Biphenylene-containing polycyclic conjugated compounds

Introduction

Acenes represent an important category of carbon-rich polycyclic aromatic hydrocarbons (PAHs) characterized by the presence of linearly fused benzene rings [1,2]. Investigating the electronic properties of acenes is essential for understanding the correlations between structure and electronic properties, as these units serve as fundamental building blocks in graphite and carbon nanotubes [3]. The limited stability of this particular class of PAHs arises as a key challenge, primarily attributed to their extended conjugation. The longer acenes exhibit increased reactivity, readily undergoing processes of oxidation and dimerization, consequently disrupting the molecular conjugation [4]. This instability poses a significant obstacle in their widespread application across various devices [4]. The decline in stability seen in larger acenes can be attributed to Clar's rule, which considers the increasing number of non-sextet rings throughout the acene series as a contributing factor (Figure 1) [5,6].

Numerous approaches have been developed to address the challenges arising from the instability and solubility issues encountered in acenes. These include the incorporation of heteroatoms within the acene backbone [7,8], stabilization of the acene core structure through the integration of diverse units [9,10], and the introduction of bulky substituents [11]. These approaches aim to maintain the desirable electronic properties of acenes while mitigating the aforementioned challenges to the best possible extent. Our focus in this review is primarily on exploring the role of biphenylenes in stabilizing the core structures of acenes and other PAHs.

Review

Biphenylenes and [N]phenylenes

Biphenylene (1), which consists of two aromatic benzene rings connected with a four-membered ring, is a highly intriguing compound in terms of its structure. It possesses a planar configuration and consists of 4n π-electrons, rendering it antiaromatic. However, despite being antiaromatic, biphenylene is more stable than other known antiaromatic compounds [12]. Research using computational methods to investigate how benzo and benzocyclobutadiene annulations impact the ring current density of biphenylene derivatives reveals that the antiaromatic (paratropic) current density in the 4-ring structure can range widely, shifting from highly antiaromatic to nonaromatic limits based on the annulation modes employed [13,14]. Since Lothrop's initial successful synthesis of biphenylene in 1941 [15], numerous studies have been carried out, highlighting four prominent synthetic approaches. These methods include flash vacuum pyrolysis [16-18], [2 + 2] cycloaddition [19,20], [2 + 2 + 2] cycloaddition [21], and the Ullmann reaction [15,22] (Scheme 1). Due to the observed low yields in flash vacuum pyrolysis, the difficulty in synthesizing starting materials, such as 3, and the impractical nature of scaling up the method for large quantities, the other three approaches have gained popularity for synthesizing biphenylene derivatives [23]. The utilization of in-situ aryne synthesis to generate biphenylene through the dimerization of arynes 2 from diverse substrates has gained popularity. However, this approach occasionally gives rise to the production of high-energy intermediates, such as benzenediazonium-2-carboxylate, and yields that are comparatively low [20]. After the Ullmann reaction was successfully employed for the first reported synthesis of biphenylene [15], subsequent studies have explored various transition-metal-mediated coupling reactions using 2,2'-dihalogenated biphenyls 4 as starting materials [24,25]. Although the cobalt-mediated alkyne trimerization route frequently used by Vollhardt and co-workers is not the first choice for the synthesis of the biphenylene itself, it has led to the synthesis of structurally demanding substituted biphenylenes and the emergence of a family of polycyclic hydrocarbons called [N]phenylenes.

The utilization of cobalt-mediated alkyne trimerization facilitated the synthesis of [N]phenylenes exhibiting diverse structural configurations, including linear 7, angular 8, zig-zag 9, bent 10, branched 11, and cyclic 12 topologies (Figure 2) [26-30]. In [N]phenylene structures, the presence of a formally antiaromatic four-membered ring leads to the localization of π-electrons on the benzene rings [31]. Despite this phenomenon, the oligomer series demonstrates a decreasing band gap, indicating the ability of electrons to transmit through the four-membered rings [32].

