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Synlett 2013; 24(6): 686-696
DOI: 10.1055/s-0032-1318157
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© Georg Thieme Verlag Stuttgart · New York

Mono- and Oligocyclic Aromatic Ynes and Diynes as Building Blocks to Approach Larger Acenes, Heteroacenes, and Twistacenes

Junbo Li
School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Ave, Singapore 639798, Singapore   Fax: +6567909081   Email: qczhang@ntu.edu.sg
,
Qichun Zhang*
School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Ave, Singapore 639798, Singapore   Fax: +6567909081   Email: qczhang@ntu.edu.sg
› Author Affiliations
Further Information

Publication History

Received: 12 December 2012

Accepted after revision: 14 January 2013

Publication Date:
11 March 2013 (online)

 
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Abstract

Although larger acenes or heteroacenes are expected to exhibit smaller band gaps, which may provide better functionality in organic optoelectronics, their lower solubility and poor stability make their syntheses more challenging. In this account, we review our recent progress in constructing larger acenes, heteroacenes, and twistacenes using different mono- and oligocyclic aromatic ynes and diynes as building blocks.

1 Overview

2 Mono- and Oligocyclic Aromatic Ynes

2.1 Benzynes

2.2 Naphthalynes

2.3 Anthracynes

2.4 Tetracyne

3 Mono- and Oligocyclic Aromatic Diynes

3.1 One-Step Diyne Precursors

3.2 Step-by-Step Diyne Precursors

4 An Expansion Method To Approach Oligoacenes

5 Conclusion and Outlook


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Key words

oligoacenes - twistacenes - heteroacenes - arynes - Diels–Alder reaction

Biographical Sketches

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Junbo Li was born in the P. R. of China in 1979. He received his Ph.D. in organic chemistry from the Institute of Chemistry, Chinese Academy of Science, in 2008. Then, he entered the Wuhan Institute of Technology as an associate professor. In 2012, he joined Professor Zhang’s group at Nanyang Technological University, Singapore, as a postdoctoral fellow. His research interests now mainly focus on the construction of p- or n-type acenes for organic electronic materials.

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Qichun Zhang obtained his B.S. from Nanjing University in the P. R. of China in 1992, his M.S. in physical organic chemistry from the Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences (ICCAS) (Professors Peiji Wu and Daoben Zhu’s group), in Beijing in 1998, his M.S. in organic chemistry (Professor Fred Wudl’s group) from the University of California, Los Angeles (UCLA), USA, in 2003, and his Ph.D. in inorganic chemistry from the University of California Riverside (Professor Pingyun Feng’s group) in 2007. Then, he joined Professor Kanatzidis’ group at Northwestern University as a postdoctoral fellow (October 2007–December 2008). Since January 2009, he has been at Nanyang Technological University (NTU), Singapore, as an assistant professor. Besides this, he also has three-year’s experience of working at the research institute of the Nanjing Chemical Industry Co. (August 1992–August 1995) and two-year’s research experience at ICCAS (August 1998–June 2000). He has published 92 papers and 4 patents. Some of his papers have been in Nature Chemistry, Angewandte Chemie, International Edition, the Journal of the American Chemical Society, ACS Nano, Advanced Functional Materials, Small, Organic Letters, Chemistry of Materials, the Journal of Materials Chemistry, Chemistry – A European Journal, Applied Physics Letters, Nanoscale, and Chemistry – An Asian Journal.

1

Overview

Acenes, consisting of linearly fused benzene rings, have attracted the interest of a lot of scientists over the past decades because they have been demonstrated to be active components in light-emitting diodes (LEDs),[ 1 ] field effect transistors (FETs),[ 2 ] and photovoltaic devices.[ 3 ] Although smaller acenes, such as naphthalene and anthracene, can be found in petroleum resources, larger acenes (more than five fused benzene rings) are not observed in nature owing to their extreme reactivity.[ 2b ] Both their nonexistence in nature and extensive interesting theoretical observations relating to such acenes[ 4 ] have encouraged scientists to try and understand these compounds through synthesis. Unfortunately, poor solubility, instability, and the tedious purification processes required for larger acenes make their formation more challenging.[ 5 ]

