Sterically Controlled 5-exo-dig Cyclization Enables Modular Synthesis of Non-benzenoid Polycyclic Aromatic Hydrocarbons with Intriguing (Anti)aromaticity and Diradical

Sterically Controlled 5-exo-dig Cyclization Enables Modular Synthesis of Non-benzenoid Polycyclic Aromatic Hydrocarbons with Intriguing (Anti)aromaticity and Diradical | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Sterically Controlled 5-exo-dig Cyclization Enables Modular Synthesis of Non-benzenoid Polycyclic Aromatic Hydrocarbons with Intriguing (Anti)aromaticity and Diradical Renqiang Yang, Liangliang Chen, Zhichun Shangguan, Tianyu Shi, and 8 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7977582/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Non-benzenoid polycyclic aromatic hydrocarbons (PAHs) containing antiaromatic indacene or pentalene and aromatic azulene subunits emerged as compelling materials, distinguished by their unique electronic configurations, exceptional optoelectronic characteristics, and potential applications in organic electronics. However, their controllable synthesis remains challenging due to inherent instability and stringent electronic requirements. Herein, we present a modular synthetic strategy that enables the construction of stable non-benzenoid PAHs ( 1 , 2 , and 3 ) featuring indacene, pentalene, and azulene motifs through a carefully designed sequence of 5-exo-dig cyclization (with controllable E/Z -selectivity), nucleophilic addition, Friedel-Crafts cyclization and oxidative dehydrogenation. Comprehensive structural and electronic analyses revealed that 1 and 2 exhibit global antiaromaticity and 2 displays a more pronounced open-shell diradical character than 1 , while 3 maintains a global aromaticity and a closed-shell structure. Notably, compound 2 demonstrated promising p -type semiconductor behavior with a hole mobility of up to 0.083 cm 2 V − 1 s − 1 . Additionally, all three compounds demonstrated remarkable stability under ambient conditions, underscoring their potential for practical applications in organic electronics. Further exploration of this synthetic strategy enabled the potential synthesis of additional non-benzenoid PAHs ( 25 − 1/2/3 ), which show strong antiaromaticity and triplet diradical characteristics, resulting in their instability. This work provides a tailorable and universal approach to designing non-benzenoid PAHs with tunable structure, aromaticity and diradical characters for functional applications. Physical sciences/Chemistry/Materials chemistry/Electronic materials Physical sciences/Chemistry/Materials chemistry/Magnetic materials Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Over the past decade, non-benzenoid polycyclic aromatic hydrocarbons (PAHs) featuring pentagonal and heptagonal rings have emerged as a vibrant research frontier in materials chemistry. 1 – 9 These structurally unique systems exhibit fundamentally different electronic properties compared to their benzenoid counterparts, owing to their characteristic structural strains, electronic character, and non-alternant π-conjugation networks. 10 – 19 Particularly noteworthy are three archetypal building blocks, antiaromatic indacene (5/6/5-fused rings with 12 peripheral π-electrons), antiaromatic pentalene (5/5-fused rings with 8 π-electrons), and aromatic azulene (5/7-fused rings with 10 π-electrons). Integrating these motifs into extended π-conjugated systems enables precise modulation of electronic structures and molecular geometries. 20 – 22 Antiaromatic indacene and pentalene units impart remarkable open-shell characteristics, facilitating unusual spin interactions and enhancing electron delocalization, 23–25 while azulene moieties introduce strong dipole moments and intramolecular charge transfer capabilities, resulting in tunable absorption and emission properties. 26 – 32 These distinctive features give rise to novel optoelectronic behaviors and spin-related phenomena, positioning non-benzenoid PAHs as promising candidates for advanced applications in organic electronics, singlet fission systems, and molecular magnetism. 33 – 37 The ability to fine-tune these properties through rational molecular design has sparked growing interest in developing synthetic methodologies for these structurally challenging yet functionally versatile materials. The formidable challenge is mainly attributed to the inherent instability and complex electronic requirements for synthesizing these subunit-based molecular materials. A notable example is the first synthesis of s -indacene by Hafner and coworkers in 1963, which yielded an unsubstituted derivative that exhibits poor stability, being highly sensitive to oxygen and acids, and was inadequately characterized. 38 – 39 Recent advances in synthetic methodologies have facilitated more precise construction of these architectures, thereby unlocking new opportunities for their practical implementation. Currently, s -indacene-based molecules can be prepared through two primary routes: (1) sequential nucleophilic addition and reductive elimination reactions starting from dicarbonyl (ketone) compounds, 23, 40 or (2) a combination of nucleophilic addition of aldehyde, Friedel-Crafts cyclization, and oxidative dehydrogenation reactions (Scheme 1a ). 24 , 41 Building on these approaches, Haley, Tobe, Chi, Müllen etc . successfully prepared a series of s -indacene derivatives exhibiting open-shell characteristics, 42–46 where the diradical character was mainly localized at the C1 position of the s -indacene core through steric protection with bulky substituents (Fig. 1 a). Similarly, another notable antiaromatic subunit, pentalene, which consists of two fused five-membered rings with 8 π-electrons, is also inherently unstable except for sterically protected derivatives or annulated with other aromatic rings. 47 – 48 Owing to the fused five-membered rings (5/5), pentalene exhibits a distinct electronic structure compared with the dicyclopenta-fused arenes, which were initially discovered by Garcia-Garibay and later pursued by Plunkett. 49 – 52 The main synthetic routes to pentalene derivatives include: (1) a one-step Pd-catalyzed or Ni-catalyzed cyclodimerization of ortho-bromophenylacetylene derivatives with low yield (Scheme 1b ), 53–54 or (2) a two-step sequence involving Pd-catalyzed cyclization followed by Fe-mediated dehydrogenation. 55 In addition, Hashmi and coworkers reported a gold-catalyzed regiospecific annulation of unsymmetrically substituted 1,2-di(arylethynyl)benzenederivatives for a geometry-controlled synthesis of linear bispentalenes. 56 Yasuda and coworkers synthesized a Dibenzo[a,f]pentalene by sequential nucleophilic addition of aldehyde, Friedel − Crafts cyclization and oxidative dehydrogenation processes, which exhibited antiaromatic and singlet biradical characters. 47 , 57 Despite these rapid developments in synthetic strategies, the central or side benzene or naphthalene moiety in these molecules (Fig. 1 b) remains strongly aromatic except for the pentalene units, preserving overall stability. Nevertheless, combining two antiaromatic subunits of s -indacene and pentalene into one molecule, simultaneously, has not been explored due to synthetic challenges. Additionally, it is also unclear how these two antiaromatic units affect each other and their impact on the overall performance of the molecule. In comparison, azulene-containing conjugated systems benefit from relatively higher stability among these three motifs, leading to the development of diverse synthetic approaches. Traditional methods for constructing azulene units, such as those employing pyridinium salts or troponoids, face limitations when applied to larger azulene-embedded PAHs due to low reactivity and challenges in accessing appropriate benzo-fused precursors. 15 Consequently, alternative strategies have emerged for in situ construction of azulene subunits within extended π-systems, including both on-surface synthesis, 58–63 and in-solution chemistry. These encompass Scholl-type cyclization, 64–66 intramolecular Friedel-Crafts reactions followed by aromatization, 31, 67 Pd-catalyzed alkyne annulation, 68–70 , etc. 71 – 73 However, these transformations typically require multi-step synthesis of specialized precursors and are often complicated by rearrangement or insertion side reactions, leading to unpredictable product distributions. 28 For more controllable synthesis of azulene-based PAHs, a common strategy involves starting with commercially available azulene and performing sequential coupling and cyclization reactions to construct azulene-terminated PAHs (Scheme 1c ). 74 – 79 However, the azulene unit in these azulene-terminated PAHs exhibits little effect on the optoelectronic properties of the whole molecules owing to the relatively independent local electronic structure characteristic. In contrast, azulene-embedded PAHs integrate azulene into the fused ring backbone, which directly tunes the π-conjugation pathway. For example, Feng and Liu recently reported a modular approach to azulene-embedded PAHs using a cascade reaction combining Suzuki coupling and Knoevenagel condensation (Scheme 1c ). 80 – 82 Nevertheless, these synthetic methods generally require complex precursor preparation and are often limited to specific substrate types. Therefore, the development of simpler, more controllable, and broadly applicable synthetic strategies for preparing non-benzenoid PAHs with tunable structures and properties is highly desirable, especially for these azulene-embedded linear PAH. Herein, we developed an innovative synthetic strategy combining 5-exo-dig cyclization of alkyne, nucleophilic addition of aldehyde, Friedel − Crafts cyclization at the exocyclic double-bond or substituted phenyl position and oxidative dehydrogenation processes to construct two highly stable antiaromatic PAH combining an indacene skeleton and two pentalene (5/5) motifs ( 1 and 2 ) and an S-shaped azulene (5/7)-embedded linear PAH ( 3 ). The product selectivity was governed by the E/Z configuration of key intermediates ( 7 , 13 , and 19 ) obtained through 5-exo-dig cyclization. Detailed structural characterization reveals that 1 and 2 exhibited global antiaromaticity, while 3 exhibits global aromaticity along its molecular periphery, supported by bond length analysis of crystal and theory calculation. Notably, 2 adopts a planar configuration and demonstrates significant open-shell character (diradical index y 0 = 0.38) with a small singlet-triplet energy gap (Δ E st = -2.02 kcal/mol). In contrast, 1 maintains a closed-shell configuration, while 3 exhibits a twisted geometry resulting from the cove edge between 2hthalene and azulene moieties, along with closed-shell characteristics. Remarkably, 2 demonstrates promising p -type transporting behavior, achieving a hole mobility up to 0.083 cm 2 V − 1 s − 1 under an air atmosphere, underscoring its potential for organic electronics. To further assess the universality of this synthetic approach, we designed two axisymmetric substrates of 22a and 22b , which successfully afforded 55/55 ( 24 − 1 ), 55/57 ( 24 − 2 ) and 57/57 ( 24 − 3 ) structures owing to the limited stereoselectivity. However, subsequent oxidative dehydrogenation failed to yield isolable non-benzenoid PAH products, which was attributed to their inherent instability arising from triplet ground states, as supported by experimental observations and DFT calculations. This modular synthetic strategy provides a versatile and controllable approach for exploring diverse 5/5 or 5/7 rings-based non-benzenoid PAHs, from chemical structure, optoelectronic properties, to applications in organic electronics. Results Synthesis and characterization The synthetic routes towards compounds 1 , 2 and 3 are illustrated in Scheme 2 . Key precursors (compounds 6 , 12 , and 18 ) were prepared through sequential Sonogashira and Suzuki coupling reactions starting from 4 , 10 , and 16 . These precursors subsequently underwent Pd(OAc) 2 -catalyzed 5-exo-dig cyclization in the presence of 2-dicyclohexylphosphino-2',6'-dimethoxybiphenyl (Sphos) and tri-n-butylphosphine ligands to afford intermediates 7 , 13 , and 19 . 