1． Introduction

Polycyclic aromatic hydrocarbons (PAHs) have attracted renewed attention in recent decades due to their intriguing and tunable electronic, optical, and magnetic properties, which render them promising for applications in advanced optoelectronic devices. 1 Large PAHs with sizes of over 1 nm are also called nanographenes or (nano)graphene molecules, whose chemical structures can be regarded as nanoscale fragments of gra-phene. 2 In the �eld of theoretical physics, such graphene frag-ments have been intensively studied as graphene quantum dots (GQDs), which are also structurally the same as large PAHs. 3

The synthesis of PAHs was pioneered by Scholl 4 and Clar 5 in the early 20th century, and the relationship between PAH structures and their properties has been continually explored. 6 The size, symmetry, and edge structure are the key factors that de�ne the chemical and physical properties of PAHs. 1b,6b In the graphene �eld, two types of edge structures are predominantly discussed, namely, armchair and zigzag edges, which corre-spond to extensions of the so─ called bay and L─ regions, respectively (see Figure 1). The properties of PAHs can also be

modulated by peripheral functionalization, 1a,7 heteroatom doping, 8 and the incorporation of nonhexagonal rings. 1g,9

In 1995, Müllen and his coworkers demonstrated a facile synthesis of hexa─ peri ─ hexabenzocoronene (HBC), which can be regarded as a hexagonal nanographene only with armchair edges, through oxidative cyclodehydrogenation of hexaphenyl-benzene. 10 Afterwards, they and others have achieved the syn-thesis of a number of extended armchair─ edged PAHs, or nanographenes, by employing tailor─ made oligophenylene precursors. 1e,1f,6a,9c,11 For nanographenes with armchair edge structures, the energy gaps between the highest occupied mole-cular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are mainly dependent on size, and good cor-relation is found between the gap and the number of carbon atoms in the aromatic core, showing decreased energy gaps as the core becomes larger. 7b,12 Currently, the largest nanogra-phene synthesized consists of 222 sp 2 carbon atoms and has a relatively small HOMO─ LUMO gap of 1.4 eV, as calculated by a density functional theory (DFT) method. 7b,13

On the other hand, PAHs with zigzag edges, such as tetra-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 with open─ shell characters (Figure 2) 22 and demonstrated a synthesis of a laterally extended heptazethrene in 2016. 23 In 2018, the long─ awaited peritetracene was achieved indepen-dently by Feng et al. 24 and Wu et al, 25 exhibiting a moderate

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, Onna─ son, Kunigami─ gun, Okinawa 904─ 0495, Japan

2＊Max Planck Institute for Polymer Research Ackermannweg 10, 55128, Mainz, Germany

(Received August 4, 2020; E─ mail: akimitsu.narita@oist.jp)

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.

Figure 1.　Schematic representation of edge types of PAH.

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biradical character and a small energy gap (1.1 eV). However, the number of PAHs with zigzag edges reported in the litera-ture is still limited, and many of them suffer from low stability, obstructing their in─ depth characterization and application. To this end, in 2017, our group reported the synthesis of dibenzo[hi,st]ovalene (DBOV) as a new PAH with a combina-tion of zigzag and armchair edges. 26 DBOV exhibits strong red �uorescence and stimulated emission, which renders it an interesting dye for light─ emitting devices, including lasers, 26 as well as for �uorescence microscopy imaging. 27

Most of the PAHs with zigzag edges thus far synthesized have a combination of zigzag edges with armchair edges or bay regions. Representative exceptions are pyrene, coronene, 28 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 (coronene) and circumnaphthalene (ovalene) (see Figure 2). Circumanthracene, as the next member of the circumarene

family, was �rst reported by Diederich and his colleagues in 1991, 32 and its soluble derivative was reported by Feng et al. in 2018 through Diels─ Alder cycloaddition at the bay regions of peritetracene (Figure 2). 24 Nevertheless, circumarene larger than circumanthracene had not been achieved before our group reported the synthesis of circumpyrene through a π ─ extension of DBOV. 33

This account focuses on the synthesis and functionalization of DBOVs and the investigation of their optoelectronic and 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-tions with solvent or atmospheric molecules. 19,34 While open─ shell PAHs are highly interesting for spintronic and even quantum information technology applications, 35 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-cence imaging. 27,38 The stability and possible open─ shell char-acter of PAHs can be qualitatively assessed according to Clar’s aromatic π ─ sextet rule. 39 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 Clar’s π ─ sextets in the closed─ shell form (Figure 3a), which was experimentally demonstrated to indeed have the open─ shell biradical character in the ground state. 20 Conversely, a stable PAH with zigzag edges can in principle be designed by making the number of Clar’s π ─ sextets larger or the same in

Figure 2.　Examples of PAHs with zigzag edges.

Figure 3.　 The resonance structures of teranthene (a) and DBOV (b) in closed─ and open─ shell forms with Clar’s π ─ sextets indicated with circles.

