Synthesis of Polycyclic Aromatic Hydrocarbons with Highly Twisted N-Doped Heptalene

: A series of N-doped heptalene-containing polycyclic aromatic hydrocarbons (PAHs) have been synthesized and characterized in comparison with the N-doped azulene analogs. The crystal structure revealed its highly twisted geometry with a dihedral angle of 105.7° in the cove region of the N-doped dibenzoheptalene backbone. In addition, the electronic structure was both theoretically and experimentally investigated compared with the PAH bearing N-doped azulene unit. Our study provides a new synthetic strategy towards N-doped heptalene-embedded PAHs, and gives insights into the electronic properties of novel π-systems with N-doped nonalternant topologies


Introduction
Polycyclic aromatic hydrocarbons (PAHs) containing nonhexagons have attracted great interests due to their intriguing topological structures and optoelectronic properties. 1 Typically, the introduction of pentagons and heptagons into π-systems brings in novel geometries and unique properties, such as the aromaticity, 2 open-shell character, 3 host-guest interactions 4 and chirality 5 .Heteroatom doping is another effective strategy to alter the electronic structures of PAHs. 6Recently, the increasing interest in N-doped PAHs containing odd-membered rings have been developed, which greatly enriched the chemistry of molecular nanocarbons. 7ulene and heptalene (Figure 1) are the representative nonalternant molecules with an aromatic and antiaromatic character, respectively, 8 in the planar geometry, which have been widely used as key building blocks to the synthesis of functional π-conjugated systems, 9 especially for helicenes 10 and curved nanographenes. 11The introduction of azulene unit, generally, resulted in unsymmetrically distributed frontier orbitals, which further affect the physical and chemical properties, including dipole moment, photophysical and redox properties. 12The heptalene moiety has a significant impact on the molecular geometries and the aromaticity of π-conjugated systems. 13By doping one nitrogen (N) atom into these two units, such as azepino [3,2,1-hi]indole I and azabenzo [ef]heptalene II, one of the bridging sp 2 -carbon atom is replaced by the sp 3 -N atom.Accordingly, one additional benzene ring is required to fuse the bicycle to guarantee the fully π-conjugated system.
Recently, examples of PAHs based on structure I have been successfully synthesized due to the well-established synthetic methodologies. 14For instance, a typical example is the N-doped nanographenes containing pentagon-heptagon pair(s), including the analogs of 1, facilely synthesized via a ring expansion strategy by our group.7b However, the π-extended derivatives based on structure II are still rarely reported probably due to the high strain when forming the adjacent heptagons.For example, Rickhaus et al. reported the synthesis of dibenzo-azabenzo [ef]heptalene derivatives in the pursuit of azatriseptane. 15During the preparation of this manuscript, azaheptalene 2 was reported by Ishigaki et al. via an one-pot method, and its chiral property was investigated. 16Herein, we report the synthesis of a series of π-extended PAHs with Ndoped heptalene or N-doped azulene, respectively, which can be viewed as benzannulated I and II, via a different synthetic strategy.Besides, the effects of heteroatom doping at different joined nonhexagons on both the geometric and electronic structures have been thoroughly investigated.

