Recent Highlights and Perspectives on Acene Based Molecules and

Perspective pubs.acs.org/cm

Recent Highlights and Perspectives on Acene Based Molecules and Materials† Qun Ye‡ and Chunyan Chi* Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore, 117543 ABSTRACT: Acenes represent a series of molecules with intriguing physical and chemical properties for applications in organic electronics. Nevertheless, the stability and solubility issues associated with longer acenes are two major obstacles for their applications. In this Perspective, we summarize the major design principles for stabilizing acenes. A variety of stable acene based derivatives are included for discussion. Finally, we highlight some research areas where breakthroughs will be critical for the further development of acene based molecules and materials.

1. INTRODUCTION Acenes are a series of laterally fused benzene rings (Figure 1). The early investigation on their synthesis and physical

ization energies, and reasonable HOMO and LUMO energy levels.4 The interest in acenes is further expanded as graphene, a monolayer of graphite, was isolated in 2004.5 Of the many theoretical investigations of graphene, a large portion was devoted to the edge structures of the 2D network.6−8 The chemistry and physics of the arm-chair and zigzag edged graphene, however, can hardly be investigated experimentally mainly due to the difficulty of unambiguous recognition of these edges.9 By studying the smaller fragments of graphene with definitive edges and sizes, the effect induced from the edges could be better understood. Acenes, especially longer acenes, are therefore investigated as graphene model compounds with well-defined zigzag edges. The chemistry and reactivity of acenes, as will be discussed below, are closely related to the chemical environment at the zigzag edges. In this Perspective, we provide a summary of recent representative studies on the chemistry and applications of acene based molecules and materials. The purpose of this Perspective is to highlight the recent important achievements in acene research and to render our outlook to any future effort that pushes the frontier further, rather than a comprehensive review of this area. Interested readers are directed to the following excellent review articles10−26 and book chapters.27

Figure 1. Chemical structure of acenes. The six-membered ring with circle denotes aromatic sextet ring.

properties was mainly carried out by Eric Clar and co-workers, who also coined the term “acene” for this series of molecules in 1939.1 Clar also contributed to the understanding of the stability issue of polycyclic aromatic hydrocarbons by introducing the concept of the aromatic sextet.2 This simple concept is a very useful qualitative tool to interpret the stability issue and many associated physical phenomena for polycyclic hydrocarbon systems. For the series of acene molecules, there can be only one aromatic sextet ring present, i.e., the aromaticity is shared by more and more rings as the acenes go longer (Figure 1). This makes the longer acenes suffer from instability, thus making the synthesis of these intriguing molecules a challenging task. More interest in acenes from the academic community was pumped by the discovery of conducting plastics in 1977.3 Since then, the semiconducting properties of conjugated systems have been intensively investigated. Pentacene and rubrene (5,6,11,12-tetraphenyltetracene) are among the topmost studied molecules used in active layer in organic field effect transistors (OFETs) with high hole mobilities. A large number of acene derivatives have been prepared thereafter to test their potential as high performance organic electronics. To achieve high performance OFETs, the acene-based molecules should have large intermolecular electronic couplings, small reorgan-

2. ACENE BASED MOLECULES AND MATERIALS 2.1. Longer Acenes. The synthesis of longer acenes still remains as a challenging task for synthetic chemists. The major challenges of preparing longer acenes include their poor stability at room temperature, especially in solution, and their poor solubility which hampers detailed characterization. By fusing one more benzene ring onto acenes, e.g., from tetracene to pentacene, both the stability and the solubility decrease Received: April 29, 2014 Revised: June 13, 2014 Published: June 19, 2014

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This Perspective is part of the Up-and-Coming series. © 2014 American Chemical Society

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Chemistry of Materials

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significantly. Both problems become more severe for acenes longer than pentacene. For parent pentacene, tremendous experimental work has been done,13,19 while for naked hexacene, heptacene, and even longer acenes, a very limited amount of literature can be found.28−38 The prevailing strategies to resolve these two problems are introduction of electron-withdrawing moieties and bulky substitutions at the periphery of the acene backbone to decrease the HOMO energy level of longer acenes, hence to enhance its stability toward oxidation and to provide sufficient solubility of the final product, respectively. Currently the state-of-art synthesis of acenes goes to nonacene.39 Purushothaman et al.39 has reported the synthesis of a series of nonacene derivatives (1a−c in Figure 2) with

