Stable Peri-hexacene: A Long-Waited Open-Shell Graphene Fragment

doi:10.21203/rs.3.rs-4954989/v1

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Stable Peri-hexacene: A Long-Waited Open-Shell Graphene Fragment

https://doi.org/10.21203/rs.3.rs-4954989/v1

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[n]Peri-acenes ([n]PA) have attracted great interest as promising candidates for nanoelectronics and spintronics. However, the synthesis of large [n]PA (n > 4) is extremely challenging due to their intrinsic open-shell radical character and high reactivity. Herein, we report the successful synthesis and isolation of a derivative (1) of peri-hexacene in crystalline form. The structure of 1 was unequivocally confirmed by X-ray crystallographic analysis. Its ground state, aromaticity and photophysical properties were systematically studied by both experimental methods and theoretical calculations. Although the parent peri-hexacene is calculated to have a very large diradical character (y₀ = 94.5%), 1 shows reasonable stability (t_1/2 = 24 h under ambient conditions) due to the kinetic blocking. 1 exhibited an open-shell singlet ground state with a small singlet-triplet energy gap (-1.33 kcal/mol from SQUID measurements). 1 has also a narrow HOMO-LUMO energy gap (1.05 eV) and displays amphoteric redox behavior. This work opens new avenues for the design and synthesis of stable zigzag-edged graphene-like molecules with significant diradical character.

Physical sciences/Chemistry

Physical sciences/Chemistry/Materials chemistry

peri-acenes

nanographenes

diradicaloids

zigzag

aromaticity

Atomically precise nanographenes (NGs) have gained intensive attention in the past decade because of their fascinating opto-electronic properties and high promise for next-generation semiconductor materials.¹ Both experimental and computational studies have demonstrated that widths and edge topologies of NGs play a pivotal role in determining their electronic and magnetic properties as well as their chemical reactivities.² Among various NG families, [n]peri-acenes ([n]PAs), where n defines the number of benzene rings formed along the zigzag axis, are a unique class of rectangle-shaped NGs consisting of two zigzag-edges and two armchair-edges, which can exhibit, if n exceeds a certain threshold (n > 3), diradical and even polyradical character upon the lateral π-extension due to the gain of additional Clar’s aromatic sextets in their open-shell resonance forms (Fig. 1).³ They can also be considered as ideal model molecules to explore the electronic structures of the large-size NGs with zigzag edges.⁴ However, their synthesis is a huge challenge owing to their intrinsic open-shell radical character, which is related to the presence of a localized non-bonding π-state around parallel zigzag edges. To date, only a few [n]PA molecules have been successfully synthesized. For example, in 2018, Feng and Wu’s groups independently reported the in-solution synthesis of peri-tetracene derivatives ([4]PA, Fig. 1a), showing large diradical character with a half-life of 3 ~ 7 h in solution under ambient conditions.^5,6 In 2021, Feng’s group described the in-solution synthesis and in-situ characterization of hitherto the largest peri-heptacene derivative ([7]PA, Fig. 1c), which demonstrated a significant tetraradical character with a half-life of ~ 25 min under inert conditions.⁷ It should be mentioned that parent peri-pentacene ([5]PA, Fig. 1b) has been accomplished on Au (III) surface under ultra-high vacuum conditions and its singlet open-shell nature was revealed by scanning tunneling spectroscopy,⁸ but all attempts so far to synthesize [5]PA or its derivatives in solution have been unsuccessful due to the poor solubility and undesired side reactions.⁹ Therefore, the solution-phase synthesis of large [n]PAs (n > 4) or their appropriate derivatives depends heavily on the development of more effective design and synthetic strategies, which need to overcome the challenges associated with the low solubility and the intrinsic high reactivity of the target molecules.

