Zethrene and its related molecules have attracted much attention from the perspectives of both synthetic chemistry and materials science. One characteristic feature of these molecules is the existence of two distinct resonance structures, a closed-shell quinoidal form and an open-shell biradical form, which will lead to a significant biradical contribution to the ground-state electronic configuration. As a result, zethrene-based molecules exhibit unique optical, electronic, and magnetic properties, which open the door for a diversity of applications in the field of electronics, nonlinear optics, and spintronics.
Zethrene (1, Fig. 1) is a name given to a unique polycyclic hydrocarbon (PH) with its structure resembling the letter “Z” [1–4]. Zethrene is a planar molecule with fixed double bonds in the central part, which represents a special case among various PHs. As revealed by its crystallographic structures shown in Fig. 1, the bond lengths for the middle part exhibit a large alternation, similar to the 1,3-butadiene. The packing mode of zethrene in single crystals is γ-packing with offset π-stacks in two directions. The π-to-π distance is 3.45 Å, and the angle between neighboring π-planes is 78.6° . Extension of the π-conjugation of zethrene either laterally or vertically will generate a number of new structures, for example, when the terminal naphthalene units in zethrene are bridged by p-quinodimethane (p-QDM) or 2,6-naphthoquinodimethane (NQDM) units, the resulting longitudinal homologues are called heptazethrene (2) or octazethrene (3, Fig. 2), respectively. If the terminal naphthalene units are replaced by anthracene or tetracene, dibenzozethrene and tetrabenzozethrene are obtained. In principle, any aromatic units can be used as terminal units to replace the naphthalene moieties in zethrene or heptazethrene and so on, which will result in a large library of expanded zethrene molecules.
A signature character for zethrene and expanded zethrenes is the existence of different resonance structures, including a closed-shell quinoidal form and an open-shell biradical form (Fig. 2), and the biradical character is closely related to the number of Clar’s aromatic sextet rings in the biradical form . In other words, additional aromatic sextet rings formed in the biradical resonance structure compared to the quinoidal form provide aromatic stabilization effect, therefore increasing the biradical contribution to the ground state. For example, zethrene possesses one aromatic sextet ring in the biradical and two aromatic sextet rings in its quinoidal form, while the heptazethrene has one more aromatic sextet ring in the biradical form than its quinoidal form (Fig. 2). As a result, the calculated biradical character y value of heptazethrene (y = 0.537) is larger than the zethrene (y = 0.407), on a basis of the occupancy numbers of spin-unrestricted Hartree–Fock natural orbitals . Another straightforward example is the comparison between the structural isomers 1,2:9,10-dibenzoheptazethrene 4 and 5,6:13,14-dibenzoheptazethrene 5 (Fig. 2). Although these two molecules own the same chemical composition, the latter has maximum five aromatic sextet rings and the former has maximum three in their biradical resonance form. As a result, the derivative of the latter shows much larger biradical character than that of the former . The investigation of biradical character in PHs is important to understand the nature of chemical bonding and many chemical/physical phenomena in π-conjugated systems, and is the basis of many related applications such as nonlinear optics  and spintronics .
Another reason that zethrene-based compounds have drawn a great deal of attention in recent years is their potential as functional materials. The possibility of zethrene as organic semiconductors is always pursued as the HOMO-LUMO gap of zethrene is almost equal to the well-known semiconductor pentacene. Moreover, in 1995, Burt et al. predicted by PPP calculations that zethrene bis(dicarboximide)s would show substantial near-infrared (NIR) absorption and emission . In 2006, Maksić et al. reported that zethrene as well as its longitudinal homologues would exhibit large absolute proton affinity (APA) and second-order hyperpolarizability (γ) based on semi-empirical AM1 calculations . All of these theoretical predictions make zethrene-based compounds promising candidates for NIR dyes and nonlinear optical materials.