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the closed─ shell form compared to that in the open─ shell form. To this end, DBOV has 4 Clar’s π ─ sextets in both the closed─ and open─ shell forms (Figure 3b), which suggests a more stable closed─ shell character.2.1　 Initial Synthesis through Oxidative Cyclodehydrogenation

The initial synthesis of DBOV was inspired by the achieve-ment of nanographenes with cove regions (see Figure 1) by Feng, Müllen, and their colleagues. 40 Pioneering synthetic works were reported by Chen, Liu, and their coworkers, who synthesized fused bichrysene 3 through PtCl 2─ catalyzed cycloaromatization 41 of bis(biaryl)diyne 1 to give bichrysene 2, followed by oxidative cyclodehydrogenation with 2,3─ dichloro─ 5,6─ dicyanobenzoquinone (DDQ)/CH 3SO 3H (Figure 4). 42 DBOV can be obtained by “bridging” the cove regions of the

fused bichrysene 3 with methine groups, thus forming zigzag edges. Therefore, the synthesis of DBOV was planned through the introduction of formyl groups at the cove regions of 3 and an intramolecular Friedel─ Crafts cyclization 43 followed by dehydroxylation 44 (see Figure 5).

The synthesis of DBOV 15a was started with the iodination of commercially available 4─ dodecylaniline (4) to give 4─ dodecyl─ 2─ iodoaniline (5), followed by Sandmeyer bromina-tion to afford 1─ bromo─ 4─ dodecyl─ 2─ iodobenzene (6) in 78% yield (Figure 5). Subsequently, Sonogashira coupling of 6 with trimethylsilyl (TMS)─ acetylene afforded 1─ bromo─ 4─ dodecyl─ 2─ (TMS─ ethynyl)benzene (7) in 96% yield, followed by boryla-tion with n ─ BuLi/triisopropyl borate and hydrolysis to obtain 4─ dodecyl─ 2─ (TMS─ ethynyl)benzeneboronic acid (8) in 94% yield.

Subsequently, a Suzuki─ Miyaura coupling of 8 and 7─ bromo─ 2─ naphthaldehyde provided TMS─ ethynylphenyl-naphthaldehyde 9 in 97% yield. A CuCl─ mediated Glaser cou-pling of 9 then gave diaryldiacetylene 10a in 98% yield, which was subjected to a PtCl 2─ catalyzed cycloaromatization 41b,42 to afford bichrysene 11a in 48% yield.

In contrast to the previous works by Chen and Liu et al. 42 and Feng and Müllen et al., 40 where fused bichrysene 3 was synthesized through the oxidative cyclodehydrogenation of bichrysene 2, fused bichrysene 14a with formyl groups could not be obtained by treating 11a under various Scholl condi-tions, presumably because of the electron─ withdrawing effect of the formyl groups. Therefore, the formyl groups of 11a were reduced with NaBH 4 and then protected with acetyl to elec-tron─ donating acetoxymethyl groups, affording 12a in 87% yield over two steps. 12a could then be cyclized to provide fused bichrysene 13a via oxidative cyclodehydrogenation using DDQ and TfOH in 33% yield. The acetoxymethyl groups of

Figure 5.　 Synthesis of DBOV 15a. PTSA: p ─ toluenesulfonic acid; ACN: acetonitrile; THF: tetrahydrofuran; DMF: N,N ─ dimethylformamide; TEA: triethylamine; NBS: N ─ bromosuccinimide; 4─ DMAP: 4─ dimethylaminopyridine; PCC: pyridinium chlorochromate; Ac 2O: acetic anhydride; TfOH: tri�ic acid.

Figure 4.　 Synthesis of fused bichrysene 3 reported by Chen, Liu, and their coworkers. 42 DDQ: dichloro─ 5,6─ dicyano─ 1,4─ benzoquinone.

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fused bichrysene 13a were subsequently transformed back to aldehyde groups to afford 14a in 70% yield over two steps.

Finally, the treatment of 14a with 2─ mesitylmagnesium bromide, followed by BF 3·OEt 2─ catalyzed Friedel─ Crafts reac-tion and oxidation by air, provided DBOV 15a in 24% yield over two steps. The relatively low yield in these �nal steps was considered to be due to the steric hindrance between the dodecyl groups at the 5 and 13 positions of DBOV 15a and the mesityl groups introduced at the 6 and 14 positions (see Figure 5). In this initial synthesis, the total yield of DBOV 15a was only 1.5% over 14 steps in linear sequence from the com-mercially available starting material 4.2.2　 Second Synthesis of DBOV through Oxidative

CyclodehydrogenationTo scale up the synthesis, DBOV derivatives 15b and 15c

without dodecyl groups at the 5 and 13 positions were next targeted to suppress the steric hindrance in the Friedel─ Crafts reaction in the �nal steps (see Figure 6). 45 Boronic acid 16 was prepared from commercially available 2─ iodobromobenzene over 2 steps in 75% yield 46 and used instead of its dodecyl─ substituted counterpart 8, which reduced the number of steps by two. However, the TMS─ protected analog of 16 was depro-tected during the subsequent Suzuki─ Miyaura coupling with 7─ bromo─ 2─ naphthaldehyde, leading to a mixture of different products, in contrast to the same reaction of boronic acid 8 in the �rst synthesis. This complication was presumably due to an increase in polarity upon removal of the dodecyl group from 8, making it prone to enter the basic aqueous phase and become deprotected; thus, triisopropylsilyl (TIPS)─ protected boronic acid 16 was employed, affording 17 in 97% yield.

