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• Benzenoid polycyclic hydrocarbons, such as acenes, have garnered significant interest for their potential as organic semiconducting materials [1-6]. Pentacene Ⅰ (Fig. 1a), a classic acene, has been extensively studied for use in organic field-effect transistors [7, 8]. Alternatively, substituting the benzenoid rings with non-benzenoid rings in acenes can generate new π-systems that offer enhanced stability, unique aromaticity, and remarkable photophysical properties [9-12]. Azulene, a prominent example of a nonalternant non-benzenoid polycyclic hydrocarbon, is an isoelectronic structure of naphthalene and has a dipole moment of approximately 1.08 D [13]. Azulene-fused acenes exhibit unique chemical structures, aromaticity, and properties [14-28]. Among the azulene-fused analogues of pentacene, Takai and coworkers reported an azulene-fused acene (Ⅱ) with a narrow band gap, good solubility and stability [11]. Liu's group developed an efficient approach for synthesizing nonalternant analogues incorporating two azulene units (Ⅲ and Ⅳ) [22]. Very recently, Zhang and Xin et al. reported another nonalternant analogue Ⅴ fusing two azulene units [24]. While various analogues of pentacene with terminal azulene units have been reported, the analogue Ⅵ of pentacene with a non-terminal azulene unit remains largely unexplored, despite existing nonalternant analogues of other acenes that fuse non-terminal azulene units [14-30]. Additionally, azulene typically react with acids due to protonation occurring at the electron-rich five-membered ring.

  Figure 1

  

  Figure 1.  (a) Pentacene and its azulene-fused analogues. (b) This work: novel type of nonalternant analogues of pentacene that incorporate a non-terminal azulene unit.

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  This reaction leads to absorption change derived from the formation of azulenium cation, making azulene-fused acenes promising stimuli-responsive materials and conducting materials [13]. However, the azulene-fused acenes with positive charges, in which the azulene unit could react with bases rather than acids, remain largely unexplored.

  Conjugated metallacycles which incorporate transition metals into the organic polycyclic framework create new d_π–p_π conjugated patterns and impart both organic and organometallic characteristics continuously attract considerable attention due to their intriguing structures, unique properties, and promising applications [31-36]. Recent progress has witnessed the development of polycyclic conjugated metallacycles including metallapentalenes [37, 38], metallanaphthalene [39, 40], metallaanthracene [41, 42], metalla-dual-azulenes [43], and others [44-49]. However, fused metallacycles with acenes are rarely explored [41-46]. Herein, we present a new type nonalternant analogue of pentacene that incorporates a non-terminal azulene unit and a conjugated metallacycle (Fig. 1b). Theoretical studies reveal that the five-membered rings exhibit antiaromatic. Different aryl substituents were introduced to alter the properties of the acene. Enhanced absorptions in the low-energy regions are observed, attributed to the extensive conjugated aryl substituents that shift the HOMOs of acenes and narrow the HOMO–LUMO energy gap. Moreover, these fused acenes readily react with strong base rather than acid, thus leading to reversible base/acid stimuli responsiveness.

  Initially, we designed the diyne ligand L containing a naphthyl group and phenyl group. The reaction of ligand L with [Ir(CH₃CN)(CO)(PPh₃)₂]BF₄ in CH₂Cl₂ at room temperature (r.t.) for 6 h formed compound 1 in 65% yield (Scheme 1). A possible mechanism for the generation of 1, involving the [2 + 2 + 1] cycloaddition of L with [Ir(CH₃CN)(CO)(PPh₃)₂]BF₄ is proposed (Fig. S1 in Supporting information) according to our previous study [41]. Treatment of 1 with aryl alkynes and AgBF₄ in the presence of HBF₄·Et₂O at r.t. in CH₂Cl₂ for 6–8 h resulted in compounds 2 with yields from 80% to 92%. The reaction involves a formal [5 + 2] cycloaddition of the phenyl allyl alcohol in 1 with aryl alkynes, constructing a seven-membered ring, and finally forming nonalternant analogues of pentacene that incorporate a non-terminal azulene unit and a conjugated metallacyclopentadiene. The proposed mechanism for formation of 2 with a formal [5 + 2] cycloaddition was shown in Fig. S2 (Supporting information). Alkyl alkynes such as 1-heptyne, propargyl bromide, and ethoxyethyne were also attempted in this cycloaddition reaction, however, no targeted products were observed.

  Scheme 1

  

  Scheme 1.  Synthetic route for nonalternant analogues of pentacene that incorporate a non-terminal azulene unit.