In 1955, Clar et al. found a small amount of deep red color hydrocarbon obtained by either catalytic dehydrogenation of acenaphthene to acenaphthylene, thermolysis of acenaphthene, or treatment of acenaphthylene [or bi(acenaphthylidene)] with NaCl and AlCl3 at 110 °C. The authentic sample was then synthesized from 2,8-dicyanochrysene with dehydrogenation as a final step, and was identified as zethrene . This pioneering work represented the first synthesis of zethrene, however, the overall yield was quite low and the parent zethrene was found to be readily decomposed under ambient conditions. Since then, the reaction yield and stability issue became the major concerns in the synthesis of zethrene-based compounds, which can only be solved by proper synthetic methods and reasonable functionalization and protection. In the subsequent 50 years, the development of zethrene-based molecules was very slow and zethrene was only separated as a byproduct in the synthesis of annulenes [14–16]. Inspiringly, the last 5 years witnessed a boost in the synthesis of zethrene derivatives and its homologues as reported by Tobe, Wu, Miao, and us. The synthetic approaches can be roughly categorized into five methods based on the final step reaction, although the synthesis of the precursors usually takes a multiple-step sequence (Scheme 1). The first method is intramolecular transannular cyclization reaction from tetradehydrodinaphthoannulene or octadehydrodinaphthoannulene precursors. The proximate triple bonds in such precursors make them reactive towards heating, light irradiation, or in the presence of electrophiles, and the intramolecular transannular cyclization usually takes place simultaneously to afford the corresponding zethrene and heptazethrene derivatives. Tobe et al. reported that the precursor tetradehydrodinaphthoannulene 6 can be isolated and converted to the 7,14-diodozethrene 7 by reaction with iodine . Our group reported a one-pot synthesis of the zethrene diimide 9 by Stille coupling/transannular cyclization from compound 8, and the reaction was supposed to go through a reactive tetradehydrodinaphthoannulene intermediate . This method is the most traditional way to form zethrenes, but the yield is normally quite low (<30 %). Using a similar concept, the heptazethrene diimide 11 was prepared by us through a reactive octadehydrodinaphthoannulene intermediate from 10 . The second method is a Wittig reaction–Heck coupling–cyclodehydrogenation sequence recently developed in Miao’s group . This approach started from a Wittig reaction between 12 and the corresponding aldehyde 13 to afford the precursor 14, followed by Heck coupling and simultaneous cyclodehydrogenation in the presence of Pd(OAc)2 to give the parent zethrene in improved yields (~70 %). The third method is a Pd-catalyzed cyclodimerization reaction from precursors such as 15 and 17, which was developed in Y. T. Wu’s group . This method allows the formation of zethrene or zethrene diimide  core and the substitution at the 7,14-positions in one step. The substitution also largely enhances the stability and solubility of the resulting zethrene derivatives, and provides an opportunity to tune the properties of the obtained zethrene materials to meet different purposes. The fourth method is a nucleophilic addition–reductive dehydroxylation sequence, which was widely used in the synthesis of substituted acenes. This approach was now adopted by us to access extended zethrenes such as the triisopropylsilyl (TIPS) ethynylene blocked heptazethrene 21, octazethrene 22, and dibenzoheptazethrene 24, and is by far one of the most efficient ways to produce extended zethrene homologues [8, 22]. The last method is oxidative dehydrogenation reaction. For example, the parent zethrene can be synthesized by Pd/C-catalyzed dehydrogenation of the hydrogenated zethrene 25 . Other examples include the synthesis of the dibenzoheptazethrene 28  and a p-QDM bridged porphyrin dimer 30  by oxidative dehydrogenation from the corresponding hydrogenated precursors, which are synthesized by intramolecular Friedel–Crafts alkylation reaction. The oxidants used in this method should be carefully chosen to ensure a sufficient conversion and avoid over-oxidation.
The reactivity of zethrene and its derivatives is strongly related to the butadiene subunit and the biradical character. As shown in Scheme 2, reduction of parent zethrene 1 by Pd-catalyzed hydrogenation leads to 4,5,6,11,12,13-hexahydrozethrene 25 . Although the mechanism of this hydrogenation is not determined, it could be either through hydrogenation and hydrogen shift from 31 or related to the biradical structure shown in Fig. 2. Another interesting reaction happens when the attempts of direct bromination of 9 are made with NBS in DMF at room temperature. Instead of bromination, an oxidation takes place to give the diketone 32 . This reaction indicates a high electron density of the central butadiene subunit, which can be easily oxidized by a weak oxidant such as NBS. The existence of a diene structure in the bay region also makes the Diels–Alder reaction possible. Miao et al. examined this reactivity by heating of zethrene with N-alkyl maleimide followed by oxidation to give 33, and further reaction with excess of N-alkyl maleimide followed by oxidation by air gave a more extended product 34 (Scheme 3). This observation demonstrates that zethrene could be treated as diene with one of its fixed double bonds and one of its naphthalene rings involved .