83–85 Interestingly, the exocyclic double bonds of compounds 7 and 13 adopted E -configuration, whereas 19 exhibited a Z -configuration due to steric hindrance imposed by its substituents. This stereochemical assignment was confirmed through comprehensive structural characterization. As shown in Figures S1 and S2, NOESY spectra of 7 and 13 revealed strong correlation signals between H c and H d , H g and H e , consistent with E -configuration. Moreover, the crystals of 7 and 13 , suitable for single-crystal structural analysis, were successfully obtained by slowly diffusing methanol into the chloroform solution, which unambiguously confirmed their structures as assigned (Scheme 2 , Figures S7 and S8). Notably, in the case of 13 , the intramolecular hydrogen bond between the aldehyde oxygen and the exocyclic double bond hydrogen was observed in the crystal structure, potentially stabilizing the E -configuration. Alternatively, intermediate 19 was determined to possess a Z -configuration, as evidenced by NOESY correlations between H c and H e , H d and H g (Figure S3) and confirmed by single-crystal analysis (Scheme 2 and Figure S9). Then, dihydro-precursors of 9 , 15 and 21 were obtained by the treatment of 7 , 13 , and 19 with mesitylmagnesium bromide (MesMgBr), followed by Friedel − Crafts cyclization with BF 3 ·OEt 2 (Scheme 2 ). Subsequently, the oxidative dehydrogenation of 9 , 15 and 21 using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) afforded the target products in moderate yield across three steps. Compound 1 was obtained as an army green solid with a yield of 17.3%, 2 as a purple solid with a yield of 63.3% and 3 as a brown solid with a yield of 19.7%. The chemical structures of 1 , 2 and 3 were thoroughly characterized using nuclear magnetic resonance (NMR) spectroscopy, high-resolution mass spectra (HRMS), and X-ray single-crystal analysis. Detailed characterization data and experimental procedures are provided in the synthesis and characterization section of the Supporting Information. Single-crystal structure analysis The single crystals of 1 and 2 were successfully grown by slowly diffusing methanol into the chloroform/CS 2 (1/1, v/v) solution at -4 o C. As shown in Fig. 1 , both 1 and 2 maintain nearly planar geometries, with significant steric hindrance from the mesityl and tert-butyl phenyl substituents preventing π-π stacking interactions between molecular backbones. 1 shows a one-dimensional parallel packing and the main interactions are multiple C-H…π with distances of 2.820–2.842 Å and C-H…O with distances of 2.483–2.854 Å (Fig. 1 c), while 2 shows a herringbone stacking and is also dominated by C-H…π interactions (with shorter distances of 2.681 Å) and C-H…C-H interaction (2.316 Å) (Fig. 1 f). Bond length analysis depicts that the C( sp 2 )-C( sp 2 ) bond lengths for the periphery of 1 and 2 exhibit typical bond-distance alternation, ranging from 1.348 Å to 1.487 Å for 1 and 1.349 Å to 1.476 Å for 2 , as expected for a closed-shell quinoid structure (Figs. 1 b and 1 e). Especially, the bond lengths of C10-C11 of the central benzene ring and naphthalene ring of 1 or 2 are 1.489 and 1.480 Å, indicating the aromaticity of the central benzene ring and naphthalene ring was disrupted, consistent with quinoid structures. The small bond length alternation and shorter distance of C10-C11 of 2 than 1 agree well with the small energy barrier for the valence tautomerization between the closed-shell quinoid and open-shell diradical structure. In addition, the distances of C9-C10 bonds in 1 and 2 are 1.390 and 1.413 Å, respectively, which fall between those of known closed-shell (1.371 Å) and open-shell (1.437 Å) Mes-substituted indenofluorene analogues. 86 – 87 The intermediate value indicates 1 and 2 are resonance hybrids, with contributions from both open-shell and closed-shell structures in their ground states. Moreover, the slightly longer C9-C10 bond length in 2 compared to 1 further demonstrates that 1 exhibits more pronounced closed-shell character, while 2 leans toward open-shell features. Similarly, single crystals of 3 suitable for X-ray diffraction analysis were obtained through slow diffusion of methanol into a chloroform solution at -4°C. The molecular structure displays a distinctive twisted S-shaped conformation (Fig. 1 j), resulting from steric repulsion between the naphthalene and azulene moieties at the cove region. Interestingly, the crystal packing exhibits an intriguing trimeric superstructure composed of repeating M1, M2, and M3 units. M1 and M3 adopt head-to-tail stacking with torsion angles of 35.1° and 20.3°, respectively, whereas M2 inserts nearly vertically between M1 and M3, with torsion angles of 22.7° (19.6°) (Fig. 1 j). The main intermolecular interactions between adjacent molecules are π-π interactions with distances of 3.250–3.398 Å and C-H…O interactions with distances of 2.317–2.719 Å (Figs. 1 i and 1 k). Bond length analysis of 3 revealed that the C-C bond length of the periphery along the S-shaped skeleton exhibited an averaged characterization with the longest bond length up to 1.478 Å (C11-C16) and the shortest bond length down to 1.346 Å (C10-C11), indicating the global aromatic character. In addition, the bond lengths along the 5/7 ring exhibit distinct bond alternation characterization, as shown in Fig. 1 h, between 1.346 and 1.478 Å, which are less averaged than that of the parent distinct azulene unit (1.387–1.427 Å) owing to the more delocalized electronic feature within the extended conjugated structure. These results indicate that the azulene motif in this molecule is not independent, which exhibits a significant influence on molecular configuration, stacking, intermolecular interactions and electronic properties. Aromaticity and electronic structure To gain deeper insights into the electronic structures and aromaticity of 1 , 2 and 3 , nucleus-independent chemical shift (NICS) calculations were performed at the (U)B3LYP/6-31G(d,p) level of theory. The obtained NICS(1) zz values of 1 are − 16.1, 8.2, 18.9 and 16.2 ppm for rings A, B, C and D (Figs. 2 a and S12), respectively, indicating that rings B, C, and D in 1 are strongly antiaromatic. Similarly, NICS(1) zz values of -14.9, 13.1, 10.7 and − 2.4 ppm for rings A, B, C and D were observed in 2 (Figs. 2 b and S12), suggesting rings B and C in 2 are also antiaromatic, while ring D is weak aromaticity. The reduced antiaromaticity of 2 compared to 1 correlates with its enhanced open-shell character. In contrast, the NICS values of B, C and D rings is smaller than the individual s -indacene (54.7, 45.0 and 54.7 ppm) and pentalene (62.4 and 62.4 ppm) motifs (see Figure S16), indicating the B, C and D rings in 1 and 2 are hybridizing structures, which weakens the s -indacene and pentalene motifs and thus improves the stability of these two molecules. To further explore the global antiaromaticity of compounds 1 and 2 , we also calculated the bis(pentalene) derivatives without the A ring. As shown in Figure S17, the NICS values of two five-membered rings are 38.8 and 37.6 ppm for bis(pentalene)-fuse benzene (compound A) and 31.9 and 29.2 ppm for bis(pentalene)-fuse naphthene (compound B), while the central benzene or naphthene are weak antiaromaticity or aromaticity (7.4 ppm for A and − 2.1 ppm for B), indicating that the two pentalene motifs in these molecules were dependent and the antiaromatic electronic property is more localized owing to the central aromatic rings. In contrast, the aromaticity of the central benzene or naphthalene rings of 1 and 2 was broken, showing quinone structure and antiaromaticity characteristics. Consequently, 1 and 2 exhibit global antiaromatic character. Conversely, the rings A and D of 3 exhibit strong aromatic features with NICS(1) zz values of -26.8 and − 20.7 ppm for rings A and D and the rings B and C of 5/7 motif are weak aromaticity with NICS(1) zz values of -9.2 and − 9.3 ppm (Fig. 2 c), indicating 5/7 motif in 3 is not independent because of the good delocalized characteristic upon the whole skeleton, aligning well with the bond analysis. Additionally, these results are consistent with the anisotropy of the induced current density (ACID) and isotropic chemical shielding surface (ICSS) analysis results. As illustrated in Figs. 2 d and 2 e, the calculated ACID plot shows counter-clockwise ring current along the periphery except for ring A of 1 and 2 , corresponding to the antiaromaticity of indacene and pentalene. On the contrary, diamagnetic ring currents are found along the periphery of 3 (Fig. 2 f), indicating global aromaticity along the periphery. These results agree well with the bond length analysis in the crystals and the ICSS calculation (Figure S11). The UV-vis absorption spectra of 1 , 2 and 3 in dichloromethane (DCM) solution at a concentration of 10 − 5 mol/L are shown in Fig. 2 g. The colors of 1 , 2 and 3 in DCM are purple, blue and claybank, respectively, as highlighted in the inset of Fig. 2 g. 1 and 2 display similar absorption patterns with two main absorption peaks based on their likely electronic structures. The DCM solution of 1 exhibits a prominent long-wavelength absorption peak at 553 nm, accompanied by a weak absorption tail extending up to 907 nm. Time-dependent (TD) DFT calculations reveal that the absorption band at 553 nm with a shoulder peak at 521 nm is mainly attributed to HOMO-3→LUMO (S 0 →S 4 ) electronic transition. The weak-tail absorption is attributed to symmetry-forbidden HOMO→LUMO electronic transition with an oscillator strength of 0.1037 (Figs. 2 i and S18), following its antiaromatic character. For 2 , the absorption spectrum exhibits a significant bathochromic shift compared to 1 . The long-wavelength absorption peak appears at 644 nm, red-shifted by 91 nm, which is mainly ascribed to HOMO-2→LUMO (S 0 →S 4 ). Interestingly, the calculated maximum absorption peak is in the intermediate between the open-shell (729 nm) and closed-shell (565 nm) states (Figure S20), illustrating that the long-wavelength absorption was attributed to both closed-shell and open-shell exciton states, consistent with the diradicaloid nature of compound 2 . The long-tail absorption also extends to 1071 nm with a larger oscillator strength of 0.2774, mainly originating from a partially allowed HOMO → LUMO electronic transition due to its open-shell feature (Figs. 2 j and S19). For compound 3 , the absorption peak at 537 nm is mainly attributed to the HOMO→LUMO + 2 (S 0 →S 3 ) electronic transition and the absorption band at 441 nm with a shoulder absorption at 415 nm originates from HOMO-1→LUMO (S 0 →S 5 ) and HOMO-1→LUMO + 2 (S 0 →S 9 ) electronic transitions (Figs. 2 k and S21). Accordingly, the optical energy gaps ( E g opt ) for 1 , 2 and 3 were determined to be 1.36, 1.15, and 2.06 eV, respectively, from the onset of their UV − vis absorption spectra. Furthermore, time-dependent UV-vis measurements were performed under ambient conditions to investigate the stability of 1 , 2 and 3 (see Figures S22 and S23). The results revealed that 1 and 3 exhibited exceptional stability, with no significant changes observed in their absorption spectra in DCM solution within 28 days. In contrast, 2 showed gradual degradation when exposed to ambient conditions, with a fitted half-life of approximately 32 days for a 10 − 5 M DCM solution. This difference in stability can be attributed to the more pronounced open-shell character of 2 , which renders it more reactive under ambient conditions. In addition, three compounds can be stored as crystalline solids under ambient conditions without any degradation. These results highlight the remarkable stability of these non-benzenoid PAHs, particularly for 1 and 3 , which maintain their structural and optical integrity even in solution over extended periods. The electrochemical behaviors of 1 , 2 and 3 were investigated by cyclic voltammetry (CV) in anhydrous DCM solution with ferrocene/ferrocenium (Fc/Fc + ) as an external standard, revealing distinct redox behaviors for each compound (Fig. 2 h). 