To date, many synthetic methods have been reported for the construction of oligoacenes. One iterative synthetic approach, which constructs acene molecules with repetitive units, most likely would be tedious for preparing larger acenes because the method only can increase the target compound by one benzene ring at a time.[ 6 ] Other approaches based on some classic organic reactions, such as the Friedel–Crafts reaction,[ 7 ] the aldol condensation reaction,[ 8 ] and the Diels–Alder reaction,[9] [10] [11] [12] [13] which can integrate two or more different units into one acene precursor, have been investigated and have proven to be effective methods for producing larger acenes. Among them, those involving the Diels–Alder reaction are the most popular choice. In our research, we have been interested in employing mono- and oligocyclic aromatic ynes and diynes as important dienophiles in Diels–Alder reactions to approach larger polycyclic aromatic hydrocarbons (PAHs).

Mono- and oligocyclic aromatic ynes and diynes are important reactive intermediates which have been widely used in organic synthesis, mechanistic studies, and the preparation of multifunctional materials.[ 14 ] In particular, they have been demonstrated as very important scaffolds for efficiently constructing larger PAHs.[ 15 ] According to the degree of conjugation, mono- and oligocyclic arynes can be divided into benzyne (n = 0), naphthalyne (n = 1), anthracyne (n = 2), and so on (Figure [1]).

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Figure 1 The structure of mono- and oligocyclic arynes

To approach larger, stable acenes, several research groups have shown that the selective addition of moieties, such as phenyl,[ 10 ] fluoro,[ 11 ] arylsulfanyl,[ 12 ] or 2-silylethynyl[ 13 ] groups, on the periphery of the conjugated acene framework can enhance the compound’s solubility and stability. In our group, we have been interested in synthesizing a series of larger acenes, heteroacenes, and twistacenes through a ‘clean reaction’ strategy.[ 16 ] As shown in Scheme [1], this strategy involves soluble 1,4-cycloaddition lactam-bridged adducts (oligoacene precursors), which can be transformed into the corresponding acenes by thermally eliminating the bridging moiety (a retro-Diels–Alder reaction).[ 16 ] The amazing advantage of this strategy is that the target products can be readily obtained in a pure state and in nearly 100% yield and tedious separation can be avoided.[ 16 ] In this account, we mainly focus on the chemistry of mono- and oligocyclic arynes and our related research on them.

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Scheme 1 The ‘clean reaction’ strategy to approach larger acenes

# 2

Mono- and Oligocyclic Aromatic Ynes

2.1

Benzynes

Although most benzyne precursors have been reviewed by Hart,[ 17 ] some novel ones were reported in the literature after his publication.[ 18 ] The methods used to trigger the release of benzyne can be divided into two types: thermally induced (decomposition) and chemically induced. Thermally regulated systems involve the extrusion of inert gases or byproducts, typically from intramolecular salts. The classic substrates for this method are diazonium carboxylate salt 2 [ 19 ] and phenyliodonium analogue 3 (Scheme [2]).[ 20 ]

As for chemically initiated systems, the mechanisms used to trigger the benzyne formation are more diverse. Under harsh conditions, benzyne can be generated via the deprotonation of bromobenzene (1, Y = Br) (Scheme [2]) using strong bases (e.g., lithium amide).[ 21 ] It is noteworthy that the conditions for this deprotonation can become much milder if the appropriate functional groups are introduced onto the benzene ring. The oxidative method to unleash benzyne from 1H-benzotriazol-1-amine (4) also works quite efficiently.[ 22 ] 2-(Trimethylsilyl)phenyl triflate (5) has been demonstrated to accept the attack of a fluoride ion to complete the formation of the didehydrobenzene.[ 23 ] Under similar conditions, phenyl[2-(trimethylsilyl)phenyl]iodonium triflate (6) seems to be a more-reactive benzyne precursor compared with compound 5.[ 24 ]

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Scheme 2 Benzyne precursors with various triggering mechanisms

With fluoride-induced benzynes as intermediates, Pérez and co-workers reported the synthesis of triphenylene (7),[ 25 ] tribenzotriphenylene (8),[ 26 ] and hexabenzotriphenylene (9)[ 26 ] using palladium catalysis (Scheme [3]). This method seems to give a higher yield than other syntheses.[ 27 ] Moreover, the benzyne precursors also reacted with alkynes[ 28 ] or o-benzoquinones[ 29 ] to produce functionalized naphthalenes.