Currently, the functionalizations of zethrene are achieved at 7,14-positions (bay region) and 3,4,10,11-positions (naphthalene unit) to afford 7,14-disubstituted zethrenes 16 [17, 20] and zethrene diimides 9, 18a–d [18, 20] (Fig. 3). The stability shows a large enhancement from 1 to 18a–d thanks to the proper blocking of the reactive bay regions and the electron-withdrawing effect of the imide groups. Impressively, the chloroform solution of 18d can survive in ambient conditions for more than 10 days with <10 % degradation, which paves the way for the practical applications of zethrene-based materials. Compounds 1 and 6 display absorption and emission in the range of 500–600 nm with moderate fluorescence quantum yields, while for zethrene diimides 9 and 18a–d, the absorption maxima are red-shifted to 622–648 nm and the emission is up to 704 nm, reaching a desirable window for biological applications. Moreover, the physical properties, such as solubility and quantum yields, are tunable by introducing different substituents on either the imide side or the bay region. For example, the 2,6-diisopropylphenyl substituent can help to suppress dye aggregation and enhance the quantum yield, and the oligo(ethylene glycol) ether chain can improve the solubility in polar solvents. The introduction of carboxylic acid group is also important because it can serve as a binding site for biomolecules such as protein. All of these qualities make zethrene diimides promising materials in bio-related applications. Notably, all of the above-mentioned compounds show typical closed-shell features, such as sharp peaks in the NMR spectra and typical p-bands similar to rylenes in the UV/vis absorption spectra.
In principle, higher-order zethrenes such as heptazethrene and octazethrene are more prone to exhibit biradical character due to the recovery of one more aromatic sextet ring from the pro-aromatic p-quinodimethane or 2,6-naphthodimethane unit in the biradical form. However, those molecules should be carefully protected by proper substituents due to the intrinsic low stability. Our group has developed new synthetic methods and synthesized three high-order zethrenes 11, 21, and 22 (Scheme 1). On the basis of the unrestricted symmetry-broken DFT calculations, the ground state (the state with the lowest energy) was determined to be closed-shell (CS) for 21 and singlet biradical (SB) for 11 and 22. The singlet-triplet energy gaps (ΔES-T) were also calculated as 5.0, 8.1, and 4.4 kcal/mol for 11, 21, and 22, respectively. It seems that the introduction of the imide group and replacement of p-quinodimethane with 2,6-naphthodimethane linker can provide better stability for the biradical resonance form, and results in the change of the ground states from 21 to 11 and 22. The experimental observations are also in good agreement with the theoretical predictions. The NMR spectrum of 21 shows sharp peaks at room temperature, on the contrary, the NMR peaks for 11 and 22 are broadened due to the presence of thermally excited triplet species, which is ascribed to the small singlet-triplet gaps. An ESR signal at g = 2.0026 was recorded for a powder sample of 22, further supporting the existence of the paramagnetic species. Meanwhile, the singlet-triplet gap of 22 was estimated by superconducting quantum interfering device (SQUID) measurements to be 2J/kB = –1946 K (0.168 eV or 3.87 kcal/mol). It is worthy to note that although compounds 11 and 21 have the same heptazethtrene core, they show different ground states due to their different electrochemical energy gaps (0.99 eV for 11 and 1.46 eV for 21), that is, a small energy gap is a prerequisite for a singlet biradicaloid. Another reliable demonstration of the biradical character is the bond length alternation. As shown in Fig. 4, the bond length alternation for the central part is larger for 21 than 22, which means 21 has more quinoidal character than 22. It is also interesting to see that both compounds form ordered one-dimensional packing structure via close π–π interactions in the single crystals (Fig. 4), implying their potential applications as charge-transporting materials in organic field effect transistors (OFETs). The physical data for the comparison of 11, 21, and 22 are tabulated in Table 1, and some of the properties, such as one-photon and two-photon absorption (TPA), are closely related to the biradical character of these molecules. It is noteworthy that both compounds 21 and 22 showed large TPA cross-sections σmax due to their moderate biradical character.
|GSa||ΔES-TCd(kcal/mol)||ΔES-TEe(kcal/mol)||yCf||yE g||λabs(nm)||ε (M–1/cm)h||σmax(GM)||EgEC(eV)|
|11||SBb||5.0||–||–||–||641, 701, 747, 827||24 382||–||0.99|
|21||CSc||8.1||–||0.16||–||584, 634||60 000||920||1.46|
|22||SB||4.4||3.87||0.43||0.56||613, 668, 719, 795||83 300||1200||1.13|
|24||SB||3.3||3.7||0.576||–||989, 919, 879, 804, 727||148 900||2800||1.02|
|28||SB||6.9||–||0.309||–||819, 687, 628||72 000||530||1.34|
fCalculated biradical character y.
gExperimental biradical character y.
hMolar extinction coefficient at the longest-wavelength absorption maximum.