1 exhibited two oxidation waves at E 1/2 ox = 1.16 and 1.39 V versus Fc/Fc + , along with two reduction waves at E 1/2 re = -0.47 and − 0.93 V, while 2 showed more complex redox activity with three oxidation peaks at 0.98, 1.22, and 1.39 V and two reduction peaks at -0.43 and − 0.72 V. In contrast, 3 displayed oxidation potentials at 1.24, 1.53, and 1.71 V and reduction potentials at -0.92, -1.45, and − 1.69 V, reflecting its different electronic structure. From the first redox couples, we estimated the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels at -5.48/-3.85 eV for 1 , -5.30/-3.89 eV for 2 , and − 5.56/-3.40 eV for 3 , yielding electrochemical band gaps ( E g el ) of 1.63, 1.41, and 2.16 eV, respectively. Diradical and charge transport characteristics The chemical and electronic structures of 1 , 2 and 3 were further investigated by variable-temperature (VT) 1 H NMR measurements. For 2 , the proton signals in C 2 D 2 Cl 4 exhibited significant broadening with the temperature increasing from 300 to 393 K, particularly for the protons b , c, d and g located on the backbone (see Figs. 3 a and S25). Upon cooling back to 300 K, the signals fully recovered to their original intensity. However, when further cooled to 243 K, the intensity of the signals gradually decreased, which was attributed to the reduced solubility of 2 at lower temperatures (Figure S26). This reversible thermal broadening strongly indicates population of a triplet state at elevated temperatures, consistent with the significant open-shell diradical character of 2 . Meanwhile, both 1 and 3 maintained sharp NMR signals throughout the same temperature range (300–393 K) (Figures S24 and S27), demonstrating their closed-shell nature with negligible thermally accessible diradical states. Complementary electron-spin resonance (ESR) and superconducting quantum interference device (SQUID) measurements provided definitive evidence for the distinct open-shell characteristics of two antiaromatic compounds of 1 and 2 . While 1 showed no solution-phase ESR signal but weak solid-state signals attributed to its small band gap (Figure S28a), 88 2 exhibited strong isotropic signals ( g e = 2.002) in both toluene solution and solid state, unambiguously confirming its paramagnetic nature and carbon-centered radical character (Figures S28b and S30). Additionally, VT-ESR studies (180–330 K for 1 and 160–220 K for 2 ) demonstrated increasing signal intensity with decreasing temperature for both compounds (Fig. 3 b and S29a), with Bleaney-Bowers analysis (the signal ( I ) x T versus T ) yielding Δ E st = -4.72 kcal/mol for 1 and − 2.02 kcal/mol for 2 (Fig. 3 c and S29b), 89–91 consistent with the more pronounced diradical character of 2 . DFT calculations at the UCAM-B3LYP/6-31G(d) level reveal 2 displays an open-shell singlet ground state, with a y 0 of 0.38, while 1 , in contrast, exhibits a negligible diradical character with a closed-shell state (Table S11). The hybridizing structure of s -indacene and pentalene allows the spin densities in 2 to be primarily localized both on C7 and C9 of the cyclopenta-rings, which is conducive to improving y 0 and stability (see Fig. 3 d). This electronic structure property is completely different from the reported individual s -indacene or pentalene structures, indicating the hybridization of the antiaromatic motifs is an effective strategy to modulate the whole aromaticity and diradical as well as molecular stability. The spin-density distribution also corresponds to the significant changes observed in the VT-NMR signals of b, c, and g hydrogens (Fig. 3 a). Additionally, SQUID magnetometry further corroborated these findings, showing a continuous increase in χₘT for 2 from 2-300 K, indicative of progressive thermal population of triplet states, whereas 1 displayed only weak, discontinuous magnetic responses characteristic owing to its close-shell structure (Figure S31). The charge transport characteristics of 1 , 2 , and 3 were evaluated through bottom-gate bottom-contact (BGBC) field-effect transistor (FET) devices fabricated under ambient conditions, with detailed device architecture illustrated in Fig. 4 a and fabrication procedures provided in the Supporting Information. While 1 showed negligible charge transport behavior, 2 demonstrated well-defined p -type semiconductor characteristics, as evidenced by the transfer and output curves (Figs. 4 a and 4 b). Analysis of the saturation regime transfer characteristics revealed impressive hole mobilities for 2 , with maximum and average values reaching 0.083 and 0.064 cm 2 V − 1 s − 1 , respectively, accompanied by an on/off current ratio exceeding 10³ (Fig. 4 c, Table S12). These values are comparable to the best-reported values for organic diradical small molecules. 92 – 93 The good carrier-transporting properties may be attributed to the good planarity, tight molecular packing, better diradical stability, and suitable frontier orbital energy levels. Universality of Synthetic Strategy Building upon the advantages of the modular synthetic approach and the promising optoelectronic characteristics of non-benzenoid PAHs incorporating indacene, pentalene, or azulene motifs, we extended our investigation to synthesize additional non-benzenoid PAHs using diverse substrates through this strategy. Here, we designed and synthesized two novel substrates ( 22a and 22b ) that successfully enabled the concurrent construction of 55/57 ( 24 − 1 ), 57/57 ( 24 − 2 ), and 55/55 ( 24 − 3 ) skeletons (Fig. 5 a). This was achieved through a series of reactions, including 5-exo-dig cyclization, aldehyde nucleophilic addition, and Friedel-Crafts cyclization, facilitated by the poor E/Z stereoselectivity of the exocyclic double bond (Fig. 5 ). We isolated the corresponding products with different yields, 55.9% and 32.2% for 24-1a and 24-3a , 25.6%, 3.3%, and 40.6% for 24-1b , 24-2b , and 24-3b , respectively. The notably low yield of 24 − 2 compared to the higher yields of 24 − 1 and 24 − 3 suggests that product formation is strongly influenced by the steric hindrance ( E - or Z -configuration) of the key intermediates ( 23 − 1 , 23 − 2 , and 23 − 3 ). However, attempts to obtain the corresponding oxidized non-benzenoid PAHs ( 25 − 1 and 25 − 2 ) through oxidative dehydrogenation using DDQ/toluene conditions were unsuccessful. While 25 − 3 could be formed under DDQ/toluene conditions, it was proved too unstable for isolation in pure form, though the HRMS corroborates the proposed structure. Unfortunately, we could not obtain the single crystal of compound 25 − 3 owing to its instability. We successfully obtained the single crystal structure of the corresponding precursor of 24-3a , which unambiguously confirmed the construction of two 5/7 motifs and locks in the core ring topology, providing high confidence in the chemical structure of 25-3a/b (see Figs. 5 and S32). These results suggested that the proposed structure of 25 − 3 is rational. To elucidate the origin of this instability, we conducted DFT calculations to examine the electronic structure and aromaticity of these compounds. As illustrated in Figs. 5 c- 5 e, these non-benzenoid systems display pronounced local antiaromaticity, particularly in the 5/5 and 5/7 motifs, as evidenced by NICS(1) zz , ICSS, and ACID analyses (see Supporting Information). This antiaromatic character disrupts electron delocalization across the molecular framework, leading to their pronounced instability under reaction conditions. Furthermore, DFT calculations revealed that these compounds adopt triplet ground states (Table S15 and Figure S34), further accounting for their instability. Surprisingly, when we subjected the asymmetric precursors 24-1a and 24-1b to oxidation, we isolated hydroxyl-substituted derivatives ( 26-1a and 26-1b ) in 8.6% and 12.1% yields, respectively. Their structures were unambiguously characterized by NMR, HRMS, and single-crystal X-ray diffraction (Figure S33). ICSS maps (Fig. 5 b) indicate that while the 5/5 rings retain their antiaromatic nature, the 5/7 rings transition to weakly aromatic behavior upon hydroxylation, thereby enhancing molecular stability. These findings underscore the critical role of E/Z -configuration stereoselectivity in the 5-exo-dig cyclization intermediates for accessing structurally diverse 5/5 or 5/7-membered ring skeletons of non-benzenoid PAHs featuring indacene, pentalene, or azulene units. Discussion In summary, we developed a modular synthetic strategy to construct three distinctive non-benzenoid polycyclic aromatic hydrocarbons (PAHs) that 1 and 2 combine an indacene core and two pentalene (5/5) subunits and 3 is an S-shaped azulene (5/7)-embedded linear PAH. The product selectivity was governed by the E/Z -configuration of key intermediates, yielding corresponding products of 1 , 2 , and 3 , as confirmed by NMR, HRMS, and X-ray crystallography. Bond length analysis, NICS(1) zz calculations, ACID plots, and ICSS maps revealed that 1 and 2 adopt planar, globally antiaromatic structures, whereas 3 exhibits a twisted, globally aromatic periphery. Intriguingly, compound 2 exhibited a pronounced open-shell feature, with a y 0 of 0.38 and a small Δ E st of -2.02 kcal/mol compared to 1 (closed-shell) and 3 (closed-shell), as confirmed by VT-NMR, VT-ESR measurements, and DFT calculations. The half-life of 2 in dichloromethane solution was approximately 32 days, while 1 and 3 showed almost no degradation within a month. Surprisingly, compound 2 demonstrates promising p -type transporting behavior, achieving a hole mobility up to 0.083 cm 2 V − 1 s − 1 under air atmosphere, underscoring its potential for organic electronics. Further extension of this method to two axisymmetric substrates yielded additional non-benzenoid PAHs, though their instability was attributed to strong antiaromaticity and triplet states, as experimental and DFT calculations confirmed. This work provides a versatile and controllable approach to access structurally diverse non-benzenoid PAHs containing indacene, pentalene, or azulene motifs, offering tunable electronic properties for functional applications. Future efforts will focus on optimizing stability and expanding the library of functional non-benzenoid architectures. Methods Synthesis of compound 1 To a 100 mL three-necked flask equipped with a magnetic stir bar, compound 9 (crude product obtained from the above-mentioned two steps) and 20 mL dry toluene were added. The mixture was degassed and charged with nitrogen for three times and stirred at 0 o C (ice-water bath) for 3 min. Then, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (250 mg, 1.1 mmol) in 5 mL dry toluene was added to the mixture and the mixture was stirred under N 2 at room temperature for 1 h (the color of the mixture turned to red-brown). Then H 2 O was added to quench the reaction. The mixture was extracted by DCM (50 mL×3), and washed with saturated sodium chloride aqueous solution (50 mL×3). The organic layer was dried by anhydrous Na 2 SO 4 . The crude product was further purified by a short silica gel column (PE/DCM = 1/1, v/v). Compound 1 was obtained as an army green solid (57 mg, 17.3% yield). Synthesis of compound 2 To a 100 mL Schlenk flask equipped with a magnetic stir bar, compound 15 (crude product obtained from the above-mentioned second step) and 30 mL dry toluene were added. Then, through a freeze-pump-thaw cycle, the tube was charged with nitrogen for three times again. After the mixture had thawed, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (430 mg, 1.89 mmol) in 4 mL dry toluene was added to the mixture and the mixture was stirred under N 2 at room temperature for 2 h (the color of the mixture turned to blue-green). After the reaction finished, the mixture was poured into hexane (300 mL) and a large amount of precipitation was generated and filtered off. The precipitation was washed with MeOH. The crude product was further purified by a short silica gel column (DCM/toluene = 4/1, v/v). Compound 2 was obtained as a purple solid (305 mg, 63.3% yield). Synthesis of compound 3 To a 50 mL Schlenk flask equipped with a magnetic stir bar, compound 21 (crude product obtained from the second step) and 15 mL dry toluene were added. Then, through a freeze-pump-thaw cycle, the tube was charged with nitrogen for three times. After the mixture had thawed, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (71.8 mg, 0.32 mmol) in 2 mL dry toluene was added to the mixture and the mixture was stirred under N 2 at room temperature for 0.5 h. Then, H 2 O (5 mL) and Et 3 N (2 mL) were added to quench the reaction. The residual was extracted by DCM, and washed with saturated sodium chloride aqueous solution. The organic layer was dried with anhydrous Na 2 SO 4 and the solvent was removed by rotary evaporation. The crude product was then purified by flash column chromatography using dichloromethane and methanol (1/0 ~ 20/1, v/v) as eluent to give a brown solid of 3 (19 mg, 19.7% yield). Declarations Contributions R. Yang conceived this project and gave constructive guidance and revised the manuscript; L. Chen did the experiments and analyzed the data and wrote the draft; Z. Shangguan helped with the calculation; L. Qin gave help for the single-crystal analysis; Y. Zeng and Q. Zhu gave help for the photophysical measurements; J. Chen, J. Liang, X. Qiu, X. Wang and D. Zhang gave valuable discussions for this article. Competing interests The authors declare no competing interest Acknowledgements The authors are deeply grateful to the National Natural Science Foundation of China (22375077, 52573276, 52203225, 52073122), Excellent Young and Middle aged Science and Technology Innovation Team Program for Universities in Hubei Province (T2023037), Department of Science and Technology of Hubei Province (2024DJC006), Key R & D Project of Hubei Province (2022BAA095), Hubei Natural Science Foundation (2022CFB903), Key Research and Development Program of Wuhan (2024010802030156), Ministry of Science and Technology of China (2021YFE0113600). Data Availability Materials and methods, experimental procedures, characterization data, copies of NMR and EPR spectra, X-ray crystallographic details, photophysical studies, electrochemical measurements, and calculations of all involved compounds and processes are provided in the Supplementary Information. Source data are provided with this paper. CIF crystallographic data files and xyz coordinates of the optimized structures are available as Supplementary Files. Crystallographic data for the structures reported in this article have been deposited at the Cambridge Crystallographic Data Center, under deposition numbers CCDC 2478865 ( 7 ), 2478866 ( 13 ), 2478867 ( 19 ), 2478868 ( 1 ), 2478869 ( 2 ), 2478870 ( 3 ), 2493835 ( 25-3a ) and 2478871 ( 26-1b ). All data are available from the corresponding authors upon request. References Pun SH, Miao Q (2018) Toward Negatively Curved Carbons. Acc Chem Res 51:1630–1642 Liu C et al (2018) Macrocyclic Polyradicaloids with Unusual Super-ring Structure and Global Aromaticity. Chem 4:1586–1595 Frederickson CK, Rose BD, Haley MM (2017) Explorations of the Indenofluorenes and Expanded Quinoidal Analogues. Acc Chem Res 50:977–987 Bally T, Chai S, Neuenschwander M, Zhu Z, Pentalene (1997) Formation, Electronic, and Vibrational Structure. 