Scheme 1. Synthesis of Parent Hexacene 6 and Its Crystal Packinga

a

Adapted with permission from ref 37. Copyright 2012 Nature Publishing Group.

physical vapor-transport method and tested for transistors. In the crystal, the hexacene packs in a herringbone motif, quite similar to that of pentacene. Hole mobility up to 4.28 cm2 V−1 s−1 with on/off ratio of 105 and a threshold voltage of 37 V were obtained. 2.2. Electron Deficient Acene-Based Molecules. Electron deficient acene-based molecules are of particular interest for the application of n-type organic field effect transistors. The chemical methodologies to reverse the electron rich nature of acenes and to prepare electron deficient acenes include introduction of a variety of electron-withdrawing groups, such as cyano groups, imide groups, fluorides, and TCNQ moieties. By attachment of electron-withdrawing groups onto the acene backbone, both the HOMO and LUMO of the acene can be lowered. Lowering the HOMO energy levels provides a thermodynamic protection of the acene backbone toward oxygen related decomposition. The frontier energy levels of an organic semiconductor have also proven to be closely related to the types of charge carriers generated in the transistor device.44 Hence the lowering of the LUMO energy levels of acene based materials would favor the electron transport in the device. 2.2.1. Halogen-Substituted Acenes. Introducing the halogen atoms into the zigzag edges of acenes is a very efficient way to make electron deficient acenes. Many research groups have contributed to this area, and large amount of works have been summarized in the recent review papers.12,15 Two representative halogenated pentacenes are shown in Figure 3. Perfluoropentacene 7 crystallized in a herringbone motif similar to that adopted by pentacene, although with a nearly 90° edgeto-face angle.45,46 Transistor devices fabricated from 7 exhibited n-type behavior with electron mobilities as high as 0.22 cm2 V−1

Figure 2. Chemical structures of reported nonacenes and heptacenes by wet chemistry.

definitive evidence to prove their existence. The successful synthesis of this series of nonacene involves a beautiful and judicious design strategy to stabilize and solubilize nonacenes. The target nonacenes were protected by four ethynylsilyl groups, two bis(trifluoromethylphenyl) groups, and eight fluorine atoms. All these protection means aim to provide thermodynamic and kinetic stability to the central nonacene core. Other than this unambiguous nonacene example, four elegant heptacenes (2, 3, and 4) were synthesized and confirmed by NMR, MS, and crystallographic analysis.40−43 The synthesis of these heptacenes was well discussed in our previous review.16 For heptacene 2,40 as it is only protected by two ethynylsilyl groups, the solution of 2 decomposed in a few hours (R = SiMe3) to 1 day (R = tBu) under ambient condition, which limits its application in the transistor device. For 3 and 4,41−43 due to the heavy substitution at the zigzag edges, effective frontier orbital overlap between the central heptacenes is inhibited. This is indeed the rational to protect the heptacene core, while from the application point of view, the charge carrier transport would be hampered. An excellent example to demonstrate the potential applications of longer acenes for organic electronics was reported by Watanabe et al. (Scheme 1).37,38 A nonsubstituted hexacene 6 was prepared by solid state synthesis via a soluble hexacene synthon 5.37 Platelet crystals were obtained by

Figure 3. Chemical structures of reported fluorinated pentacenes. 4047



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Scheme 3. Synthesis of Cyanated Tetracene and Pentacenea

s−1. The partially fluorinated pentacene 8 showed p-type behavior in air.47 The hole mobility is increased from 8a to 8b due to more ordered packing for 8b in the crystal. 2.2.2. Cyano Acenes. Construction of cyano groups on aromatic systems can be achieved in a number of ways, and new methods involving this chemical transformation are also being developed.48−58 The commonly used methodologies to prepare aromatic nitriles by functional group interconversions from substrates with halogen atom, azide, aldehyde, amide, primary amine, and primary alcohol have been developed. A recent methodology58 involving a direct generation of nitriles from methyl groups sheds more light on the preparation of nitrile containing aromatics due to the wider availability of methyl based precursors. Currently the most commonly used method to introduce cyano groups is by replacement of halogen atoms with/without palladium catalysis.48,52 The problem of this approach for the preparation of cyanated longer acenes is the nonselective halogenation reaction on pentacene and longer acenes. For the bromination on tetracene, two isomers can be isolated,59−62 and direct bromination on pentacene has never been reported. Our group recently reported two cyanated pentacene diimides (13, Scheme 2).63,64 To achieve the synthesis of

a

Below is the crystal packing of 19. Adapted with permission from ref 65. Copyright 2011 American Chemical Society.