Phenylene-containing oligoacenes (POAs)

The localization of π-electrons and the consequent decrease in band gaps observed in the [N]phenylene series have sparked interest in exploring acene–biphenylene hybrid structures. If this trend could be maintained in practical applications, it would offer the opportunity to retain the desirable electronic properties while mitigating the inherent stability concerns associated with acenes. The underlying principle guiding the design is to maximize stability by incorporating the largest possible number of Clar sextets, while concurrently minimizing any adverse impact on electronic properties resulting from reduced electron delocalization. In 1983, McOmie and co-workers reported the first synthesis of phenylene-containing oligoacenes (POAs) [33]. The primary objective of their work was to establish an alternative synthetic approach to the existing methods for biphenylene synthesis described in the literature, rather than focusing on the optoelectronic properties of the resulting compounds. Small quantities of the key starting material benzocyclobutene-1,2-dione (BBD, 13) were obtained through the pyrolysis of indane-1,2,3-trione. When BBD 13 was subjected to reflux conditions with bis(cyanomethyl) compounds 14a (a benzene derivative) and 14b (a naphthalene derivative) in acetonitrile, the desired POAs 15a and 15b were obtained with yields of 25% and 48%, respectively (Scheme 2). It was also reported in the same study that the yield of 15b increased up to 62% when the water formed during the reaction was removed with CaH₂.

The initial results on biphenylene and its more extensively fused counterparts led to another notable study conducted by Jensen and Coleman in 1959 [34]. By subjecting α,α,α′,α′-tetrabromo-o-xylene (16) to t-BuOK to anhydrous conditions, biphenylene derivative 17 was obtained with a yield of 69% (Scheme 3). In the final step of the synthesis, a halogen–lithium exchange was carried out, followed by treatment with MeOH, resulting in a 79% yield of benzo[b]biphenylene (18).

In their efforts to find a more general method for the synthesis of benzo[b]biphenylenes, Barton and co-workers were able to synthesize benzo[b]biphenylene (18) in 71% yield by the reaction of equal molar amounts of tetrabromo compound 16 and 1,1,2,2-tetrabromo-1,2-dihydrobenzocyclobutene (19) in the presence of activated zinc dust in THF (Scheme 4) [35]. By making minor adjustments to the reaction conditions, such as employing DMF as the solvent and raising the temperature to 100 °C, along with utilizing compound 20 as the starting material, Barton and his team achieved the synthesis of POA 21. This POA featured an extended conjugation and was obtained in 30% yield.

In a study released by Swager and Parkhurst in 2012 [36], the term POA has been used for the first time in the literature. In this study, they successfully synthesized POAs 25a and 25b by employing sequential Diels–Alder reactions (Scheme 5). Furthermore, the researchers conducted a comprehensive investigation into the optical and electrochemical characteristics of these compounds. The key component employed in the production process, known as 3,4-bis(methylene)cyclobutene, was generated through the application of flash vacuum pyrolysis to 1,5-hexadiyne. When 3,4-bis(methylene)cyclobutene undergoes a reaction with dienes like 1,3-diphenylisobenzofuran and 1,3-diphenylisonaphthofuran, it selectively produces compounds 22a and 22b. In the subsequent step, the exocyclic methylidene groups react with bisarynes, which are in situ formed from 23, resulting in the formation of symmetric polycyclic structures 24a and 24b. These isomers obtained as a mixture are then subjected to treatment with p-TsOH in acetic acid, without the need for further purification, to yield the desired products 25a and 25b in 71 and 42% yields, respectively. When comparing compounds 25a and 25b, UV–vis and fluorescence studies (λ_max = 500 nm, λ_em = 502 nm, Φ_em = 0.45 for 25a; λ_max = 513 nm, λ_em = 517 nm, Φ_em = 0.26 for 25b; λ_max = 442 nm, λ_em = 444 nm, Φ_em = 0.97 for 9,10-bis((triisopropylsilyl)ethynyl)anthracene – blue-colored) provide clear evidence of a bathochromic shift and a reduction in the optical band gap. These results support the idea that the introduction of biphenylene linkages may decrease delocalization in the structure. However, despite this reduction, there is still communication between the acene units evident from the decreased band gaps in compounds 25a and 25b.