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Scheme 3 The palladium-catalyzed cyclotrimerization of arynes from their precursors

In practice, the most convenient way to generate benzyne is by the debromination of bromobenzene or 1,2-dibromobenzene in the presence of a strong base. Employing this type of benzyne precursor, Herwig and Müllen reported the synthesis of soluble acene precursor 11 [ 30 ] (Scheme [4]), which can be converted into pentacene on heating.

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Scheme 4 The synthetic route to pentacene

Using the same benzyne precursor, Uno and co-workers produced a series of 1,2-diketones,[ 31 ] which could be transformed into the corresponding pentacene or its derivatives through photo-decarbonylation. A similar strategy was employed by Neckers and co-workers to prepare hexacene precursor 12 [ 32 ] (Scheme [5]), which can form hexacene in a polymer matrix through photo-decarbonylation. Interestingly, the so-prepared hexacene can survive for more than 12 hours in the polymer matrix under ambient conditions. More recently, Tönshoff and Bettinger employed similar benzyne precursors to prepare the 1,2-diketone intermediates of octacene and nonacene;[ 33 ] these intermediates can be converted into the corresponding reactive acenes on reaction at 30 kelvin in a polymer matrix under an argon atmosphere.

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Scheme 5 The synthetic route to a hexacene precursor

In our group, we have been more interested in synthesizing acenes or twistacenes using a so-called ‘clean reaction’ method.[ 16 ] The key reactants in our synthetic strategy are (hetero)cyclic dienes, such as cyclopentadienones 13 and 15, mesoionic compound 14, and isoquinolinone 16 (Figure [2]), which can trap in situ generated benzyne. Unfortunately, these dienes are not stable in strong bases. Thus, the conditions required to release benzyne from bromobenzene or 1,2-dibromobenzene have posed a limitation on exploiting the use of these precursors with our chosen dienes.

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Figure 2 The structures of (hetero)cyclic dienes (Ph = phenyl, Me = methyl)

Luckily, in our research, we found that the benzynes generated from iodonium triflate 6 [and 2-(trimethylsilyl)phenyl triflate (5)], diazonium carboxylate 2, and 1H-benzotriazol-1-amine (4)[ 34 ] can be trapped by (hetero)cyclic dienes 13–16 through [4+2] cycloaddition reactions (e.g., Scheme [6]). Our success has paved a way to address the challenge of synthesizing larger oligoacenes.

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Scheme 6 Probing the reactivity of mesoionic compound 14 with three different benzyne precursors

To quickly and logically approach larger acenes or twistacenes, the use of ‘step-by-step’ benzdiyne precursors may be more helpful and reasonable (see Section 3.2). In 2010, we synthesized step-by-step benzdiyne precursor 17 from a commercially available dimethyl aminoterephthalate in four steps.[35] [36] Following our designed synthetic strategy, new compound tetracene 18 was prepared via a [4+2] cycloaddition reaction involving an in-situ-generated aryne (derived from substrate 17) as a dienophile and diphenylpyrenocyclopentadienone 15 as a diene (Scheme [7]).[ 36 ] More interestingly, the performance of organic light-emitting devices based on this tetracene indicates that this material has a bipolar transporting behavior in such systems.[ 36 ]

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Scheme 7 The synthetic route to tetracene 18

# 2.2

Naphthalynes

There are two simple and convenient methods to generate 2,3-didehydronaphthalene (naphthalyne): one is to treat 2,3-dibromonaphthalene with a strong base[ 37 ] and the other is to react 3-amino-2-naphthoic acid with isoamyl nitrite. Using 3-amino-2-naphthoic acid as the starting material, Wudl and co-workers successfully synthesized a new heteroacene, isoquinolin-3(2H)-one derivative 19, in short steps.[ 38 ] Interestingly, compound 19 showed amazing dual fluorescence emission, where the band positions and intensities strongly depended on the choice of solvents.[ 38 ] Further studies indicated that this behavior arises from the fact that the molecule has two valence tautomers: an inner salt (I) and an isoquinolinone structure (II) (Scheme [8]).

