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Ovalene As a Highly Fluorescent Polycyclic Aromatic Hydrocarbon and Its Π ─ Extension to Circumpyrene Xiushang Xu 1, Qiang Chen 2, and Akimitsu Narita 1,2＊

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Ovalene As a Highly Fluorescent Polycyclic Aromatic Hydrocarbon and Its Π ─ Extension to Circumpyrene Xiushang Xu 1, Qiang Chen 2, and Akimitsu Narita 1,2＊ Synthesis and Characterization of Dibenzo[hi,st]ovalene as a Highly Fluorescent Polycyclic Aromatic Hydrocarbon and Its π ─ Extension to Circumpyrene Xiushang Xu 1, Qiang Chen 2, and Akimitsu Narita 1,2＊ 1＊ Organic and Carbon Nanomaterials Unit, Okinawa Institute of Science and Technology Graduate University 1919 ─ 1 Tancha,2＊ Onna ─ son, Kunigami ─ gun, Okinawa 904 ─ 0495, Japan Max Planck Institute for Polymer Research Ackermannweg 10, 55128, Mainz, Germany (Received August 4, 2020; E ─ mail: [email protected]) Abstract: Polycyclic aromatic hydrocarbons (PAHs) with zigzag edges have attracted increasing attention for their unique optical and electronic properties. This account describes our synthetic approaches to dibenzo[hi,st]- ovalene (DBOV) as a novel PAH with a combination of armchair and zigzag edges and the elucidation of its unique optoelectronic and photophysical properties, such as strong red emission with a uorescence quantum yield of up to 0.89 and stimulated emission. Furthermore, DBOV demonstrated the so ─ called uorescence blinking that enables its application as a uorophore in single ─ molecule localization microscopy, which is one of the modern superresolution uorescence microscopy methods. The self ─ assembly of a DBOV derivative bearing two 3,4,5 ─ tris(dodecyloxy)phenyl groups was also investigated, showing the formation of helical columnar stacks. On the other hand, the regioselective bromination of DBOV was achieved, allowing the postsynthetic functionalization and modulation of the optoelectronic properties. Moreover, π ─ extension of the DBOV at the bay regions led to circumpyrene, the largest circumarene synthesized to date. modulated by peripheral functionalization, 1a,7 heteroatom 1． Introduction 8 1g,9 doping, and the incorporation of nonhexagonal rings. Polycyclic aromatic hydrocarbons (PAHs) have attracted In 1995, Müllen and his coworkers demonstrated a facile renewed attention in recent decades due to their intriguing and synthesis of hexa ─ peri ─ hexabenzocoronene (HBC), which can tunable electronic, optical, and magnetic properties, which render be regarded as a hexagonal nanographene only with armchair them promising for applications in advanced optoelectronic edges, through oxidative cyclodehydrogenation of hexaphenyl- devices. 1 Large PAHs with sizes of over 1 nm are also called benzene. 10 Afterwards, they and others have achieved the syn- nanographenes or (nano)graphene molecules, whose chemical thesis of a number of extended armchair ─ edged PAHs, or structures can be regarded as nanoscale fragments of gra- nanographenes, by employing tailor ─ made oligophenylene phene. 2 In the eld of theoretical physics, such graphene frag- precursors. 1e,1f,6a,9c,11 For nanographenes with armchair edge ments have been intensively studied as graphene quantum dots structures, the energy gaps between the highest occupied mole- (GQDs), which are also structurally the same as large PAHs. 3 cular orbital (HOMO) and the lowest unoccupied molecular The synthesis of PAHs was pioneered by Scholl 4 and Clar 5 orbital (LUMO) are mainly dependent on size, and good cor- in the early 20th century, and the relationship between PAH relation is found between the gap and the number of carbon structures and their properties has been continually explored. 6 atoms in the aromatic core, showing decreased energy gaps as The size, symmetry, and edge structure are the key factors that the core becomes larger. 7b,12 Currently, the largest nanogra- dene the chemical and physical properties of PAHs. 1b,6b In the phene synthesized consists of 222 sp 2 carbon atoms and has a graphene eld, two types of edge structures are predominantly relatively small HOMO ─ LUMO gap of 1.4 eV, as calculated by discussed, namely, armchair and zigzag edges, which corre- a density functional theory (DFT) method. 7b,13 spond to extensions of the so ─ called bay and L ─ regions, On the other hand, PAHs with zigzag edges, such as tetra- respectively (see Figure 1). The properties of PAHs can also be cene, 14 dibenzo[b,def]chrysene, 15 bisanthene, 16 ovalene, 17 and benzo[rst]pentaphene 18 (Figure 2), display intriguing proper- ties, such as higher chemical reactivity, smaller HOMO ─ LUMO energy gaps, and higher uorescence quantum yields than other PAHs with similar sizes (i.e., number of carbon atoms) but without zigzag edges. 19 Moreover, in contrast to armchair ─ edged PAHs, some PAHs with zigzag edges show open ─ shell character. For example, Kubo and his coworkers reported the syntheses of teranthene 20 and quarteranthene, 21 revealing their open ─ shell ground state (Figure 3). On the other hand, Wu’s group pioneered the syntheses of zethrenes 22 with open ─ shell characters (Figure 2) and demonstrated a synthesis of a laterally extended heptazethrene in 2016. 