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J Am Chem Soc 145:3891–3896 Zong CY et al (2022) Isomeric dibenzooctazethrene diradicals for high-performance air-stable organic field-effect transistors. Chem Sci 13:11442–11447 Lin Z et al (2020) Tuning Biradical Character to Enable High and Balanced Ambipolar Charge Transport in a Quinoidal pi-System. Org Lett 22:2553–2558 Schemes Schemes 1 and 2 are available in the Supplementary Files section Additional Declarations There is NO Competing Interest. Supplementary Files SupportinginformationRenqiangYang.docx Detailed synthesis and characteristic data Schemes.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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07:13:03","extension":"html","order_by":19,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":189214,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7977582/v1/10540555c8b1f4fb5df0fc12.html"},{"id":95359318,"identity":"121f1aae-73e7-4f10-a9b8-2d9ac450b70b","added_by":"auto","created_at":"2025-11-07 07:13:02","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":415347,"visible":true,"origin":"","legend":"\u003cp\u003eX-ray single-crystal crystallographic analysis. Molecular structure of top/side view of (a) \u003cstrong\u003e1\u003c/strong\u003e, (d) \u003cstrong\u003e2\u003c/strong\u003e and (g) \u003cstrong\u003e3\u003c/strong\u003e. The bond-length analysis of (b) \u003cstrong\u003e1\u003c/strong\u003e, (e) \u003cstrong\u003e2\u003c/strong\u003e and (h) \u003cstrong\u003e3\u003c/strong\u003e. The intermolecular packing mode and interaction of (c) \u003cstrong\u003e1\u003c/strong\u003e, (f) \u003cstrong\u003e2\u003c/strong\u003e and (i) \u003cstrong\u003e3\u003c/strong\u003e. (j) The three molecular twist conformation and the dihedral angle of \u003cstrong\u003e3\u003c/strong\u003e in a cell unit. (k) The intermolecular packing mode and interaction of the trimer of \u003cstrong\u003e3\u003c/strong\u003ein a cell unit.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7977582/v1/b735f8c0df8626e1d5c93003.png"},{"id":95359320,"identity":"23f9c68e-045f-4f7b-86ec-453c106e0080","added_by":"auto","created_at":"2025-11-07 07:13:02","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":344092,"visible":true,"origin":"","legend":"\u003cp\u003eThe calculated NICS(1)\u003csub\u003ezz\u003c/sub\u003e values at 1 Å of Z axis of (a) \u003cstrong\u003e1\u003c/strong\u003e, (b) \u003cstrong\u003e2\u003c/strong\u003e and (c) \u003cstrong\u003e3\u003c/strong\u003e. The calculated ACID plots of (d) \u003cstrong\u003e1\u003c/strong\u003e, (e) \u003cstrong\u003e2\u003c/strong\u003e and (f) \u003cstrong\u003e3\u003c/strong\u003e. The clockwise ring current represents aromaticity and the counter-clockwise ring current represents antiaromaticity. (g) The UV–vis absorption spectra of \u003cstrong\u003e1\u003c/strong\u003e, \u003cstrong\u003e2\u003c/strong\u003e and \u003cstrong\u003e3\u003c/strong\u003e (10\u003csup\u003e-5\u003c/sup\u003e mol/L in DCM solution). (h) Cyclic voltammograms (CV) curves of \u003cstrong\u003e1\u003c/strong\u003e, \u003cstrong\u003e2\u003c/strong\u003e and \u003cstrong\u003e3\u003c/strong\u003e. The calculated absorption transitions of (i) \u003cstrong\u003e1\u003c/strong\u003e, (j) \u003cstrong\u003e2\u003c/strong\u003e and (k) \u003cstrong\u003e3\u003c/strong\u003e based on TD-DFT calculations.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7977582/v1/3e77c34dc18ced4bab576bdf.png"},{"id":95525933,"identity":"79aee509-bd31-4d6b-a2b5-abd1be6ebd60","added_by":"auto","created_at":"2025-11-10 10:05:53","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":233426,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The resonance structures of \u003cstrong\u003e2\u003c/strong\u003e with open-shell diradical and close-shell quinoid structure (left) and the VT \u003csup\u003e1\u003c/sup\u003eH NMR of \u003cstrong\u003e2\u003c/strong\u003e from 300 to 393 K in C\u003csub\u003e2\u003c/sub\u003eD\u003csub\u003e2\u003c/sub\u003eCl\u003csub\u003e4\u003c/sub\u003e (right). (b) Temperature dependence of the ESR signals of \u003cstrong\u003e2\u003c/strong\u003e in powder state. (c) The integrated signal multiplied by temperature and fitted with the Bleaney–Bowers equation of \u003cstrong\u003e2\u003c/strong\u003e. (d) Spin density surfaces calculated by DFT UB3LYP-D3(BJ)/6-31G(d,p) in the open-shell singlet state. The isovalue is 0.005.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7977582/v1/1cae163bd928be150568c1c6.png"},{"id":95359321,"identity":"96e4bbb8-7576-407a-b131-ac744facd441","added_by":"auto","created_at":"2025-11-07 07:13:02","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":252927,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Transfer characteristics of OFETs based on \u003cstrong\u003e2\u003c/strong\u003e (the inset is the BGBC OFET device structure). (b) Typical output curve of OFETs based on \u003cstrong\u003e2\u003c/strong\u003e. The width (\u003cem\u003eW\u003c/em\u003e) and length (\u003cem\u003eL\u003c/em\u003e) of the conductive channel are 1440 and 5 μm, respectively.(c) The mobility distribution of 13 FET devices based on \u003cstrong\u003e2\u003c/strong\u003e.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7977582/v1/46f4e8f038c80a1ab7b01433.png"},{"id":95524723,"identity":"99429e53-d8ff-4dbd-8afa-d2cf49530ca7","added_by":"auto","created_at":"2025-11-10 10:03:19","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":503922,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The synthetic route of more non-benzenoid PAH based on \u003cstrong\u003e22a\u003c/strong\u003e and \u003cstrong\u003e22b\u003c/strong\u003e. The ICSS maps of (b) \u003cstrong\u003e26-1b\u003c/strong\u003e, (c) \u003cstrong\u003e25-1b\u003c/strong\u003e, (d) \u003cstrong\u003e25-2b\u003c/strong\u003e and (e) \u003cstrong\u003e25-3b\u003c/strong\u003e.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7977582/v1/b5d9846ef220956eb4ca09bb.png"},{"id":97671023,"identity":"cbe26746-b102-4b41-aec1-0902b6330c1a","added_by":"auto","created_at":"2025-12-08 09:31:46","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2296733,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7977582/v1/20551edc-7a00-4e01-9358-02ab36145cb7.pdf"},{"id":95359345,"identity":"dde99c2d-fafb-45e1-b58e-66fa50416b41","added_by":"auto","created_at":"2025-11-07 07:13:04","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":69108202,"visible":true,"origin":"","legend":"Detailed synthesis and characteristic data","description":"","filename":"SupportinginformationRenqiangYang.docx","url":"https://assets-eu.researchsquare.com/files/rs-7977582/v1/78e62855d2ccf4f36e1dadbc.docx"},{"id":95525962,"identity":"aac8c2df-6cdd-4e0b-b536-4d567ee2d861","added_by":"auto","created_at":"2025-11-10 10:05:58","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":814371,"visible":true,"origin":"","legend":"","description":"","filename":"Schemes.docx","url":"https://assets-eu.researchsquare.com/files/rs-7977582/v1/3e66e714d6f505e65dc056c3.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Sterically Controlled 5-exo-dig Cyclization Enables Modular Synthesis of Non-benzenoid Polycyclic Aromatic Hydrocarbons with Intriguing (Anti)aromaticity and Diradical","fulltext":[{"header":"Introduction","content":"\u003cp\u003eOver the past decade, non-benzenoid polycyclic aromatic hydrocarbons (PAHs) featuring pentagonal and heptagonal rings have emerged as a vibrant research frontier in materials chemistry.\u003csup\u003e\u003cspan additionalcitationids=\"CR2 CR3 CR4 CR5 CR6 CR7 CR8\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e These structurally unique systems exhibit fundamentally different electronic properties compared to their benzenoid counterparts, owing to their characteristic structural strains, electronic character, and non-alternant π-conjugation networks.\u003csup\u003e\u003cspan additionalcitationids=\"CR11 CR12 CR13 CR14 CR15 CR16 CR17 CR18\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e Particularly noteworthy are three archetypal building blocks, antiaromatic indacene (5/6/5-fused rings with 12 peripheral π-electrons), antiaromatic pentalene (5/5-fused rings with 8 π-electrons), and aromatic azulene (5/7-fused rings with 10 π-electrons). Integrating these motifs into extended π-conjugated systems enables precise modulation of electronic structures and molecular geometries.\u003csup\u003e\u003cspan additionalcitationids=\"CR21\" citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e Antiaromatic indacene and pentalene units impart remarkable open-shell characteristics, facilitating unusual spin interactions and enhancing electron delocalization,\u003csup\u003e23\u0026ndash;25\u003c/sup\u003e while azulene moieties introduce strong dipole moments and intramolecular charge transfer capabilities, resulting in tunable absorption and emission properties.\u003csup\u003e\u003cspan additionalcitationids=\"CR27 CR28 CR29 CR30 CR31\" citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e These distinctive features give rise to novel optoelectronic behaviors and spin-related phenomena, positioning non-benzenoid PAHs as promising candidates for advanced applications in organic electronics, singlet fission systems, and molecular magnetism.\u003csup\u003e\u003cspan additionalcitationids=\"CR34 CR35 CR36\" citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e The ability to fine-tune these properties through rational molecular design has sparked growing interest in developing synthetic methodologies for these structurally challenging yet functionally versatile materials.\u003c/p\u003e\u003cp\u003eThe formidable challenge is mainly attributed to the inherent instability and complex electronic requirements for synthesizing these subunit-based molecular materials. A notable example is the first synthesis of \u003cem\u003es\u003c/em\u003e-indacene by Hafner and coworkers in 1963, which yielded an unsubstituted derivative that exhibits poor stability, being highly sensitive to oxygen and acids, and was inadequately characterized.\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e Recent advances in synthetic methodologies have facilitated more precise construction of these architectures, thereby unlocking new opportunities for their practical implementation. Currently, \u003cem\u003es\u003c/em\u003e-indacene-based molecules can be prepared through two primary routes: (1) sequential nucleophilic addition and reductive elimination reactions starting from dicarbonyl (ketone) compounds,\u003csup\u003e23, 40\u003c/sup\u003e or (2) a combination of nucleophilic addition of aldehyde, Friedel-Crafts cyclization, and oxidative dehydrogenation reactions (Scheme \u003cspan refid=\"Sch1\" class=\"InternalRef\"\u003e1a\u003c/span\u003e).\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e Building on these approaches, Haley, Tobe, Chi, M\u0026uuml;llen \u003cem\u003eetc\u003c/em\u003e. successfully prepared a series of \u003cem\u003es\u003c/em\u003e-indacene derivatives exhibiting open-shell characteristics,\u003csup\u003e42\u0026ndash;46\u003c/sup\u003e where the diradical character was mainly localized at the C1 position of the \u003cem\u003es\u003c/em\u003e-indacene core through steric protection with bulky substituents (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea).\u003c/p\u003e\u003cp\u003eSimilarly, another notable antiaromatic subunit, pentalene, which consists of two fused five-membered rings with 8 π-electrons, is also inherently unstable except for sterically protected derivatives or annulated with other aromatic rings.\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e Owing to the fused five-membered rings (5/5), pentalene exhibits a distinct electronic structure compared with the dicyclopenta-fused arenes, which were initially discovered by Garcia-Garibay and later pursued by Plunkett.\u003csup\u003e\u003cspan additionalcitationids=\"CR50 CR51\" citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e The main synthetic routes to pentalene derivatives include: (1) a one-step Pd-catalyzed or Ni-catalyzed cyclodimerization of ortho-bromophenylacetylene derivatives with low yield (Scheme \u003cspan refid=\"Sch1\" class=\"InternalRef\"\u003e1b\u003c/span\u003e),\u003csup\u003e53\u0026ndash;54\u003c/sup\u003e or (2) a two-step sequence involving Pd-catalyzed cyclization followed by Fe-mediated dehydrogenation.\u003csup\u003e\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e\u003c/sup\u003e In addition, Hashmi and coworkers reported a gold-catalyzed regiospecific annulation of unsymmetrically substituted 1,2-di(arylethynyl)benzenederivatives for a geometry-controlled synthesis of linear bispentalenes.