Introduction

Atomically precise nanographenes (NGs) have gained intensive attention in the past decade because of their fascinating optoelectronic properties and high promise for next-generation semiconductor materials1,2,3,4,5,6,7,8,9,10. 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 reactivities11,12,13. 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 arm-chair-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)14,15,16,17,18,19,20,21. They can also be considered as ideal model molecules to explore the electronic structures of the large-size NGs with zigzag edges22,23,24. 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 conditions25,26. 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 conditions27. 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 spectroscopy28, 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 reactions29,30. 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.

Fig. 1: Representative closed-shell and open-shell diradical resonance forms of peri-acenes.
figure 1

a [4]PA derivatives; b parent [5]PA; c [7]PA derivative, and d the new [6]PA derivative. The substituents in [4]PA, [7]PA and [6]PA are omitted in the open-shell resonance structures for clarity. The rings shaded in blue denote aromatic sextets. e Calculated (UB3LYP/6-31G(d,p)) SOMO-α, SOMO-β and spin density distribution map of singlet biradical (SB) form of parent [6]PA.

Full size image

Peri-hexacene ([6]PA, Fig. 1d), which has been achieved difficultly 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). To obtain preliminary insights into the stability and properties of 1, Clar’s aromatic π-sextet rule was utilized. The Clar’s rule is a fundamental concept in the chemistry of polycyclic aromatic hydrocarbons (PAHs) and posits that the Kekulé resonance structure with the highest number of disjoint benzene-like aromatic π-sextets—referred to as “Clar’s sextets”—has the most contribution to the properties of the PAH. Generally, an isomeric PAH with more Clar’s sextets is expected to exhibit higher stability14,15,16. Recent studies have highlighted the importance of additional aromatic sextet rings on stabilizing the diradical structure25,26,27,28,31,32,33,34,35,36,37,38. Specifically, the gain of one or more additional Clar’s sextets in the open-shell form is crucial for achieving a stable ground-state singlet diradicaloid with high diradical character. According to this rule, 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 form 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/6-31 G(d,p) on the parent [6]PA indicated that the frontier SOMO-α and SOMO-β orbitals have significant disjoint features, 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))39, 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.

Results

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. The incorporation of CP rings in these bay regions not only enhances solubility but also restrain the Diels-Alder reaction in these areas, a phenomenon observed in the previous report concerning the synthesis of peri-tetracene derivative25. 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 Fig. 2, 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 procedure40,41. The monobromo CP-fused perylene 3 was firstly 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. This method differs from the stepwise syntheses typically reported for peri-acene derivatives25,26,27. Our one-pot strategy not only streamlines the procedure but also enhances overall efficiency, thereby providing an alternative pathway for the construction of other graphene fragments. The structure of 7 was confirmed by single-crystal analysis (Supplementary Fig. 22). 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. Because 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 (t1/2) of about 24 h when monitored at 617 nm (Supplementary Fig. 1). During the submission of our work, Wu et al. reported the synthesis of both peri-pentacene and peri-hexacene derivatives utilizing a similar synthetic strategy42. Their studies indicated that the peri-hexacene derivative exhibited remarkable stability, with its absorbance at the maximum decreasing by only 1% after 72 hours of exposure to ambient air and light, which surpassed the stability of our peri-hexacene derivative 1 under similar conditions. The exceptional stabilities observed in their work can be ascribed to the protection of the zigzag edges with bulky groups through a synergistic captodative effect, wherein the electron donor and acceptor collaboratively stabilize the radicals by forming a zwitterionic radical structure. In contrast to the stabilization strategy based on kinetic blocking combined with a synergistic captodative effect, our design employed kinetic protection of both zigzag edges and bay regions by bulky groups with similar electronic characteristics, resulting in reasonable stability. These findings offer valuable insights for the synthesis of a broader range of graphene fragments with zigzag edges in the future.

Fig. 2: Synthetic route toward peri-hexacene derivative 1.
figure 2

a Br2, DCM, 0 °C, 30 min; 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) KOBut, 18-crown-6, THF, rt; ii) p-chloranil. SPhos: 2-Dicyclohexylphosphino-2’,6’-dimethoxybiphenyl; DCE: 1,2-dichloroethane; THF: tetrahydrofuran; DCM: dichloromethane.

Full size image

Single crystals of 1 suitable for X-ray crystallographic analysis were grown by slow diffusion of acetonitrile into its solution in toluene. The molecule has a centrosymmetric geometry with a slightly distorted skeleton (Fig. 3a, b). The dihedral angles between the π-conjugated skeleton and its 2,6-dichlorophenyl substituents range from 83.1 to 84.5o (Fig. 3a). Bond length analysis of the 1 backbone revealed that the bond lengths of the CC bonds (the bonds shown in red, Fig. 3c) 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)26, indicating enhanced electronic coupling between two hexacene units. The CC bonds shown in blue (Fig. 3c) 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) values43 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. 3c), indicating a large aromatic character. The rings C/D/N/O have much smaller HOMA values (0.38 ~ 0.44, Fig. 3c) while the other benzenoid rings have a HOMA value of 0.65–0.80 (Fig. 3c), 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)44 calculations show that these five benzenoid rings have large negative NICS(1)zz values (−20.49 ppm to −22.20 ppm), the rings B/E/H/J/M/P have moderate negative NICS(1)zz values (−11.93 ppm to −16.00 ppm) while the rings C/D/N/O are almost non-aromatic (Fig. 4a). 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 (Supplementary Fig. 23).