Despite the achievements in the synthesis of [N]phenylene and POAs, the challenges of low yield and stability issues, particularly with the starting materials employed in the syntheses, prompted researchers to explore alternative and improved methods. In this context, Xia and co-workers drew inspiration from their prior investigations on palladium-catalyzed annulation reactions concerning ladder polymers [37]. They envisioned that by making minor modifications to the starting materials, they could readily access structurally complex POAs. The researchers efficiently conducted palladium-catalyzed C–H activated annulation reactions, involving oxanorbornadiene derivative 26 and aryl bromides including dibromoanthracene 27 [38]. Subsequent aromatization reactions were then carried out, resulting in the successful synthesis of the target POAs with high yields. The study involved the synthesis of numerous polycyclic hydrocarbons containing both electron-withdrawing and -donating side groups. Among the various compounds synthesized, a particularly noteworthy achievement was the successful synthesis of acene-type compound 29 using 2,6-dibromo-9,10-bis(triisopropylsilylethynyl)anthracene (27), which was accomplished with high yields (Scheme 6).

The presence of two methyl groups in the bridge-head positions of compound 26 is crucial in these annulation reactions. These groups play a vital role in preventing undesired side reaction pathways, as their absence would lead to the failure in the formation of the desired target products. Upon comparing the UV–vis absorbance graphs of compounds 28 and 29, POA 29 (λ_max = 500 nm), which was obtained through the aromatization of compound 28, exhibited a significant bathochromic shift. These observations further support the hypothesis that electrons can indeed be delocalized through the 4-membered ring system in the POAs.

Following their work reported in 2017 [38], Xia and his group directed their attention towards refining the electronic properties of POAs through structural variations [39]. Using catalytic annulation reactions involving 1,4-dibromo-2,5-dichlorobenzene (31) and substituted oxanorbornenes (26 and 30b,c), products 32a–c, with R groups representing (a) H, (b) F, and (c) –OCH₂O–, were successfully synthesized, albeit with moderate yields (Scheme 7).

Following that, compounds 32a–c underwent derivatization through Sonogashira cross-coupling reactions with alkynes featuring different protecting groups such as TIPS, TES, and TIBS. Scheme 7 illustrates the derivatization process using one of the chosen examples, specifically the TIPS group. Accordingly, the cross-coupling products 33a–c were obtained in yields ranging between 45% and 60%. The last step of the sequential reactions is the aromatization step and the target POAs 34a–c were obtained in yields between 80–84%. UV–vis investigations conducted on compounds 34a–c revealed absorption bands that align well with acene structures. While 34a and 34b displayed nearly identical absorption profiles with a maximum absorption at λ_max = 515 nm, the incorporation of donor groups in compound 34c led to a noteworthy bathochromic shift with a maximum absorption at λ_max = 534 nm. Additionally, it was reported that all three compounds, 34a–c, demonstrated remarkable stability, showing no signs of degradation over an extended period when kept in the dark, both in solid form and in solution under air. In the final phase of the study, the authors investigated the charge-transport properties of compond 34a in OFET. Since the charge-transport properties are significantly affected by the molecular packing, they have modified compound 34a using different protecting groups. In this context, triethylsilyl (TES), triisopropylsilyl (TIPS), and triisobutylsilyl (TIBS) groups were incorporated into the structure considering the increased dimensions. Thus, derivatives 34a-TES and 34a-TIPS showed hole mobilities of 0.075 cm² V⁻¹ s⁻¹ and 0.19 cm² V⁻¹ s⁻¹, respectively, while the highest value was noted with 34a-TIBS at 0.52 cm² V⁻¹ s⁻¹.

Later on, Xia and his colleagues demonstrated that curved PAH structures can be synthesized by employing their own developed palladium-catalyzed arene–oxanorbornadiene annulation reactions [40]. This study involved converting PAHs obtained through annulation and aromatization steps into curved PAH structures using metal-catalyzed cycloaddition reactions pioneered by Vollhart [41] and Kotora [42]. Unlike previous studies that reported cycloadditions from bay regions of [N]phenylenes, metal-catalyzed cycloadditions with diphenylacetylene occurred exclusively in the non-bay region, which allowed for straightforward syntheses of curved structures. Moreover, the presence of methyl groups in the structure facilitated the controlled activation of desired cyclobutadiene units, enabling precise modifications. A selected example from the study is summarized in Scheme 8. The angular structure 36 was prepared through annulation reaction between oxanorbornene 26 and 1,8-dibromobiphenylene (35), followed by aromatization via treatment with HCl in CHCl₃ and iPrOH, resulting in the formation of compound 37 in 49% yield. In the final step, Ir-catalyzed cycloaddition reaction with diphenylacetylene (tolane) led to PAH 38 in 47% yield.