Peri-hexacene ([6]PA, Fig. 1d), which has never been achieved by solution-mediated or surface-assisted synthesis, is the next important target of synthesis and isolation, and its realization is also crucial for elucidating the structure-property relationship of this family from both fundamental and applied perspectives. In this context, herein we report the solution-

phase synthesis and characterization of a stable peri-hexacene derivative (1, Fig. 1d). According to Clar’s aromatic π-sextet rule,^3b the molecule can be drawn in at least three different resonance forms: a closed-shell form with two aromatic sextet rings (the hexagons shaded in blue, form A), an open-shell diradical forms with five aromatic sextet rings (form B), and an open-shell tetraradical form with six aromatic sextet rings (form C). The gain of three/four Clar’s aromatic sextet rings in the open-shell resonance forms B/C implies a large diradical character and moderate tetraradical character. Our spin-unrestricted theoretical calculations (UB3LYP/6-31G(d,p) on the parent [6]PA indicated that the frontier SOMO-α and SOMO-β orbitals have significant disjoint feature, and the high spin densities are mainly distributed along the two zigzag edges (Fig. 1e). Thus, the diradical character (y₀) and tetraradical character (y₁) of the parent [6]PA were calculated to be 94.5% and 24.5%, respectively, by the Natural orbital occupation number (NOON) calculations (UCAM-B3LYP/6-31G(d,p)),¹⁰ which is much larger than [4]PA (y₀ = 52.5%) and [5]PA (y₀ = 75.4%) under the same level of theory. The significant open-shell radical character of [6]PA suggests the need to kinetically block the most reactive zigzag edges by bulky groups and to develop an innovative strategy to build up the framework. In response to these challenges, a soluble and stable [6]PA derivative 1 was successfully synthesized and isolated in a crystalline form. Its ground state, aromaticity and photophysical properties were systematically explored by both experimental methods and theoretic calculations, and compared with those of [4]PA and [7]PA derivatives.

To realize the synthesis of [6]PA with sufficient stability and solubility, our strategy is to introduce four bulky and electron-withdrawing 2,6-dichlorophenyl groups at the most reactive sites of the two zigzag edges and to incorporate C(sp³)-bridged cyclopenta (CP) rings at the bay regions of [6]PA where each sp³ carbon substituted by two 3,5-di-tert-butylphenyl groups. In addition to these substituents, four additional 3,5-di-tert-butylphenyl groups are also attached onto the π-conjugated framework to further improve solubility. As depicted in Scheme 1, the key intermediate for the synthesis of the target molecule 1 is the partially fused dibromo- intermediate 7, which is prepared via multiple cross-coupling, regioselective cyclization and oxidative dehydrogenation reactions. The CP ring-fused perylene 2 was synthesized from parent perylene according to our reported procedre.¹¹ The monobrom CP-fused perylene 3 was firsty synthesized by regio-selective bromination with bromine under controlled conditions at the bay-region, and then Miyaura borylation reaction of 3 with bis(pinacolato)diboron (B₂(pin)₂)in the presence of palladium catalyst provided monoboronic ester 4 in 63% yield. After that, the Suzuki coupling of 5 with 4 yielded the intermediate 6 in 68% yield. Next, the key intermediate 7 was obtained by the treatment of 6 with CuBr₂ in the presence of base where a one-pot reaction of

benzannulation and intramolecular oxidative cyclodehydrogenation was achieved. The structure of 7 was confirmed by single-crystal analysis (Figure S9). Afterwards, lithium-halogen exchange reaction by treatment of 7 with n-BuLi proceeds and subsequent quenching with 2,6-dichlorobenzaldehyde to generate the diol, which underwent an intramolecular Friedel-Crafts cyclization in the presence of trifluoromethanesulfonic acid (TfOH) to afford the dihydro- precursor 8 in 53% yield over three steps from 7. Finally, the deprotonation of 8 with ^tBuOK and 18-crown-6 in THF followed by the oxidation with p-chloranil furnished the target peri-hexacene derivative 1 in 73% yield. Beacause two cyclopenta (CP) ring was attached onto the arm-chair edges and the four bulky and electron withdrawing 2,6-dichlorophenyl groups are attached onto the most reactive zig-zag edges, compound 1 shows good solubility and stability, and can be purified by triethylamine deactivated silica gel column. Stability test of the solution of 1 in carbon tetrachloride under ambient air and light conditions followed by UV/Vis-NIR absorption measurements revealed a half-life time (t_1/2) of about 24 h when monitored at 617 nm (Figure S1 in the Supporting Information (SI)).