The TIPS protecting group was removed with tetra─ n ─ butylammonium �uoride (TBAF) in 92% yield, which added one step compared with the initial synthetic route, and a subse-quent CuCl─ mediated Glaser coupling gave diaryldiacetylene 10b in 98% yield. Then, the formyl groups of 10b were reduced with NaBH 4 and protected with acetyl to provide 19 in 87% yield. Bichrysene 12b was prepared by the PtCl 2─ catalyzed cycloaromatization of 19, followed by oxidative cyclodehydro-genation with DDQ and TfOH at -78 ℃ to yield fused bichry-sene 13b. Compound 13b was converted to fused bichrysene 14b with two formyl groups, which was treated with 2─ mesityl─ and phenylmagnesium bromide, followed by intramolecular Friedel─ Crafts cyclization and dehydrogenation to afford DBOV derivatives 15b and 15c in 56% and 80% yields over two steps, respectively. The relatively higher yield of 15c than 15b was presumably due to the smaller steric hindrance of phenyl groups compared with the mesityl groups in the Friedel─ Crafts reaction. The yields in these �nal steps were indeed signi�-cantly improved from that for 15a (24%). However, the total yields of DBOV 15b and 15c in this second synthesis route were 1.6% and 2.2%, respectively, over 13 steps in linear sequence from the commercially available starting material, which were not signi�cantly higher than the 1.5% total yield in the �rst synthetic route.2.3　 Improved Synthesis of DBOV via Photocyclization

The low total yields of DBOV 15 in the �rst and second synthetic routes could be mainly ascribed to the low yields in the Pt─ catalyzed cycloaromatization and the Scholl reaction steps as well as the necessity of transformation between the formyl and acetoxymethyl groups before and after the Scholl reaction. To achieve more ef�cient synthesis of DBOV deriva-tives, a sequence of cycloaromatizations with ICI, 41a,47 which allows simultaneous regioselective iodination, and subsequent Pd─ catalyzed cyclization 48 was next considered. 49 Diaryldi-acetylene 10b was thus reacted with ICl to provide iodinated bichrysene 20 in 76% yield (Figure 7). Pd─ catalyzed cyclization of 20 was then attempted, but only deiodinated product was observed without the occurrence of cyclization. Interestingly, however, during thin─ layer chromatography (TLC) analysis of a reaction mixture, the spot of starting material 20 gradually became red after irradiation with UV light. This observation indicated a photoinduced reaction of 20 to form a π ─ extended compound and prompted us to conduct the photocyclization of 20.

Dehydrohalogenative photocyclization was reported by Henderson and Zweig in 1967 50 and by Sato et al. in 1970, 51 which enabled photocyclization without the use of oxidants. Recently, Morin and his colleagues synthesized a series of PAHs fused with either electron─ rich (thiophene) or electron─ poor (pyridine) rings through the photochemical cyclodehy-drochlorination of carefully designed aryl chlorides in high yields. 52 Moreover, Alabugin et al. demonstrated a sequence of ICl─ promoted cycloaromatizations of bis(biaryl)acetylene precursors and subsequent photochemical cyclodehydroiodin-ation to provide π ─ extended [5]helicene derivatives. 49d These previous reports also suggested that photochemical cyclodehy-drohalogenation could be an ef�cient alternative method for the synthesis of the key intermediate 14.

Indeed, the photochemical cyclodehydroiodination of iodinated bichrysene 20 in acetone in the presence of triethyl-amine (TEA) gave fused bichrysene 14b in 86% yield

Figure 6.　 Synthesis of DBOV derivatives 15b and 15c. TBAF: tetra─ n ─ butylammonium �uoride. TFMSA: tri�uoromethanesulfonic acid.

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(Figure 7). 53 TEA was added as the base to trap the generated HI to prevent an intramolecular Friedel─ Crafts reaction of 14b to give an insoluble diketone byproduct. The yield of 14b from 10b was thus improved to 65% over two steps from 4.2% over six steps in the previous synthetic route via the Scholl reaction. This simpli�ed and ef�cient synthetic route allowed the preparation of the key intermediate 14b on a gram scale and the subsequent syntheses of a series of DBOV derivatives with different meso ─ substituents, namely, dodecyl (15c), aryl (15d─ f and 15h─ i), and TIPS─ ethynyl (15g) groups. The total yields of 15c─ i were in the range of 14─36% over 9 steps in lin-ear sequence from the commercially available starting material, signi�cantly better than the ～2% total yield in the �rst and second synthetic routes over 13─14 steps.