Scheme 2. Synthesis of Cyanated Pentacene Diimide

and ambipolar behavior was observed with electron/hole mobility of the order of 10−3 cm2 V−1 s−1. 2.2.3. Acene Imides. The chemical strategies to introduce imide groups to acenes and the reported acene imides have been well discussed in ref 16. Here we would like to put more emphasis on the energy level control by imide groups. The energy levels of some reported acene imide molecules have been summarized in Figure 4.63−68 There are two places where

cyanated pentacene, the bromine atoms were first introduced to an anthracene derivative 9, and the subsequent benzyl bromination and aromatization afforded the 6,13-dibromo pentacene derivative 12. Cyanation with Pd2(dba)3/dppf as catalyst gave the final cyanated pentacene derivative 13 with two imides fused on two sides. This material has been tested in OFETs and electron mobility value up to 0.08 cm2 V−1 s−1, current on/off ratio of 106 to 107, and small threshold voltage of −5 to 0 V were achieved in solution processed transistors. Due to the low-lying LUMO energy level (∼4.15 eV), the transistor devices showed little degradation at ambient conditions. Yamada et al. reported a series of cyanated acenes (Scheme 3).65 Treating the vicinal diols 14 with o-iodoxybenzoic acid afforded the tetracene dialdehyde 15 and pentacene dialdehyde 16. This creative method is of great importance as very few examples demonstrate a direct functionalization on the 6,13position on pentacene. The subsequent reaction between the aldehydes with NH2OH·HCl in pyridine generated dihydroxyiminomethyl derivatives, which were then converted to dicyanotetracene 19 and dicyanopentacene 20 in the presence of acetic anhydride or mesityl chloride. The dicyanotetracene 19 was found to pack in a slipped face-to-face motif with an intermolecular distance of 3.472 Å. The transistor devices based on the dicyanated tetracene and pentacene were investigated,

Figure 4. Energy levels of acene imide molecules and 20.

an imide group can be fused to the acene backbone. One is fusion at the zigzag edge via a six-membered ring, and the other is fusion on the two sides via a five-membered ring. These two approaches, as suggested by Mohebbi et al.,66 have different involvement in the electron delocalization of the acene backbone. On the basis of DFT calculations at the B3LYP/631G* level for compounds 21 and 22, Mohebbi et al. suggests that, for the 6-membered ring fused imide anthracene 21, the nitrogen atom is directly involved in the π-electron delocalization while a similar LUMO profile is not observed for 5-membered imide anthracene 22. Another possibility to explain this discrepancy is the intramolecular hydrogen bonding 4048



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between the carbonyl oxygen and the hydrogen atoms at the zigzag edge for the acenes with 6-membered imides. Such hydrogen bonding is envisaged to play an important role for the physical properties of a series of aza-tetracene diimide.69 Intramolecular hydrogen bonding would provide extra conjugation pathways and hence make the π-system more delocalized while for acenes with 5-membered imides such intramolecular hydrogen bonding is hardly possible due to much larger O−H distance. There is another example to illustrate the effect of fusion pattern of imides on the acenes. For the two cyano-acenes 13 and 20 that we have discussed previously, their chemical structures are essentially the same except for the two imides attached on the two sides on 13. Imide groups are supposed to be very electron deficient, and they are envisaged to lower the electron density of the π-system significantly. However, the LUMO energy levels for compounds 13 and 20 are comparable to each other, and the difference is within the experimental error range of cyclic voltammetry. The attachment of two imide groups at the two sides induces a hypsochromic shift of the absorption for ca. 50 nm. This interesting phenomenon caused by the imide groups agrees well with the theoretical results investigated by Medina et al.70 It is pointed out that the substituent effects are not additive on pentacene. Attachment of electron-withdrawing groups at the 2,3,9,10-position of pentacene, i.e., the two sides of the acene backbone, would cause a hypsochromic shift. This is indeed what has been observed experimentally. The imide groups on the pentacene, however, play an essential role in optimizing the morphology and solid state packing of the materials in the thin film. As branched alkyl chains could be conveniently attached on the imide nitrogen, it helps to solubilize the molecule and to improve the overall quality of the thin film. This enhances the homogeneity and long-range packing of 13. The much improved mobility of 13 compared with 20 was mainly due to higher crystallinity and improved morphology of the thin film. 2.2.4. TCNQ−Acenes. 7,7,8,8-Tetracyanoquinodimethane (TCNQ)-embedded acenes represent another interesting type of electron deficient acene-based molecules (Figures 5 and 6 and Scheme 4). It should be noted that, as TCNQ is introduced into the acene backbone, the π-conjugation of the TCNQ−acene belongs to a cross-conjugation (germinal connectivity) instead of a linear conjugation (vicinal con-