Single crystals of 1 suitable for X-ray crystallographic analysis were grown by slow diffusion of acetonitrile into its solution in toluene.¹² The molecule has a centrosymmetric geometry with a slightly distorted skeleton (Figs. 2a,b). The dihedral angles between the π-conjugated skeleton and its 2,6-dichlorophenyl substituents range from 83.1 to 84.5^o (Fig. 2b). Bond length analysis of the 1 backbone revealed that the bond lengths of the CC bonds (the bonds shown in red, Fig. 2c) linking the two hexacene units were in the range of 1.409–1.429 Å, which are shorter than that in [4]PA derivative (1.442–1.459 Å),⁶ indicating enhanced electronic coupling between two hexacene units. The CC bonds shown in blue (Fig. 2c) are much longer (1.464 Å) like a typical C(sp²)-C(sp²) single bond, which could be attributed to the elongation induced by the steric hindrance between the neighboring 2,6-dichlorophenyl substituenets. The calculated harmonic oscillator model of aromaticity (HOMA) values¹³ based on the X-ray structures revealed that the four benzenoid rings at the termini (rings A/F/L/Q) have largest HOMA values (0.87/0.84, Fig. 2c), indicating a large aromatic character. The rings C/D/N/O have much smaller HOMA values (0.38 ~ 0.44, Fig. 2c) while the other benzenoid rings have a HOMA value of 0.65 ~ 0.80 (Fig. 2c), indicating significant contribution of the open-shell diradical form B with five localized aromatic sextet rings shown in Fig. 1d to the ground-state structure. On the other hand, nucleus independent chemical shift (NICS)¹⁴ calculations show that these five benzenoid rings have large negative NICS(1)_zz values (-20.49 ppm ~ -22.20 ppm), the rings B/E/H/J/M/P have moderate negative NICS(1)_zz values (-11.93 ppm ~ -16.00 ppm) while the rings C/D/N/O are almost non-aromatic (Fig. 3a). All these suggest that [6]PA possesses a large diradical character, which can be stabilized by five aromatic sextet rings. In addition, no intermolecular π-π interaction was observed in the 3D packing structure (Figure S10).

To gain a deeper understanding of the electronic structure and aromaticity of compound 1, we carried out anisotropy of induced current-density (ACID)¹⁵ and isochemical shielding surface (ICSS)¹⁶ calculations (B3LYP/6-31G(d,p)). ACID plot shows obvious clockwise diatropic ring current circuit along the periphery (Fig. 3b), indicating that the molecule is globally aromatic and the two hexacene units are coupled well with each other, in accordance with the bond length analysis. Notably, the central anthracene units show clockwise diatropic ring current (Fig. 3b), in agreement with the HOMA value and NICS analysis. Furthermore, the calculated 2D ICSS map also shows that the rings A/F/L/Q have the strongest shielding, the rings B/E/H/J/M/P have moderate shielding, and there is the least shielding in other benzenoid rings (Fig. 3c), consistent with the NICS and ACID calculations. For comparison, although the open-shell diradical form of [4]PA exhibits a localized aromatic character (Fig. 1a), the central benzenoid ring connecting the two tetracene units in[4]PA is almost non-aromatic. The central benzenoid rings connecting the two hexacene/heptacene units in [6]PA and [7]PA are aromatic. This difference could be ascribed to the much larger diradical character of [6]PA and [7]PA compared to [4]PA. Therefore, the lateral extension in [6]PA and [7]PA provides the opportunity to form local aromatic sextets along the central row of the backbone, resulting in different electronic structure.