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  Single-crystal X-ray diffraction analysis reveals that compound 2a comprises an azulene-fused acene with a conjugated five-membered metallacycle Fig. 2. In the five-membered metallacycle, the bond length of Ir–C1 (2.090 Å) is similar to that of Ir1–C10 (2.201 Å). The C–C bond lengths within the two fused five-membered rings (1.335(5)–1.472(5) Å) and the newly formed seven-membered ring (1.382(5)–1.463(5) Å) fall between those of typical C–C single and C=C double bond lengths, indicating a delocalized structure [22, 23, 41]. The planarity of seven-membered ring is manifested by the mean deviation of atoms C5–C11 from the least-squares plane (0.087 Å). The C–C bond distances and the planarity suggest it is an aromatic tropylium motif[47] and a significant contribution from the resonance form 2′.

  Figure 2

  

  Figure 2.  X-ray molecular structure of 2a (the phenyl groups in the PPh₃ moieties are omitted for clarity). Selected bond lengths (Å) and angles (degrees). Ir1−C1 2.090(4), Ir1−C10 2.201(3), C1−C2 1.335(5), C1−C13, 1.462(5), C2−C11 1.466(5), C3−C4, 1.431(5), C5−C6 1.382(5), C6−C7 1.384(5), C7−C8 1.441(5), C8−C9 1.463(5), C9−C10 1.437(5), C10−C11 1.411(5), C12−O1 1.132(5), C13−N1 1.284(5), Ir1−C1−C2 115.6(3), Ir1−C1−C13 119.1(3), Ir1−C10−C11 108.7(2), C1−Ir1−C10 79.7(14).

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  The unique conjugated structure promoted us to investigate aromaticity of compounds 2 using nucleus independent chemical shift (NICS) values [50, 51] and the anisotropy of the current-induced density (ACID) calculations [52, 53]. In general, negative NICS values and clockwise ring currents indicate aromaticity. Simplified model complex 2-PH₃ was employed where the PPh₃ ligands were replaced by PH₃ groups and aryl substituents were omitted to evaluate the aromaticity of compounds 2 (Fig 3a). As shown in Fig. 3b, the NICS(1)_ZZ values, which represents the out-of-plane zz components' contribution of the NICS tensor calculated at 1 Å above the ring plane, are negative for rings A, B, D, E, indicating these rings are aromatic. However, NICS(1)_ZZ values of five membered rings C and F are positive, suggesting weak antiaromaticity. Therefore, the skeleton of compounds 2 contains both aromatic rings and antiaromatic rings. ACID results are consistent with the NICS(1)_ZZ values: the obvious diatropic ring currents (clockwise vectors) confirm the aromaticity of the rings A, B, D, E, while paratropic ring currents (counterclockwise vectors) are observed in rings C and F. In comparison, the aromaticity of organic analogues 3 and 4 was also investigated. Compound 3 exhibits global aromaticity as evidenced by both negative NICS(1)_ZZ values and diatropic ring currents, similar to the other azulene-fused analogues of pentacene [11, 22, 24]. Compound 4 features an additional conjugated five-membered ring fused to the edge of azulene and can be regarded as having the iridium fragment of 2-PH₃ replaced by a CH group. Positive NICS value and paratropic ring currents in ring C indicate its antiaromaticity, in sharp contrast to its aromaticity in compound 3, whereas the other rings in compound 4 remain aromatic as in compound 3. The differing aromaticity of ring C in compounds 3 and 4 can be explained by the inherent aromaticity of azulene. Azulene exhibits aromaticity due to the formation of a 6π-electron cyclopentadienyl anion and a 6π-electron tropylium cation through intramolecular charge transfer. In molecule 4, we propose that a competing intramolecular charge transfer occurs from the electron-rich seven membered ring D to the electron deficient five membered rings C and F [23]. Since ring F can participate in the global aromaticity, as evidenced by ACID plots, the charge transfer from ring D is more likely to favor ring F over ring C. This results in aromaticity in ring F and antiaromaticity in ring C. Comparing 2-PH₃ and 4, the introduction of a metal fragment into the ring F may decrease π conjugation compared with organic systems. The ring C fused with ring F can be regarded as an analogue of metal-containing pentalene, exhibiting antiaromaticity. Thus, the unusual structure results in the unique aromaticity.

  Figure 3

  

  Figure 3.  Aromaticity evaluations of model complex 2-PH₃, organic azulene-fused acenes 3 and 4. (a) Chemical structures. (b) NICS(1)_zz values. (c) ACID plots: red lines represent diatropic ring currents (clockwise vectors) and blue lines represent paratropic ring currents (counterclockwise vectors).