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Scheme 8 The neutral and zwitterionic tautomeric forms of heteroacene 19

In our research, we found that 3-amino-2-naphthoic acid was insufficient for constructing larger conjugated polycyclic compounds owing to there being few protecting groups on its backbone and its lower solubility. To address this problem, a novel naphthalyne precursor, 3-amino-5,6,7,8-tetraphenylnaphthalene-2-carboxylic acid (21), was designed and successfully synthesized.[ 39 ] Employing the step-by-step benzdiyne precursor 17, methyl 3-nitro-5,6,7,8-tetraphenylnaphthalene-2-carboxylate (20) was prepared in 70% yield from commercially available compound tetraphenylcyclopentadienone (13). After reduction and hydrolyzation, 3-amino-5,6,7,8-tetraphenylnaphthalene-2-carboxylic acid (21) was obtained in 86% yield (Scheme [9]).[ 39 ]

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Scheme 9 The synthetic route to naphthalyne precursor 21

Following this, new heteroacene compound 2-methyl-1,4,6,7,8,9-hexaphenylbenzo[g]isoquinolin-3(2H)-one (22) was successfully synthesized via the cycloaddition reaction between substrate 21 and mesoionic compound 14 in the presence of isoamyl nitrite (Scheme [10]). Interestingly, compound 22 displayed diverse colors in different solvents (aprotic and protic) owing to its pyridone terminal unit. Moreover, we also fabricated a simple heterojunction photovoltaic device using 22 as an electron donor and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor. Our results showed that this type of heteroacene could be a good charge transport material in organic semiconductor devices.[ 39 ]

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Scheme 10 The synthetic route to heteroacene 22

# 2.3

Anthracynes

Although it is well-known that 2,3-didehydroanthracene (anthracyne) can be generated from 2,3-dibromoanthrancene using phenyllithium as a strong base,[ 40 ] such conditions have been demonstrated to destroy several important dienes, such as tetraphenylcyclopentadienone (13), mesoionic compound 14, and isoquinolinone 16. Because we already proved that benzyne or naphthalyne, produced from 2-aminobenzoic acid (anthranilic acid) or 3-amino-2-naphthoic acid derivatives, respectively, can be trapped by mesoionic compound 14 [34] [38] or isoquinoline 16 [34] [38] through 1,4-dipolar cycloadditions, we believed that anthracyne generated from 3-aminoanthracene-2-carboxylic acid[ 41 ] could also be trapped by heterocyclic compounds through [4 + 2] cycloaddition reactions.

In addition, we hoped that the success of these reactions would allow us to approach larger conjugated polycyclic systems more easily and quickly. However, to our knowledge, with increasing linear annulations of polyacenes comes a dramatic decrease in the stability and solubility of such compounds. Introducing optimal substituents at key positions of oligoacenes not only resolves the problem of solubility, but also protects the compounds from dimerization and oxidation. Thus, it was necessary to introduce phenyl groups at the 9- and 10-positions of anthracyne for the purpose of protection.

The synthetic route to the substituted anthracyne precursor 2-amino-9,10-diphenylanthracene-3-carboxylic acid (26) is shown in Scheme [11]. The initial benzyne produced from step-by-step benzdiyne precursor 17 was trapped by mesoionic compound 14 to form two isomers I and II; elimination of the lactam bridge at 180 °C gave isoquinolinones 23a and 23b. The structure of compound 23a was confirmed by X-ray crystallography (Figure [3]). Compound 24 was obtained by a [4 + 2] cycloaddition reaction involving in-situ-generated benzyne from anthranilic acid and functionalized isoquinolinones 23a and 23b. After thermally eliminating the bridge in compound 24, compound 25 was formed in high yield. Through reduction with hydrogen and palladium on carbon followed by hydrolyzation, compound 25 was converted into 2-amino-9,10-diphenylanthracene-3-carboxylic acid (26), a phenyl-substituted precursor of anthracyne.