Figure 6. Crystal structure and packing of compounds 25 (a and c) and 26 (b and d). Adapted with permission from ref 73. Copyright 2013 The Royal Society of Chemistry.

Scheme 4. Synthesis of Bispentacene TCNQ 32

nectivity).71 The cross conjugation allows effective conjugation from the groups attached to the double bond but not between the groups. Hence it is envisaged that introduction of TCNQ into acenes would significantly alter the electronic properties of acenes and a hypsochromic shift of the absorption spectrum is expected. Currently the most convenient way to introduce TCNQ moiety to the acene backbone is via condensation reaction between carbonyls and malononitriles. One major drawback of this reaction is the unwanted nucleophilic attack to the TCNQ moieties by excess malononitriles. Hence the reaction temperature and duration have to be well controlled in order to optimize the yield of the final product. A series of TCNQ-embedded heptacene and nonacene derivatives (25−28 in Figure 5) have been reported by us following this strategy.72,73 Other than that, Li et al. and Wu et al. reported a series of pentacene diimides with TCNQ moieties 29.74,75 Tanaka et al.76 synthesized a TCNQ−bispentacene derivative 32 as shown in Scheme 4. Their success is mainly due to a new way to construct the precursor quinone 31, which is of high purity for the next condensation reaction. Otherwise the product would be very difficult to purify due to its low solubility and low yield. The geometry of TCNQ−acene molecules is of particular interest (Figure 6). For TCNQ-embedded thiophene fused heptacene derivative 28,72 a saddle-shaped conformation with lowest energy was obtained by TD-DFT calculations at the B3LYP/61-G* level to account for its much better solubility. For a similar TCNQ-heptacene derivative 25,73 single crystal showed that this molecule indeed possesses a saddle-shaped geometry with a bent angle of 139° (Figure 6), which is similar to that of reported TCNQ-acene derivatives.77−80 The molecule packs in a herringbone manner in the solid state, with minimal π-overlapping observed. For heptacene containing two TCNQ moieties 26, the single crystal structure was

Figure 5. Chemical structures of reported longer TCNQ−acene molecules. 4049



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backbone also affects their electron deficiency. The hexa-aza pentacenes 38 with alternating fused aza-rings were prepared by Li et al.84 They have the same number of imine-type nitrogen atoms along the pentacene backbone, but the LUMO energy level is around −3.60 eV, indicating that they are less electron deficient than 35−37. Engelhart et al. reported the attempts to prepare diaza hexacene and heptacene 39 and 41 (Figure 8).85 The presence

obtained as well. In the crystal, the two TCNQ moieties were found to point toward opposite directions, i.e., trans to each other. The molecule hence has a zigzag conformation and adopts a “slipped” face-to-face packing motif. A number of TCNQ−acene derivatives have been tested in OFETs, and the best performance with electron mobility value up to 0.01 cm2 V−1 s−1 is obtained for compound 28. Compounds 25, 26, and 27 possess poor or no electron mobility under electric field. The major reason is the poor quality and low crystallinity of the thin films compared with that of 28. 2.2.5. Aza-acenes. Nitrogen-substituted acenes, or azaacenes, have attracted intensive attention mainly because of their potential to serve as n-type organic field effect transistor materials. The synthesis, chemistry, and transistor performance of electron deficient aza-acenes have been well summarized in the literature.20−24 Herein we would like to highlight a number of recently reported longer aza-acenes and their stability issue. Lindner et al.81 reported the synthesis of a series of azahexacenes (33 and 34 in Figure 7). Compound 33 with four

Figure 8. Chemical structure and aza-hexacene and heptacene and their degraded dimers.