\u003csup\u003e\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e\u003c/sup\u003e Yasuda and coworkers synthesized a Dibenzo[a,f]pentalene by sequential nucleophilic addition of aldehyde, Friedel\u0026thinsp;\u0026minus;\u0026thinsp;Crafts cyclization and oxidative dehydrogenation processes, which exhibited antiaromatic and singlet biradical characters.\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e, \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e\u003c/sup\u003e Despite these rapid developments in synthetic strategies, the central or side benzene or naphthalene moiety in these molecules (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb) remains strongly aromatic except for the pentalene units, preserving overall stability. Nevertheless, combining two antiaromatic subunits of \u003cem\u003es\u003c/em\u003e-indacene and pentalene into one molecule, simultaneously, has not been explored due to synthetic challenges. Additionally, it is also unclear how these two antiaromatic units affect each other and their impact on the overall performance of the molecule.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eIn comparison, azulene-containing conjugated systems benefit from relatively higher stability among these three motifs, leading to the development of diverse synthetic approaches. Traditional methods for constructing azulene units, such as those employing pyridinium salts or troponoids, face limitations when applied to larger azulene-embedded PAHs due to low reactivity and challenges in accessing appropriate benzo-fused precursors.\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e Consequently, alternative strategies have emerged for in situ construction of azulene subunits within extended π-systems, including both on-surface synthesis,\u003csup\u003e58\u0026ndash;63\u003c/sup\u003e and in-solution chemistry. These encompass Scholl-type cyclization,\u003csup\u003e64\u0026ndash;66\u003c/sup\u003e intramolecular Friedel-Crafts reactions followed by aromatization,\u003csup\u003e31, 67\u003c/sup\u003e Pd-catalyzed alkyne annulation,\u003csup\u003e68\u0026ndash;70\u003c/sup\u003e, etc.\u003csup\u003e\u003cspan additionalcitationids=\"CR72\" citationid=\"CR71\" class=\"CitationRef\"\u003e71\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e73\u003c/span\u003e\u003c/sup\u003e However, these transformations typically require multi-step synthesis of specialized precursors and are often complicated by rearrangement or insertion side reactions, leading to unpredictable product distributions.\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e For more controllable synthesis of azulene-based PAHs, a common strategy involves starting with commercially available azulene and performing sequential coupling and cyclization reactions to construct azulene-terminated PAHs (Scheme \u003cspan refid=\"Sch1\" class=\"InternalRef\"\u003e1c\u003c/span\u003e).\u003csup\u003e\u003cspan additionalcitationids=\"CR75 CR76 CR77 CR78\" citationid=\"CR74\" class=\"CitationRef\"\u003e74\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR79\" class=\"CitationRef\"\u003e79\u003c/span\u003e\u003c/sup\u003e However, the azulene unit in these azulene-terminated PAHs exhibits little effect on the optoelectronic properties of the whole molecules owing to the relatively independent local electronic structure characteristic. In contrast, azulene-embedded PAHs integrate azulene into the fused ring backbone, which directly tunes the π-conjugation pathway. For example, Feng and Liu recently reported a modular approach to azulene-embedded PAHs using a cascade reaction combining Suzuki coupling and Knoevenagel condensation (Scheme \u003cspan refid=\"Sch1\" class=\"InternalRef\"\u003e1c\u003c/span\u003e).\u003csup\u003e\u003cspan additionalcitationids=\"CR81\" citationid=\"CR80\" class=\"CitationRef\"\u003e80\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR82\" class=\"CitationRef\"\u003e82\u003c/span\u003e\u003c/sup\u003e Nevertheless, these synthetic methods generally require complex precursor preparation and are often limited to specific substrate types. Therefore, the development of simpler, more controllable, and broadly applicable synthetic strategies for preparing non-benzenoid PAHs with tunable structures and properties is highly desirable, especially for these azulene-embedded linear PAH.\u003c/p\u003e\u003cp\u003eHerein, we developed an innovative synthetic strategy combining 5-exo-dig cyclization of alkyne, nucleophilic addition of aldehyde, Friedel\u0026thinsp;\u0026minus;\u0026thinsp;Crafts cyclization at the exocyclic double-bond or substituted phenyl position and oxidative dehydrogenation processes to construct two highly stable antiaromatic PAH combining an indacene skeleton and two pentalene (5/5) motifs (\u003cb\u003e1\u003c/b\u003e and \u003cb\u003e2\u003c/b\u003e) and an S-shaped azulene (5/7)-embedded linear PAH (\u003cb\u003e3\u003c/b\u003e). The product selectivity was governed by the \u003cem\u003eE/Z\u003c/em\u003e configuration of key intermediates (\u003cb\u003e7\u003c/b\u003e, \u003cb\u003e13\u003c/b\u003e, and \u003cb\u003e19\u003c/b\u003e) obtained through 5-exo-dig cyclization. Detailed structural characterization reveals that \u003cb\u003e1\u003c/b\u003e and \u003cb\u003e2\u003c/b\u003e exhibited global antiaromaticity, while \u003cb\u003e3\u003c/b\u003e exhibits global aromaticity along its molecular periphery, supported by bond length analysis of crystal and theory calculation. Notably, \u003cb\u003e2\u003c/b\u003e adopts a planar configuration and demonstrates significant open-shell character (diradical index \u003cem\u003ey\u003c/em\u003e\u003csub\u003e\u003cem\u003e0\u003c/em\u003e\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.38) with a small singlet-triplet energy gap (Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003est\u003c/sub\u003e = -2.02 kcal/mol). In contrast, \u003cb\u003e1\u003c/b\u003e maintains a closed-shell configuration, while \u003cb\u003e3\u003c/b\u003e exhibits a twisted geometry resulting from the cove edge between 2hthalene and azulene moieties, along with closed-shell characteristics. Remarkably, \u003cb\u003e2\u003c/b\u003e demonstrates promising \u003cem\u003ep\u003c/em\u003e-type transporting behavior, achieving a hole mobility up to 0.083 cm\u003csup\u003e2\u003c/sup\u003eV\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003es\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e under an air atmosphere, underscoring its potential for organic electronics. To further assess the universality of this synthetic approach, we designed two axisymmetric substrates of \u003cb\u003e22a\u003c/b\u003e and \u003cb\u003e22b\u003c/b\u003e, which successfully afforded 55/55 (\u003cb\u003e24\u0026thinsp;\u0026minus;\u0026thinsp;1\u003c/b\u003e), 55/57 (\u003cb\u003e24\u0026thinsp;\u0026minus;\u0026thinsp;2\u003c/b\u003e) and 57/57 (\u003cb\u003e24\u0026thinsp;\u0026minus;\u0026thinsp;3\u003c/b\u003e) structures owing to the limited stereoselectivity. However, subsequent oxidative dehydrogenation failed to yield isolable non-benzenoid PAH products, which was attributed to their inherent instability arising from triplet ground states, as supported by experimental observations and DFT calculations. This modular synthetic strategy provides a versatile and controllable approach for exploring diverse 5/5 or 5/7 rings-based non-benzenoid PAHs, from chemical structure, optoelectronic properties, to applications in organic electronics.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003eSynthesis and characterization\u003c/p\u003e\u003cp\u003eThe synthetic routes towards compounds \u003cb\u003e1\u003c/b\u003e, \u003cb\u003e2\u003c/b\u003e and \u003cb\u003e3\u003c/b\u003e are illustrated in Scheme \u003cspan refid=\"Sch2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Key precursors (compounds \u003cb\u003e6\u003c/b\u003e, \u003cb\u003e12\u003c/b\u003e, and \u003cb\u003e18\u003c/b\u003e) were prepared through sequential Sonogashira and Suzuki coupling reactions starting from \u003cb\u003e4\u003c/b\u003e, \u003cb\u003e10\u003c/b\u003e, and \u003cb\u003e16\u003c/b\u003e. These precursors subsequently underwent Pd(OAc)\u003csub\u003e2\u003c/sub\u003e-catalyzed 5-exo-dig cyclization in the presence of 2-dicyclohexylphosphino-2',6'-dimethoxybiphenyl (Sphos) and tri-n-butylphosphine ligands to afford intermediates \u003cb\u003e7\u003c/b\u003e, \u003cb\u003e13\u003c/b\u003e, and \u003cb\u003e19\u003c/b\u003e.\u003csup\u003e83\u0026ndash;85\u003c/sup\u003e Interestingly, the exocyclic double bonds of compounds \u003cb\u003e7\u003c/b\u003e and \u003cb\u003e13\u003c/b\u003e adopted \u003cem\u003eE\u003c/em\u003e-configuration, whereas \u003cb\u003e19\u003c/b\u003e exhibited a \u003cem\u003eZ\u003c/em\u003e-configuration due to steric hindrance imposed by its substituents. This stereochemical assignment was confirmed through comprehensive structural characterization. As shown in Figures \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e and S2, NOESY spectra of \u003cb\u003e7\u003c/b\u003e and \u003cb\u003e13\u003c/b\u003e revealed strong correlation signals between H\u003csub\u003ec\u003c/sub\u003e and H\u003csub\u003ed\u003c/sub\u003e, H\u003csub\u003eg\u003c/sub\u003e and H\u003csub\u003ee\u003c/sub\u003e, consistent with \u003cem\u003eE\u003c/em\u003e-configuration. Moreover, the crystals of \u003cb\u003e7\u003c/b\u003e and \u003cb\u003e13\u003c/b\u003e, suitable for single-crystal structural analysis, were successfully obtained by slowly diffusing methanol into the chloroform solution, which unambiguously confirmed their structures as assigned (Scheme \u003cspan refid=\"Sch2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, Figures S7 and S8). Notably, in the case of \u003cb\u003e13\u003c/b\u003e, the intramolecular hydrogen bond between the aldehyde oxygen and the exocyclic double bond hydrogen was observed in the crystal structure, potentially stabilizing the \u003cem\u003eE\u003c/em\u003e-configuration. Alternatively, intermediate \u003cb\u003e19\u003c/b\u003e was determined to possess a \u003cem\u003eZ\u003c/em\u003e-configuration, as evidenced by NOESY correlations between H\u003csub\u003ec\u003c/sub\u003e and H\u003csub\u003ee\u003c/sub\u003e, H\u003csub\u003ed\u003c/sub\u003e and H\u003csub\u003eg\u003c/sub\u003e (Figure S3) and confirmed by single-crystal analysis (Scheme \u003cspan refid=\"Sch2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and Figure S9).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThen, dihydro-precursors of \u003cb\u003e9\u003c/b\u003e, \u003cb\u003e15\u003c/b\u003e and \u003cb\u003e21\u003c/b\u003e were obtained by the treatment of \u003cb\u003e7\u003c/b\u003e, \u003cb\u003e13\u003c/b\u003e, and \u003cb\u003e19\u003c/b\u003e with mesitylmagnesium bromide (MesMgBr), followed by Friedel\u0026thinsp;\u0026minus;\u0026thinsp;Crafts cyclization with BF\u003csub\u003e3\u003c/sub\u003e\u0026middot;OEt\u003csub\u003e2\u003c/sub\u003e (Scheme \u003cspan refid=\"Sch2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Subsequently, the oxidative dehydrogenation of \u003cb\u003e9\u003c/b\u003e, \u003cb\u003e15\u003c/b\u003e and \u003cb\u003e21\u003c/b\u003e using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) afforded the target products in moderate yield across three steps. Compound \u003cb\u003e1\u003c/b\u003e was obtained as an army green solid with a yield of 17.3%, \u003cb\u003e2\u003c/b\u003e as a purple solid with a yield of 63.3% and \u003cb\u003e3\u003c/b\u003e as a brown solid with a yield of 19.7%. The chemical structures of \u003cb\u003e1\u003c/b\u003e, \u003cb\u003e2\u003c/b\u003e and \u003cb\u003e3\u003c/b\u003e were thoroughly characterized using nuclear magnetic resonance (NMR) spectroscopy, high-resolution mass spectra (HRMS), and X-ray single-crystal analysis. Detailed characterization data and experimental procedures are provided in the synthesis and characterization section of the Supporting Information.\u003c/p\u003e\u003cp\u003eSingle-crystal structure analysis\u003c/p\u003e\u003cp\u003eThe single crystals of \u003cb\u003e1\u003c/b\u003e and \u003cb\u003e2\u003c/b\u003e were successfully grown by slowly diffusing methanol into the chloroform/CS\u003csub\u003e2\u003c/sub\u003e (1/1, v/v) solution at -4 \u003csup\u003eo\u003c/sup\u003eC. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, both \u003cb\u003e1\u003c/b\u003e and \u003cb\u003e2\u003c/b\u003e maintain nearly planar geometries, with significant steric hindrance from the mesityl and tert-butyl phenyl substituents preventing π-π stacking interactions between molecular backbones. \u003cb\u003e1\u003c/b\u003e shows a one-dimensional parallel packing and the main interactions are multiple C-H\u0026hellip;π with distances of 2.820\u0026ndash;2.842 \u0026Aring; and C-H\u0026hellip;O with distances of 2.483\u0026ndash;2.854 \u0026Aring; (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec), while \u003cb\u003e2\u003c/b\u003e shows a herringbone stacking and is also dominated by C-H\u0026hellip;π interactions (with shorter distances of 2.681 \u0026Aring;) and C-H\u0026hellip;C-H interaction (2.316 \u0026Aring;) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef). Bond length analysis depicts that the C(\u003cem\u003esp\u003c/em\u003e\u003csup\u003e2\u003c/sup\u003e)-C(\u003cem\u003esp\u003c/em\u003e\u003csup\u003e2\u003c/sup\u003e) bond lengths for the periphery of \u003cb\u003e1\u003c/b\u003e and \u003cb\u003e2\u003c/b\u003e exhibit typical bond-distance alternation, ranging from 1.348 \u0026Aring; to 1.487 \u0026Aring; for \u003cb\u003e1\u003c/b\u003e and 1.349 \u0026Aring; to 1.476 \u0026Aring; for \u003cb\u003e2\u003c/b\u003e, as expected for a closed-shell quinoid structure (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb and \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee). Especially, the bond lengths of C10-C11 of the central benzene ring and naphthalene ring of \u003cb\u003e1\u003c/b\u003e or \u003cb\u003e2\u003c/b\u003e are 1.489 and 1.480 \u0026Aring;, indicating the aromaticity of the central benzene ring and naphthalene ring was disrupted, consistent with quinoid structures. The small bond length alternation and shorter distance of C10-C11 of \u003cb\u003e2\u003c/b\u003e than \u003cb\u003e1\u003c/b\u003e agree well with the small energy barrier for the valence tautomerization between the closed-shell quinoid and open-shell diradical structure. In addition, the distances of C9-C10 bonds in \u003cb\u003e1\u003c/b\u003e and \u003cb\u003e2\u003c/b\u003e are 1.390 and 1.413 \u0026Aring;, respectively, which fall between those of known closed-shell (1.371 \u0026Aring;) and open-shell (1.437 \u0026Aring;) Mes-substituted indenofluorene analogues.\u003csup\u003e\u003cspan citationid=\"CR86\" class=\"CitationRef\"\u003e86\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR87\" class=\"CitationRef\"\u003e87\u003c/span\u003e\u003c/sup\u003e The intermediate value indicates \u003cb\u003e1\u003c/b\u003e and \u003cb\u003e2\u003c/b\u003e are resonance hybrids, with contributions from both open-shell and closed-shell structures in their ground states. Moreover, the slightly longer C9-C10 bond length in \u003cb\u003e2\u003c/b\u003e compared to \u003cb\u003e1\u003c/b\u003e further demonstrates that \u003cb\u003e1\u003c/b\u003e exhibits more pronounced closed-shell character, while \u003cb\u003e2\u003c/b\u003e leans toward open-shell features.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eSimilarly, single crystals of \u003cb\u003e3\u003c/b\u003e suitable for X-ray diffraction analysis were obtained through slow diffusion of methanol into a chloroform solution at -4\u0026deg;C. The molecular structure displays a distinctive twisted S-shaped conformation (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ej), resulting from steric repulsion between the naphthalene and azulene moieties at the cove region. Interestingly, the crystal packing exhibits an intriguing trimeric superstructure composed of repeating M1, M2, and M3 units. M1 and M3 adopt head-to-tail stacking with torsion angles of 35.1\u0026deg; and 20.3\u0026deg;, respectively, whereas M2 inserts nearly vertically between M1 and M3, with torsion angles of 22.7\u0026deg; (19.6\u0026deg;) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ej). The main intermolecular interactions between adjacent molecules are π-π interactions with distances of 3.250\u0026ndash;3.398 \u0026Aring; and C-H\u0026hellip;O interactions with distances of 2.317\u0026ndash;2.719 \u0026Aring; (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ei and \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ek). Bond length analysis of \u003cb\u003e3\u003c/b\u003e revealed that the C-C bond length of the periphery along the S-shaped skeleton exhibited an averaged characterization with the longest bond length up to 1.478 \u0026Aring; (C11-C16) and the shortest bond length down to 1.346 \u0026Aring; (C10-C11), indicating the global aromatic character. In addition, the bond lengths along the 5/7 ring exhibit distinct bond alternation characterization, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eh, between 1.346 and 1.478 \u0026Aring;, which are less averaged than that of the parent distinct azulene unit (1.387\u0026ndash;1.427 \u0026Aring;) owing to the more delocalized electronic feature within the extended conjugated structure. These results indicate that the azulene motif in this molecule is not independent, which exhibits a significant influence on molecular configuration, stacking, intermolecular interactions and electronic properties.\u003c/p\u003e\u003cp\u003eAromaticity and electronic structure\u003c/p\u003e\u003cp\u003eTo gain deeper insights into the electronic structures and aromaticity of \u003cb\u003e1\u003c/b\u003e, \u003cb\u003e2\u003c/b\u003e and \u003cb\u003e3\u003c/b\u003e, nucleus-independent chemical shift (NICS) calculations were performed at the (U)B3LYP/6-31G(d,p) level of theory. The obtained NICS(1)\u003csub\u003ezz\u003c/sub\u003e values of \u003cb\u003e1\u003c/b\u003e are \u0026minus;\u0026thinsp;16.1, 8.2, 18.9 and 16.2 ppm for rings A, B, C and D (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea and S12), respectively, indicating that rings B, C, and D in \u003cb\u003e1\u003c/b\u003e are strongly antiaromatic. Similarly, NICS(1)\u003csub\u003ezz\u003c/sub\u003e values of -14.9, 13.1, 10.7 and \u0026minus;\u0026thinsp;2.4 ppm for rings A, B, C and D were observed in \u003cb\u003e2\u003c/b\u003e (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb and S12), suggesting rings B and C in \u003cb\u003e2\u003c/b\u003e are also antiaromatic, while ring D is weak aromaticity. The reduced antiaromaticity of \u003cb\u003e2\u003c/b\u003e compared to \u003cb\u003e1\u003c/b\u003e correlates with its enhanced open-shell character. In contrast, the NICS values of B, C and D rings is smaller than the individual \u003cem\u003es\u003c/em\u003e-indacene (54.7, 45.0 and 54.7 ppm) and pentalene (62.4 and 62.4 ppm) motifs (see Figure S16), indicating the B, C and D rings in \u003cb\u003e1\u003c/b\u003e and \u003cb\u003e2\u003c/b\u003e are hybridizing structures, which weakens the \u003cem\u003es\u003c/em\u003e-indacene and pentalene motifs and thus improves the stability of these two molecules. To further explore the global antiaromaticity of compounds \u003cb\u003e1\u003c/b\u003e and \u003cb\u003e2\u003c/b\u003e, we also calculated the bis(pentalene) derivatives without the A ring. As shown in Figure S17, the NICS values of two five-membered rings are 38.8 and 37.6 ppm for bis(pentalene)-fuse benzene (compound A) and 31.9 and 29.2 ppm for bis(pentalene)-fuse naphthene (compound B), while the central benzene or naphthene are weak antiaromaticity or aromaticity (7.4 ppm for A and \u0026minus;\u0026thinsp;2.1 ppm for B), indicating that the two pentalene motifs in these molecules were dependent and the antiaromatic electronic property is more localized owing to the central aromatic rings. In contrast, the aromaticity of the central benzene or naphthalene rings of \u003cb\u003e1\u003c/b\u003e and \u003cb\u003e2\u003c/b\u003e was broken, showing quinone structure and antiaromaticity characteristics. Consequently, \u003cb\u003e1\u003c/b\u003e and \u003cb\u003e2\u003c/b\u003e exhibit global antiaromatic character. Conversely, the rings A and D of \u003cb\u003e3\u003c/b\u003e exhibit strong aromatic features with NICS(1)\u003csub\u003ezz\u003c/sub\u003e values of -26.8 and \u0026minus;\u0026thinsp;20.7 ppm for rings A and D and the rings B and C of 5/7 motif are weak aromaticity with NICS(1)\u003csub\u003ezz\u003c/sub\u003e values of -9.2 and \u0026minus;\u0026thinsp;9.3 ppm (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec), indicating 5/7 motif in \u003cb\u003e3\u003c/b\u003e is not independent because of the good delocalized characteristic upon the whole skeleton, aligning well with the bond analysis. Additionally, these results are consistent with the anisotropy of the induced current density (ACID) and isotropic chemical shielding surface (ICSS) analysis results. As illustrated in Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee, the calculated ACID plot shows counter-clockwise ring current along the periphery except for ring A of \u003cb\u003e1\u003c/b\u003e and \u003cb\u003e2\u003c/b\u003e, corresponding to the antiaromaticity of indacene and pentalene. On the contrary, diamagnetic ring currents are found along the periphery of \u003cb\u003e3\u003c/b\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef), indicating global aromaticity along the periphery. These results agree well with the bond length analysis in the crystals and the ICSS calculation (Figure S11).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe UV-vis absorption spectra of \u003cb\u003e1\u003c/b\u003e, \u003cb\u003e2\u003c/b\u003e and \u003cb\u003e3\u003c/b\u003e in dichloromethane (DCM) solution at a concentration of 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e mol/L are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg. The colors of \u003cb\u003e1\u003c/b\u003e, \u003cb\u003e2\u003c/b\u003e and \u003cb\u003e3\u003c/b\u003e in DCM are purple, blue and claybank, respectively, as highlighted in the inset of Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg. \u003cb\u003e1\u003c/b\u003e and \u003cb\u003e2\u003c/b\u003e display similar absorption patterns with two main absorption peaks based on their likely electronic structures. The DCM solution of \u003cb\u003e1\u003c/b\u003e exhibits a prominent long-wavelength absorption peak at 553 nm, accompanied by a weak absorption tail extending up to 907 nm. Time-dependent (TD) DFT calculations reveal that the absorption band at 553 nm with a shoulder peak at 521 nm is mainly attributed to HOMO-3\u0026rarr;LUMO (S\u003csub\u003e0\u003c/sub\u003e\u0026rarr;S\u003csub\u003e4\u003c/sub\u003e) electronic transition. The weak-tail absorption is attributed to symmetry-forbidden HOMO\u0026rarr;LUMO electronic transition with an oscillator strength of 0.1037 (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ei and S18), following its antiaromatic character. For \u003cb\u003e2\u003c/b\u003e, the absorption spectrum exhibits a significant bathochromic shift compared to \u003cb\u003e1\u003c/b\u003e. The long-wavelength absorption peak appears at 644 nm, red-shifted by 91 nm, which is mainly ascribed to HOMO-2\u0026rarr;LUMO (S\u003csub\u003e0\u003c/sub\u003e\u0026rarr;S\u003csub\u003e4\u003c/sub\u003e). Interestingly, the calculated maximum absorption peak is in the intermediate between the open-shell (729 nm) and closed-shell (565 nm) states (Figure S20), illustrating that the long-wavelength absorption was attributed to both closed-shell and open-shell exciton states, consistent with the diradicaloid nature of compound \u003cb\u003e2\u003c/b\u003e. The long-tail absorption also extends to 1071 nm with a larger oscillator strength of 0.2774, mainly originating from a partially allowed HOMO \u0026rarr; LUMO electronic transition due to its open-shell feature (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ej and S19). For compound \u003cb\u003e3\u003c/b\u003e, the absorption peak at 537 nm is mainly attributed to the HOMO\u0026rarr;LUMO\u0026thinsp;+\u0026thinsp;2 (S\u003csub\u003e0\u003c/sub\u003e\u0026rarr;S\u003csub\u003e3\u003c/sub\u003e) electronic transition and the absorption band at 441 nm with a shoulder absorption at 415 nm originates from HOMO-1\u0026rarr;LUMO (S\u003csub\u003e0\u003c/sub\u003e\u0026rarr;S\u003csub\u003e5\u003c/sub\u003e) and HOMO-1\u0026rarr;LUMO\u0026thinsp;+\u0026thinsp;2 (S\u003csub\u003e0\u003c/sub\u003e\u0026rarr;S\u003csub\u003e9\u003c/sub\u003e) electronic transitions (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ek and S21). Accordingly, the optical energy gaps (\u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e\u003csup\u003eopt\u003c/sup\u003e) for \u003cb\u003e1\u003c/b\u003e, \u003cb\u003e2\u003c/b\u003e and \u003cb\u003e3\u003c/b\u003e were determined to be 1.36, 1.15, and 2.06 eV, respectively, from the onset of their UV\u0026thinsp;\u0026minus;\u0026thinsp;vis absorption spectra. Furthermore, time-dependent UV-vis measurements were performed under ambient conditions to investigate the stability of \u003cb\u003e1\u003c/b\u003e, \u003cb\u003e2\u003c/b\u003e and \u003cb\u003e3\u003c/b\u003e (see Figures S22 and S23). The results revealed that \u003cb\u003e1\u003c/b\u003e and \u003cb\u003e3\u003c/b\u003e exhibited exceptional stability, with no significant changes observed in their absorption spectra in DCM solution within 28 days. In contrast, \u003cb\u003e2\u003c/b\u003e showed gradual degradation when exposed to ambient conditions, with a fitted half-life of approximately 32 days for a 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e M DCM solution. This difference in stability can be attributed to the more pronounced open-shell character of \u003cb\u003e2\u003c/b\u003e, which renders it more reactive under ambient conditions. In addition, three compounds can be stored as crystalline solids under ambient conditions without any degradation. These results highlight the remarkable stability of these non-benzenoid PAHs, particularly for \u003cb\u003e1\u003c/b\u003e and \u003cb\u003e3\u003c/b\u003e, which maintain their structural and optical integrity even in solution over extended periods.