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  Based on the abnormal structure and aromaticity, we further investigated the physiochemical properties of compounds 2, including their thermal stability, electrochemical properties, and photophysical properties. Compounds 2 start to decompose after heating at 140 ℃ for four hours under an air atmosphere (Table S3 in Supporting information). The electrochemical properties of compounds 2 were investigated by cyclic voltammetry. No reversible cyclic voltammetry was observed probably due to two positive charges in these compounds (Fig. S13 in Supporting information), while reversible electrochemical behaviors are typical in many organic azulene-fused acenes [54, 55]. The absorption spectra of compounds 1 and 2 were also measured (Fig. 4a). The absorptions of compounds 2 are red-shifted compared to that of compound 1 due to the presence of the conjugated azulene unit. Specifically, the absorption of compound 1 extends up to 606 nm and display an absorption maximum at 511 nm with a molar absorption coefficient value (ε) of 4.33 × 10³ L mol⁻¹ cm⁻¹, however, the absorption of compound 2a reaches up to 663 nm with two absorption peaks at 472 nm (ε = 1.01 × 10⁴ L mol⁻¹ cm⁻¹) and 583 nm (ε = 3.4 × 10³ L mol⁻¹ cm⁻¹). Moreover, due to the enhanced conjugation of the substituted aryl group at C7 position, the absorption spectra become stronger and red-shifted. For instance, phenanthrene-substituted acene 2e shows absorption peaks at 470 nm and 604 nm with higher coefficient values of 1.45 × 10⁴ and 5.80 × 10³ L mol⁻¹ cm⁻¹, respectively; the pyrene-substituted acene 2f displays three peaks at 467 nm (ε = 1.68 × 10⁴ L mol⁻¹ cm⁻¹), 590 nm (ε = 7.20 × 10³ L mol⁻¹ cm⁻¹), and 650 nm (ε = 6.00 × 10³ L mol⁻¹ cm⁻¹). The absorption spectra of nonalternant analogues of pentacenes (Ⅱ-Ⅴ) are generally below 550 nm [11, 22, 24], while those of compounds 2 are red-shifted due to the d_π–p_π conjugation between the metallacycle and acene unit [56], as well as spatial separation of LUMO and HOMO depending on the π-extension [57]. To gain insights of the absorption spectra, time-dependent density functional theory (TD-DFT) calculations were performed. All the absorption bands in the low-energy absorption regions can be assigned to HOMO to LUMO transition (Table S2 in Supporting information). The calculated results indicate that while the energy levels of the LUMOs are nearly identical across the compounds, the energy levels of the HOMOs noticeably increase with the degree of conjugation of aryl substituents (Fig. 4b). This feature favors a narrowed HOMO–LUMO energy gap, particularly evident in compound 2f, thus leading to the red-shifted absorption band compared to that of 2a. Surface plots of the HOMO and the LUMO of compounds provide more information on this feature (Fig. 4c). For compound 2a, featuring a single phenyl group at the C7 position, the LUMO is predominantly delocalized over the seven membered ring, the terminal phenyl group and part of metallacycle while HOMO distributions are delocalized over the naphthyl ring and the metallacycle. In contrast, the HOMO surfaces of compounds 2d-2f migrate from the naphthyl group to the aryl substituted groups while the LUMO distributions remain unchanged. Consequently, the overlap between the HOMO and LUMO is significantly minimized by the effective separation of their electron densities due to larger conjugation of aryl substituted groups which enables an increase of HOMO energy levels. Compounds 2 are virtually nonfluorescent and display very weak fluorescence under 400 nm light irradiation (Fig. S9 in Supporting information).

  Figure 4

  

  Figure 4.  (a) Absorption spectra of compounds 2, M⁻¹ cm⁻¹ = L mol⁻¹ cm⁻¹. (b) Calculated LUMO and HOMO energy levels. (c) Isodensity surface plots of the LUMO and the HOMO (isosurface value = 0.03, hydrogen atoms were omitted for clarity).

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  The electron-rich cyclopentadiene ring in azulene can undergo protonation by strong acids like trifluoroacetic acid (TFA), forming a stable tropylium cation. This cation can subsequently be neutralized by a base, thereby regenerating azulene [13]. However, no protonation reactions were observed upon adding TFA to the solution of compounds 2. This can be rationalized by two positive charges in these analogues and the existence of resonance structure 2′ containing a tropylium cation moiety. A similar phenomenon is also observed in azulene-pyridine-fused heteroaromatics reported by Gao and Swager et al., where the nitrogen atom on pyridine unit was protonated rather than the azulene moiety and the electron-withdrawing protonated nitrogen deactivates the azulene core and prevents protonation [29]. Inspired by the resonance structure of 2′, we envision that compounds 2 can react with hydroxide ion at the tropylium cation moiety. Taking compound 2f, which exhibits strongest absorption in the low-energy region, as an example, the yellow-green solution of 2f turned red immediately after the addition of NaOH. Compound 5 was isolated in 95% yield (Fig. 5a) and further verified by NMR spectra and HRMS data. The carbon signal of C5 at 72.9 ppm, compared to 141.4 ppm in 2f, indicates the disruption of conjugation and blue-shifted optical absorption.