Although DBOV derivatives 15a─ c from the �rst and sec-ond syntheses could not be crystallized, the improved synthetic method enabled the exploration of a wider variety of DBOV derivatives with different substituents. A single crystal suitable for X─ ray diffraction analysis could thus be obtained for DBOV 15e with two 2,6─ dimethylphenyl groups, revealing the precise structure of the DBOV core (Figure 8a─ b). 53 Interest-ingly, 15e exhibited a herringbone π ─ stacking motif in the crystal, with a face─ to─ face distance between two DBOV cores

of 3.13 Å (Figure 8c). The CH─ π interaction between the DBOV cores and the 4─ position of the 2,6─ dimethylphenyl groups was suggested by a distance of 3.25 Å, which might have facilitated the crystallization of DBOV 15e in comparison to DBOV 15b with mesityl groups.2.4　 Optoelectronic and Photophysical Properties of DBOV

The UV/Vis absorption and �uorescence spectra of DBOV 15a exhibit sharp absorption and emission peaks at 625 and 637 nm, respectively, with clear vibronic progressions and a small Stokes shift of 301 cm -1 (Figure 9a). 26 In comparison with fused bichrysene 14a with cove edges instead of zigzag edges, the absorption maximum of 15a is redshifted by 88 nm, and the molar extinction coef�cient is enhanced by approxi-mately �ve times. The absolute photoluminescence quantum yield (PLQY) of 15a was determined to be 0.79, indicating the potential application of DBOV as a red emitter.

DBOV 15b displays the longest─ wavelength absorption maximum at 609 nm (2.04 eV), which is 16 nm shorter than that of DBOV 15a (635 nm; 1.98 eV), suggesting that the addi-tion of dodecyl chains at the 5 and 13 positions lowers the optical energy gap by 0.06 eV. 45b DBOV derivatives 15b and 15c─ h all show very similar absorption spectra with maxima in the range of 607─611 nm (2.03─2.04 eV), which indicates that the aryl and alkyl groups at the 6 and 14 positions have a negli-gible in�uence on the electronic properties of the DBOV core, most likely because of the large dihedral angles prohibiting π ─ conjugation for the aryl groups (Figure 9b). 53 For the redshift observed for 15a, there might also be an interaction between the neighboring dodecyl and mesityl groups affecting the dihe-dral angles of the latter and/or the planarity of the DBOV core.

In contrast, the absorption maximum of DBOV 15h with TIPS─ ethynyl groups is observed at 647 nm (1.92 eV), which is redshifted by approximately 40 nm (0.12 eV). This result dem-onstrates an ef�cient π ─ conjugation between the DBOV core and the acetylene moieties and the consequent potential of modulating the optoelectronic properties of DBOV by prop-

Figure 9.　 (a) UV/Vis absorption and �uorescence spectra of fused bichrysene 14a and 15a (10 -5 M in toluene for all measure-ments). Reproduced with permission 26. Copyright (2017) Wiley Library. (b) UV/Vis absorption and �uorescence spectra of DBOV derivatives 15d─ h (10 -5 M in toluene for all measurements) 53. (c) Fluorescence spectra of two single 15e molecules embedded in a Zeonex �lm at 296 K (λ exc=561 nm) and 4.5 K (shown in the inset; λ exc=565 nm) 53. (d) Simpli�ed energy level scheme to high-light the relevant photophysical transitions in 15e 53.

Figure 7.　 Synthesis of DBOV derivatives 15d─ i through a sequence of ICl─ promoted cycloaromatization and photocyclization. MSA: methanesulfonic acid.

Figure 8.　 Single─ crystal structure of 15e: (a) front view; (b) side view; (c) packing pattern. 53

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erly selecting the substituents. According to DFT calculations, the HOMO─ LUMO energy gaps of 15d─ g are approximately 2.1 eV, while 15h has a smaller energy gap of 1.9 eV, which is in very good agreement with the experimental observations. DBOV derivatives 15d─ g display strong red �uorescence with maximum emission peaks located in the range of 611─617 nm (2.01─2.03 eV) with a high relative PLQY of 0.79─0.89 (Figure 9b). 53 The emission peak of DBOV 15h was redshifted to 650 nm, similar to the absorption spectra, while its PLQY was 0.67 and relatively lower than the values of the other derivatives, which can be ascribed to increased vibronic cou-pling and enhanced intersystem crossing.

To further reveal the photophysical properties of DBOV, single─ molecule spectroscopy (SMS) of 15e was performed at room temperature (296 K) and 4.5 K by embedding 15e in a polymer matrix in collaboration with Thomas Basché and his colleagues 53. SMS enables to study the optical properties of single molecules without intermolecular interactions, and the spectral broadening due to the vibronic coupling can be sup-pressed by measurement at low temperatures, elucidating the purely optical transitions. 54 The high photostability of DBOV is advantageous for the SMS to collect different information during the measurements. The single─ molecule �uorescence spectrum of 15e at 296 K (Figure 9c) is very similar to the spectrum measured in solution (Figure 9b). In contrast, the single─ molecule spectrum at 4.5 K displays a series of narrow vibronic transitions, providing evidence that the broad emis-sion bands observed in solution and for the single molecule at room temperature are due to the thermal broadening of several vibronic transitions. Furthermore, �uorescence correlation spectroscopy was performed to study the rates of the photo-physical transitions of DBOV 15e. At room temperature, a three─ level system with a singlet ground state (S 0), singlet excited state (S 1), and triplet excited state (T 1) was applied, and the photophysical parameters of 15e could be determined as the �uorescence decay rate k 21=1.5×10 8 s -1, intersystem cross-ing (ISC) rate k 23=2.2×10 5 s -1, and triplet decay rate k 31