According to the X-ray analysis results, it is evident that the structure of compound 38 is far from planarity, and the phenanthrene moiety exhibits a dihedral angle of approximately 22°. Upon comparing the UV–vis spectra of the angular structures 37 and 38, it was observed that after the Ir-catalyzed cycloaddition reaction, the λ_max of product 38 considerably blue shifted in comparison to the λ_max of 37.

Xia et al. also conducted a synthesis of [3]naphthylene regioisomers through Pd-catalyzed annulation reactions, employing 2,7-, 1,5-, and 1,7-dibromonaphthalenes (Scheme 9) [43].

These annulation reactions involving 2,7-, 1,5-, and 1,7-dibromonaphthalene with different benzoxanorbornadienes (R = (a) TIPSA, (b) 2,6-(CH₃)₂C₆H₃, (c) H), followed by aromatization in acidic conditions, resulted in the formation of three [3]naphthalene regioisomers 43a–c, 44a–c, and 45a–c with excellent yields of up to 94%. The synthesized PAHs 43a, 44a, and 45a with diverse geometries exhibited interesting absorption and emission characteristics, making them highly intriguing for further study and potential applications. Among the regioisomers in the series, the linear isomer 43a displayed the highest quantum yield (Φ_em = 0.64). Additionally, its absorption and emission max values (λ_max and λ_max,em) were determined to be 476 and 477 nm, respectively. Compound 44a exhibited no alteration in the absorption maxima; however, there was a considerable bathochromic shift in the emission maxima (λ_max = 476 nm and λ_max,_em = 587 nm for 44a). In contrast to compound 43a, Φ_em of compound 44a decreased significantly to 0.08. Likewise, compound 45a (λ_max = 402 nm, λ_max,em = 524 nm, Φ_em = 0.16) also demonstrated a substantially reduced quantum yield when compared to compound 43a. Theses regioisomers with varying geometric structures provided strong evidence that the antiaromaticity of the cyclobutadiene (CBD) ring can be tuned up with appropriate structural designs. NICS (nucleus independent chemical shift) analysis revealed that the linear structures within the series exhibited reduced antiaromaticity in the CBD ring compared to other structural arrangements.

By employing a methoxy group on the oxonorbornene, the methyl ether moiety can be removed and the phenol subsequently converted to a triflate. The Pd-catalyzed annulation approach can be conducted sequentially, facilitating the synthesis of polyaromatic hydrocarbons, particularly unsymmetrical ones (Scheme 10) [44]. In this way, compound 48 was synthesized through a two-step process involving the Pd-catalyzed annulation between compounds 46 and 47, followed by aromatization in the presence of PPTS. The methoxy group was then cleaved using HBr in Aliquat-336, and the resulting product was converted to the triflate under basic conditions, resulting in a regioisomeric mixture of 49 with a yield of 81% over the two steps. To obtain the PAH 51, a second Pd-catalyzed annulation and subsequent aromatization of 50 were performed in an acidic medium. Ultimately, the desired compound, PAH 51 bearing TIPS groups, was obtained in 82% yield as the final product. UV–vis and fluorescence studies were conducted to gather insights into the optoelectronic characteristics of PAH 51. The obtained data revealed that the synthesized compound possesses a unique absorption and emission profile, highlighting its distinctive optical properties (λ_max = 470 nm, λ_max,em = 470 nm, Φ_em = 0.12).