It is well known that the Clar’s Aromatic Sextet Rule can nicely predict the stability and reactivity of polycyclic aromatic hydrocarbons with same chemical composition, and recent studies on open-shell systems such as high-order zethrenes and anthenes [24, 25] have shown that this rule can also predict the stability of biradical resonance forms and therefore the biradical contribution to the ground state. In order to examine more accurately the effect of aromatic sextet rings in open-shell systems, two dibenzoheptazethrene derivatives 24 and 28 are prepared . The cores of these two molecules have the same chemical composition, however, in their respective biradical resonance structures, we can draw maximum three aromatic sextet rings for 28, while maximum five for 24 (Fig. 2). Based on this difference, the calculated biradical character y is determined to be 0.309 for 28 and 0.576 for 24. This result was further evidenced by the bond lengths obtained from X-ray analysis of single crystals and the calculated NICS(1)zz values (Fig. 5), which indicate that the central benzenoid ring is almost aromatic for 24 while at a half-way between aromatic and quinoidal for 28. The ground states of both 28 and 24 are calculated to be singlet biradical with triplet states located 6.9 and 3.3 kcal/mol above. In line with this, 24 shows line broadening in NMR spectrum at room temperature while 28 does not. A broad ESR signal is observed for toluene solution of 24 with g = 2.0027 and the SQUID measurements give 2J/kB = –1859 K (–3.7 kcal/mol). Notably, 24 displays an intense NIR absorption at 804 nm and a large TPA cross-section of 2800 GM at 1600 nm, which are related to its larger biradical character (Table 1).
A zethrene-type porphyrin dimer 30 was recently synthesized by us, representing the first example of zethrene homologues with other aromatic units. Although both quinoidal and biradical resonance forms can be drawn for this p-QDM bridged porphyrin dimer, DFT calculations as well as NMR measurements point out a closed-shell electronic structure in the ground state. The molecule has a distorted structure due to steric congestion. The calculated bond length alternation and the positive NICS value of +1.41 ppm for the central benzenoid ring manifest a clear quinoidal electronic structure. Compound 30 exhibits intense one-photon absorption (maximum at 955 nm) and large two-photon absorption cross-section (2080 GM at 1800 nm), as a result of its largely delocalized electronic structure and the quinoidal character .
The studies on the material applications of the zethrene-based PHs are scarce. A few patents report the use of zethrene derivatives in organic electronic devices, but the preparative methods are not mentioned. Recently, Miao et al. fabricated OFET devices based on parent zethrene 1 and its diimide derivative 34 (R = C6H13). The devices were fabricated in a top-contact bottom-gate configuration, and it was found that the zethrene behaves as a p-type organic semiconductor with field effect mobility in the range of 0.01–0.05 cm2 V/s. Under vacuum, the OFETs based on 34 exhibited n-channel behavior with electron mobility up to 2 × 10−4 cm2 V/s and a threshold voltage larger than 40 V . The closed 3D packing structure for 21 and 22 in single crystals also implies their potential applications for OFETss and this work is underway in our laboratory. In addition, the recently synthesized far-red emission dyes such as 18a–d open the opportunity for bio-imaging applications.
Zethrene-based PHs have attracted much attention in recent years driven by the desire to understand their electronic structures and the fundamental structure–physical properties relationship. The main concerns of the production of zethrene-based compounds include the scalable and high-efficient synthesis, as well as the material stability. With the current synthetic methods on hand and more to come, many more new structures become feasible to explore. The library of zethrene-based PHs will be a young and strong force in the field of materials science.
The studies on the applications of zethrene-based materials are still in the early stage, and only a few attempts are made to utilize them in organic electronics. However, their great potential in the field of nonlinear optics, spintronics, and bio-imaging cannot be overlooked. There is no doubt that the material applications of zethrene-based materials will be continuously fueled by the emergence of new functionalized molecules.
This work was financially supported by MOE Tier 2 grant (MOE2011-T2-2-130), A*STAR BMRC grant (10/1/21/19/642), MINDEF-NUS-JPP grant (12/02/05), and IMRE core funding (IMRE/13-1C0205).