\u003c/p\u003e\u003cp\u003eThe electrochemical behaviors of \u003cb\u003e1\u003c/b\u003e, \u003cb\u003e2\u003c/b\u003e and \u003cb\u003e3\u003c/b\u003e were investigated by cyclic voltammetry (CV) in anhydrous DCM solution with ferrocene/ferrocenium (Fc/Fc\u003csup\u003e+\u003c/sup\u003e) as an external standard, revealing distinct redox behaviors for each compound (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eh). \u003cb\u003e1\u003c/b\u003e exhibited two oxidation waves at \u003cem\u003eE\u003c/em\u003e\u003csub\u003e1/2\u003c/sub\u003e\u003csup\u003eox\u003c/sup\u003e = 1.16 and 1.39 V versus Fc/Fc\u003csup\u003e+\u003c/sup\u003e, along with two reduction waves at \u003cem\u003eE\u003c/em\u003e\u003csub\u003e1/2\u003c/sub\u003e\u003csup\u003ere\u003c/sup\u003e = -0.47 and \u0026minus;\u0026thinsp;0.93 V, while \u003cb\u003e2\u003c/b\u003e showed more complex redox activity with three oxidation peaks at 0.98, 1.22, and 1.39 V and two reduction peaks at -0.43 and \u0026minus;\u0026thinsp;0.72 V. In contrast, \u003cb\u003e3\u003c/b\u003e displayed oxidation potentials at 1.24, 1.53, and 1.71 V and reduction potentials at -0.92, -1.45, and \u0026minus;\u0026thinsp;1.69 V, reflecting its different electronic structure. From the first redox couples, we estimated the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels at -5.48/-3.85 eV for \u003cb\u003e1\u003c/b\u003e, -5.30/-3.89 eV for \u003cb\u003e2\u003c/b\u003e, and \u0026minus;\u0026thinsp;5.56/-3.40 eV for \u003cb\u003e3\u003c/b\u003e, yielding electrochemical band gaps (\u003cem\u003eE\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e\u003csup\u003eel\u003c/sup\u003e) of 1.63, 1.41, and 2.16 eV, respectively.\u003c/p\u003e\u003cp\u003eDiradical and charge transport characteristics\u003c/p\u003e\u003cp\u003eThe chemical and electronic structures of \u003cb\u003e1\u003c/b\u003e, \u003cb\u003e2\u003c/b\u003e and \u003cb\u003e3\u003c/b\u003e were further investigated by variable-temperature (VT) \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003eH NMR measurements. For \u003cb\u003e2\u003c/b\u003e, the proton signals in C\u003csub\u003e2\u003c/sub\u003eD\u003csub\u003e2\u003c/sub\u003eCl\u003csub\u003e4\u003c/sub\u003e exhibited significant broadening with the temperature increasing from 300 to 393 K, particularly for the protons \u003cem\u003eb\u003c/em\u003e, \u003cem\u003ec, d\u003c/em\u003e and \u003cem\u003eg\u003c/em\u003e located on the backbone (see Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea and S25). Upon cooling back to 300 K, the signals fully recovered to their original intensity. However, when further cooled to 243 K, the intensity of the signals gradually decreased, which was attributed to the reduced solubility of \u003cb\u003e2\u003c/b\u003e at lower temperatures (Figure S26). This reversible thermal broadening strongly indicates population of a triplet state at elevated temperatures, consistent with the significant open-shell diradical character of \u003cb\u003e2\u003c/b\u003e. Meanwhile, both \u003cb\u003e1\u003c/b\u003e and \u003cb\u003e3\u003c/b\u003e maintained sharp NMR signals throughout the same temperature range (300\u0026ndash;393 K) (Figures S24 and S27), demonstrating their closed-shell nature with negligible thermally accessible diradical states.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eComplementary electron-spin resonance (ESR) and superconducting quantum interference device (SQUID) measurements provided definitive evidence for the distinct open-shell characteristics of two antiaromatic compounds of \u003cb\u003e1\u003c/b\u003e and \u003cb\u003e2\u003c/b\u003e. While \u003cb\u003e1\u003c/b\u003e showed no solution-phase ESR signal but weak solid-state signals attributed to its small band gap (Figure S28a),\u003csup\u003e88\u003c/sup\u003e \u003cb\u003e2\u003c/b\u003e exhibited strong isotropic signals (\u003cem\u003eg\u003c/em\u003e\u003csub\u003ee\u003c/sub\u003e = 2.002) in both toluene solution and solid state, unambiguously confirming its paramagnetic nature and carbon-centered radical character (Figures S28b and S30). Additionally, VT-ESR studies (180\u0026ndash;330 K for \u003cb\u003e1\u003c/b\u003e and 160\u0026ndash;220 K for \u003cb\u003e2\u003c/b\u003e) demonstrated increasing signal intensity with decreasing temperature for both compounds (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb and S29a), with Bleaney-Bowers analysis (the signal (\u003cem\u003eI\u003c/em\u003e) x \u003cem\u003eT\u003c/em\u003e versus \u003cem\u003eT\u003c/em\u003e) yielding Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003est\u003c/sub\u003e = -4.72 kcal/mol for \u003cb\u003e1\u003c/b\u003e and \u0026minus;\u0026thinsp;2.02 kcal/mol for \u003cb\u003e2\u003c/b\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec and S29b),\u003csup\u003e89\u0026ndash;91\u003c/sup\u003e consistent with the more pronounced diradical character of \u003cb\u003e2\u003c/b\u003e. DFT calculations at the UCAM-B3LYP/6-31G(d) level reveal \u003cb\u003e2\u003c/b\u003e displays an open-shell singlet ground state, with a \u003cem\u003ey\u003c/em\u003e\u003csub\u003e\u003cem\u003e0\u003c/em\u003e\u003c/sub\u003e of 0.38, while \u003cb\u003e1\u003c/b\u003e, in contrast, exhibits a negligible diradical character with a closed-shell state (Table S11). The hybridizing structure of \u003cem\u003es\u003c/em\u003e-indacene and pentalene allows the spin densities in \u003cb\u003e2\u003c/b\u003e to be primarily localized both on C7 and C9 of the cyclopenta-rings, which is conducive to improving \u003cem\u003ey\u003c/em\u003e\u003csub\u003e\u003cem\u003e0\u003c/em\u003e\u003c/sub\u003e and stability (see Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed). This electronic structure property is completely different from the reported individual \u003cem\u003es\u003c/em\u003e-indacene or pentalene structures, indicating the hybridization of the antiaromatic motifs is an effective strategy to modulate the whole aromaticity and diradical as well as molecular stability. The spin-density distribution also corresponds to the significant changes observed in the VT-NMR signals of b, c, and g hydrogens (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). Additionally, SQUID magnetometry further corroborated these findings, showing a continuous increase in \u003cem\u003eχₘT\u003c/em\u003e for \u003cb\u003e2\u003c/b\u003e from 2-300 K, indicative of progressive thermal population of triplet states, whereas \u003cb\u003e1\u003c/b\u003e displayed only weak, discontinuous magnetic responses characteristic owing to its close-shell structure (Figure S31).\u003c/p\u003e\u003cp\u003eThe charge transport characteristics of \u003cb\u003e1\u003c/b\u003e, \u003cb\u003e2\u003c/b\u003e, and \u003cb\u003e3\u003c/b\u003e were evaluated through bottom-gate bottom-contact (BGBC) field-effect transistor (FET) devices fabricated under ambient conditions, with detailed device architecture illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea and fabrication procedures provided in the Supporting Information. While \u003cb\u003e1\u003c/b\u003e showed negligible charge transport behavior, \u003cb\u003e2\u003c/b\u003e demonstrated well-defined \u003cem\u003ep\u003c/em\u003e-type semiconductor characteristics, as evidenced by the transfer and output curves (Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb). Analysis of the saturation regime transfer characteristics revealed impressive hole mobilities for \u003cb\u003e2\u003c/b\u003e, with maximum and average values reaching 0.083 and 0.064 cm\u003csup\u003e2\u003c/sup\u003eV\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003es\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively, accompanied by an on/off current ratio exceeding 10\u0026sup3; (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec, Table S12). These values are comparable to the best-reported values for organic diradical small molecules.\u003csup\u003e\u003cspan citationid=\"CR92\" class=\"CitationRef\"\u003e92\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR93\" class=\"CitationRef\"\u003e93\u003c/span\u003e\u003c/sup\u003e The good carrier-transporting properties may be attributed to the good planarity, tight molecular packing, better diradical stability, and suitable frontier orbital energy levels.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eUniversality of Synthetic Strategy\u003c/p\u003e\u003cp\u003eBuilding upon the advantages of the modular synthetic approach and the promising optoelectronic characteristics of non-benzenoid PAHs incorporating indacene, pentalene, or azulene motifs, we extended our investigation to synthesize additional non-benzenoid PAHs using diverse substrates through this strategy. Here, we designed and synthesized two novel substrates (\u003cb\u003e22a\u003c/b\u003e and \u003cb\u003e22b\u003c/b\u003e) that successfully enabled the concurrent construction of 55/57 (\u003cb\u003e24\u0026thinsp;\u0026minus;\u0026thinsp;1\u003c/b\u003e), 57/57 (\u003cb\u003e24\u0026thinsp;\u0026minus;\u0026thinsp;2\u003c/b\u003e), and 55/55 (\u003cb\u003e24\u0026thinsp;\u0026minus;\u0026thinsp;3\u003c/b\u003e) skeletons (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). This was achieved through a series of reactions, including 5-exo-dig cyclization, aldehyde nucleophilic addition, and Friedel-Crafts cyclization, facilitated by the poor \u003cem\u003eE/Z\u003c/em\u003e stereoselectivity of the exocyclic double bond (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). We isolated the corresponding products with different yields, 55.9% and 32.2% for \u003cb\u003e24-1a\u003c/b\u003e and \u003cb\u003e24-3a\u003c/b\u003e, 25.6%, 3.3%, and 40.6% for \u003cb\u003e24-1b\u003c/b\u003e, \u003cb\u003e24-2b\u003c/b\u003e, and \u003cb\u003e24-3b\u003c/b\u003e, respectively. The notably low yield of \u003cb\u003e24\u0026thinsp;\u0026minus;\u0026thinsp;2\u003c/b\u003e compared to the higher yields of \u003cb\u003e24\u0026thinsp;\u0026minus;\u0026thinsp;1\u003c/b\u003e and \u003cb\u003e24\u0026thinsp;\u0026minus;\u0026thinsp;3\u003c/b\u003e suggests that product formation is strongly influenced by the steric hindrance (\u003cem\u003eE\u003c/em\u003e- or \u003cem\u003eZ\u003c/em\u003e-configuration) of the key intermediates (\u003cb\u003e23\u0026thinsp;\u0026minus;\u0026thinsp;1\u003c/b\u003e, \u003cb\u003e23\u0026thinsp;\u0026minus;\u0026thinsp;2\u003c/b\u003e, and \u003cb\u003e23\u0026thinsp;\u0026minus;\u0026thinsp;3\u003c/b\u003e). However, attempts to obtain the corresponding oxidized non-benzenoid PAHs (\u003cb\u003e25\u0026thinsp;\u0026minus;\u0026thinsp;1\u003c/b\u003e and \u003cb\u003e25\u0026thinsp;\u0026minus;\u0026thinsp;2\u003c/b\u003e) through oxidative dehydrogenation using DDQ/toluene conditions were unsuccessful. While \u003cb\u003e25\u0026thinsp;\u0026minus;\u0026thinsp;3\u003c/b\u003e could be formed under DDQ/toluene conditions, it was proved too unstable for isolation in pure form, though the HRMS corroborates the proposed structure. Unfortunately, we could not obtain the single crystal of compound \u003cb\u003e25\u0026thinsp;\u0026minus;\u0026thinsp;3\u003c/b\u003e owing to its instability. We successfully obtained the single crystal structure of the corresponding precursor of \u003cb\u003e24-3a\u003c/b\u003e, which unambiguously confirmed the construction of two 5/7 motifs and locks in the core ring topology, providing high confidence in the chemical structure of 25-3a/b (see Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e and S32). These results suggested that the proposed structure of \u003cb\u003e25\u0026thinsp;\u0026minus;\u0026thinsp;3\u003c/b\u003e is rational. To elucidate the origin of this instability, we conducted DFT calculations to examine the electronic structure and aromaticity of these compounds. As illustrated in Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec-\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee, these non-benzenoid systems display pronounced local antiaromaticity, particularly in the 5/5 and 5/7 motifs, as evidenced by NICS(1)\u003csub\u003ezz\u003c/sub\u003e, ICSS, and ACID analyses (see Supporting Information). This antiaromatic character disrupts electron delocalization across the molecular framework, leading to their pronounced instability under reaction conditions. Furthermore, DFT calculations revealed that these compounds adopt triplet ground states (Table S15 and Figure S34), further accounting for their instability. Surprisingly, when we subjected the asymmetric precursors \u003cb\u003e24-1a\u003c/b\u003e and \u003cb\u003e24-1b\u003c/b\u003e to oxidation, we isolated hydroxyl-substituted derivatives (\u003cb\u003e26-1a\u003c/b\u003e and \u003cb\u003e26-1b\u003c/b\u003e) in 8.6% and 12.1% yields, respectively. Their structures were unambiguously characterized by NMR, HRMS, and single-crystal X-ray diffraction (Figure S33). ICSS maps (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb) indicate that while the 5/5 rings retain their antiaromatic nature, the 5/7 rings transition to weakly aromatic behavior upon hydroxylation, thereby enhancing molecular stability. These findings underscore the critical role of \u003cem\u003eE/Z\u003c/em\u003e-configuration stereoselectivity in the 5-exo-dig cyclization intermediates for accessing structurally diverse 5/5 or 5/7-membered ring skeletons of non-benzenoid PAHs featuring indacene, pentalene, or azulene units.