of diazahexacene 39 was validated by spectroscopy data while during the crystal growing process only the Diels−Alder dimerization product 40 was obtained. The Diels−Alder reaction happened between one diazahexacene core and the triisopropylsilyl (TIPS)-ethynyl group of a second molecule. This decomposition pathway is very similar to that of the reported bis(ethynyl) substituted hexacene.86 For the tetraazahexacene 33, no such degradation pathways were observed. Isolation of the neat diazaheptacene 41 was unsuccessful, and only the Diels−Alder dimerization product 42 was isolated and confirmed. A bulkier substitution, as suggested by the authors, would render extra steric protection of the conjugated backbone toward the unwanted Diels−Alder degradation pathways. Other larger aza-acene derivatives including pyrene units or with fused phenanthrene at the ends were also reported.87−90 Such molecules have been investigated for applications in phototransistors and anion recognition.87,88 To summarize, introduction of imine-type nitrogen atoms into the acene backbone generates a new type of electron deficient molecule. For aza-hexacene and heptacene, they possess interesting stability issues associated with their structural properties, and many detailed mechanisms remain obscure. A better understanding of these issues would help to improve their stability and test their potential in the transistor device. So far, transistors based on aza-acenes have reached an electron mobility value up to 2.5 cm2 V−1 s−1 for a TIPS− tetraazapentacene sample with suitable interface engineering adopted.91 It is hence a promising research direction which is worth more effort in the future. 2.3. Other Types of Acenes. 2.3.1. Acene Dimers. Dimeric acenes with the two acene backbones in close contact with each other draw much attention for the study of the electronic interaction between two acenes and the preparation

Figure 7. Chemical structures of aza-hexacenes and pentacenes.

nitrogen atoms present in the acene backbone was stable, while for compound 34, as six nitrogen atoms were present, it turned out to be sensitive and was converted to its dihydrogenated amine form in both solid and solution state. The obscure stability issue of the hexa-aza hexacene 34 is of great interest for the exploitation of nitrogen-rich electron deficient molecules. A similar phenomenon was observed as well by He et al.82 A series of hexa-aza pentacenes 35−37 were prepared by He and his co-workers (Figure 7). For this series of molecules with ultrahigh electron deficiency, they were found to gradually convert from the imine form to the dihydrogenated amine form, which is very similar to 34. At least two questions need to be addressed regarding this issue: first, what is the driving force for the reduction/hydrogenation to occur on the imine-based nitrogens? Second, where do the hydrogen atoms come from? Imine itself is stable and in many cases even stable in the presence of acids.83 Our recent findings on a cyanated azatetracene diimide example suggests that hydrogenation on the imine containing conjugated systems might be related to the electron deficiency of the system.69 As the electron deficiency of the conjugated system increases, there is a higher tendency for the hydrogenation/reduction reaction to occur. However, more detailed mechanical study is in need to confirm all these assertions. The compounds 35−37 with three continuous fused aza-rings have a very low LUMO energy level below −4.50 eV. This is the main reason for their instability in ambient conditions. The arrangement of aza-rings in the acene 4050



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of new electronic materials. The chemistry involved in the construction of acene dimers is also worth discussion here. Two pentacenes can be linked together at the 6 and 13 positions, which render the higher stability for the dimer. Zhang et al.92 reported a pentacene dimer via a bis(pentacene) quinone synthon 43 (Scheme 5). Note that for this type of

Scheme 6. Synthesis of a Dibromo Pentacene Dimer

Scheme 5. Synthesis of TIPS Substituted Pentacene Dimer 44 and Its Crystal Packinga

Scheme 7. Synthesis of [2.2]Cyclophane Linked Pentacene Dimer

a

Adapted with permission from ref 92. Copyright 2010 Wiley-VCH Verlag GmbH and Co.