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Scheme 11 The synthetic route to anthracyne precursor 26
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Figure 3 The crystal structure of diphenylisoquinolinone 23a

As shown in Scheme [12, 9],10-diphenyl-2,3-didehydroanthrancene could be effectively trapped by our frequently used dienes, cyclopentadienones 13 and 15, mesoionic compound 14, and isoquinolinone 16, to form the corresponding phenyl-substituted acene 29, twistacene 30, and acene precursors 27 and 28, respectively.[ 42 ]

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Scheme 12 The chemistry of anthracyne

# 2.4

Tetracyne

To synthesize larger conjugated oligoacenes and shorten the required synthetic steps, it is more logical to prepare longer oligocyclic aryne precursors. Clearly, 2,3-didehydrotetracene (tetracyne or naphthacyne) is a promising dienophile candidate for the construction of higher acenes or twistacenes. In Section 2.1, we described the synthesis of compound 18, which can be converted into new tetracyne precursor 31 by reduction and hydrolyzation (Scheme [13]). By treating precursor 31 with isoamyl nitrite in boiling solvents, the corresponding tetracyne can be generated in situ and, furthermore, trapped by several different dienes to produce the corresponding larger heteroacenes, twistacenes, or their precursors. For example, 1,2,3,4,6,15-hexaphenyldibenzo[jk,op]pentacene (32) was prepared through benzyne-trapping chemistry between tetracyne 31 and tetraphenylcyclopentadienone (13) (Scheme [13]).

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Scheme 13 The synthetic route to pentatwistacene 32

The single-crystal structure analysis of 32 clearly showed that this material has a twisted structure with the torsion angle as high as 23.0°.[ 43 ] The performance of organic light emitting devices using 32 as the emitter indicated that such twistacenes could enhance the efficiency of the systems by decreasing the aggregation-caused quenching effect.[ 43 ]

Encouraged by this result, a larger heteroacene, 2-methyl-1,4,6,7,8,9-hexaphenylbenzo[g]isoquinolin-3(2H)-one (22),[ 39 ] was employed in the reaction with tetracyne precursor 31 to successfully form the desired heptacene precursor, octaphenyl-substituted bridged derivative 33 (Scheme [14]).[ 16 ] Elimination of the bridge in diphenyl ether proved to be an effective method to form heptatwistacene 34.

Heptatwistacene 34 is the longest such compound recorded for a type II system;[ 16 ] it is noteworthy that it took almost 50 years to achieve this record. The novel, stable, green twistacene possesses a low highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap, which is close to that of the reported hexacene. Crystallographic analysis revealed the presence of the conjugated heptacene chromophores with a twisted shape.[ 16c ]

Continuing in this direction, stable, green twisted heteroacene 36 was obtained via the cycloaddition reaction between tetracyne 31 and mesoionic compound 14 (Scheme [15]). Our research showed that the HOMO–LUMO gap of compound 36 is close to that of hexacene, which makes it a potential candidate for application in electronic devices.[ 16 ]

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Scheme 14 The synthetic route to heptatwistacene 34
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Scheme 15 The synthetic route to twisted heteroacene 36
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Scheme 16 The synthetic route to nonatwistacene 38

Having twisted heteroacene 36 in hand, we were strongly driven to construct the novel larger nonatwistacene 38 via the 1,4-cycloaddition reaction of the heteroacene and tetracyne 31 (Scheme [16]).[ 16a ] Fortunately, the reaction worked very well and the desired precursor 37 was obtained in 22% yield. Through the ‘clean reaction’ strategy, nonatwistacene 38 was obtained in nearly 100% yield by thermally eliminating the lactam bridge of precursor 37 in diphenyl ether at 330 °C (in a sealed tube). The structure of 38 was confirmed by single-crystal X-ray analysis.[ 16a ] It is noteworthy that nonatwistacene 38 is the longest such compound recorded for a type I system and that it took almost 50 years to achieve this record.