23 In 2018, the long ─ awaited peritetracene was achieved indepen- 24 25 Figure 1. Schematic representation of edge types of PAH. dently by Feng et al. and Wu et al, exhibiting a moderate 1094 （ 96 ） J. Synth. Org. Chem., Jpn. biradical character and a small energy gap (1.1 eV). However, family, was rst reported by Diederich and his colleagues in the number of PAHs with zigzag edges reported in the litera- 1991, 32 and its soluble derivative was reported by Feng et al. in ture is still limited, and many of them suffer from low stability, 2018 through Diels ─ Alder cycloaddition at the bay regions of 24 obstructing their in ─ depth characterization and application. peritetracene (Figure 2). Nevertheless, circumarene larger To this end, in 2017, our group reported the synthesis of than circumanthracene had not been achieved before our dibenzo[hi,st]ovalene (DBOV) as a new PAH with a combina- group reported the synthesis of circumpyrene through a π ─ tion of zigzag and armchair edges. 26 DBOV exhibits strong red extension of DBOV. 33 uorescence and stimulated emission, which renders it an This account focuses on the synthesis and functionalization 26 interesting dye for light ─ emitting devices, including lasers, as of DBOVs and the investigation of their optoelectronic and well as for uorescence microscopy imaging. 27 photophysical properties. After establishing a scalable syn- thetic route towards DBOV derivatives, the regioselective bro- mination of DBOV was achieved, enabling postsynthetic functionalization at the bay regions and modulation of their optoelectronic properties. The self ─ assembly behavior of a DBOV derivative bearing two tridodecyloxyphenyl groups was also explored. Furthermore, π ─ extension at the bay regions, taking advantage of dibrominated DBOV, provided circumpy- rene, which represents the largest circumarene synthesized to date. 2． Synthesis and Characterizations of DBOV In view of the in ─ depth studies on their intriguing proper- ties and potential applications, stability is a crucial factor for PAHs with zigzag edges. Their low stability is mainly attributed to their potential open ─ shell character, which makes them sus- ceptible to oxidation, dimerization, and/or other possible reac- 19,34 tions with solvent or atmospheric molecules. While open ─ shell PAHs are highly interesting for spintronic and even 35 quantum information technology applications, closed ─ shell PAHs with long ─ wavelength absorption, strong uorescence and high chemical stability can be more useful for other appli- cations, such as in (opto)electronics, 36 photonics, 37 and uores- 27,38 cence imaging. The stability and possible open ─ shell char- acter of PAHs can be qualitatively assessed according to Clar’s 39 aromatic π ─ sextet rule. In general, a PAH with a given num- ber of aromatic π ─ sextets is more stable than its isomers with fewer aromatic π ─ sextets. On the other hand, open ─ shell PAHs typically have more Clar’s π ─ sextets in the open ─ shell forms than in the closed ─ shell forms. For example, teranthene has 6 Clar’s π ─ sestets in the open ─ shell form in comparison to 3 Figure 2. Examples of PAHs with zigzag edges. Clar’s π ─ sextets in the closed ─ shell form (Figure 3a), which was experimentally demonstrated to indeed have the open ─ Most of the PAHs with zigzag edges thus far synthesized shell biradical character in the ground state. 20 Conversely, a have a combination of zigzag edges with armchair edges or bay stable PAH with zigzag edges can in principle be designed by 28 regions. Representative exceptions are pyrene, coronene, making the number of Clar’s π ─ sextets larger or the same in acenes, 19 anthanthrene, 29 and ovalene 17 (Figure 2), which can be considered classical examples of zigzag ─ edged PAHs with- out any bay region, which we call zigzag PAHs in this account. Very recently, Wu et al. demonstrated the syntheses and char- acterizations of a series of parallelogram ─ shaped PAHs with four ─ zigzag ─ edges, including peripentacenopentacene (Figure 2), which showed global aromaticity and moderate energy gaps. 30 Notably, in 2019, they successfully fabricated organic distributed feedback (DFB) laser devices using anthanthrene and several other parallelogram ─ shaped zigzag PAHs in the active layer. 31 Circumarene is a subclass of PAHs, structures with small central aromatic cores surrounded by one outer layer of annu- lene. The classical examples of circumarene are circumbenzene Figure 3. The resonance structures of teranthene (a) and DBOV (b) (coronene) and circumnaphthalene (ovalene) (see Figure 2). in closed ─ and open ─ shell forms with Clar’s π ─ sextets Circumanthracene, as the next member of the circumarene indicated with circles. Vol.78
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