=2.2×10 3 s -1 (Figure 9d). At 4.5 K, the zero─ �eld splitting of the triple state (T 1) into triplet sublevels t xy and t z was consid-ered, and the ISC and triplet delay rates could be determined as k xy 23=2.0×10 4 s -1, k z 23=1.6×10 3 s -1, k xy 31=2.0×10 3 s -1, and k z 31=4.4×10 2 s -1 (Figure 9d). The triplet decay rate remained almost unchanged at 296 and 4.5 K, while the ISC rate increased by more than one order of magnitude at 296 K. Moreover, a single molecule of DBOV 15e showed high─ con-trast photon antibunching and single─ photon emission, which rendered DBOV of potential interest as a single─ photon source for quantum information technology applications. 53,55

The photophysical properties of DBOV 15a were also studied in detail by ultrafast transient absorption spectroscopy in collaboration with Francesco Scotognella and his col-leagues. 26,45b,56 Transient absorption spectra of 15a in solution displayed i) a negative signal at approximately 450 nm, corre-sponding to photoinduced absorption from S 1 to higher excited states; ii) positive signals at 570 and 650 nm due to photo-bleaching, namely, the depletion of the ground state upon excitation; and iii) a positive signal at 695 nm, which could be assigned to the stimulated emission (SE) (Figure 10a). The observation of SE indicated the potential of DBOV for appli-cation to be optical gain materials, for example, in laser and optical ampli�ers.

Further transient absorption studies comparing DBOV derivatives 15a─ c revealed that the lifetime of SE depends on the concentration and aggregation tendency because of the competition of the optical gain and intermolecular charge transfer. 45b DBOV 15a with two mesityl and two dodecyl groups thus demonstrated the longest SE lifetimes at lower concentrations, while the less substituted 15b and especially 15c displayed shorter SE lifetimes.

The SE signal of DBOV 15a─ c vanished in their neat �lms along with the aggregation─ induced quenching of the �uores-cence. Nevertheless, the �uorescence and SE in thin �lms could be recovered by embedding 15a─ c in polystyrene (PS) (Figure 10b). Notably, ampli�ed stimulated emission (ASE) could be observed from �lms containing 1 wt% 15a (Figure 10c) and 15b in the PS matrix. The ASE threshold was also dependent on the aggregation tendency of the DBOV derivatives, similar to the observation of the SE lifetimes, and thus the threshold of 15a (60 μJcm -2) was lower than that of 15b (180 μJcm -2), and no ASE could be achieved for 15c. These results indicate that the photophysical properties of DBOV can be tuned by engineering the peripheral substituents, which is highly advantageous for their applications and makes it indispensable to develop further synthetic methods to func-tionalize the DBOV core.2.5　 Self─ assembly Behavior of DBOV with

Tridodecyloxyphenyl GroupsPAHs show strong intermolecular π ─ π stacking, and their

self─ assembly behavior has been extensively explored, along with the construction of various supramolecular structures by using different PAH cores and varying peripheral substitu-ents. 57 For example, columnar self─ assemblies are observed for PAHs with rigid cores that are substituted with �exible ali-phatic chains at the peripheral positions, displaying discotic liquid crystalline (LC) properties. 7a,58 Triphenylene, perylene, and HBC derivatives are representative examples, demonstrat-ing columnar supramolecular structures, which have also been studied for applications in �eld─ effect transistors. 59 On the other hand, the self─ assembly of PAHs on surfaces or at solid/liquid interfaces has been extensively investigated by scanning probe microscopy, demonstrating the formation of various two─ dimensional (2D) supramolecular nanoarchitectures that can be �ne─ tuned, for example, by the choice of PAH core, peripheral substituents, and substitution pattern.

The self─ assembly behavior of DBOV was investigated by using 15i with two 3,4,5─ tris(dodecyloxy)phenyl groups at the meso ─ positions (see Figure 7) in collaboration with the labora-tories of Wojciech Pisula and Steven de Feyter. 60 Differential

Figure 10.　 (a,b) Ultrafast transient spectra of 15a in (a) solution and (b) a 1 wt% blend with PS. (c) Photoluminescence spectra of 15a in a 1 wt% blend with PS taken at laser power �u-ences below and above the ASE threshold. Reproduced with permission. 26 Copyright (2017) Wiley Library.