 R. Umeda, D. Hibi, K. Miki, Y. Tobe. Pure Appl. Chem.82, 871 (2010). Search in Google Scholar
 Z. Sun, J. Wu. J. Mater. Chem.22, 4151 (2012). Search in Google Scholar
 Z. Sun, Q. Ye, C. Chi, J. Wu. Chem. Soc. Rev.41, 7857 (2012). Search in Google Scholar
 L. Shan, Z. Liang, X. Xu, Q. Tang, Q. Miao. Chem. Sci.4, 3294 (2013). Search in Google Scholar
 A. Shimizu, Y. Hirao, T. Kubo, M. Nakano, E. Botek, B. Champagne. AIP Conf. Proc.1504, 399 (2012). Search in Google Scholar
 M. Nakano, R. Kishi, A. Takebe, M. Nate, H. Takahashi, T. Kubo, K. Kamada, K. Ohta, B. Champagne, E. Botek. Comput. Lett.3, 333 (2007). Search in Google Scholar
 Z. Sun, S. Lee, K. H. Park, X. Zhu, W. Zhang, B. Zheng, P. Hu, Z. Zeng, S. Das, Y. Li, C. Chi, R.-W. Li, K.-W. Huang, J. Ding, D.-H. Kim, J. Wu. J. Am. Chem. Soc.135, 18229 (2013). Search in Google Scholar
 K. Kamada, K. Ohta, T. Kubo, A. Shimizu, Y. Morita, K. Nakasuji, R. Kishi, S. Ohta, S. I. Furukawa, H. Takahashi, M. Nakano. Angew. Chem., Int. Ed.46, 3544 (2007). Search in Google Scholar
 Y. W. Son, M. L. Cohen, S. G. Louie. Phys. Rev. Lett.97, 216803 (2006). Search in Google Scholar
 D. Désilets, P. M. Kazmaier, R. A. Burt. Can. J. Chem.73, 319 (1995). Search in Google Scholar
 A. Knežević, Z. B. Maksić. New J. Chem. 30, 215 (2006). Search in Google Scholar
 E. Clar, K. F. Lang, H. Schulz-Kiesow. Chem. Ber. 88, 1520 (1955). Search in Google Scholar
 H. A. Staab, A. Nissen, J. Ipaktschi. Angew. Chem., Int. Ed. Engl.7, 226 (1968). Search in Google Scholar
 R. H. Mitchell, F. Sondheimer. Tetrahedron 26, 2141 (1970). Search in Google Scholar
 H. A. Staab, J. Ipaktschi, A. Nissen. Chem. Ber. 104, 1182 (1971). Search in Google Scholar
 R. Umeda, D. Hibi, K. Miki, Y. Tobe. Org. Lett.11, 4104 (2009). Search in Google Scholar
 Z. Sun, K.-W. Huang, J. Wu. Org. Lett.12, 4690 (2010). Search in Google Scholar
 Z. Sun, K.-W. Huang, J. Wu. J. Am. Chem. Soc.133, 11896 (2011). Search in Google Scholar
 T. C. Wu, C. H. Chen, D. Hibi, A. Shimizu, Y. Tobe, Y. T. Wu. Angew. Chem., Int. Ed.49, 7059 (2010). Search in Google Scholar
 Z. Sun, J. Wu. J. Org. Chem.78, 9032 (2013). Search in Google Scholar
 Y. Li, W.-K. Heng, B. S. Lee, N. Aratani, J. L. Zafra, N. Bao, R. Lee, Y. M. Sung, Z. Sun, K.-W. Huang, R. D. Webster, J. T. Löpez Navarrete, D.-H. Kim, A. Osuka, J. Casado, J. Ding, J. Wu. J. Am. Chem. Soc.134, 14913 (2012). Search in Google Scholar
 W. Zeng, M. Ishida, S. Lee, Y. M. Sung, Z. Zeng, Y. Ni, C. Chi, D. Kim, J. Wu. Chem.—Eur. J.19, 16814 (2013). Search in Google Scholar
 A. Konishi, Y. Hirao, M. Nakano, A. Shimizu, E. Botek, B. Champagne, D. Shiomi, K. Sato, T. Takui, K. Matsumoto, H. Kurata, T. Kubo. J. Am. Chem. Soc.132, 11021 (2010). Search in Google Scholar
 A. Konishi, Y. Hirao, K. Matsumoto, H. Kurata, R. Kishi, Y. Shigeta, M. Nakano, K. Tokunaga, K. Kamada, T. Kubo. J. Am. Chem. Soc.135, 1430 (2013). Search in Google Scholar
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