pentacene dimer, the two pentacene units have a tilt angle of 78° as evidenced from the single crystal, hence limiting the effective π-conjugation between them. This is also envisaged by its absorption behavior. The TIPS−pentacene shows its p-band absorption at 642 nm. For TIPS−pentacene dimer 44, the absorption is slightly blue-shifted to 637 nm. The frontier orbital energy levels of the dimer 44 are also similar to those of TIPS−pentacene while the photostability of the dimer 44 turned out to be much better than that of TIPS−pentacene. Two π−π stacking axes were observed in crystals, and FET hole mobility up to 0.11 cm2 V−1 s−1 was obtained on vapordeposited thin films. This molecule represents an interesting example demonstrating a cruciform molecule which shows good performance in the transistor device. Detailed investigation on the frontier orbital interaction at solid state of this type of cross-shaped semiconductor under field effect is needed to shed light on a better understanding of the charge transport mechanism of organic semiconductors. Tanaka et al.76 prepared pentacene dimers via a soluble bispentacenequinone precursor 46 (Scheme 6). They took the advantage of the unique properties of bicyclo[2.2.2]octadiene and applied the chemistry herein. Note that by this approach a 13,13′-dibromo-6,6′-bipentacene building block 48 could be achieved. Further chemistry on the two halogen positions would render more functionalized pentacene dimers, oligomers, polymers, and even graphene ribbons.93 Bula et al.94 reported a pentacene dimer with two pentacene units linked by a [2.2]paracyclophane (Scheme 7). Starting from [2.2]paracyclophane, the pentacenophane-6,13-bisquinone 50 was obtained in four steps. The quinones were then attacked by lithium triisopropylsilylacetylide followed by reduction of the generated tetraol with tin(II) chloride in acidic media to afford the target pentacene dimer protected by

four TIPS−acetyl groups. The λmax of the p-band of 51 is located at 663 nm, which is roughly 20 nm bathochromic shifted compared that of TIPS−pentacene. This bathochromic shift was believed to originate from the frontier orbital coupling of the two pentacene molecules via the cyclophane linkage and was validated by theoretical computation results at the RICC2/ def2-TZVP level of theory. The solid state packing and the charge transport behavior of this molecule are of great interest and not reported yet. Lehnherr et al.95 reported the synthesis a pentacene dimer linked via a diyne (Scheme 8). The synthesis begins with desymmetrizing 6,13-pentacenequinone 52 by treating it with different types of lithium reagent. The key building block 53 with terminal alkyne was synthesized by four steps. The following Cadiot-Chokiewicz coupling reaction afforded the precursor of pentacene dimer 54 which was then converted to the target molecule 55 by reductive aromatization in the presence of tin(II) chloride. The solubility of the dimer was found to be low, mainly due to the extended π-conjugation. The absorption maximum of the p-band of the pentacene dimer 55 was centered at 740 nm, about 100 nm red shifted compared to that of TIPS-pentacene. The pentacene dimer 55 packs in a 2D slipped stacking motif in the single crystal. Along the b axis minimal slippage was found with an interplanar spacing distance at 3.40 Å. This makes it a promising candidate for transistor materials. 4051



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Scheme 8. Synthesis of Diyne-Linked Pentacene Dimer and Its Solid State Packinga

a


Scheme 9. Synthesis of twisted nonacene and its crystal packinga

a


sides were interacting with each other with a π−π distance of 3.44 Å. The absorption maximum of this twisted nonacene goes to 739 nm, about 275 nm blue-shifted compared with the real nonacene 1.39 The four benzene rings fused on the zigzag edges were found to contribute to the frontier orbitals by DFT calculations while the two terminal double bonds in the end of molecule remain intact. From this point of view, the electronic properties of this twisted nonacene 59 is more close to a tetrabenzo[a,c,p,r]heptacene, instead of a neat nonacene. The HOMO−LUMO band gap of 59 is around 1.68 eV; it is much larger than 1.2 eV of nonacene 1. The value is between that of heptacene and hexacene.40,41,43 Both 59 and 1 are more stable in the crystalline state than that in solution. Twisted HOMO and LUMO profiles of 59 are attractive features. Unfortunately, as evidenced from the crystal information, the frontier orbital overlapping is insufficient and the electronic applications of this novel twisted nonacene are to be explored.

2.3.2. Twisted Acenes. Twisted acenes are of particular interest for understanding of aromaticity in nonplanar, distorted π-systems. The backbone of nonsubstituted acenes is essentially planar. However, as the acenes are heavily substituted, the backbone can be twisted, mainly due to the steric congestion on the periphery. All kinds of twisted acene systems have been summarized in the comprehensive reviews.25,26 Here we would like to highlight some recent development on this area. Xiao et al.96 recently reported the synthesis of a pyrene terminated twisted nonacene 59 (Scheme 9) by a “clean reaction” strategy. The key step in this method is a retro-Diels− Alder process involving the thermal elimination of lactam bridges from soluble acene precursor 58. The target acene derivative 59 was generated in a pure state and in near quantitative yield from the precursor 58. The tedious separation was avoided in this case. Such a “clean reaction” has been also used to synthesize other acene derivatives.97−99 Single crystal data of 59 shows that the nonacene backbone is twisted about 26°. π−π interaction from the central pentacene core is found to be minimal while the pyrene units at the two 4052