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# 3

Mono- and Oligocyclic Aromatic Diynes

To prepare larger acenes more quickly and efficiently, benzdiynes and their analogues can be a good choice of substrate owing to their bidirectional ability. Benzdiyne precursors can be divided into two types: type A is the so-called ‘one-step’ benzdiyne precursors (see Scheme [17, ]Section 3.1) and type B is the step-by-step kind (see Scheme [22, ]Section 3.2).

3.1

One-Step Diyne Precursors

Using type A precursors (Scheme [17]), the corresponding 1,4-diynes are generated sequentially in one pot without controlling the second yne equivalent. This class of benzdiyne precursors is frequently employed to trap only one type of diene.

For example, Pascal and co-workers synthesized the new twistacene 9,11,20,22-tetraphenyltetrabenzo[a,c,l,n]pentacene (44) by bis-cycloaddition between phenanthrene-derived cyclone 43 and the in situ generated benzdiyne formed from diiodonium dicarboxylate 42 (Scheme [18]).[ 44 ] However, the isolated yield of compound 44 was only 1.2%.

In 2003, Wudl and co-workers used tetrabutylammonium fluoride to trigger the in situ formation of benzdiyne from compound 41 at room temperature. The intermediate was trapped by two equivalents of pyreno-fused diphenylcyclopentadienone 15 to afford compound 45 as a bright-orange powder (Scheme [19]).[ 45 ] Compound 45 has strong fluorescence and has been employed as a dopant in a polymer-based LED to emit white light.[ 46 ]

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Scheme 17 Type A benzdiyne precursors

In 2008, Wudl’s group successfully synthesized and fully characterized several stable substituted heptacenes 48 via the reaction of diyne precursors anthracenes 46 or 47 with an isobenzofuran (Scheme [20]).[ 10a ]

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Scheme 18 The synthetic route to twistacene 44
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Scheme 19 The synthetic route to twistacene 45
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Scheme 20 The synthetic route to heptacenes 48

More recently, Parkhurst and Swager reported the synthesis of phenylene-containing oligoacenes 49 employing a benzdiyne as an important building block (Scheme [21]).[ 47 ] In our research, we are trying to use benzdiynes, such that formed from 39 and 41, as important intermediates to trap several heteroacenes, such as 22 and 36, to address the challenge of synthesizing larger acenes and twistacenes. This research is still in progress.

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Scheme 21 The synthetic route to oligoacenes 49

# 3.2

Step-by-Step Diyne Precursors

As shown in Scheme 22, type B are step-by-step benzdiyne precursors. In some cases, stoichiometric modulation has been desperately employed to extract an aryne (one triple bond) from compound 40 (see Scheme 17). More importantly, the step-by-step diyne precursors could offer more chances for synthetic chemists to rationally design their targeted compounds (especially asymmetric acenes) and control their syntheses.���

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Scheme 22 Type B benzdiyne precursors

In 1986, Rickborn and co-workers showed that a pentacene precursor could be synthesized using 1,4-dibromobenzene (40a) as a benzdiyne precursor. Either the one-step or step-by-step method could be used by controlling the stoichiometric ratio of the reactants and the strong base lithium 2,2,6,6-tetramethylpiperidide (Scheme [23]).[ 48 ] However, to use 1,4-dibrombenzene (40a) as a bis-dienophile both dienes and the first cycloadducts must be stable in a strong base.

Alternatively, there are some other known precursors that can generate aromatic diynes step-by-step. The first is the commercially available compound 2-amino-4-bromobenzoic acid (50) (Scheme [22]). Wudl’s group used it to prepare further diyne precursors, anthracenes 46, which can generate the corresponding diynes in situ under strong basic conditions (see Scheme [20]).[ 10a ] These active anthracene-related diynes could be trapped by 1,3-diphenylisobenzofuran to form heptacene precursors, which can be further converted into stable heptacenes by reduction.[ 10a ]

Another interesting benzdiyne precursor is the intermolecular iodonium salt 51 (Scheme [22]), a modification of Kitamura’s hypervalent iodine.[ 24 ] The yields for the cycloaddition reactions between this precursor and a variety of heterocyclic dienes were extremely high, offering a possible method to construct larger acenes.[ 49 ]

In our group, we have been more interested in step-by-step benzdiyne precursor 17 (Scheme [22]) because it offers us more opportunity to design and control our target compounds. As previously mentioned, this benzdiyne precursor has been successfully employed in the construction of several acenes, heteroacenes, and twistacenes.[16] [36] [39] [43] Research on larger acenes is still underway.