Fig. 3: X-ray crystallographic analyses of 1.
figure 3

a Top view and b side view of 1. c The selected bond lengths (in Å) of the backbone. The pink numbers in hexagons are the calculated HOMA values; the hexagons shaded in gray with large HOMA values indicate the aromatic sextet rings.

Full size image
Fig. 4: Aromatic properties of 1.
figure 4

a Calculated NICS(1)zz values (the numbers in the rings) of 1. b Calculated ACID plot of 1 (isovalue: 0.02). The magnetic field is perpendicular to the XY plane and point out through the paper. c Calculated 2D ICSS map of parent peri-hexacene at 1 Å above the XY plane.

Full size image

To gain a deeper understanding of the electronic structure and aromaticity of compound 1, we carried out anisotropy of induced current-density (ACID)45 and isochemical shielding surface (ICSS)46,47 calculations (B3LYP/6-31 G(d,p)). ACID plot shows obvious clockwise diatropic ring current circuit along the periphery (Fig. 4b), 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. 4b), 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. 4c), 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-aromatic26. The central benzenoid rings connecting the two hexacene/heptacene units in [6]PA and [7]PA27 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 structures.

To further understand the ground-state electronic structure of compound 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 (Supplementary Fig. 3). 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 EPR measurements on 1 in solid and solution. The EPR intensities of compound 1 in powder state and dilute toluene solution decreased with the lowering of temperature (Fig. 5b and Supplementary Fig. 6a), and fitting of the data in both solid state and frozen solution by using Bleaney–Bowers equation48 gave a singlet-triplet energy gap (∆ES-T) of −1.41 kcal/mol and −1.74 kcal/mol, respectively (Fig. 5c and Supplementary Fig. 6b). In the solid states, we could observe the weak axially symmetric triplet state EPR signals corresponding to the ΔMs = ±1 transition (Fig. 5a), and the spectral simulation showed the spin-Hamiltonian parameters of S = 1, g = 2.0022, |D/hc| = 0.00825 cm−1, and |E/hc| = 0.00191 cm−1. Accordingly, the spin distance was determined to be 6.9 Å, which close to distance of the two zigzag edges of hexacene (7.06 Å) in the crystals. Rabi cycles at 70 K by an echo-detected nutation experiment of compound 1 was conducted (Fig. 5d), and a stable tris(2,4,6-trichlorophenyl)methyl (TTM, S = 1/2) radical is employed as an external standard. The results show that the slope of the linear dependence of Rabi frequency on the B1 magnetic field is approximately \(\sqrt{2}\) times that of a doublet species (S = 1/2), indicating the triplet species derived by thermal excitation (Fig. 5e)49,50. The spin-lattice relaxation time (T1) and spin coherence time (Tm) of 1 were obtained using the inversion recovery method and Hahn-echo sequences (Supplementary Fig. 4 and Supplementary Fig. 5). The spin-echo decay curves of 1 were fitted with an exponential decay to extract T1 of 16.7 ms and Tm of 0.85 μs at 70 K. Compared to most reported radicaloids (Supplementary Table 2)51,52,53,54,55, peri-hexacene 1 exhibits a longer spin-lattice relaxation time (T1 = 16.7 ms) and a comparable spin coherence time (Tm = 0.85 μs), with Tm significantly exceeding the duration of spin manipulation pulses (on the order of 10 ns) by nearly two orders of magnitude. These characteristics highlight its potential applications in quantum information processing and two-qubit systems56,57,58,59,60. Furthermore, we also carried out superconducting quantum interference device (SQUID) measurements for the microcrystalline sample of 1 at 2–300 K. The magnetic susceptibility increased with increasing temperature, and the singlet-triplet energy gap (∆ES-T) was estimated to be −1.33 kcal/mol by careful fitting of the data by using the Bleaney–Bowers48 equation (Fig. 5f), consistent with the above EPR results. 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)26 showed a larger ∆ES-T value, in consistency with its smaller diradical character. On the other hand, broken-symmetry DFT calculations (UB3LYP/6-31 G(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 (∆ES-T) is calculated to be −2.08 kcal/mol, which is consistent with the above experimental results. In addition, DFT calculations also gave a triplet-quintet energy gap (∆ET-Q) of −21.79 kcal/mol for 1. This large ∆ET-Q value is in accordance with its small tetraradical character.

