A Nonbenzenoid 3D Nanographene Containing 5/6/7/8-Membered Rings

Abstract

Nanographenes (NGs) have attracted continuous attention in recent years owing to their opened bandgaps and optoelectronic applications. Especially, nonbenzenoid NGs containing non-six-membered rings have been developed rapidly due to their unique structures and properties. In this work, we employ nonbenzenoid acepleiadylene (APD) and the cyclooctatetraene (COT) moiety to construct the first three-dimensional (3D) NG containing 5/6/7/8-membered rings in one molecule (COT-APD). The calculated results prove that COT-APD has a saddle-like configuration similar to that of other COT-type molecules. Each APD segment in COT-APD keeps the inherent aromaticity of the APD moiety. Compared with other COT-type molecules, COT-APD shows a narrower bandgap, which indicates the superiority of APD in bandgap regulation. Furthermore, four reversible reductive waves are observed in electrochemical characterizations, demonstrating the excellent electron-accepting capability of COT-APD.

Key words

nanographenes - nonbenzenoid arenes - cyclooctatetraene - acepleiadylene

Introduction

Nanographenes (NGs) with opened bandgaps have played a significant role in the development of next-generation semiconductors.[1] The bottom-up chemical syntheses of NGs have not only provided atomically precise structures,[2] but also enabled the creation of NGs with novel topologies.[3] For example, nonbenzenoid NGs with non-six-membered rings have been extensively developed[4] in recent years, showing intriguing properties and applications.[5] However, the reported ring topologies have still been limited. An intriguing type of nonbenzenoid NGs with multiple different rings (e.g. 5/6/7/8-membered rings) merged in a single molecule has remained elusive.

Acepleiadylene (APD) is a nonbenzenoid arene containing 5/6/7-membered rings, and has been employed as a unique building block for NGs and optoelectronic materials.[6] On the other hand, cyclooctatetraene (COT), an 8π nonaromatic ring, has often been used for the construction of 3D saddle-shaped conjugated molecules.[7] Nevertheless, only very few examples of COT-based NGs have been reported, such as the acenaphthylene-fused COT (COT-AC) by Whalley et al. and its imide-functionalized derivatives (COT-ACI) by Stępień et al. ([Figure 1]), as well as the hexa-peri-hexabenzocoronene-fused COT by Martín et al.[8] Thus, COT-based 3D NGs, especially nonbenzenoid NGs, have been largely underexplored.

Figure 1 Previously reported COT-cored 3D NGs based on acenaphthylene and the new COT-APD hybrid containing 5/6/7/8-membered rings in this work.

Herein, we report the synthesis and characterizations of a new COT-APD hybrid as a nonbenzenoid 3D NG, which represents the first NG bearing 5/6/7/8-membered rings in the same backbone ([Figure 1]). Compared with COT-AC, COT-APD has a larger π-system and a narrower bandgap. Furthermore, as a pure hydrocarbon molecule, COT-APD exhibits four reversible reductive waves in cyclic voltammetry (CV) characterizations, indicating its strong electron-accepting capability.

Results and Discussion

The synthetic route to COT-APD is shown in [Scheme 1]. A Yamamoto coupling of monobrominated APD 1 gave APD dimer 2 in 91% yield, which was subsequently brominated on the two five-membered rings to afford compound 3 in 68% yield. Another Yamamoto coupling of two brominated APD dimers achieved the target molecule in 44% yield.[9] The three-step route realized the synthesis of the nonbenzenoid 3D NG in a 27% overall yield. All compounds were characterized by NMR and HRMS to verify their chemical structures. The solubility of COT-APD in common organic solvents is relatively low (e.g. 0.26 mg · mL⁻¹ in toluene), which hampered the growth of single crystals for X-ray analysis.

Scheme 1 Synthetic route to COT-APD.

As shown in [Figure 2a], COT-APD exhibits similar ¹H NMR chemical shifts compared with its monomer APD, which implies that the chemical environment of each APD moiety in COT-APD is similar to that of the monomer. The slight upfield shift of all proton signals in COT-APD results from the stronger electron-donating effect of the sp² carbon in the COT ring than hydrogen in APD. The HRMS spectrum of COT-APD shows a molecular ion signal at m/z = 800.2488 Da, in good agreement with the expected molecular mass of 800.2499 Da for C₆₄H₃₂ with the relative error of 1.4 ppm. The experimental isotopic distribution also matches well with the simulated pattern, clearly proving the chemical identity of COT-APD ([Figure 2b]).