  Figure 5

  

  Figure 5.  Reversible stimuli-responsive properties. (a) The reaction of compound 2f with sodium hydroxide yields compound 5, and treatment of compound 5 with tetrafluoroboric acid regenerates compound 2f. (b) UV–vis absorption spectra and the visual color changes of compound 2f and compound 5 in 1, 2-dichloroethane upon addition of sodium hydroxide and tetrafluoroboric acid. (c) The base/acid induced change of absorbance at 650 nm for five cycles. Conditions: DCE, 25 ℃, 1 × 10⁻⁴ mol/L.

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  The presence of hydroxyl was evidenced by its proton signal (2.89 ppm) and HRMS data (m/z calculated for [5]⁺, 1364.4271; found, 1364.4265, Fig. S57 in Supporting information). Once the acid (HBF₄) was injected into the red solution, the solution color instantly reverted to yellow-green (Fig. 5b). The fast (< min) and repeatable (≥5 cycles) change of absorption spectra induced by base and acid could potentially be exploited for switchable optical materials (Fig. 5c and Fig. S11 in Supporting information).

  In summary, a novel analogue of pentacene that incorporates a non-terminal azulene unit and one antiaromatic ring is constructed. The absorption spectra of these nonalternant acenes are red-shifted due to the d_π–p_π conjugation between the metallacycle and acene unit and spatially separated LUMO and HOMO depending on the π-extension. In particular, the aryl substituents with large conjugations in these analogues indeed change the HOMO distributions from the naphthyl ring and the metallacycle to aryl substituents and the HOMO energy levels increase along with the expansion of conjugation, thus leading to strong absorption in the low-energy absorption regions. Additionally, these nonalternant acenes exhibit high reactivity towards base rather than acid, and reversible stimuli responsiveness was achieved. These azulene-fused acenes with tunable photophysical properties provide the potential for future applications in switchable photofunctional materials.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Youxiang He: Data curation. Yongfa Zhu: Formal analysis. Ming Luo: Writing – original draft. Haiping Xia: Writing – review & editing, Conceptualization.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (Nos. 92156021, 22350009, and 22101115), Financial Support for Outstanding Talents Training Fund in Shenzhen, the Guangdong Provincial Key Laboratory of Catalysis (No. 2020B121201002), high level of special funds (No. G03050K003) and Introduction of Major Talent Projects in Guangdong Province (No. 2019CX01C079). The theoretical work was supported by the Center for Computational Science and Engineering at SUSTech.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110463.

  
  
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• 

  Figure 1  (a) Pentacene and its azulene-fused analogues. (b) This work: novel type of nonalternant analogues of pentacene that incorporate a non-terminal azulene unit.

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  Scheme 1  Synthetic route for nonalternant analogues of pentacene that incorporate a non-terminal azulene unit.

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  Figure 2  X-ray molecular structure of 2a (the phenyl groups in the PPh₃ moieties are omitted for clarity). Selected bond lengths (Å) and angles (degrees). Ir1−C1 2.090(4), Ir1−C10 2.201(3), C1−C2 1.335(5), C1−C13, 1.462(5), C2−C11 1.466(5), C3−C4, 1.431(5), C5−C6 1.382(5), C6−C7 1.384(5), C7−C8 1.441(5), C8−C9 1.463(5), C9−C10 1.437(5), C10−C11 1.411(5), C12−O1 1.132(5), C13−N1 1.284(5), Ir1−C1−C2 115.6(3), Ir1−C1−C13 119.1(3), Ir1−C10−C11 108.7(2), C1−Ir1−C10 79.7(14).

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  Figure 3  Aromaticity evaluations of model complex 2-PH₃, organic azulene-fused acenes 3 and 4. (a) Chemical structures. (b) NICS(1)_zz values. (c) ACID plots: red lines represent diatropic ring currents (clockwise vectors) and blue lines represent paratropic ring currents (counterclockwise vectors).

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  Figure 4  (a) Absorption spectra of compounds 2, M⁻¹ cm⁻¹ = L mol⁻¹ cm⁻¹. (b) Calculated LUMO and HOMO energy levels. (c) Isodensity surface plots of the LUMO and the HOMO (isosurface value = 0.03, hydrogen atoms were omitted for clarity).

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  Figure 5  Reversible stimuli-responsive properties. (a) The reaction of compound 2f with sodium hydroxide yields compound 5, and treatment of compound 5 with tetrafluoroboric acid regenerates compound 2f. (b) UV–vis absorption spectra and the visual color changes of compound 2f and compound 5 in 1, 2-dichloroethane upon addition of sodium hydroxide and tetrafluoroboric acid. (c) The base/acid induced change of absorbance at 650 nm for five cycles. Conditions: DCE, 25 ℃, 1 × 10⁻⁴ mol/L.

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