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scanning calorimetry (DSC) and polarized optical microscopy (POM) revealed the melting of 15i into the LC phase upon heating to 158 ℃, and two─ dimensional wide─ and small─ angle X─ ray scattering (2D─ WAXS and 2D─ SAXS) analyses indicated self─ assembly into columnar stacks arranged in a hexagonal lattice, with an intracolumnar π ─ π distance of 0.36 nm. After cooling the sample to the crystalline phase at 30 ℃, the molecular motion of 15i inside the supramolecular organization was reduced, demonstrating a higher long─ range order and slightly increased π ─ stacking distance of 0.37 nm. The intercolumnar arrangement changed from hexagonal to a square lattice, and helical intracolumnar packing with a rota-tion of 12° between adjacent molecules as well as a very long helical pitch of 5.55 nm were elucidated (Figure 11a).

The 2D self─ assembly behavior of 15i was studied at the interface of highly oriented pyrolytic graphite (HOPG) and 1─ phenyloctane by scanning tunneling microscopy (STM), which revealed molecular layers with DBOV cores aligned in a row─ like fashion (Figure 11b). Interestingly, 15i preferentially formed molecular bilayers even in a dilute solution of 6×10 -6 mol L -1, in contrast to the majority of previously studied PAHs, which predominantly showed self─ assembled monolay-ers at solid─ liquid interfaces. Moreover, the orientation of the DBOV cores was perpendicular to the row in the second layer, but they were rotated by approximately 40° with respect to the row in the �rst layer. These results indicated that the self─ assembled structure of the second layer was affected by the �rst layer and that the interactions between the molecules of 15i were stronger than the molecule─ surface interactions. It should be noted that the layers above the second layer were probably removed during the STM measurements, as the STM tip needed to come closer to the HOPG to have suf�cient tun-neling currents. The self─ assembly of 15i into well─ organized columnar and 2D supramolecular structures supports the potential of such DBOV derivatives for optoelectronic device applications.2.6　 Application of DBOV in Superresolution Microscopy

Superresolution �uorescence microscopy, such as single─ molecule localization microscopy (SMLM) and stimulated emission depletion (STED) microscopy, allows visualization with higher resolutions than conventional light microscopy, going beyond the optical diffraction limit. 61 The SMLM is typically based on the acquisition of thousands of �uorescence images, detecting the emission from single �uorophore mole-

cules and determining their precise locations. By using �uoro-phores with the so─ called blinking properties, which switches between on and off states under continues optical excitation, each �uorescence image shows locations of different sets of the �uorophore molecules without spatial overlaps, and �tting of the data provides the high─ resolution images. With the view of applying DBOV as a �uorophore for optical imaging, espe-cially in SMLM, the blinking properties of 15b were investi-gated in collaboration with Xiaomin Liu, Mischa Bonn, and their colleagues. 27 DBOV 15b indeed demonstrated blinking with a blinking time of 87 ms, which is approximately 1.3─ fold longer than that of Alexa 647 (69 ms), one of the most widely used �uorophores for SMLM. Remarkably, the blinking of 15b could be observed under various environments, including under air and in a PS matrix, in stark contrast to Alexa 647, which needs a special buffer for blinking. Moreover, 15b dis-played highly stable blinking behavior, showing comparable numbers of photons per blinking event after storing samples over several months. The applicability of 15b in the SMLM could also be con�rmed by the visualization of nanoscale crevices on a glass surface, which agreed very well with the atomic force microscopy images. These results indicate the great potential of DBOV as a �uorescent probe in imaging applications, especially in bioimaging.

3． Regioselective Bromination of DBOV

After establishing the scalable synthetic method for DBOV derivatives 15 with various meso ─ substituents and revealing their highly intriguing optoelectronic and photophysical prop-erties as well as self─ assembly behavior, direct postsynthetic substitution of the DBOV core was next considered. To this end, meso ─ mesityl substituted DBOV 15b was employed, and different bromination conditions were applied. 62 The treatment of 15b with bromine apparently resulted in the oxidation of 15b, and the use of NBS in a mixture of chloroform and N,N ─ dimethylformamide led to no reaction. Nevertheless, the reac-tion of 15b with NBS in tetrahydrofuran successfully provided dibrominated DBOV 21 in 79% yield (Figure 13). The struc-ture of 21 was con�rmed by 1 H NMR analyses in views of disappearance of the doublet peak of 15b at 9.21 ppm and the presence of 6 doublet peaks (Figure 14a─ b), as well as by sin-

Figure 11.　 (a) Schematic illustration of the helical organization of 15i at 30 ℃. (b) High─ resolution STM images superim-posed on a tentative model of the molecular assembly. Reproduced with permission. 60 Copyright (2019) Royal Society of Chemistry.

Figure 13.　 Synthesis of DBOV derivatives 22 by the selective bro-mination of DBOV─ Mes at the peripheral positions and transition─ metal─ catalyzed cross─ coupling reactions.

（ 102 ） J. Synth. Org. Chem., Jpn.1100

gle─ crystal X─ ray diffraction analysis that clearly revealed the two bromo groups at the 3─ and 11─ positions (Figure 14c─ d). Calculation of the Mulliken charge distributions on the DBOV core indicated higher electron density at the 3─ and 11─ posi-tions, accounting for the selective bromination at these posi-tions.