To further understand the ground-state electronic structure of compond 1, we conducted magnetic measurements such as variable temperature (VT) NMR, electron spin resonance (ESR) and superconducting quantum interference device (SQUID) measurements. The ¹H NMR spectrum of 1 in THF-d₈ at room temperature exhibited one almost flat baseline, and cooling of sample to -80 ^oC also did not obtain full resolution (Figure S3). The NMR broadening can be explained by the thermal population of triplet diradical species at elevated temperatures, as expected for species with strong diradical character. Further experimental evidence was obtained by ESR measurement on 1 in solid, showing strong ESR signal at room temperature with a g-factor of 2.003 (Figure S4). Furthermore, we also carried out superconducting quantum interference device (SQUID) measurement for the microcrystalline powder sample of 1 at 2–300 K. The magnetic susceptibility increased with increasing temperature, and the singlet-triplet energy gap (∆E_S-T) was estimated to be -1.33 kcal/mol by careful fitting of the data by using the Bleaney–Bowers¹⁷ equation (Fig. 4). All these indicate that compound 1 is an open-shell singlet diradicaloid and exhibits a large diradical character with a small singlet-triplet energy gap. For comparison, the [4]PA derivative (-2.5 kcal/mol)⁶ showed a larger ∆E_S-T value, in consistence with its smaller diradical character. On the other hand, broken-symmetry DFT calculations (UB3LYP/6-31G(d,p)) also predict that the open-shell singlet biradical state of 1 has lower energy compared to the triplet biradical state, and the singlet–triplet energy gap (∆E_S-T) is calculated to be -2.08 kcal/mol, which is consistent with the above experimental results.

The UV-vis absorption spectra of 1 was recorded in carbon tetrachloride and compared to that of precursor 8 in DCM, as illustrated in Fig. 5a. Compound 8 exhibits a well-resolved absorption maximum at 632 nm along with a small shoulder band peaking at 580 nm. In contrast, the absorption spectrum of 1 displays a broad band in the near-infrared (NIR) region with a maximum at 789 nm, which is significantly redshifted compared with that of precursor 8. Moreover, 1 shows a weak absorption band in the NIR region with maximum (λ_max) at 1090 nm along with a shoulder (λ_sh) at 1197 nm (Fig. 5a). 1 also displays multiple intense absorption bands in the visible region. The NIR absorption band has a similar structure to that in [4]PA derivative⁶ and [7]PA derivative,⁷ however, the intensity is much weaker. Furthermore, the characteristic long-wavelength shoulder peak for 1 is strong indication that it has an open-shell singlet ground state, and the band is originated from the ground-state HOMO,HOMO→LUMO,LUMO double excitation.¹⁸ The optical band gap (E_g^opt) of 1 from the onset of its UV/Vis absorption is roughly estimated to be 0.99 eV, which is smaller than that of [4]PA derivative (1.12 eV)⁶ due to the π-extension.

The electrochemical property of 1 was investigated by differential pulse voltammetry (DPV) and cyclic voltammetry (CV) measurements in dichloromethane (Fig. 5b). Compound 1 displays two reversible oxidation waves with halfwave potentials E_1/2^ox at 0.04 V and 0.32 V, and two reversible reduction waves with half-wave potentials E_1/2^red at − 1.03 V and − 1.24 V (vs Fc/Fc⁺). Compared with [4]PA derivative, the neighboring oxidation or reduction waves of 1 shows a smaller segregation, which can be explained by the larger π-conjugated backbone that reduces the intramolecular Coulomb charge repulsion. In addition, according to the onset potentials of the first oxidation/reduction waves, the HOMO and LUMO energy levels of 1 are estimated to be -4.82 eV and − 3.76 eV, respectively. Thus, the corresponding electrochemical energy gap (E_g^EC) is estimated to be 1.06 eV, which is consistent with the optical energy gap. On the other hand, compound 1 can be chemically oxidized into its corresponding radical cation and dication with NO•SbF₆ in anhydrous DCM. The radical cation exhibits multiple intense absorption bands in the NIR region with λ_max at 858 nm, 1210 nm and 1590 nm, respectively (Figure S2), and a strong ESR signal was observed for the radical cation (Figure S4). The dication shows an intense absorption band with λ_max at 845 nm in the NIR region (Figure S2). All absorption spectra are in agreement with the time-dependent (TD) DFT calculations (see SI).

