 À 3 ), the second one at 553 nm (j Δɛ j = 70.1 M À 1 cm À 1 , g abs = 6.6 × 10 À 3 ), which is comparable with the reported heptagonembedded helicenes. [3b, 15d] Besides, two minor bands with opposite Cotton effect at 318 nm (j Δɛ j = 41.3 M À 1 cm À 1 ,