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eIn summary, we developed a modular synthetic strategy to construct three distinctive non-benzenoid polycyclic aromatic hydrocarbons (PAHs) that \u003cb\u003e1\u003c/b\u003e and \u003cb\u003e2\u003c/b\u003e combine an indacene core and two pentalene (5/5) subunits and \u003cb\u003e3\u003c/b\u003e is an S-shaped azulene (5/7)-embedded linear PAH. The product selectivity was governed by the \u003cem\u003eE/Z\u003c/em\u003e-configuration of key intermediates, yielding corresponding products of \u003cb\u003e1\u003c/b\u003e, \u003cb\u003e2\u003c/b\u003e, and \u003cb\u003e3\u003c/b\u003e, as confirmed by NMR, HRMS, and X-ray crystallography. Bond length analysis, NICS(1)\u003csub\u003ezz\u003c/sub\u003e calculations, ACID plots, and ICSS maps revealed that \u003cb\u003e1\u003c/b\u003e and \u003cb\u003e2\u003c/b\u003e adopt planar, globally antiaromatic structures, whereas \u003cb\u003e3\u003c/b\u003e exhibits a twisted, globally aromatic periphery. Intriguingly, compound \u003cb\u003e2\u003c/b\u003e exhibited a pronounced open-shell feature, with a \u003cem\u003ey\u003c/em\u003e\u003csub\u003e\u003cem\u003e0\u003c/em\u003e\u003c/sub\u003e of 0.38 and a small Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003est\u003c/sub\u003e of -2.02 kcal/mol compared to \u003cb\u003e1\u003c/b\u003e (closed-shell) and \u003cb\u003e3\u003c/b\u003e (closed-shell), as confirmed by VT-NMR, VT-ESR measurements, and DFT calculations. The half-life of \u003cb\u003e2\u003c/b\u003e in dichloromethane solution was approximately 32 days, while \u003cb\u003e1\u003c/b\u003e and \u003cb\u003e3\u003c/b\u003e showed almost no degradation within a month. Surprisingly, compound \u003cb\u003e2\u003c/b\u003e demonstrates promising \u003cem\u003ep\u003c/em\u003e-type transporting behavior, achieving a hole mobility up to 0.083 cm\u003csup\u003e2\u003c/sup\u003eV\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003es\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e under air atmosphere, underscoring its potential for organic electronics. Further extension of this method to two axisymmetric substrates yielded additional non-benzenoid PAHs, though their instability was attributed to strong antiaromaticity and triplet states, as experimental and DFT calculations confirmed. This work provides a versatile and controllable approach to access structurally diverse non-benzenoid PAHs containing indacene, pentalene, or azulene motifs, offering tunable electronic properties for functional applications. Future efforts will focus on optimizing stability and expanding the library of functional non-benzenoid architectures.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003eSynthesis of compound \u003cb\u003e1\u003c/b\u003e\u003c/p\u003e\u003cp\u003eTo a 100 mL three-necked flask equipped with a magnetic stir bar, compound \u003cb\u003e9\u003c/b\u003e (crude product obtained from the above-mentioned two steps) and 20 mL dry toluene were added. The mixture was degassed and charged with nitrogen for three times and stirred at 0 \u003csup\u003eo\u003c/sup\u003eC (ice-water bath) for 3 min. Then, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (250 mg, 1.1 mmol) in 5 mL dry toluene was added to the mixture and the mixture was stirred under N\u003csub\u003e2\u003c/sub\u003e at room temperature for 1 h (the color of the mixture turned to red-brown). Then H\u003csub\u003e2\u003c/sub\u003eO was added to quench the reaction. The mixture was extracted by DCM (50 mL\u0026times;3), and washed with saturated sodium chloride aqueous solution (50 mL\u0026times;3). The organic layer was dried by anhydrous Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e. The crude product was further purified by a short silica gel column (PE/DCM\u0026thinsp;=\u0026thinsp;1/1, v/v). Compound \u003cb\u003e1\u003c/b\u003e was obtained as an army green solid (57 mg, 17.3% yield).\u003c/p\u003e\u003cp\u003eSynthesis of compound \u003cb\u003e2\u003c/b\u003e\u003c/p\u003e\u003cp\u003eTo a 100 mL Schlenk flask equipped with a magnetic stir bar, compound \u003cb\u003e15\u003c/b\u003e (crude product obtained from the above-mentioned second step) and 30 mL dry toluene were added. Then, through a freeze-pump-thaw cycle, the tube was charged with nitrogen for three times again. After the mixture had thawed, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (430 mg, 1.89 mmol) in 4 mL dry toluene was added to the mixture and the mixture was stirred under N\u003csub\u003e2\u003c/sub\u003e at room temperature for 2 h (the color of the mixture turned to blue-green). After the reaction finished, the mixture was poured into hexane (300 mL) and a large amount of precipitation was generated and filtered off. The precipitation was washed with MeOH. The crude product was further purified by a short silica gel column (DCM/toluene\u0026thinsp;=\u0026thinsp;4/1, v/v). Compound \u003cb\u003e2\u003c/b\u003e was obtained as a purple solid (305 mg, 63.3% yield).\u003c/p\u003e\u003cp\u003eSynthesis of compound \u003cb\u003e3\u003c/b\u003e\u003c/p\u003e\u003cp\u003eTo a 50 mL Schlenk flask equipped with a magnetic stir bar, compound \u003cb\u003e21\u003c/b\u003e (crude product obtained from the second step) and 15 mL dry toluene were added. Then, through a freeze-pump-thaw cycle, the tube was charged with nitrogen for three times. After the mixture had thawed, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (71.8 mg, 0.32 mmol) in 2 mL dry toluene was added to the mixture and the mixture was stirred under N\u003csub\u003e2\u003c/sub\u003e at room temperature for 0.5 h. Then, H\u003csub\u003e2\u003c/sub\u003eO (5 mL) and Et\u003csub\u003e3\u003c/sub\u003eN (2 mL) were added to quench the reaction. The residual was extracted by DCM, and washed with saturated sodium chloride aqueous solution. The organic layer was dried with anhydrous Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e and the solvent was removed by rotary evaporation. The crude product was then purified by flash column chromatography using dichloromethane and methanol (1/0\u0026thinsp;~\u0026thinsp;20/1, v/v) as eluent to give a brown solid of \u003cb\u003e3\u003c/b\u003e (19 mg, 19.7% yield).\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eContributions\u003c/h2\u003e\n\u003cp\u003eR. Yang conceived this project and gave constructive guidance and revised the manuscript; L. Chen did the experiments and analyzed the data and wrote the draft; Z. Shangguan helped with the calculation; L. Qin gave help for the single-crystal analysis; Y. Zeng and Q. Zhu gave help for the photophysical measurements; J. Chen, J. Liang, X. Qiu, X. Wang and D. Zhang gave valuable discussions for this article.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interest\u003c/p\u003e\n\u003ch2\u003eAcknowledgements\u003c/h2\u003e\n\u003cp\u003eThe authors are deeply grateful to the National Natural Science Foundation of China (22375077, 52573276, 52203225, 52073122), Excellent Young and Middle aged Science and Technology Innovation Team Program for Universities in Hubei Province (T2023037), Department of Science and Technology of Hubei Province (2024DJC006), Key R \u0026amp; D Project of Hubei Province (2022BAA095), Hubei Natural Science Foundation (2022CFB903), Key Research and Development Program of Wuhan (2024010802030156), Ministry of Science and Technology of China (2021YFE0113600).\u003c/p\u003e\n\u003ch3\u003eData Availability\u003c/h3\u003e\n\u003cp\u003eMaterials and methods, experimental procedures, characterization data, copies of NMR and EPR spectra, X-ray crystallographic details, photophysical studies, electrochemical measurements, and calculations of all involved compounds and processes are provided in the Supplementary Information. Source data are provided with this paper. CIF crystallographic data files and xyz coordinates of the optimized structures are available as Supplementary Files. Crystallographic data for the structures reported in this article have been deposited at the Cambridge Crystallographic Data Center, under deposition numbers CCDC 2478865 (\u003cstrong\u003e7\u003c/strong\u003e), 2478866 (\u003cstrong\u003e13\u003c/strong\u003e), 2478867 (\u003cstrong\u003e19\u003c/strong\u003e), 2478868 (\u003cstrong\u003e1\u003c/strong\u003e), 2478869 (\u003cstrong\u003e2\u003c/strong\u003e), 2478870 (\u003cstrong\u003e3\u003c/strong\u003e), 2493835 (\u003cstrong\u003e25-3a\u003c/strong\u003e) and 2478871 (\u003cstrong\u003e26-1b\u003c/strong\u003e). All data are available from the corresponding authors upon request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003ePun SH, Miao Q (2018) Toward Negatively Curved Carbons. 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Org Lett 22:2553\u0026ndash;2558\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"},{"header":"Schemes","content":"\u003cp\u003eSchemes 1 and 2 are available in the Supplementary Files section\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-7977582/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7977582/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eNon-benzenoid polycyclic aromatic hydrocarbons (PAHs) containing antiaromatic indacene or pentalene and aromatic azulene subunits emerged as compelling materials, distinguished by their unique electronic configurations, exceptional optoelectronic characteristics, and potential applications in organic electronics. However, their controllable synthesis remains challenging due to inherent instability and stringent electronic requirements. Herein, we present a modular synthetic strategy that enables the construction of stable non-benzenoid PAHs (\u003cb\u003e1\u003c/b\u003e, \u003cb\u003e2\u003c/b\u003e, and \u003cb\u003e3\u003c/b\u003e) featuring indacene, pentalene, and azulene motifs through a carefully designed sequence of 5-exo-dig cyclization (with controllable \u003cem\u003eE/Z\u003c/em\u003e-selectivity), nucleophilic addition, Friedel-Crafts cyclization and oxidative dehydrogenation. Comprehensive structural and electronic analyses revealed that \u003cb\u003e1\u003c/b\u003e and \u003cb\u003e2\u003c/b\u003e exhibit global antiaromaticity and \u003cb\u003e2\u003c/b\u003e displays a more pronounced open-shell diradical character than \u003cb\u003e1\u003c/b\u003e, while \u003cb\u003e3\u003c/b\u003e maintains a global aromaticity and a closed-shell structure. Notably, compound \u003cb\u003e2\u003c/b\u003e demonstrated promising \u003cem\u003ep\u003c/em\u003e-type semiconductor behavior with a hole mobility of up to 0.083 cm\u003csup\u003e2\u003c/sup\u003eV\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003es\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Additionally, all three compounds demonstrated remarkable stability under ambient conditions, underscoring their potential for practical applications in organic electronics. Further exploration of this synthetic strategy enabled the potential synthesis of additional non-benzenoid PAHs (\u003cb\u003e25\u0026thinsp;\u0026minus;\u0026thinsp;1/2/3\u003c/b\u003e), which show strong antiaromaticity and triplet diradical characteristics, resulting in their instability. This work provides a tailorable and universal approach to designing non-benzenoid PAHs with tunable structure, aromaticity and diradical characters for functional applications.\u003c/p\u003e","manuscriptTitle":"Sterically Controlled 5-exo-dig Cyclization Enables Modular Synthesis of Non-benzenoid Polycyclic Aromatic Hydrocarbons with Intriguing (Anti)aromaticity and Diradical","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-07 07:12:58","doi":"10.21203/rs.3.rs-7977582/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"7c6878b7-13fe-4917-b94b-c804943c4402","owner":[],"postedDate":"November 7th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":57560779,"name":"Physical sciences/Chemistry/Materials chemistry/Electronic materials"},{"id":57560780,"name":"Physical sciences/Chemistry/Materials chemistry/Magnetic materials"}],"tags":[],"updatedAt":"2025-12-05T14:52:07+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-07 07:12:58","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7977582","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7977582","identity":"rs-7977582","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}