Biphenylene-containing azaacenes

Bunz and co-workers incorporated biphenylene units into azaacene structures to enhance their stability [45]. Initially, their early attempts focused on synthesizing unsymmetrical azaacenes containing biphenylene units. Through the condensation of ortho-diamine compounds 53a–c derived from benzene (53a), naphthalene (53b), and anthracene (53c) with compound 52, the desired target azaacenes 54a–c were successfully obtained in yields ranging from 68% to 84% (Scheme 11). As anticipated, there was a notable red shift observed from 54a to 54c, which can be attributed to the expansion of the acene structure (λ_max = 441 nm, λ_max,em = 444 nm for 54a; λ_max = 541 nm, λ_max,em = 550 nm for 54b; λ_max = 648 nm, λ_max,em = 655 nm for 54c).

Next, the Bunz group focussed on symmetric azaacenes, which present a greater challenge in synthesis compared to their non-symmetric counterparts (Scheme 12) [46]. This complexity arises from factors such as limited substrate versatility and the difficulty in incorporating solubilizing groups into the symmetric azaacene framework. In the initial stage, the condensation reactions between biphenylene-2,3-dione (52) and diaminothiadiazoles 55a,b resulted in the formation of polycyclic structures 56a and 56b containing thiadiazole units, in yields of 57% and 34%, respectively. Subsequently, by selectively cleaving the thiadiazole ring using LiAlH₄, followed by further condensation reaction with dione 52, the desired symmetric azaacenes 58a and 58b with R groups representing (a) isopropyl and (b) sec-butyl, were successfully obtained in 84% and 61% yields, respectively. The incorporation of a biphenylene group into the azaacene structure did not result in any significant impact on the electrochemical properties. The electrochemical analysis revealed the presence of two reduction potentials commonly observed in azaacenes, suggesting that the modification did not alter this characteristic feature. For compound 58a, λ_max was observed at 600 nm, and λ_max,em was at 614 nm. On the other hand, for compound 58b, λ_max was found at 606 nm, and λ_max,em occurred at 616 nm. Based on the photophysical properties of compounds 54a and 54b, which were synthesized in the previous study (Scheme 11), it is evident that the addition of the second biphenylene-fused pyrazine group to the structure leads to a substantial red shift towards the NIR region. This observation indicates that the incorporation of biphenylene-containing groups has a positive impact on the optoelectronic properties of these structures, while also promoting their stability.

With appropriate substrate choices, Xia and co-workers were able to apply the Pd-catalyzed annulation strategy that they developed for the synthesis of biphenylene-containing azaacene structures [47] (Scheme 13). The methodology used in this study closely resembled their previous work (Scheme 9) [43].

The method used offered a distinct advantage compared to previous approaches [45], as it enabled the synthesis of not only linear azaacene structures but also non-linear structures as shown in Scheme 13. This versatility allowed for a broader range of conjugated molecules to be successfully prepared. The target azaacene analogues 62–64, featuring 2,6-(CH₃)₂C₆H₃ groups were obtained in yields ranging from 50 to 85% through a series of steps. First, Pd-catalyzed annulation reactions of oxanorbornene 39b and dibromophenazine derivatives 59–61 were carried out. Subsequently, aromatization reactions were performed in the presence of HCl to yield the desired products. The conjugated structures with distinct geometries demonstrated significant absorption and fluorescence characteristics. Among the synthesized compounds, linear derivative 62 exhibited the most intense absorption peak (λ_max = 503 nm), whereas the other two compounds 63 and 64 possessed slightly blue-shifted absorption maxima at 487 nm and 501 nm, respectively. When the fluorescence properties of compounds 62, 63, and 64 were examined, it was observed that compound 62 exhibited strong fluorescence, making it an excellent fluorophore (λ_max,em = 508 nm, Φ_em = 0.58) . On the other hand, compound 63 showed weaker fluorescence compared to 62 (λ_max,em = 666 nm, Φ_em = 0.07), and compound 64 displayed almost no fluorescence emission (λ_max,em = 497 nm, Φ_em = NA).