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3. FUTURE WORK 3.1. Creative Ways To Stabilize Acene. Acenes, especially longer acenes, are thermodynamically unstable in an environment of oxidizing species (O2). The dimerization of the acene backbone is also a problem when acene molecules are dissolved in organic solutions. Furthermore, the strong tendency of aggregation due to π−π interaction makes naked longer acenes hardly soluble. Current chemical methodologies to stabilize and solubilize acenes have been comprehensively described in previous texts. Nevertheless, by now the examples of isolated derivatives of heptacene, octacene, and nonacene are still very limited. There are still many questions that remained unresolved, e.g., the ground state of nonacene. Hence more effort is in need to prepare longer acene examples to demonstrate their physical properties. Creative ways to stabilize acenes, for example, supramolecular encapsulation,100 solid state synthesis,18,19,27,37 and charging the acenes,101 are required to address both the solubility and stability issues of acenes. 3.2. Polyacene. So far, the synthesis of the acene series goes to nonacene. Polyacenes (Figure 9) still remain as a topic

Figure 10. Chemical structures of anthenes and periacenes.

successful story of teranthene and quarteranthene by Kubo and co-workers116−119 has provoked more interest in periacenes, which might serve as biradical materials. Understanding of their physical properties is of great importance to disclose the edge effect and size effect on graphene and related hydrocarbons. A number of attempts to synthesize periacenes have been reported109,120 but yet the fully cyclized species has not been achieved. The major issue on synthesis of large periacenes is their stability and solubility. The suitable design of this kind of molecules by blocking the reactive sites and properly decreasing the HOMO energy level is necessary. The stabilized periacenes are believed to be able to serve as useful materials for spintronic, electronic, optical, and magnetic devices. 3.4. Cyclacenes. [n]Cyclacenes (Figure 11) only have zigzag edges and can be envisioned as subunits of zigzag carbon

Figure 9. Chemical structure of polyacene.

of theoretical interest,102−108 and no experimental evidence is present to prove their existence. As the conjugation length of acenes increases, the ground state will gradually change from closed-shell to open-shell.102−108 Polyacenes are believed to have an open-shell ground state, and experimental proof is essential for the understanding of the electronic behavior of this kind of conjugated π-system. From the viewpoint of synthesis, not only the stability and solubility are of concern but also the synthetic method to construct the acene backbone itself also needs improvement. Conventional chemical methodologies to synthesize acene include Diels−Alder reaction or aldol condensation reaction to construct the acene quinone first and then nucleophilic attack with lithium or Grignard reagent followed by reductive aromatization to afford the conjugated acene. This approach has a very serious side reaction, that is, the unwanted Michael addition of the lithium or Grignard reagent on the β-position of the carbonyls, instead of on the carbonyl carbons. This side reaction is more serious when the conjugation along the quinone extends.72,109,110 So far this problem has not yet been satisfactorily solved. As suggested by Sun et al.,16 steric protection of the β-position by attachment of sterically demanding substituents at the δ-position should be taken in consideration as one is to apply this methodology to synthesize large conjugated systems. In addition, polyacene with a quinoidally conjugated structure is also interesting and may show very different physical properties from the normal acenes with a diene conjugation. 3.3. Periacenes. Periacenes represent a series of conjugated systems with two rows of acenes peri-fused together (Figure 10). Periacenes can be considered as rectangular polycyclic aromatic hydrocarbons (PAHs) with both zigzag and armchair edges, which are essential structural elements in graphene. The small analogues, perylene and bisanthene, have been known for long time. Nevertheless, the large congeners, peritetracene and peripentacene, have not yet been synthesized so far.111−115 The

Figure 11. Chemical structure of [n]cyclacene.