With dibrominated DBOV 21 in hand, further edge functionalization of DBOV at these bay positions was envis-aged. As an initial test, DBOV 21 was subjected to Suzuki─ Miyaura coupling with three arylboronic acids and Sono-gashira coupling with TIPS─ acetylene, which afforded 3,11─ diaryl─ substituted DBOV 22a─ c in 33─ 69% yield and 3,11─ bis(TIPS─ ethynyl)─ substituted DBOV 22d in 31% yield, respectively (Figure 13). The UV─ Vis absorption spectra of 22a─ c were similar to those of DBOV derivatives 15 with absorption maxima at 625─633 nm (1.96─1.98 eV), which were redshifted by 10─18 nm (0.04─0.06 eV) in comparison to 15b. The small redshift might be due to slightly extended π ─ conju-gation between the DBOV core and the aryl groups at the bay positions. TIPS─ ethynyl─ substituted DBOV 22d displayed an absorption maximum at 646 nm (1.92 eV) with a larger redshift of 35 nm (0.10 eV) than that of 15b, similar to the observation for DBOV 15h with TIPS─ ethynyl groups at the meso ─ posi-tions. Notably, 22a─ c demonstrated an improved relative PLQY of 0.91─0.97, which might be a result of limited inter-molecular interactions by the additional bulky substituents as well as distortion of the planarity of the DBOV core by bay substitution. The PLQY of 22d was 0.69, similar to that of 15h. This method paves the way towards the bay─ position functionalization of DBOV with various substituents for mod-ulation of the optoelectronic properties, as well as building up complicated molecular systems incorporating DBOV units.

4． Synthesis of Circumpyrene from Brominated DBOV

The bay regions of PAHs are often used for the further π ─ extension of their π ─ conjugated structures, for example, by intramolecular Friedel─ Crafts cyclization, 63 Diels─ Alder cyclo-addition, 64 and Pd─ catalyzed direct annulation. 65 To this end, π ─ extension of DBOV through the introduction of two addi-tional C=C bonds at its bay regions can lead to the long─ awaited circumpyrene. 33 For the synthesis of circumpyrene 24,

the Diels─ Alder cycloaddition of DBOV 15b with diphenyl-acetylene was initially attempted, but there was no reaction even upon heating at 180 ℃ in o ─ dichlorobenzene for 24 h, most likely because of the weak diene character of 15b (Figure 15). In an alternative approach, TIPS─ ethynyl─ substi-tuted DBOV 22d was deprotected with TBAF to provide ethy-nyl─ substituted DBOV 25 in 32% yield. Subsequently, the PtCl 2─ catalyzed cycloaromatization of the ethynyl groups of 25 afforded circumpyrene 26 in 46% yield. However, the poor solubility of 26 prohibited further characterization of its struc-ture and properties, for example, by 13 C NMR, single─ crystal X─ ray analysis, and cyclic voltammetry. On the other hand, as the second synthetic approach, the Pd─ catalyzed direct benzannulation of 21 and diphenylacetylene was carried out, which provided benzo[bc]naphtho[2,1,8,7─ stuv]ovalene (BNOV) 23 and circumpyrene 24 in 40% and 15% yields, respectively.

The 1 H NMR spectrum of BNOV 23 exhibited �ve new signals from the aromatic core due to the lower symmetry after fusing one C=C bond to DBOV 15b (Figure 16a). In contrast, the 1 H NMR spectrum of circumpyrene 24 showed one singlet peak and four sets of doublet peaks from the aromatic core, and their chemical shifts were shifted down�eld with respect to the corresponding peaks observed in the spectra of 15b and 23. The structure of circumpyrene 24 was also demonstrated by single─ crystal X─ ray diffraction analysis (Figure 16b─ d). The plane─ to─ plane distance of 24 is 4.73 Å, indicating no π ─ π interactions (Figure 16d). The short distance between the CH bonds on the phenyl rings and the core of the neighboring cir-cumpyrene molecules (2.75 Å) indicates the existence of CH─ π interactions, which can be responsible for the highly ordered packing mode in the single crystals (Figure 16d).

Interestingly, the UV/Vis absorption spectra revealed that the longest absorption wavelength of 23 (555 nm; 2.23 eV) was blueshifted by 56 nm in comparison with that of 15b (611 nm; 2.03 eV) (Figure 17a), indicating an increase in the optical energy gap by 0.20 eV upon π ─ extension. This observation could be understood in terms of a decreased number of π ─ electrons in the conjugation pathway of 23 according to the anisotropy of the induced current density (ACID) calculations and was also in line with an increased number of Clar’s π ─ sex-

Figure 14.　 1 H NMR spectra of DBOV─ Mes 15b (a) and 21 (b). Single crystallographic structure of 21 (c) front view and (d) side view. Reproduced with permission. 62 Copyright (2019) Wiley Library.

Figure 15.　 Synthesis of circumpyrenes 24 and 26. TABF: tetra─ n ─ butylammonium �uoride.