Naphthazarin–biphenylene hybrid structures

Taking the advantage of naphthazarin's bifunctional Diels–Alder reactivity, Swager and his team succeeded in the synthesis of POA-type structures incorporating naphthazarin and triptycene units (Scheme 14) [48]. Naphthazarin derivatives are known to complex with boron moieties and metals to form electron-poor acene units. Through the Diels–Alder reaction involving dienophile 66, which was formed via the tautomerization of compound 65, and diene 67, compound 68 was successfully obtained in 30% yield. In the final step, the target POA-type structure containing naphthazarin 69 was synthesized by aromatization under acidic conditions. The attempts to conduct complexation experiments using BF₃·OEt₂ to obtain a stable BF₂ complex were unsuccessful, primarily due to solubility problems encountered with substrate 69 and its BF₂ complex. Based on the UV–vis analysis of compounds 65 and 69, it was observed that compound 65 displayed a low-energy absorption band at λ_max = 568 nm, whereas compound 69 exhibited absorption at λ_max = 557 nm. Unfortunately, the unsuccessful BF₂ complexation step to isolate stable BF₂ complex of 69, likely attributable to solubility issues encountered with substrate 69, prevented the incorporation of the acene backbone into the structure. Consequently, the anticipated red-shift in the absorption spectrum could not be observed as expected.

Boron-doped phenylene-containing oligoacenes

Following the evaluation of the POA concept combined with azaacenes and naphthazarin derivatives, the impact of boron doping on the optoelectronic properties of POA structures was also targeted [49]. In the study, the envisaged POA 73 was successfully synthesized in three steps (Scheme 15). The initial step involved the synthesis of compound 71 in 64% yield using a cobalt-catalyzed cyclotrimerization reaction between 1,2-diethynylbenzene (5) and bis(trimethylsilyl)acetylene (70), a method commonly employed in [N]phenylene synthesis. Subsequently, treatment of compound 71 with excess BBr₃ in n-hexane, dibrominated intermediate 72 was obtained in 57% yield. In the final step, mesitylation was conducted utilizing mesitylcopper, leading to the successful access of the desired boron-doped POA 73 in 80% yield.

The red-colored POA 73 is noted for its absence of well-resolved absorption bands above 450 nm. Instead, the compound's color is attributed to plateau-shaped weak absorption bands, extending to approximately 570 nm. Surprisingly, POA 73 exhibited nonfluorescent behavior, in contrast to previously known POAs described in the literature. The hypothesis suggesting that this phenomenon is a result of non-radiative deactivation was corroborated by detailed computational chemistry studies.

In continuation of the aforementioned work by Wagner and co-workers [49], they have synthesized also "v" and "z"-shaped POAs, wherein biphenylene groups are angularly incorporated into the 1,4-dibora-2,5-cyclohexadiene structure (Scheme 16) in addition to a π-extended linear POA (Scheme 17) [50]. The preparation of v- and z-shaped POAs 77 and 78 was carried out starting from 2-bromobiphenylene (74). Initial steps involved ortho-directed lithiation and subsequent treatment with Me₃SiCl, resulting in the formation of compound 75. Further transformation through lithium–halogen exchange, followed by reaction with Me₃SiCl, yielded bis(trimethylsilylated) intermediate 76 in a 90% yield. By utilizing the conditions outlined in the prior investigation [49], compound 76 was subjected to a reaction with BBr₃. In the final step, the integration of mesityl groups into the molecular structure, resulted in the production of both compounds 77 and 78, adopting v and z-configurations, respectively, in a 4 to 1 ratio.

The synthesis of linear compound 84, characterized by a more extensive conjugated system in comparison to the boron-modified POA 73 outlined in Scheme 15, was accomplished through a sequence of five steps, starting from 2,3-dihydroxynaphthalene (79, Scheme 17). Following the successful synthesis of the nonaflate during the initial phase starting from 79, the subsequent step involved the synthesis of compound 81 through the utilization of the Negishi cross-coupling reaction and then the removal of TMS groups from this intermediate was achieved using TBAF, resulting in the formation of diyne 82 in 65% yield. The progression towards the synthesis of biphenylene-containing substrate 83 was achieved through a Co-mediated alkyne trimerization process. Finally, the synthesis of the targeted boron-doped extended POA 84 was carried out with a yield of 61%, following a series of reactions including cyclocondensation with BBr₃ and mesitylation.