Results and Discussion
The synthetic methodologies toward tribenzo [b,d,f]azepine derivatives have been well established, 17 which provide guidance to the synthesis of N-containing heptalenes.Initially, the direct synthesis of 2 started from dibromo-aniline 3 was attempted using Palladium (Pd)-catalyzed double annulations (Route A, Scheme 1).However, it failed to obtain 2, instead, mono-annulated product 5 was obtained in 21-58% yield.Therefore, stepwise synthetic strategy (Routes B and C) from 5 Template for Organic Materials © Thieme Stuttgart • New York 2023-09-02 page 2 of 4 towards 2 was adopted.Copper-catalyzed Ullmann reaction of 5t Bu with 4 successfully afforded triarylated amine 6.However, the subsequent Pd-catalyzed direct arylation gave 1t Bu with one newly-formed pentagon in a total yield of 54% over two steps.There was no reaction further using Scholl reaction of 1t Bu toward 10 in the presence of 2,3-dichloro-5,6-dicyano-1,4benzoquinone (DDQ)/trifluoromethanesulfonic acid (TfOH).Pd-catalyzed decarboxylative annulation of 2-aminobenzoic acids with cyclic hypervalent diaryliodonium reagents has been reported to construct N-doped seven-membered rings. 18nspired by this method, cyanation of 5 with CuCN, followed by hydrolysis with strong base, afforded the precursors 8 with the carboxylic acid functional group in a total yield of 47-66%.
Finally, the desired N-doped heptalene-containing 2 successfully obtained via Pd-catalyzed decarboxylative annulation in a yield of 6-16%.All the targets were soluble in common organic solvents and were unambiguously characterized by NMR and high-resolution mass spectrometry.
Single crystals of 2t Bu suitable for X-ray diffraction analysis were obtained by slow evaporation of the chloroform/acetonitrile solutions (Figure 2).Compound 2t Bu demonstrates a highly twisted structure of C2 symmetry with a dihedral angle (C1-C2-C3-C4) of 105.7°, which is much larger than that of the cove region in the heptalene-embedded PAHs (46.9°).11b This further supports the large racemization barrier for 2 (49.4 kcal mol -1 , Figure S5), which indicates a high configurational stability that allows for chiral separation. 16ond length analysis on the N-doped heptagonal rings indicates an obvious bond length alternation on the C-C bonds due to πlocalization of fused benzene rings (Figure 2a), and the C-N bond lengths (1.42 Å) are shorter than the C-N single bond length (1.48 Å).A pair of enantiomers of 2t Bu with different helical chirality exist in the packing model, in which they are aligned on the ac plane (Figure 2c).Owing to the twisted structure and the bulky tert-butyl group, no obvious π−π interaction between molecules was observed in the solid state.Theoretical calculations on 1t Bu and 2t Bu were carried out at the B3LYP/6-31G(d,p) levels to gain insight into the differences on the electronic structures.Compound 2t Bu exhibits slightly higher highest occupied molecular orbital (HOMO, -5.19 eV) and lowest unoccupied molecular orbital (LUMO, -1.00 eV) energy levels than those of 1t Bu (HOMO: -5.25 eV, LUMO: -1.10 eV) (Figure S2).The HOMO of 2t Bu is located on the inner backbone, and the LUMO is distributed on the peripheral rings around the N atom.The embedded N atom in both 1t Bu and 2t Bu contributes little to their LUMOs.Nuclear independent chemical shift (NICS, Figure 3a) calculations of both compounds were further performed.The values of the pentagon and heptagon in 1t Bu are -9.7 and 14.4, respectively, suggesting the weak aromatic character for the pentagon and antiaromatic character for the heptagon.Interestingly, the two heptagons in 2t Bu have small values of 9.8, indicating a weak antiaromatic character.The weak aromatic/antiaromatic characters of these non-hexagonal rings could be attributed to the π-localization of the peripherally fused benzene rings.These results are further supported by the anticlockwise ring current flow in the heptagons and the diatropic ring current circuits in the peripheral benzene rings as shown in the anisotropy of the induced current density (ACID, Figure 3b) maps.
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Accepted Manuscript
Template for Organic Materials © Thieme Stuttgart • New York 2023-09-02 page 3 of 4 The photophysical and electrochemical properties of 1t Bu and 2 were further compared (Figure 4 and Table 1). 19ompound 1t Bu shows two major absorptions at 310 nm and 350 nm, respectively.Compound 2 with different substituents exhibit identical absorptions with a hypsochromically shifted wavelength maximum at 315 nm (Figure 4a).All compounds have a blue emission in solution.Interestingly, the emissions of 2, ranging from 446 nm to 459 nm, are more redshifted compared to that of 1t Bu (438 nm), probably due to the large conformation changes upon photoexcitation. 20Cyclic voltammetry measurements of these two compounds 1t Bu and 2t Bu in dichloromethane revealed one reversible oxidation wave ( 1/2  ) at 0.79 V and 0.76 V, respectively.The HOMO energy levels are estimated to be -5.49eV for 1t Bu and -5.48 eV for 2t Bu, respectively, which are much higher than that of other Ndoped analogs.7b

Conclusions
In summary, we demonstrated the synthesis and characterization of a series of N-doped heptalene derivatives, which can be viewed as the benzannulation of four benzene rings with azabenzo[ef]heptalene core.The crystal structure revealed the molecular backbone possessing a highly twisted geometry with a dihedral angle of 105.7°, and theoretical calculations suggested the weak antiaromatic character for the inner heptagons.In addition, the π-extended N-doped heptalene exhibited similar electronic structures of the N-doped azulene analogs, such as the redox properties and molecular orbital energy levels.Our research reported herein opens a new door for the synthesis of novel graphene nanostructures with Ndoped heptalene, and related works are ongoing in our group.