Vol.78 No.11 2020 （ 103 ） 1101

tets from 15b (four) to 16 (�ve). By comparison, circumpyrenes 24 and 26 displayed distinct UV/Vis absorption patterns with the longest─ wavelength bands at 549 and 558 nm, respectively, which seemed to be forbidden transitions. This result agreed very well with the theoretical prediction by Lischka et al., who reported that the longest─ wavelength absorption band of cir-cumpyrene should arise from the forbidden HOMO─ 1(H─ 1)→LUMO (L) and H→L+1 transitions based on density functional theory/multireference con�guration interaction (DFT/MRCI) calculations. 66 In the �orescence spectra, the maximum emission peaks were located at 614 nm (15b), 563 nm (23), 555 nm (24), and 566 nm (26) with small Stokes shifts (Figure 17b). The absolute PLQYs of 15b, 23, 24, and 26

were 0.79, 0.42, 0.11, and 0.17, respectively, which dropped drastically after fusing extra double bonds, highlighting the sensitive dependence of the optical properties on the PAH structure. Furthermore, cyclic voltammetry analyses of 15b, 23, and 24 allowed the estimation of the electrochemical energy gaps based on the onset potentials of the �rst oxidation and reduction peaks to be 1.81, 2.06, and 2.16 eV, respectively (Figure 17c). This trend was consistent with the HOMO─ LUMO energy gaps based on the DFT calculations (Figure 17d), as well as the optical energy gap estimated from the UV/Vis absorption spectra, which could be understood in terms of the decreased number of π ─ electrons in the conjuga-tion pathway after fusing C=C bonds.

5． Conclusion

The successful synthesis of DBOV 15, a new PAH with zigzag and armchair edges revealed its intriguing optical prop-erties, including strong red �uorescence, stimulated emission, and �uorescence blinking under both air and polymer matrix, which enabled the application of 15 as a �uorophore in SMLM imaging. Although the initial synthetic routes through oxida-tive cyclodehydrogenation to prepare 14 provided 15 in total yields of only ～2% over 13─14 steps, a simpli�ed and scalable synthesis of DBOV 15 was developed based on the iodination─ benzannulation of diyne 10b to give iodinated bichrysene 20, followed by photochemical cyclodehydroiodination to yield 14b on a gram scale. DBOV 15 could thus be obtained in a total yield of 14─36% over 9 steps, which also allowed the exploration of its postsynthetic substitution, leading to the regioselective bromination of DBOV at the 3 and 11 positions. Moreover, circumpyrene could be achieved as the largest syn-thesized circumarene to date through π ─ extension at the bay regions of the DBOV core via the transition─ metal─ catalyzed alkyne benzannulation of dibrominated DBOV 21. These results indicate the high potential of DBOV as a highly �uores-cent and stable PAH for optoelectronic, photonic, and imaging applications as well as the possibility of developing a wider variety of unprecedented PAH structures starting from DBOV.

AcknowledgmentsWe acknowledge all of our distinguished collaborators and

dedicated colleagues who enabled the achievements described in this article. We appreciate the �nancial support from the Okinawa Institute of Science and Technology Graduate Uni-versity, the Max Planck Society, the Marie Curie ITN project iSwitch (GA No. 642196), and the ANR─ DFG NLE grant GRANAO by DFG 431450789.

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PROFILE

Xiushang Xu received his Ph.D. degree at the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences under the su-pervision of Professor Lixiang Wang in Janu-ary 2019. Since April 2019, he has been a postdoctoral researcher in the Narita Unit at the Okinawa Institute of Science and Tech-nology Graduate University (OIST). He was also a guest scientist at the Max Planck Insti-tute for Polymer Research (MPIP) in Mainz, Germany, for one year starting in April 2019. His current research interest focuses on the bottom─ up synthesis of novel nanocarbon materials with atomically precise structures for the elucidation of their optical and elec-tronic properties and applications for opto-electronic devices and bioimaging.

Qiang Chen obtained his Master’s degree at Nankai University in 2014 under the supervi-sion of Professor Jianyu Zheng, where he studied S NAr reactions of porphyrin. He then joined the group of Professor Klaus Müllen at the Max Planck Institute for Polymer Re-search in 2015, working in the subgroup of Dr. Akimitsu Narita, and obtained his Ph.D. degree in chemistry in 2019. Since December 2019, he has been a postdoctoral researcher in the same group. His current research fo-cuses on the bottom─ up syntheses of carbon─ rich materials, including nanographene mole-cules and graphene nanoribbons, with well─ de�ned edge structures, as well as investigat-ing their optoelectronic properties and appli-cations.

Akimitsu Narita studied chemistry at the University of Tokyo, where he received his Bachelor’s (2008) and Master’s (2010) degrees under the supervision of Professor Eiichi Nakamura. He then joined the group of Pro-fessor Klaus Müllen at MPIP and obtained his Ph.D. in Chemistry in 2014, granted by the Johannes Gutenberg University of Mainz. Since 2014, he has been a group lead-er at MPIP. In 2018, he joined OIST as an Assistant Professor (Adjunct) and became an Assistant Professor in 2020, leading the Or-ganic and Carbon Nanomaterials Unit. His current research focuses on the synthesis and characterization of large polycyclic aromatic hydrocarbons as nanographenes. He received the Chemical Society of Japan Award for Young Chemists for 2017.

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