Fig. 5: Magnetic properties of 1.
figure 5

a X-band cw EPR spectra of compound 1 in toluene at 293 K (red line: experiment; black line: simulation; simulation parameters: S = 1, g = 2.0022, |D/hc| = 0.00825 cm−1, and |E/hc| = 0.00191 cm−1). b Variable-temperature EPR spectra of compound 1 in solid form. c fitted I×T-T curve by using Bleaney−Bowers equation. I: EPR intensity, T: temperature in K. Inset is the calculated spin density distribution of the singlet diradical form. d Dependence of the Rabi oscillations of 1 on the microwave power attenuation. e Corresponding dependence of the Rabi frequency (Ω) on the B1 field. Key: standard radical TTM (black line; the molecular structure of TTM is shown in Supplementary Fig. 4d), 1 (red line). f χM T-T plot in the SQUID of measurement of the power 1, the solid line is the fitting curve according to Bleaney–Bowers equation.

Full size image

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. 6a. 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 (λmax) at 1090 nm along with a shoulder (λsh) at 1197 nm (Fig. 6a). 1 also displays multiple intense absorption bands in the visible region. The NIR absorption band has a similar structure to that in [4]PA derivative26 and [7]PA derivative27, 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 excitation61. The optical band gap (Egopt) 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)26 due to the π-extension.

Fig. 6: Electronic, optic and redox properties of 1.
figure 6

a UV/Vis-NIR absorption spectra of 1 in DCM at room temperature. b Cyclic voltammogram and differential pulse voltammogram of 1 in DCM with 0.1 M Bu4N·PF6 as supporting electrolyte; scan rate of 100 mV/s for CV.

Full size image

The electrochemical property of 1 was investigated by differential pulse voltammetry (DPV) and cyclic voltammetry (CV) measurements in dichloromethane (Fig. 6b). Compound 1 displays two reversible oxidation waves with halfwave potentials E1/2ox at 0.04 V and 0.32 V, and two reversible reduction waves with half-wave potentials E1/2red at −1.03 V and −1.24 V (vs Fc/Fc+). Compared with [4]PA derivative, the neighboring oxidation or reduction waves of 1 show 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 (EgEC) 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 λmax at 858 nm, 1210 nm and 1590 nm, respectively (Supplementary Fig. 2), and a strong EPR signal was observed for the radical cation (Supplementary Fig. 6c). The dication shows an intense absorption band with λmax at 845 nm in the NIR region (Supplementary Fig. 2). All absorption spectra are in agreement with the time-dependent (TD) DFT calculations (Supplementary Figs. 8 to 21).

To reveal the excited-state dynamics of compound 1, its femtosecond transient absorption (fs-TA) spectra have been recorded in degassed DCM (Fig. 7). As shown in Fig. 7a, upon photoexcitation, two photo-induced bleaching (PIB) bands centered at ~600 and ~644 nm were observed for fs-TA spectra of 1. The proximity of these two bands to the position of the ground-state absorption peak suggests they are originated from ground-state bleaching (GSB). Furthermore, two excited-state absorption (ESA) bands centered at ~470 and ~520 nm at blue region, as well as a broad band after 660 nm at the red region were observed, which should be originated from its singlet state. With the delay of the time, the TA signal recovered into the ground state without no further spectral evolution, suggesting that no new excited state was formed. From the fitting result of the single-wavelength dynamics of 470 nm (Fig. 7b), we can see that the singlet state decays rapidly into the ground state with a time constant of 26.4 ps. Notably, no triplet state signal was observed in the evolution process of the singlet state, which is consistent with the uphill thermodynamic driving force (1.33 kcal/mol).

Fig. 7: Excited state dynamics of 1.
figure 7

a fs-TA 2D spectra and time-resolved spectra of compound 1 in degassed DCM (λex = 610 nm). b Single-wavelength dynamics probed at 470 nm.

Full size image

Discussion

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 tetra radical character but 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. Notably, a long spin-lattice relaxation time of 16.7 ms and quantum phase memory time of 0.85 µs was realized in a 1 solution, suggesting its potential application in organic spintronics. 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.

Methods

Synthesis of 1

In a nitrogen-filled glove box, under argon atmosphere and in the dark, a 20 ml vial containing a magnetic stir bar was charged with 8 (50 mg, 0.018 mmol), tert-butoxide potassium (20.4 mg, 0.182 mmol), 18-crown-6 (48.0 mg, 0.182 mmol) and THF (10 mL). The resulting mixture was stirred at room temperature for 6 h, then p-chloranil (11.6 mg, 0.047 mmol) was added. The mixture was kept stirred for 20 min and then the solvent was removed under reduced pressure at room temperature. The resulting residue was purified by flash chromatography (silica gel was neutralized with Et3N, Hexane/DCM = 3:1) to give 1 (36.5 mg) as a cyan solid in 73% yield. Full experiment details can be found in the Supplementary Information.