General information
Solvents were purified and dried by standard methods prior to use.All commercially available reagents were used without further purification unless otherwise noted.H and 13 C NMR data were recorded on a 400 or 500 MHz spectrometer using CDCl3 or CD2Cl2 as solvent at room temperature.The chemical shifts (δ) are reported in ppm and coupling constants (J) in Hz.Mass spectra were obtained on a Bruker Q-Tof Maxis II mass spectrometer and MALDI-TOF MS system, Bruker ultrafleXtreme.Melting points were measured on the MELTING POINT SMP1 from Stuart Scientific Co. LTD.The UV-Vis spectroscopy was performed on the Agilent Cary 60.The photoluminescence spectra and quantum yields were conducted on Cary Eclipse Fluoroscence Spectrofluorometer, Agilent Cary G9800AA.Cyclic voltammetry (CV) measurements were carried out on a CHI660E (CH Instruments, USA) in a three electrode cell in an anhydrous dichloromethane (DCM) solution of tetrabutylammonium hexafluorophosphate (n-Bu4NPF6, 0.1 M) with a scan rate of 100 mV/s at room temperature.All potentials were further calibrated against ferrocene/ferrocenium (Fc/Fc + ).Single crystals data collections were performed on a Bruker D8 Venture with a CuKα (λ = 1.5406Å) X-ray source.All calculations were performed using the SHELXL and the crystal structure crystallographic software package.
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Preparation of starting materials
Compound 9 was prepared according to the literature reported methods.S1

Synthesis of compound 1-t Bu
To a 10 mL sealed tube, 5-t Bu (206 mg, 0.54 mmol, 1 eq), 4 (0.4 mL), Cu powder (173 mg, 2.70 mmol, 5 eq), K2CO3 (149 mg, 1.08 mmol, 2 eq) were added under N2 atmosphere.The mixture was heated to 200 °C for 20 h.Upon completion, the mixture was cooled to room temperature and washed with DCM.The solution was filtrated, and the filtrate was evaporated to dryness.The residue was purified by the column chromatography on silica gel (petroleum ether/CH2Cl2 = 4:1) to afford compound 6 as white solid in 60% yield.It was directly utilized to the next step without further purification.

Theoretical calculations
For all titled molecules, DFT calculations were performed using the Gaussian 09 software package.S2 The geometries were optimized with the B3LYP functional and 6-31G(d,p)/ 6-311G(d,p) basis set.Anisotropy of the induced current density (ACID) was calculated by adopting Herges's method S3 and standard gauge invariant atomic orbital (GIAO) S4-S6 method at B3LYP functional.The NICS calculations for nonplanar and fused pi-conjugated skeletons were performed according to the reported literature S7 , in which NICS(1)ave = [NICS(-1)+NICS( 1)]/2.The coordinates of the points above and below the plane were obtained based on the optimized structures with the Multiwfn software S8 , and further calculated by Gaussian 09.Finally, the NICS( 1)_ZZ values were evaluated by the calculated shielding tensors with the Multiwfn software.This article is protected by copyright.All rights reserved.This article is protected by copyright.All rights reserved.This article is protected by copyright.All rights reserved.

S14
Table S2.The cartesian coordinates of some optimized structures are listed as follows: This article is protected by copyright.All rights reserved.
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Accepted Manuscript

Figure 2
Figure 2 Top view (a) and side view (b) of 2-t Bu with thermal ellipsoids at the 30% probability level.(c) Molecular packing of the enantiomers 2-t Bu.Phelical is shown in blue, M-helical is shown in pink.Hydrogen atoms are omitted for clarity.

a
Measured in dichloromethane (1.0 × 10 −5 M); b  1/2  is the half-wave potential of the oxidative waves with potentials vs Fc/Fc + couple.c HOMO energy levels were calculated according to equations: HOMO = −(4.8+    potentials of the first oxidative redox wave.d Obtained from the edge of the absorption spectra according to  g  = (1240/λonset).

Figure S1 .
Figure S1.Crystallographic structure of 2-t Bu with thermal ellipsoids at the 30% probability level.
Figure S8. 1 H NMR spectrum of 5-Cl measured in CDCl3 at 298 K.