In summary, a soluble and stable peri-hexacene derivative 1 was successfully synthesized and isolated in crystalline form. It exhibits large diradical character and moderate tetraradical is still reasonably stable as a result of kinetic blocking of the most reactive sites at the two zigzag edges with the bulky and electron-withdrawing 2,6-dichlorophenyl groups. Analysis shows that it displays a unique electronic structure with five localized aromatic sextets. It exhibits characteristic absorption of singlet open-shell diradicaloids in the NIR region and has a smaller singlet-triplet energy gap. It also shows a global aromatic character that is similar to those of peri-tetracene and peri-heptacene analogues. On the other hand, it has a small energy gap, showing amphoteric redox behavior and magnetic activity. Our synthetic strategy and physical studies not only reveal the fundamental change of electronic structures and aromaticity of peri-acenes upon lateral extension, but also provide some insights into the synthesis of larger size peri-acene molecules or other zigzag-edged graphene-like molecules with significant diradical character.

ASSOCIATED CONTENT

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website.

Synthetic procedures and characterization data of all new compounds; details for all physical characterizations and theoretical calculations; and additional spectroscopic and X-ray crystallographic data (PDF).

Accession Codes

CCDC number 2327038 (7) and 2337978 (1) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 336033.

AUTHOR INFORMATION

Corresponding Author

Wangdong Zeng – School of Materials Science and Engineering, Hunan University of Science and Technology, Xiangtan 411201, China; E-mail: [email protected]

Baishu Zhengn – Key Laboratory of Theoretical Organic Chemistry and Function Molecule of Ministry of Education, Hunan Provincial Key Laboratory of Controllable Preparation and Functional Application of Fine Polymers, School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China; E-mail: [email protected]

Qing Jiang – College of Chemistry and Bioengineering, Hunan University of Science and Engineering, Yongzhou, 425100, China; E-mail: [email protected]

Guangwu Li – Center of Single-Molecule Sciences, Institute of Modern Optics, Tianjin Key Laboratory of Micro-Scale Optical Information Science and Technology, College of Electronic Information and Optical Engineering, Nankai University, 38 Tongyan Road, Jinnan District, Tianjin 300350, China; E-mail: [email protected]

Author Contributions

‡These authors contributed equally.

Notes
The authors declare no competing financial interest.

ACKNOWLEDGMENT

W. Zeng acknowledges the financial support from National Natural Science Foundation of China (22175061), Excellent Youth Foundation of Hunan Scientific Committee (2022JJ10025), Key research and development projects of Hunan Province (2022GK2035) and Key research and development projects of Xiangtan City (ZX-ZD20221006). Q. Jiang thanks the construct program of applied characteristic discipline in Hunan Province and the High-level Talent Program from Hunan University of Science and Engineering for financial support. G. Li acknowledges financial supports from the National Natural Science Foundation of China (22205117), the Fundamental Research Funds for the Central Universities (63223054, 63233092, 63243139).

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Scheme 1 is available in the Supplementary Files section.

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image2.png
Scheme 1. Synthetic route toward peri-hexacene derivative 1. (a) Br₂, DCM, 0 °C, 30min; (b) Pd(dppf)Cl₂, B₂(pin)₂, KOAc, dioxane, 90 °C; (c) Pd(PPh₃)₄, SPhos, K₃PO₄ , toluene/H₂O, 90 °C; (d) CuBr₂, K₃PO₄, DCE, reflux; (e) i) n-BuLi, -30°C; ii) 2,6-dichlorobenzaldehyde, rt; iii) TfOH, DCM, rt; (f) i) KOBu^t, 18-crown-6, THF, rt; ii) p-chloranil. SPhos: 2-Dicyclohexylphosphino-2',6'-dimethoxybiphenyl; DCE: 1,2-dichloroethane; THF: tetrahydrofuran; DCM: dichloromethane.
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Stable Peri-hexacene: A Long-Waited Open-Shell Graphene Fragment

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