Figure 2 (a) Partial ¹H NMR spectra of COT-APD and APD with a mark indicating the residual solvent CHCl₃. (b) HRMS spectrum of COT-APD showing the experimental and simulated isotopic distributions.

In order to further understand the configuration and properties of COT-APD, density functional theory (DFT) calculations in the gas phase were performed ([Figure 3]). The optimized geometry suggests that the molecular size of COT-APD is in nanoscale. Furthermore, COT-APD distorts apparently with a dihedral angle of 103.3° between the two opposite APD moieties. It shows a saddle-like configuration, and belongs to the D _2d point group. The harmonic oscillator model of aromaticity (HOMA) values[10] were calculated according to the optimized structure. The positive values ([Figure 3b]) of the rings in the APD moiety prove that the aromaticity of APD is kept in COT-APD. The anisotropy of the induced current density (ACID) plot[11] ([Figure 3c]) of COT-APD suggests that each APD segment maintains the intrinsic aromaticity of its monomer with the clockwise ring current flow in each segment, while the central COT ring does not display continuous ring current flow, exhibiting nonaromaticity which is a typical feature for COT. The 3D iso-chemical shift shielding surface (ICSS)[12] indicates that the space above and below each APD segment is the shielding region (yellow isosurface), while the space around each APD segment is the deshielding region (blue isosurface).

Figure 3 DFT-calculated results of COT-APD. (a) Top view and (b) side view of the optimized configuration of COT-APD. The blue numbers are the HOMA values of different rings in the APD moiety. (c) The ACID plot of COT-APD. The magnetic field is perpendicular to the XY-plane and points out through the paper. The red arrows indicate the clockwise (diamagnetic) current flow. (d) The 3D-ICSS of COT-APD. The yellow color represents the shielding region and the cyan color indicates the deshielding region.

In order to study the photophysical properties, the UV-Vis absorption spectrum of COT-APD in a toluene solution was recorded ([Figure 4a]). The absorption spectrum of COT-APD features an absorption maximum at 386 nm with additional fine structures at 474, 554, and 604 nm. The absorption onset of COT-APD (639 nm) is red-shifted compared with that of its monomer APD (565 nm), indicating a narrower optical gap upon COT fusion (APD: 2.19 eV; COT-APD: 1.94 eV). Compared with other COT-based molecules, COT-APD also exhibits an apparently narrower optical gap (Table S1). The fluorescence spectrum shows an emission maximum at 787 nm in the near-infrared region (Figure S2).

Figure 4 (a) The UV-Vis absorption spectrum of COT-APD in a 5 × 10⁻⁶ M toluene solution. (b) The simulated absorption spectrum calculated by the TD-DFT method at the B3LYP/6 – 31 G(d) level. The full-width at half-maximum is set as 0.25 eV. (c) The orbital distributions and energy levels of COT-APD with the enlarged view of the APD moieties shown in the boxes.

To shed light on the absorption feature of COT-APD, time-dependent (TD) DFT calculations were carried out. The simulated spectrum resembles well the experimental result ([Figure 4b]). The transition from S₀ to S₁ is forbidden with an oscillator strength of 0. The S₀ → S₂ and S₀ → S₃ excitations are equal in energy and are mainly contributed by the transitions from degenerate HOMO − 1 and HOMO − 2 to LUMO, respectively (Table S2). Similarly, the S₀ → S₆ and S₀ → S₇ excitations are mainly contributed by the transitions from the HOMO to degenerate LUMO +1 and LUMO +2, respectively. The degenerate orbitals of COT-APD stem from its high symmetry. Note that the HOMO is mainly localized on the COT moiety, while the LUMO is distributed over the whole backbone ([Figure 4c]). Therefore, the COT ring has a significant contribution to the absorption feature of COT-APD.