Since the "v" and "z"-shaped POAs could not be separated by physical methods, it was not possible to study their photophysical properties separately. Nonetheless, comparison of the properties of POAs 73 and 84 has provided essential data. Closely resembling compound 73, the red color of POA 84 is attributed to an extensively broad absorption band, with an onset wavelength at around 570 nm. Although POA 73 does not exhibit any fluorescence characteristics, compound 84 displays a red emission, which can be attributed to the presence of two benzene rings integrated within its structure (λ_max,em = 646 nm, Φ_em = 0.12). The fluorescence properties of the mixture 77/78 were markedly enhanced in comparison to those of linear POAs 73 and 84, owing to the distinctive geometric arrangement inherent in their structures (λ_max,em = 506 nm, Φ_em = 0.65).

On-surface synthesis of phenylene-containing oligoacenes

The POA syntheses discussed in this review thus far have predominantly involved solution chemistry. However, recent reports have demonstrated the feasibility of producing biphenylene-containing polycyclic aromatic compounds through on-surface chemistry techniques. One of the initial examples was documented by Fasel and Meunier in 2017 [51]. In their study, they effectively synthesized POA 87 utilizing 2,3-dibromotetracene (85) as substrate at a temperature of 430 K, employing ultra high vacuum conditions on Ag(111).

Subsequent scanning tunneling microscopy (STM) analyses unveiled not only the linear POA 87 resulting from surface-catalyzed formal [2 + 2] cycloaddition reactions but also the emergence of tetracene trimer 86 and tetramer 88 stemming from [2 + 2 + 2] cycloaddition reactions (Scheme 18). It is proposed that an aryne intermediate is formed after thermal activation and that the observed end products are formed from arynes via [2 + 2] and [2 + 2 + 2] cycloadditions.

In a closely related study conducted by Grill et al., the behavior of 2,3-dibromoanthracene (89) was examined on two distinct surfaces [Au(100) and Au(111)] (Scheme 19) [52]. Notably, on the Au(111) substrate, nearly equivalent quantities of dimer 91 resulting from a [2 + 2] cycloaddition and trimer 90 formed via a [2 + 2 + 2] cycloaddition pathway were observed. In contrast, when applied to Au(100), only the dimer structure 91 was generated through the [2 + 2] cycloaddition process.

In a recent study in this field, Izydorczyk et al. were able to selectively synthesize compound 87 through a hybrid approach involving the integration of both solution and surface chemistry techniques [53]. The key compound 96 to be used in the synthesis of POA 87 was synthesized in two steps. In the first step, 94 was obtained using a double Sonogashira cross-coupling reaction, followed by a Au(I)-catalyzed [4 + 2] cycloaddition reaction to afford the target substrate 96 and its regioisomer 95 in a 2:1 ratio (Scheme 20). POA 87 was obtained on Au(111) at 610 K after Ullmann-type coupling and aromatic dehydrogenation of compound 96. Apart from these studies, the synthesis of smaller POA units has been accomplished by similar methods starting from naphthalene percursors on the Ag(111) surface [54]. The synthesis of nanoribbons containing biphenylene units was also achieved on the Ag(111) surface at 475 K by using 2,3,8,9-tetrabromotetracene as the substrate [55].

Conclusion

In conclusion, even though considerable time has elapsed since its initial synthesis in 1941, biphenylene continues to be a significant synthetic target, with a notable increase in research activity in recent times. Despite this long history, the synthetic methods used in the synthesis of biphenylene are largely limited to flash vacuum pyrolysis, [2 + 2] cycloaddition, [2 + 2 + 2] cycloaddition, and the Ullmann reaction. This shows how open this field is to further development and indicates the significant potential for new methodologies to be developed. In particular, the utilization of biphenylene units for stabilizing polycyclic aromatic compounds, along with the instances elucidated in this review where the electron delocalization occurs through the 4-membered ring, amplifies the intrinsic value of these structures. Methods that combine heteroatoms with biphenylene moieties have also been employed to alter the electronic characteristics and enhance the stability of polycyclic aromatic (POA) structures. Over the recent years, it has been shown that synthetically demanding biphenylene-containing polycyclic aromatic compounds can be achieved in a controlled manner not only by solution chemistry but also by on-surface chemistry. All these results, which are open for further development, confirm that biphenylene structures are very important synthetic units and will be used as tools for the synthesis of more complex structures in the future.