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# 4

An Expansion Method To Approach Larger Oligoacenes

As shown in Scheme [24, a] new route to approach larger oligoacenes starts with the important intermediate compound 23a, which can trap 9,10-diphenylanthracyne (generated from precursor 26) or 5,6,7,8-tetraphenylnaphthalyne (generated from precursor 21).[ 42 ] Thus, compound 55 was prepared using 23a to trap the in situ generated anthracyne from precursor 26. Continuing with this method, larger odd-numbered oligoacenes could be obtained. To synthesize even-numbered oligoacenes, compound 23a was used to trap the naphthalyne generated from compound 21 to form a tetracene precursor, which could be converted into tetracene 56 on heating.

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Scheme 23 The synthetic route to pentacene precursor 53
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Scheme 24 The expansion method to larger oligoacenes

# 5

Conclusion and Outlook

In this account, we have summarized two methods to approach larger stable acenes: the first one involves synthesizing different lengths of oligocyclic arynes and the second one traps different aromatic diynes using various dienes. Because a tetracyne has been successfully employed in the construction of a nonatwistacene, it is hoped that longer aryne precursors could be used to approach larger acenes through our ‘clean reaction’ strategy. Our future research will focus on employing aromatic diynes as building blocks to form challenging longer acenes, heteroacenes, and twistacenes.


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Acknowledgment

Q.Z. thanks Professor Wudl for his kindness in allowing him to use some previous work done by his group at UCLA. Q.Z. acknowledges financial support from MOE (Ministry of Education) AcRF Tier 1 (RG 18/09) and MOE Tier 2 (MOE2012-T2-1-019, ARC 20/12), the CREATE program (Nanomaterials for Energy and Water Management) of the National Research Foundation, and the New Initiative Fund of ­Nanyang Technological University, Singapore.



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Figure 1 The structure of mono- and oligocyclic arynes
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Scheme 1 The ‘clean reaction’ strategy to approach larger acenes
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Scheme 2 Benzyne precursors with various triggering mechanisms
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Scheme 3 The palladium-catalyzed cyclotrimerization of arynes from their precursors
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Scheme 4 The synthetic route to pentacene
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Scheme 5 The synthetic route to a hexacene precursor
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Figure 2 The structures of (hetero)cyclic dienes (Ph = phenyl, Me = methyl)
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Scheme 6 Probing the reactivity of mesoionic compound 14 with three different benzyne precursors
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Scheme 7 The synthetic route to tetracene 18
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Scheme 8 The neutral and zwitterionic tautomeric forms of heteroacene 19
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Scheme 9 The synthetic route to naphthalyne precursor 21
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Scheme 10 The synthetic route to heteroacene 22
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Scheme 11 The synthetic route to anthracyne precursor 26
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Figure 3 The crystal structure of diphenylisoquinolinone 23a
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Scheme 12 The chemistry of anthracyne
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Scheme 13 The synthetic route to pentatwistacene 32
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Scheme 14 The synthetic route to heptatwistacene 34
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Scheme 15 The synthetic route to twisted heteroacene 36
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Scheme 16 The synthetic route to nonatwistacene 38
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Scheme 17 Type A benzdiyne precursors
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Scheme 18 The synthetic route to twistacene 44
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Scheme 19 The synthetic route to twistacene 45
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Scheme 20 The synthetic route to heptacenes 48
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Scheme 21 The synthetic route to oligoacenes 49
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Scheme 22 Type B benzdiyne precursors
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Scheme 23 The synthetic route to pentacene precursor 53
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Scheme 24 The expansion method to larger oligoacenes