To characterize the redox properties of COT-APD, CV and differential pulse voltammetry (DPV) measurements were conducted ([Figure 5a]). COT-APD exhibits an irreversible oxidation wave with an onset at 0.47 V, which is lower than that of APD (0.58 V) (Figure S3). Interestingly, the 3D NG shows four reversible reductive waves with the half-wave potentials at −1.65 V, −1.84 V, −2.38 V, and −2.56 V, demonstrating the excellent electron-accepting capability of COT-APD, which may come from the electron-accepting capability of APD as revealed by Müllen et al. in 1980.[13] Four reductive peaks are also observed in DPV measurement, in accordance with the CV result. Because the oxidation is irreversible, the HOMO and LUMO energy levels of COT-APD are determined by the onsets of oxidative and reductive waves, respectively, with ferrocene as an external standard. COT-APD exhibits a higher HOMO level and a lower LUMO level than the monomer APD, indicating the extended π-conjugation in the tetramer ([Figure 5b]). Compared with COT-AC, COT-APD shows a similar LUMO energy level (−3.22 eV for COT-APD, −3.26 eV for COT-AC), but a higher HOMO energy level (−5.25 eV for COT-APD, −5.37 eV for COT-AC). As a result, COT-APD displays a narrower HOMO–LUMO gap than COT-AC, which is consistent with the optical gap results.

Figure 5 (a) The electrochemical reduction properties of COT-APD. (b) The electrochemical energy gap of APD, COT-APD, and COT-AC.

Conclusions

In summary, the synthesis of the first NG containing 5/6/7/8-membered rings in the same backbone has been achieved by fusing the APD and COT moieties. Theoretical calculations reveal that COT-APD displays a saddle-shaped configuration. The absorption spectrum illustrates the narrow optical gap of COT-APD, which is in accordance with the electrochemical HOMO–LUMO gap. Furthermore, four reversible reductive waves of COT-APD demonstrate its strong electron-accepting capability, significantly different from the previously reported COT-based NGs. This work not only provides a novel nonbenzenoid NG with 3D configuration, but also demonstrates the high potential of the novel COT-APD hybrid as functional materials.

Funding Information

This work was financially supported by the National Natural Science Foundation of China (22 071 120, 92 256 304, 22 221 002), the National Key R&D Program of China (2020YFA0 711 500), and the Fundamental Research Funds for the Central Universities.

Conflict of Interest

The authors declare no conflict of interest.

Acknowledgment

The authors extend their gratitude to Ms. Shudi Ren from Shiyanjia Lab (www.shiyanjia.com) for providing invaluable assistance with the HRMS tests.

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• 9 Cyclooctatetraacepleiadylene (COT-APD) was prepared by the Yamamoto coupling of compound 3. A typical procedure for the synthesis of COT-APD is described as follows. To a Schlenk flask charged with compound 3 (50.4 mg, 0.0900 mmol), Ni(cod)₂ (49.5 mg, 0.180 mmol), 1, 5-cyclooctadiene (19.5 mg, 0.180 mmol) and 2, 2′-bipyridine (28.1 mg, 0.180 mmol) was added dioxane (4.5 mL) under argon. Then the mixture was heated to 100 °C for 16 h. After cooling to room temperature, the mixture was filtrated and the residue was sequentially washed by MeOH, HCl (2 M), EtOH, and CH₂Cl₂. The obtained solid was dried in vacuo to afford 15.9 mg (yield: 44‍%) of COT-APD as a black solid. ¹HNMR (400 MHz, CS₂/CDCl₃, 297 K, ppm) δ 8.300 (d, J = 7.5 Hz, 8H), 7.961 (d, J = 7.6 Hz, 8H), 7.793 – 7.710 (m, 8H), 6.958–6.881 (m, 8H). ¹³CNMR (101 MHz, CS₂/CDCl₃, 297 K, ppm) δ 137.98, 136.96, 135.49, 127.54, 127.06, 126.68. HRMS (MALDI) m/z: Calcd. for C₆₄H₃₂ ⁺, 800.2499; found: 800.2488 [M]⁺.
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Publication History

Received: 01 January 2024

Accepted after revision: 22 February 2024

Accepted Manuscript online:
20 March 2024

Article published online:
30 April 2024