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

Abstract

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.

Key words

heptalene - nitrogen doping - non-alternant PAHs - optical properties - fluorescence

Introduction

Polycyclic aromatic hydrocarbons (PAHs) containing non-hexagons 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.[6] Recently, N-doped PAHs containing odd-membered rings have been developed, which greatly enriched the chemistry of molecular nanocarbons.[7]

Azulene and heptalene ([Figure 1]) are the representative nonalternant molecules with an aromatic and an antiaromatic character, respectively,[8] in the planar geometry, which have been widely used as key building blocks in the synthesis of functional π-conjugated systems,[9] especially for helicenes[10] and curved nanographenes.[11] The introduction of azulene unit, generally, resulted in unsymmetrically distributed frontier orbitals, which further affect the physical and chemical properties, including dipole moment and photophysical and redox properties.[12] The heptalene moiety has a significant impact on the molecular geometries and the aromaticity of π-conjugated systems.[13] By 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 sp²-carbon atoms is replaced by the sp³-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.[14] For 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.[15] During the preparation of this manuscript, azaheptalene 2 was reported by Ishigaki et al. via a one-pot method, and its chiral property was investigated.[16] Herein, we report the synthesis of a series of π-extended PAHs with N-doped 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.

Figure 1 Molecular design of N-doped azulene derivatives 1 and heptalene derivatives 2.

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 starting 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, a stepwise synthetic strategy (Routes B and C) from 5 towards 2 was adopted. Copper-catalyzed Ullmann reaction of 5- ^t Bu with 4 successfully afforded triarylated amine 6. However, the subsequent Pd-catalyzed direct arylation gave 1- ^t Bu with one newly formed pentagon in a total yield of 54% over two steps. There was no reaction further using the Scholl reaction of 1- ^t Bu toward 10 in the presence of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (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.[18] Inspired by this method, cyanation of 5 with CuCN, followed by hydrolysis with a 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 was 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.

Scheme 1 Synthesis of N-doped azulene-containing PAH 1 and N-doped heptalene derivatives 2.

Single crystals of 2- ^t Bu suitable for X-ray diffraction analysis were obtained by slow evaporation of the chloroform/acetonitrile solutions ([Figure 2]). Compound 2- ^t Bu demonstrates a highly twisted structure of C ₂ 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.[16] Bond length analysis on the N-doped heptagonal rings indicates an obvious bond length alternation on the C–C bonds due to π-localization of fused benzene rings ([Figure 2]a), 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 2- ^t Bu with different helical chirality exist in the packing model, in which they are aligned on the ac plane ([Figure 2]c). Owing to the twisted structure and the bulky tert-butyl group, no obvious π−π interaction between molecules was observed in the solid state.

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. P-helical is shown in blue; M-helical is shown in pink. Hydrogen atoms are omitted for clarity.

Theoretical calculations on 1- ^t Bu and 2- ^t Bu were carried out at the B3LYP/6 – 31 G(d,p) levels to gain insight into the differences on the electronic structures. Compound 2- ^t Bu exhibits slightly higher HOMO (−5.19 eV) and LUMO (−1.00 eV) energy levels than those of 1- ^t Bu (HOMO: −5.25 eV, LUMO: −1.10 eV) (Figure S2). The HOMO of 2- ^t 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 1- ^t Bu and 2- ^t Bu contributes little to their LUMOs. Nuclear-independent chemical shift (NICS; [Figure 3]a) calculations of both compounds were further performed. The values of the pentagon and heptagon in 1- ^t Bu are −9.7 and 14.4, respectively, suggesting the weak aromatic character for the pentagon and an antiaromatic character for the heptagon. Interestingly, the two heptagons in 2- ^t 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 3]b) maps.

Figure 3 (a) NICS(1)_zz values and (b) ACID plots of 1- ^t Bu (left) and 2- ^t Bu (right).

The photophysical and electrochemical properties of 1- ^t Bu and 2 were further compared ([Figure 4] and [Table 1]).[19] Compound 1- ^t Bu shows two major absorptions at 310 nm and 350 nm. Compound 2 with different substituents exhibit identical absorptions with a hypsochromically shifted wavelength maximum at 315 nm ([Figure 4]a). 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 those of 1- ^t Bu (438 nm), probably due to the large conformation changes upon photoexcitation.[20] Cyclic voltammetry measurements of these two compounds 1- ^t Bu and 2- ^t Bu in dichloromethane revealed one reversible oxidation wave (E _1/2 ^ox) at 0.79 V and 0.76 V, respectively. The HOMO energy levels are estimated to be −5.49 eV for 1- ^t Bu and −5.48 eV for 2- ^t Bu, respectively, which are much higher than those of other N-doped analogs.[7b]

Figure 4 (a) Absorptions (solid line) and emissions (dashed line) of compounds 1- ^t Bu and 2 measured in dichloromethane at 298 K. (b) Cyclic voltammograms of 1- ^t Bu and 2- ^t Bu in anhydrous dichloromethane with n-Bu₄NPF₆ (0.1 M) as the electrolyte versus Fc/Fc⁺. Scan rate: 0.1 V · s⁻¹.

Table 1 Photophysical and electrochemical properties of 1- ^t Bu and 2

Compound	λab a (nm)	λem a (nm)	E 1/2 ox b (V)	HOMOc (eV)	E g opt d
aMeasured in dichloromethane (1.0 × 10−5 M). b E 1/2 ox is the half-wave potential of the oxidative waves with potentials vs. Fc/Fc+ couple. cHOMO energy levels were calculated according to the equation: HOMO = −(4.8 + E onset ox), where E onset ox values are the onset potentials of the first oxidative redox wave. dObtained from the edge of the absorption spectra according to E g opt = (1240/λonset).
1- t Bu	350	438	0.79	−5.49	3.17
2-H	315	447	–	–	3.34
2-Cl	315	459	–	–	3.34
2- t Bu	315	446	0.76	−5.48	3.34

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 an 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 N-doped heptalene, and related works are ongoing in our group.

Funding Information

This work was supported by the Hong Kong Research Grants Council (27 301 720, 17 304 021) and the National Natural Science Foundation of China (22 122 114). J. L. is grateful for the funding from The University of Hong Kong (HKU) and ITC to the SKL.

Conflict of Interest

The authors declare no conflict of interest.

Acknowledgment

We thank the UGC funding administered by HKU for supporting the Time-of-Flight Mass Spectrometry Facilities under the Support for Interdisciplinary Research in Chemical Science. We acknowledge the computer cluster (HPC2021) of HKU for generous allocations of compute resources.

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Publication History

Received: 03 July 2023

Accepted after revision: 30 August 2023

Accepted Manuscript online:
08 September 2023

Article published online:
12 December 2023