nanotubes (CNTs). They remain a puzzle unresolved until now.121,122 Pioneering work123−127 and theoretical study128−130 on cyclacenes suggest that the [n]cyclacenes would be too reactive to be obtained experimentally. The dream of cyclacene is awakened by the recent success on the synthesis of [n]cyclopara-phenylene131−136 and its application in a template-driven growth of carbon nanotubes.136 The [n]cyclo-para-phenylenes can be regarded as subunits of armchair CNTs which have vastly different properties with zigzag CNTs. Zigzag CNTs behave as semiconductors and armchair CNTs have metallic character.137 Therefore, synthesis of [n]cyclacenes is significantly important to understanding their physical applications. Nevertheless, as great challenges are envisaged to handle the ultrareactive cyclacene, more creative chemistry and technology have to be developed to fulfill this dream. Potential applications of [n]cyclacenes include utilization as ionophores, receptors, organic semiconductors, organic magnetic materials, and molecular electronic devices.128 3.5. Singlet Fission Phenomenon on Acenes. Singlet fission is a process in which a conjugated organic chromophore in an excited singlet state shares its excitation energy with a neighboring ground-state chromophore and both are converted into triplet excited states.138 This spin-allowed process is of particular interest since it can, in principle, lead to photochemical quantum yields greater than one. This theory was first proposed to explain the photophysics of anthracene and tetracene crystals.139−142 Recent interest of this phenomenon mainly stems from the hypothesis for the improvement of the 4053



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solar efficiency of organic photovoltaic (OPV) cells.143 However, the detailed understanding of the mechanisms involved in this photon-matter interaction process is still lacking. For the further development of the theory itself, as well as its application in OPV materials, more model compounds are in need. Acene based derivatives144−150 are good candidates for this approach.

supervision of Professor Gerhard Wegner and postdoctorate research with Professor Guillermo C. Bazan in the University of California at Santa Barbara. In 2011 she was appointed as an assistant professor in the Department of Chemistry, National University of Singapore. Her research interests include development of novel π-systems and synthesis of high performance conjugated materials for organic electronics and sensors.

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4. CONCLUSIONS Acene based molecules and materials have been intensively investigated in the past decades to fully understand their chemical and physical properties and to prepare high performance organic electronic materials. Larger acenes up to nonacenes have been prepared. Various electron-deficient acenes with high electron affinity were synthesized and used as semiconductors for n-channel OFETs. Other type acenes such as acene dimers and twisted acenes were also developed. Acene and its derivatives represent one of the best types of semiconductors for organic OFETs. The physical properties of acenes have been widely studied and provided the important information to disclose the edge effect and size effect on graphene. All of these exciting achievements on acene based molecules and materials stimulate a scientist’s interest in deeply understanding the unsolved problems in this area. For example, it is a public task to design and prepare long acenes (above pentacene) with good solubility and stability for solutionprocessed electronic devices. Some old challenges concerning the experimental capture of some basic but important structures such as polyacenes, periacenes, cyclacenes, and so forth still remain unresolved. These are unfulfilled dreams for many chemists. Many questions regarding the chemical and physical properties of acene based materials, e.g., ground state of longer acenes, singlet fission phenomenon of acenes, stability of longer acenes and electron deficient acenes, solution of the Michael-type addition on acene quinones, etc., also remain obscure. Dreams and curiosity are the impetus that have stimulated the acene research so far, and they will continue to push the frontier further.

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ACKNOWLEDGMENTS This work was financially supported by the National University of Singapore Start-Up Grant (R-143-000-486-133) and MOE AcRF Tier 1 Grants (R-143-000-510-112 and R-143-000-573112).

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AUTHOR INFORMATION

Corresponding Author

*(C.C.) E-mail: [email protected]. Present Address ‡

(Q.Y.) Institute of Materials Research and Engineering, A*Star, 3 Research Link, Singapore, 117602

Notes

The authors declare no competing financial interest. Biographies Qun Ye was born in Hubei province, China, in 1986. He received his bachelor degree with Honours (2009) in Chemistry from National University of Singapore with financial support from the MOE SM3 scholarship. He then continued his Ph.D. study in the groups of Prof. Chunyan Chi and Prof. Jishan Wu in the same University. In 2013, he was appointed as a research scientist in the Institute of Materials Research and Engineering, A*STAR, Singapore. He is interested in the design and synthesis of π-conjugated molecules and materials. Chunyan Chi was born in Harbin, China, in 1975. She received a Bachelor’s degree from Qiqihar University in 1998 and a Master’s degree from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, in 2001. She then conducted her Ph.D. research in the Max-Planck-Institute for Polymer Research under the 4054



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